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Bioturbation in Cambrian siliciclastic shelf strata: paleoecological, paleoenvironmental, and temporal patterns
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Bioturbation in Cambrian siliciclastic shelf strata: paleoecological, paleoenvironmental, and temporal patterns
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Content
BIOTURBATION IN CAMBRIAN SILICICLASTIC SHELF STRATA:
PALEOECOLOGICAL, PALEOENVIRONMENTAL, AND TEMPORAL
PATTERNS
by
Katherine Nicholson Marenco
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(GEOLOGICAL SCIENCES)
December 2008
Copyright 2008 Katherine Nicholson Marenco
ii
ACKNOWLEDGEMENTS
I am deeply grateful to my committee for their guidance throughout this
process, from the qualifying exams to the defense. Each member posed challenging
questions that forced me to re-examine my project and, ultimately, improve it. Al
Fischer and Donn Gorsline generously shared their wealth of sedimentological,
paleoenvironmental, and paleoecological knowledge. Wiebke Ziebis contributed an
invaluable modern marine biological perspective. Frank Corsetti lent his expertise on
Death Valley stratigraphy, paleoecology, and field localities. And, most importantly,
Dave Bottjer helped me maintain both a paleoecological focus and a positive
outlook. I owe my fascination with Cambrian bioturbation to Dave and am thankful
for his guidance and support throughout my graduate career.
My fieldwork in Wisconsin would not have been possible without Whitey
Hagadorn’s generosity. Thanks to Whitey, I have had the privilege of studying some
of the largest and most fascinating bedding planes in the world. I am also grateful to
Dan Damrow for his hospitality during my Wisconsin fieldwork.
Per Ahlberg and Sören Jensen both helped me arrange my fieldwork in
Sweden. I have benefited greatly over the past few years from Sören’s expert advice
on trace fossils and bedding-plane-rich localities. Sören gave me invaluable guidance
on localities and logistics as I planned this trip. I am particularly indebted, however,
to Per Ahlberg, who devoted so much of his time and energy to making my stay in
iii
Sweden comfortable, productive, and enjoyable. I am also grateful to Mikael Calner
for showing me around the Hardeberga Quarry and taking an interest in my work.
The field activities and analyses I conducted over the course of this project
were made possible by funding from the Evolving Earth Foundation, the Geological
Society of America, Sigma Xi, the Paleontological Society, the International
Association of Sedimentologists, and the University of Southern California.
The love and support of my family has helped me throughout my time in
graduate school. I am particularly grateful to my husband, Pedro, for working
tirelessly as my field assistant and always cheering me on.
iv
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
LIST OF TABLES viii
LIST OF FIGURES ix
ABSTRACT xix
CHAPTER I 1
Introduction 1
Purpose 1
Trace Fossils and Bioturbation 1
Ecosystem Engineering in the Fossil Record 4
Introduction 4
Paleocommunity Reconstruction 5
Identifying Ecosystem Engineers in the Fossil Record 8
The Cambrian Period 11
Early Metazoan Allogenic Engineers 15
Early Metazoan Autogenic Engineers 23
Summary 27
Dissertation Summary 29
CHAPTER II 34
Quantitative Analysis of Bioturbation on Bedding Planes 34
Introduction and Previous Work 34
Methodology 43
Testing the Cell and Intersection Methods 48
Results 51
Test 1: Vertical Bioturbation 51
Test 2: Horizontal Bioturbation 51
Discussion – Evaluation of the Cell and Intersection Methods 56
Practical Considerations for Applying the Intersection Method 64
CHAPTER III 68
Bioturbation in the Lower Cambrian Succession of the Death Valley area,
CA-NV 68
Introduction and Previous Work 68
Paleogeography and Tectonic Reconstruction of the Lower
Cambrian Succession in the Western United States 70
Geological Setting 71
v
Introduction 71
Previous Work 75
Wood Canyon Formation 77
Zabriskie Quartzite 84
Methods 89
Introduction 89
Field Methods 90
Laboratory Methods 96
Data Analysis 98
Results 98
Trace Fossils 98
Data from Meter Sections and Samples: 104
Sample Data: General Patterns 105
Titanothere Canyon 113
Echo Canyon 113
Montgomery Mountains 127
Emigrant Pass 149
Winters Pass Hills 155
Northern Salt Spring Hills 156
Bedding Plane Grid Analysis 164
Discussion 176
Volborthella 176
Bioturbation in the Lower Cambrian of the Death Valley
Region 179
Comparison with Studied Material from the White-Inyo
Mountains 191
Implications 194
CHAPTER IV 197
Bioturbation in the Lower Cambrian Succession of Scania, Southern
Sweden 197
Introduction 197
Early Cambrian Paleogeography of Scania 197
Geological Setting 198
Overview 198
Hardeberga Sandstone 203
Methods 207
Introduction 207
Field Methods and Localities 207
Laboratory Methods 211
Results 211
Trace Fossils 211
Vik 213
Brantevik 222
vi
Sample Analysis 227
Discussion 236
Hardeberga Sandstone 236
Norretorp Formation and Rispebjerg Sandstone 238
Comparison with the Death Valley Succession 238
CHAPTER V 241
Exceptional Bedding Plane Exposures in the Upper Cambrian of Central
Wisconsin 241
Introduction and Previous Work 241
Upper Cambrian Paleogeography of Central Wisconsin 243
Geological Setting 245
Introduction 245
Mt. Simon Sandstone 245
Wonewoc Sandstone 250
Erosional Outliers 251
Methods 253
Field Localities and Methods 253
Laboratory Methods 256
Results 257
Trace Fossils 257
Krukowski Quarry Surface 260
Nemke Quarry Surface 263
Pointe Quarry Surface 276
Discussion 279
Krukowski Quarry Surface 279
Nemke Quarry Surface 282
Pointe Quarry Surface 284
CHAPTER VI 287
Conclusions 287
Quantitative Methods for the Study of Bioturbation on Bedding
Planes 287
Bioturbation in the Lower Cambrian Succession of the Death
Valley Region 290
Bioturbation in the Lower Cambrian Succession of Southern
Sweden 299
Extensive Bedding Plane Exposures in the Upper Cambrian of
Wisconsin 302
Synthesis 305
BIBLIOGRAPHY 309
vii
APPENDIX I 338
Locality Information 338
Death Valley Region 338
Southern Sweden 340
Wisconsin 340
APPENDIX II 341
Death Valley Grid Method Data 341
APPENDIX III 382
Nemke Quarry, Wisconsin, Grid Method Data 382
viii
LIST OF TABLES
TABLE A2 – Field and intersection grid method data from the Lower
Cambrian succession in the Death Valley region 342
TABLE A3 – Field and intersection grid method data from the studied Nemke
Quarry bedding plane 383
ix
LIST OF FIGURES
FIGURE 1.1 – Characteristic features of Lower Cambrian rocks 13
FIGURE 1.2 – Illustration of typical Precambrian “matground” seafloors and
post-Cambrian “mixground” seafloors 16
FIGURE 1.3 – Trace fossils preserved in Lower Cambrian and Eocene rocks 18
FIGURE 1.4 – Schematic illustration of the Late Neoproterozoic to post-
Cambrian shift in the dominant processes controlling seafloor conditions 22
FIGURE 1.5 – Views of an archaeocyath-calcimicrobial reef 25
FIGURE 2.1 – Illustrated key for use in determining bedding-plane bioturbation
indices (BPBIs) in the field 37
FIGURE 2.2 – Photographs illustrating the effect of lighting on bioturbation
visibility 40
FIGURE 2.3 – Tracing technique for estimating the percentage of a bedding
surface that contains bioturbation 41
FIGURE 2.4 – Bedding-plane-surface expressions of vertical and horizontal
bioturbation 44
FIGURE 2.5 – Illustrations of three methods for estimating percentage cover 46
FIGURE 2.6 – Black-and-white images of “bioturbation” used to test the
intersection and cell grid methods 49
FIGURE 2.7 – Analysis of the “vertically bioturbated” image at three grid scales 52
FIGURE 2.8 – Three additional grid scales used to analyze the “vertically-
bioturbated” image 53
FIGURE 2.9 – Results of Test 1 (“vertical bioturbation”) 54
FIGURE 2.10 – Analysis of the “horizontally-bioturbated” image at three grid
scales 55
x
FIGURE 2.11 – Three additional grid scales used to analyze the “horizontally-
bioturbated” image 56
FIGURE 2.12 – 128x128-cell grid analyses of the “horizontally-bioturbated”
image 58
FIGURE 2.13 – Results of Test 2 (“horizontal bioturbation”) 59
FIGURE 2.14 – Results of Tests 1 and 2, plotted together 60
FIGURE 2.15 – A fractured bedding plane surface in the upper member of the
Lower Cambrian Wood Canyon Formation 66
FIGURE 3.1 – Early Cambrian paleogeography 72
FIGURE 3.2 – Generalized stratigraphy of the Lower Cambrian successions in
the White-Inyo Mountains and the Death Valley region 73
FIGURE 3.3 – Locality map for the Death Valley region 91
FIGURE 3.4 – 600cm
2
frames used for bedding plane analysis 94
FIGURE 3.5 – Illustrated key for use in assigning the five ichnofabric indices 95
FIGURE 3.6 – Field photographs of Planolites and Rusophycus 99
FIGURE 3.7 – Field photographs of Psammichnites and Skolithos 101
FIGURE 3.8 – Field photographs of Psammichnites 102
FIGURE 3.9 – Photomicrographs of features observed in the thin section of a
sample from the upper member Wood Canyon Formation, Echo Canyon 106
FIGURE 3.10 – Photomicrographs of bioturbation structures in thin section 108
FIGURE 3.11 – Photomicrographs of probable Volborthella fragments in thin
section 111
FIGURE 3.12 – Lower member Wood Canyon Formation bedding plane
sample from Titanothere Canyon 114
FIGURE 3.13 – Bedding plane samples collected from Echo Canyon 116
xi
FIGURE 3.14 – Echo Canyon, Meter A (upper member Wood Canyon
Formation) 119
FIGURE 3.15 – Echo Canyon, Meter B (upper member Wood Canyon
Formation) 123
FIGURE 3.16 – Montgomery Mountains, Meter A (lower member Wood
Canyon Formation) 129
FIGURE 3.17 – Montgomery Mountains, Meter B (lower member Wood
Canyon Formation) 133
FIGURE 3.18 – Montgomery Mountains, Meter C (middle member Wood
Canyon Formation) 136
FIGURE 3.19 – Montgomery Mountains, Meter D (middle member Wood
Canyon Formation) 140
FIGURE 3.20 – Montgomery Mountains, Meter E (middle member Wood
Canyon Formation) 146
FIGURE 3.21 – Bedding plane samples collected from Emigrant Pass 150
FIGURE 3.22 – Rusophycus-bearing bed sole in the Zabriskie Quartzite 152
FIGURE 3.23 – Bedding plane samples collected from the Wood Canyon
Formation in the Northern Salt Spring Hills 157
FIGURE 3.24 – Northern Salt Spring Hills, Meter (upper member Wood
Canyon Formation) 160
FIGURE 3.25 – Intersection grid method data from the Death Valley region,
graphed in order of increasing area bioturbated 166
FIGURE 3.26 – Cross plot of field BPBI estimates and intersection grid
method percentage estimates 167
FIGURE 3.27 – Intersection grid method data from the Death Valley region,
graphed in bins of 5% bioturbation 168
FIGURE 3.28 – Intersection grid method results from the lower member of the
Wood Canyon Formation 169
xii
FIGURE 3.29 – Intersection grid method results from the middle member of
the Wood Canyon Formation 170
FIGURE 3.30 – Intersection grid method results from the upper member of the
Wood Canyon Formation 171
FIGURE 3.31 – Intersection grid method results from the Zabriskie Quartzite 172
FIGURE 3.32 – Intersection grid data from both formations, graphed together 173
FIGURE 3.33 – Intersection grid data from both formations, grouped by
bedding plane grain size 175
FIGURE 3.34 – Examples of probable Volborthella fragments in x-
radiographs of Death Valley region samples 178
FIGURE 3.35 – Field photographs of a shell bed in the upper member of the
Wood Canyon Formation in Echo Canyon 180
FIGURE 3.36 – Field photographs of the Rusophycus-bearing bed sole in the
Zabriskie Quartzite 187
FIGURE 3.37 – Bedding plane bioturbation index data from the Lower
Cambrian successions in Death Valley and the White-Inyo Mountains 190
FIGURE 4.1 – Early Cambrian paleogeography 199
FIGURE 4.2 – Generalized stratigraphy of the Lower Cambrian succession in
Scania, southern Sweden 201
FIGURE 4.3 – Map of field localities in Scania, southern Sweden 208
FIGURE 4.4 – Field photograph of the Vik locality 209
FIGURE 4.5 – Field photograph of the Brantevik locality 210
FIGURE 4.6 – Field photographs of trace fossils found in the Hardeberga
Sandstone 212
FIGURE 4.7 – Field photograph of the studied meter-thick section at Vik 214
FIGURE 4.8 – Field photograph of intensely-bioturbated beds within the 60-
70 centimeters of outcrop below base of studied meter at Vik 215
xiii
FIGURE 4.9 – Close-up view of herringbone cross-stratification within the
studied meter-thick section at Vik 217
FIGURE 4.10 – Field photographs of studied bedding planes and trace fossils
within measured meter-thick section at Vik 218
FIGURE 4.11 – Field photographs of an intensely-bioturbated outcrop in the
Vik Member of the Hardeberga Sandstone at Vik 221
FIGURE 4.12 – Field photograph of a block of Hardeberga Sandstone that
contains abundant Psammichnites gigas 223
FIGURE 4.13 – Studied meter-thick section of the Tobisvik Member of the
Hardeberga Sandstone at Brantevik 224
FIGURE 4.14 – Bedding planes within the measured meter at Brantevik 226
FIGURE 4.15 – Field photograph of a bedding plane surface in the Norretorp
Formation at Brantevik 228
FIGURE 4.16 – Field photographs of the bedding plane surface at the top of
the Rispebjerg Sandstone at Brantevik 229
FIGURE 4.17 – Samples collected from the measured meter at Vik 230
FIGURE 4.18 – Sample collected from the Didymaulichnus-bearing bedding
plane in the studied meter at Brantevik 235
FIGURE 5.1 – Late Cambrian paleogeography 244
FIGURE 5.2 – Location map for study area in north-central Wisconsin 246
FIGURE 5.3 – Generalized stratigraphy of the Upper Cambrian succession in
Wisconsin 247
FIGURE 5.4 – Impression of a scyphozoan medusa on a quarried bedding
plane surface of Mt. Simon-Wonewoc Sandstone 252
FIGURE 5.5 – Views of quarries that contain studied bedding planes 254
FIGURE 5.6 – Trace fossils on quarried surfaces of Mt. Simon-Wonewoc
Sandstones 258
xiv
FIGURE 5.7 – Field photograph of studied bedding plane in the Krukowski
Quarry 261
FIGURE 5.8 – Graph of number of Climactichnites traces intersecting grid
cells on the Krukowski Quarry bedding plane surface 262
FIGURE 5.9 – Schematic representation of the Krukowski Quarry bedding
plane surface 264
FIGURE 5.10 – Measured widths of Climactichnites burrows on studied
Krukowski Quarry bedding plane 265
FIGURE 5.11 – Field photograph of a portion of the studied Krukowki Quarry
bedding plane in which earlier episodes of burrowing activity by
Climactichnites tracemakers are visible 266
FIGURE 5.12 – BPBI data from the Krukowski Quarry bedding plane 267
FIGURE 5.13 – Field photograph of Nemke Quarry bedding plane surface 268
FIGURE 5.14 – Field photograph of putative sand stromatolites on a portion
of the Nemke Quarry bedding plane 269
FIGURE 5.15 – Field photograph of “small” and “large” Gordia on a portion
of the Nemke Quarry bedding plane 271
FIGURE 5.16 – Gordia width measurements from the Nemke Quarry bedding
plane surface 272
FIGURE 5.17 – Schematic representation of the Nemke Quarry bedding plane 273
FIGURE 5.18 – Cross plot of field BPBI estimates and intersection grid
method estimates for selected Nemke Quarry grid cells 274
FIGURE 5.19 – Intersection grid method results from randomly-selected grid
cells on the Nemke Quarry bedding plane 275
FIGURE 5.20 – Field photograph of the studied bedding plane surface in
Pointe Quarry 277
FIGURE 5.21 – Schematic representation of the Pointe Quarry bedding plane 278
FIGURE 5.22 – Gordia width measurements from the Pointe Quarry bedding
plane surface and combined with Nemke bedding plane surface data 280
xv
FIGURE 5.23 – Field photographs of portions of the Nemke Quarry bedding
plane in which Gordia traces are sparse 285
FIGURE 6.1 – Comparison of Lower Cambrian bedding plane bioturbation
data obtained from the Death Valley region in this study and
ichnofabric index data compiled from the western U.S. by Droser and
Bottjer (1989) 295
FIGURE A2.1 – Emigrant Pass, Meter 1, bedding plane at 0cm (EP01-0) 345
FIGURE A2.2 – Emigrant Pass, Meter, bedding plane at 91cm (EP01-91s1) 346
FIGURE A2.3 – Emigrant Pass, Rusophycus-bearing bedding plane (EPRBS) 347
FIGURE A2.4 – Emigrant Pass, Psammichnites-bearing bedding plane
(SEPP1) 348
FIGURE A2.5 – Montgomery Mtns, Meter D, bedding plane at 18cm
(MM01-18) 349
FIGURE A2.6 – Montgomery Mtns, Meter D, bedding plane at 100cm
(MM01-100) 350
FIGURE A2.7 – Montgomery Mtns, Meter C, bedding plane at 8cm (MM02-8) 351
FIGURE A2.8 – Montgomery Mtns, Meter C, bedding plane at 100cm
(MM02-100) 352
FIGURE A2.9 – Montgomery Mtns, Meter E, bedding plane at 65cm
(MM03-65) 353
FIGURE A2.10 – Montgomery Mtns, Meter E, bedding plane at 74cm
(MM03-74) 354
FIGURE A2.11 – Montgomery Mtns, Meter E, bedding plane at 78cm
(MM03-78) 355
FIGURE A2.12 – Montgomery Mtns, Meter E, bedding plane at 100cm
(MM03-100) 356
FIGURE A2.13 – Montgomery Mtns, Meter B, bedding plane at 100cm
(MM04-100) 357
xvi
FIGURE A2.14 – Montgomery Mtns, Meter A, bedding plane at 78cm
(MM05-78) 358
FIGURE A2.15 – Montgomery Mtns, Meter A, bedding plane at 100cm
(MM05-100) 359
FIGURE A2.16 – Montgomery Mtns, Outcrop, bedding plane (MMBP-1) 360
FIGURE A2.17 – Montgomery Mtns, Outcrop, bedding sole (MMBP-2) 361
FIGURE A2.18 – Echo Canyon, bedding plane EC01A 362
FIGURE A2.19 – Echo Canyon, bedding plane ECZ01 363
FIGURE A2.20 – Echo Canyon, bedding plane ECBP01 364
FIGURE A2.21 – Echo Canyon, Meter A, bedding plane (ECBP02) 365
FIGURE A2.22 – Echo Canyon, Meter B, bedding plane at 29cm (ECBP03) 366
FIGURE A2.23 – Echo Canyon, Meter B, bedding plane at 35cm (ECBP04) 367
FIGURE A2.24 – Echo Canyon, Meter B, bedding plane at 45cm (ECBP05) 368
FIGURE A2.25 – Echo Canyon, Meter B, bedding plane at 61cm (ECBP06) 369
FIGURE A2.26 – Titanothere Canyon, bedding plane TIT2 370
FIGURE A2.27 – Winters Pass Hills, lower bedding plane (WPH1) 371
FIGURE A2.28 – Winters Pass Hills, upper bedding plane (WPH2) 372
FIGURE A2.29 – Northern Salt Spring Hills, bedding plane B (NSSH01) 373
FIGURE A2.30 – Northern Salt Spring Hills, bedding plane A (NSSH02) 374
FIGURE A2.31 – Northern Salt Spring Hills, bedding plane C (NSSH03) 375
FIGURE A2.32 – Northern Salt Spring Hills, Meter, bedding plane (NSSH04) 376
FIGURE A2.33 – Northern Salt Spring Hills, bedding plane D (NSSH05) 377
FIGURE A2.34 – Northern Salt Spring Hills, bedding plane F (NSSH06) 378
xvii
FIGURE A2.35 – Northern Salt Spring Hills, bedding plane E (NSSH07) 379
FIGURE A2.36 – Northern Salt Spring Hills, bedding plane G (NSSH08) 380
FIGURE A2.37 – Northern Salt Spring Hills, bedding plane H (NSSH09) 381
FIGURE A3.1 – Cell K8 386
FIGURE A3.2 – Cell J20 387
FIGURE A3.3 – Cell B1 388
FIGURE A3.4 – Cell T8 389
FIGURE A3.5 – Cell C1 390
FIGURE A3.6 – Cell N24 391
FIGURE A3.7 – Cell T15 392
FIGURE A3.8 – Cell P24 393
FIGURE A3.9 – Cell N30 394
FIGURE A3.10 – Cell T6 395
FIGURE A3.11 – Cell H18 396
FIGURE A3.12 – Cell E1 397
FIGURE A3.13 – Cell K11 398
FIGURE A3.14 – Cell B11 399
FIGURE A3.15 – Cell N16 400
FIGURE A3.16 – Cell B14 401
FIGURE A3.17 – Cell L2 402
FIGURE A3.18 – Cell A15 403
FIGURE A3.19 – Cell P5 404
xviii
FIGURE A3.20 – Cell G20 405
FIGURE A3.21 – Cell A10 406
FIGURE A3.22 – Cell G11 407
FIGURE A3.23 – Cell H14 408
FIGURE A3.24 – Cell I14 409
FIGURE A3.25 – Cell H8 410
xix
ABSTRACT
Cambrian rocks record the morphological and behavioral diversification of
early metazoans. Bioturbation was predominantly bedding-parallel during the Early
Cambrian due to a combination of evolutionary and ecological factors.
Consequently, trace fossils are typically preserved on bedding planes in Lower
Cambrian siliciclastic strata. The overarching goal of this work was to gain a better
understanding of the agronomic revolution as it occurred in shallow marine to
transitional environments by studying the bioturbation preserved on Lower Cambrian
bedding plane exposures. The objectives of this study were as follows: (1) develop a
precise, quantitative method for evaluating bioturbation on bedding planes; (2) apply
this and other methods to Lower Cambrian rocks of the Death Valley region and
southern Sweden, which represent similar depositional settings; (3) compare the
resulting bedding plane bioturbation data with data previously collected from the
Lower Cambrian succession in the White-Inyo Mountains; and (4) study the
distribution of bioturbation across exceptionally-large bedding planes in the Upper
Cambrian of Wisconsin to determine whether small bedding planes effectively
sample the bioturbation that is present across sedimentary horizons.
The intersection grid method was developed to estimate the percentage area
of bioturbation on bedding planes. A test of the method demonstrated that results
within 5-10 percent error can be obtained even at relatively coarse grid scales. This
method was applied to bedding planes from the Lower Cambrian succession in the
xx
Death Valley region. Results of these analyses, which have a moderately broad
distribution with a cluster between 5-25 percent bioturbation, are typical of Lower
Cambrian rocks, in which bioturbation intensities are relatively low. However, an
abundance of vertical burrows in coarse-grained rocks in the Death Valley region
and southern Sweden indicates that vertical bioturbation, rather than horizontal
bioturbation, was dominant in Early Cambrian nearshore settings. In addition,
common vertical burrows in rocks of heterogeneous grain size in the Death Valley
Lower Cambrian succession suggests that adaptations to burrowing vertically into
fine-grained material may have first appeared in nearshore settings. Data from large
Upper Cambrian exposures in Wisconsin demonstrate that ancient bedding planes,
like modern intertidal zones, can exhibit considerable heterogeneity.
1
CHAPTER I
Introduction
Purpose
Many previous studies of biogenic structures have taken a taxonomic
approach, focusing on the classification of morphological variations. In contrast, the
aim of this study was to examine the temporal and environmental distribution of both
well-defined trace fossils and indistinct bioturbation, which together reflect complex
interactions between organisms and their environment. In Cambrian rocks,
bioturbation represents a significant source of information because body fossils are
often sparse. Due to a unique combination of evolutionary and ecological factors,
bioturbation in Lower Cambrian rocks is typically preserved on bedding plane
surfaces. A variety of methods were employed to obtain ichnological data from
bedding planes and field samples. These methods include photography, image
analysis, petrography, and x-radiography. Although considerable sedimentological
data was generated in the course of this study and is presented here, data relevant to
the understanding of bioturbation patterns were prioritized. Future studies will
address the sedimentological aspects of this dataset in greater detail.
Trace Fossils and Bioturbation
Trace fossils, or ichnofossils, are the preserved products of organisms’ life
activities (Bromley, 1996). At its broadest extent, this definition encompasses such
2
disparate biogenic structures as ant mounds (e.g., Turner, 2003), coprolites (e.g.,
Rodriguez-de la Rosa et al., 1998), and gnaw marks on seeds (Collinson and Hooker,
2000). In each case, the trace fossil records information about behavior and habits of
the tracemaker. When body fossils are absent or, as is often the case, the identity of
tracemakers cannot be ascertained from their trace fossils, behavioral information
contained within a suite of trace fossils can be vital for reconstructing ecospace
utilization and trophic relationships within a paleocommunity (Frey, 1978).
In the field of invertebrate paleoecology, trace fossils that record the
activities of animals on or within a sediment substrate have particular significance.
These biogenic sedimentary structures (Frey, 1978) become preserved in the rock
record of marine, non-marine, and terrestrial environments and can convey a
considerable amount of information about past ecosystems. For example, the
abundance, diversity, and size of the trace fossils preserved in a sedimentary unit
may reflect the degree of physical and competitive stress placed on members of the
paleocommunity. In marine and transitional environments, factors such as predation
(e.g., Jensen, 1990; Dzik, 2005), oxygen availability (e.g., Ekdale and Mason, 1988;
Savrda and Ozalas, 1993), temperature (e.g., Pearson and Gingras, 2006), salinity
(e.g., Zorn et al., 2006), nutrient supply (e.g., Wetzel and Uchman, 1998), substrate
stability (e.g., Droser and Bottjer, 1989a), and current energy (e.g., Pemberton and
MacEachern, 1997) can limit the development of a benthic community and,
consequently, the trace fossils that its members produce.
3
The ichnological characteristics of a marine sedimentary unit also inevitably
reflect the evolutionary development of the community that lived on and within the
sediment. As epibenthic organisms expanded upward into higher tiers during the
Paleozoic, infaunal communities gradually delved deeper into the substrate (Ausich
and Bottjer, 1982; Bottjer and Ausich, 1986; Droser and Bottjer, 1993). The effects
of increased biological sediment disturbance, or bioturbation (Frey, 1978), are
considerable, not only for benthic communities but also for the eventual preservation
of sedimentary structures and trace fossils. Bioturbation imparts a variety of physical
effects on the substrate, including geochemical zonation (e.g., Ziebis et al., 1996;
Zorn et al., 2006) and changes in sediment cycling (e.g., Graf and Rosenberg, 1997;
Graf, 1999) and sediment stability/erodibility (Rhoads and Young, 1970; Paterson,
1997). When bioturbation depth and intensity are at their upper limits, as is the case
in most modern marine substrates, the likelihood of preserving sedimentary
structures and individual trace fossils is greatly reduced, and much behavioral
diversity data is lost as a result. Lower Paleozoic rocks represent the opposite end of
the bioturbation spectrum. Marine bioturbation depths and intensities are at their
lowest levels of the Phanerozoic in Cambrian rocks (Droser, 1987; Droser and
Bottjer, 1988; Droser and Bottjer, 1993), with the exception of post-extinction
recovery intervals (Twitchett and Wignall, 1996; Twitchett, 1999; Twitchett and
Barras, 2004; Barras and Twitchett, 2007). Individual trace fossils and sedimentary
structures are generally well-preserved in Cambrian rocks due to a combination of
physical factors, such as firmer substrates, and the shallow average depth of
4
bioturbation (Droser et al., 2000). Thus, Cambrian rocks provide an interesting
window into the environments and activities of early benthic communities, including
two of the earliest examples of metazoan “ecosystem engineering” (Jones et al.,
1994).
Ecosystem Engineering in the Fossil Record
Introduction: Jones and colleagues (1994) define ecosystem engineers as
“organisms that directly or indirectly modulate the availability of resources (other
than themselves) to other species, by causing physical state changes in biotic or
abiotic materials. In so doing they modify, maintain and/or create habitats.” Until
recently, the ecosystem engineering concept had been applied only to modern
environments, in which biological interactions can be observed directly. Modern
examples of ecosystem engineering have been described from a diverse array of
habitats and at a range of scales (e.g., Flecker, 1996; Gurney and Lawton, 1996;
Gutiérrez et al., 2003; Talley and Crooks, 2007). Only in the past few years,
however, have paleoecologists taken notice of the ecosystem engineering concept
and begun to identify examples from the fossil record (e.g., Hasiotis, 2001; Curran
and Martin, 2003; Nicholson and Bottjer, 2004; Nicholson and Bottjer, 2005; Gibert
and Netto, 2006; Parras and Casadio, 2006; Marenco and Bottjer, 2008). Although
the identification of ancient ecosystem engineers can often be facilitated by
comparisons with modern analogues (e.g., burrowing behavior in modern and
Pleistocene decapod crustaceans; Curran and Martin, 2003), the task is invariably
5
challenging. Considerable ecological information is lost during the processes by
which living organisms and their surroundings become preserved. Despite the
obstacles presented by the fossil record, searching for ancient examples of ecosystem
engineering is worthwhile because it helps to improve our understanding of
ecological relationships and evolutionary trends throughout the history of life.
Paleocommunity Reconstruction: In modern environments, it is possible to
directly observe and document the activities of organisms, the effects of those
activities on the distribution of resources and, in turn, the impact of changes in
resource supply on the ecosystem as a whole. These observations permit the
identification of modern ecosystem engineers (Jones et al., 1994). When examining
the fossil record for evidence of ancient ecosystem engineering, paleoecologists must
use the limited information preserved within rocks to reconstruct paleocommunities
and interactions among community members.
The first task in this process is to determine the type of environment in which
the rocks were deposited, whether terrestrial, marine, or transitional. This is best
accomplished by examining the rocks for sediment characteristics and sedimentary
structures that reflect environment-specific physical processes and for fossil
organisms that inhabited a limited range of environments. Echinoderms, for example,
are known to have lived almost exclusively in marine settings since their appearance
over 500 million years ago (Brusca and Brusca, 2003).
Second, the fossils preserved within a rock unit must be identified.
Paleoecological data that can be obtained directly from body fossils include
6
estimates of community diversity, relative abundances of species or groups, and
occupation of morphospace, or the set of all theoretically-possible body plans
(morphotypes) (e.g., Thomas et al., 2000). Life habits of community members can be
inferred from body fossils using functional morphology, in which comparisons
between analogous body structures in modern and ancient organisms facilitate the
interpretation of fossil behavior (e.g., Brenchley and Harper, 1998). Trace fossils are
useful indicators of the range of benthic activities that took place within ancient
environments. In most cases, trace fossils are the only preserved evidence for the
presence of soft-bodied (non-skeletonized) organisms within a rock unit, unless
exceptional conditions at the time of deposition permitted the preservation of soft
tissues (Bottjer et al., 2002).
Third, the physical condition of the fossils and their distribution within the
rock unit must be taken into account. For example, a marine rock unit that contains
abundant shells may represent one of two types of deposits, depending on the
condition and orientation of the shells: a shell bed (a dense accumulation of wave-
transported and often fragmentary shell material), or an in situ community with high
population density. Determining whether marine organisms were transported away
from their habitat prior to preservation is more difficult when studying ancient soft
substrate environments, such as muddy seafloors, than hard substrate environments,
such as reefs and carbonate hardgrounds, in which many benthic organisms lived
permanently or semi-permanently attached to hard surfaces. In most instances of
hardgrounds in the fossil record, surface-attaching benthic organisms are preserved
7
in life position, providing opportunities for the study of spatial relationships among
many members of the benthic community (Taylor and Wilson, 2003). Although reefs
generally require more reconstruction due to their tendency to break apart prior to
preservation, evidence of original spatial relationships may be found in rare well-
preserved fossil reefs (e.g., Wood, 1999).
Throughout the process of paleocommunity reconstruction, caution must be
taken to avoid misinterpreting evidence obtained from rocks and fossils. One of the
more troublesome factors to take into account is the rate of sediment accumulation
versus the rates of erosion and bioturbation in a given environment. In modern
environments, sediment accumulation rates rarely remain constant for extended
periods of time. A meter-thick unit of rock may in one location represent 5 million
years of slow sediment accumulation and in another area merely 500,000 years if
sediment accumulation was more rapid. In addition, the rate of erosion may
temporarily exceed the rate of sediment accumulation, leaving a gap in the rock
record. Bioturbation intensity often appears greater during periods of slow sediment
accumulation, when the seafloor experiences prolonged exposure to benthic activity.
In addition, benthic organisms that engage in vertical burrowing may transport
younger material down into older layers of sediment, and vice versa. Thus,
fluctuations in rates of erosion, bioturbation, and sediment accumulation may lead to
time-averaging of body fossils, or the adjacent preservation of organisms that did not
coexist in life (e.g., Brenchley and Harper, 1998).
8
A second complication to be dealt with is the incompleteness of the fossil
record, even after hiatuses and time-averaging have been taken into consideration.
As is the case in modern environments, many ancient organisms were soft-bodied, or
non-skeletonized. Soft tissues are much more susceptible to decay than skeletal
components and are not preserved unless decay is inhibited through exceptional
circumstances. Thus, most fossil-bearing rock units contain body fossils only of
skeletonized organisms. Trace fossils are excellent clues to this “missing” diversity
because they record the activities of both skeletonized and soft-bodied organisms.
Identifying Ecosystem Engineers in the Fossil Record: Jones and colleagues
(1994) define two distinct categories of ecosystem engineers: “autogenic engineers,”
whose biogenic structures (living and dead) change the environment; and “allogenic
engineers,” whose activities alter the physical condition of pre-existing materials,
thereby changing the environment. Ecosystem engineers and the products of their
engineering vary in their potential to be preserved in the fossil record.
Autogenic engineers of hard structures, such as reefs, can readily be
recognized in the fossil record because their often-substantial engineering products
have high preservation potential, their original shape can usually be reconstructed,
and in most cases they reflect the taxonomic affinities of the engineers that built
them. Biogenic hard structures often are preserved with other organisms still
attached, in life position. This type of preservation facilitates interpretation of inter-
species relationships and paleocommunity trophic structure.
9
Some autogenic engineers are soft-bodied, such as aquatic plants
(“macrophytes”) and most sponges. Aquatic plants may grow densely in bodies of
fresh water. In doing so, they may affect the environment, for instance by changing
the amount of light that reaches the bottom (Carpenter and Lodge, 1986). However,
they are unlikely to be preserved as fossils unless anoxic bottom conditions,
generated by stagnation of the water column, inhibit the decay of plant material (e.g.,
Brenchley and Harper, 1998). Sponges alter fluid flow in marine environments,
through both their passive filtration systems and their physical presence on the
seafloor. Some sponges also create nutrient-rich habitats for fish and other animals
(Saito et al., 2003). Sponge construction, which typically consists of soft tissue
surrounding a matrix of unarticulated skeletal elements or “spicules” (e.g., Brusca
and Brusca, 2003), is not conducive to complete fossilization. Only the spicules are
typically preserved, and these often become scattered and mixed in with sediment
grains, leaving no record of the sponge’s original structure or life position. Rare
examples of spiculate sponges preserved intact have been found in sedimentary
deposits that appear to have formed under exceptional circumstances, such as the
Lower Cambrian Chengjiang Biota of southern China (Hou et al., 2004; Xiao et al.,
2005). Thus, physical evidence for the influence of macrophytes and sponges on
benthic communities usually is absent or limited to indirect sources, such as scattered
spicules.
Allogenic engineers that construct macroscopic burrows (e.g., fiddler crabs;
Bertness, 1985) or mounds (e.g. thalassinidean shrimp; Ziebis et al., 1996) or graze
10
on hard surfaces (e.g., periwinkles; Bertness, 1984) may have their activities
recorded as trace fossils. However, a problem may arise if the allogenic engineer is a
soft-bodied organism, such as a polychaete worm. In such a case, the engineering
activity itself may be identifiable from trace fossils, but the identity of the engineer
will likely remain unknown. The reverse of this situation is also possible: an
engineer may have a preservable skeleton but produce an ephemeral structure (e.g.
skeletonized diatoms produce mucilaginous mats; Winterwerp and van Kesteren,
2004). If no modern analogues for such an allogenic engineer are known, then its
engineering behavior may never come to light.
An additional complication is that the quantity and depth of bioturbation in
marine environments have increased throughout the past approximately 540 million
years concurrently with a gradual rise in benthic biodiversity (Ausich and Bottjer,
1982; Droser and Bottjer, 1993). Over time, as bioturbation structures began to
extend to greater sediment depths and occur in greater densities, individual burrows
and tunnels became obliterated. The resulting sediment and sedimentary rocks are
left with a nearly-homogeneous appearance. Trace fossils produced by one seafloor
community often are overprinted later by a different set of structures as
environmental conditions change (e.g., Orr, 1994). Thus, it may be difficult to
discern the preserved work of a single allogenic engineer from that of hundreds of
other benthic bioturbators within a rock unit.
Allogenic engineers that either are microscopic (e.g., meiofauna,
zooplankton) or engage in engineering activities that do not result in the production
11
of physical structures (e.g., chemical effects), or both, can be very difficult to
identify in the fossil record. Zooplankton concentrate organic matter into fecal
pellets, which assist in the vertical transport of material to the seafloor (e.g., Dunbar
and Berger, 1981). Although individual pellets are not preserved in the rock record
under normal conditions, the rise of zooplankton in ancient oceans is reflected in
marine rocks by a change in the δ
13
C ratios of preserved organic matter (Logan and
Butterfield, 1998). In most cases, however, biochemical engineering effects are too
subtle to be recorded in rocks and fossils. If such effects are, in fact, recorded, they
may mistakenly be attributed to abiotic causes.
Incomplete fossil preservation of animals and their behavior precludes the
identification of more than a fraction of the ecosystem engineers that once existed.
At the same time, many well-preserved examples of ecosystem engineering in the
fossil record have yet to be recognized. Two of the earliest examples of metazoan
allogenic and autogenic engineering in the history of life, both from the Cambrian
Period, are described below.
The Cambrian Period: The Cambrian Period (ca. 542-500 Ma) was an
important time of transition in ecological and evolutionary history. Mineralized
skeletons and skeletal elements, such as “small shelly fossils” and sponge spicules,
appeared in the earliest Cambrian but did not become widespread and diverse until
the end of the Cambrian (Brasier and Hewitt, 1979; e.g., Brasier et al., 1997). A wide
variety of soft-bodied fossils have been described from the exceptionally-preserved
Early Cambrian Chengjiang Biota in southern China, suggesting that non-
12
mineralized metazoans constituted a substantial component of Early Cambrian
benthic communities (Hou et al., 2004). Biomineralizing organisms, with predator-
and pressure-resistant skeletons, were capable of occupying a greater range of niches
than their soft-bodied counterparts, and this competitive advantage allowed the
populations of such organisms to expand into a variety of marine environments (e.g.,
Vermeij, 1989). Paralleling the trend toward widespread biomineralization among
metazoans was the rapid diversification of metazoan body plans known as the
Cambrian explosion (e.g., Thomas et al., 2000; Conway Morris, 2006; Marshall,
2006). Metazoan body plans in the earliest Cambrian were commonly simple and
limited to few types, whereas by the latest Cambrian, most of the biological
“architecture” considered characteristic of the major metazoan groups had already
become established (Sepkoski, 1979; Thomas et al., 2000).
Lower Cambrian rocks deposited in shallow subtidal marine environments
typically contain limited disruption of sedimentary layers, which reflects a lack of
vertically-oriented bioturbation, and common microbially-mediated sedimentary
structures in siliciclastic facies (Hagadorn and Bottjer, 1997) (Fig. 1.1). Microbially-
mediated sedimentary structures are thought to represent the effects of sediment
binding by microbial mats (Noffke et al., 1996; Hagadorn and Bottjer, 1997; 1999),
namely cohesive sediment behavior (Schieber, 1999). Support for this interpretation
comes from observations of modern microbial mats (e.g., Gerdes et al., 1993;
Hagadorn and Bottjer, 1999). The surfaces of many such modern mats strikingly
resemble the strange features preserved in Lower Cambrian rocks, including
13
FIGURE 1.1 – Characteristic features of Lower Cambrian rocks. (Left) Positive x-
radiograph of a vertically-sectioned siliciclastic sample from the Lower Cambrian
Campito Formation showing finely-laminated sediment that was not disrupted by
vertical bioturbation. Scale bar = 1 cm. (Right) Lower Cambrian bedding plane
exposure showing “wrinkle structures” (microbially mediated sedimentary
structures), which are thought to have formed due to the compaction of a seafloor
microbial mat. Superimposed on the wrinkle structures are examples of the simple
horizontal trace fossil Planolites (arrows). Planolites likely represents the work of a
shallow-burrowing organism that may have fed on nutrients associated with the
microbial mat.
14
“wrinkle structures” (Hagadorn and Bottjer, 1997) (Fig. 1.1), “elephant skin”
(Gehling, 1999), “domal structures” (Schieber, 1999), and “syneresis cracks”
(Pflüger, 1999).
In Lower Cambrian carbonate rocks deposited primarily in shallow water,
microbial structures are common (e.g., Rowland and Shapiro, 2002). Microbialites,
structures that formed through precipitation of carbonate in the presence of (and
often triggered by) benthic microbial communities, first appeared approximately 3.5
billion years ago (Wood, 1999). Prior to the earliest Cambrian, the dominant form of
microbialite was the stromatolite, a laminated structure produced primarily by
photosynthetic cyanobacteria (Wood, 1999). Thrombolites, non-laminated
microbialites with “clotted” textures, appeared in the Neoproterozoic (ca. 1000-542
Ma) but did not become abundant until the earliest Cambrian (Wood, 1999). The first
true reefs were constructed by microbial communities in the Neoproterozoic, and
stromatolite-thrombolite reefs persisted into the earliest Cambrian (e.g., Rowland
and Shapiro, 2002). The rise of metazoan reefs in the Early Cambrian likely
contributed to the decline of microbialites in many shallow marine environments
(Zhuravlev, 2001).
Studies to date have shown that microbially-mediated sedimentary structures
and microbialites are common in Neoproterozoic and Lower Cambrian rocks but are
comparatively scarce in younger rocks (Hagadorn and Bottjer, 1997; e.g., Gehling,
1999; 1999). This implies that most seafloor sediments in the Neoproterozoic
through Early Cambrian were bound together by microbial filaments, making them
15
firmer and more cohesive than those of the modern oceans (e.g., Hagadorn and
Bottjer, 1997; Gehling, 1999). These “matgrounds” would likely have been difficult
or impossible for benthic metazoans to penetrate, and the combination of ubiquitous
mats and a lack of infaunal bioturbation would have prevented aeration of the
sediment, allowing an oxic-anoxic boundary to develop in the sediment close to the
sediment-water interface (McIlroy and Logan, 1999). As a result, most metazoan
activity likely took place on the top surfaces of mats, within mats, or immediately
beneath them.
Seilacher (1999) proposed four guilds to characterize the categories of
metazoan activity that took place in late Neoproterozoic shallow marine benthic
communities (Fig. 1.2). These are “mat encrusters,” organisms that lived
permanently attached to the mat surface; “mat scratchers,” mobile organisms that
scavenged or hunted for food on the surface of the mat without damaging it; “mat
stickers,” suspension feeders that used conical shells to maintain an upright
orientation in the surface of the mat; and “undermat miners,” burrowers that tunneled
directly beneath the mat and fed on detritus from the layers above (Seilacher, 1999).
These mat-associated lifestyles persisted into the Early Cambrian but gradually
disappeared from open marine environments along with the microbial mats
themselves (Dornbos and Bottjer, 2001).
Early Metazoan Allogenic Engineers: The absence of vertical bioturbation in
siliciclastic rocks deposited during the Neoproterozoic and Early Cambrian indicates
that conditions beneath the seafloor surface may have been unfavorable for metazoan
16
FIGURE 1.2 – Illustration of typical Precambrian “matground” seafloors and post-
Cambrian “mixground” seafloors with their associated communities. Matgrounds
supported a specialized community of “mat scratchers,” “mat encrusters,” “mat
stickers,” and “undermat miners” (Seilacher, 1999). Post-Cambrian mixgrounds are
characterized by a diverse community of organisms that were active both on the
surface of the substrate and infaunally. Modified from Seilacher (1999).
17
activity (Bottjer et al., 2000). Although limited food resources within the sediment
may have provided little incentive for organisms to burrow infaunally, considerable
evidence, including the presence of abundant microbially mediated sedimentary
structures, suggests that physical and adaptive limitations were primarily responsible
for restricting benthic organisms to epifaunal habitats (e.g., Bottjer et al., 2000).
The fossil record of bioturbation exhibits a prominent trend over time toward
increasing trace fossil complexity, density, and penetration depth beneath the
seafloor (Ausich and Bottjer, 1982; Droser and Bottjer, 1993). The earliest-known
macroscopic trace fossils are found in rocks that were deposited during the late
Neoproterozoic (e.g., Jensen, 2003; Jensen et al., 2006). Most of these early biogenic
structures consist of simple, bilaterally-symmetrical, bedding-parallel forms that
likely represent the activities of soft-bodied vermiform organisms on or just beneath
the seafloor surface or beneath microbial mats (Valentine, 1995; Collins et al., 2000).
Trace fossils do not begin to exhibit a vertically-oriented component until the
Neoproterozoic-Cambrian boundary (ca. 542 Ma), when Treptichnus pedum, a trace
fossil that consists of a series of shallow scoop-like marks, appears in rocks
representing shallow marine environments (e.g., Droser et al., 1999; Gehling et al.,
2001) (Fig. 1.3). Although deeply-vertical burrows occurred in nearshore and
shoreface environments in the earliest Cambrian ("Skolithos piperock;" Droser,
1991), shallow burrow structures with little or no verticality were the dominant form
of bioturbation in subtidal environments until the Middle to Late Cambrian (Bottjer
et al., 2000) (Fig. 1.3). The gradual increase in bioturbation depth in shallow marine
18
FIGURE 1.3 – Trace fossils preserved in Lower Cambrian (A, B) and Eocene (C)
rocks. (A) Treptichnus pedum, the first trace fossil to exhibit a vertically-oriented
component, preserved upside-down on the bottom of a Lower Cambrian sedimentary
rock unit. The nested lobes (arrows) of the trace likely represent systematic probing
of the seafloor sediment by a priapulid-like deposit-feeding organism. (B) A Lower
Cambrian bedding plane surface that contains abundant horizontal trace fossils
(Planolites; arrows). (C) Abundant vertically-oriented trace fossils in Eocene
exposures near San Diego, CA. Conostichus (black arrow), a large lobe-shaped
burrow, is produced by anemones and other stationary benthic suspension feeders
during sediment influx. Ophiomorpha (white arrows), a deep mud-lined burrow, is
produced by many types of benthic suspension-feeding crustaceans, which require
stable semi-permanent dwellings.
19
environments from the Early to Late Cambrian has been demonstrated by Droser
(1987) and Droser and Bottjer (1988; 1989b). Rocks that were deposited in shallow
marine settings of the Late Cambrian through the Modern display a very different set
of characteristics from those representing Early Cambrian seafloors, including
visibly-disrupted sedimentary layers, common vertical burrows that may overprint
earlier bioturbation structures, and absent microbially-mediated sedimentary
structures (Bottjer et al., 2000) (Fig. 1.3). Thus, a transition occurred during the
Cambrian Period between seafloors that were characterized by primarily horizontal
bioturbation and extensive microbial mats (as reflected by the abundance of
microbially-mediated sedimentary structures and dearth of vertical sediment
disruption in Lower Cambrian rocks) and those that were characterized by extensive
vertical bioturbation and absent microbial mats.
Seilacher and Pflüger (1994) proposed the agronomic revolution hypothesis
to explain how and why this transition in seafloor conditions and benthic behavior
took place. According to this hypothesis, benthic metazoans acquired evolutionary
adaptations during the Cambrian explosion that allowed them to burrow vertically
into matgrounds. Bioturbation depth and intensity increased, eventually disrupting
the layered structure of the microbial mats and increasing the water and oxygen
content of the seafloor sediment. Mat development was relegated to marginal
environments in the wake of the agronomic revolution, and the seafloor took on
characteristics more typical of post-Cambrian marine settings, such as improved
20
nutrient distribution and an indistinct water-sediment boundary (Bottjer et al., 2000)
(Fig. 1.1).
The ecological and evolutionary effects of the agronomic revolution are
reflected in the record of body and trace fossils and have collectively been termed
the Cambrian substrate revolution (Bottjer et al., 2000). Among the more significant
effects were those felt by the matground community (Seilacher, 1999). Mat
scratchers and undermat miners were better equipped than the other guilds for
adjusting to new seafloor conditions because their mobile lifestyles allowed them to
reposition themselves in response to changes in oxygen availability and substrate
consistency (Bottjer et al., 2000). However, mat scratchers were adapted to living
and feeding on cohesive sediment surfaces, and the disappearance of such surfaces
from open marine environments forced many species to migrate into more restricted
areas where hard substrates were common, such as rocky coastlines and the deep
ocean (Bottjer et al., 2000). Mat encrusters and mat stickers faced a greater challenge
due to their specialized sessile lifestyles. Lacking a means of migrating to more
suitable environments, many of these groups evolved stems or direct attachment
mechanisms that allowed them to utilize the limited hard surfaces that were available
in shallow marine settings (Bottjer et al., 2000). Not all such groups were successful,
however. The mat-sticking helicoplacoid echinoderms, for example, did not adapt to
the new substrate conditions and became extinct before the end of the Cambrian
(Bottjer et al., 2000; Dornbos and Bottjer, 2000; 2001).
21
The agronomic and Cambrian substrate revolutions together represent the
earliest-known instance of allogenic ecosystem engineering by metazoans in the
history of life. With increasing depth and intensity of bioturbation, benthic
metazoans brought about a dramatic change in shallow subtidal seafloors of the
Early Cambrian. These organisms supplanted microbes as the dominant biotic factor
influencing substrate conditions and made a variety of previously-inaccessible
resources and ecological niches available to other members of the community
(Bottjer et al., 2000; Dornbos et al., 2004) (Fig. 1.4). The transformative effects of
bioturbation have been recognized in a wide variety of modern ecosystems as well
(e.g., Meysman et al., 2006).
In this case of allogenic ecosystem engineering, as in some of the examples
discussed earlier, the engineers themselves were not necessarily preserved, but the
impact of the engineering activity can easily be recognized in rocks. Efforts to
identify the ecosystem engineer(s) of the agronomic revolution are in their early
stages, although soft-bodied metazoans are likely candidates based on their
abundance in exceptionally-preserved deposits such as the Chengjiang Biota (Hou et
al., 2004). Given the scarcity of preserved soft tissues in the fossil record, studying
the distribution and abundance of trace fossils in Lower Cambrian rocks may be the
best way to determine the role soft-bodied organisms may have played in
engineering Early Cambrian ecosystems. A study of Lower Cambrian shallow
marine rocks in eastern California demonstrates that the simple horizontal trace fossil
Planolites, likely the product of shallow burrowing by soft-bodied vermiform
22
FIGURE 1.4 – A schematic illustration of the shift, between the Late Neoproterozoic
and post-Cambrian, in the dominant processes that controlled seafloor conditions. As
indicated in the triangular diagrams, Late Neoproterozoic seafloor conditions were
controlled by physical and microbial processes. Microbial influence gradually
decreased during the Neoproterozoic-Phanerozoic transition, when metazoan
bioturbation became more abundant and disruptive. In the post-Cambrian, abundant
and extensive bioturbation by metazoans was the primary factor, in addition to
physical processes, that governed seafloor conditions. Modified from Bottjer et al.
(2000).
23
metazoans, was the most abundant type of bioturbation present on bedding plane
surfaces throughout the rocks examined (Marenco, 2006; Marenco and Bottjer,
2008). The proliferation of Planolites burrows on Early Cambrian seafloors likely
reflects the presence of a steady nutrient supply, generated by widespread microbial
mats, that was capable of sustaining a diverse matground community. Other evidence
for the existence of such a community in these Lower Cambrian rock units includes
the common occurrence of Volborthella, a small enigmatic Cambrian fossil
interpreted as the skeleton of a matground-adapted animal (e.g., Seilacher, 1999),
and shell casts and molds of possible linguliform brachiopods, which may have been
adapted to life in the low-oxygen conditions promoted by microbial mats (Bailey et
al., 2006). Despite the abundance of horizontal bioturbation, microbial activity was
likely still the dominant factor influencing substrate conditions in these particular
Early Cambrian shallow marine environments prior to the agronomic revolution
(Bailey et al., 2006).
Early Metazoan Autogenic Engineers: As mentioned above, the earliest reefs
known from the fossil record were constructed entirely by microorganisms (e.g.,
Grotzinger, 1989). These microbial reefs, with frameworks consisting entirely of
stromatolitic and thrombolitic fabrics, were dominant in marine settings until the
Early Cambrian. The transition toward a new style of reef building was gradual,
beginning in the Neoproterozoic and extending 10-15 million years into the
Cambrian Period (Copper, 2001). In the Late Neoproterozoic (ca. 550 million years
ago), the first metazoans to construct calcium carbonate skeletons appeared
24
(Grotzinger et al., 1995). These include two possible cnidarian forms: Cloudina, a
tube-building organism (Grant, 1990) and Namacalathus, a goblet-shaped organism
(Grotzinger et al., 2000). Although fossil evidence suggests that these and other early
skeletonized animals commonly lived within microbial reefs or constructed small
“thickets” and mounds independently, their skeletons were not substantial enough to
constitute a primary reef framework (Wood, 1999). It was not until approximately
530 million years ago that metazoans began to play a more significant role in reef
construction.
Archaeocyath sponges were the first skeletonized metazoan components of
Early Cambrian reefs (Wood, 1999; Copper, 2001). These sponges lacked spicules,
having instead a calcified skeleton with a complex internal structure (Wood, 1999).
Typical archaeocyath skeletons were cone- or cup-shaped with double or single walls
constructed of calcite (Copper, 2001). In double-walled forms, the two walls
commonly were joined together by septa. This septate region, the intervallum, likely
housed soft tissue (Wood, 1999). Archaeocyaths were solitary or colonial, and their
skeletal morphologies varied widely from single cones to branching or sheet-like
forms (Copper, 2001) (Fig. 1.5). Archaeocyath skeletons, significantly more robust
than those of earlier reef-associated metazoans, not only enhanced their preservation
potential but likely also helped the animals deter potential encrusters and competitors
(Zhuravlev, 2001). Although they rarely grew to more than 20 cm in height (Wood,
1999), archaeocyaths were capable of constructing substantial reef frameworks
(Zhuravlev, 2001). Unlike later reef-building organisms, however, archaeocyaths
25
FIGURE 1.5 – Views of an archaeocyath-calcimicrobial reef, Stewart’s Mill, NV.
(Top) A branching archaeocyath sponge, which appears to have been preserved in
life position. Surrounding the archaeocyath is carbonate sediment that may have
accumulated slowly while it was alive, allowing it to be preserved in place. Scale bar
= 1 cm. (Bottom) Accumulated fragments of skeletal material, including that of
archaeocyaths (arrows), and microbial structures surrounded by carbonate sediment.
The sharp boundary in the upper portion of the photograph (arrowheads) likely
represents the floor of a reef cavity, which would have harbored organisms that were
specially adapted to life in these cryptic settings. Photo by Matthew Clapham.
26
required the assistance of calcified microorganisms to build reefs (Wood, 1999;
Rowland and Shapiro, 2002).
Calcified microorganisms, or calcimicrobes, likely were cyanobacteria
preserved by the precipitation of calcium carbonate around their extracellular sheaths
(Wood, 1999). These “skeletal” microorganisms rose to prominence as reef-builders
in the Late Neoproterozoic, constructing mounds on the order of several hundred
meters thick and one kilometer wide (e.g., Aitken, 1989). Calcimicrobes differ from
stromatolites in their growth morphology, which commonly is clumpy or shrub-like
(Wood, 1999), and probably also in their mode of carbonate precipitation, which is
unknown but may have been influenced by environmental factors (Copper, 2001).
The three main categories of calcimicrobes are Renalcis, a globular form comprised
of clumps or clots of fine-grained calcite; Epiphyton, a shrub-like colonial form; and
Girvanella, a sheet- or crust-like form (Wood, 1999).
The presence of well-established, abundant calcimicrobes in earliest
Cambrian seafloor communities facilitated the reef-building success of
archaeocyaths 10 to 15 million years later (Wood, 1999). Calcimicrobes appear to
have become cemented in calcium carbonate during active reef growth (e.g., Kruse et
al., 1995), which would have lent added strength to any of their associated structures.
In high-energy shallow-water environments, calcimicrobial crusts likely stabilized
the seafloor sediment, allowing archaeocyaths to become established (Wood, 1999).
The presence of complexly intergrown calcimicrobes and archaeocyaths in fossil
reefs suggests that calcimicrobes strengthened archaeocyath frameworks at later
27
stages of reef growth (e.g., Zhuravlev, 2001). Thus, calcimicrobes served as non-
metazoan autogenic engineers by facilitating the growth of the earliest substantial
metazoan reefs.
As reef builders, archaeocyaths were autogenic ecosystem engineers in their
own right. The growth of archaeocyath-calcimicrobial reefs expanded benthic
ecospace on Early Cambrian seafloors. In addition to increasing available surface
area for organism attachment, these reefs promoted diversification of the benthic
community by dividing the habitat into open-surface and cryptic settings and
increasing the number of energy-dependent microhabitats through varied topography
(Wood, 1999; Zhuravlev, 2001) (Fig. 1.5). Among the groups that colonized
archaeocyath-calcimicrobial reefs were brachiopods, echinoderms, gastropods, and
trilobites, many of which evolved reef-specific adaptations (Zhuravlev, 2001). For
example, the trilobite genus Giordanella became specialized as a reef-dwelling
stationary suspension-feeder (Zhuravlev, 2001). Other groups, such as sponges and
metazoan microburrowers, became specialized inhabitants of reef cavities (Kobluk,
1988; Wood, 1999). Thus, in archaeocyath-calcimicrobial reefs, we have a well-
preserved multi-stage example of early autogenic engineering, which likely
contributed to the Cambrian explosion of marine animal diversity.
Summary: Recognizing ancient examples of ecosystem engineering in the
fossil record is challenging due to the loss of primary ecological information that
occurs during preservation. Problems such as time-averaging, fluctuating sediment
accumulation rates, and preferential preservation of skeletonized organisms can
28
hamper paleoecological investigations. Evidence for engineering behavior, or for the
presence of engineers themselves, may be impossible to obtain from the fossil record
unless exceptional conditions prevailed at the time of preservation. Autogenic
engineers, which altered the environment through their biogenic structures, are
generally more apparent in the fossil record than allogenic engineers, which altered
the environment through the transformation of pre-existing materials; this “bias”
may become more apparent as the study of ancient ecosystem engineering
progresses.
The Early Cambrian agronomic revolution and development of archaeocyath-
calcimicrobial reefs are two of the earliest examples of allogenic and autogenic
engineering in the history of life. By expanding benthic ecospace, these instances of
engineering had broad ecological and evolutionary effects. Erwin (2005) argues that
the construction of new ecological niches is essential if organisms’ genetic
inventions are to become successful innovations that persist in communities through
time. The development of new niches via the agronomic revolution and the
expansion of reefs likely helped facilitate the Cambrian explosion of marine
innovations.
The search for ancient ecosystem engineers is in its early stages, but it
promises to greatly improve our understanding of community ecology over broad
timescales. Paleoecologists must continue to refine and build upon current strategies
for identifying examples of ancient ecosystem engineering in the fossil record.
29
Dissertation Summary
The Cambrian Period was a pivotal time in the history of marine benthic
communities. Trace fossils and other bioturbation structures preserved in Cambrian
rocks are an excellent source of information about the ecological and evolutionary
changes that occurred as the first Paleozoic communities took shape. The
overarching objective of this study was to obtain a better understanding of animal-
substrate interaction in shallow marine environments during and after the agronomic
revolution by developing and implementing new analytical techniques in the field
and laboratory.
One of the challenges of studying Lower Cambrian bioturbation is that much
of it is nearly horizontal, or bedding-parallel, due to the evolutionary and ecological
constraints placed on early benthic animals. Horizontal bioturbation, even in large
quantities, commonly results in minimal visible disruption of primary bedding. Thus,
assessments of primarily-horizontal bioturbation from vertical outcrop exposures are
of little use. Bedding plane exposures are the best medium for studying the nature
and quantity of horizontal bioturbation in Lower Cambrian rocks and thereby
achieving a better understanding of the ichnofabric that is present. If an accurate
picture of Early Cambrian bioturbation is to be obtained, precise methods for
documenting and quantifying horizontal bioturbation from bedding plane surfaces
are required. Many existing methods for evaluating the nature and quantity of both
vertical and horizontal bioturbation in outcrops, such as ichnofabric indices (Droser
and Bottjer, 1986) and bedding plane bioturbation indices (Miller and Smail, 1997),
30
are geared toward scoring using visual estimates of bioturbation intensity. In these
methods, each numerical score corresponds to a range of bioturbation percentages.
For example, bedding plane bioturbation index (BPBI) “three” corresponds to
bedding surfaces that are 10-40 percent bioturbated. Thus, different bedding planes
with the same score can vary in bioturbation intensity by up to 30 percent, making
comparisons imprecise. An objective of this study was to explore and test image-
analysis-based methods of assessing bedding plane bioturbation in an effort to make
meaningful comparisons among datasets possible. The “intersection” method, a grid-
based presence-absence method for analyzing field photographs, achieves a
satisfactory balance between precision and efficiency. This method can be used as a
lab-based complement to the field-based bedding plane bioturbation index method or
as a stand-alone means of arriving at accurate percentages of bioturbation on studied
bedding planes.
Although much work has addressed both ichnofossil diversity (e.g., Alpert,
1974; Langille, 1974) and the challenges of estimating bioturbation intensity in
Lower Cambrian rocks (Droser and Bottjer, 1986; Miller and Smail, 1997), few
studies have focused on the broader ichnofossil-sediment record in the context of the
agronomic and Cambrian substrate revolutions (Marenco, 2006; Marenco and
Bottjer, 2008). A recent preliminary study (Marenco, 2006; Marenco and Bottjer,
2008) focused on assessing the nature and abundance of bioturbation in siliciclastic
units of the Lower Cambrian succession in the White-Inyo Mountains, eastern
California, with the goal of testing the hypothesis that trilobites were the primary
31
substrate engineers of the Early Cambrian. Results from fine-grained units suggest
that soft-bodied worm-like organisms, represented by the horizontal trace fossil
Planolites, were in fact the primary sources of sediment disruption in the Early
Cambrian shallow subtidal marine environments represented by fine-grained Lower
Cambrian strata in the White-Inyo Mountains. In contrast, analysis of coarser-
grained quartzites from the Poleta Formation indicates that deeply-vertical
bioturbation, in the form of the trace fossils Skolithos and Diplocraterion, was
already common in higher-energy environments in the Early Cambrian. Thus, it
appears that the nature of the agronomic revolution may have varied among different
depositional environments.
A number of new hypotheses grew out of the results of this earlier study. (1)
Horizontal bioturbation, in the form of Planolites-type traces, was the primary mode
of substrate engineering in shallow subtidal siliciclastic marine environments early in
the agronomic revolution. (2) In higher-energy siliciclastic environments, such as
nearshore settings, the transition from primarily-horizontal to deeply vertical
bioturbation was much more abrupt than in lower-energy facies. (3) Trilobites were
largely restricted to more nearshore settings than those represented in the Lower
Cambrian succession of the White-Inyo Mountains. In order to test these hypotheses
and further explore the environmental, temporal, and geographical scale of the
agronomic revolution, field studies were conducted in the Lower Cambrian
successions of the Death Valley region, CA-NV, and southern Sweden. These rocks
together represent a variety of shallow subtidal marine depositional settings. The
32
Lower Cambrian succession in the Death Valley region is a thinner, more proximal
equivalent of the Lower Cambrian succession in the White-Inyo Mountains (Nelson,
1978) and thus provides an excellent opportunity to test whether the trends observed
in the White-Inyo Mountains represent localized, environment-specific phenomena.
The Lower Cambrian succession in Scania, southern Sweden, consists primarily of
units deposited under moderate-energy nearshore conditions, which can readily be
compared with similar units in the Death Valley succession to test hypothesis 2. In
addition to field data collection, bedding plane photographs from all Death Valley
region localities were analyzed digitally using the point-intercept method. Results of
this work indicate that Planolites-type traces indeed represent the dominant form of
bioturbation in Early Cambrian shallow subtidal siliciclastic settings, but vertical
burrows, such as Skolithos and Diplocraterion, had a more significant impact on the
substrate in coeval moderate- to high-energy nearshore settings.
Bedding plane exposures are a superior medium for studying the nature and
quantity of horizontal bioturbation in Cambrian siliciclastic strata. In most deposits,
however, exposed bedding planes are few in number and generally small, rarely
exceeding one square meter in size. In central Wisconsin, quarries of Upper
Cambrian (Dresbachian-Franconian) sandstones expose bedding planes that
commonly extend for many tens of square meters. These well-preserved surfaces,
reflecting deposition in an intertidal setting that experienced intermittent subaerial
exposure, contain an ichnofauna that is predominantly bedding-parallel and
characterized by moderate diversity and moderate to high abundance (Hagadorn et
33
al., 2002). Individual traces vary in diameter from a few millimeters (Gordia-type) to
more than ten centimeters (Climactichnites). Extensive bedding plane exposures
such as these provide a rare opportunity for detailed study of bioturbation intensity
and patchiness at a variety of scales. Several of these large bedding planes were
analyzed in detail using grid-based methods. The results of this work illuminate the
complex problem of scale for quantitative analysis of horizontal bioturbation,
particularly in Cambrian shallow to marginal marine deposits.
34
CHAPTER II
Quantitative Analysis of Bioturbation on Bedding Planes
Introduction and Previous Work
In Lower Cambrian siliciclastic rocks deposited in shallow subtidal marine
environments, preserved bioturbation structures typically are bedding-parallel and
minimally disruptive to primary bedding. Thus, bedding plane exposures have far
greater utility for ichnological studies than vertical outcrop exposures in rocks of this
age and depositional context. In post-Cambrian rocks, however, with the exception
of those deposited during extinction recovery intervals (e.g., Twitchett and Barras,
2004) or in restricted environments (e.g., Taylor and Goldring, 1993; Wignall, 1993;
Taylor et al., 2003), preserved bioturbation is dominantly vertical and can be studied
effectively from vertical outcrop exposures. Consequently, relatively little attention
has been given to developing precise methods for describing bioturbation on bedding
planes. Lower Cambrian bedding plane exposures record the expansion and
behavioral diversification of skeletonized and soft-bodied organisms during the
Cambrian explosion. As such, bedding planes are of critical importance for
understanding how the earliest benthic communities evolved, especially given the
dearth of body fossils in most Lower Cambrian deposits.
Inherent in bedding-plane-focused bioturbation studies are a number of
interpretive challenges. For example, the diversity and abundance of trace fossils on
a bedding plane surface is more likely to be a function of the sediment accumulation
35
rate at the time of deposition than a reflection of the true diversity and density of the
benthic population (e.g., Howard, 1975; Frey, 1978). Longer exposures on the
seafloor typically give the appearance of more intense bioturbation and/or greater
numbers of bioturbating organisms, while shorter exposures may diminish apparent
benthic diversity and abundance. Thus, bioturbation abundance and diversity data are
most likely to be meaningful when considered in large quantities, from a variety of
depositional environments, and over long timescales, which should help to minimize
the sediment accumulation rate signal.
Diffuse or indistinct biogenic sediment disruption, in which individual trace
fossils cannot be identified, is another factor that complicates the study of
bioturbation on bedding planes. Although this type of biogenic feature provides
information about substrate consistency, specifically the water content of the
sediment prior to lithification, it conveys little about tracemaker behavior and defies
description by most ichnological methods. When little or no ichnotaxonomic data
can be obtained from a bedding plane to make qualitative comparisons possible, the
surface must be described in terms of the quantity and distribution of the bioturbation
present.
The primary established method for assessing the quantity of bioturbation on
bedding planes is the bedding plane bioturbation index (BPBI) method of Miller and
Smail (1997). Similar to the ichnofabric index method of Droser and Bottjer (1986),
the bedding plane bioturbation index method is a semiquantitative method in which a
bedding plane is scored through visual comparison of the bedding surface with
36
diagrams depicting varying amounts of “bioturbation” (Fig. 2.1). The possible BPBI
scores are as follows: “one” (0% bioturbation), “two” (0-10%), “three” (10-40%),
“four” (40-60%), and “five” (60-100%) (Miller and Smail, 1997).
The primary advantages of the bedding plane bioturbation index method are
its ability to accommodate indistinct biogenic sediment disruption, because it
involves a subjective assessment of the bedding plane surface, and its efficiency for
use in the field. The drawbacks of the method are more numerous. First, no two
BPBIs represent an equal range of bioturbation percentages: “one” has no range (it is
fixed at 0%), “two” has a range of 10%, “three” has a range of 30%, “four” has a
range of 20%, and “five” has a range of 40%. Thus, the five bedding plane
bioturbation indices make up an ordinal scale, or a series of numbered categories that
are ranked but are unequal to one another (e.g., Stevens, 1946; Jager and Looman,
1995). Consequently, it is reasonable to find the median and frequencies of a set of
BPBI results, but the mean and standard deviation are meaningless from a statistical
perspective (Jager and Looman, 1995). In addition, the precision of BPBI analyses is
limited, not only because each BPBI above “one” encompasses a bioturbation range
of 10% or more, but also because scores are assigned subjectively. Miller and Smail
(1997) provided four sets of bioturbation diagrams to account for a range of possible
differences in burrow morphology (including size and shape) and burrow distribution
(patchy or even) among bedding planes encountered in the field (Fig. 2.1). In reality,
however, actual bedding plane exposures rarely compare well to any of these guides.
Even when a good match is possible, the scale and intensity of bioturbation on a
37
FIGURE 2.1 – One of two illustrated keys for use in determining bedding-plane
bioturbation indices (BPBIs) in the field. Central column of numbers (1-5) are
BPBIs. The range of each BPBI is indicated by brackets and dashed lines.
Corresponding percentages of bioturbation are listed in outer columns. Column of
thumbnails at left represents hypothetical bedding planes containing evenly-
distributed trace fossils that vary in size and shape; column at right shows uneven
distributions of morphologically-heterogeneous trace fossils. Modified from Miller
and Smail (1997).
38
bedding plane exposure can be misleading, as Miller and Smail (1997) discussed at
length. One or two large burrows may cover the same total area as many widely-
dispersed small burrows, but the latter will likely appear to achieve greater coverage
of the bedding plane surface (Miller and Smail, 1997).
Clearly, the bedding plane bioturbation index method falls short of being an
optimal technique for evaluating bioturbated bedding planes. An ideal method would
have the following characteristics: objective data collection procedures, precise
quantitative means of assessing the abundance and spatial distribution of
bioturbation, accommodation of indistinct bioturbation, consistent units of
measurement, reproducibility, and efficiency. No single method is likely to meet all
of these criteria. For example, a method that achieves complete objectivity is
improbable because trace fossils and indistinct bioturbation occur in endlessly varied
forms that require subjective scrutiny. Nevertheless, reducing subjectivity as much as
possible will improve the likelihood of obtaining reproducible and comparable
results.
Computer-based image analysis can help to minimize inconsistencies in data
due to worker error (e.g., Francus, 2001). A number of ichnological methods employ
image analysis to assess the quantity of bioturbation in sedimentary rocks (e.g.,
Magwood and Ekdale, 1994; Francus, 2001; Löwemark, 2003; Honeycutt and
Plotnick, 2006). In most cases, software is used to enhance the contrast of digital
images of bioturbation so that bioturbated and non-bioturbated areas can be
distinguished from one another, whether automatically by the computer (Löwemark,
39
2003) or manually, with computer assistance (e.g., Magwood and Ekdale, 1994).
Images may be obtained for analysis from outcrop or core photographs (e.g.,
Magwood and Ekdale, 1994), x-radiographs of core material (e.g., Löwemark, 2003),
or thin sections of bioturbated material (Francus, 2001). Invariably, images that
contain at least some initial contrast between bioturbation structures and surrounding
material produce more robust image analysis results (e.g., Magwood and Ekdale,
1994). Vertical outcrop exposures and vertically-sliced core slabs typically work
well for image analysis because vertical burrows often pipe sediment down into
layers of contrasting lithology, limiting the need for digital enhancement. On the
other hand, bedding planes usually are lithologically homogeneous, unless vertical
burrows intersect the horizon from above. On most bedding plane exposures,
differences in relief between bioturbated and non-bioturbated areas offer the only
source of contrast for recognizing bioturbation. Thus, the quantity and angle of
available natural light can determine how well biogenic structures stand out on a
bedding plane surface, in a photograph of the surface, and in an image analysis
program (Fig. 2.2).
Trace fossils often are not preserved on bedding planes with sufficient relief
to stand out well in photographs, even under the best lighting conditions. Manually
tracing bioturbated zones on a digital bedding plane photograph is one way to ensure
that low-relief structures are included in later bioturbation estimates (Fig. 2.3). Miller
and Smail (1997) recommended using this tracing technique to double-check the
accuracy of results obtained with the bedding plane bioturbation index method.
40
FIGURE 2.2 – Photographs of the same Lower Cambrian bedding plane taken at
different times of day to illustrate the effect of lighting on bioturbation visibility.
Bedding plane is oriented at a high angle. Inner dimensions of frame are 24x25
centimeters. (Top) Early evening, sun behind photographer. (Bottom) Late morning,
sun almost directly overhead.
41
FIGURE 2.3 – Tracing technique for estimating the percentage of a bedding surface
that contains bioturbation. (Top) Digital photograph of a Lower Cambrian bed sole
on which distinct trace fossils (black) and indistinct biogenic disruption (blue) have
been traced. Sample area is 12x25 centimeters. (Bottom) A stand-alone black-and-
white image of the shaded areas can be analyzed to determine the proportion of black
to white pixels within the sample area, which is equivalent to the percentage of
bioturbation present.
42
Tracing bioturbation is time-intensive, however, especially when many small
burrows must be outlined one-by-one. Indistinct bioturbation also presents a problem
for tracing because the boundaries between bioturbated and non-bioturbated zones
may be difficult to pinpoint, leading to a more subjective assessment of total
bioturbation.
Grid- or quadrat-based quantitative methods have been used widely in marine
ecology (e.g., Bakus, 1990, pp. 1-18, 47-62, and references therein; Sullivan and
Chiappone, 1993) and plant ecology (e.g., Floyd and Anderson, 1987, and references
therein) for estimating the relative abundance, distribution, and percentage cover of
species in a habitat. Paleoecological studies involving fossiliferous bedding plane
exposures have employed grid-based methods for nearest neighbor analysis (e.g.,
Clapham et al., 2003), neighbor proximity analysis (Leighton and Schneider, 2004),
and relative abundance (e.g., Ager, 1963, pp. 220-223). Although grid-based
methods are common in vertebrate ichnology for the study of trackways (e.g., Mezga
and Bajraktarevi ć, 1999; Swanson and Carlson, 2002; Getty, 2005), few invertebrate
ichnological studies have made use of such techniques (e.g., Pickett, 1972; Miller,
1977; Pemberton and Frey, 1984; Miller et al., 2002). Most grid-based studies in
invertebrate ichnology have examined the spatial relationships among preserved
vertical burrows, such as Skolithos and Diplocraterion (Pickett, 1972; Miller, 1977;
Pemberton and Frey, 1984). Vertical trace fossils are well-suited for nearest-
neighbor grid analysis because most represent semi-permanent dwelling and feeding
structures that record the approximate life positions of their tracemakers (Pemberton
43
and Frey, 1984) (Fig. 2.4). The entrances to vertical burrows typically are small,
well-defined, and non-overlapping, making precise measurements of their spatial
relationships possible. Conversely, most bedding-parallel trace fossils are elongated
with respect to the bedding plane and commonly cross-cut one another (Fig. 2.4).
Thus, grid-based methods that emphasize presence/absence and percentage cover are
more appropriate for studying bedding-parallel bioturbation than the nearest-
neighbor approach.
The objective of this study was to develop an efficient and precise grid-based
method for determining the percentage area of a bedding plane that contains
bioturbation. The new method is designed to facilitate accurate quantitative
comparisons between bioturbated bedding planes and serve as a complement to
qualitative and semiquantitative bedding plane data.
Methodology
Each studied bedding plane should first be described thoroughly and
photographed in the field, using a square or rectangular frame to designate a portion
of the surface for later grid analysis. Although more efficient than tracing, small-
scale grid-based methods require too much time and care to be practical for field
execution. Field time should instead be devoted to describing and photographing as
many different bedding planes as possible. Later, a grid can be superimposed
digitally onto the framed portion of each bedding plane photograph using a drawing
program such as Adobe Illustrator. Once the grid has been applied, the same drawing
44
FIGURE 2.4 – Vertical bioturbation and horizontal bioturbation have different
bedding-plane-surface expressions, and this necessitates the use of different
analytical approaches. (Top) Illustration of the nearest-neighbor method as applied to
a surface that contains abundant Skolithos-type vertical burrow entrances. Numbers
indicate distances, in millimeters, between each burrow entrance and its “nearest
neighbor,” identified by an arrow. Modified from Pemberton and Frey (1984).
(Bottom) Illustration of a “typical” Lower Cambrian bedding plane. Only horizontal
bioturbation is present, in the form of arthropod-type and Planolites-type trace
fossils. Because bedding-parallel burrows may overlap one another, the nearest-
neighbor approach cannot be applied to most Lower Cambrian bedding planes.
45
program can be used to mark portions of the gridded photograph as they are
analyzed. The resulting image can then be saved as a permanent record of the
analysis.
Estimating the percentage cover of plant species is a technique used widely in
plant ecology for determining the relative importance of species in an ecosystem
(Floyd and Anderson, 1982). Percentage ground or canopy cover is an effective
means of comparing plant species with disparate growth forms because all species,
regardless of morphology or distribution, are treated equally (Mueller-Dombois and
Ellenberg, 1974; Whittaker, 1975). A number of methods for estimating percentage
plant cover have been developed and tested. The most commonly cited of these are
the line intercept, point intercept, and quadrat-based cover class methods. The line
intercept method (Canfield, 1941) entails stretching a thin line across a study area
and measuring the length of each line increment that crosses each species of interest
(Fig. 2.5). Dividing the total length of line each species intercepts by the total length
of line examined gives an estimate of the percentage cover of each species. Floyd
and Anderson (1982; 1987) found that the line intercept method generally
overestimates the percentage cover of species because it assumes that each plant
creates a solid canopy from edge to edge. In addition, much more sampling time is
required to achieve a satisfactory estimate using the line intercept method than using
the grid-based point intercept method (Floyd and Anderson, 1982; 1987). The point
intercept method (Goodall, 1952; Floyd and Anderson, 1982; Greig-Smith, 1983;
Floyd and Anderson, 1987), involves counting the number of points or vertical pins,
46
A)
B)
C)
FIGURE 2.5 – Illustrations of three methods for estimating percentage cover,
originally developed for use in plant ecology studies. (A) Line-intercept method. The
total length of line that cross patches of vegetation (red) is divided by the full line
length. (B) Point intercept method (modified as “intersection method” in this study).
(C) “Cell method.”
47
arranged at the intersections of the lines in a grid, that touch each species of interest
(Fig. 2.5). Dividing each species total by the total number of points or pins examined
provides an estimate of percentage cover for each species. The quadrat-based cover
class method produces similar results to the point intercept method (Floyd and
Anderson, 1987), although the former is more subjective. In the cover class method,
a square or rectangular quadrat is placed over one portion of the study area, and the
percentage area covered by each species of interest is estimated visually and
assigned one of several cover classes (Daubenmire, 1959). Similarly to the bedding
plane bioturbation index method, each cover class encompasses a range of
percentages (Daubenmire, 1959).
Of the three methods described above, the point intercept method was chosen
to be adapted and tested based on its potential for use at the scale of a bedding plane.
This method was modified and renamed the “intersection method.” The intersection
method differs from the point intercept method only in the means by which grids are
applied (digitally versus manually). A second method was chosen to be tested against
the intersection method. Renamed the “cell method” (Fig. 2.5), this approach is
similar to a method described by Korva (1996), in which percentage cover is found
by counting grid cells that contain 50-100 percent vegetative cover and then dividing
this number by the total number of cells in the grid. For maximum efficiency, grid
cells analyzed using the cell method were counted based on the presence or absence
of bioturbation.
48
Testing the Cell and Intersection Methods
The cell and intersection methods can be applied to grids of the same size and
shape, making side-by-side comparisons possible. In order to determine which
method is more precise and efficient, the two methods were tested on two black and
white digital images. Because each image consists of a matrix of black and white
pixels, the exact percentage area of “bioturbation” in each image can be determined
using the histogram function in Adobe Photoshop. The first image, a hypothetical
bedding plane containing only identical vertical burrows (e.g., Skolithos), is Miller
and Smail’s (1997) visual guide for 10 percent bioturbation by morphologically-
similar, evenly-distributed trace fossils (Fig. 2.6). This image is 9.91%
“bioturbated.” The second image is a drawing of randomly-distributed, horizontal
trace fossils (e.g., Planolites), which differ in length but not in width (Fig. 2.6). This
image is 6.30% “bioturbated.”
The test itself was designed based on the following prediction: repeatedly
increasing the number of subdivisions (cells) in a fixed-dimension grid should result
in a proportional increase in the precision of each subsequent grid analysis. In other
words, each time the total number of available data points (cells or grid intersections)
increases, more of these points will fall within bioturbated portions of the
photograph, refining the estimated proportion of bioturbated to non-bioturbated area.
Of course, the time investment required to conduct an analysis also increases each
time the grid is subdivided further. To test the accuracy of this prediction and find a
balance between analytical precision and efficiency, a series of grids were
49
FIGURE 2.6 – Black-and-white digital images of “bioturbation” used to test the
intersection and cell methods for estimating the percentage area of bioturbation on
bedding planes. (Top) “Vertical bioturbation” in the form of Skolithos-type burrow
entrances. Modified from Miller and Smail’s (1997) visual BPBI guide for 10
percent bioturbation by morphologically-similar, evenly-distributed trace fossils.
(Bottom) “Horizontal bioturbation,” made up of randomly-distributed, Planolites-
type trace fossils of identical width. 6.30% of the image contains “bioturbation.”
50
superimposed onto each black-and-white image. Beginning with a simple 2x2-cell
grid (four total cells), the total number of cells was quadrupled eight times,
producing additional grids with the following dimensions (in terms of cells): 4x4,
8x8, 16x16, 32x32, 64x64, 128x128, 256x256, and 512x512. Each image was tested
at grid scales 2x2 through 128x128 using both the intersection and cell methods. The
“vertically-bioturbated” image was tested additionally at the 256x256-cell scale
using both methods and at the 512x512-cell scale using only the cell method. These
additional tests were conducted to establish the grid scale at which the results using
each method became accurate within one percent error. Additional tests were not
conducted on the “horizontally-bioturbated” image due to the prohibitively-long
timeframes required for these detailed analyses (the 512x512-cell grid contains
262,144 cells).
During this experiment, only those grid-line intersections that join four grid
cells were included in analyses using the intersection method. In other words,
intersections between grid lines and the frame of the grid were excluded. As a result,
the total number of available intersections in each grid is smaller than the total
number of cells. The difference between total cells and total intersections on each
grid is equal to the number of intersections on one side of the next-more-subdivided
grid. The implications of this difference in available data points for the two methods
are discussed below.
51
Results
Test 1: Vertical Bioturbation: Estimates of bioturbation using the cell method
are 100% for the first two grids (2x2 and 4x4) because each cell contains no fewer
than two “burrow entrances” (Fig. 2.7, 2.9). In contrast, only one intersection
touches a black dot on either of these grids (Fig. 2.7). The cell method first captures
gaps in “bioturbation” on the 8x8-cell grid (Fig. 2.7). On grid 16x16, no cell contains
more than one black dot or portion thereof, and 25 of the 51 dots fall on grid-line
intersections (Fig. 2.8). The width of one cell in the 32x32-cell grid is slightly greater
than the average dot diameter (Fig. 2.8). At this grid scale, each of the black dots
touches 1-4 intersections and overlaps 4-7 cells. Between grids 64x64 (Fig. 2.8) and
256x256 (or 512x512 for the cell method), the number of “bioturbated” cells and
intersections increases, but there are no significant changes in the pattern by which
new data points are added. Using the intersection method, the estimate of total
bioturbation becomes accurate within one percent of the actual bioturbation
percentage as of the 64x64-cell grid (Fig. 2.9). This degree of accuracy is not
achieved with the cell method until the 512x512-cell grid (Fig. 2.9).
Test 2: Horizontal Bioturbation: The cell method again generates an estimate
of 100% bioturbation for both the 2x2- and 4x4-cell grids (Fig. 2.10, 2.13), while
results from the intersection method jump from zero percent on the 2x2-cell grid to
22.22% on the 4x4-cell grid (Fig. 2.10, 2.13). The first gaps in “bioturbation” shown
by the cell method again appear on the 8x8-cell grid (Fig. 2.10). The 32x32-cell grid
is the first in which the cell method results illustrate a visually-realistic proportion of
52
A)
B)
C)
FIGURE 2.7 – Three grid scales used to analyze the “vertically bioturbated” image.
(A) 2x2 cells. (B) 4x4 cells. Cell analysis at left, intersection analysis at right. (C)
8x8 cells.
53
A)
B)
C)
FIGURE 2.8 – Three grids used to analyze the “vertically-bioturbated” image. Cell
method analyses at left, intersection method analyses at right. (A) 16x16 cells. (B)
32x32 cells. (C) 64x64 cells.
54
0
10
20
30
40
50
60
70
80
90
100
2X2
4X4
8X8
16X16
32X32
64X64
128X128
256X256
512X512
Percentage
Cell
Intersection
FIGURE 2.9 – Results of Test 1 (“vertical bioturbation”) obtained using the cell and
intersection grid methods. Grid dimensions are listed on the x axis in order of
increasing subdivision.
55
A)
B)
C)
FIGURE 2.10 – Three grid scales used to analyze the “horizontally-bioturbated”
image. (A) 2x2 cells. No analysis shown due to the small number of available grid
cells/intersections. (B) 4x4 cells. Cell analysis at left, intersection analysis at right.
(C) 8x8 cells.
56
bioturbated to non-bioturbated area (Fig. 2.11). Results using the intersection method
begin to reflect the elongated morphology of the “trace fossils” by the 16x16-cell
grid (Fig. 2.11); this trend continues through the remainder of the grids. However,
maximum coverage of the “bioturbation” by intersections does not occur until the
128x128-cell grid, when the width of a “trace fossil” exceeds the width of a grid cell
(Fig. 2.12). Accuracy within one percent of the actual value is achieved on the
128x128-cell grid using the intersection method (Fig. 2.13). The result for the
128x128-cell grid using the cell method overestimates total bioturbation by 5.13%
(Fig. 2.12, 2.13).
Discussion – Evaluation of the Cell and Intersection Methods
These results demonstrate that increasingly subdividing a fixed-dimension
grid indeed facilitates more precise analyses of bioturbation. The degrees of
precision obtained using the two methods are disparate, however (Fig. 2.14). The
intersection method out-performed the cell method by a significant margin in both
tests. Although neither method produced realistic results at the coarsest grid scales
(2x2 and 4x4), the intersection method generated results within eight percent of the
actual value for both images by the 8x8-cell grid (Fig. 2.14). In contrast, estimates
made using the cell method did not fall below 20% bioturbation for either image
until the 64x64-cell grid (Fig. 2.14). Thus, the offset in total numbers of data points
between the cell and intersection methods as tested here appears to be
57
A)
B)
C)
FIGURE 2.11 – Three grids used to analyze the “horizontally-bioturbated” image.
Cell method analyses at left, intersection method analyses at right. (A) 16x16 cells.
(B) 32x32 cells. (C) 64x64 cells.
58
FIGURE 2.12 – 128x128-cell grid analyses of the “horizontally-bioturbated” image.
(Top) Cell method analysis. (Bottom) Intersection method analysis.
59
0
10
20
30
40
50
60
70
80
90
100
2X2
4X4
8X8
16X16
32X32
64X64
128X128
256X256
512X512
Percentage
Cell
Intersection
FIGURE 2.13 – Results of Test 2 (“horizontal bioturbation”) obtained using the cell
and intersection grid methods. Grid dimensions are listed on the x axis in order of
increasing subdivision.
60
0
10
20
30
40
50
60
70
80
90
100
2X2
4X4
8X8
16X16
32X32
64X64
128X128
256X256
512X512
Percentage
Cell T1
Cell T2
Intersection T1
Intersection T2
FIGURE 2.14 – Results of Tests 1 and 2, plotted together on a single graph for
comparison. Grid dimensions are listed on the x axis in order of increasing
subdivision.
61
inconsequential. The intersection method produced results that were consistently
more precise despite smaller numbers of available data points.
In general, the intersection method produces conservative estimates of total
bioturbation, and the cell method does the opposite, because of the difference
between the point and cell as units of data. Each point (or grid-line intersection)
theoretically has no dimensions and, as such, can record only the presence or absence
of bioturbation (Greig-Smith, 1983, pp. 5-6). In practice, grid lines must be wide
enough to be visible for analysis; digital grid application presents an advantage over
field application in this regard because digital grid lines can be reduced in size well
below the diameter of the finest-gauge fishing line. Each cell, in contrast, consists of
a finite area that can record presence, absence, or any amount of partial bioturbation
coverage. When cells are marked on a presence/absence basis, which helps to
expedite analysis, considerable precision is lost. The amount of precision lost
decreases as the ratio of cell size to “trace fossil” size decreases, or as the grid is
further subdivided. However, any one data point in cell method analysis will always
be less precise than its equivalent in intersection method analysis. Thus, the
intersection method is superior to the cell method in terms of precision.
As the tests confirmed, increasing the number of subdivisions in a grid leads
to an increase in both precision and the length of time required to conduct each
analysis. Based on the results of the two tests, the following can be used as a guide
for constructing a grid that will maximize precision and efficiency in a given
situation: the average width/diameter of the trace fossils to be analyzed should equal
62
or exceed the distance between any two parallel grid lines. Adjusting a grid in this
fashion limits the amount of bioturbation that is “lost” in gaps between intersections.
This guideline also sets a minimum number of grid subdivisions to achieve
reasonable precision. If desired, more subdivisions can be added to further increase
the precision of an analysis.
Scale and intensity are important considerations for the intersection method,
as for all methods that estimate percentage cover. The relationship between the
average size of a trace fossil or patch of indistinct bioturbation and the size of the
sample area, as defined by a square or rectangular frame, has important implications
for intersection analysis. The number of grid subdivisions required to produce a
precise bioturbation estimate will be small when the average trace fossil is large
relative to the sample area. For example, the diameter of each of the “vertical trace
fossils” in the first test image is nearly three times the width of one “horizontal trace
fossil” in the second image. Using the metric described in the previous paragraph,
the optimal grids for evaluating these two test images are the 32x32-cell and
128x128-cell grids, respectively. Analysis of the “vertically bioturbated” image
using the 32x32-cell grid produced a bioturbation estimate of 11.86 percent, which is
1.95 percent higher than the actual percentage bioturbation (Fig. 2.9). The 128x128-
cell grid produced a bioturbation estimate for the “horizontally-bioturbated” image
of 6.89 percent, which is 0.5 percent higher than the actual percentage bioturbation
(Fig. 2.13). Thus, it is likely that applying this metric to any bedding plane
photograph will result in a bioturbation estimate that is accurate within five percent.
63
Bioturbation intensity relative to the size of the sample area can also have an
effect on how intersection analysis proceeds. If bioturbation covers much more than
half of the sample area, for example, counting intersections that fall on gaps in
bioturbation may be more efficient than the usual approach. If trace fossils are both
abundant and small relative to the sample area, the amount of time required to
conduct a grid analysis may approach that required for tracing the bioturbation.
Under the latter circumstances, the intersection method does not present an
advantage over other methods in terms of efficiency.
The intersection method compares favorably to the “ideal method,” defined
above, for evaluating bioturbation on bedding planes. Although the data collection
procedures of the intersection method are not fully objective, they leave much less
room for inconsistency and error than do those of the bedding plane bioturbation
index method. Results of the two tests demonstrate that the intersection method
attains a high level of precision in estimating the abundance of bioturbation on
bedding planes. Results obtained using this method are both quantitative and unitless
and can, therefore, be compared and manipulated in large datasets. An analysis using
the intersection method results in a map of the spatial distribution of bioturbation
within a sample area. This method also accommodates indistinct bioturbation by
reducing the amount of subjectivity required for its evaluation. Instead of having to
decide whether an area of the bedding plane is completely bioturbated and where the
boundaries of that area should be drawn, an analyst can count only those isolated
points that come in contact with bioturbation. The intersection method is readily
64
reproducible, and a record of each analysis can be saved and shared for maximum
transparency. This method is necessarily less efficient than semiquantitative methods
because the results it produces are more precise. The efficiency of the method
depends, to some extent, on the scale and intensity of bioturbation relative to the size
of the sample area. If the trace fossils in a sample area are of moderate size and
abundance, then the intersection method can be an efficient means of estimating the
percentage of bioturbation present. Extremes of scale and/or intensity may require
adjustments to the method or the use of an alternative method. In most cases,
however, the intersection method should serve as a precise means of evaluating
bioturbation on bedding planes and an effective means of refining estimates made in
the field.
Practical Considerations for Applying the Intersection Method
The intersection grid method was applied to photographs of studied bedding
planes from the Lower Cambrian of the Death Valley region and the Upper
Cambrian of Wisconsin. These results are presented and discussed in Chapters III
and V, respectively. Application of this method to actual bedding plane surfaces
prompted a few additional modifications to the method beyond those discussed
above.
Bedding planes exposed in the field are often small (but see Chapter V) and
poorly or incompletely preserved. Obtaining standardized 600-square-centimeter
sample areas for bedding plane analysis often necessitates the selection of surfaces
65
that are fractured or partially covered by soil or vegetation (Fig. 2.15). Gaps in the
studied bedding plane area represent missing information. On some surfaces, the
dominant pattern of bioturbation may indicate clearly whether gaps originally
contained bioturbation. For example, well-defined bedding-parallel traces may be
interrupted by small fractures but still maintain a consistent trajectory, indicating that
the traces were once continuous. In a majority of cases, however, little evidence is
available to support the likely presence or absence of bioturbation on missing
portions of bedding planes. If grid-line intersections overlie gaps in the bedding
plane surface, these points must be treated as unknowns and excluded from the total
number of grid-line intersections (Fig. 2.15). At the same time, the possibility that
some or all of the missing bedding plane area once contained bioturbation must be
addressed.
In this study, the following procedures were used to account for missing
portions of studied bedding planes. A hypothetical minimum percentage area
bioturbated was calculated for each incomplete surface based on the assumption that
the missing bedding plane area was completely devoid of bioturbation. The
“unknown” grid intersections were included in the total number of possible
intersections (i.e. the grid was considered to be complete); the total number of
bioturbated intersections recorded during analysis was then divided by the total
number of possible intersections. Similarly, a hypothetical maximum percentage area
bioturbated was calculated based on the assumption that the missing area was
homogeneously bioturbated. The “unknown” grid intersections were again included
66
FIGURE 2.15 – A fractured bedding plane surface in the upper member of the Wood
Canyon Formation (Lower Cambrian). Inner dimensions of frame are 24x25
centimeters. (Top) Unaltered digital photograph. (Bottom) Photograph with 80x80-
intersection grid superimposed. Green dots mark grid-line intersections below which
the bedding plane surface is missing.
67
within the total number of possible intersections. The total number of unknowns was
added to the total bioturbated intersections prior to dividing by the total number of
possible intersections. Finally, a probable percentage area bioturbated was calculated
by subtracting the total number of unknown grid intersections from the total number
of possible intersections and then dividing the total bioturbated intersections by the
adjusted number of possible intersections. This final calculation produces an estimate
of the percentage bioturbation within visible portions of the bedding plane only. As
long as the missing area does not constitute a substantial portion of the studied 600
square centimeters, the probable percentage area bioturbated should represent a
reasonable estimate for the entire studied region of the surface. For this reason, the
probable percentage area bioturbated is plotted as the primary estimate for each
incomplete studied bedding plane. Positive and negative error bars, bounded by the
hypothetical maximum and minimum percentages of bioturbation, accompany the
probable percentage and are used to indicate the degree to which the original
percentage of bioturbation, when the surface was complete, might have varied from
the grid estimate. These error bars do not take into account the potential analytical
error that is inherent in intersection grid analysis. Based on the results of the tests
presented above, the probable percentage area bioturbated is unlikely to be an
underestimate for the known bedding plane area, but it may be a slight overestimate
(of approximately five percent or less).
68
CHAPTER III
Bioturbation in the Lower Cambrian Succession of the Death Valley area, CA-NV
Introduction and Previous Work
Much work has addressed both ichnofossil diversity (e.g., Alpert, 1974;
Langille, 1974) and the challenges of estimating bioturbation intensity in Lower
Cambrian rocks (Droser and Bottjer, 1986; Miller and Smail, 1997). However, few
studies have focused on the broader ichnofossil-sediment record in the context of the
agronomic and Cambrian substrate revolutions (Marenco, 2006; Marenco and
Bottjer, 2008). A recent study (Marenco, 2006; Marenco and Bottjer, 2008) focused
on assessing the nature and abundance of bioturbation in siliciclastic units of the
Lower Cambrian succession in the White-Inyo Mountains, eastern California, with
the goal of testing the hypothesis that trilobites were the primary ecosystem
engineers on Early Cambrian seafloors. Results from fine-grained units suggest that
soft-bodied worm-like organisms, represented by the horizontal trace fossil
Planolites, were in fact the primary sources of sediment disruption in the Early
Cambrian shallow marine environments represented by fine-grained Lower
Cambrian strata in the White-Inyo Mountains. Data from bedding planes indicate an
increase in the intensity (quantity) of horizontal bioturbation, dominated by
Planolites-type behavior, up-section (through time). However, examples of vertical
bioturbation are rare in these units, most likely due to the limitations posed by
extensive microbial mats at the sediment-water interface. In contrast, analysis of
69
coarser-grained quartzites from the Poleta Formation indicates that deeply-vertical
bioturbation, in the form of the trace fossils Skolithos and Diplocraterion, was
already common in higher-energy environments in the Early Cambrian. Frequent
disruption by strong currents may have prevented microbial mats from becoming
well-established in higher-energy settings, resulting in substrate conditions that were
conducive to deep burrowing activity. Thus, it appears that the nature of the
agronomic revolution may have varied among different depositional environments.
A number of new hypotheses were prompted by the results of this recent
study. (1) Horizontal bioturbation, in the form of Planolites-type traces, was the
primary mode of substrate engineering in shallow subtidal siliciclastic marine
environments early in the agronomic revolution. (2) In higher-energy siliciclastic
environments, such as nearshore settings, the transition from primarily-horizontal to
deeply vertical bioturbation was much more abrupt than in lower-energy facies. (3)
Trilobites were largely restricted to more nearshore settings than those represented in
the Lower Cambrian succession of the White-Inyo Mountains. In order to test these
hypotheses and further explore the environmental and geographical scale of the
agronomic revolution, field studies were conducted in the Lower Cambrian
succession of the Death Valley region, California and Nevada. The Lower Cambrian
succession in the Death Valley region is a thinner, more proximal equivalent of the
Lower Cambrian succession in the White-Inyo Mountains (Nelson, 1978) and thus
provides an excellent opportunity to test whether the trends observed in the White-
Inyo Mountains represent localized, environment-specific phenomena. In addition to
70
field data collection, bedding plane photographs from all localities were analyzed
digitally using the intersection grid method described in Chapter II.
Paleogeography and Tectonic Reconstruction of the Lower Cambrian Succession in
the Western United States
The Lower Cambrian succession of the Death Valley region was deposited
within the western portion of the Cordilleran miogeocline (Mount, 1982b), a
westward-thickening wedge of terrigenous detrital material that originated in
shallow-water environments (Stewart and Suczek, 1977). Deposition of the
terrigenous detrital sequence was preceded by the emplacement of a diamictite-and-
volcanic sequence in the Precambrian and followed by the deposition of a carbonate
sequence later in the Cambrian (Stewart and Suczek, 1977). These three sequences
were deposited along a rifted continental margin (e.g. Stewart, 1972; Burchfiel and
Davis, 1975; Stewart and Suczek, 1977).
Stewart and Suczek (1977) proposed a tectonic model for the development of
the Precambrian-Cambrian succession in western North America in which the onset
of continental rifting, around 850 Ma, caused the crust to become thermally elevated
and then to gradually subside as it cooled and moved away from the developing
spreading center. Soon after the onset of continental rifting, the Precambrian
sequence of diamictite and volcanic rocks collected locally in rift basins, and the
thermally-elevated crust began to erode (Stewart and Suczek, 1977). Erosion and
shallow-water deposition of continental material continued, building up the
terrigenous clastic sequence along the continental margin, until the continental crust
71
reached an equilibrium height through a combination of erosion and subsidence
(Stewart and Suczek, 1977). An eastward marine transgression began at this stage,
eventually leading to the formation of a shallow epicontinental sea, and the carbonate
sequence was deposited in shallow water on the fully-formed miogeoclinal shelf
(Stewart and Suczek, 1977).
In the Early Cambrian, the region that is now western North America is
thought to have been migrating northward toward the equator as part of the
continental landmass Laurentia (McKerrow et al., 1992) (Fig. 3.1). The
paleocoastline was oriented roughly northeast-southwest at this time (McKerrow et
al., 1992). Using modern climate patterns, Rowland (1978) surmised that western
North America in the Early Cambrian would have had tropical environmental
conditions with a narrow range of annual temperatures.
Geological Setting
Introduction: The Lower Cambrian succession in the Death Valley region is a
thinner, proximal equivalent of the Lower Cambrian succession in the White-Inyo
Mountains (Nelson, 1978) and is comprised of the Wood Canyon Formation, the
Zabriskie Quartzite, and the lower portion of the Carrara Formation (Fig. 3.2). This
shallow subtidal marine to braidplain succession is dominated by siliciclastic
deposition with minor pulses of carbonate (Stewart, 1970). The Wood Canyon
Formation and Zabriskie Quartzite are the focus of this study and will be discussed
in detail below. Because siliciclastic units in the lower portion of the Carrara
72
FIGURE 3.1 – Early Cambrian paleogeography. Red dot marks approximate location
of the Death Valley region. Modified from McKerrow et al. (1992) and Scotese
(2001).
73
FIGURE 3.2 – Generalized stratigraphy of the Lower Cambrian successions in the
White-Inyo Mountains, eastern California, and the Death Valley region, California
and Nevada. Correlation between successions is based on trilobite zonation.
Montenegro Member of Campito Formation is abbreviated as “Mont.,” Zabriskie
Quartzite as “Zab.,” and Carrara Formation as “Carr.” After Nelson (1962), Stewart
(1970), Moore (1976), Palmer and Halley (1979), Mount (1982), Hunt (1990),
Hagadorn et al. (2000), and Corsetti and Hagadorn (2003).
74
Formation consist of slope-forming shale (Palmer and Halley, 1979), bedding plane
outcrops of this unit are rare and, where present, are too small for analysis.
The lower member of the Wood Canyon Formation is comprised of three
parasequences, each bounded by shallow subtidal siliciclastic rocks at the base and
shallow marine carbonates at the top (Prave et al., 1991). The Precambrian-Cambrian
boundary is located near the base of the third parasequence, where Treptichnus
pedum occurs in basal siliciclastic strata (Horodyski et al., 1994) and a prominent
carbon isotope excursion, which correlates to other boundary sections, has been
recorded (Corsetti and Hagadorn, 2000). The middle member of the Wood Canyon
Formation is predominantly comprised of non-marine to transitional conglomerates
(Fedo and Cooper, 1990) and contains no body fossils (Corsetti and Hagadorn,
2000). A return to marine-dominated sedimentation as a result of transgression is
recorded by fine- to medium-grained quartzites, sandstones, and siltstones, with
minor dolomites, in the upper member of the Wood Canyon Formation (Diehl,
1979).
Ediacaran-type fossils, including Swartpuntia (Hagadorn and Waggoner,
2000) and Ernietta (Horodyski, 1991), occur sparsely in the first parasequence of the
lower member of the Wood Canyon Formation, along with small calcareous cones
that resemble the earliest-known biomineralized fossil, Cloudina (Hagadorn and
Waggoner, 2000). Present in the upper member are a variety of characteristically-
Cambrian body fossils, including archaeocyathids (e.g., Hunt and Mabey, 1966),
echinoderm fragments (e.g., Diehl, 1976), trilobites (e.g., Hunt, 1990), and
75
inarticulate brachiopods (e.g, Hazzard, 1937). Trace fossils in the Wood Canyon
Formation include Treptichnus pedum, which defines the Precambrian-Cambrian
boundary (Narbonne et al., 1987); Rusophycus, an arthropod trace fossil, which
occurs in the upper member (e.g. Langille, 1974); and Skolithos, a deep vertical tube,
which occurs in the middle (Jensen et al., 2002) and upper (e.g. Langille, 1974)
members.
The Zabriskie Quartzite is a cliff-forming, pink quartz arenite that was
deposited in a nearshore to distal fluvial setting (Prave, 1984). Prave (1988)
interpreted the facies succession in the Zabriskie Quartzite to reflect a
“progradational-transgressive event.” No body fossils are known from the formation
(Stewart, 1970). However, Skolithos is conspicuously abundant in some of the
quartzite units, and Rusophycus occurs in dense associations along at least one
laterally-persistent horizon (Jensen et al., 2002b).
Previous Work: Nolan (1924; 1929) and Hazzard (1937) were the first to
describe in detail the stratigraphy of the Lower Cambrian succession in the Death
Valley region. Based on exposures in the northwestern Spring Mountains, Nolan
(1924; 1929) named the Johnnie Formation, Stirling Quartzite, and Wood Canyon
Formation. Hazzard (1937) correlated these three formations to a section in the
southern Nopah Range, where he described and named the Noonday Dolomite and
the Zabriskie Quartzite, treating the latter as a unit within the Wood Canyon
Formation. Wheeler (1948) elevated the Zabriskie Quartzite to a formation. Later,
Cornwall and Kleinhampl (1961) grouped the Wood Canyon Formation strata above
76
the Zabriskie Quartzite with the overlying Cadiz Formation (Hazzard, 1937) and
named these units the Carrara Formation.
Since Nolan and Hazzard, many workers have focused on the Lower
Cambrian succession in the Death Valley region. Due to the large volume of studies,
only a selection are mentioned here (for additional references, see Palmer (1971) and
Stewart (1970)). The sedimentology and stratigraphy of the Death Valley succession
have been investigated at a regional scale by Wheeler (1943; 1948), Wright and
Troxel (1956; 1966), Hunt and Mabey (1966), Troxel (1967), Reynolds (1969),
Stewart (1970), Mount and Rowland (1981), Prave (1992) and others (1991),
Corsetti (1998), Mount and Bergk (1998), Fedo and Cooper (2001), and Perkins
(2003). Regional correlations have been refined by Wheeler (1943; 1948), Stewart
(1966; 1970; 1982), Fritz (1975), Prave (1992), and Corsetti (1998). Mount (1982a)
conducted regional facies analysis, and Corsetti (1998) and Corsetti and Hagadorn
(2000; 2003) have used chemostratigraphy to correlate the Death Valley succession
with other Precambrian-Cambrian boundary successions around the world. Detailed
study of Wood Canyon Formation sedimentology and stratigraphy has been done by
Diehl (1976; 1979). Fedo and Cooper (1990) analyzed and modeled the depositional
facies of the middle and upper members of the Wood Canyon Formation. The
Zabriskie Quartzite has been studied extensively by Crews (1980) and Prave (1984;
1988; 1992). Bates (1965), Halley (1974), Palmer and Halley (1979), Palmer (1982),
and Adams and Grotzinger (1996) have studied the stratigraphy of the Carrara
Formation.
77
A number of workers have focused on the paleontology and paleoecology of
the Death Valley Lower Cambrian succession. Trilobites and trilobite biostratigraphy
have been studied by Palmer (1971; 1977; 1982), Palmer and Halley (1979), and
Hunt (1990). Corsetti (1998) incorporated biostratigraphy of pre-trilobitic strata into
his litho- and chemostratigraphic studies of the Death Valley succession. Gangloff
(1975; 1976) studied the distribution of archaeocyaths in the central and
southwestern Great Basin. Rowell (1977) studied the regional distribution of
brachiopods. Ediacaran-type fossils (Hagadorn et al., 2000; Hagadorn and
Waggoner, 2000) and problematic fossils (Langille, 1974; Hagadorn and Waggoner,
2002) have been described from the Precambrian-Cambrian boundary interval in the
Death Valley region. Langille (1974), Sundberg (1982; 1983), Signor (1994), and
Jensen (2002b) have described the trace fossils and Droser (1987; 1991) and Droser
and Bottjer (1988) the ichnofabrics of the Death Valley Lower Cambrian succession.
Wood Canyon Formation: Following the work of Nolan (1924; 1929) and
Hazzard (1937), Stewart (1966, p. C71) subdivided the Wood Canyon Formation
into lower, middle, and upper informal members, which are regionally-persistent
(Fig. 3.2). Diehl (1979) used a four-member subdivision scheme in his study of the
formation, dividing the middle member of Stewart (1966) into two members. This
latter scheme has not, however, been widely adopted. The Early Cambrian age of the
Wood Canyon Formation is corroborated by the presence of Treptichnus pedum in
the lower member (Horodyski et al., 1994) and trilobites and archaeocyaths in the
upper member. The Fallotaspis and Nevadella trilobite zones, in their entirety, and
78
the lowermost portion of the Bonnia-Olenellus trilobite zone occur in the upper
member (Hunt, 1990) (Fig. 3.2).
The contact between the Wood Canyon Formation and the underlying
Stirling Quartzite is conformable (Stewart, 1970) or disconformable (Corsetti and
Hagadorn, 2000) in much of the Death Valley region, although in some areas the
contact is either erosional, in which the lower member is missing, or transitional
(Stewart, 1970). Where it is exposed, the lower member of the Wood Canyon
Formation (the “lower carbonate-bearing member” of Diehl (1979)) ranges in
thickness from approximately 15 meters in the southeast to more than 430 meters in
the northwest (Stewart, 1970). The lower member consists of greenish-gray and
yellowish laminated fine sandstone and siltstone with thin interbeds of laminated
fine-grained quartzite and three or more prominent units of thinly-bedded quartz-
sand-bearing dolomite (Stewart, 1970; Diehl, 1979) (Fig. 3.2). Diehl (1979)
describes a gradual decrease in sand content and a concomitant increase in siltstone
above the first prominent dolomite unit in the lower member. The frequency of
cross-bedding also decreases up-section as the sand units become thinner and less
continuous (Diehl, 1979). Below the middle dolomite unit, flaser and lenticular
bedding are found in siltstone-dominated strata (Diehl, 1979). The middle and upper
dolomite units contain silty and sandy laminations that stand out prominently against
the fine- to medium-grained dolomite (Diehl, 1979). Below the first occurrence of
Treptichnus pedum, simple bedding-parallel trace fossils such as Planolites occur in
fine-grained strata ("worm borings or castings," Stewart, 1970; Diehl, 1979; Corsetti
79
and Hagadorn, 2000; Jensen et al., 2002b). The presence of wave and interference
ripples, desiccation cracks, and load casts led Diehl (1979) to conclude that the lower
member was deposited in an intertidal to mud flat environment. The dolomite units
likely represent deposition on a “low intertidal” carbonate bank (Diehl, 1979). Mount
(Mount, 1982a) interpreted the uppermost portion of the lower member (above the
third dolomite unit) to represent the initiation of a terrigenous clastic half-cycle in the
Death Valley and White-Inyo Mountains regions as a continental shelf developed.
The middle member of the Wood Canyon Formation is found at all localities
where the formation is exposed and ranges in thickness from 100 meters in the
southeast to more than 500 meters in the northwest (Stewart, 1970). The lower
portion of the middle member, the “conglomeratic arkose member” of Diehl (1979),
is a prominent, resistant unit comprised of reddish-purple conglomerates and
conglomeratic quartzites (Stewart, 1970; Diehl, 1979) (Fig. 3.2) and is cross-
stratified throughout (Diehl, 1979). Both planar and trough cross-stratified sets are
present (Stewart, 1970; Diehl, 1979). Interbeds of reddish-purple micaceous siltstone
up to 15cm thick occur rarely near the base of the member but increase in frequency
up-section (Stewart, 1970; Diehl, 1979). Diehl (1979) and Fedo and Cooper (1990)
interpreted this lower half of the middle member to be a distal braided river or
alluvial fan deposit, which most likely formed due to local tectonic uplift.
The upper half of the middle member, the “arkose-feldspathic quartzite
member” of Diehl (1979), consists of reddish sandstones, siltstones, quartzites, and,
in the lower portion, conglomerates (Diehl, 1979) (Fig. 3.2). Diehl (1979) identified
80
many well-developed fining upward sequences, typically 1-3 meters thick, in this
portion of the middle member. A layer of coarse-grained sandstone or conglomerate,
containing subangular siltstone clasts, is found at the base of each fining-upward
sequence (Diehl, 1979). Stewart (1970) similarly reported small (2-4cm diameter)
chips of siltstone and silty sandstone at the bases of cross-stratified bed sets. Each
fining-upward sequence grades into cross-stratified coarse-grained quartzite,
complexly-cross-laminated medium-grained quartzite, laminated fine-grained
sandstone, and finally reddish-purple siltstone (Diehl, 1979). Diehl (1979)
interpreted these fining-upward sequences as deposits generated by migrating
channels on a distal delta plain. Stewart (1970) reported isolated occurrences of
“indistinct vertical ‘worm borings’ resembling Scolithus tubes” in the siltstones.
Diehl (1979) reported Skolithos from only one bed: a one-meter-thick fine- to
medium-grained dark grayish-purple quartzite, which marks the top of his “arkose-
feldspathic quartzite member.” These burrows are evidence for a non-marine to
intertidal transition (Diehl, 1979), which Fedo and Cooper (1990) interpreted, in the
Marble Mountains, as a fluvially-dominated braid delta system with limited tidal
influence.
The upper member of the Wood Canyon Formation, the “upper carbonate-
bearing member” of Diehl (1979), is present at all Wood Canyon Formation
localities and ranges in thickness from less than 30 meters in the southeast to more
than 300 meters in the northwest (Stewart, 1970). In general, the lithologies and
sedimentary structures of the upper member resemble those of the lower member
81
(Diehl, 1979) (Fig. 3.2). Greenish- to brownish-gray siltstones and yellowish fine-
grained quartzites occur throughout the upper member, and yellowish-brown
dolomite and minor limestone are present in a regionally-traceable unit in the upper
half of the member (Stewart, 1970; Diehl, 1979).
The lower portion of the upper member consists predominantly of massive,
evenly laminated, and rarely cross-stratified quartzites (Stewart, 1970; Diehl, 1979).
Micaceous fine-grained sandstone and siltstone interbeds in the quartzites contain a
variety of sedimentary structures, such as wrinkle structures ("microripples,"
Stewart, 1970; "wrinkle marks," Diehl, 1979) and ripples, and arthropod trace fossils,
such as Cruziana and Rusophycus, and simple Planolites-like trace fossils (Stewart,
1970; Diehl, 1979; Jensen et al., 2002b), all of which become more common in the
upper half of the member (Diehl, 1979). Diehl (1979) also observed the arthropod
traces Diplichnites and Monomorphichnus, and possible examples of the anthozoan
dwelling trace Bergaueria, on bedding planes of the fine sandstone interbeds.
Psammichnites gigas, a large horizontal burrow (2-3cm in diameter) with transverse
striations and a lateral furrow, occurs locally in the upper member in the Death
Valley region (Jensen et al., 2002b). Diehl (1979) and Fedo and Cooper (1990)
interpreted the lower portion of the upper member to have been deposited in a
shallow subtidal to shoreface environment, which Mount (1982a) considered a
transitional setting between the terrigenous clastic and carbonate half-cycles in the
region.
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In quartzite beds in the middle of the upper member, Skolithos burrows are
common and trilobite fragments occur along laminations (Stewart, 1970; Diehl,
1979); Stewart (1970) considered these trilobite-bearing quartzites to be diagnostic
of the upper member. Skolithos is also common in the upper portion of the member
(Stewart, 1970; Diehl, 1979).
The regionally-persistent carbonate unit in the upper portion of the upper
member typically is 15-50 meters thick and almost entirely dolomitic, with many
oolitic layers, at most localities (Stewart, 1970). Exposures of the carbonate unit in
the northwestern part of the Death Valley region are typically thicker (150-250
meters) and limestone-dominated (Stewart, 1970); archaeocyaths are present low in
the unit at many of these localities (e.g., Hunt and Mabey, 1966; Stewart, 1970;
Diehl, 1979). Diehl (1979) recognized a cyclic pattern in the carbonate unit
consisting of a thin siltstone at the base overlain by a calcareous-cemented
sandstone, a cross-laminated dolomitic sandstone, a cross-laminated dolomite with
sandy stringers, and a massive dolomite at the top. Echinoderm and trilobite
fragments and molds of inarticulate brachiopods are common in the carbonate unit
(Stewart, 1970; Diehl, 1979). The presence of ooids, sand ribbons, and cross
laminations led Diehl (1979) to interpret the carbonate unit as a shallow-subtidal to
intertidal carbonate bank deposit. Mount (1982a) regarded this carbonate unit as the
inception of a carbonate half-cycle in the Death-Valley-White-Inyo region, which
would become further developed in the Carrara Formation above.
83
Above the carbonate unit is a brownish-yellow, medium-grained micaceous
sandstone that breaks apart relatively easily (Diehl, 1979). In this unit are quartzite
interbeds that contain trilobite fragments and closely-spaced Skolithos burrows and
interbeds of slope-forming micaceous siltstone (Diehl, 1979).
Diehl (1979) noted the presence of “unidentified spiral mold fossil traces” in
cross-laminated quartzites of the upper half of the member at all of his measured
sections. Diehl (1979) described the attributes of the specimens as follows: straight
or curved, up to 8.8cm in length, and between 1.5 and 2.5mm in diameter; randomly
oriented relative to bedding and non-disruptive to laminations; and ornamented with
“septa-like partitions” that intersect the main shaft obliquely. Signor (1994)
interpreted these structures as trace fossils (Harlaniella confusa). Jensen and
colleagues (2002a; 2002b) subsequently argued that the corkscrew-like surface
ornamentation could not have been produced by a spirally-burrowing animal and
that, instead, a body fossil interpretation of the structure should be explored.
At sections in the southern Nopah Range and Salt Spring Hills, Diehl (1979,
p. 60) identified a bed in the lower half of the upper member that contains “densely
packed, tiny (3-5cm long, 1mm basal diameter), conical casts” that are nearly
circular in cross section and have “evidence of a central structure.” Diehl (1979)
likened these cone-shaped structures to the enigmatic Lower Cambrian fossil
Volborthella, which is also conical, rather than to hyoliths, which have been reported
from the upper member by other workers (e.g., Hazzard, 1937; Stewart, 1970).
84
Zabriskie Quartzite: The Zabriskie Quartzite ranges in thickness from 3-250
meters in the Death Valley region (Prave, 1988). The formation is characterized by
pale pinkish fine- to medium-grained quartzite, which forms prominent cliffs
(Stewart, 1970) (Fig. 3.2). Through petrographic analysis, Stewart (1970) determined
that the average quartzite composition is 97 percent silica, making the Zabriskie
Quartzite more silica-rich than any other unit in the Death Valley Neoproterozoic-
Lower Cambrian succession. Because it has yielded no body fossils, the Zabriskie
Quartzite is considered to be Lower Cambrian on the basis of its stratigraphic
position between the trilobite-bearing Wood Canyon and Carrara formations
(Stewart, 1970) (Fig. 3.2).
The contact between the upper member of the Wood Canyon Formation and
the Zabriskie Quartzite is transitional except in the eastern portion of the Death
Valley region, where it is most likely erosional (Stewart, 1970). Consequently, there
has been debate over how the base of the formation should be defined (Stewart,
1970; Prave, 1984; 1988). In his original definition of the Zabriskie, Hazzard (1937)
included a unit, consisting of interbedded siltstones and Skolithos-bearing quartzites,
that Diehl (1979) later described as the uppermost portion of the Wood Canyon
Formation. Crews (1980) used Hazzard’s definition in her study of the Zabriskie
Quartzite, while Prave (1984; 1988) chose to follow Diehl’s interpretation, placing
the boundary at the top of the “highest, thick (greater than 50cm), laterally-
continuous, dusky brown weathering, highly-burrowed (Skolithus) quartz arenite
bed” (Prave, 1984, p. 19). The contact between the Zabriskie and the overlying
85
Carrara Formation has also undergone revision. Hazzard (1937) placed the boundary
at the first occurrence of siltstone or shale above the thick, pure Zabriskie quartzites.
However, several quartzite units occur above this boundary. Cornwall and
Kleinhampl (1961) treated these strata as a “transition zone” in the basal Carrara
Formation, but Stewart (1970) recognized that it might have been more reasonable to
include them in the Zabriskie Quartzite. In their study of the Carrara Formation,
Palmer and Halley (1979) formally revised the Zabriskie-Carrara boundary, placing
the upper Zabriskie-like quartzites and their associated siltstones and fine sandstones
within the Zabriskie Quartzite as the Emigrant Pass Member. The Emigrant Pass
Member ranges in thickness from 0-51 meters in the Death Valley region (Palmer
and Halley, 1979). Prave (1984) later informally renamed the remainder of the
Zabriskie strata (between his defined lower boundary and the base of the Emigrant
Pass Member) the Resting Spring member.
Prave (1984) subdivided the Resting Spring Member of the Zabriskie
Quartzite into four units (in ascending stratigraphic order): the transitional sandstone
and siltstone unit, the dark laminated quartz arenite unit, the massive to laminated
quartz arenite unit, and the coarse quartz arenite unit. The transitional sandstone and
siltstone unit is characterized by thinly-bedded quartz arenite with siltstone and
sandstone interbeds (Prave, 1984). Brownish red and yellowish brown shaly and silty
sandstones, often either rippled or laminated, make up the lower portion of the unit
(Prave, 1984). Small (1-10cm thick, up to several tens of meters in length) light gray
to brown quartz arenite lenses occur as sharply-defined interbeds within these lower
86
fine sandstones (Prave, 1984). In the upper portion of the transitional sandstone and
siltstone unit, quartz arenite beds are the dominant feature, and the finer-grained
interbeds are mostly discontinuous (Prave, 1984). Laminated, cross-bedded, and
massive quartz arenites, which commonly contain Skolithos and horizontal traces on
bedding surfaces, are present in the unit; Prave (1984) interpreted the massive beds
to be the product of vertical bioturbation. Based on the presence of small-scale cross
bedding, well-defined quartzite lenses, and Skolithos burrows, Prave (1984)
interpreted the transitional sandstone and siltstone unit as a middle shoreface deposit
that was periodically influenced by storms.
The dark laminated quartz arenite unit is composed of fine- to medium-
grained purplish quartz arenite that is typically laminated or hummocky cross-
stratified (Prave, 1984). In the lower portion of the unit, coarse-grained lags are
common at the base of laminations, ripples are often present, and Skolithos burrows
are rare and shorter than elsewhere in the Zabriskie Quartzite (Prave, 1984). The
upper portion of the dark laminated quartz arenite unit is lighter in color and contains
no hummocky cross-stratification (Prave, 1984). Small ripples and centimeter-thick
coarse-grained lenses are common, and trace fossils are absent (Prave, 1984). Prave
(1984) interpreted the presence of hummocky cross stratification, coarse-grained
lags, plane-parallel laminae truncated at low angles, and absent fine-grained material
in the lower portion of the unit as evidence for a swashface to nearshore marine
depositional setting. The upper portion of the unit represents an upper foreshore to
87
backshore setting in which coarse-grained lags may have formed due to eolian
processes (Prave, 1984).
The massive to laminated quartz arenite unit is predominantly composed of
fine- to medium-grained pinkish quartz arenite, throughout which coarse- to pebble-
sized grains of chert, quartz, and quartz sandstone are scattered (Prave, 1984). These
coarser grains are also found concentrated on the tops of some beds (Prave, 1984).
Bedding is poorly-developed and generally lenticular to faintly laminated, and trace
fossils are absent (Prave, 1984). Prave (1984) rejected the typical shallow marine
environmental interpretation for this portion of the Zabriskie Quartzite, suggesting
instead that the bimodal grain size distribution (no sorting), coarse-grained bed tops,
massive to parallel-laminated beds, and absence of trace fossils indicate an alluvial
braidplain setting, although he notes that no unquestionably-continental sedimentary
structures are present.
The coarse quartz arenite unit consists of light to dark brown, medium-
grained to pebble-sized quartz arenite, in which pebbles are composed of quartz,
quartz arenite, and chert (Prave, 1984). Bedding is sharply-defined and generally
lenticular, and layers of siltstone commonly occur in between beds (Prave, 1984).
Poorly-developed fining-upward sequences, laminations, and trough cross-bedding
are also common (Prave, 1984). Prave (1984) reported observing “a few burrows” in
the uppermost portion of the unit, which is slightly finer-grained. The coarse quartz
arenite unit is interpreted as a distal braided river to braidplain deposit in which the
88
final stage of regression was followed by initial transgression as indicated by the
return of bioturbation (Prave, 1984).
Palmer and Halley (1979) recognized two distinct units within the Emigrant
Pass Member of the Zabriskie Quartzite at many localities. Prave (1984) identified
these as the mixed siltstone and sandstone unit and the quartzose sandstone unit. The
lower portion of the mixed unit consists of greenish- and yellowish-gray siltstones
and thinly-bedded sandstones with lenses of fine- to medium-grained quartz arenite
that become more common up-section (Prave, 1984). Bioturbation, mostly bedding-
parallel and including arthropod traces, is abundant throughout the lower portion of
the mixed unit (Prave, 1984). A light red to brown fine- to medium-grained silty
sandstone, which is typically well-bioturbated and massive, is present in the middle
portion of the mixed unit (Prave, 1984). Above this reddish sandstone are
bioturbated greenish and reddish shales and siltstones that contain lenses of
calcareous sandstone and localized mudcracks (Prave, 1984). Prave (1984)
interpreted the abundance of bioturbation, dominance of fine-grained material, and
localized mudcracks as evidence for a tidal flat to lagoonal depositional
environment.
The quartzose sandstone unit of the Emigrant Pass Member is predominantly
comprised of light brown to brownish-gray, fine- to medium-grained quartzose
sandstone that forms continuous, cross-bedded to laminated beds, often with rippled
tops (Prave, 1984). Thin shaly layers commonly separate these beds (Palmer and
Halley, 1979). Bioturbation, including Skolithos, is abundant and widespread
89
throughout the quartzose sandstone unit, and raindrop impressions are present on
isolated bedding surfaces (Prave, 1984). The greenish siltstones and shales of the
lowermost Carrara Formation sharply overlie the slightly coarser-grained uppermost
beds of the Emigrant Pass Member (Prave, 1984). Prave (1984) interpreted the
quartzose sandstone unit to represent a transgressional, tidally-influenced,
intermittently-exposed barrier shoal environment based on the presence of cross-
bedding and ripple marks, abundant bioturbation, and subaerial exposure features.
This barrier shoal would have sheltered the tidal flat to lagoonal setting that is
represented by the mixed siltstone and sandstone unit (Prave, 1984).
Methods
Introduction: Studying the distribution and abundance of specific types of
trace fossils in Lower Cambrian rocks provides the most accurate picture available of
the role soft-bodied organisms may have played in engineering Early Cambrian
ecosystems. Lower Cambrian trace fossils can readily be studied through
examination of bedding-plane exposures, upon which cross cutting relationships and
trace ornamentation are often easy to observe. However, collecting such data from
bedding planes alone cannot result in a complete understanding of the degree and
diversity of tracemaker activity, and the impact of this activity on the substrate,
within a given stratigraphic interval. Instead, methods that clarify the sedimentary
context of bedding planes and provide a three-dimensional understanding of the
90
bioturbation present in an outcrop are a necessary complement to bedding plane
observations.
Examining and sampling the sedimentary strata that occur immediately above
and below each bedding plane helps to establish the depositional regime in which
bioturbation took place. Cross-sectional information about horizontal trace fossils
and ichnofabric can be obtained from views perpendicular to bedding, either at the
outcrop or by cutting and polishing rocks in hand sample. However, some traces may
either be too small or too indistinct to be recognized in outcrop or hand sample,
especially when the trace fill resembles the surrounding sediment. Petrography and
x-radiography can provide information in much greater detail than is available from
hand sample examination. Petrography is useful for identifying the mineralogical
components of trace fill and for recognizing features associated with microbial mats,
such as concentrations of heavy mineral grains and the alignment of elongated grains
such as mica (Schieber, 1999). X-radiography provides a sense of depth (three-
dimensionality) and density contrast at the macroscopic level, revealing bioturbation
patterns that extend beyond the thin section field of view.
Field Methods: All primary data for this project were collected from outcrops
of Lower Cambrian rocks in the Death Valley region of California and Nevada (Fig.
3.3). Locality information was obtained from Troxel (1967), Stewart (1970), Diehl
(1979), Prave (1984), Hunt (1990), Jensen (2002b), and Corsetti (pers. comm.) (see
Appendix I for additional locality information). Due to the nature of this project and
91
FIGURE 3.3. – Locality map for the Death Valley region. Red labels indicate
localities at which Lower Cambrian bedding planes were described, with the number
of studied bedding planes in parentheses. Blue labels indicate localities at which no
suitable bedding planes were found. The Horse Thief Canyon locality (White-Inyo
Mountains succession), at which 21 bedding planes were studied during the
preliminary study (Marenco and Bottjer 2008), is included for reference. Of the
localities shown here, the Winters Pass Hills is the most proximal and Horse Thief
Canyon is the most distal. After Stewart (1970), Prave (1984), and Hunt (1990).
92
the volume of previous studies concerning this succession, detailed stratigraphic
information was recorded from meter-thick intervals of section only.
Localities were chosen based on the probability of finding one or more
bedding plane exposures of at least 600cm
2
in size. Bedding planes with smaller
proportions than this supply insufficient information for evaluating trace fossil
diversity and the percentage of bioturbation present. Descriptive locality information
was compared with formation thicknesses and dip-slope relationships using maps of
the area (Jennings et al., 1962; Streitz and Stinson, 1974) to determine which
localities had the potential to contain extensive bedding-plane exposures. In spite of
this research, many promising localities were found to lack adequate bedding plane
exposures (Fig. 3.3). Factors such as lithology, structure, and weathering rates can
determine whether bedding planes will be exposed. Consequently, searching for
bedding planes is often hit-or-miss in siliciclastic strata. In the Death Valley region,
the relative thinness of the proximal Lower Cambrian succession also limits the
potential for bedding plane exposures.
To arrive at a three-dimensional perspective of Lower Cambrian bioturbation
in strata of the Death Valley succession, a method of data collection was employed
to assess simultaneously the fine-scale sedimentological and ichnological
characteristics of associated vertical and bedding-plane exposures. This method was
applied to eight one-meter-thick vertical outcrop exposures in which one or more
bedding planes of suitable size are also exposed. A portion of each bedding plane,
600cm
2
in area, was chosen for analysis at random by tossing one of two forms onto
93
the bedding surface. A 24x25cm form was used to evaluate broad bedding planes;
another, 10x60cm, was used on bedding planes with widths smaller than 24cm (Fig.
3.4). More than one random selection was made on larger bedding planes in order to
assess lateral variations in bioturbation across the surface.
Several types of data were recorded from the studied 600cm
2
portion of each
bedding plane. Sedimentary characteristics were described, including an estimate of
average grain size and composition. Ichnofossils were identified to the ichnogeneric
level when possible, and the dominant type of ichnofossil within the 600cm
2
area
was visually determined. On some bedding planes, the diameter range of the trace
fossils present was estimated. The bedding plane bioturbation index (BPBI) method
of Miller and Smail (1997) was used to estimate the quantity of bioturbation present
within the studied bedding plane area. Because BPBIs 2-5 each represent a range of
bioturbation percentages, a visual estimate of the percentage area bioturbated was
made using guidelines provided by Miller and Smail (1997). Following BPBI
analysis, photographs were taken of the 600cm
2
area framed by the form.
Each one-meter-thick vertical exposure was also examined in detail.
Sedimentary characteristics, including grain size and sedimentary structures, were
logged at a centimeter scale. Vertical bioturbation intensity was evaluated using the
ichnofabric index method of Droser and Bottjer (1986), in which vertical exposures
are scored based on the extent to which primary bedding was disrupted by
bioturbation. The ichnofabric indices range from “one” (no disruption) to “five”
(complete homogenization of the sediment) (Fig. 3.5). The position of each studied
94
FIGURE 3.4 – 600cm
2
frames used to randomly select portions of bedding planes for
analysis. (Top) 24x25-centimeter frame. Bedding plane is from the upper member of
the Wood Canyon Formation. (Bottom) Half of 10x60-centimeter frame. Bedding
plane is from the middle member of the Wood Canyon Formation.
95
FIGURE 3.5 – Illustrated key for use in assigning the five ichnofabric indices.
Ichnofabric index 1 = 0% bioturbation; 2 = 0-10%; 3 = 10-40%; 4 = 40-60%; and 5 =
60-100%. Modified from Droser and Bottjer (1986).
96
bedding plane within each meter section was recorded in centimeters from the base
of the meter. Where conditions permitted, a photograph of the complete meter
section was taken. Each meter section was also sampled at a minimum frequency of
every 10 cm, and all studied bedding planes were sampled.
At some localities, meter sections could not be analyzed because of
insufficient exposure. In such instances, individual bedding planes were evaluated
using the procedures discussed above.
Laboratory Methods: All of the samples collected from the eight studied one-
meter-thick sections were analyzed in detail using petrography and x-radiography.
The procedures followed for each of these analyses are described below.
At least one thin section was made from each sample. Each sample was cut,
perpendicularly to bedding, to the following approximate dimensions: 24x40mm for
smaller samples and 45x70mm for larger samples. The sample number and
orientation of bedding were noted on each thin section. When possible, thin section
blanks were prepared so that the thin sections would bisect one or more surface
traces. Digital images of the cut blanks were obtained using a standard optical
scanner prior to thin-sectioning.
Each thin section was examined using a petrographic microscope equipped
with a camera and imaging software. The following information was recorded for
each thin section, when applicable: general mineralogy, including the most
prominent mineral components; evidence of diagenetic and/or metamorphic
alteration; evidence, in the form of preferential mineral concentrations and/or grain
97
alignments, for the influence of a microbial mat; evidence of mineral concentrations
and/or grain alignments due to tracemaker activity (e.g. burrow fill); and descriptions
of structures made up of opaque and/or heavy mineral grains. Digital
photomicrographs were taken of enigmatic structures and other features requiring
further examination.
Once thin sections were made, the remaining billet from each sample was x-
rayed using a Penetrex industrial x-ray unit and 25.4x30.5 cm Kodak Industrex M x-
ray film. Film was exposed at 96kVA/8mA in three-minute increments for a total of
9, 12, 15, 18, 21, 24, 27, or 30 minutes depending on sample thickness and density.
Billets were x-rayed in place of the larger original samples because their
standardized dimensions, particularly thicknesses of approximately one centimeter,
made it easier to determine optimal exposure lengths for many samples at once. In
addition, the billets, which have regular surfaces, generated x-radiographs with fewer
artifacts than did samples having irregular surfaces.
All of the thin section billets were x-rayed perpendicularly to bedding,
producing x-radiographs that resemble vertical cross-sections. Digital images of the
x-radiographs were obtained by scanning the developed x-radiographic film. Both
negative and positive digital images were generated from each x-radiograph.
Features observed in x-ray, particularly sedimentary structures and evidence of
bioturbation reflected by density contrasts, were recorded and compared with those
documented from the thin sections and hand samples.
98
Data Analysis: Digital photographs of studied bedding planes were analyzed
using the intersection grid method to precisely estimate the percentage of each
600cm
2
sample area that is bioturbated. The percentages obtained using this method
were compared with the visual field estimates of bedding plane bioturbation indices
to test for analytical consistency. Bioturbation percentages obtained using the
intersection method were then used in place of the field estimates for further data
analysis.
The bioturbation percentages recorded using the intersection method from all
studied bedding planes were compiled and grouped according to their occurrence
within each formation (the Wood Canyon Formation is subdivided into lower,
middle, and upper members for this purpose). The mean bioturbation percentage
from each formation was also calculated. All results from the Death Valley region
were then graphed together to reveal any trends in bioturbation intensity that might
be present.
Results
Trace Fossils: The ichnogenus Planolites was first described by Nicholson
(1873) as “…wandering tunnels excavated by the worm in its search after
food….filled up with the sand or mud which the worm has passed through its
alimentary canal” (Fig. 3.6). Nicholson's intention, in publishing his description of
Planolites, was to preserve the similarly-described Palaeophycus as a botanical
genus (Keighley and Pickerill, 1995). However, confusion ensued over the criteria
99
FIGURE 3.6 – Field photographs of Planolites (top) and Rusophycus (bottom).
100
by which the two genera should be distinguished (see Keighley and Pickerill, 1995,
for a review). Many attempts have since been made to settle the dispute (e.g.,
Osgood, 1970; Pemberton and Frey, 1982; Fillion and Pickerill, 1990). At present,
Planolites and Palaeophycus are chiefly distinguished from each other by the
presence (Palaeophycus) or absence (Planolites) of a burrow lining (Keighley and
Pickerill, 1995; Bromley, 1996). Characteristics shared by both genera include sub-
horizontality and non-systematic meandering (Keighley and Pickerill, 1995).
Keighley and Pickerill (1995) regard burrow size and presence/absence of branching
as superfluous considerations for identifying trace fossils as Palaeophycus or
Planolites.
Rusophycus is considered an arthropod trace fossil based on its bilaterally-
arranged scratchmark ornamentation (Fig. 3.6). Originally interpreted as a resting
trace (Seilacher, 1955), Rusophycus is now more commonly considered to be a
predatory trace, reflecting a trilobite-worm predator-prey relationship (e.g., Jensen,
1990), although some are skeptical of this interpretation (Rydell et al., 2001). A
similar, shallower trace, Cruziana, has also been attributed to arthropods; Seilacher
(2007, p. 31-41) discussed at length the morphological variations of this ichnogenus
and their associated behavioral implications.
Psammichnites (gigas) is an unusually-large and complex trace fossil by
Lower Cambrian standards (Seilacher, 2007) (Fig. 3.7, 3.8). Seilacher (2007, p.80)
wrote, about a display specimen, “[it] looks as if a cowboy had dropped his lasso on
101
FIGURE 3.7 – Field photographs of Psammichnites (top) and Skolithos (bottom;
bedding plane view at left, cross-sectional view at right).
102
FIGURE 3.8 – Field photographs of Psammichnites on bedding planes of the upper
member Wood Canyon Formation at the Emigrant Pass locality. (Top) Close-up
view showing fine transverse corrugations and rope-like burrow crossings. (Bottom)
View of several burrow segments that are partially overlain and obscured by the
lighter-colored sediment of the bedding plane surface.
103
the ancient bedding plane.” Psammichnites reflects “scribbling” behavior, in which
the tracemaker crosses back over previously-covered territory (Seilacher, 2007). The
trace is typically preserved with a medial furrow, which may be sinuous, and delicate
transverse corrugations (Seilacher, 2007) (Fig. 3.8). Seilacher (2007, p. 80-81)
discussed the preservational variations of this trace at some length and tentatively
attributed it to a shell-less mollusk with a prominent “snorkel” that was used both for
respiration and surface detritus feeding. Psammichnites gigas typically is found in
upper Lower Cambrian strata (Jensen et al., 2002b), in which it has been used for
local stratigraphic correlation (Álvaro and Vizcaïno, 1999).
Skolithos is a “vertical unlined dwelling shaft” (Bromley, 1996) (Fig. 3.7)
that in Cambrian rocks often occurs in dense accumulations known as “piperock,” a
term first used by Peach and Horne (1884) and made popular by Hallam and Swett
(1966). Piperock is considered a typical feature of Cambrian high-energy nearshore
clastic deposits (e.g., Hallam and Swett, 1966) and rarely is found in younger rocks
(Droser, 1991). Skolithos is the characteristic trace fossil of Seilacher’s (1967)
ichnofacies of that name, which is characterized by alternating erosional and
depositional events in moderate to high-energy nearshore environments with
dynamic substrates. However, low-density Skolithos assemblages occur throughout
the Phanerozoic in a range of environments, including storm-event deposits (e.g.,
Vossler and Pemberton, 1988; Frey, 1990), restricted marginal marine settings (e.g.,
de Gibert and Ekdale, 1999), and transitional-to-nonmarine environments (e.g.,
Buatois et al., 1998).
104
Data from Meter Sections and Samples: Below are field data recorded from
each of the studied one-meter-thick sections and additional individual bedding
planes. Data from corresponding studied samples follow each field data entry. These
data are grouped in ascending stratigraphic order by locality, and localities are listed
according to their geographical location, from northwest to southeast in the Death
Valley region. No samples were collected from the “Outcrop” in the Montgomery
Mountains, the Meter at Emigrant Pass, the bedding planes in the Winters Pass Hills,
or bedding planes D, G, and H from the Northern Salt Spring Hills. Unless noted
otherwise, the 24x25-centimeter form was used to designate the studied portion of
each bedding plane. Photographs of all studied bedding planes are located in
Appendix II. Samples were analyzed using petrography and x-radiography. Three
views of each sample are figured: the cut surface of the sample, the thin section, and
the x-radiograph. Each of these images was produced using a flat-bed optical
scanner. The cut surface of each sample was immersed in water prior to scanning in
order to improve the clarity of the resulting image. Consequently, air bubbles appear
in some of the sample images. The top of each thin section is indicated by a notch.
Most of the thin sections are incomplete relative to the billets and x-radiographs. X-
radiographic negatives were scanned and then inverted to positives, in which darker
tones indicate greater density.
105
Sample Data: General Patterns
Petrographic analysis revealed that the samples generally resemble one
another in terms of sedimentary characteristics despite common, distinct color
differences among samples from different outcrops. Quartz is the predominant
mineral throughout the samples, although the grains vary considerably in size.
Elongated mineral grains, predominantly mica, are present in fine-grained, organic-
rich layers within many of the samples and appear to have been aligned by post-
depositional alteration. Limited quantities of pyrite, hematite, and chlorite occur in
some samples. Opaque minerals, often including pyrite, are concentrated in some
samples and stand out in x-radiograph. A few samples from the upper member of the
Wood Canyon Formation contain large quantities of calcite, primarily in the form of
echinoderm ossicles, and dolomite, in the form of individual rhombs and dolomitized
ooids (Fig. 3.9).
Sedimentary structures are visible in many samples and in some instances are
defined by concentrations of opaque mineral grains or layers of very fine-grained
organic-rich material that are visible in thin section and x-radiograph. In other
samples, bedding is less prominent due to the relative compositional homogeneity of
the constituent grains.
Evidence of bioturbation is common and varies from well-defined structures
to diffusely disrupted bedding. Horizontal burrows often are visible in cross-section,
appearing as ovate structures filled with clean quartz grains that are distinctly coarser
than the surrounding material (Fig. 3.10). Such burrows are typically surrounded by
106
FIGURE 3.9 – Photomicrographs, in plane light, of features observed in the thin
section of the 15-centimeter sample from Meter B, upper member Wood Canyon
Formation, Echo Canyon. (A) Calcitic echinoderm ossicles. (B) Archaeocyath
skeletal fragment. (C) and (D) Dolomitized ooids.
A B
C D
107
FIGURE 3.10 – Photomicrographs, in plane light, of bioturbation structures observed
in thin sections of Wood Canyon Formation samples from the Death Valley region.
All burrows are filled with material that is coarser-grained and more quartzose than
the surrounding sediment. (A) Cross section of a bedding-parallel burrow in fine-
grained matrix. Upper member, Echo Canyon, Meter B, 90cm. (B) Lozenge-shaped
cross-sections of three burrows in contrasting matrix. Flattening of burrows likely
due to post-depositional compaction. Lower member, Montgomery Mountains,
Meter B, 60cm. (C) Sub-vertical meandering burrow in very fine-grained matrix.
Lower member, Montgomery Mountains, Meter A, 20cm. (D) and (E) Vertical
burrows with burrowing-induced downwarping of adjacent finer-grained strata. D:
Upper member, Echo Canyon, Meter A, 100cm. E: Middle member, Montgomery
Mountains, Meter E, 40cm. (F) Close-up of contact between vertical burrow (left)
and fine-grained matrix (right). Middle member, Montgomery Mountains, Meter C,
59cm.
108
FIGURE 3.10 – Photomicrographs of bioturbation structures
A B
C D
F E
109
sediment that is fine-grained and organic-rich. Vertical and sub-vertical burrows are
also made visible by distinct grain-size contrasts, particularly when they occur at the
base of coarse-grained, quartz-rich layers that overlie finer-grained layers (Fig. 3.10).
In such instances, vertical and sub-vertical burrows extend downward into the finer-
grained layer and are filled with coarser-grained material from above. Diffuse
bioturbation is more apparent in the x-radiographs because subtle density contrasts in
x-radiograph are more prominent than subtle grain-size contrasts in thin section.
Ovate to conical or globular structures composed primarily of opaque mineral
grains occur in a number of samples from the Death Valley region and are
interpreted to be fragments of the enigmatic Lower Cambrian fossil Volborthella
(Fig. 3.11). These structures are visible in thin sections and x-radiographs of one
sample from the Montgomery Mountains (MM03-80), one sample from Echo
Canyon (EC01A), and several samples from the Northern Salt Spring Hills
(NSSH03, NSSH04-4, -25, -50, -70, -80, and -92). In most cases, the edges of these
opaque-grain-filled structures are moderately well-defined in both thin section and x-
radiograph. Many dense structures interpreted to be skeletal material are visible in x-
radiographs of the following additional samples: from Echo Canyon, ECBP01 and
ECM02-5 and -15; from Emigrant Pass, EPRUS1; from the Montgomery Mountains,
MM01-18 and -58, MM02-20, and MM03-90 and -100; and from the Northern Salt
Spring Hills, NSSH04-40, -62, and -100.
110
FIGURE 3.11 – Photomicrographs, in plane light, of probable Volborthella
fragments in thin sections of Wood Canyon Formation samples from the Death
Valley region. (A) and (B) Cone-shaped specimens within a larger cluster of
Volborthella fragments. Middle member, Montgomery Mountains, Meter E, 80cm.
(C) Concentrically-laminated transverse cross-section of a Volborthella cone. Upper
member, Echo Canyon, bedding plane EC01A. (D) and (E) Longitudinal and
transverse cross-sections, respectively, of portionsof a Volborthella cone. Gap
between two halves in D represents open central shaft of fossil. Upper member,
Northern Salt Spring Hills, bedding plane C. (F) Large cluster of Volborthella
fragments interpreted to be a skeletal lag. Upper member, Northern Salt Spring Hills,
Meter, 25cm. (G) Upper right extension of cluster of fragments shown in F. (H)
Close-up of largest specimen in G, showing “V”-like termination of cone. (I) Small
cluster of Volborthella fragments. Part of open central shaft is visible in largest,
cone-shaped fragment. Upper member, Northern Salt Spring Hills, Meter, 50cm. (J)
Close-up of largest fragment in I. (K) Upper middle extension of cluster of fragments
shown in F. (L) Transverse cross-section of cone. Upper Member, Northern Salt
Spring Hills, Meter, 92cm.
111
FIGURE 3.11 – Photomicrographs of probable Volborthella fragments
A
B
C D
F E
112
FIGURE 3.11 (continued) – Photomicrographs of probable Volborthella fragments
G H
I J
L K
113
Titanothere Canyon
Bedding Plane – Wood Canyon Formation, lower member (Fig. 3.12)
Located in the lower member of the Wood Canyon Formation, most likely
above the third prominent carbonate unit, this bedding plane consists of a slightly
muddy, heavily-varnished surface that tops a bed composed of silty fine sandstone.
Half of the 10x60-centimeter frame was used for analysis due to the limited
dimensions of the bedding surface. A BPBI of three was assigned based on the
presence of small Planolites-type and bilobed trace fossils.
The top of this sample corresponds to the bedding plane surface. No
bioturbation is visible within the billet, thin section, or x-radiograph of this sample.
However, fine-grained opaque material highlights small-scale cross-bedding in the
upper portion of the sample.
Echo Canyon
Bedding Plane EC01A – Wood Canyon Formation, upper member (Fig. 3.13)
This bedding plane was described from an outcrop located in a small canyon
that diverges from Echo Canyon toward the north. The surface consists of greenish,
micaceous, silty fine sandstone. Circular burrow entrances, each approximately one
centimeter in diameter, and scattered clusters of scratchmarks constitute the
bioturbation on this surface. A BPBI of two was assigned.
Abundant hematite gives this sample a reddish cast. No bioturbation is
apparent. Skeletal fragments, including trilobite material, are scattered throughout
114
FIGURE 3.12 – Lower member Wood Canyon Formation bedding plane sample
from Titanothere Canyon. Top of sample corresponds to the bedding plane. Left to
right: billet scan, thin section, and x-radiograph. Sample is cut approximately to
standard thin section size (24x40mm).
115
FIGURE 3.13 – Bedding plane samples collected from Echo Canyon. EC01A and
ECBP01 occur in the upper member of the Wood Canyon Formation and are
standard-size thin section samples (24x40mm). ECZ01 occurs in the Zabriskie
Quartzite and is an oversized sample (approximately 45x70mm). Left to right: billet
scan, thin section, x-radiograph.
116
FIGURE 3.13 – Bedding plane samples collected from Echo Canyon
117
the sample and are most clearly visible in the thin section. One circular fragment
with concentric laminations, visible only under magnification, is interpreted to be a
cross-section of Volborthella.
Bedding Plane ECBP01 – Wood Canyon Formation, upper member (Fig. 3.13)
Located on the north side of the main canyon, this bedding plane consists of
pale yellowish to orange, hematite-rich silty fine sandstone. A BPBI of four was
assigned based on abundant, small trace fossils, both well-defined and indistinct.
Planolites and circular burrow entrances are present, in addition to one large
nondescript trace.
Hematite is abundant in this sample. Several horizontal trace fossils and one
shallow sub-vertical trace are visible in cross-section; these traces are filled with
hematite-coated quartz sediment and surrounded by finer-grained material. A few
very small (one millimeter or less in diameter) possible skeletal fragments are visible
in the x-radiograph.
Meter A – Wood Canyon Formation, upper member (Fig. 3.14)
The base of this meter is located approximately one meter above bedding
plane ECBP01. The bedding plane ECBP02 occurs at 15 centimeters above the base
of the meter. Siltstone, silty fine- to medium-grained sandstone, and skeletal
sandstone characterize this meter. Skeletal- and quartz-rich material occurs from
approximately 28-35, 45-50, and 82-82 centimeters within the meter. An isolated
“stringer” of millimeter-scale skeletal fragments occurs at 65 centimeters, and lags of
skeletal material occur in the troughs of cross beds between 52 and 56 centimeters.
118
FIGURE 3.14 – Upper member Wood Canyon Formation, Echo Canyon, Meter A.
Numbers at left indicate height, in centimeters, of each sample above base of meter
but do not constitute a scale. Samples are arranged in descending stratigraphic order.
All samples are cut approximately to standard thin section size (24x40mm). Left to
right: billet scan, thin section, x-radiograph.
119
FIGURE 3.14 – Echo Canyon, Meter A
120
FIGURE 3.14 (continued) – Echo Canyon, Meter A
121
Rippled bedding and cross-bedding are scattered throughout the meter, of which the
remainder is planar-bedded. An ichnofabric index of two was recorded for the
following intervals within the meter: 3-34, 35-40, 70-72, 76-78, 88-90, 91-92, and
98-100 centimeters. All other strata were interpreted to reflect ichnofabric index one.
Bedding plane ECBP02, at 15 centimeters, is composed of pale orangish-
yellow, micaceous fine sandstone; the surface is gently undulating. Although
bioturbation is generally indistinct, a few Planolites-type traces and circular burrow
entrances, which vary considerably in diameter, are visible on the surface. A BPBI of
four was assigned. Elsewhere on the surface, two horizontal traces with apparent
bulbous ends occur.
All of the samples collected from this meter have a reddish-orange cast due to
abundant hematite. Samples 39, 45, 65A, 65B, and 77 are characterized by uniform
grain size, laminated bedding, and the absence of bioturbation. Horizontal burrows
are visible in samples zero and 10. Within sample 95, hematite is concentrated in a
well-defined ovate structure that may be the cross-section of a horizontal burrow.
Sample 15 is indistinctly bioturbated, and vertical burrows are visible in samples 34
and 100 and in the x-radiograph of sample 95. Also present in sample 34 are
scattered dolomite rhombs and skeletal fragments.
Meter B – Wood Canyon Formation, upper member (Fig. 3.15)
This base of this meter occurs fewer than 25 centimeters above the top of
Meter A. The lowermost ten centimeters, composed of broadly cross-bedded
medium-grained sandstone, are distinct from the remainder of the meter, which is
122
FIGURE 3.15 – Upper member Wood Canyon Formation, Echo Canyon, Meter B.
Numbers at left indicate height, in centimeters, of each sample above base of meter
but do not constitute a scale. Samples are arranged in descending stratigraphic order.
All samples, except the 15-centimeter sample, are cut approximately to standard thin
section size (24x40mm). The 15-centimeter oversized sample is cut approximately to
45x70mm. Left to right: billet scan, thin section, x-radiograph.
123
FIGURE 3.15 – Echo Canyon, Meter B
124
FIGURE 3.15 (continued) – Echo Canyon, Meter B
125
dominated by fine sandstone with a few thin siltier layers. These lowermost ten
centimeters appear to be non-bioturbated (ichnofabric index one). Ripples and cross-
bedding are common between 16 and 30 centimeters. An oolitic interval occurs
between 10 and 15 centimeters and is capped by two centimeters of rippled siltstone.
An ichnofabric index of two was recorded from 15 to 60 centimeters. Possible
skeletal fragments are present at 24 centimeters. Quartz-rich fine sandstone lenses
occur within siltier fine sandstone between 37 and 60 centimeters. The upper portion
of the meter, between 60 and 100 centimeters, could not be described in detail due to
the steepness of the outcrop; however, examination at a distance indicated that the 40
centemeters are predominantly composed of fine-grained sandstone.
Bedding planes occur at 29, 35, 45, and 61 centimeters above the base of the
meter. ECBP03, at 29 centimeters, is composed of yellowish silty fine sandstone
with a very thin muddier surface layer. Most of the bioturbation present is indistinct,
with the exception of a few Planolites-type traces. A BPBI of four was assigned to
this surface. Bedding plane ECBP04 occurs at 35 centimeters; only half of the
24x25-centimeter frame was used due to its limited size. This surface is yellowish
and slightly siltier than ECBP03. Aside from indistinct bioturbation, a few small
Planolites-type traces and a possible cluster of scratchmarks are visible. A BPBI of
three was assigned to this surface. Bedding plane ECBP05, at 45 centimeters, is a
tan, silty surface that contains indistinct bioturbation and small, abundant traces,
most of which resemble Planolites. Half of the 24x25-centimeter grid was used, and
a BPBI of four was assigned to this surface. Bedding plane ECBP06 occurs at 61
126
centimeters. The surface is orangish and silty with scattered patches of a yellowish,
muddy surface layer. Using half of the 24x25-centimeter grid and based on abundant
but mostly indistinct bioturbation, a BPBI of four was assigned.
Abundant hematite is present in all of the samples in this meter. Hematite-
coated dolomite rhombs are present in samples five and 15. Sample 15 also contains
isolated archaeocyath fragments and abundant dolomitized ooids, echinoderm
ossicles, and assorted skeletal grains. Indistinct bioturbation occurs in samples 22,
29, 35, 40, and 55. Horizontal burrows are present in samples 35 and 90. Bedding
appears undisturbed in samples 61 and 100.
Bedding Plane ECZ01 – Zabriskie Quartzite (Fig. 3.13)
This bedding plane is located on the south side of Echo Canyon in a narrow
wash. Stratigraphically, this surface is within a few meters of the Zabriskie-Carrara
contact. The bedding plane occurs within a darkly-weathered quartzite unit that
contains Skolithos in moderate abundance (BPBI three). Skolithos burrows
intersecting this surface have diameters of 4-10 millimeters.
Portions of at least three Skolithos burrows are visible in the billet, thin
section, and x-radiograph of this sample. Each burrow is filled with clean quartz
grains that are coarser than the material adjacent to the burrow. The x-radiograph
reveals the full extent of the Skolithos burrow on the right. This burrow is oriented
slightly obliquely to the cut surface of the billet as indicated by the incomplete
expression of the burrow in both the billet and thin section.
127
Montgomery Mountains
Meter A – Wood Canyon Formation, lower member (Fig. 3.16)
This meter occurs approximately five meters above the third (uppermost)
prominent carbonate in the lower member. The lowermost 35 centimeters are
dominated by alternating thin layers of quartzite and silty fine-grained sandstone;
flaser bedding is common, and wavy laminations occur sporadically between 20 and
35 centimeters. The intervals between 35-52 and 62-70 centimeters are dominated by
planar-bedding shaley fine sandstone. Thicker quartzite beds alternate with thinner
fine sandstone layers between 70 and 100 centimeters; wavy and crinkly bedding is
scattered throughout. Bioturbated layers (ichnofabric index two) occur between 7-8,
10-15, 15-18, and 52-55 centimeters. The remainder of the meter is non-bioturbated.
Two bedding planes occur within this meter, at 78 and 100 centimeters. The
78-centimeter bedding plane (possibly an “underbed” below the true bedding plane)
is an irregular, tan to reddish brown surface. Little bioturbation is present (BPBI
two), and most is indistinct. The second bedding plane, at 100 centimeters, consists
of a reddish-tan, silty surface that contains few, poorly-defined Planolites-type
traces. A BPBI of two was assigned to this bedding plane.
Samples from this meter are tan, brown, and/or deep green in color.
Bioturbation is common; all of the samples appear to contain at least some biogenic
disruption. Burrows are particularly well-defined in samples 5, 10, 20, and 50.
Macroscopic pyrite grains are present in sample 70, and aligned muscovite grains
define some laminations in samples 91 and 100.
128
FIGURE 3.16 – Lower member Wood Canyon Formation, Montgomery Mountains,
Meter A. Numbers at left indicate height, in centimeters, of each sample above base
of meter but do not constitute a scale. Samples are arranged in descending
stratigraphic order. All samples, except the 100-centimeter sample, are cut
approximately to standard thin section size (24x40mm). The 100-centimeter
oversized sample is cut to approximately 45x70mm. Left to right: billet scan, thin
section, x-radiograph.
129
FIGURE 3.16 – Montgomery Mountains, Meter A
130
FIGURE 3.16 (continued) – Montgomery Mountains, Meter A
131
Meter B – Wood Canyon Formation, lower member (Fig. 3.17)
This meter occurs approximately 15 meters above the third prominent
carbonate unit and close to the base of the middle member. Wrinkle structures are
present approximately one meter above the top of Meter B. The dominant lithology
of Meter B is fine-grained sandstone, either planar- or cross-bedded at a small scale.
Thin muddy siltstone horizons are distributed throughout the meter section, although
they are thickest and most abundant in the 60-70 centimeter interval, which also
includes thin flaser-bedded layers. Flaser bedding occurs between 53 and 60
centimeters as well. These flaser-bedded intervals appear to be bioturbated
(ichnofabric index two); no other bioturbation was observed.
One bedding plane, at 100 centimeters, was analyzed using half of the 10x60-
centimeter frame. The surface undulates gently and is composed of fine sandstone
that is stained reddish in patches. Bioturbation consists entirely of Planolites-type
traces. A BPBI of two was assigned to this surface.
All samples from this meter are greenish in color. Chlorite is present in
samples 60, 80, and 100. Bioturbation occurs only in samples 60 and 100 and
consists primarily of bedding-parallel burrows. Well-defined laminations are visible
in all of the remaining samples except 40, in which no sedimentary structures are
visible. One set of small-scale, low-angle cross beds is present in sample 50. Sample
20 was cut slightly obliquely to bedding, but laminations are visible.
132
FIGURE 3.17 – Lower member Wood Canyon Formation, Montgomery Mountains,
Meter B. Numbers at left indicate height, in centimeters, of each sample above base
of meter but do not constitute a scale. Samples are arranged in descending
stratigraphic order. All samples, except the 80-centimeter sample, are cut
approximately to standard thin section size (24x40mm). The 80-centimeter oversized
sample is cut to approximately 45x70mm. Left to right: billet scan, thin section, x-
radiograph.
133
FIGURE 3.17 – Montgomery Mountains, Meter B
134
Meter C – Wood Canyon Formation, middle member (Fig. 3.18)
This meter occurs in the upper portion of the middle member, within the
transition from a non-marine to an intertidal setting. The lower 54 centimeters
consist of cross-bedded, pinkish fine-grained quartzite; individual cross beds are
approximately 10 centimeters thick. Massive, medium-grained quartzite comprises
the remainder of the meter. No bioturbation is visible throughout the vertical
exposure of the meter, although the surface is considerably varnished.
Two bedding planes were described from this meter, at eight and 100
centimeters above the base. The eight-centimeter bedding surface is of limited size
and required use of half of the 10x60-centimeter form. The bedding plane, which
consists of fine-grained quartzite, contains no apparent bioturbation. The 100-
centimeter bedding surface is draped with a thin layer of mud that is interrupted by
abundant circular burrow entrances, which are filled with fine-grained quartzite.
Some of these vertical burrows appear to be joined to one another. A small number
of Planolites-type burrows are also present. A BPBI of three was assigned to this
surface.
These samples are purplish-gray to deep purple in color. No bioturbation is
visible in samples 10 and 20; opaques define small-scale cross beds in sample 10 and
laminations in sample 20. All of the other samples contain vertical bioturbation;
burrows are clearly defined in all samples except for 90. Samples 80 and 90 are
dominated by medium to coarse quartz grains; consequently, contrast within the thin
sections of these samples is limited. Burrows are well-defined in samples 59 and 100
135
FIGURE 3.18 – Middle member Wood Canyon Formation, Montgomery Mountains,
Meter C. Numbers at left indicate height, in centimeters, of each sample above base
of meter but do not constitute a scale. Samples are arranged in descending
stratigraphic order. The 10-, 20-, and 59-centimeter samples are cut approximately to
standard thin section size (24x40mm). The 80-, 90-, and 100-centimeter samples are
cut to approximately 45x70mm (oversized). Left to right: billet scan, thin section, x-
radiograph.
136
FIGURE 3.18 – Montgomery Mountains, Meter C
137
FIGURE 3.18 (continued) – Montgomery Mountains, Meter C
138
due to the presence of fine-grained material in addition to the coarser-quartz-rich
burrow fill. A possible Volborthella fragment is visible in the x-radiograph of sample
20.
Meter D – Wood Canyon Formation, middle member (Fig. 3.19)
The base of this meter is approximately five centimeters above Meter C. The
basal 14 centimeters consist of medium-grained quartzite with a few thin stringers of
coarser sand throughout. Small-scale hummocky cross-stratification is present in
medium-grained quartzite from 14 to 18 centimeters. No bioturbation was observed
within the lowermost 18 centimeters. The interval between 18 and 40 centimeters is
covered. Between 40 and 64 centimeters, the dominant lithology is tan medium-
grained quartzite. Small-scale cross-bedding occurs from 42 to 43 centimeters and
from 50 to 55 centimeters; thin siltier layers are present at 45, 46, 51, 55, and 58
centimeters. 64-65 centimeters is covered. The remainder of the meter consists of
varnished fine-grained quartzite with occasional thin silty interbeds and small-scale
cross-bedding. Small fining-upward sequences, each approximately four centimeters
thick, occur between 80 and 100 centimeters. No bioturbation was observed in the
vertical exposures of the upper portion of the meter.
Two bedding planes were described, at 18 and 100 centimeters. Both required
use of half of the 10x60-centimeter frame. The 18-centimeter bedding plane consists
of a silty, fine-grained, oscillation-rippled surface. Bioturbation consists
predominantly of burrow entrances that vary in shape and diameter and appear to be
filled with coarser-grained material. One small bilobed trace and few very small
139
FIGURE 3.19 – Middle member Wood Canyon Formation, Montgomery Mountains,
Meter D. Numbers at left indicate height, in centimeters, of each sample above base
of meter but do not constitute a scale. Samples are arranged in descending
stratigraphic order. The 85-, 90-, and 100-centimeter samples are cut to
approximately 45x70mm (oversized). The remaining samples are cut approximately
to standard thin section size (24x40mm). Left to right: billet scan, thin section, x-
radiograph.
140
FIGURE 3.19 – Montgomery Mountains, Meter D
141
FIGURE 3.19 (continued) – Montgomery Mountains, Meter D
142
FIGURE 3.19 (continued) – Montgomery Mountains, Meter D
143
Planolites-type traces are also present. A BPBI of two was assigned to this surface.
The 100-centimeter bedding plane consists of a muddy interference-rippled surface;
the muddy layer is missing in some patches, revealing fine-grained quartzite beneath.
Bioturbation again consists mainly of circular burrow entrances that vary in diameter
from five to eight millimeters and are filled with fine-grained quartzite. Rare
Planolites-type traces are also present. A BPBI of three was assigned to this surface.
All of these samples are dark purple in color. A thin (1-3 millimeters thick)
layer of very fine-grained micaceous and opaque-rich material is present at the top of
sample 18, which corresponds to one of the two studied bedding plane surfaces
within this meter. Indistinct bioturbation is present in the lower few millimeters of
sample 18. Samples 50, 58, and 70 appear to lack bioturbation; opaques outline some
laminations and, in sample 50, small-scale cross beds. Samples 85, 90, and 100
contain a mixture of indistinct and well-defined bioturbation, including both
horizontal and vertical burrows. The visibility of these burrows is enhanced by the
presence of fine-grained interbeds within the samples. Many of the burrows appear
to extend downward two or more centimeters from their point of origination. The
most prominent burrow visible in the x-radiograph of sample 85 extends at least four
centimeters down from the top of the sample. At least three of the burrows visible in
the thin section of sample 100 appear to originate from the 100-centimeter bedding
plane that tops the meter. Possible fragments of Volborthella are visible in the x-
radiographs of samples 18 and 58.
144
Outcrop – Wood Canyon Formation, middle member
This small outcrop is located approximately five meters up-section from
Meter D and consists predominantly of cross-bedded medium-grained quartzite.
Possible desiccation cracks are present on finer-grained interbeds within the outcrop.
A bedding plane and a bed sole that has been overturned (treated here as a bedding
plane) were described. The true bedding plane is an uneven surface that is made up
of patches of non-bioturbated mud interspersed with patches that contain abundant
quartzite-filled burrow entrances. A BPBI of two was assigned to this surface. The
bed sole consists of a muddy layer with a cluster of apparent desiccation cracks on
the right side. A nicely-preserved Rusophycus trace, small Planolites-type traces, and
quartzite-filled burrow entrances occur on the surface. A BPBI of three was assigned
to this bed sole.
Meter E – Wood Canyon Formation, middle member (Fig. 3.20)
This meter section occurs 1-2 meters above the Outcrop. The lowermost eight
centimeters consists of the upper portion of a thicker cross-bedded interval composed
of pinkish fine-grained sandstone. A coarsening-upward sequence occurs between
eight centimeters (very fine grained sandstone) and 74 centimeters (medium-grained
cross-bedded quartzite). Another coarsening-upward sequence begins at 74
centimeters (siltstone) and continues through the top of the meter (planar to cross-
bedded fine-grained quartzite). Bioturbation is quite variable within this meter.
Vertical burrows occur sporadically in the lowermost eight centimeters (ichnofabric
index two). No bioturbation was observed between eight and 20 centimeters.
145
FIGURE 3.20 – Middle member Wood Canyon Formation, Montgomery Mountains,
Meter E. Numbers at left indicate height, in centimeters, of each sample above base
of meter but do not constitute a scale. Samples are arranged in descending
stratigraphic order. The 55- and 74-centimeter samples are cut to approximately
45x70mm (oversized). The remaining samples are cut approximately to standard thin
section size (24x40mm). Left to right: billet scan, thin section, x-radiograph.
146
FIGURE 3.20 – Montgomery Mountains, Meter E
147
FIGURE 3.20 (continued) – Montgomery Mountains, Meter E
148
Scattered vertical burrows occur between 20 and 30 centimeters (ii two). No
bioturbation is present between 30 and 34 centimeters. Burrows are common
between 34 and 60 centimeters (ii three), decrease in frequency between 60 and 65
centimeters (ii two), and are absent between 65 and 76 centimeters. Burrows are
present again between 76 and 78 centimeters (ii two) but are absent from the
remainder of the meter.
Four bedding planes were described from Meter E. These surfaces occur at
65, 74, 78, and 100 centimeters above the base. The 65-centimeter bedding plane
consists of a slightly undulatory, varnished, pinkish-tan muddy surface. Circular
burrow entrances filled with quartzite constitute the only bioturbation on the bedding
plane. The burrow entrances vary in diameter from four to 11 millimeters. A BPBI of
three was assigned to this surface. The 74-centimeter bedding plane consists of a
flakey, muddy surface that undulates gently. Patches of the surface in which the mud
layer is missing reveal few poorly-defined, quartzite-filled burrow entrances. A
BPBI of three was assigned to this surface. The 78-centimeter bedding plane consists
of an interference-rippled, silty fine sandstone surface. Burrow entrances, which vary
between 3 and 10 millimeters in diameter, are the only bioturbation present (BPBI
two). The 100-centimeter bedding plane consists of an interference-rippled, muddy
surface. Bioturbation is limited (BPBI two); few Planolites traces, scratchmarks of
unknown affinity, and vertical burrow entrances are present. Burrow entrances range
in diameter from 0.5 to 1.75 centimeters.
149
Samples in this meter are purplish-gray in color. Sample zero was cut
obliquely to bedding and appears to lack bioturbation. Only one other sample, 90, is
non-bioturbated. Distinct vertical burrows are visible in samples 20, 40, 55 (x-
radiograph), 74 (x-radiograph), and 100. Bioturbation is predominantly diffuse in
samples 30 and 65. Well-defined horizontal burrows are present in sample 80.
Opaque-rich structures, identified as fragments of Volborthella, are present in
samples 80 and 90, although those within sample 90 are visible only in the x-
radiograph. A possible additional fragment is present in sample 100.
Emigrant Pass
Psammichnites Bedding Plane – Wood Canyon Formation, upper member (Fig.
3.21)
Jensen and colleagues (2002b) documented Psammichnites gigas from this
locality. The large trace is found in the middle portion of the upper member, on
laterally-persistent bedding surfaces composed of dark purplish medium-grained
quartzite. No other bioturbation is apparent on these bedding surfaces. Individual
Psammichnites burrows possess fine transverse ridges and exhibit “scribbling”
patterns (Seilacher, 2007), in which the trace crosses itself (Fig. 3.7, 3.8). Aspects of
trace preservation at this outcrop support Seilacher’s (2007, p. 80) interpretation of
Psammichnites as a “shadow trace,” or an infaunal burrow that “pushed through” to
an overlying (or underlying) bedding plane. In places, bedding plane sediment
appears to lap up against the burrows, and some burrow loops appear narrower
150
FIGURE 3.21 – Bedding plane samples collected from Emigrant Pass. The
Psammichnites bedding plane sample was collected from the middle upper member
of the Wood Canyon Formation. The Rusophycus bed sole occurs in the upper
portion of the Zabriskie Quartzite. Both are standard-sized thin section samples
(24x40mm). Left to right: billet scan, thin section, x-radiograph.
151
because they are partially covered by the sediment of the bedding surface (Fig. 3.8).
In addition, burrow crossings, in Seilacher’s (2007, p. 80) words, “look like a rope
passing over another.”
A portion of this bedding plane horizon was selected for analysis. At least
eight Psammichnites burrow segments visibly intersect this portion of the outcrop,
and no other bioturbation is present. A BPBI of three was assigned to the surface.
A Psammichnites burrow is visible in cross-section within the billet, thin
section, and x-radiograph of this sample. The burrow appears as a crescent-shaped
area that is oriented at an angle to bedding and is denser and richer in hematite than
the surrounding material. The upper surface of the burrow, which curves downward
into the sample from the bedding plane horizon, is accented by a thin (approximately
two millimeters thick) layer of concentrated hematite. The lower edge of the burrow
is distinct but is less sharply defined. No internal structure is discernable within the
burrow.
Rusophycus Bed Sole – Zabriskie Quartzite (Fig. 3.21, 3.22)
Abundant Rusophycus (and probably Cruziana) occur as the only
bioturbation on the sole of a thick, partially cross-bedded, pink quartzite bed located
between the two most prominent Zabriskie quartzite units at the Emigrant Pass
locality (Fig. 3.22). The top surface of this bed contains Skolithos burrow entrances
and few Planolites. Only one Rusophycus-bearing horizon was observed within the
Zabriskie Quartzite. This horizon appears to persist for several hundred meters in the
Emigrant Pass area; a single identical horizon was observed in Zabriskie outcrops on
152
FIGURE 3.22 – Rusophycus-bearing bed sole in the Zabriskie Quartzite, Emigrant
Pass. (Top) View of quartzite outcrop that contains bed sole, visible within gap
between beds. (Bottom) Close-up of bed sole, showing Rusophycus-Cruziana
ichnofabric and a well-preserved individual Rusophycus in the foreground.
153
the opposite side of the Old Spanish Trail highway. The Rusophycus-bearing horizon
does not have a counterpart in the underlying bed. In many cases, a gap of a several
millimeters or more separates the base of the Rusophycus-bearing bed from the top
of the underlying bed. Although many Zabriskie Quartzite outcrops have this slightly
blocky appearance, the gap may represent a thin fine-grained layer that has eroded
completely. If the Rusophycus tracemakers burrowed into a now-absent layer, then
the lack of a counterpart exposure is more plausible.
A portion of this horizon, preserved on a large inverted block of float
adjacent to the in situ exposure, was selected for analysis. Arthropod bioturbation
dominates the bed sole; few individual traces can be discerned because of multiple
overlaps. The surface appears to be completely bioturbated (BPBI five). The most
prominent, distinct Rusophycus is partially filled with coarse quartz and chert grains
(Fig. 3.6).
The base of this sample corresponds to the Rusophycus-bearing bed sole. No
bioturbation is visible within the sample, however. Opaques outline several cross
beds. Several fragments of denser skeletal material, possibly of Volborthella, are
present in the sample.
Meter – Zabriskie Quartzite
This section, which is incomplete at 91 centimeters, is exposed within the
most prominent ridge-forming portion of the Zabriskie Quartzite at Emigrant Pass.
Sedimentary structures, if present, are not visible due to varnish on this outcrop. The
lithology of the studied interval appears to be medium- to coarse-grained quartzite.
154
Bioturbation, in the form of Skolithos burrows, is abundant throughout (ichnofabric
index three-four). Burrows originate from multiple horizons within the studied
section; the products of later burrowing episodes extend downward into earlier-
bioturbated material, giving the appearance of ubiquitous bioturbation. The longest
single burrow appears to extend nine centimeters downward from the horizon at
which it originated. Based on measurements made from the vertical exposure,
Skolithos burrow diameters appear to vary between 2.5 and 10 millimeters.
Two bedding planes were examined, at the base and top of this 91-
centimeters-thick section. The basal bedding plane consists of an unevenly-
weathered quartzite surface that has weathered to a dark color pinkish color.
Skolithos burrow entrances, the only bioturbation present, stand out as pale spots
against the surrounding material. These burrows vary in diameter from three to 15
millimeters. A BPBI of three was assigned to this surface. The bedding plane
exposed at the top of the section resembles the surface at the base. Two portions of
this upper bedding plane were chosen for analysis. Both contain bioturbation only in
the form of Skolithos burrows; diameters of these burrows vary between three and
12.5 millimeters in both bedding plane areas. A BPBI of three was assigned to the
first bedding plane section, and a BPBI of two was assigned to the second section.
155
Winters Pass Hills
Bedding Planes – Wood Canyon Formation, middle member
Two bedding planes were analyzed from outcrops of the upper portion of the
middle member. These outcrops consist predominantly of dark-brown-weathering
fine-grained quartzite; burrows are filled with medium-grained quartzite. Mud chips
are present in non-bioturbated strata approximately 10 centimeters below the first
(lower) bedding plane.
The lower bedding plane contains Skolithos and several apparent pairs of
burrow entrances, which resemble Diplocraterion (Fig. 3.7) A few of the pairs
appear to be connected by a thin strip of medium-grained quartzite, which weathers
less prominently than the burrow fill but more prominently than the surrounding
bedding plane surface. This strip of material may be the preserved remnant of
spreiten. No spreiten are visible in the vertical exposure below this surface, however.
Burrow diameters on this surface range from three to 12 millimeters. A BPBI of
three was assigned. The upper bedding plane, approximately two meters up-section
from the first surface, contains abundant and densely-packed burrow entrances
(BPBI four). Color contrast between the bedding surface and burrow fill is poor on
this surface, making the true burrow density difficult to estimate. Diameters of well-
defined burrow entrances range from five to eight millimeters.
156
Northern Salt Spring Hills
Bedding Plane A – Wood Canyon Formation, middle member (Fig. 3.23)
This bedding plane, in the upper portion of the middle member, consists of
heavily-varnished fine- to medium-grained quartzite. Bioturbation on the surface
consists of Skolithos burrow entrances, which are filled with slightly coarser material
than the bedding plane itself. A BPBI of three was assigned.
This sample contains indistinct bioturbation. Contrast is poor due to the high
abundance of quartz within the sample. The upper surface of the sample, which
corresponds to the bedding plane surface, is draped by a thin layer of fine-grained
material.
Bedding Plane B – Wood Canyon Formation, upper member (Fig. 3.23)
This bedding plane consists of a silty to muddy surface that appears to have
been distorted as a result of regional metamorphism. Planolites-type traces and
arthropod scratchmarks are the primary types of bioturbation on the surface. A BPBI
of three was assigned.
Limited bedding-parallel bioturbation is present within the upper portion of
this sample. A 2-3-centimeters-thick layer of fine-grained material is present at the
top of the sample, which corresponds to the bedding plane surface.
Bedding Plane C – Wood Canyon Formation, upper member (Fig. 3.23)
This bedding plane consists of an irregular silty fine sandstone surface that
weathers to a dark reddish gray. Circular burrow entrances with diameters of 1-5
157
FIGURE 3.23 – Bedding plane samples collected from the Wood Canyon Formation
in the Northern Salt Spring Hills. Bedding plane A occurs in the middle member. All
other bedding planes are found in the upper member of the formation. All of the
samples are cut to standard thin section size (24x40mm). Left to right: billet scan,
thin section, x-radiograph.
158
millimeters are abundant, and few very small Planolites-type traces and small
clusters of scratchmarks are also present. A BPBI of four was assigned.
The uppermost five millimeters of this sample contain two layers of fine-
grained material separated by a thin layer of quartz grains. Below, however, the
sample is dominated by a mixture of medium and coarse quartz grains. At least three
likely fragments of Volborthella are present within the lower portion of the sample.
The lowermost of these fragments appears to be a near-perfect lateral cross-section
through a portion of the conical fossil, showing the open central shaft that has been
described from many Volborthella specimens.
Meter – Wood Canyon Formation, upper member (Fig. 3.24)
Few sedimentary structures are visible within this meter-thick section due to
ubiquitous, dark varnish. The meter is dominated by silty fine sandstone; layers of
siltstone or silty mudstone occur at 10 centimeters, between 20 and 25 centimeters, at
40 centimeters, and within the intervals of 69-71 and 92-94 centimeters. A small
bedding plane, at 58 centimeters, contains abundant Planolites-type traces and other
indistinct bioturbation. Analysis was done on the bedding plane that tops this meter.
Most of the bioturbation present is indistinct, with the exception of a few Planolites-
type traces and possible small circular burrow entrances. A BPBI of four was
assigned.
All of the samples in this meter are dark bluish-gray. Opaque skeletal grains,
interpreted as fragments of Volborthella, are particularly abundant within this meter.
Volborthella fragments can be recognized within the thin sections of samples four,
159
FIGURE 3.24 – Upper member Wood Canyon Formation, Northern Salt Spring
Hills, Meter. Numbers at left indicate height, in centimeters, of each sample above
base of meter but do not constitute a scale. Samples are arranged in descending
stratigraphic order. All samples are cut approximately to standard thin section size
(24x40mm). Left to right: billet scan, thin section, x-radiograph.
160
FIGURE 3.24 – Northern Salt Spring Hills, Meter
161
FIGURE 3.24 (continued) – Northern Salt Spring Hills, Meter
162
25, 50, 70, 80, and 92. X-radiographs revealed the presence of additional fragments
within the remaining three samples: 40, 62, and 100. The fragments found within
these samples vary considerably in size, from less than one millimeter to several
millimeters in length. Fragments range in shape from ovate to cylindrical or conical.
Particularly dense concentrations of Volborthella fragments are present in samples
25 and 50; the fragments in sample 25 are iron-stained and appear red in the billet
and thin section. Bioturbation is very limited within this meter. Samples 40, 70, 92,
and 100 contain small amounts of indistinct bioturbation. Bedding is generally
poorly defined within these samples. Laminated bedding is visible within the lower
portion of sample 40, the upper half of sample 50, and the entirety of sample 62.
Bedding Plane D – Wood Canyon Formation, upper member
This bedding plane is exposed several meters up-section from the top of the
studied meter section. The surface consists of silty fine sandstone that weathers light
brown and appears to have been distorted slightly. Bioturbation consists of apparent
Monomorphichnus and few small Planolites-type traces and circular burrow
entrances. A BPBI of three was assigned.
Bedding Plane E – Wood Canyon Formation, upper member (Fig. 3.23)
This bedding plane occurs three to five meters up-section from Bedding
Plane D. The surface consists of mottled dark brown fine sandstone. Most of the
bioturbation present is indistinct, with the exception of very few small Planolites-
type traces and possible Monomorphichnus. A BPBI of four was assigned.
163
This sample is fine-grained and contains no bioturbation. A one-millimeter-
thick layer of very fine-grained material drapes the top of the sample. Opaque grains
define some of the gently-undulating layers within the sample.
Bedding Plane F – Wood Canyon Formation, upper member (Fig. 3.23)
Located a few centimeters above Bedding Plane E, this surface consists of
light brown silty fine sandstone. In addition to a few small indistinct patches of
bioturbation, a large horizontal trace crosses the studied bedding plane area. This
trace is unornamented and Planolites-like at one end and becomes bilobed near the
other. A BPBI of two was assigned to this surface.
Biotite and chlorite are present within this sample. Laminations are well-
defined and are cut, near the center of the specimen, by an indistinct vertical burrow.
Bedding Plane G – Wood Canyon Formation, upper member
This bedding plane occurs in float but appears to have remained close to its
outcrop of origin. The surface is dark gray with iron-stained patches. Planolites-type
and nondescript small traces are present (BPBI three). Outside of the studied area,
Rusophycus and arthropod scratchmarks are present.
Bedding Plane H – Wood Canyon Formation, upper member
This bedding plane consists of a slightly irregular, brown fine sandstone
surface. Planolites-type traces are present; some are indistinctly preserved. A BPBI
of four was assigned to this bedding plane.
164
Bedding Plane Grid Analysis: Photographs of the 37 bedding planes studied
in the field were analyzed using the intersection grid method, which is described in
Chapter II. For each studied bedding plane, an unaltered photograph and a
photograph with the grid analysis superimposed (where applicable) are located in
Appendix II. Grids used to analyze 10x60-centimeter bedding plane areas varied in
size from 22x88 to 50x100 intersections (edges excluded). Grids used to analyze
24x25-centimeter bedding plane areas varied from 31x31 (Psammichnites-bearing
surface) to 80x80 intersections. Despite the asymmetrical dimensions of the 24x25-
centimeter frame, symmetrical grids were used for analysis of 24x25-centimeter
bedding plane areas due to the difficulty of assembling and modifying grids with the
appropriate inequal proportions. Each grid was stretched proportionally to match the
24-centimeter dimension and centered with respect to the 25-centimeter dimension.
Two of the studied bedding planes were not subjected to grid analysis. The
first, located eight centimeters above the base of Meter 2 (middle member Wood
Canyon Formation) at the Montgomery Mountains locality, contained no
bioturbation (BPBI one). The second, an exposure of the Rusophycus-bearing bed
sole within the Zabriskie Quartzite at Emigrant Pass, was judged to be 100 percent
bioturbated (BPBI five). The intersection grid method was used to analyze the
remaining 35 bedding surfaces, which were found to be between three and 69
percent bioturbated, with an average of 27 percent bioturbation. A spreadsheet
containing all grid-based data is located in Appendix II. The results of the
intersection grid analyses, plus the two end-member bedding planes, are shown in
165
Figure 3.25. Each data point represents either the exact percentage area bioturbated
(complete bedding plane surfaces) or the “probable” percentage area bioturbated
(incomplete bedding planes; defined as the total number of bioturbated grid-line
intersections divided by the difference of total possible intersections and non-
bedding-plane intersections; see Chapter II). For each incomplete bedding plane,
positive and negative error bars reflect the maximum and minimum possible
percentage bioturbated area, respectively. Figure 3.26 is a cross-plot of grid-method
and field BPBI estimates for bedding planes from the Death Valley region. Forty-one
percent of the 32 field BPBI estimates agree with the corresponding grid-method
estimates.
In Figure 3.27, the intersection method data are binned to show the frequency
at which bedding planes with similar bioturbation percentages occur within the
dataset. Note that separate bins are used for zero and 100 percent bioturbation and
that a single bin encompasses 70-99 percent bioturbation. For each bin, the lower end
of the percentage range is excluded, and the upper end is included. Fifty-eight
percent of the studied bedding planes (a total of 21) are five to 25 percent
bioturbated. No substantial peaks or clusters are present in the remainder of the
graph.
Results are plotted individually and in binned groups by formation (Zabriskie
Quartzite) and member (Wood Canyon Formation) in Figures 3.28-3.31. These
results are combined in Figure 3.32. Among studied bedding planes from the Wood
Canyon Formation, the average area bioturbated is 14 percent in the lower member,
166
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Percentage bioturbation
FIGURE 3.25 – Intersection grid method estimates of percentage bioturbation for
studied bedding planes in the Lower Cambrian succession of the Death Valley
region, graphed in order of increasing area bioturbated. Data points from two
bedding planes not subjected to grid analysis are also included; these surfaces
contain 0% and 100% bioturbation. Data points are “probable” percentages based on
visible bedding plane area; error bars are bounded by “maximum” and “minimum”
percentages, which include missing bedding plane area in the estimate, treat the area
as either completely bioturbated (maximum) or devoid of bioturbation (minimum).
N=37.
167
0%
10%
20%
30%
40%
50%
60%
70%
12 345
BPBI (field estimate)
% (grid estimate)
0% 0-10% 10-40% 40-60% 60-100%
FIGURE 3.26 – Cross plot of field BPBI estimates and intersection grid method
percentage estimates for all bedding planes that were subjected to grid analysis
(N=35). Each box outlines the range of percentages on the y axis that corresponds to
the BPBI indicated on the x axis. Data points that fall within the boxes represent
bedding planes for which field BPBI estimates and grid percentage estimates are in
agreement.
168
0
1
2
3
4
5
6
0%
1-5%
5-10%
10-15%
15-20%
20-25%
25-30%
30-35%
35-40%
40-45%
45-50%
50-55%
55-60%
60-65%
65-70%
70-99%
100%
Percentage Bioturbation
Number of Bedding Planes
FIGURE 3.27 – The data shown in Figure 3.25 are graphed here in bins of 5%
bioturbation. Because no data fall between 70-99%, this range of percentages is
shown as a single bin. The single 0% and 100% data points are treated as separate
bins.
169
0%
5%
10%
15%
20%
25%
Percentage Bioturbation
0
1
2
0%
1-5%
5-10%
10-15%
15-20%
20-25%
25-30%
30-35%
35-40%
40-45%
45-50%
50-55%
55-60%
60-65%
65-70%
70-99%
100%
Percentage Bioturbation
Number of Bedding Planes
FIGURE 3.28 – Intersection grid method results from the lower member of the
Wood Canyon Formation. (Top) Graph of individual bedding plane data points.
(Bottom) Data grouped into 5% bins. N=4.
170
0%
10%
20%
30%
40%
50%
60%
Percentage Bioturbation
0
1
2
3
0%
1-5%
5-10%
10-15%
15-20%
20-25%
25-30%
30-35%
35-40%
40-45%
45-50%
50-55%
55-60%
60-65%
65-70%
70-99%
100%
Percentage Bioturbation
Number of Bedding Planes
FIGURE 3.29 – Intersection grid method results from the middle member of the
Wood Canyon Formation. (Top) Graph of individual bedding plane data points.
(Bottom) Data grouped into 5% bins. N=13.
171
0%
10%
20%
30%
40%
50%
60%
70%
Percentage Bioturbation
0
1
2
3
4
0%
1-5%
5-10%
10-15%
15-20%
20-25%
25-30%
30-35%
35-40%
40-45%
45-50%
50-55%
55-60%
60-65%
65-70%
70-99%
100%
Percentage Bioturbation
Number of Bedding Planes
FIGURE 3.30 – Intersection grid method results from the upper member of the
Wood Canyon Formation. (Top) Graph of individual bedding plane data points.
(Bottom) Data grouped into 5% bins. N=16.
172
0%
20%
40%
60%
80%
100%
Percentage Bioturbation
0
1
2
0%
1-5%
5-10%
10-15%
15-20%
20-25%
25-30%
30-35%
35-40%
40-45%
45-50%
50-55%
55-60%
60-65%
65-70%
70-99%
100%
Percentage Bioturbation
Number of Bedding Planes
FIGURE 3.31 – Intersection grid method results from the Zabriskie Quartzite. (Top)
Graph of individual bedding plane data points. (Bottom) Data grouped into 5% bins.
N=4.
173
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Percentage Bioturbation
Zabriskie
umWCF
mmWCF
lmWCF
0
1
2
3
4
5
6
0%
1-5%
5-10%
10-15%
15-20%
20-25%
25-30%
30-35%
35-40%
40-45%
45-50%
50-55%
55-60%
60-65%
65-70%
70-99%
100%
Percentage Bioturbation
Number of Bedding Planes
Zabriskie
umWCF
mmWCF
lmWCF
FIGURE 3.32 – Intersection grid data from both formations, graphed together and
color-coded by formation/member. (Top) Individual data points. (Bottom) Data
grouped into 5% bins. N=37.
174
29 percent in the middle member, and 26 percent in the upper member. The range of
bioturbation percentages is 5-25 in the lower member, 0-55 in the middle member,
and 1-70 in the upper member of the Wood Canyon Formation. Of the 13 bedding
planes studied from the middle member of the Wood Canyon Formation, six are 0-25
percent bioturbated and the remaining seven are 30-55 percent bioturbated. In
contrast, eleven (or 69 percent) of the sixteen bedding planes analyzed from the
upper member of the Wood Canyon Formation are 1-25 percent bioturbated. All of
the bedding planes in the lower member of the Wood Canyon Formation and the
Zabriskie Quartzite are less than 25 percent bioturbated, with the exception of the
Rusophycus-bearing bed sole in the Zabriskie Quartzite, which is 100 percent
bioturbated. The average area bioturbated on studied bedding planes from the
Zabriskie Quartzite is 41 percent, and the range of percentages is 15-100.
In Figure 3.33, the intersection method data are plotted according to bedding
plane grain size. The average area bioturbated on bedding planes composed of shale
is 37 percent, and the range is 17-50 percent. On bedding planes composed of
siltstone, bioturbation percentages range from 6-54 percent, with an average of 24
percent. The largest range of values, 0-68 percent, is found in the fine sandstone
category. However, most of these data cluster between 0-20 percent, and the average
is 19 percent. Bioturbation percentages on the studied medium sandstone bedding
planes range from 22-100 percent, with an average of 47 percent.
175
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Percentage Bioturbation
FIGURE 3.33 – Intersection grid data from both formations, grouped by bedding
plane grain size. Three studied bedding planes, all from the middle member of the
Wood Canyon Formation, are composed of shale. The 17 studied siltstone bedding
planes were described from the lower, middle, and upper members of the Wood
Canyon Formation. The 11 studied fine sandstone bedding planes were described
from all members of the Wood Canyon Formation and the Zabriskie Quartzite. The
six studied medium sandstone bedding planes were described from the middle and
upper members of the Wood Canyon Formation and the Zabriskie Quartzite.
mud silt
fine
sand
medium
sand
176
Discussion
Volborthella: As noted in the results section, a number of the specimens
collected from the Wood Canyon Formation contain one or more very small (less
than two millimeters in length) conical, ovate, and circular structures that are
composed predominantly of dense, often opaque grains. Some of the structures are
slightly curved, and others appear to be fragments of larger skeletal elements (Fig.
3.11). X-radiographs show slight density variations along the lengths of some of the
larger, more visible specimens. These fossils are interpreted to be examples of
Volborthella, an enigmatic mineralized fossil that appears to be restricted to the
Lower Cambrian (Signor and Ryan, 1993; Hagadorn and Waggoner, 2002).
According to Signor and Ryan (1993) and Hagadorn and Waggoner (2002),
Volborthella tubes are agglutinated, having been assembled from detrital grains by
the producing animal. Volborthella specimens from the White-Inyo Mountains are
composed predominantly of aligned zircon, magnetite, and pyrite grains (Hagadorn
and Waggoner, 2002). Thus, it appears that the producer of Volborthella
preferentially selected heavy mineral grains for the construction of its skeletal
material (Hagadorn and Waggoner, 2002). This is reflected by the sharp density
contrast between the Volborthella fossils and the surrounding matrix in samples from
the Wood Canyon Formation. Signor and Ryan (1993) proposed the hypothesis that
Volborthella fossils are sclerites that served as multiple pieces of armor for a larger
organism. Yet, as Hagadorn and Waggoner (2002) note, only one specimen has been
found that indicates such an arrangement. Other interpretations hold that
177
Volborthella was a “mat sticker” (Seilacher, 1999) or a mat scratcher (Bailey et al.,
2006). Because Volborthella has only been identified from Lower Cambrian rocks, it
is plausible that this animal was a member of the Early Cambrian matground
community proposed by Seilacher (1999).
Many of the probable Volborthella specimens examined in this study are
concentrated together in randomly-oriented clumps that resemble skeletal lags (Fig.
3.11, 3.34). Compared to specimens from the White-Inyo Mountains, the Wood
Canyon Formation Volborthella specimens are poorly preserved. Most of the latter
specimens are incomplete and have rougher edges than those studied from the
White-Inyo Mountains. Internal structural organization is rarely visible. Unlike some
of the White-Inyo Mountains specimens, none of the Wood Canyon Formation
specimens is known to be associated with microbially-mediated sedimentary
structures. Based on the available evidence, it appears likely that most of the
Volborthella specimens from the Wood Canyon Formation were reworked to a
greater degree than those found in the White-Inyo Mountains samples. Given the
more proximal depositional setting of the Wood Canyon Formation, nearshore
processes likely contributed to the degradation of these fossils and to their later
concentration within skeletal lags.
The cone-shaped structures reported by Diehl (1979, p. 60) from the lower
portion of the upper member Wood Canyon Formation are unlikely to be specimens
of Volborthella as it is defined by Signor and Ryan (1993) and Hagadorn and
Waggoner (2002). Diehl (1979) described the fossils as being three to five
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FIGURE 3.34 – Examples of probable Volborthella fragments in x-radiographs of
samples from the Wood Canyon Formation in the Death Valley region. All samples
are standard thin section size (24x40mm). (A) Cluster of skeletal fragments in upper
right corner of sample. Middle member, Montgomery Mountains, Meter 3, 80cm.
(B) Few scattered fragments in medium-grained matrix. Upper member, Northern
Salt Spring Hills, bedding plane 3. (C) Concentrations of skeletal fragments along
several horizons. Upper member, Northern Salt Spring Hills, Meter 4, 50cm. (D)
Dense concentration of many very small fragments in upper right corner. Upper
member, Northern Salt Spring Hills, Meter 4, 25cm. (E) Scattered fragments along
several horizons within thinly-bedded material. Middle member, Montgomery
Mountains, Meter 3, 90cm.
A B
E
D C
179
centimeters in length; such dimensions dwarf those of all agglutinated structures
observed in thin sections and x-radiographs of Wood Canyon Formation samples. A
shell bed that fits Diehl’s description was observed in strata of the upper member
Wood Canyon Formation in Echo Canyon. Macroscopic conical structures occur in
dense accumulations within a reddish, sandy, carbonate-rich matrix (Fig. 3.35). Most
structures are a centimeter or more in length and white in color, indicating a
composition very different from those observed in the studied samples. The affinity
of these larger structures is unclear. The cones of the Wood Canyon Formation
structures are larger, broader, and straighter than examples of Salterella, another
enigmatic Lower Cambrian conical fossil, which occur in dense shell beds in the
upper portion of the Harkless Formation in the White-Inyo Mountains. Given the
abundance, distribution, and variety of conical fossils in Lower Cambrian units,
particularly of the Great Basin, it is likely that the fossils found in the upper member
of the Wood Canyon Formation served a similar function to that of the others,
whether as sclerites (Signor and Ryan, 1993), mat-sticking anchors (Seilacher,
1999), or protective shells (Bailey et al., 2006).
Bioturbation in the Lower Cambrian of the Death Valley Region: As
discussed in the Geological Setting section, this bedding plane dataset is small due to
the relative thinness of the Death Valley Lower Cambrian succession. Patterns
observed within these data are not, therefore, statistically significant. However, as
will be discussed, the results do reflect observable variations in the quantity and type
of bioturbation present in these strata.
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FIGURE 3.35 – Field photographs of a shell bed in the upper member of the Wood
Canyon Formation in Echo Canyon. (Top) Close-up of a conical fossil in the reddish,
carbonate-rich matrix of the shell bed. (Bottom) Cross-sections of conical fossils and
other fragments visible in outcrop view of shell bed.
181
All of the studied bedding planes within the predominantly fine-grained
lower member of the Wood Canyon Formation are less than 25 percent bioturbated.
Such low bioturbation percentages are consistent with the stratigraphic position of
these bedding surfaces just above the Neoproterozoic-Cambrian boundary.
Planolites-type burrows are the most common identifiable trace fossils on these
bedding planes. Unexpectedly, however, bioturbation is visible within vertical
outcrop exposures and samples collected from the lower member. Most of the
structures appear to be bedding parallel, but at least one sub-vertical trace is visible
in a sample collected a few meters above the third prominent carbonate unit in the
Montgomery Mountains. Thus, bioturbation-induced sediment disruption was
already imparting an effect on shallow marine siliciclastic substrates early in the
Early Cambrian in the Death Valley region.
Most of the bedding planes analyzed from the middle member of the Wood
Canyon Formation are dominated by the circular entrances of vertical burrows.
However, the units in which these bedding planes occur do not always bear the
typical characteristics of Skolithos piperock. Bedding plane surfaces are commonly
very fine-grained; burrow entrances stand out due to the grain-size contrast between
the quartzose burrow fill and the dark, silty surrounding sediment. Small bilobed
trace fossils and Planolites-type traces often co-occur with circular burrow entrances
on the fine-grained bedding planes. Although fine- to medium-grained quartzite is
the dominant lithology of the upper portion of the middle member, very fine-grained
material is also common within these units, as thin sections of samples from the
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Montgomery Mountains demonstrate. Vertical burrows often cross-cut these fine-
grained layers, and bedding-parallel burrows are visibly abundant within the fine-
grained portions of many middle member samples.
Most of the vertical burrow entrances observed on bedding plane surfaces in
the middle member of the Wood Canyon Formation appear to correspond to
individual Skolithos burrows. However, in an outcrop of the middle member in the
Winters Pass Hills, many paired associations between adjacent burrow entrances are
readily apparent (Fig. 3.7). The burrow entrances that comprise several of these
likely Diplocraterion burrows appear to be conjoined, indicating the presence of
spreiten. However, no spreite-bearing vertical burrows or simple “U”-tubes are
visible in the vertical outcrop exposures associated with these bedding planes.
Definitive Diplocraterion burrows have been observed in Skolithos-bearing
quartzites of the Poleta Formation in the White-Inyo Mountains. However, this
outcrop in the Winters Pass Hills is the only convincing instance of Diplocraterion
observed in the Death Valley region during this study. Further discussion of this
phenomenon will follow in Chapter IV.
The intersection grid method data for the middle member of the Wood
Canyon Formation do not cluster strongly within narrow ranges of bioturbation
percentages. However, roughly half of the bedding plane data fall between 0-25
percent bioturbation and the other half between 30-55 percent bioturbation, and only
two bedding planes were found to contain less than 15 percent bioturbation (Fig.
3.29). Bedding planes on which vertical burrow entrances were the dominant type of
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bioturbation are present in both halves of the dataset, although the two bedding
planes with the highest percentages of bioturbation are both dominated by vertical
burrows. A data distribution such as this one is reasonable for a unit that contains
more vertical than bedding-parallel bioturbation. The entrances of vertical burrows
are generally compact and nearly symmetrical. Consequently, vertical burrows must
be concentrated in high densities in order to achieve bedding plane coverage that
approaches 50 percent. Vertical trace fossils such as Skolithos and Diplocraterion are
associated with moderate- to high-energy depositional environments in which
sediment accumulation rates are high (e.g., Droser, 1991). Intervals of section
deposited during times of particularly high sedimentation rates may contain sparse
vertical burrows or lack bioturbation altogether. Thus, it is not improbable to observe
bedding planes that are less than 15 percent bioturbated and others that are 50
percent bioturbated within the same unit.
Grid analysis results from the upper member of the Wood Canyon Formation
cover a broader range of percentages than the results from the middle and lower
members (Fig. 3.30). At the same time, nine of the 16 data points cluster between
one and 15 percent bioturbation. Five of these points fall between one and 10
percent. Many of the bedding planes in this data cluster contain few, if any,
identifiable trace fossils. However, indistinct bioturbation and structures of
questionable biogenicity are common to abundant. At a glance, such a surface may
appear to be well-bioturbated, but under careful scrutiny during grid analysis, many
of the questionable structures may be ruled abiogenic due to insufficient evidence.
184
Four bedding planes from the upper member of the Wood Canyon Formation
contain 50-70 percent bioturbation. This cluster may be significant due to its
isolation from the remainder of the upper member data, although it is much less
robust than the 1-15 percent cluster. Three of these well-bioturbated bedding planes
were described from Echo Canyon. Most of the studied bedding planes at Echo
Canyon contain a combination of common circular burrow entrances and abundant
Planolites-type and indistinct bedding-parallel bioturbation, which occur in high
concentrations that impart considerable surface coverage. Consisting predominantly
of silty fine-grained sandstone, many of the upper member units closely resemble
portions of the lower member of the Wood Canyon Formation, although Diehl
(1979) interpreted the upper member as a slightly deeper-water deposit. Bioturbation
is much more limited and indistinct within thin sections and x-radiographs from
upper member samples compared to those of the middle member, although one or
two distinct sub-vertical burrows are visible in samples from the upper member. This
reduction in overall bioturbation depth from the middle to upper members likely
reflects facies control. Based on the abundance in upper member samples of
echinoderm and trilobite skeletal fragments, as well as other skeletal material of
unknown affinity, metazoans were well established in the depositional environment
represented by fine-grained units in the upper member.
The presence of Psammichnites in the middle portion of the upper member
Wood Canyon Formation at Emigrant Pass is a strong indication of the extent of
metazoan behavioral specialization by this point in the Early Cambrian.
185
Psammichnites is one of the largest Cambrian fossils of any kind. As discussed
earlier in the chapter, this large looping trace fossil was likely produced infaunally a
few centimeters below the sediment-water interface. Based on the Psammichnites
bedding plane sample from Emigrant Pass, the sediment was cohesive enough to
preserve active backfill as a well-defined structure, which is crescent-shaped in cross
section. At the same time, the sediment was not so cohesive that a large tracemaker
could not wedge its way through and extend a snorkel up to the sediment-water
interface. Thus, the depositional environment of the middle portion of the upper
member was suitable for large shallow-infaunal burrowers, although infaunal
bioturbation appears to have been limited in the member overall. The distribution of
Psammichnites within the Death Valley Lower Cambrian succession appears to be
limited, although the horizon at Emigrant Pass that contains these large burrows can
be traced for several meters in either direction. An animal as large as the
Psammichnites tracemaker would likely have had a significant, if localized, impact
on the substrate.
The bedding plane grid data from the Zabriskie Quartzite are too sparse to be
interpreted effectively. However, three of the four bedding plane data points cluster
between 15 and 25 percent bioturbation, which overlaps strongly with data from the
Skolithos-bearing middle member of the Wood Canyon Formation (Fig. 3.31). Due
to the difficulty of collecting and preparing samples of well-cemented quartzites,
only one sample of Skolithos piperock was obtained and sectioned in the course of
this study (Fig. 3.13). Burrows visible within this sample appear to be larger than the
186
Skolithos burrows in all other samples from the Death Valley region, although
compaction likely had a more pronounced effect on the finer-grained, lithologically
heterogeneous material of the Wood Canyon Formation than on the pure quartzites
of the Zabriskie Quartzite.
The presence of extremely abundant arthropod bioturbation, primarily
consisting of Rusophycus and Cruziana, at the base of a thick quartzite bed in the
Zabriskie Quartzite (Fig. 3.9) is surprising given the dominance of vertical
bioturbation throughout most of the formation. Bioturbation is so concentrated
within portions of the arthropod-trace-bearing bed sole that individual traces cannot
be distinguished (Fig. 3.36). Like the Skolithos piperock that is present elsewhere
within the Zabriskie Quartzite, this high concentration of arthropod traces constitutes
a distinctive, ichnotaxonomically-homogeneous ichnofabric that can be traced
laterally for meters to tens of meters. Based on accumulated observations, a similar
co-occurrence of abundant Skolithos and arthropod trace fossils is not present in the
Lower Cambrian succession of the White-Inyo Mountains. This association may
instead be restricted to the more proximal, higher-energy facies represented in the
Death Valley region. For example, medium- to coarse-grained quartzite units rarely
preserve body fossils, but trilobite fragments occur along laminations in the middle
portion of the upper member of the Wood Canyon Formation (Stewart, 1970; Diehl,
1979). Stewart (1970) considered such trilobite-bearing quartzites to be a marker for
the upper member of the Wood Canyon Formation.
187
FIGURE 3.36 – Field photographs of the Rusophycus-bearing bed sole in the
Zabriskie Quartzite, visible on overturned blocks at Emigrant Pass. Few complete
individual arthropod traces are discernable.
188
Despite the high concentration of arthropod traces at Emigrant Pass,
Rusophycus, Cruziana, and other trilobite-associated arthropod trace fossils are
generally uncommon in both the Death Valley and White-Inyo Mountains Lower
Cambrian successions. Aside from isolated scratchmarks and a dubious specimen of
Monomorphichnus, only two examples of arthropod trace fossils were observed in
the Death Valley region. A single Rusophycus trace was observed on the base of an
overturned bed in the middle member of the Wood Canyon Formation in the
Montgomery Mountains, and a second isolated specimen was observed in the upper
member of the Wood Canyon Formation in the Northern Salt Spring Hills. One
explanation for the apparent scarcity of arthropod trace fossils in the Death Valley
and White-Inyo successions may be preservation. Rusophycus is typically preserved
as a positive hyporelief on bed soles, which are easily overlooked unless they have
been overturned in float. Thus, studying the top surfaces of in situ bedding planes
may lead to results that are biased in favor of non-arthropod trace fossils.
When examined together, bedding plane data from the Lower Cambrian of
the Death Valley region have a broad distribution but also contain a robust cluster
between 5-25 percent bioturbation (Fig. 3.32). A peak located well below 50 percent
bioturbation can be expected from a Lower Cambrian bedding plane bioturbation
dataset. Variable quantities of bioturbation may be present on bedding planes due to
facies-related factors such as the degree of microbial influence on sediment
consistency. Whether microbial communities played a prominent role in shaping
substrate conditions in the Death Valley succession is difficult to judge. Isolated
189
occurrences of wrinkle structures and suspected microbially-mediated sedimentary
structures were observed within the Wood Canyon Formation, but co-occurrences of
these structures with bioturbation were not observed. Although nearshore settings
offer plentiful sunlight for photosynthesizing microorganisms, it is unlikely that
extensive microbial mats grew in the depositional environments represented by the
middle member of the Wood Canyon Formation and the Zabriskie Quartzite. Some
species of mat-constructing bacteria are phototactic and can adjust their position in
the sediment relative to light, but the high rates of sediment accumulation that are
characteristic of nearshore settings exceed the top speed at which these microbes can
grow upward through the sediment (Noffke et al., 2001). Mat development is more
likely to have occurred within the lower and upper members of the Wood Canyon
Formation. However, the presence of Psammichnites within the upper member
indicates that microbial mats may not have been ubiquitous.
The results of the grid analyses highlight two significant flaws in the bedding
plane bioturbation index method. First, the cluster of data discussed in the preceding
paragraph is lost in the BPBI “three” bin (10-40 percent bioturbation) when the
intersection grid data are converted to bedding plane bioturbation indices and plotted
(Fig. 3.37). Bins as large as 30 percentage points obscure fine-scale patterns that may
be essential for realistic data interpretation. Second, the cross plot of grid results
versus field BPBI estimates illustrates the significant potential for inconsistency
when using visual scoring to obtain bioturbation data. Forty percent of the field and
grid estimates were in agreement, but many of the field BPBI determinations are
190
0
2
4
6
8
10
12
14
16
18
20
1 2 345
Bedding Plane Bioturbation Index
Number of Bedding Planes
Zabriskie
umWCF
mmWCF
lmWCF
0
5
10
15
20
25
30
35
12 345
Bedding Plane Bioturbation Index
Number of Bedding Planes
Harkless
Poleta
Campito
0% 0-10% 10-40% 40-60% 60-100%
FIGURE 3.37 – Bedding plane bioturbation index data from the Lower Cambrian
successions in Death Valley (top) and the White-Inyo Mountains (bottom), color-
coded by formation and, for the Wood Canyon Formation, member. N=37 for Death
Valley dataset; N=79 for White-Inyo Mountains dataset.
191
gross over- or under-estimates. As mentioned above, some bedding planes within the
upper member of the Wood Canyon Formation contain a mixture of indistinct
bioturbation and structures that cannot be classified with confidence as either
biogenic or abiogenic. During field scoring, these bedding surfaces were assessed to
be well-bioturbated (BPBI four). However, the intersection grid method forces the
analyst to examine every part of the studied bedding plane area carefully. Because
the tendency of the intersection method is toward minor overestimation, the most
precise, conservative results can be obtained by marking only those grid-line
intersections that overlie convincing bioturbation structures. Thus, many of the
questionably-biogenic structures on the studied upper member Wood Canyon
Formation bedding planes were not counted as bioturbation during grid analysis. For
these and other reasons, the intersection grid method represents a significant
improvement over semiquantitative methods, which are inherently imprecise and
subjective.
Comparison with Studied Material from the White-Inyo Mountains: Bedding
plane data from the Lower Cambrian successions in Death Valley and the White-
Inyo Mountains can be compared only on the basis of bedding plane bioturbation
indices because intersection grid method data are not yet available for the White-
Inyo Mountains. An early version of the “cell” grid method was tested on a number
of bedding planes from the White-Inyo Mountains, but, as discussed in Chapter II,
the cell method greatly over-exaggerates the percentage area bioturbated unless very
finely-subdivided grids are used.
192
Plots of the complete BPBI datasets from the Lower Cambrian successions in
the Death Valley region and the White-Inyo Mountains both resemble bell curves
(Fig. 3.37). The White-Inyo Mountains curve is slightly smoother because the
numbers of bedding planes in the BPBI two and BPBI four bins are larger relative to
the peak in the BPBI three bin. (Note that the BPBI three peak in the Death Valley
region data is equal in size to the BPBI four bin in the White-Inyo Mountains plot.)
As discussed above, it is reasonable for a Lower Cambrian bioturbation dataset to be
weighted toward lower bioturbation percentages. The White-Inyo Mountains Lower
Cambrian succession contains a larger proportion of fine-grained to coarse-grained
units than does the Death Valley succession. Therefore, it is reasonable to expect that
a considerable number of White-Inyo Mountains bedding planes will be scored as
BPBI two or three. Based on previous work, the White-Inyo Mountains succession
also appears to surpass the Death Valley succession in the number of bedding planes
that are completely covered by bedding-parallel bioturbation (BPBIs four and five).
This difference likely reflects lengthier periods of slow sediment accumulation in the
deeper-water facies of the White-Inyo Mountains succession.
Based on the relatively limited number of White-Inyo Mountains samples
analyzed using petrography and x-radiography, bioturbation is less common,
extensive, and distinct overall than in the Death Valley region samples. These
regional variations are due, in part, to facies differences. Much of the White-Inyo
Mountains Lower Cambrian succession consists of interbedded fine-grained
sandstones and siltstones that resemble the finest-grained portions of the lower and
193
upper members of the Wood Canyon Formation. However, the entire Wood Canyon
Formation was deposited within a more proximal environment than the
stratigraphically-equivalent Campito and lower Poleta formations in the White-Inyo
Mountains. Not only do the fine-grained units within the Wood Canyon Formation
contain common bioturbation structures, but also many more traces are present in the
coarser-grained, higher-energy deposits in the formation. A particularly striking
distinction between the two successions is the abundance of vertical burrow
entrances in the Death Valley succession, both in coarser- and finer-grained units.
Vertical burrow entrances occur very rarely outside of the coarse-grained quartzite
units of the White-Inyo Mountains succession. The relative abundance of vertical
burrows throughout the Death Valley Lower Cambrian succession likely reflects a
combination of environmental suitability (potentially fewer well-developed
microbial mats, as mentioned above) and nearshore origination of adaptations to
burrowing vertically within firmer substrates. The first occurrence of abundant
Skolithos burrows in the Death Valley Lower Cambrian succession roughly
coincides with the earliest-known occurrence of trilobite body fossils in the region.
Skolithos does not appear in the White-Inyo Mountains succession until the upper
Poleta Formation, although this may be due entirely to facies control rather than to
nearshore origination.
Other types of bioturbation seem to have appeared almost simultaneously in
the Death Valley and White-Inyo Mountains Lower Cambrian successions. Although
relatively uncommon, arthropod trace fossils first occur within a few tens of meters
194
of the Neoproterozoic-Cambrian boundary in both successions. Taphrhelminthopsis,
a large bilobed trace fossil found in the White-Inyo Mountains succession, shares
many characteristics with Psammichnites; these traces both have limited stratigraphic
distributions and occur, respectively, in the middle siliciclastic portion of the Poleta
Formation and the middle upper member of the Wood Canyon Formation, which are
approximately correlative.
Implications: This research was designed to address the following
hypotheses: (1) horizontal bioturbation, in the form of Planolites-type traces, was the
primary mode of substrate engineering in shallow subtidal siliciclastic marine
environments early in the agronomic revolution; (2) the transition from bedding-
parallel to deeply vertical bioturbation was much more abrupt in higher-energy
nearshore siliciclastic environments than in lower-energy facies; and (3) trilobites
were largely restricted to more nearshore settings than those represented in the
Lower Cambrian succession of the White-Inyo Mountains. The results presented
here support the first hypothesis. Planolites-type traces are indeed the most abundant
type of bioturbation in fine-grained subtidal units within the Death Valley Lower
Cambrian succession. The lower portion of the upper member of the Wood Canyon
Formation is likely the most distal facies within the Wood Canyon Formation and
Zabriskie Quartzite. Planolites-type traces and very limited amounts of vertical
bioturbation characterize this unit. In addition, cross-sections of bedding-parallel
trace fossils are very common among the samples collected and reflect the likely
influence of horizontally-burrowing organisms on substrate consistency within finer-
195
grained units. Although Diehl (1979) interpreted the lower member of the Wood
Canyon Formation as an intertidal to mudflat deposit, sedimentary structures and
bioturbation, dominated by Planolites, are very similar to those observed in the
upper member. All other marine-influenced units within the Death Valley Lower
Cambrian succession, excluding the lower Carrara Formation, likely represent
intertidal to transitional facies. Vertical burrows, such as Skolithos, are more
abundant and disruptive than Planolites-type traces and constitute the primary type
of substrate engineering in these nearshore depositional settings.
The second hypothesis is more difficult to test. The first appearance of
vertical burrows occurs considerably earlier in the Death Valley succession than in
the White-Inyo Mountains succession based on trilobite zonation. Skolithos burrows
abruptly appear in the upper middle member of the Wood Canyon Formation,
although their appearance coincides with the establishment of the first Early
Cambrian moderate- to high-energy nearshore depositional setting in the region. The
hypothesis that bioturbation changed abruptly from bedding-parallel to vertical in
high-energy nearshore settings can only be tested effectively under very specific
conditions. The presence of a high-energy nearshore facies that persists across the
Neoproterozoic-Cambrian boundary is required in order to pinpoint the onset of
bioturbation and track any changes in its expression through Lower Cambrian units.
Because no such facies is present in either the Death Valley or White-Inyo
Mountains succession, this hypothesis will require further testing elsewhere.
196
Based on the presence of arthropod trace fossils in the Lower Cambrian
successions of the Death Valley region and the White-Inyo Mountains, trilobites
were not completely restricted to nearshore settings in the Early Cambrian. However,
as discussed above, the extremely high concentration of arthropod traces along a
single persistent horizon in the Zabriskie Quartzite and the well-documented
occurrence of trilobite skeletal fragments in Skolithos-bearing upper Wood Canyon
Formation quartzites suggest that trilobites (and/or other arthropods) were
particularly abundant in moderate- to high-energy nearshore depositional settings
during the Early Cambrian. During periods of slower sediment accumulation,
bioturbation by arthropods may have had a more pronounced influence on substrate
consistency than the activities of vertical burrowers.
197
CHAPTER IV
Bioturbation in the Lower Cambrian Succession of Scania, Southern Sweden
Introduction
The Lower Cambrian succession in Scania, southern Sweden, is well-suited
for comparison with the Death Valley region Lower Cambrian succession because
both were deposited in siliciclastic-dominated shallow marine to back-barrier
settings. The Hardeberga Sandstone (Lower Cambrian of Sweden) and Zabriskie
Quartzite (Lower Cambrian of CA-NV) are quartz arenites that contain abundant and
intensive bioturbation by deep-burrowing organisms. Arthropod trace fossils have
been identified from both formations (Bergström, 1970; Prave, 1984), and bedding-
parallel bioturbation is an important component of the associated units in both
successions. In this chapter, field and laboratory data from Lower Cambrian strata in
southern Sweden are presented, followed by a discussion of the implications of these
data, particularly for the Death Valley Lower Cambrian succession.
Early Cambrian Paleogeography of Scania
Scania, the southernmost province in Sweden, is positioned on the edge of
the Fennoscandian Shield (Bergström and Kornfält, 1998), which underlies Norway,
Sweden, Finland, and western Russia. In the Late Neoproterozoic and Early
Cambrian, the Fennoscandian Shield formed the core of the microcontinent Baltica,
which was located at mid latitudes in the southern hemisphere (McKerrow et al.,
198
1992) (Fig. 4.1). In contrast, the continent of Laurentia, which included present-day
North America, was near the equator in the Early Cambrian (McKerrow et al., 1992)
(Fig. 4.1). Distinct differences between the Cambrian trilobite faunas of Baltica and
Laurentia reflect both latitude (Conway Morris and Rushton, 1988) and separation
by the Iapetus Ocean (Bergström and Gee, 1985; McKerrow et al., 1992). However,
the small shelly fossils Platysolenites, Mobergella, and Volborthella are found on
both continents, indicating that the separation was not extreme (Brasier, 1989;
McKerrow et al., 1992).
In the Late Neoproterozoic, much of the Fennoscandian Shield was eroded
extensively, leaving a broad, relatively flat, peneplained surface (Martinsson, 1974,
and references therein). A stable passive margin setting developed along the edge of
the Fennoscandian Shield by the Early Cambrian (Bergström and Gee, 1985).
Evidence for at least three regional transgressive-regressive cycles is found in the
Lower Cambrian succession of Sweden, which is condensed and contains a number
of hiatuses (Martinsson, 1974; Bergström and Gee, 1985).
Geological Setting
Overview: The Lower Cambrian succession in Scania is estimated to be at
least 120 meters thick based on a drill core from the Hardeberga area (Martinsson,
1974), although it may be closer to 200 meters in total thickness (Bergström and
Gee, 1985). The Hardeberga Sandstone is the basal formation in the succession and,
at greater than 100 meters in thickness, dominates the stratigraphy (Ahlberg and
199
FIGURE 4.1 – Early Cambrian paleogeography. Red dot marks approximate location
of southern Sweden. Modified from McKerrow et al. (1992) and Scotese (2001).
200
Bergström, 1998) (Fig. 4.2). Composed primarily of quartz sandstone, the
Hardeberga Sandstone rests unconformably on the extensively weathered and eroded
Proterozoic crystalline basement of the Baltic Shield and represents the initiation of
northward-directed regional transgression (Martinsson, 1974; Bergström and Gee,
1985). The Hardeberga Sandstone contains abundant ichnofossils, including
Diplocraterion, Skolithos, Syringomorpha, Psammichnites, Planolites, and
Didymaulichnus (Hamberg, 1991). Rusophycus parallelum has been found in the
upper portion of the formation in the Hardeberga Quarry in southwestern Scania
(Bergström, 1970). Body fossils are comparatively rare in the Hardeberga Sandstone
and are limited to hyoliths, a possible trilobite impression (Ahlberg et al., 1986), and
acritarchs (Ahlberg and Bergström, 1998). The acritarch assemblage found in the
formation reflects the lower Lower Cambrian Skiagia ornata – Fimbriaglomerella
membranacea Zone of Poland, which has been correlated with the Schmidtiellus
mickwitzi trilobite Zone of the Baltoscandian Platform (e.g., Moczydlowska, 1998).
The Hardeberga Sandstone will be discussed in greater detail below.
The Norretorp Formation transitionally or disconformably overlies the
Hardeberga Sandstone and is 4-15 meters thick (Bergström, 1970; Lindström and
Staude, 1971) (Fig. 4.2). This formation is characterized by hummocky cross-
stratified greenish phosphorite- and glauconite-rich siltstones and sandstones
(Ahlberg and Bergström, 1998). Olenellid trilobites of the Schmidtiellus mickwitzi
Zone have been found in the Norretorp Formation as well as obolellid brachiopods,
hyoliths, Volborthella tenuis, and acritarchs of the Skiagia ornata –
201
FIGURE 4.2 – Generalized stratigraphy of the Lower Cambrian succession in
Scania, southern Sweden. Trilobite zones are shown at left. The Brantevik Member
of the Hardeberga Sandstone is abbreviated as “Br.” The following additional
abbreviations are used: Norretorp Formation = “Norre.,” Rispebjerg Sandstone =
“Risp.,” and Gislov Formation = “Gis.” After Bergström et al. (1982) and Hamberg
(1991).
202
Fimbriaglomerella membranacea Zone (e.g, Ahlberg and Bergström, 1998;
Moczydlowska, 1998). Arthropod and other trace fossils have also been found in the
Norretorp Formation (Ahlberg and Bergström, 1998).
The Rispebjerg Sandstone disconformably overlies the Norretorp Formation
and is a thin (1-3 meters thick; Bergström and Gee, 1985) series of phosphatic and
calcareous coarse-grained sandstones (Lindström and Staude, 1971; Ahlberg and
Bergström, 1998) (Fig. 4.2). The formation contains only unidentifiable fossil
fragments and, near the top, large bedding-parallel trace fossils (Bergström and Gee,
1985; Ahlberg and Bergström, 1998).
The Gislöv Formation, which completes the Lower Cambrian succession in
Scania, disconformably overlies the Rispebjerg Sandstone (Bergström and Ahlberg,
1981) and is a thin (0-2 meters; Bergström and Gee, 1985) transitional unit between
the predominantly quartzose Lower Cambrian strata and younger clay- and
carbonate-rich deposits (Ahlberg and Bergström, 1998) (Fig. 4.2). The formation
consists of a sequence of silty limestone, siltstone, and phosphatic shale (Bergström
and Gee, 1985). The upper boundary of the Gislöv Formation, a regional
unconformity, constitutes the Lower-Middle Cambrian boundary in Scania
(Bergström and Ahlberg, 1981). A variety of body fossils have been found in the
Gislöv Formation, including trilobites, brachiopods, hyoliths, helcionellids, a
lapworthellid (Bengtson, 1977; 1980), a hyolithelminthid, possible conodonts,
echinoderm fragments (Marino, 1980), and rare bradoriids (Bergström and Ahlberg,
1981). Trilobites found in the Gislöv Formation indicate the upper Lower Cambrian
203
Holmia-kjerulfi and Proampyx linnarssoni Zones (Moczydlowska, 1998). Only two
acritarch species are found in the Gislöv Formation, but one of the species, Skiagia
ciliosa, is found in both the Heliosphaeridium dissimilare – Skiagia ciliosa and
Volkovia dentifera – Liepaina plana acritarch Zones, which are correlated to the two
trilobite zones indicated for the Gislöv Formation (Moczydlowska, 1998).
Hardeberga Sandstone: Hamberg (1991, p. 256) interpreted the depositional
setting of the Hardeberga Sandstone as “a tidally-influenced, backbarrier, sand-
dominated system with coast-parallel channels and shoals situated just behind the
barrier island.” The formation is subdivided into four members: (from base to top)
Lunkaberg, Vik, Brantevik, and Tobisvik (Bergström and Ahlberg, 1981; Hamberg,
1991) (Fig. 4.2). Three marine shallowing-upward sequences, separated by erosional
surfaces, are represented within the Hardeberga Sandstone, as well as a 15-25 meters
thick nonmarine sandstone unit at the base of the formation (Hamberg, 1991). The
Lunkaberg Member is comprised of this basal nonmarine unit, followed by the first,
condensed, marine sequence and the lower portion of the second sequence. The Vik
Member represents the conclusion of the second marine sequence and is bounded at
the top by an erosional surface. The Brantevik and Tobisvik members comprise the
third marine sequence.
The basal nonmarine unit is made up of a conglomeratic arkosic sandstone
succeeded by a coarse-grained cross-bedded sandstone in which sets of cross beds
are separated by layers of pebble-sized grains (Hamberg, 1991). Hamberg (1991)
204
interpreted this unit as a braid delta deposit. The unit is capped by an erosional
surface.
Hamberg (1991) identified five facies associations within each of the three
marine sequences in the Hardeberga Sandstone, which are best developed in the
Brantevik and Tobisvik members: inner shelf to shoreface, tidal inlet, upper
shoreface, barrier tidal creek, and backbarrier tidal channel. The inner shelf to
shoreface association directly overlies each of the erosional sequence boundaries. In
the lower portion of this association, glauconitic mudstone is interbedded with well-
defined 2-10 centimeters thick sand layers (Hamberg, 1991). Above, fine-grained
hummocky cross-stratified beds alternate with beds containing the bedding-parallel
trace fossils Planolites, Palaeophycus, and Didymaulichnus (Hamberg, 1991).
Coarse-grained, rippled interbeds become more common up-section. At the top of
the facies association are 3-4-meters-thick coarse-grained sandstones that are trough
cross-bedded, rippled, and covered by fine-grained hummocky cross-stratified
sandstones (Hamberg, 1991). This coarsening-upward facies association is
interpreted to reflect the progradation of the shoreface over the inner shelf (Hamberg,
1991).
The tidal inlet facies association is separated from the inner shelf to shoreface
association by an erosional surface. A medium- to coarse-grained pebbly sandstone
unit, 7-8 meters thick and made up of beds that are inclined and cross-bedded,
overlies the erosional surface (Hamberg, 1991). This unit is interpreted to represent
deposition in a laterally-migrating tidal inlet (Hamberg, 1991).
205
The upper shoreface facies association is a 2.5-meters-thick medium- to
coarse-grained cross-bedded sandstone unit interpreted to have been produced by the
migration of small underwater dunes in the troughs of sand bars on the upper
shoreface (Hamberg, 1991). Commonly cutting into the upper shoreface deposits is
the barrier tidal creek facies association, which is characterized by a 1.5 to 2-meters-
thick sandstone body that is inclined and internally cross-bedded (Hamberg, 1991).
This sand body fines both upward and laterally, from coarse-grained to fine-grained
graded beds in which an increasing number of discontinuity surfaces are present
(Hamberg, 1991). The finer-grained beds are often topped by thin, well-bioturbated
mud layers. Hamberg (1991) interpreted this facies association to represent the
lateral development of a marine point bar in a high-sinuosity, storm-influenced, tidal
creek channel on the remains of a barrier island.
The backbarrier tidal channel facies association consists of two units: a
lower, 2 to 6-meters-thick, cross-bedded sandstone unit representing channel
development; and an upper, 1 to 4-meters-thick, unit of alternating cross-bedded and
bioturbated sandstone beds representing channel abandonment (Hamberg, 1991).
These two units occur as repeated couplets within deposits that represent this facies
association, such as the Vik Member (Hamberg, 1991). The channel development
units consist of cross-bedded medium-grained quartzose sandstones that contain
sporadic Diplocraterion and occasional Skolithos (Hamberg, 1991). Burrows become
more common as each channel development unit grades upward into a channel
abandonment unit. Each channel abandonment unit consists of alternating cross-
206
bedded and bioturbated sandstone layers; the bioturbated layers occur as two distinct
types (Hamberg, 1991). The first type of bioturbated bed consists of 20 to 30-
centimeters-thick, thoroughly-bioturbated layers that in some instances overlap one
another to create even thicker bioturbated intervals (Hamberg, 1991). Diplocraterion
and Skolithos dominate the bioturbated layers, and individual burrows extend to
depths as great as 25-30 centimeters (Hamberg, 1991). Individual trace fossils can
only be distinguished near the base of each bioturbated layer, where the traces come
in contact with the underlying cross-bedded sandstone bed (Hamberg, 1991).
Bedding-parallel trace fossils, such as Didymaulichnus and rarely Planolites, occur
on the tops of the bioturbated layers, along with small ripples and, in some instances,
desiccation cracks (Hamberg, 1991). Hamberg (1991) interpreted the first type of
bioturbated bed to represent colonization of the stabilized channel during a period of
low sediment accumulation, such as the summer. The underlying cross-bedded layers
may represent high-energy winter storm and tidal deposition, which may have
prevented burrowing organisms from becoming established (Hamberg, 1991). The
second type of bioturbated bed is characterized by 30 to 35-centimeters-thick tidally-
bedded sandstone layers that contain dispersed vertical burrows (Hamberg, 1991).
Multiple tidal couplets, each consisting of two mud-draped sandstone layers of
unequal size, occur within each bed. Hamberg (1991) interpreted this second type of
bioturbated bed to represent consistent summer tidal deposition in an abandoned
channel.
207
Methods
Introduction: The methods employed during this portion of the study are
similar to those used in the Death Valley area. In the course of the fieldwork in
Scania, only one of the two meter-thick sections of outcrop was sampled extensively.
Samples from Scania were not thin-sectioned due to their relative compositional
uniformity. In addition, only bedding plane bioturbation indices were used to
estimate the amount of bioturbation present on each bedding surface.
Field Methods and Localities: Fieldwork was conducted at two coastal
localities in eastern Scania (Fig. 4.3). See Appendix I for additional locality
information. The first locality, adjacent to the fishing village of Vik, contains
laterally-extensive meters-thick coastal exposures of the Vik Member of the
Hardeberga Sandstone (Fig. 4.4). At the Vik locality, a single one-meter-thick
vertical section of outcrop, with associated bedding planes, was measured, described,
photographed, and sampled according to the procedures discussed in Chapter III.
Because bedding plane and vertical exposures can easily be traced laterally for
meters along the shoreline at Vik, this measured one-meter-thick section includes
bedding planes that occur up to five meters away from the sampled exposures.
The second locality is located beside the fishing village of Brantevik (Fig.
4.3). The Tobisvik Member of the Hardeberga Sandstone is well-exposed for 600
meters along the shoreline south of the harbor (Fig. 4.5). A fault cuts the Tobisvik
Member section at its southern end, juxtaposing the younger Norretorp Formation
and the Tobisvik Member sandstones. The Rispebjerg Sandstone and the Gislöv
208
FIGURE 4.3 – Map of field localities in Scania, southern Sweden. Stippling shows
approximate extent of Lower Cambrian exposures in southern Scania. Although no
field data were collected from the Hardeberga Quarry, it is shown for reference as
the type locality of the Hardeberga Sandstone. Inset map shows location of Scania
within Scandinavia. Modified from Hamberg (1991).
209
FIGURE 4.4. – Field photograph of the Vik locality showing extensive coastal
outcrops of the Vik Member of the Hardeberga Sandstone, including location of
studied meter-thick section in foreground.
210
FIGURE 4.5 – Field photograph of the Brantevik locality, showing extensive, nearly
flat-lying outcrops of the Tobisvik Member of the Hardeberga Sandstone.
211
Formation are exposed, as part of the faulted-in sequence, farther south along the
shore. One meter-thick section was measured from the Tobisvik Member of the
Hardeberga Sandstone. The bedding plane associated with the meter section was
sampled, but no other samples were collected. One bedding plane was photographed
and described from the Norretorp Formation and another from the Rispebjerg
Sandstone.
Laboratory Methods: A one-centimeter-thick slab was cut, perpendicularly to
bedding, from each of the samples collected from Scania. These slabs were scanned
on one cut surface using an optical scanner and x-rayed according to the procedures
described in Chapter III. Digital images of the x-radiographs were obtained by
scanning the developed x-radiographic film. Features observed in x-ray, particularly
sedimentary structures and evidence of bioturbation, were recorded and compared
with those documented from the hand samples.
Results
Trace Fossils: The bedding-perpendicular, “U”-shaped, spreite-bearing trace
fossil Diplocraterion has typically been interpreted as the dwelling burrow of a
suspension-feeding organism (e.g., Seilacher, 1967; Fürsich, 1974; Cornish, 1986;
Seilacher, 2007) (Fig. 4.6). The spreiten reflect vertical adjustment of the burrow,
usually in response to sediment deposition or erosion at the sediment-water interface
(Goldring, 1964; Seilacher, 2007). Although Diplocraterion is present in rocks of
Lower Cambrian through Tertiary age (Häntzschel, 1966; Crimes, 1977), it is
212
FIGURE 4.6 – Field photographs of trace fossils found in the Hardeberga Sandstone.
(Top) Diplocraterion in the Vik Member at Vik. (Bottom) Didymaulichnus in the
Tobisvik Member of the Hardeberga Sandstone, exposed in a tide pool at Brantevik.
213
particularly common in Cambrian rocks interpreted as intertidal deposits (e.g.,
Goodwin and Anderson, 1974; Crimes et al., 1977; Hereford, 1977; Cornish, 1986;
Hamberg, 1991). Fürsich (1974) discussed the behavioral and preservational
implications of Diplocraterion morphology, and Cornish (1986) explored the
paleoecological aspects of the trace.
The smoothly-bilobed, looping trace fossil Didymaulichnus (Fig. 4.6) was
first described by Young (1972) from the uppermost Precambrian of the Miette
Group in eastern British Columbia and western Alberta. An intrastratal trace fossil
similar to Psammichnites and Taphrhelminthopsis (S. Jensen, pers. comm.),
Didymaulichnus has been reported from rocks of upper Precambrian to Cretaceous
age (Young, 1972; Pickerill et al., 1984; Vossler et al., 1989).
Vik: The one-meter-thick vertical section measured at Vik is typical of the
backbarrier tidal channel facies association of Hamberg (1991). Large-scale cross-
bedding, including herringbone cross stratification, is present within the medium-
grained sandstones of the meter and is punctuated along at least three distinct
horizons by abundant Diplocraterion (Fig. 4.7). Although bedding is never
completely obliterated by bioturbation, this meter is interpreted to represent the well-
bioturbated abandoned channel fill facies described by Hamberg (1991).
Bioturbation, in the form of Diplocraterion, is most abundant in the
lowermost 10 centimeters of the meter, from which an ichnofabric index of four was
recorded. Extensive bioturbation is also present within the 60-70 centimeters of
outcrop located below the base of the measured meter (Fig. 4.8). Between 10 and 50
214
FIGURE 4.7 – Field photograph of the studied meter-thick section in the Vik
Member of the Hardeberga Sandstone at Vik. Top of outcrop corresponds to top of
meter (100cm). Base of meter is located approximately 10cm below end of hammer
handle. Note herringbone cross-stratification within non-bioturbated interval below
hammer head.
215
FIGURE 4.8 – Field photograph of intensely-bioturbated beds within the 60-70
centimeters of outcrop below base of studied meter at Vik. No individual
Diplocraterion burrows can be discerned in the strata above the pen, although
individual burrows are clearly visible within the bed below.
216
centimeters above the base of the meter, large-scale cross-bedding is present, taking
the form of herringbone cross-stratification between 10 and 40 centimeters (Fig. 4.9).
This interval of the meter is devoid of bioturbation (ichnofabric index one).
Moderately abundant Diplocraterion extend downward from two gently undulatory
horizons at 66 and 73 centimeters. These two horizons converge near the righthand
(eastern) edge of the outcrop (Fig. 4.7). The lower of these two horizons can be
traced laterally toward the left, where nicely-defined individual Diplocraterion
burrows occur one meter away from the measured meter-thick section (Fig. 4.6).
Because the vertical burrows disrupt the cross-bedding to a limited degree, the
ichnofabric index of this bioturbated interval was recorded as three. A fourth
bioturbated horizon occurs at the top of the meter, where it is exposed as a
moderately-eroded bedding plane. The bioturbated interval below this horizon (80-
100 centimeters above the base) also reflects ichnofabric index three.
Three bedding planes were described from this one-meter-thick vertical
section of outcrop. They occur at 58, 73, and 100 centimeters above the base of the
meter section. All three surfaces likely represent actual, if slightly eroded, bed tops
because Diplocraterion burrows originate at these horizons. (The bedding plane at
58 centimeters represents the continuation, downward and to the left, of the
burrowed horizon measured at 66 centimeters.)
The bedding plane at 58 centimeters contains several well-defined, paired
Diplocraterion burrow entrances and few possible horizontal burrows (Fig. 4.10).
Fifteen percent of the surface was estimated to be bioturbated, and a BPBI of three
217
FIGURE 4.9 – Close-up view of herringbone cross-stratification within non-
bioturbated interval of studied meter at Vik. Bottom of photograph approximately
corresponds to base of meter.
218
A)
B)
FIGURE 4.10 – Field photographs of studied bedding planes and trace fossils within
measured meter at Vik. Inner dimensions of frame are 24x25cm. (A) 58-centimeter
bedding plane. (B) 73-centimeter bedding plane. (C) Large horizontal trace and
smaller bilobed traces on 58-centimeter bedding plane. (D) 100-centimeter bedding
plane.
219
C)
D)
FIGURE 4.10 (continued) – Field photographs of Vik meter bedding planes
220
was recorded. The bedding plane at 73 centimeters (Fig. 4.10) is poorly weathered,
although several Diplocraterion burrow entrances, discontinuous segments of a large
(1.5-2 centimeters in diameter) bedding-parallel trace (Fig. 4.10), and a few possible
bilobed traces are visible. The large bedding-parallel trace may be Psammichnites,
although none of its characteristic ornamentation, such as lateral striations, is
preserved. The possible bilobed traces may be Didymaulichnus or very poorly
weathered Diplocraterion. The length of an average observed bilobed trace is
approximately the same as the distance between the paired burrow entrances of
Diplocraterion. Approximately 20 percent of this surface was estimated to be
bioturbated, and a BPBI of three was assigned. Although the bedding plane at 100
centimeters (Fig. 4.10) extends for several meters along the top of the outcrop and
varies in its quality of preservation, the amount of bioturbation present is relatively
consistent across the surface, ranging from approximately 30-40 percent, or BPBI
three. At one location, bioturbation was estimated to be slightly more abundant,
covering greater than 40 percent of the surface, or BPBI four. Diplocraterion was the
only trace fossil identified across the bedding surface.
Several tens of meters northward of the measured meter-thick section, at the
inland edge of the outcrops, are exposures of the well-bioturbated endmember of the
abandoned channel fill facies (Fig. 4.11). Cross-bedding in the medium- to coarse-
grained sandstones is completely obliterated by bioturbation, mostly in the form of
Diplocraterion. A weathered horizontal surface that is not a true bed top contains
possible bedding-parallel burrows in addition to Diplocraterion burrow entrances
221
FIGURE 4.11 – Field photographs of an intensely-bioturbated outcrop in the Vik
Member of the Hardeberga Sandstone at Vik. (Top) Intensely-bioturbated layer is
topped by an non-bioturbated bed. (Bottom) Close-up of bioturbated layer and sharp
boundary with non-bioturbated material beneath.
222
(Fig. 4.11). If these structures are indeed horizontal burrows, then they were
produced several centimeters below the surface of the bed based on the stratigraphic
position of this weathered surface.
Brantevik: In addition to a measured one-meter-thick section in the Tobisvik
Member of the Hardeberga Sandstone, bedding surfaces were described from the
Norretorp Formation and the Rispebjerg Sandstone at Brantevik. Although the large
horizontal trace fossil Psammichnites gigas has been reported from the lower portion
of the Tobisvik Member (Hamberg, 1991), no in situ bedding surfaces containing the
trace were found, and only one poorly-weathered block was observed in float near
the water line. The lower portion of the member is exposed close to Brantevik
harbor, where a large block containing nicely-preserved examples of Psammichnites
gigas is on display (Fig. 4.12). Development in the harbor area may have led to the
coverage or removal of most of the Psammichnites-bearing exposures.
The one-meter-thick section measured from the Tobisvik Member of the
Hardeberga Sandstone is interpreted to be a portion of the inner shelf to shoreface
facies association of Hamberg (1991) (Fig. 4.13). A fine-grained, well-bioturbated
bedding surface occurs in the lower half of the meter, and the overlying beds become
progressively coarser. At the top of the meter, medium- to coarse-grained hummocky
cross-stratified sandstone is succeeded by a large-scale oscillation-rippled surface
(Fig. 4.13). Hamberg (1991) interpreted such features to reflect deposition in a
storm-influenced shoreface setting. A raised network of slightly sinuous cracks,
concentrated in ripple troughs on a fine-grained bedding plane, was observed within
223
FIGURE 4.12 – Field photograph of a block from the Tobiskvik member of the
Hardeberga Sandstone that contains abundant Psammichnites gigas. This block is on
display in the harbor in the fishing village of Brantevik.
224
FIGURE 4.13 – Studied meter-thick section of the Tobisvik Member of the
Hardeberga Sandstone at Brantevik. (Top) Photograph of meter. (Bottom) Large-
scale oscillation-rippled surface, which corresponds to the top of the measured meter.
225
the meter, approximately 30 centimeters below the bioturbated bedding surface (Fig.
4.14). Hamberg (1991) did not mention cracks of any kind in his description of the
Tobisvik Member, although he documented several occurrences of desiccation
cracks within the Vik Member. Given the lack of additional evidence for subaerial
exposure, these structures are interpreted to be syneresis cracks that formed either
intrastratally, as a result of compaction, or at the sediment-water interface due to a
brief salinity change. Syneresis cracks are typically sinuous and, when formed
intrastratally, may point upward (Plummer and Gostin, 1981, and references therein).
Crack formation in the troughs of ripples is also considered typical of syneresis
processes (Plummer and Gostin, 1981).
The lowermost 10 centimeters of this one-meter-thick section is characterized
by fine- to medium-grained, planar-bedded sandstone that appears featureless in
outcrop (Fig. 4.13). The syneresis-crack-bearing surface occurs at 10 centimeters, in
a millimeters-thick fine-grained layer that separates the lower sandstone bed from
several similar beds above, which extend from 10 to 30 centimeters above the base
of the meter. An interval of gently undulating, thinly-bedded, finer-grained sandstone
is present between 30 and approximately 47 centimeters. The bioturbated bedding
plane is found within this interval, at 40 centimeters. Between 47 and 76 centimeters
is an interval of medium- to coarse-grained, hummocky cross-stratified sandstone.
The remainder of the meter consists of a massive coarse-grained sandstone bed
topped with large-scale oscillation ripples.
226
FIGURE 4.14. – Bedding planes within the measured meter at Brantevik. Inner
dimensions of frame are 24x25cm. (Top) Syneresis cracks on bedding plane at 10
centimeters. (Bottom) 40-centimeter bedding plane containing Didymaulichnus.
227
The bedding plane at 40 centimeters is a darkly-weathered fine sandstone that
is moderately well-bioturbated (40-50%, BPBI four) by small (3-4 millimeters in
diameter), looping, bilobed trace fossils identified as Didymaulichnus (Fig. 4.14).
Another exposure 20 to 30 meters south along the shoreline, which likely represents
the same stratigraphic horizon, is slightly undulatory but contains a similar
concentration of bilobed burrows.
South of the fault at the Brantevik locality, a well-weathered bedding surface
of Norretorp Formation is exposed near the base of the section. Within the greenish-
gray siltstone, apparent Planolites-like trace fossils of varying size are preserved and
cover an estimated 15 percent of the surface (BPBI 3) (Fig. 4.15). These traces have
no relief relative to the bedding plane but are visible because they contain fill that is
darker than the surrounding matrix.
Further south along the coastal outcrops, the thin Rispebjerg Sandstone is
exposed. The top of the formation, a coarse-grained sandstone surface, contains
abundant, indistinctly-preserved, bedding-parallel trace fossils, 1-1.5 centimeters in
average diameter, that overlap one another extensively (Fig. 4.16). These trace
fossils appear to cover at least 50 percent of the surface (BPBI 4), although their
poor preservation may obscure the true burrow density.
Sample Analysis: Five samples were collected from the one-meter-thick
section that was measured within the Vik Member of the Hardeberga Sandstone at
Vik (Fig. 4.17). The top surfaces of two of these samples correspond to the bedding
plane surfaces at 58 and 73 centimeters. The remaining three samples were collected
228
FIGURE 4.15 – Field photograph of a bedding plane surface in the Norretorp
Formation at Brantevik that contains horizontal trace fossils. Inner dimensions of
frame are 24x25cm.
229
FIGURE 4.16 – Two views of the bedding plane surface at the top of the Rispebjerg
Sandstone at Brantevik showing abundant horizontal trace fossils. Inner dimensions
of frame are 24x25cm.
230
A)
B)
FIGURE 4.17 – Samples collected from the measured meter at Vik. (A) 50-
centimeter sample. Largest dimension of sample is 5cm. (B) 58-centimeter sample,
bedding plane at top. Largest dimension of sample is 5.5cm. (C) 73-centimeters,
bedding plane at top. Largest dimension of sample is 5.5cm. (D) 90-centimeter
sample. Largest dimension of sample is 5cm. (E) 97-centimeter sample. Largest
dimension of sample is 6.3cm. (F) Uncut side of 97-centimeter sample, showing
portions of Diplocraterion burrows outlined by dark material.
231
C)
D)
E)
FIGURE 4.17 (continued) – Samples from the measured meter at Vik.
232
F)
FIGURE 4.17 (continued) – Samples from the measured meter at Vik.
233
at 50, 90, and 97 centimeters above the base of the meter. All of the samples are
composed of medium- to coarse-grained quartzose sandstone.
The cut surfaces of the Vik hand samples revealed no visible bioturbation
when scanned, with the exception of the 58- and 97-centimeter samples. The upper
two centimeters of the 58-centimeter sample appear slightly mottled, and the contact
between the upper mottled material and lower cross-bedded sandstone is irregular
(Fig. 4.17). Indistinct bioturbation at such a fine scale is not apparent in vertical
outcrop exposures. The cut surface of the 97-centimeter sample, which is in two
pieces, contains an indistinct, darker area near the center. This region is interpreted
to be an oblique cross section through a Diplocraterion burrow based on the
presence of distinct burrow segments on the opposite, uncut side of the sample. On
the uncut surface, oblique sections through Diplocraterion burrows are highlighted
against the pale sandstone by dark staining (Fig. 4.17). This staining appears to be
concentrated between the quartz grains and may be composed of organic-rich, and
possibly opaque-rich, material. The Vik samples either display cross-bedding (58-,
73-centimeter samples) or no visible sedimentary structures (50-, 90-, 97-centimeter
samples).
X-radiographs of the Vik samples revealed opaques that are scattered in most
cases but which define cross beds in the 58-centimeter sample (Fig. 4.17).
Centimeter-scale opaque patches in the 90 and 97 centimeter samples appear to
represent diagenetic concentrations of hematite, which are visible upon examination
of the hand samples. Mottling is apparent within the upper two centimeters of the 58-
234
centimeter sample, but this feature is equally indistinct in both the x-radiograph and
the scan of the cut surface. Vague mottling is also present in x-radiographs of the 73,
90, and 97-centimeter samples. Opaques concentrated near the center of the 97-
centimeter sample may correspond to the potential cross-section of Diplocraterion
discussed above. A vertically-oriented, 0.3-0.5-centimeter-thick structure in the 73-
centimeter sample likely corresponds to a vertical fracture that extends downward
from the top of the sample.
The cut surface of the Didymaulichnus bedding plane sample, from the
Tobisvik Member of the Hardeberga Sandstone at Brantevik, displays regular planar
bedding (Fig. 4.18). Diagenetic iron (hematite) is conspicuous within the sample and
is concentrated in a nodule, which may have a phosphatic core, at three centimeters
above the base. Horizontal burrows visibly intersect the cut surface in two places but
do not appear to disrupt the underlying bedding. In x-radiograph, opaques define
some of the planar beds, and scattered concentrations of hematite are apparent. No
internal structure is visible in the hematite-rich nodule. The elongated opaque
structure at the top of the sample may be an infaunal burrow, but it is unclear from
the hand sample where the potential burrower might have entered or whether the
“burrow” disrupts the adjacent bedding. The pair of opaque structures near the center
of the top edge appears to correspond to one of the Didymaulichnus burrows,
although it is likely that the opaque material is a diagenetic concentration of hematite
near the burrow rather than primary burrow fill.
235
FIGURE 4.18 – Sample collected from the Didymaulichnus-bearing bedding plane in
the meter measured at Brantevik. Bedding plane surface is at top of sample. Largest
dimension of sample is 12cm.
236
Discussion
Hardeberga Sandstone: Too few bioturbated bedding planes were available
within the coastal outcrops of this formation to permit the assembly of a robust
dataset. However, the BPBI analyses from the Vik locality indicate that the
concentration of vertical bioturbation (Diplocraterion) in the Vik Member of the
Hardeberga Sandstone is relatively uniform across large bedding plane surfaces.
Different portions of the same bedding plane surface are commonly weathered to
different degrees, in some cases giving the illusion of a lateral change in the
percentage of bioturbation present. Little actual change across individual bedding
plane surfaces is inferred, however, based on the relatively consistent lateral
distribution of Diplocraterion burrows along horizons, including the tops of large
cross-bed sets, that are visible in vertical exposures. Weathering can also make
Diplocraterion spreiten resemble bedding-parallel trace fossils on bedding plane
surfaces. Aside from the faintly-preserved bilobed traces (likely Didymaulichnus)
and the larger burrow segments (likely heavily-weathered Psammichnites) preserved
on the 73-centimeter bedding plane surface within the Vik meter, no definitive
bedding-parallel trace fossils could be identified in the Vik Member. The heavily-
bioturbated portion of the outcrop examined several tens of meters away from the
studied meter-thick section is too chaotic to permit identification of individual trace
fossils other than Diplocraterion. It is surprising that any individual traces can be
discerned within the heavily-bioturbated layer given that sedimentary structures have
been obliterated completely.
237
Samples collected from the Vik Member one-meter-thick section exhibit very
little contrast, either in scans or x-radiographs, due to the extreme purity of the
medium- to coarse-grained quartz arenite that comprises the member. Consequently,
few traces are clearly visible within the samples, although small-scale cross beds are
highlighted by light iron staining (opaques) and organic material in samples 58 and
73. Sample 58 also contains small-scale indistinct bioturbation below the 58-
centimeter bedding plane surface. This relatively thin bioturbated interval is not
consistent with the overall pattern of bioturbation in the meter and may represent a
brief pause in sediment accumulation that permitted limited biogenic reworking to a
depth of only two centimeters. The only clear example of an individual
Diplocraterion burrow occurs in sample 97. Three short burrow segments are clearly
apparent on the uncut surface of the sample due to haloes of black material that
surround each burrow. A subtle grain-size contrast between burrow fill and
surrounding matrix, further enhanced by weathering, may have promoted the
preferential accumulation of organic matter, whether through algal growth or wave
deposition, within and around the burrow fill. The U-shaped basal portion of one of
the Diplocraterion burrows is faintly visible on the opposite, cut side of the sample.
The meter-thick section analyzed within the Tobisvik Member of the
Hardeberga Sandstone at Brantevik reflects relatively low-energy deposition
succeeded by a high-energy storm event, which resulted in the deposition of the
hummocky cross-stratified and oscillation-rippled beds near the top of the meter.
Based on the uniform appearance of the beds within the lowermost 30 centimeters of
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the meter, the syneresis cracks at 10 centimeters may reflect compaction of a thin
clay-rich layer deposited during a brief pause in sedimentation. Subaerial exposure is
unlikely to have occurred during deposition of the Tobisvik Member judging by the
absence of typical nearshore sedimentary structures and the considerable storm
influence. The Didymaulichnus-bearing bedding plane horizon, at 40 centimeters,
likely was exposed at the sediment-water interface during a quiescent period between
storm events. The shallow-infaunal Didymaulichnus traces were likely produced by
deposit-feeding organisms.
Norretorp Formation and Rispebjerg Sandstone: The two types of bedding-
parallel trace fossils present within these formations appear to be morphologically
simple, although all available examples of both types are poorly weathered. Both
trace types consist of relatively short burrows (10-15 centimeters in length) that do
not branch or curve. Consequently, similar behaviors may have generated these
traces despite the considerable difference in average grain size between the two
bedding plane surfaces in which they occur. The tracemakers may have been
opportunists that were able to exploit a wide range of environmental conditions. Or,
given the restriction of burrows to the top of the Rispebjerg Sandstone, the return of
this particular tracemaker may signal the re-establishment of the slower sediment
accumulation rates that were more characteristic of the Norretorp Formation.
Comparison with the Death Valley Succession: The Lower Cambrian
successions in the Death Valley region and southern Sweden were both deposited in
passive margin settings and reflect similar ranges of facies, from low-energy shallow
239
subtidal to high-energy nearshore and non-marine. Separation by the Iapetus Ocean
resulted in the development of distinct trilobite and acritarch faunas associated with
Laurentia and Baltica. At the same time, Volborthella and other small shelly fossils
and the complex trace fossil Psammichnites are present in the Lower Cambrian
successions of both paleocontinents.
The Vik Member of the Hardeberga Sandstone and the upper portion of the
Emigrant Pass Member of the Zabriskie Quartzite have both been interpreted as
nearshore barrier system deposits. The Vik Member represents deposition in
backbarrier tidal channels, while the upper Emigrant Pass Member represents a
tidally-influenced barrier shoal deposit. Both units are composed of quartzose
sandstone that is laminated to cross-bedded with abundant vertical burrows.
Interestingly, bioturbation in the uppermost Zabriskie Quartzite consists
almost exclusively of Skolithos, while bioturbation in the Vik Member is dominated
by Diplocraterion. Both Skolithos and Diplocraterion represent the semi-permanent
dwelling/feeding burrows of suspension feeding organisms that thrived in moderate-
to high-energy settings. Skolithos consists of a straight, vertical tube that has a single
opening at the sediment-water interface; Diplocraterion consists of a U-shaped tube
with dedicated intake and outlet openings. These two ichnogenera co-occur
infrequently in sedimentary units despite the similarities in the tracemakers’
physiological requirements (Cornish, 1986). As discussed in Chapter III, small
numbers of Diplocraterion have been found in close association with Skolithos in the
middle member of the Wood Canyon Formation. Diplocraterion has also been
240
observed in limited quantities within the Skolithos-dominated quartzite unit of the
Poleta Formation in the White-Inyo Mountains. Thus, the two traces are not
mutually-exclusive.
Slight differences in current energy among similar environments may favor
one type of vertical suspension-feeding trace over another. A U-shaped burrow is a
more sophisticated structure than a single-opening vertical tube because a
unidirectional water current reduces the likelihood that food and waste particles will
mix. A flow-through system is also more hydrodynamic, requiring the tube-dweller
to expend less energy to maintain constant flow in the tube. Therefore, U-shaped
burrows may function more effectively in slightly lower-energy settings than single-
opening vertical tubes. Diplocraterion may be the dominant type of trace fossil
within the Vik Member of the Hardeberga Sandstone because of seasonal cyclicity in
current energy within the back-barrier tidal channels. Based on Hamberg’s (1991)
facies model, sediment accumulation is greatly reduced or ceases altogether in
abandoned tidal channels during the summer season, resulting in near-complete
biogenic reworking of the upper portion of the substrate. The barrier shoal setting
represented by the Emigrant Pass Member of the Zabriskie Quartzite was likely
exposed to incoming waves on a constant basis. No evidence for seasonal cyclicity,
and associated periodic reductions in current energy, has been reported from the
upper Zabriskie Quartzite. Persistent high-energy conditions may have favored
colonization by Skolithos tracemakers over Diplocraterion tracemakers in the upper
Zabriskie Quartzite depositional setting.
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CHAPTER V
Exceptional Bedding Plane Exposures in the Upper Cambrian of Central Wisconsin
Introduction and Previous Work
In the Cambrian Period, the average depth and intensity of marine
bioturbation was lower than at any other time in the Phanerozoic (Droser, 1987;
Droser and Bottjer, 1988; Droser and Bottjer, 1993), with the exception of post-
extinction recovery intervals (Twitchett and Wignall, 1996; Twitchett, 1999;
Twitchett and Barras, 2004; Barras and Twitchett, 2007). Thus, detailed study of
bioturbation in Cambrian rocks often depends on the availability of bedding plane
exposures, which can preserve the activities of epibenthic and shallow-burrowing
animals. Bedding surface exposures larger than one to two square meters are rare,
however, due to such factors as local structural complexity, lithological variation,
and weathering rates. Consequently, little information concerning the lateral
variability of bedding-parallel bioturbation is available from typical outcrops.
Estimates of bioturbation intensity must be made based on samples that are
disproportionately small and localized compared to the extent and heterogeneity of
the originating depositional environment. Modern nearshore shelf settings, for
example, have been shown to exhibit considerable ecological patchiness (Dörjes,
1972; Howard and Reineck, 1972; Thrush, 1991, and references therein).
Few ichnological studies have examined the lateral variability of
ichnofaunas, either within single outcrops or at larger scales. Most such studies have
242
focused upon variations in ichnodiversity (Bromley, 1967; Palmer and Palmer, 1977;
Goldring et al., 1998; Uchman, 2001) or on the spatial relationships among
individuals in a taxonomically uniform ichnoassemblage (Pickett, 1972; Miller,
1977; Pemberton and Frey, 1984). McIlroy (2007), in a study of lateral ichnofabric
variability in vertical exposures of Lower Jurassic shales, found significant
patchiness within individual beds and cautioned against making overly-specific
ichnofabric characterizations based on small areas of outcrop or core slabs. The few
studies that have explored the spatial distribution of bioturbation across bedding
surfaces have focused on the distribution of vertical burrow entrances, rather than on
bedding-parallel trace fossils (Pickett, 1972; Miller, 1977; Pemberton and Frey,
1984).
This study is the first to examine the spatial distribution and concentration of
bedding-parallel bioturbation across large (greater than five square meters) bedding
surfaces. Extensive bedding planes of the Upper Cambrian Mt. Simon and Wonewoc
Sandstones are exposed in several quarries in central Wisconsin, near the town of
Mosinee. Many of these surfaces contain bedding-parallel trace fossils, such as
Climactichnites and Gordia, and sedimentary structures indicative of an intertidal to
transitional setting, such as interference ripples and adhesion structures. Studying the
distribution of bioturbation across these large bedding planes provides clues about
the heterogeneity of ancient intertidal zones and helps put data collected from
smaller bedding planes into perspective.
243
Upper Cambrian Paleogeography of Central Wisconsin
In the Late Cambrian, the continent of Laurentia straddled the equator
(Torsvik et al., 1996). Wisconsin, on the North American craton within Laurentia,
was located at approximately ten degrees south latitude (Dott, 1974) (Fig. 5.1). A
relatively stable tectonic framework, the Sauk Sequence (Sloss, 1950), was in place
on the craton from Middle Cambrian through Early Ordovician time. Broad shelves
existed over much of the North American craton during this period and, in
Wisconsin, sloped toward the south (Lochman-Balk, 1971). The Wisconsin dome, an
uplifted area of Late Neoproterozoic origin located in present-day northern
Wisconsin, was tectonically neutral and lacking in significant relief during the Late
Cambrian (Lochman-Balk, 1971). The Wisconsin dome was an important source of
clastic sediment for the Late Cambrian coastal areas (Runkel et al., 1998), although
extensive reworking of sand and silt sized grains occurred in the nearshore zone
(Lochman-Balk, 1971). The broad, low-lying craton was inundated by several
transgressions brought on by eustatic sea level fluctuations during the Sauk Sequence
(Lochman-Balk, 1971). These transgressions became progressively more extensive
as the highlands and coastal plains gradually eroded (Lochman-Balk, 1971). The
Upper Cambrian Mt. Simon and Wonewoc (Galesville Member) Sandstones were
deposited at the maximum extent of a transgression that began in the early
Dresbachian (Lochman-Balk, 1971). These formations accumulated within the
Hollandale Embayment, a broad topographic low that was bordered on the northwest
244
FIGURE 5.1 – Late Cambrian paleogeography. Red dot marks approximate location
of Wisconsin. Modified from Scotese (2001).
245
by the Transcontinental Arch and on the northeast by the Wisconsin dome and arch
(Runkel et al., 1998).
Geological Setting
Introduction: In north-central Wisconsin, the Upper Cambrian succession
thins from southwest to northeast, largely disappearing against the Precambrian
crystalline basement of the Wisconsin Highlands. A few small erosional outliers of
this succession are preserved in the northern half of the state, of which a number
have been quarried for flagstone (Fig. 5.2). The quarries discussed in this chapter
expose fewer than 20 meters of the uppermost Mt. Simon Sandstone and the
lowermost Wonewoc Sandstone (Hagadorn et al., 2002a) (Fig. 5.3). In more
complete localities further to the southwest, these two similar sandstone formations
can be differentiated based on lithology, facies, and biostratigraphy, in addition to
their positions relative to the thin Eau Claire Formation, which separates the two
(Hagadorn et al., 2002a). However, the Eau Claire Formation is not present within
the erosional outliers, and the limited thicknesses of the outlier sections preclude
definitive stratigraphic correlation. Thus, all strata discussed from these outliers will
be treated as combined Mt. Simon and Wonewoc material.
Mt. Simon Sandstone: The Mt. Simon Sandstone is a fine- to coarse-grained,
submature to mature quartz arenite that rests directly, and unconformably, on the
Precambrian crystalline basement (Lochman-Balk, 1971; Driese et al., 1981) (Fig.
5.3). Driese and colleagues (1981) subdivided the formation into lower, middle, and
246
FIGURE 5.2 – Location map for study area in north-central Wisconsin (marked by
red star). Dashed line indicates inferred position of paleoshoreline during Late
Cambrian time. Modified from Dott (1974), Dott et al. (1986), and Hagadorn et al.
(2002).
247
FIGURE 5.3 – Generalized stratigraphy of the Upper Cambrian succession in
Wisconsin. The St. Lawrence Formation is abbreviated as “St. Lawr.” After
Hagadorn et al. (2002).
248
upper facies. The lower facies is 15-27 meters thick and is more variable in
composition than the remainder of the formation (Driese et al., 1981). Much of the
lower facies is characterized by tabular and trough cross-bedded medium- to very
coarse-grained quartz arenites, in which individual cross beds are often graded
(Driese et al., 1981). Clay intraclasts and thin clay laminae occur within this portion
of the facies, as well as scattered Skolithos and Arenicolites (Driese et al., 1981).
Less common within the lower facies are planar-bedded, fine- to medium-grained
feldspathic arenites and micaceous siltstones and shales, which range in thickness
from 1-30 centimeters (Driese et al., 1981). Driese and colleagues (1981) interpret
the lower, coarser-grained portion of the facies as a braided fluvial deposit; the
burrowed beds as a high-energy foreshore deposit; and the finer-grained remainder
of the facies as a shallow subtidal, storm- and tide-influenced deposit.
The middle facies is typically 20-35 meters thick, although it is often poorly
exposed (Driese et al., 1981). Driese and colleagues (1981) recognized two distinct
associations between sedimentary structures and trace fossils within this facies. The
first, which is similar to the dominant lithology of the lower facies, is a tabular and
trough cross-bedded quartz arenite with graded cross laminae and both symmetrical
and asymmetrical ripple marks (Driese et al., 1981). Abundant Skolithos and
Arenicolites occur in the upper portions of some cross beds, where they are often
truncated and may obscure cross-bedding completely (Driese et al., 1981). Driese
and colleagues (1981) interpret this association to represent deposition in high-
velocity tidal channels. The second association consists of planar-bedded and ripple-
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cross-stratified fine- to medium-grained feldspathic arenites that are 1-15 centimeters
in thickness (Driese et al., 1981). Thin micaceous siltstone and shale interbeds
contribute to the development of lenticular and flaser bedding. Rusophycus,
Cruziana, Planolites, and “meandering bilobate traces of uncertain affinity” occur
within this second association (Driese et al., 1981). Driese and colleagues (1981)
interpret the second association as a lower tidal flat deposit in which the sandstone
and siltstone interbeds correspond to high and low tides, respectively. The dearth of
subaerial exposure features in the middle facies indicates that deposition occurred
low in the intertidal zone (Driese et al., 1981).
The upper facies, which is typically 10-20 meters thick, is dominated by fine-
to coarse-grained quartz arenites with abundant Skolithos (Driese et al., 1981).
Driese and colleagues (1981) estimate the densest concentrations of Skolithos at 5-7
burrows per square centimeter, and this intensity of bioturbation commonly imparts a
massive appearance to the sandstones. Tabular and trough cross-bedding and other
features similar to those of the lower facies sandstones are visible in moderately-
burrowed or undisturbed strata of the upper facies (Driese et al., 1981). Desiccation
cracks are present on the crests of some ripples (Driese et al., 1981). A second, less
common lithology is fine-grained sandstone interbedded with thin siltstone and shale
layers. Valves of the inarticulate brachiopod Obolus occur within cross laminae 2-5
meters below the contact between the Mt. Simon Sandstone and the Eau Claire
Formation (Driese et al., 1981). Driese and colleagues (1981) interpret the upper
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facies as a middle tidal flat deposit in which sediment accumulation was relatively
slow, allowing the beds to become extensively burrowed.
Wonewoc Sandstone: The Wonewoc Sandstone, which spans portions of the
Late Cambrian Crepicephalus, Aphelaspis, Dunderbergia, and Elvinia trilobite
Zones (Runkel et al., 1998), is a light-colored, medium- to coarse-grained, well-
sorted, pure quartz arenite that is ubiquitously trough cross-bedded at a medium scale
(Dott et al., 1986) (Fig. 5.3). Dott and colleagues (1986) identified three facies
within the formation: the large-scale cross-stratified facies, the planar-and-channeled
facies, and the burrowed and cross-stratified facies. The large-scale cross-stratified
facies is characterized by cross beds that are 4-5 meters thick (Dott et al., 1986).
Also present in the facies are common adhesion ripples and grainflow structures and
less common planar stratification (Dott et al., 1986). Dott and colleagues (1986)
interpret the abundant adhesion structures, large-scale cross-bedding, and absence of
fossils to indicate deposition in a non-marine setting comprised of aeolian dunes and
ephemeral interdune channels and ponds. Runkel and colleagues (1998) identified a
portion of this lithological facies as a shoreface deposit, in which large
Climactichnites traces reflect brief forays onto subaerially-exposed tidal flats
(Yochelson and Fedonkin, 1993; Hagadorn et al., 2002b).
The planar-and-channeled facies overlies the large-scale cross-stratified
facies and contains 15-30-centimeters-thick trough cross-bedded sets interbedded
with planar-stratified, channel-bearing sandstone beds (Dott et al., 1986). Ripples,
polygonal cracks, intraclasts, and rare adhesion structures are also present. Dott and
251
colleagues (1986) interpret this facies to represent aeolian sand sheets distributed
over a broad plain and separated by braided rivers.
The uppermost portion of the Wonewoc Sandstone is comprised of the
burrowed and trough cross-stratified facies, which is characterized by small- to
medium-scale trough cross-stratification and both massive and distinctly-bioturbated
interbeds (Dott et al., 1986). Skolithos, Arenicolites, and Planolites are common in
the bioturbated beds. Coquinas composed of the inarticulate brachiopod Lingulepis
and cephalons of the trilobite Camaraspis occur in some areas (Dott et al., 1986).
Dott and colleagues (1986) interpret the burrowed and trough cross-stratified facies
to represent a shallow subtidal to intertidal, storm-influenced marine depositional
setting.
Erosional Outliers: In the erosional outliers of north-central Wisconsin,
Hagadorn and colleagues (Hagadorn et al., 2002a) have reported impressions of
scyphozoan medusae preserved in epirelief on several large bedding surfaces in
flagstone quarries (Fig. 5.4). These structures, which average 25 centimeters in
diameter, are found on medium-grained quartzose sandstone bedding planes that may
be planar-bedded or rippled (Hagadorn et al., 2002b). Medusae-bearing bedding
surfaces are non-bioturbated and often contain microbially-mediated sedimentary
structures, including elephant skin, domal sand stromatolites, and sand chips
(Hagadorn et al., 2002a; 2003). These features reflect a shallow intertidal to lagoonal
setting that was intermittently exposed (Hagadorn et al., 2002a).
252
FIGURE 5.4 – Impression of a scyphozoan medusa on a quarried bedding plane
surface of Mt. Simon-Wonewoc Sandstone.
253
Hagadorn and colleagues (2002a) have described a more diverse suite of
trace fossils from the erosional outliers than has been reported from the Mt. Simon
and Wonewoc Sandstones by other workers. In addition to Climactichnites,
Arenicolites, Planolites, and Rusophycus, Hagadorn and colleagues (2002a) have
reported Helminthoida, Palaeophycus, Monomorphichnus, Gordia, Protichnites,
Diplichnites, and bilobate trails. Greater observed ichnodiversity in the erosional
outliers is likely a function of the anomalously-large bedding plane exposures made
available by quarrying activities.
Methods
Field Localities and Methods: Three large bedding planes were selected for
study from the Krukowski, Nemke, and Pointe Quarries near Mosinee, Wisconsin
(Fig. 5.2). Additional locality information is found in Appendix I. The Krukowski
Quarry, the largest and most active of the three, contains dozens of bedding plane
surfaces exposed for tens of square meters (Fig. 5.5). Many of these surfaces are
rippled, and a number preserve impressions of medusae. The Nemke Quarry, a small
private quarry, contains a few large bedding surfaces in slightly finer-grained
sandstones than those in the Krukowski Quarry (Fig. 5.5). The Pointe Quarry is a
hillside excavation that contains several terraced bedding plane surfaces (Fig. 5.5).
Each bedding surface was first cleared of debris and swept thoroughly to
maximize the visibility of surface features. Chalk lines were used to superimpose a
grid onto each surface. Grid lines were placed 25 centimeters apart, producing square
254
A)
B)
FIGURE 5.5 – Views of quarries that contain studied bedding planes. Studied
bedding plane is in foreground in each photograph. (A) Krukowski Quarry. (B)
Nemke Quarry. (C) Pointe Quarry.
255
C)
FIGURE 5.5 (continued) – Views of quarries that contain studied bedding planes
256
cells, each with an area of 625 square centimeters. A letter-and-number coordinate
system was used to identify cells within each large-scale grid.
Each grid cell was examined individually. If bioturbation was present, then
the type(s) of traces within each grid cell were noted. Also noted from the grid cells
on the Krukowski Quarry surface were the number of Climactichnites traces
intersecting each cell and the width of one of these traces. A random number
generator was used to produce sets of random grid cell coordinates for bedding plane
bioturbation index and intersection grid analysis. The diameter of one Gordia trace
was measured from each bioturbated, randomly-selected grid cell on the Nemke
Quarry surface and from every distinctly-bioturbated cell on the considerably-
smaller Pointe Quarry surface. Following data collection, grid cells were
photographed systematically in groups of four, and overview photographs were taken
of larger portions of each bedding plane.
Laboratory Methods: Each randomly-selected grid cell from the Nemke
Quarry bedding surface was analyzed using the intersection grid method described in
Chapter II. Analyzed photographs of these cells, and a spreadsheet of the intersection
grid method data, are located in Appendix III. An average percentage bioturbation
for the surface was calculated using the results of the grid analyses. Using the field
data, a small grid was drawn based on each of the three bedding surfaces and color-
coded according to the types of data recorded from each grid cell. These grids were
examined for patterns in the spatial distribution of bioturbation on each surface. Data
on trace widths/diameters was compiled, plotted, and examined for patterns. Several
257
types of image-stitching software were tested for their capacity to produce
photographic composites of the bedding surfaces. Although two programs showed
promise, none generated images that were practical for use in these analyses.
Results
Trace Fossils: By far the most impressive trace fossil observed on the
quarried bedding surfaces is Climactichnites, which strikingly resembles a tire track
(Fig. 5.6). This trace has relatively low relief and consists of a wavy or rippled
central “tread” that is bounded by two narrow longitudinal ridges. In some examples,
the “tread” appears to be symmetrical about a center line. Occasionally, an ovate
structure is preserved at the terminal end of the trace. Climactichnites has been
interpreted in a variety of ways, as Yochelson and Fedonkin (1993) discussed in their
detailed examination of the ichnogenus. Previous workers have envisioned an
arthropod, mollusk, or worm tracemaker (references in Yochelson and Fedonkin,
1993), although Yochelson and Fedonkin (1993) preferred a sluglike animal that
propelled itself with anterior, protruding appendages. Seilacher (2007, p. 180-81)
interpreted the tracemaker as a large mollusk, perhaps a Kimberella-like halkieriid,
that “plow[ed] along” through the sediment via the mucociliatory action of a
muscular foot. Regardless of its origin, Climactichnites is significant, not only
because of its considerable size in comparison to most other Cambrian trace (and
body) fossils, but also because it may reflect early evolutionary adaptations that
258
A)
B)
FIGURE 5.6 – Trace fossils on quarried surfaces of Mt. Simon-Wonewoc
Sandstones. (A) Climactichnites. (B) Diplichnites. (C) Gordia.
259
C)
FIGURE 5.6 (continued) – Trace fossils on quarried surfaces in the Mt. Simon-
Wonewoc Sandstones
260
made survival in subaerial conditions possible, if only for short periods (Hagadorn et
al., 2002b).
Diplichnites and Protichnites have been interpreted as arthropod trackways.
Häntzschel (1966, p. 191) described Diplichnites as a “rather nondescript biserial
walking track…with numerous steps” (Fig. 5.6). Protichnites resembles Diplichnites,
with the addition of a continuous or interrupted narrow central furrow that is
typically interpreted as a tail or telson drag mark, produced in some cases for self-
stabilization (e.g., Seilacher, 2007, p. 28-30). Both of these ichnogenera have been
documented from deposits representing marginal marine or freshwater to subaerial
(high intertidal, aeolian dune) settings (Braddy, 2004).
Gordia is a small, looping trace that bears little, if any, ornamentation and
was actively backfilled (Seilacher, 2007) (Fig. 5.6). Seilacher (2007, p. 96-97)
included Gordia in his “horizontal scribbles” category along with other traces made
by organisms that “bypassed the difficulty to maintain constant whorl distances” and
repeatedly crossed over their earlier paths.
Krukowski Quarry Surface: The gridded portion of this surface is 12 square
meters in size and is composed of fine- to medium-grained sandstone (Fig. 5.7). A
12x16-cell grid (192 total cells) was superimposed on the bedding plane. Every cell
in the grid was found to contain bioturbation, in the form of Climactichnites traces.
One cell, I14, also contains indistinctly-preserved Protichnites. As many as six
Climactichnites traces intersect each cell within the grid. Plotted in Fig 5.8 are
numbers of cells that are intersected by one, two, three, four, five, or six
261
FIGURE 5.7 – Field photograph of studied bedding plane in the Krukowski Quarry,
which contains abundant Climactichnites.
262
0
10
20
30
40
50
60
70
80
123456
Number of Traces Intersecting Each Cell
Number of Cells
FIGURE 5.8 – Graph of number of Climactichnites traces intersecting grid cells on
the Krukowski Quarry bedding plane surface. N=192.
263
Climactichnites traces. Figure 5.9, a representation of the bedding plane grid, shows
the spatial distribution of these intersections. Climactichnites traces occur in the
greatest densities within and around cells A6-7 and D16. Based on a total of 155
measurements made between the lateral ridges of each trace (inner edge to inner
edge), Climactichnites burrow widths on this surface range from 7-11.1 centimeters,
with an average width of 8.6 centimeters (also the median) (Fig. 5.10). More than
one successive “generation” of traces is visible on the bedding plane surface; traces
from earlier episodes of bioturbation appear less distinct than the final group (Fig.
5.11). Ten grid cells were selected at random for bedding plane bioturbation index
analysis. A BPBI of three was recorded from cell K5; cells F4, J10, and L2 were
interpreted to reflect BPBI four; and cells A6, A11, B9, C3, F8, and K8 were
interpreted to reflect BPBI five (Fig. 5.12). The average of the recorded BPBIs is
five.
Nemke Quarry Surface: A 24x32-cell grid (768 total cells) was superimposed
on this 48-square-meter surface (Fig. 5.13). The bedding plane is composed of fine
sandstone and is covered in large part by flat-topped asymmetrical ripples. Small
(fewer than five millimeters in diameter) raised papillae are clustered in ripple
troughs in some portions of the surface (Fig. 5.14). These structures have been
interpreted as sand stromatolites (J.W. Hagadorn, pers. comm.) and may be evidence
for microbial stabilization of the rippled surface. Two larger conical mounds, up to
15 centimeters in diameter, occur on the bedding surface. Similar structures found in
this and other outlier quarries have been interpreted as fluid- or gas-escape structures
264
FIGURE 5.9 – Schematic representation of the Krukowski Quarry bedding plane
surface. Cells are color-coded based on the number of Climactichnites traces that
intersect each cell.
265
0
2
4
6
8
10
12
Climactichnites Burrow Widths (cm)
0
2
4
6
8
10
12
7
7.2
7.4
7.6
7.8
8
8.2
8.4
8.6
8.8
9
9.2
9.4
9.6
9.8
10
10.2
10.4
10.6
10.8
11
Climactichnites Burrow Width (cm)
Number of Burrows
FIGURE 5.10 – Measured widths of Climactichnites burrows on Krukowski Quarry
bedding plane. Each measurement was made between the inner edges of the lateral
ridges on each trace. No measurements were made from grid cells in which trace
boundaries are indistinct. N=155. (Top) Data plotted in ascending order. Red line
indicates average measurement.
= 10 measurements
266
FIGURE 5.11 – Field photograph of a portion of the Krukowki Quarry bedding
plane in which well-preserved traces cross over indistinct traces that represent earlier
episodes of burrowing activity by Climactichnites tracemakers. Each well-preserved
Climactichnites trace is approximately 8.5 centimeters wide.
267
0
1
2
3
4
5
6
7
12 3 45
BPBI
Number of Cells
FIGURE 5.12 – BPBI data from cells selected at random from the Krukowski
Quarry bedding plane. N=10.
268
FIGURE 5.13 – Field photograph of Nemke Quarry bedding plane surface. Small rocks indicate boundaries of studied area.
269
FIGURE 5.14 – Field photograph of a portion of the Nemke Quarry bedding plane
that contains small, papillar structures interpreted to be sand stromatolites. These
structures often cluster in ripple troughs, as seen here. Gordia is also present.
270
that formed in the presence of a surface microbial mat (Dornbos et al., 2007). An
underlying bedding surface, fewer than three centimeters below the studied surface,
is exposed in one corner of the grid (coordinates A-D, 29-32). This surface is
interference-rippled and contains small Gordia and Diplichnites traces.
Bioturbation on the studied bedding surface is dominated by Gordia. Two
distinct size classes of Gordia traces were observed (Fig. 5.15): “small,” which
average two millimeters in width; and “large,” which range from 4-10 millimeters in
width based on data from the 40 randomly-selected grid cells. The average width of
the large Gordia traces is 6.42 millimeters, and the median (also the width with the
highest frequency among the measured traces) is seven millimeters (Fig. 5.16). The
“small” Gordia occur in four discrete patches, each consisting of three or more grid
cells and separated from adjacent patches by at least one unbioturbated cell, and in
four isolated one- to two-cell outliers (Fig. 5.17). Considered together, the Gordia
traces exhibit limited patchiness; the only true patch, or a cluster of bioturbated cells
separated from similar nearby clusters by the width of at least one non-bioturbated
cell, occurs in the upper left corner of the grid. At this grid scale, the remainder of
the bioturbated area constitutes one large patch or network of cells.
Forty grid cells were selected at random for bedding plane bioturbation index
and intersection grid analysis (Fig. 5.17). Field BPBIs from these grid cells ranged
from one to three, and BPBI two (0-10% bioturbation) was the most common. Grid
analyses largely corroborated these field estimates (Fig. 5.18, 5.19); one field
estimate was too high (BPBI three versus two), and one too low (BPBI two versus
271
FIGURE 5.15 – Field photograph of a portion of the Nemke Quarry bedding plane
that contains “small” and “large” Gordia. Most of the traces in this view are “small”
Gordia. Arrow points to single “large” Gordia trace.
272
0
1
2
3
4
5
6
7
8
4567 8 9 10
Gordia Widths (mm)
Number of Gordia Traces
FIGURE 5.16 – Gordia width measurements from the Nemke Quarry bedding plane
surface. One burrow was measured from each randomly-selected grid cell that
contains bioturbation. N=24.
273
FIGURE 5.17 – Schematic representation of the Nemke Quarry bedding plane
surface. Cells are color-coded based on the presence (and type) or absence of
bioturbation. Orange cells contain both “small” and “large” Gordia.
274
0%
5%
10%
15%
20%
25%
12 345
BPBI (field)
Grid percentage
FIGURE 5.18 – Cross plot of field BPBI estimates and intersection grid method
percentage estimates for all randomly-selected grid cells that contain bioturbation
(N=25). Each box outlines the range of percentages on the y axis that corresponds to
the BPBI indicated on the x axis. Data points that fall within the boxes represent
bedding planes for which field BPBI estimates and grid percentage estimates are in
agreement. The data point plotted at BPBI 1 (0% bioturbation) was estimated to be
0.26% bioturbated using the intersection grid method.
275
0%
10%
20%
30%
40%
50%
Percentage bioturbation
0
2
4
6
8
10
12
14
0%
0 - 1%
1 - 2%
2 - 3%
3 - 4%
4 - 5%
5 - 6%
6 - 7%
7 - 8%
8%+
Percentage bioturbation
Number of grid squares
FIGURE 5.19 – Intersection grid method results from randomly-selected grid cells
on the Nemke Quarry bedding plane. Three of the 40 random selections, which fell
on gaps in the bedding plane surface, are excluded. N=37. (Top) Individual data
points plotted in ascending order. Data points are “probable” percentages based on
visible bedding plane area; error bars are bounded by “maximum” and “minimum”
percentages, which include missing bedding plane area in the estimate, treat the area
as either completely bioturbated (maximum) or devoid of bioturbation (minimum).
(Bottom) Data grouped into 1% bins, with exception of 0% and 8+% bins.
276
three, although the grid result was 11.14%). Because many of the randomly-selected
grid cells were incomplete due to fractures in the bedding surface, probable
bioturbation percentages were calculated by excluding the non-bedding-plane grid
intersections from the total number of possible intersections (see Chapter II).
Minimum and maximum percentages were also calculated to bracket the potential
error. The “probable” percentages are taken to be the closest estimates of actual
bioturbation within these grid cells.
Twelve of the randomly-selected grid cells contain no bioturbation, and
another seven are less than one percent bioturbated (Fig. 5.19). Based on presence-
absence data from the entire bedding plane, approximately 64.29 percent of the
surface is bioturbated by Gordia traces (Fig. 5.17). Of that area, 9.01 percent
consists, at least in part, of “small” Gordia. Thus, twelve of the forty grid cells
selected at random captured gaps in bioturbation on the bedding surface. At the same
time, only two of the grid cells analyzed using the intersection method were
estimated to be more than ten percent bioturbated (Fig. 5.19).
Pointe Quarry Surface: This surface, at six square meters, is considerably
smaller than the Krukowski and Nemke surfaces (Fig. 5.20). An 8x12-cell grid (96
total cells) was superimposed on the bedding plane, which consists of symmetrically-
rippled fine sandstone. No other identifiable sedimentary structures were observed,
and Gordia (“large”) is the only trace fossil present on the surface (Fig. 5.21). The
width of one Gordia trace was measured from each bioturbated grid cell. Widths
range from 4-7 millimeters, with an average width of 4.96 millimeters and a median
277
FIGURE 5.20 – Field photograph of the studied bedding plane surface in Pointe
Quarry.
278
FIGURE 5.21 – Schematic representation of the Pointe Quarry bedding plane
surface. Cells are color-coded based on the presence or absence of bioturbation, in
the form of Gordia.
279
of five millimeters (Fig. 5.22). Thus, Gordia tends to be slightly smaller on this
surface than the “large” Gordia on the Nemke surface. Five grid cells were selected
at random for bedding plane bioturbation index analysis; only one of these contains
bioturbation (BPBI two) (Fig. 5.21). Patches of Gordia bioturbation are well-defined
and discrete on this surface. The width of at least one grid cell, if not more, separates
each patch from others nearby.
Discussion
Krukowski Quarry Surface: This bedding plane surface is remarkable because
of the scale of bioturbation that it contains. Climactichnites, one of the largest
invertebrate trace fossils known, covers all but a few square centimeters of this 12-
square-meter bedding plane. Many of the individual Climactichnites burrows can be
followed for meters across this surface. The sharply-defined “tread” and lateral
ridges on many of the Climactichnites burrows, and the fact that earlier
“generations” of burrows had not been completely obliterated by tidal inundation,
likely reflects unusual preservational conditions, such as sediment stabilization by
microbial binding. No microbially-mediated sedimentary structures were observed
on this bedding plane surface. However, such structures are present on other bedding
plane exposures in the Krukowski Quarry. The middle to high intertidal and lagoonal
depositional setting inferred for the erosional outliers likely would have been
conducive to the establishment of microbial films and mats.
280
0
2
4
6
8
10
12
14
4567
Gordia Widths (mm)
Number of Gordia Traces
0
2
4
6
8
10
12
14
456789 10
Gordia Widths (mm)
Number of Gordia Traces
Nemke
Pointe
FIGURE 5.22 – Gordia width measurements from the Pointe Quarry bedding plane
surface (top) and from the Pointe and Nemke bedding plane surfaces combined
(bottom). N=31 for Pointe dataset. N=24 for Nemke dataset.
281
The Climactichnites burrow width data are continuous at a 0.1 centimeter
resolution between seven and 10 centimeters (Fig. 5.10). No burrows were measured
to be exactly 10.1 centimeters, but burrow widths of 10.2-10.6 were measured. The
maximum burrow width in the dataset (11.1 centimeters) likely represents a
procedural error. No clear patterns or clusters were observed in these burrow width
data despite the moderate size of the dataset and the limited range of widths
represented (3.6 centimeters). However, the continuous distribution of data within a
fairly narrow range, coupled with a lack of significant variation in the expression of
the Climactichnites traces on this surface, suggests that the Climactichnites
tracemakers active on this surface were similar to one another in size and body plan
and, therefore, may have been the same species.
An average bedding plane bioturbation index of five is consistent with the
density of traces on this surface and the frequency with which they overlap and cross
one another. Most of the grid cells are intersected by three or four Climactichnites
burrow segments (Fig. 5.9). Interestingly, the upper left portion of the grid, bounded
by row F and column 13, shows a relatively uniform and low-density distribution of
burrows. Only 13 of the 91 squares in this portion of the grid are intersected by four
Climactichnites segments. Of the remainder, 45 cells are intersected by three
segments, 31 by two segments, and two by one segment. Outside of the area defined
by F and 13, no cells are intersected by only one burrow segment, and only seven of
the 101 total cells are intersected by two Climactichnites segments. Sixty-seven of
the cells are intersected by four or more burrow segments.
282
Despite the presence of areas that have high concentrations of burrows, the
pattern of bioturbation on this surface probably does not reflect gregarious behavior
on the part of the Climactichnites tracemakers. Instead, portions of the grid that are
intersected by multiple burrow segments represent locations that have been visited
repeatedly by individuals over some period of time. An individual Climactichnites
burrow commonly follows a very straight path for several meters across the surface
and then makes a smooth, arcing turn back in the direction of its origin. Many of
these turns fall within the “high burrow density” portion of the bedding plane outside
grid row F and column 13. Judging by the non-meandering paths taken by many
tracemakers to reach this portion of the surface, the journeys to and from this area
appear to have been purposeful. Given the evidence for microbially-enhanced
preservation of this surface, the destination area may have been covered by a well-
developed microbial mat that would have been an attractive food source for grazing
Climactichnites tracemakers.
Nemke Quarry Surface: The two size classes of Gordia present on this
bedding surface likely reflect the work of adult (“large”) and juvenile (“small”)
Gordia tracemakers. The “small” Gordia occur in compact patches that are scattered
across the surface, but these patches fit well into the overall distribution of Gordia
traces on the bedding plane. The distribution of width measurements for the “large”
Gordia traces is continuous from four to ten millimeters, and no significant peaks are
present in the data, indicating that these traces were likely produced by a population
of morphologically-similar tracemakers that may have belonged to the same species.
283
Unlike Climactichnites, Gordia is a “scribbling” trace; the Gordia present on this
surface likely reflect non-systematic feeding behavior by gregarious surface-deposit-
feeding organisms.
Twelve of the forty grid cells selected at random for BPBI analysis contain
no bioturbation and, thus, capture gaps in or areas outside of the apparent network of
Gordia bioturbation on this surface. The 25 bioturbated grid cells within the
randomly-selected group contain very low percentages of bioturbation (all less than
10 percent). Although large areas of this bedding plane do contain small amounts of
bioturbation, the BPBI results do not accurately reflect the full range of bioturbation
densities present on this surface. Scale is a factor. Areas of high burrow density
together constitute a very small proportion of the total bedding plane area.
Consequently, the probability that a randomly-selected set of coordinates will fall
within one of these high-density areas is very low. A much larger random sample
than the 40 cells would be required to achieve a satisfactory estimate of the range of
bioturbation on this surface.
At this grid scale, the Gordia traces present on this surface exhibit limited
patchiness. With the exception of a small patch in the upper left corner of the grid,
the bioturbated area of this surface constitutes a network of interconnected
bioturbation, or one large patch. This pattern would not change if the missing
bedding plane areas were restored as either bioturbated or non-bioturbated cells.
However, increasing the number of subdivisions within the grid, as discussed for
small bedding plane sample areas in Chapter II, would change the apparent
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distribution of bioturbation (and increase the precision of the estimate) by singling
out more gaps between traces and between small clusters of Gordia. For example,
reducing the dimensions of a grid cell by half would help to isolate the sparse Gordia
traces present in bedding plane areas B-D 5-7 and J-L 9-11 (Fig. 5.23). Multiple
discrete patches of bioturbation would likely emerge as the number of subdivisions
in the grid was increased. However, due to the three-orders-of-magnitude difference
between the width of a single Gordia trace and the length of the studied bedding
plane area, there is a practical limit to the degree of precision that can be achieved
for this surface. The metric established in Chapter II for balancing precision and
efficiency fails in this case. Thus, there is a tradeoff inherent in the study of very
large bedding plane exposures: the volume of information available on a large
bedding plane surface may exceed the investigator’s capacity (in terms of time and
resources) to evaluate it effectively.
Pointe Quarry Surface: This studied bedding plane area is one-eighth the size
of the studied Nemke Quarry surface. Despite the use of the same grid cell size in
this grid and in the Nemke grid, five discrete patches of bioturbation by Gordia
emerged on the Pointe surface. A majority of the patches are made up of relatively
dense associations of Gordia rather than isolated traces. Greater consolidation of
Gordia on the Pointe surface than on the Nemke surface is most likely the reason for
the marked difference in the pattern of bioturbation between these two surfaces.
Environmental factors may have influenced the degree to which the Gordia
tracemakers moved apart from one another on the Pointe and Nemke surfaces.
285
FIGURE 5.23 – Field photographs of portions of the Nemke Quarry bedding plane in
which Gordia traces are sparse. Each grid cell is 25x25 centimeters. Coordinates
listed do not include small portions of additional cells at edges of view. (Top) B-D,
5-7. (Bottom) J-L, 9-11.
286
The Gordia width measurements obtained from the Pointe surface were
graphed together with those from the Nemke surface for comparison (Fig. 5.22).
More Gordia traces were measured from the Pointe surface than from the Nemke
surface because one measurement was obtained from each bioturbated cell in the
smaller Pointe grid. The larger Pointe dataset exhibits a narrower distribution of
Gordia widths and a smaller average width (4.96 millimeters) than the Nemke
dataset. These differences may reflect taxonomic differences between the two
populations. However, both datasets are statistically insignificant because of their
small size; increasing the total numbers of burrow measurements from the two
surfaces would likely reveal whether these differences reflect true distinctions
between populations.
287
CHAPTER VI
Conclusions
Quantitative Methods for the Study of Bioturbation on Bedding Planes
An ideal method for the study of bioturbation on bedding planes would have
the following characteristics: objective data collection procedures, precise and
quantitative means of assessing the abundance and spatial distribution of
bioturbation, accommodation of indistinct bioturbation, consistent units of
measurement, reproducibility, and efficiency. The intersection grid method, a
variation on the point-intercept method developed for percentage cover estimation in
plant ecology studies (Goodall, 1952; Floyd and Anderson, 1982; Greig-Smith,
1983; Floyd and Anderson, 1987), achieves a satisfactory balance of objectivity,
precision, and efficiency. This method also accommodates indistinct bioturbation
effectively and produces reproducible, quantitative results.
A test of the intersection grid method using hypothetical bedding plane
images with known percentages of bioturbation demonstrated that this method
consistently out-performs the “cell” method, producing results within 10 percent of
the actual bioturbation percentage at grid scales as coarse as 8x8 cells. The “cell”
method is inherently less precise than the intersection method because its
fundamental unit, the grid cell, is itself an area that may contain internal variation.
Theoretically, each unit in the intersection method is a dimensionless point, which
288
can record only presence or absence. Thus, the potential for error due to
methodological limitations is reduced if the intersection method is used.
The relationship between the average size of a trace fossil or patch of
indistinct bioturbation and the size of the sample area, as defined by a square or
rectangular frame, has important implications for intersection grid analysis. The
number of grid subdivisions required to produce a precise bioturbation estimate will
be small when the average trace fossil is large relative to the sample area, and vice
versa. The following can be used as a guide for constructing a grid that will
maximize precision and efficiency based on the ratio of trace size to sample area: the
average width/diameter of the trace fossils to be analyzed should equal or exceed the
distance between any two parallel grid lines. This metric can be applied regardless of
the size of the sample area, although it is less effective when the trace-to-area ratio is
particularly large or small. Similarly, the concentration of bioturbation relative to the
size of the sample area can influence the effectiveness of the intersection method. If
trace fossils are both abundant and small relative to the sample area, the amount of
time required to conduct a grid analysis may approach that required for tracing the
bioturbation. Thus, at certain scales, the intersection method does not present an
advantage over other methods in terms of efficiency.
The intersection method is an effective means of evaluating bioturbation on
bedding planes and meets many of the criteria for an “ideal” method. Some of the
data collection procedures used in the intersection method are subjective, but the
likelihood of obtaining inconsistent or erroneous results using the intersection
289
method is considerably smaller than if a semiquantitative method is used. The
intersection method achieves a high level of precision in estimating the abundance of
bioturbation on bedding planes. Results obtained using this method are both
quantitative and unitless and can, therefore, be compared and manipulated in large
datasets. An analysis using the intersection method results in a map of the spatial
distribution of bioturbation within a sample area. This method also accommodates
indistinct bioturbation by reducing the amount of subjectivity required for its
evaluation. Analyses conducted using the intersection method are readily
reproducible, and a record of each analysis can be saved and shared for maximum
transparency. This method is necessarily less efficient than semiquantitative methods
because the former produces results that are more precise. The efficiency of the
method depends, to some extent, on the scale and intensity of bioturbation relative to
the size of the sample area. If the trace fossils in a sample area are of moderate size
and abundance, then the intersection method can be an efficient means of estimating
the percentage of bioturbation present. Extremes of scale and/or intensity may
require adjustments to the method or the use of an alternative method. In most cases,
however, the intersection method should serve as a precise, efficient means of
evaluating bioturbation on bedding planes and an effective means of refining
estimates made in the field.
290
Bioturbation in the Lower Cambrian Succession of the Death Valley Region
Low bioturbation percentages (less than 25 percent) obtained from the fine-
grained lower member of the Wood Canyon Formation are consistent with the
stratigraphic position of these bedding surfaces just above the Neoproterozoic-
Cambrian boundary. Planolites-type burrows are the most common identifiable trace
fossils on these bedding planes. Unexpectedly, however, bioturbation is visible
within vertical outcrop exposures and samples collected from the lower member.
Most of the structures appear to be bedding parallel, but at least one sub-vertical
trace was observed. Thus, bioturbation-induced sediment disruption was already
imparting an effect on shallow marine siliciclastic substrates early in the Early
Cambrian in the Death Valley region.
Most of the bedding planes analyzed from the middle member of the Wood
Canyon Formation are dominated by the circular entrances of vertical burrows.
However, the units in which these bedding planes occur do not always bear the
typical characteristics of Skolithos piperock. Bedding plane surfaces are commonly
very fine-grained, and limited numbers of bedding-parallel trace fossils often co-
occur with circular burrow entrances on these surfaces. Abundant vertical trace
fossils in units that contain some fine-grained material may reflect the development,
in nearshore settings, of adaptations to burrowing vertically into sediment of variable
grain size.
Intersection grid method data from the middle member of the Wood Canyon
Formation, of which half fall below 25 percent and half between 30-55 percent,
291
display a distribution that is reasonable for a unit that contains more vertical than
bedding-parallel bioturbation. Vertical burrows cover little surface area on bedding
planes compared to bedding-parallel traces and, thus, must be concentrated in high
densities in order to achieve bedding plane coverage that approaches 50 percent.
Small numbers of vertical burrows may be preserved within a bed if the sediment
was deposited during a time of particularly rapid sediment accumulation. Thus, it is
not improbable to observe bedding planes that are less than 15 percent bioturbated
and others that approach 50 percent bioturbation within the same unit.
Grid analysis results from the upper member of the Wood Canyon Formation
cover a broader range of percentages than the results from the middle and lower
members. At the same time, nine of the 16 data points cluster between one and 15
percent bioturbation. This range of data reflects considerable variability among the
bedding surfaces. Bioturbation is much more limited and indistinct within thin
sections and x-radiographs from upper member samples compared to those of the
middle member, although one or two distinct sub-vertical burrows are visible in
samples from the upper member. This reduction in overall bioturbation depth from
the middle to upper members likely reflects facies control. Based on the abundance
in upper member samples of echinoderm and trilobite skeletal fragments, as well as
other skeletal material of unknown affinity, metazoans were well established in the
depositional environment represented by fine-grained units in the upper member.
The presence of Psammichnites in the middle portion of the upper member
Wood Canyon Formation at Emigrant Pass is a strong indication of the extent of
292
metazoan behavioral specialization by this point in the Early Cambrian. The
presence of this large, shallow-infaunal trace fossil also indicates that the
depositional environment of the middle portion of the upper member was suitable for
infaunal burrowing, even though infaunal bioturbation appears to have been limited
in the member overall. Given the limited distribution of Psammichnites within the
Death Valley Lower Cambrian succession, the Psammichnites tracemaker likely had
a significant, if localized, impact on the substrate.
Probable specimens of Volborthella were found in a number of samples
collected from the Wood Canyon Formation. Many of these specimens are
concentrated together in randomly-oriented clumps that resemble skeletal lags and,
compared to specimens from the White-Inyo Mountains, the Wood Canyon
Formation Volborthella specimens are poorly preserved. Based on the available
evidence, it appears likely that most of the Volborthella specimens from the Wood
Canyon Formation were reworked to a greater degree than those found in the White-
Inyo Mountains samples. Given the more proximal depositional setting of the Wood
Canyon Formation, nearshore processes likely contributed to the degradation of these
fossils and to their later concentration within skeletal lags.
Despite the paucity of data from the Zabriskie Quartzite, three of the four
bedding plane data points overlap strongly with data from the Skolithos-bearing
middle member of the Wood Canyon Formation. Burrows visible within a Zabriskie
Quartzite sample appear to be larger than the Skolithos burrows in any other sample
from the Death Valley region, although compaction likely had a more pronounced
293
effect on the finer-grained and lithologically heterogeneous material of the Wood
Canyon Formation than on the pure quartzites of the Zabriskie quartzite.
Extremely abundant arthropod bioturbation, primarily consisting of
Rusophycus and Cruziana, at the base of a single thick quartzite bed in the Zabriskie
Quartzite constitutes a distinctive, taxonomically-homogeneous ichnofabric that can
be traced laterally for meters to tens of meters. Bioturbation is so concentrated within
portions of the arthropod-trace-bearing bed sole that individual traces cannot be
distinguished. The co-occurrence of abundant Skolithos and arthropod trace fossils
has not been observed in the Lower Cambrian succession of the White-Inyo
Mountains. This association may instead be restricted to the more proximal, higher-
energy facies represented in the Death Valley region. The presence of trilobite
fragments in quartzites of the middle upper member of the Wood Canyon Formation
is additional evidence that trilobites may have been active in these nearshore settings.
At the same time, trilobite-associated arthropod trace fossils are generally
uncommon in both the Death Valley and White-Inyo Mountains Lower Cambrian
successions, possibly due to issues of preservation. Rusophycus is typically
preserved as a positive hyporelief on bed soles, which are easily overlooked unless
they have been overturned in float. Thus, studying the top surfaces of in situ bedding
planes may lead to results that are biased in favor of non-arthropod trace fossils.
When examined together, bedding plane data from the Lower Cambrian of
the Death Valley region have a broad distribution but also contain a robust cluster
between 5-25 percent bioturbation. Such a distribution can be expected from a Lower
294
Cambrian bedding plane bioturbation dataset. Variable quantities of bioturbation
may be present on bedding planes due to facies-related factors such as the degree of
microbial influence on sediment consistency. Isolated occurrences of wrinkle
structures and suspected microbially-mediated sedimentary structures were observed
within the lower and upper members of the Wood Canyon Formation, although the
presence of Psammichnites within the upper member indicates that microbial mats
may not have been ubiquitous. Due to high-energy conditions, it is unlikely that
extensive microbial mats grew in the depositional environments represented by the
middle member of the Wood Canyon Formation and the Zabriskie Quartzite.
The cross plot of grid results versus field BPBI estimates reflects the
potential for inconsistency when using visual scoring to obtain field data. Forty
percent of the field and grid estimates were in agreement, but many of the field BPBI
determinations are gross over- or under-estimates. Some of the studied bedding
planes contain a combination of abundant, indistinct bioturbation and structures that
cannot be classified with confidence as either biogenic or abiogenic. At a glance,
such surfaces appear to be heavily bioturbated. However, careful scrutiny of the
studied bedding plane area during intersection grid analysis can reveal distinctions
between bioturbation and pseudo-bioturbation. Thus, the intersection method can
greatly refine field estimates of bedding plane bioturbation.
In Figure 6.1, maximum intersection grid method results from the Lower
Cambrian succession in the Death Valley region are plotted, for comparison, with
ichnofabric index results from the western United States that were compiled by
295
FIGURE 6.1 – Comparison of Lower Cambrian bedding plane bioturbation data
obtained from the Death Valley region in this study using the intersection grid
method (blue) and ichnofabric index data compiled from the western U.S. by Droser
and Bottjer (1989) (black). Ichnofabric data are averages and are separated into two
bins: pre-trilobite Lower Cambrian (PTLC) and trilobite-bearing Lower Cambrian
(TLC). The lower (lmWCF) and middle (mmWCF) members of the Wood Canyon
Formation constitute the pre-trilobite Lower Cambrian in the Death Valley region;
the upper member of the Wood Canyon Formation (umWCF), the Zabriskie
Quartzite (ZQ), and the lower portion of the Carrara Formation (not shown)
constitute the trilobite-bearing portion of the Lower Cambrian. The maximum
percentage of bedding plane bioturbation for each subdivision of the stratigraphy is
shown. Note that the two y axes are not scaled equivalently. Although ichnofabric
indices 1-5 encompass 0-100 percent bioturbation, the percentage ranges to which
the five indices correspond are of unequal size: ii 1 = 0 percent, ii 2 = 0-10 percent, ii
3 = 10-40 percent, ii 4 = 40-60 percent, and ii 5 = 60-100 percent. Figure modified
from Droser and Bottjer (1989, 1993).
296
Droser and Bottjer (1989b; 1993). As shown, the maximum percentage of bedding
plane bioturbation increases, along with the average ichnofabric index, throughout
the Lower Cambrian. This is an expected consequence of the Cambrian explosion
and the appearance of trilobites in the fossil record. The bedding plane grid data
obtained in the course of this study improve the resolution of this pattern of
increasing bioturbation, however.
Plots of the complete BPBI datasets from the Lower Cambrian successions in
the Death Valley region and the White-Inyo Mountains both resemble bell curves,
although the White-Inyo Mountains curve is slightly smoother due to larger numbers
of data in the BPBI two and BPBI four bins. Because the White-Inyo Mountains
Lower Cambrian succession contains a larger proportion of fine-grained units to
coarse-grained units than the Death Valley succession, it is likely that a considerable
number of White-Inyo Mountains bedding planes would be scored as BPBI two or
three. Based on previous work, the White-Inyo Mountains succession also appears to
contain a larger number of bedding planes that are completely covered by bedding-
parallel bioturbation (BPBIs four and five) than the Death Valley succession. This
difference likely reflects lengthier periods of slow sediment accumulation in the
deeper-water facies of the White-Inyo Mountains succession.
Infaunal bioturbation, as indicated by traces preserved in thin section and x-
radiograph, is less common, extensive, and distinct overall in the White-Inyo
Mountains samples than in the Death Valley region samples. These regional
variations are due, in part, to facies differences. Much of the White-Inyo Mountains
297
Lower Cambrian succession consists of interbedded fine-grained sandstones and
siltstones that resemble the finest-grained portions of the lower and upper members
of the Wood Canyon Formation. However, the entire Wood Canyon Formation was
deposited within a more proximal environment than the stratigraphically-equivalent
Campito and lower Poleta formations in the White-Inyo Mountains. Not only do the
fine-grained units within the Wood Canyon Formation contain common bioturbation
structures, but also traces are much more abundant in the coarser-grained, higher-
energy deposits in the formation. Vertical burrow entrances are particularly abundant
in the Death Valley succession, both in coarser- and finer-grained units, but are rare
in the White-Inyo Mountains succession except in coarse-grained quartzite units. The
relative abundance of vertical burrows throughout the Death Valley Lower Cambrian
succession likely reflects a combination of environmental suitability and nearshore
origination of adaptations to burrowing vertically within firmer substrates.
The first occurrence of abundant Skolithos burrows in the Death Valley
Lower Cambrian succession roughly coincides with the earliest-known occurrence of
trilobite body fossils in the region. Skolithos does not appear in the White-Inyo
Mountains succession until the upper Poleta Formation, although this may be due
entirely to facies control rather than to nearshore origination. Distinctive types of
bedding-parallel bioturbation seem to have appeared almost simultaneously in the
Death Valley and White-Inyo Mountains Lower Cambrian successions, indicating
that some complex behavioral patterns may not have originated preferentially in
nearshore settings. Although relatively uncommon, arthropod trace fossils first occur
298
within a few tens of meters of the Neoproterozoic-Cambrian boundary in both
successions. The similar traces Taphrhelminthopsis and Psammichnites both have
limited stratigraphic distributions and occur, respectively, in the middle siliciclastic
portion of the Poleta Formation and the middle upper member of the Wood Canyon
Formation, which are approximately correlative.
The results presented here support the hypothesis that horizontal bioturbation,
in the form of Planolites-type traces, was the primary mode of substrate engineering
in shallow subtidal siliciclastic marine environments early in the agronomic
revolution. Planolites-type traces are indeed the most abundant type of bioturbation
in the fine-grained subtidal units within the Death Valley Lower Cambrian
succession, the lower member and lower upper member of the Wood Canyon
Formation. All other marine-influenced units within the Death Valley Lower
Cambrian succession, excluding the lower Carrara Formation, likely represent
intertidal to transitional facies. Skolithos and other vertical burrows constitute the
primary type of substrate engineering in these nearshore depositional settings.
The hypothesis that bioturbation changed abruptly from bedding-parallel to
vertical in high-energy nearshore settings in the Early Cambrian can only be tested
effectively under very specific conditions. A high-energy nearshore facies that
persists across the Neoproterozoic-Cambrian boundary is required in order to
pinpoint the onset of bioturbation and track any changes in its expression through
Lower Cambrian units. Because no such facies is present in either the Death Valley
or White-Inyo Mountains succession, this hypothesis must be tested elsewhere.
299
Based on the presence of arthropod trace fossils in the Lower Cambrian
successions of the Death Valley region and the White-Inyo Mountains, trilobites
were not completely restricted to nearshore settings in the Early Cambrian. However,
the extremely high concentration of arthropod traces along a single persistent horizon
in the Zabriskie Quartzite and the well-documented occurrence of trilobite skeletal
fragments in Skolithos-bearing upper Wood Canyon Formation quartzites suggest
that trilobites (and/or other arthropods) were particularly abundant in moderate- to
high-energy nearshore depositional settings during the Early Cambrian. During
periods of slower sediment accumulation, bioturbation by arthropods may have had a
more pronounced influence on substrate consistency than the activities of vertical
burrowers.
Bioturbation in the Lower Cambrian Succession of Southern Sweden
The concentration of vertical bioturbation (Diplocraterion) in the Vik
Member of the Hardeberga Sandstone is relatively uniform across large bedding
plane surfaces. Different portions of the same bedding plane surface are commonly
weathered to different degrees, in some cases giving the illusion of a lateral change
in the percentage of bioturbation present. Little actual change across individual
bedding plane surfaces is inferred, however, based on the relatively consistent lateral
distribution of Diplocraterion burrows along horizons, including the tops of large
cross-bed sets, that are visible in vertical exposures. Aside from faintly-preserved
bilobed traces (likely Didymaulichnus) and larger burrow segments (likely heavily-
300
weathered Psammichnites), no definitive bedding-parallel trace fossils could be
identified in the Vik Member.
Samples collected from the Vik Member one-meter-thick section exhibit very
little contrast, either in scans or x-radiographs, due to the extreme purity of the
medium- to coarse-grained quartz arenite that comprises the member. Consequently,
few traces are clearly visible within the samples. Small-scale indistinct bioturbation
visible below the 58-centimeter bedding plane surface is not consistent with the
overall pattern of bioturbation in the meter and may represent a brief pause in
sediment accumulation that permitted limited biogenic reworking to a depth of only
two centimeters. The only clear example of Diplocraterion in the samples is
accentuated by dark coloration that is concentrated around the burrow. A subtle
grain-size contrast between burrow fill and surrounding matrix, further enhanced by
weathering, may have promoted the preferential accumulation of organic matter
within and around the burrow fill.
The meter-thick section analyzed within the Tobisvik Member of the
Hardeberga Sandstone at Brantevik reflects relatively low-energy deposition
succeeded by a high-energy storm event, which resulted in the deposition of the
hummocky cross-stratified and oscillation-rippled beds near the top of the meter. The
Didymaulichnus-bearing bedding plane horizon likely was exposed at the sediment-
water interface during a quiescent period between storm events.
The bedding-parallel trace fossils present within the Norretorp Formation and
Rispebjerg Sandstone are similar in size and morphological simplicity.
301
Consequently, similar behaviors may have generated these traces despite the
considerable difference in average grain size between the two bedding plane surfaces
in which they occur. The tracemakers may have been opportunists or, given the
restriction of burrows to the top of the Rispebjerg Sandstone, the appearance of this
particular tracemaker in the coarser-grained formation may signal the re-
establishment of slower sediment accumulation rates that were more characteristic of
the Norretorp Formation.
The Lower Cambrian successions in the Death Valley region and southern
Sweden were both deposited in passive margin settings and reflect similar ranges of
facies, from low-energy shallow subtidal to high-energy nearshore and non-marine.
The Vik Member of the Hardeberga Sandstone and the upper portion of the Emigrant
Pass Member of the Zabriskie Quartzite have both been interpreted as nearshore
barrier-system deposits. Both units are composed of quartzose sandstone that is
laminated to cross-bedded and contains abundant vertical burrows. Bioturbation in
the uppermost Zabriskie Quartzite consists almost exclusively of Skolithos, while
bioturbation in the Vik Member is dominated by Diplocraterion. U-shaped burrows
such as Diplocraterion, which channel water into one opening and out of the other,
likely function more effectively in slightly lower-energy settings than single-opening
vertical tubes. Consequently, Diplocraterion may be the dominant type of trace fossil
within the Vik Member of the Hardeberga Sandstone because of seasonal cyclicity in
current energy within the back-barrier tidal channels of this depositional setting. The
barrier shoal setting represented by the Emigrant Pass Member of the Zabriskie
302
Quartzite, in contrast, was likely exposed to incoming waves on a constant basis.
Such persistent high-energy conditions may have favored colonization by Skolithos
tracemakers in the upper Zabriskie Quartzite depositional setting.
Extensive Bedding Plane Exposures in the Upper Cambrian of Wisconsin
The sharply-defined “tread” and lateral ridges on many of the Climactichnites
burrows on the Krukowski Quarry surface, and the fact that earlier “generations” of
burrows had not been completely obliterated by tidal inundation, likely reflects
unusual preservational conditions, such as sediment stabilization by microbial
binding. Although no microbially-mediated sedimentary structures were observed on
this bedding plane surface, such structures are present on other bedding plane
exposures in the Krukowski Quarry and in the other local quarries. The middle to
high intertidal and lagoonal depositional setting inferred for the erosional outliers
likely would have been conducive to the establishment of microbial films and mats.
No clear patterns or clusters were observed in the Climactichnites burrow
width data despite the moderate size of the dataset and the limited range of widths
represented (3.6 centimeters). However, the continuous distribution of data within a
fairly narrow range, coupled with a lack of significant variation in the expression of
the Climactichnites traces on this surface, suggests that the Climactichnites
tracemakers active on this surface were similar to one another in size and body plan
and, therefore, may have been the same species.
303
An average bedding plane bioturbation index of five is consistent with the
density of traces on the Krukowski Quarry surface and the frequency with which
traces overlap and cross one another. Most of the grid cells are intersected by three
or four Climactichnites burrow segments. Higher numbers of burrow intersections
were recorded from cells in the lower and righthand portions of the gridded surface,
while the remainder of the grid contains uniformly lower numbers of burrow
intersections. The pattern of bioturbation on this surface likely represents repeated
visits to particular locations over a period of time rather than gregarious behavior.
An individual Climactichnites burrow commonly follows a very straight path for
several meters across the surface and then makes a smooth, arcing turn back in the
direction of its origin. Many of these turns fall within the “high burrow density”
portion of the bedding plane. This area may have been covered by a well-developed
microbial mat that would have been an attractive food source for grazing
Climactichnites tracemakers.
The two size classes of Gordia present on the Nemke Quarry surface likely
reflect the work of adult (“large”) and juvenile (“small”) Gordia tracemakers. The
“small” Gordia occur in compact patches that are scattered across the surface, but
these patches fit well into the overall distribution of Gordia traces on the bedding
plane. The distribution of width measurements for the “large” Gordia traces is
continuous from four to ten millimeters, and no significant peaks are present in the
data, indicating that these traces were likely produced by a population of
morphologically-similar tracemakers that may have belonged to the same species.
304
The Gordia present on this surface likely reflect non-systematic feeding behavior by
gregarious surface-deposit-feeding organisms.
The BPBI results for the Nemke Quarry surface, which fall below 10 percent
bioturbation, do not accurately reflect the full range of bioturbation densities present
on this surface. Areas of high burrow density together constitute a very small
proportion of the total bedding plane area. Consequently, the probability that a
randomly-selected set of coordinates will fall within one of these high-density areas
is very low. A much larger random sample than the 40 cells would be required to
achieve a satisfactory estimate of the range of bioturbation on this surface.
At this grid scale, the Gordia traces present on the Nemke Quarry surface
exhibit limited patchiness. With the exception of a small patch in one corner of the
grid, the bioturbated area of this surface constitutes a network of interconnected
bioturbation, or one large patch. This pattern would not change if the missing
bedding plane areas were restored as either bioturbated or non-bioturbated cells.
However, increasing the number of subdivisions within the grid would change the
apparent distribution of bioturbation (and increase the precision of the estimate) by
singling out more gaps between traces and between small clusters of Gordia.
Multiple discrete patches of bioturbation would likely emerge as the number of
subdivisions in the grid was increased. However, due to the three-orders-of-
magnitude difference between the width of a single Gordia trace and the length of
the studied bedding plane area, there is a practical limit to the degree of precision
that can be achieved for this surface. The metric established in Chapter II for
305
balancing precision and efficiency fails in this case. Thus, there is a tradeoff inherent
in the study of very large bedding plane exposures: the volume of information
available on a large bedding plane surface may exceed the investigator’s capacity (in
terms of time and resources) to evaluate it effectively.
Grid analysis revealed five discrete patches of bioturbation by Gordia on the
Pointe Quarry surface. A majority of the patches are made up of relatively dense
associations of Gordia rather than isolated traces. Greater consolidation of Gordia on
the Pointe surface compared to the Nemke surface is most likely the reason for the
marked difference in the pattern of bioturbation between these two surfaces despite
use of an identical grid scale on both bedding planes. Environmental factors may
have influenced the degree to which the Gordia tracemakers moved apart from one
another on the Pointe and Nemke surfaces.
More Gordia traces were measured from the Pointe surface than from the
Nemke surface, yet the Pointe dataset exhibits a narrower distribution of Gordia
widths and a smaller average width (4.96 millimeters). These differences may reflect
taxonomic differences between the two populations, although a larger dataset would
be needed to make this determination.
Synthesis
Not only do Cambrian sedimentary rocks record the dramatic expansion of
early metazoan body plans but they also capture the development of fundamental
behavioral patterns among early invertebrates, particularly soft-bodied forms that are
306
little known from the fossil record. The character and expression of these early
biogenic sedimentary structures differ greatly from those observed in post-Cambrian
rocks due to a combination of evolutionary and ecological factors unique in all of the
Phanerozoic. Abundant evidence from Cambrian rocks suggests that microbial
communities exerted greater influence on substrates in open marine settings during
the Cambrian than at any later time. Against this backdrop, early metazoans evolved
a series of adaptations that allowed them, first, to tolerate or even take advantage of
microbially-dominated substrates and then, later, to assume dominance themselves.
The agronomic revolution is perhaps the most significant example of metazoan
ecosystem engineering in the Phanerozoic because it set the ecological pattern for all
subsequent marine communities.
This study is only the second to address our limited understanding of how the
agronomic revolution unfolded. Because invaluable information on early animal-
substrate interaction is contained within Lower Cambrian bedding plane exposures,
effective means of analyzing these exposures are essential for studying Cambrian
substrate change. The intersection grid method is the first quantitative ichnological
method designed specifically for evaluating bedding-parallel bioturbation. The
ability to obtain precise estimates of percentage bedding plane area bioturbated
opens up the potential for generating and manipulating large datasets, accurately
comparing data collected from widely-separated regions by different workers, and,
ultimately, uncovering bioturbation patterns at large geographical and temporal
scales. Although future methods will likely improve upon the intersection grid
307
method, particularly in the areas of scale and efficiency, this study represents a
starting point for the quantitative study of bedding plane bioturbation.
Rarely are bedding planes exposed at sizes exceeding a few square meters.
Consequently, most analyses of bedding plane bioturbation are conducted from tiny
snapshots of much larger horizons. Data from exceptionally large exposures in the
Upper Cambrian of Wisconsin demonstrate that ancient bedding planes, like modern
intertidal zones, can exhibit considerable heterogeneity, even within the space of a
few square meters. These particular Upper Cambrian exposures reveal dissimilar
patterns of bioturbation from one bedding plane to the next and even between
bedding surfaces dominated by the same type of burrowing behavior. These results
have important implications for all studies of bedding plane bioturbation. The small
bedding plane samples analyzed in the Death Valley and Sweden portions of this
study and in other Cambrian bioturbation studies are legitimate sources of
bioturbation data, but the patterns they contain most likely are not representative of
the larger horizons of which they are part. Without the assistance of mechanical
excavation, the distribution of bioturbation across most horizons cannot be
determined. Thus, datasets that combine samples from many small bedding planes
are likely to better approximate the true range of bioturbation present within the
studied units.
Study of the Lower Cambrian successions in the Death Valley region, the
White-Inyo Mountains, and southern Sweden demonstrates that, in addition to
evolutionary and ecological factors, facies differences played an important role in
308
determining the character and distribution of bioturbation in shallow marine settings
during the agronomic revolution. Based on the abundance of Skolithos and
Diplocraterion in medium- to coarse-grained units in all three successions, vertical
bioturbation was dominant in high-energy nearshore settings during the Early
Cambrian, where it constituted the primary form of substrate engineering. Primarily
bedding-parallel bioturbation characterizes units that were deposited in lower-energy
subtidal to low intertidal environments dominated by fine-grained sediment. In these
settings, substrate engineering takes the form of horizontal, Planolites-type
bioturbation. There are exceptions to this pattern, however. Common to abundant
vertical burrows occur in proximal Death Valley-area units characterized by a
mixture of fine- and medium-grained sediment. Arthropod-type trace fossils are
highly abundant on at least one horizon within the Zabriskie Quartzite, which
represents a high-energy nearshore facies. Perhaps adaptations to deep vertical
burrowing into finer-grained material originated in nearshore settings, and trilobites
were particularly abundant in the high-energy shoreface despite being present in
shelf settings as well. The distribution of bioturbation across shallow marine
environments thus appears to be more complex than expected. Additional work will
be needed to elucidate whether these observations constitute widespread patterns in
Lower Cambrian strata.
309
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338
APPENDIX I
Locality Information
Death Valley Region
Last Chance Range: From Death Valley, take Death Valley Road north out of the
main valley. This road eventually curves westward and crosses the Last Chance
Range. Turn left onto South Eureka Road and continue until directly north of the
Eureka Dunes. Lower Cambrian stratigraphy is exposed on the northern, eastern, and
southeastern sides of the Dunes.
Titanothere Canyon: From Death Valley, take Daylight Pass Road (374) toward
Beatty, NV. Approximately 6.5 miles beyond the CA-NV border, turn left onto Titus
Canyon Road (one-way). Follow this road until approximately the following
coordinates: 36° 49' 42" N, 117° 1' 18". Park along the road and hike southeast along
the wash located on the south side of the road. The Lower Cambrian section begins
on the eastern side of the wash where the wash widens and is intersected by a
northeast-southwest-trending wash. The studied meter-thick section was measured at
36° 49' 5.6" N, 117° 0' 37.5" W. Continue west along Titus Canyon Road toward
Death Valley to exit.
Boundary Canyon: From Death Valley, take Daylight Pass Road (374) toward
Beatty, NV. Approximately one mile beyond the Daylight Pass Cutoff, park beside
the road. Hike north toward the large ridge. The Precambrian-Cambrian boundary is
located in the upper portion of the ridge flank. The top of the section has the
following coordinates: 36° 45' 9.2" N, 116° 58' 5.2" W.
Mosaic Canyon: The entrance to Mosaic Canyon is located on the southwestern
outskirts of Stovepipe Wells along 178/190. From Stovepipe Wells, turn left onto
Mosaic Canyon Road. Park in the parking area at the end of the road (canyon
entrance) and then hike back out toward the valley. Lower Cambrian rocks are
exposed and accessible on the northeast side of the road and along the northward-
facing portion of the ridge.
Echo Canyon: From the Park Service headquarters at Furnace Creek, head south on
178/190 until the two routes split. Bear left at the split (190), and continue until the
sign for Echo Canyon (2.07 miles beyond the split). Turn left onto the Echo Canyon
Road, cross the alluvial fan, and continue until the canyon widens considerably.
Meters A and B were measured, and bedding plane ECBP01 described, along the
northwestern side of the canyon, where extensive bedding planes are exposed: 36°
339
28' 52" N, 116° 44' 43.8" W. Bedding plane EC01A was described in a small side
canyon, northwest of the main canyon: 36° 29' 37.3" N, 116° 44' 38.4" W. Bedding
plane ECZ01 was described from an outcrop on the southeastern side of the main
canyon: 36° 29' 20.2" N, 116° 44' 12.9" W.
Trail Canyon: From Furnace Creek, take 178 south. Turn right on West Side
Highway and continue until Trail Canyon Road. Turn right on Trail Canyon Road
and follow this road into the canyon. Lower Cambrian rocks are exposed on the
north side of the canyon within one mile of the point at which the canyon forks and
narrows considerably near its upper end.
Montgomery Mountains (NV): From Pahrump, NV, drive north on 160 past the
ghost town of Johnnie. Turn left and drive through the gate that is located at the
following coordinates: 36° 26' 40.2" N, 116° 6' 0.4" W. Turn left on the dirt road that
crosses the main road approximately 0.2 miles after the gate. Follow this road along
the wash until it forks. Take the right fork of the wash and follow it as it curves
southward. The Lower Cambrian section is located along the top of the ridge at the
south end of the wash. Some outcrops of the Zabriskie Quartzite are located on the
eastern side of the wash. The approximate coordinates of the Precambrian-Cambrian
boundary in this section are as follows: 36° 25' 46.5" N, 116° 5' 56.9" W.
Emigrant Pass: From Baker (I-15), take 127 north to the Old Spanish Trail Highway.
Head east on the OSTH for approximately 13 miles, and then park along the road.
Lower Cambrian sections are exposed north and south-southwest of the road. The
studied meter-thick section in the Zabriskie Quartzite was measured at the following
coordinates: 35° 53' 31.8" N, 116° 4' 41.9" W. The Rusophycus-bearing bed sole was
photographed at the following coordinates: 35° 53' 32.8" N, 116° 4' 43.1" W. The
Psammichnites-bearing outcrop is located at the following coordinates: 35° 53' 22.5"
N, 116° 5' 30.6" W.
Northern Salt Spring Hills: From Baker (I-15), take 127 north. 0.18 miles after the
turnoff for Harry Wade Road (south entrance to Death Valley), turn right into the
Little Dumont Dunes OHV area. Park in the OHV area and hike east around the
south end of the dunes and up to the top of the ridge. Accessible exposures of Lower
Cambrian rocks are located along the top of the ridge and on the gentler, east-facing
slope. Bedding planes A, B, and C were described at the following coordinates,
respectively: 35° 38' 51.5" N, 116° 16' 14" W; 35° 38' 47.7" N, 116° 16' 14.8" W;
35° 38' 50.2" N, 116° 16' 8.2" W.
Winters Pass Hills: From Barstow, take I-15 east and exit at Kingston Road/Cima
Road (35° 26' 35.6" N, 115° 40' 31.2" W). Drive north on Kingston Road until it
splits (35° 36' 34.2" N, 115° 43' 55.1" W). Take the left fork (Excelsior Mine Road).
Turn right, at the following coordinates, onto the County Road 20918 cutoff: 35° 42'
340
59.4" N, 115° 48' 18.4" W. Park along the road shortly after the cutoff joins County
Road 20918. The Lower Cambrian section is on the southeast side of the road, along
the ridge. The two Skolithos-bearing bedding planes were described from an outcrop
located at the following coordinates: 35° 46' 16.3" N, 115° 43' 42.2" W.
Southern Sweden
Vik: Drive north from Simrishamn on Highway 9 for approximately 7km, and then
turn right at the sign for Vik. Follow the signs for “Prästens badkar” (vicar’s bathtub)
leading toward a parking area. Park, and walk down to the shoreline. Outcrops
continue south of this point along the shore. The studied meter-thick section was
measured at the following coordinates: 55° 36' 41.8" N, 14° 17' 53.7" E.
Brantevik: Drive south from Simrishamn on Highway 9/11 for approximately
3.25km. Follow signs for Brantevik. Once in the town, drive south of the southern
harbor and park in the public parking area. Lower Cambrian rocks are exposed for
more than 600 meters south along the shoreline. The studied meter-thick section was
measured at the following coordinates: 55° 30' 41" N, 14° 20' 55.1" E. The second
occurrence of the Didymaulichnus-bearing bedding plane surface is located at 55°
30' 29.3" N, 14° 20' 45.9" E. The Rispebjerg Sandstone and Norretorp Formation
outcrop near the following coordinates: 55° 30' 21.8" N, 14° 20' 22" E.
Wisconsin
*Note: All quarries are private
Krukowski Quarry: From Mosinee, drive south on I-39 and exit at Balsam
Road/Route 34. Head west on Balsam Road for less than a mile and turn left onto
County Road DB. At the Knowlton crossroads, turn left onto County Road C. Travel
8.1 miles east on County Road C and then turn right at the access road to the quarry.
Nemke Quarry: This quarry is located off of County Road C on a private road
approximatey two miles west of the Krukowski Quarry.
Pointe Quarry: This quarry is located approximately 0.5 miles southeast of the
Krukowski Quarry on a small dirt road that intersects County Road C.
341
APPENDIX II
Death Valley Grid Method Data
TABLE A2 – Field and intersection grid method data compiled from all studied
Lower Cambrian bedding planes in the Death Valley region. Bedding planes are
grouped by locality and color-coded by formation and/or member. The color codes
are as follows: blue = lower member Wood Canyon Formation, green = middle
member Wood Canyon Formation, yellow = upper member Wood Canyon
Formation, orange = Zabriskie Quartzite. The columns include the following
information, from left to right: bedding plane label, dimensions of grid used for
analysis, field BPBI estimate, number of grid-line intersections corresponding to
missing bedding plane area, total number of possible grid-line intersections, total
number of bioturbated intersections, hypothetical minimum percentage bioturbation,
hypothetical maximum percentage bioturbation, probable percentage bioturbation,
average of hypothetical minimum and maximum percentages, BPBI estimate based
on grid analysis, and bedding plane grain size. (See Chapter II for discussion of
calculating hypothetical minimum and maximum percentages and probable
percentage.) Bedding planes are labeled in this table using original field codes; refer
to the subsequent figures to obtain equivalent updated labels. The row for sample
MM02-8 is colored gray to indicate that this sample contains no bioturbation. The
following abbreviations are used in the grain size column: fss = fine sandstone, mss=
medium sandstone.
342
B.P. Grid Field BI # Non-BP Intersects Total Biot. Min Grid % Max Grid % Prob. % Average Grid BI GrnSize
EP01-0 70X70 3 13 4900 1192 24.33% 24.59% 24.39% 24.46% 3 mss
EP01-91s1 70X70 3 112 4900 1040 21.22% 23.51% 21.72% 22.37% 3 mss
EP01-91s2 NA 2 NA NA NA NA NA NA NA NA NA
EPRBS NA 5 NA NA NA NA NA NA 100% 5 mss
SEPP1 31X31 3 54 961 545 56.71% 62.33% 60.09% 59.52% 4 to 5 mss
MM01-18 22X88 2 16 1936 323 16.68% 17.51% 16.82% 17.10% 3 silt
MM01-100 22X88 3 5 1936 831 42.92% 43.18% 43.03% 43.05% 4 mud
MM02-8 1 fss
MM02-100 80X80 3 26 6400 3437 53.70% 54.11% 53.92% 53.91% 4 silt
MM03-65 80X80 3 NA 6400 2567 40.11% NA NA 40.11% 3 to 4 silt
MM03-74 60X60 3 40 3600 747 20.75% 21.86% 20.98% 21.31% 3 silt
MM03-78 60X60 2 NA 3600 370 10.28% NA NA 10.28% 2 to 3 silt
MM03-100 60X60 2 NA 3600 626 17.39% NA NA 17.39% 3 mud
MM04-100 42X168 2 12 7056 779 11.04% 11.21% 11.06% 11.13% 3 fss
MM05-78 60X60 2 6 3600 359 9.97% 10.14% 9.99% 10.06% 2 to 3 silt
MM05-100 40X40 2 NA 1600 386 24.13% NA NA 24.13% 3 silt
MMBP-1 60X60 2 23 3600 721 20.03% 20.67% 20.16% 20.35% 3 silt
MMBP-2 42X168 3 76 7056 2243 31.79% 32.87% 32.13% 32.33% 3 silt
EC01A 40X40 2 58 1600 165 10.31% 13.94% 10.70% 12.13% 2 to 3 silt
ECZ01 70X70 NA 79 4900 791 16.14% 17.76% 16.41% 16.95% 3 fss
ECBP01 80X80 4 NA 6400 3375 52.73% NA NA 52.73% 4 silt
ECBP02 80X80 4 NA 6400 4197 65.58% NA NA 65.58% 5 fss
ECBP03 80X80 4 27 6400 4337 67.77% 68.19% 68.05% 67.98% 5 fss
TABLE A2 – Field and intersection grid method data from the Lower Cambrian succession in the Death Valley region
343
B.P. Grid Field BI # Non-BP Intersects Total Biot. Min Grid % Max Grid % Prob. % Average Grid BI GrnSize
ECBP04 40X80 3 20 3200 780 24.38% 25.00% 24.53% 24.69% 3 silt
ECBP05 70X140 4 329 9800 1297 13.23% 16.59% 13.69% 14.91% 3 fss
ECBP06 50X100 4 123 5000 1715 34.30% 36.76% 35.17% 35.53% 3 silt
TIT2 42X168 3 177 7056 797 11.30% 13.80% 11.59% 12.55% 3 fss
WPH1 70X70 NA 102 4900 1944 39.67% 41.76% 40.52% 40.71% 3 to 4 mss
WPH2 22X88 NA 19 1936 690 35.64% 36.62% 35.99% 36.13% 3 mss
NSSH01 60X60 3 NA 3600 910 25.28% NA NA 25.28% 3 silt
NSSH02 22X88 3 77 1936 934 48.24% 52.22% 50.24% 50.23% 4 mud
NSSH03 80X80 4 555 6400 813 12.70% 21.38% 13.91% 17.04% 3 silt
NSSH04 80X80 4 68 6400 306 4.78% 5.84% 4.83% 5.31% 2 fss
NSSH05 80X80 3 76 6400 485 7.58% 8.77% 7.67% 8.17% 2 fss
NSSH06 60X60 2 313 3600 320 8.89% 17.58% 9.74% 13.24% 2 to 3 fss
NSSH07 80X80 4 6 6400 415 6.48% 6.58% 6.49% 6.53% 2 silt
NSSH08 80X80 3 14 6400 881 13.77% 13.98% 13.80% 13.88% 3 silt
NSSH09 60X60 4 19 3600 140 3.89% 4.42% 3.91% 4.15% 2 fss
TABLE A2 (continued) – Field and intersection grid method data from the Lower Cambrian succession in the Death Valley
region
344
In the subsequent figures, photographs of all studied bedding planes are
shown. In each case, the upper photograph is unaltered, and the bottom photograph
(if present) contains the intersection grid analysis. Bedding planes EPRBS and
MM02-8 were not subjected to grid analysis. Grid-line intersections marked with
yellow dots are interpreted to overlie bioturbation; those marked with green dots
correspond to points where the bedding plane surface is absent. Dimensions of the
frames used are 24x25 centimeters and 10x60 centimeters. The midpoint of each
frame axis is labeled in centimeters.
345
FIGURE A2.1 – Emigrant Pass, Meter 1, bedding plane at 0cm (EP01-0)
346
FIGURE A2.2 – Emigrant Pass, Meter, bedding plane at 91cm (EP01-91s1)
347
FIGURE A2.3 – Emigrant Pass, Rusophycus-bearing bedding plane (EPRBS)
348
FIGURE A2.4 – Emigrant Pass, Psammichnites-bearing bedding plane (SEPP1)
349
FIGURE A2.5 – Montgomery Mtns, Meter D, bedding plane at 18cm (MM01-18)
350
FIGURE A2.6 – Montgomery Mtns, Meter D, bedding plane at 100cm (MM01-100)
351
FIGURE A2.7 – Montgomery Mtns, Meter C, bedding plane at 8cm (MM02-8)
352
FIGURE A2.8 – Montgomery Mtns, Meter C, bedding plane at 100cm (MM02-100)
353
FIGURE A2.9 – Montgomery Mtns, Meter E, bedding plane at 65cm (MM03-65)
354
FIGURE A2.10 – Montgomery Mtns, Meter E, bedding plane at 74cm (MM03-74)
355
FIGURE A2.11 – Montgomery Mtns, Meter E, bedding plane at 78cm (MM03-78)
356
FIGURE A2.12 – Montgomery Mtns, Meter E, bedding plane at 100cm (MM03-
100)
357
FIGURE A2.13 – Montgomery Mtns, Meter B, bedding plane at 100cm (MM04-
100)
358
FIGURE A2.14 – Montgomery Mtns, Meter A, bedding plane at 78cm (MM05-78)
359
FIGURE A2.15 – Montgomery Mtns, Meter A, bedding plane at 100cm (MM05-
100)
360
FIGURE A2.16 – Montgomery Mtns, Outcrop, bedding plane (MMBP-1)
361
FIGURE A2.17 – Montgomery Mtns, Outcrop, bedding sole (MMBP-2)
362
FIGURE A2.18 – Echo Canyon, bedding plane EC01A
363
FIGURE A2.19 – Echo Canyon, bedding plane ECZ01
364
FIGURE A2.20 – Echo Canyon, bedding plane ECBP01
365
FIGURE A2.21 – Echo Canyon, Meter A, bedding plane (ECBP02)
366
FIGURE A2.22 – Echo Canyon, Meter B, bedding plane at 29cm (ECBP03)
367
FIGURE A2.23 – Echo Canyon, Meter B, bedding plane at 35cm (ECBP04)
368
FIGURE A2.24 – Echo Canyon, Meter B, bedding plane at 45cm (ECBP05)
369
FIGURE A2.25 – Echo Canyon, Meter B, bedding plane at 61cm (ECBP06)
370
FIGURE A2.26 – Titanothere Canyon, bedding plane TIT2
371
FIGURE A2.27 – Winters Pass Hills, lower bedding plane (WPH1)
372
FIGURE A2.28 – Winters Pass Hills, upper bedding plane (WPH2)
373
FIGURE A2.29 – Northern Salt Spring Hills, bedding plane B (NSSH01)
374
FIGURE A2.30 – Northern Salt Spring Hills, bedding plane A (NSSH02)
375
FIGURE A2.31 – Northern Salt Spring Hills, bedding plane C (NSSH03)
376
FIGURE A2.32 – Northern Salt Spring Hills, Meter, bedding plane (NSSH04)
377
FIGURE A2.33 – Northern Salt Spring Hills, bedding plane D (NSSH05)
378
FIGURE A2.34 – Northern Salt Spring Hills, bedding plane F (NSSH06)
379
FIGURE A2.35 – Northern Salt Spring Hills, bedding plane E (NSSH07)
380
FIGURE A2.36 – Northern Salt Spring Hills, bedding plane G (NSSH08)
381
FIGURE A2.37 – Northern Salt Spring Hills, bedding plane H (NSSH09)
382
APPENDIX III
Nemke Quarry, Wisconsin, Grid Method Data
TABLE A3 – Field and intersection grid method data compiled from grid cells
selected at random from the studied Nemke Quarry bedding plane. Columns in the
table include the following information, from left to right: grid cell coordinates
(photograph number in parentheses), dimensions of grid used for analysis, field
BPBI estimate, number of grid-line intersections corresponding to missing bedding
plane area, total number of possible grid-line intersections, total number of
bioturbated intersections, hypothetical minimum percentage bioturbation,
hypothetical maximum percentage bioturbation, probable percentage bioturbation,
average of hypothetical minimum and maximum percentages, and BPBI estimate
based on grid analysis. (See Chapter II for discussion of calculating hypothetical
minimum and maximum percentages and probable percentage.) Grid cells not
subjected to intersection grid analysis are indicated by gray shading across the row.
Rows of Xs in the Grid and Field BI columns indicate that the bedding plane surface
is missing from at least half of the grid cell in question.
383
Cell Grid Field BI # Non-BP Intersects Total Biot. Min Grid % Max Grid % Prob. % Average Grid BI
K8 (3409) 89X89 2 213 7921 177 2.23% 4.92% 2.30% 3.58% 2
J20 (3399) 69X69 3 90 4761 286 6.01% 7.90% 6.12% 6.95% 2
B1 (3326) 89X89 2 NA 7921 59 0.74% NA NA NA 2
T8 (3473) 89X89 2 54 7921 286 3.61% 4.29% 3.64% 3.95% 2
J28 (3403) NA 1 NA NA NA NA NA NA NA NA
C1 (3342) 96X96 2 NA 9216 50 0.54% NA NA NA 2
D10 (3346) NA 1 NA NA NA NA NA NA NA NA
N24 (3433) 89X89 2 325 7921 152 1.92% 6.02% 2.00% 3.97% 2
F4 (3359) NA 1 NA NA NA NA NA NA NA NA
T15 (3477) 89X89 2 NA 7921 27 0.34% NA NA NA 2
P24 (3449) 89X89 2 NA 7921 319 4.03% NA NA NA 2
H6 (3376) NA 1 NA NA NA NA NA NA NA NA
N30 (3436) 80X80 2 91 6400 111 1.73% 3.16% 1.76% 2.45% 2
I24 (3401) X XXXXXX XX XXXXXXXX NA NA NA NA NA NA NA NA
T6 (3472) 70X70 2 1382 4900 28 0.57% 28.78% 0.80% 14.67% 2 to 3
X10 (3506) NA 1 NA NA NA NA NA NA NA NA
A13 (3332) NA 1 NA NA NA NA NA NA NA NA
H18 (3382) 70X70 2 30 4900 359 7.33% 7.94% 7.37% 7.63% 2
E1 (3358) 85X85 1 NA 7225 19 0.26% NA NA NA 2
I10 (3394) X XXXXXX XX XXXXXXXX NA NA NA NA NA NA NA NA
K11 (3411) 85X85 2 NA 7225 407 5.63% NA NA NA 2
A12 (3331) NA 1 NA NA NA NA NA NA NA NA
TABLE A3 – Field and intersection grid method data from the studied Nemke Quarry bedding plane
384
Cell Grid Field BI # Non-BP Intersects Total Biot. Min Grid % Max Grid % Prob. % AverageGrid BI
B11 (3331) 50X50 2 NA 2500 11 0.44% NA NA NA 2
N16 (3429) 50X50 2 116 2500 84 3.36% 8.00% 3.52% 5.68% 2
I4 (3391) NA 1 NA NA NA NA NA NA NA NA
B14 (3332) 70X70 2 NA 4900 391 7.98% NA NA NA 2
H10 (3378)X XXXXXX XX XXXXXXXX NA NA NA NA NA NA NA NA
L2 (3406) 50X50 2 NA 2500 14 0.56% NA NA NA 2
A15 (3333) 50X50 3 127 2500 485 19.40% 24.48% 20.44% 21.94% 3
P5 (3440) 50X50 2 NA 2500 37 1.48% NA NA NA 2
G20 (3383) 50X50 2 211 2500 255 10.20% 18.64% 11.14% 14.42% 3
P28 (3451) NA 2 NA NA NA NA NA NA NA 1
J6 (3392) NA 1 NA NA NA NA NA NA NA NA
H3 (3375) NA 1 NA NA NA NA NA NA NA NA
A10 (3330) 50X50 2 NA 2500 106 4.24% NA NA NA 2
G11 (3379) 70X70 2 477 4900 184 3.76% 13.49% 4.16% 8.62% 2 to 3
H14 (3380) 70X70 2 2247 4900 62 1.27% 47.12% 2.34% 24.19% 2 to 4
I14 (3396) 50X50 2 389 2500 153 6.12% 21.68% 7.25% 13.90% 2 to 3
R12 (3459) NA 1 NA NA NA NA NA NA NA NA
H8 (3377) 145X145 2 3099 21025 394 1.87% 16.61% 2.20% 9.24% 2 to 3
TABLE A3 (continued) – Field and intersection grid method data from the studied Nemke Quarry bedding plane
385
In the subsequent figures, photographs of all bioturbated, randomly-selected
grid cells are shown. Non-bioturbated cells are not shown, and grid cells I24, I10,
and H10 are not shown because of insufficient bedding plane surface area. In each
figure, the upper photograph is unaltered, and the bottom photograph contains the
intersection grid analysis. Grid-line intersections marked with blue dots are
interpreted to overlie bioturbation; those marked with yellow dots correspond to
points where the bedding plane surface is absent. The corners of each 25x25
centimeter grid cell are marked in the unaltered photographs.
386
FIGURE A3.1 – Cell K8
387
FIGURE A3.2 – Cell J20
388
FIGURE A3.3 – Cell B1
389
FIGURE A3.4 – Cell T8
390
FIGURE A3.5 – Cell C1
391
FIGURE A3.6 – Cell N24
392
FIGURE A3.7 – Cell T15
393
FIGURE A3.8 – Cell P24
394
FIGURE A3.9 – Cell N30
395
FIGURE A3.10 – Cell T6
396
FIGURE A3.11 – Cell H18
397
FIGURE A3.12 – Cell E1
398
FIGURE A3.13 – Cell K11
399
FIGURE A3.14 – Cell B11
400
FIGURE A3.15 – Cell N16
401
FIGURE A3.16 – Cell B14
402
FIGURE A3.17 – Cell L2
403
FIGURE A3.18 – Cell A15
404
FIGURE A3.19 – Cell P5
405
FIGURE A3.20 – Cell G20
406
FIGURE A3.21 – Cell A10
407
FIGURE A3.22 – Cell G11
408
FIGURE A3.23 – Cell H14
409
FIGURE A3.24 – Cell I14
410
FIGURE A3.25 – Cell H8
Abstract (if available)
Abstract
Cambrian rocks record the morphological and behavioral diversification of early metazoans. Bioturbation was predominantly bedding-parallel during the Early Cambrian due to a combination of evolutionary and ecological factors. Consequently, trace fossils are typically preserved on bedding planes in Lower Cambrian siliciclastic strata. The overarching goal of this work was to gain a better understanding of the agronomic revolution as it occurred in shallow marine to transitional environments by studying the bioturbation preserved on Lower Cambrian bedding plane exposures. The objectives of this study were as follows: (1) develop a precise, quantitative method for evaluating bioturbation on bedding planes
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Asset Metadata
Creator
Marenco, Katherine Nicholson
(author)
Core Title
Bioturbation in Cambrian siliciclastic shelf strata: paleoecological, paleoenvironmental, and temporal patterns
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Geological Sciences
Publication Date
10/10/2008
Defense Date
08/21/2008
Publisher
University of Southern California
(original),
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(digital)
Tag
agronomic revolution,bedding planes,bioturbation,California,Cambrian,Death Valley,ecosystem engineering,Nevada,OAI-PMH Harvest,sweden,trace fossils,White-Inyo Mountains,Wisconsin
Place Name
California
(states),
mountains: Inyo Mountains
(geographic subject),
mountains: White Mountains
(geographic subject),
Nevada
(states),
Sweden
(countries),
valleys: Death Valley
(geographic subject),
Wisconsin
(states)
Language
English
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Electronically uploaded by the author
(provenance)
Advisor
Bottjer, David J. (
committee chair
), Corsetti, Frank A. (
committee member
), Fischer, Alfred G. (
committee member
), Gorsline, Donn S. (
committee member
), Ziebis, Wiebke (
committee member
)
Creator Email
kanichol@usc.edu,kathnich81@hotmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-m1652
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Marenco, Katherine Nicholson
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Repository Email
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Tags
agronomic revolution
bedding planes
bioturbation
Cambrian
ecosystem engineering
trace fossils
White-Inyo Mountains