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Lower Cambrian trace fossils of the White-Inyo Mountains, eastern California: Engineering an ecological revolution
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Lower Cambrian trace fossils of the White-Inyo Mountains, eastern California: Engineering an ecological revolution
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Content
LOWER CAMBRIAN TRACE FOSSILS OF THE WHITE-INYO
MOUNTAINS, EASTERN CALIFORNIA: ENGINEERING AN
ECOLOGICAL REVOLUTION
by
Katherine Nicholson Marenco
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
May 2006
Copyright 2006 Katherine Nicholson Marenco
UMI Number: 1437589
1437589
2006
Copyright 2006 by
Marenco, Katherine Nicholson
UMI Microform
Copyright
All rights reserved. This microform edition is protected against
unauthorized copying under Title 17, United States Code.
ProQuest Information and Learning Company
300 North Zeeb Road
P.O. Box 1346
Ann Arbor, MI 48106-1346
All rights reserved.
by ProQuest Information and Learning Company.
ii
ACKNOWLEDGEMENTS
I would like to thank Dr. David Bottjer for sparking my interest in Early
Cambrian bioturbation and for providing invaluable guidance and encouragement
throughout the course of this project. Thanks to Dr. Donn Gorsline for teaching me
how to properly expose, develop, scan, and interpret x-radiographs of samples and
for serving on my thesis committee. Thanks to committee member Dr. Frank Corsetti
for providing advice on many aspects of the project, including but not limited to
White-Inyo Mountain stratigraphy, field localities, photomicrography, and figure
design. I am very thankful to have had the assistance of such an expert committee.
Thanks to Jake Bailey for sharing his ideas about Volborthella and
matground community structure and for helping me better understand the
geochemical implications of a microbial-mat-dominated substrate.
Special thanks to Pedro Marenco, my once-and-future field assistant, for
carrying many a load of rocks without complaint, providing an extra pair of eyes
when the stratigraphy would not make sense, and encouraging me at all times. I look
forward to many more field seasons.
This project was made possible by grants from the Geological Society of
America, Sigma Xi, the Paleontological Society, and the USC Department of Earth
Sciences Graduate Student Research Fund. Thanks also to the Natural History
Museum of Los Angeles County for granting me access to their collection of Lower
Cambrian trace fossil specimens from the White-Inyo Mountains.
iii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS .........................................................................................ii
LIST OF FIGURES .....................................................................................................v
ABSTRACT...............................................................................................................vii
CHAPTER I .................................................................................................................1
Introduction and Previous Work ......................................................................1
Microbially Mediated Sedimentary Structures ..................................13
Wrinkle structures ..................................................................14
Elephant skin..........................................................................14
Domal structures ....................................................................14
Syneresis cracks .....................................................................14
CHAPTER II..............................................................................................................17
Geological Setting..........................................................................................17
Introduction........................................................................................17
Paleogeography and Tectonic Reconstruction of the Lower
Cambrian Succession in the Western United States ..........................20
Previous Work on the Lower Cambrian Succession in the White-
Inyo Mountains ..................................................................................21
Campito Formation ............................................................................23
Poleta Formation ................................................................................29
Harkless Formation ............................................................................32
CHAPTER III ............................................................................................................34
Bedding Plane Studies ...................................................................................34
Methods..............................................................................................34
Introduction............................................................................34
Field Methods ........................................................................34
Laboratory Methods...............................................................40
Data Analysis .........................................................................41
Results................................................................................................44
Introduction............................................................................44
Meter Sections........................................................................46
Petrography ............................................................................75
X-radiography ........................................................................80
Planolites Diameters ..............................................................85
iv
CHAPTER IV ............................................................................................................91
Piperock Studies.............................................................................................91
Introduction........................................................................................91
Background ........................................................................................91
Previous Work....................................................................................93
Methods..............................................................................................94
Results................................................................................................96
CHAPTER V............................................................................................................102
Discussion and Future Work........................................................................102
Introduction......................................................................................102
Microbial Influence on the Substrate ...............................................102
Microbial Trapping and Binding..........................................102
Characteristics of Bedding Plane Surfaces vs. Underlying
Bedding ................................................................................103
Opaque-grain-filled Structures—Volborthella?...............................105
Trace Fossils and Bioturbation ........................................................109
Dominance of Planolites......................................................109
Planolites Burrow Diameters...............................................112
Bedding-plane Bioturbation Indices ....................................114
X-radiography and Petrography—Vertical Bioturbation.....116
Implications for the Relationship between Substrate
Consistency, Microbial Activity, and Deep-burrowing
Behavior—Skolithos Piperock .............................................117
Future Work .....................................................................................120
CHAPTER VI ..........................................................................................................122
Conclusions..................................................................................................122
REFERENCES CITED............................................................................................128
APPENDIX..............................................................................................................138
Locality Information ....................................................................................138
Campito Formation Localities .........................................................138
Poleta Formation Localities .............................................................140
Harkless Formation Localities .........................................................142
v
LIST OF FIGURES
FIGURE 1 – Schematic Representation of the Agronomic Revolution ......................3
FIGURE 2 – Illustrations of Ichnofabric Indices.........................................................8
FIGURE 3 – Illustrations of Bedding Plane Bioturbation Indices.............................10
FIGURE 4 – Field Photographs of Planolites ...........................................................12
FIGURE 5 – Field Photographs of Wrinkle Structures .............................................15
FIGURE 6 – Map of Lower Cambrian Outcrops in the White-Inyo Mountains .......18
FIGURE 7 – Stratigraphy of the Lower Cambrian Succession in the White-Inyo
Mountains ......................................................................................................19
FIGURE 8 – Map of Studied Localities.....................................................................35
FIGURE 9 – Forms Used to Obtain Standardized Bedding Plane Data....................37
FIGURE 10 – Photograph of a Meter Logged at Centimeter Scale...........................39
FIGURE 11 – Photograph of Bedding Plane Containing Measured Planolites
Burrows .........................................................................................................43
FIGURE 12 – Photograph of a Bedding Plane Prepared for Image Analysis ...........45
FIGURE 13 – X-radiographs and Bedding Plane Photographs from Meter PC02....47
FIGURE 14 – X-radiographs and Bedding Plane Photographs from Meter FC01....49
FIGURE 15 – X-radiographs and Bedding Plane Photographs from Meter HT01 ...51
FIGURE 16 – Bedding Plane Bioturbation Index Data from the Campito
Formation.......................................................................................................53
FIGURE 17 – Bedding Plane Bioturbation Index Data from the Poleta Formation..54
FIGURE 18 – Bedding Plane Bioturbation Index Data from the Harkless
Formation.......................................................................................................55
vi
FIGURE 19 – Combined Bedding Plane Bioturbation Index Data ...........................56
FIGURE 20 – Photomicrograph of Fine-grained Material at a Bedding Plane
Surface in Thin Section .................................................................................77
FIGURE 21 – Photomicrograph of Cross-bedding in Thin Section ..........................78
FIGURE 22 – Photomicrograph of a Possible Horizontal Burrow in Thin Section ..79
FIGURE 23 – Photomicrographs of Opaque-grain-filled Structures in Thin
Section ...........................................................................................................81
FIGURE 24 – X-radiographs of Samples that Contain Visible Sedimentary
Structures .......................................................................................................82
FIGURE 25 – X-radiographs of Samples that Contain Visible Trace Fossils...........83
FIGURE 26 – X-radiographs of Samples that Contain Dense Conical Structures....84
FIGURE 27 – Planolites Burrow Diameter Data from the Campito Formation .......86
FIGURE 28 – Planolites Burrow Diameter Data from the Poleta Formation ...........87
FIGURE 29 – Planolites Burrow Diameter Data from the Harkless Formation .......88
FIGURE 30 – Combined Planolites Burrow Diameter Data.....................................90
FIGURE 31 – Field Photographs of Skolithos Piperock............................................92
FIGURE 32 – Photomicrographs of Skolithos Burrows in Thin Section...................97
FIGURE 33 – Scanned Image of a Thin Section in which a Skolithos Burrow is
Visible............................................................................................................98
FIGURE 34 – X-radiograph of a Sample of Skolithos Piperock .............................100
FIGURE 35 – X-radiographs of a Sample that Contains Abundant Dense
Conical Structures........................................................................................106
FIGURE 36 – Photomicrograph of Volborthella in Thin Section ...........................108
FIGURE 37 – Field Photographs of Planolites Burrows of Contrasting Size.........113
vii
ABSTRACT
The objective of this research was to gain an understanding of the early
stages of the Cambrian transition from “matground” to “mixground” substrates by
examining the Lower Cambrian succession in the White-Inyo Mountains, eastern
California. Lower Cambrian siliciclastic strata in this succession represent a range of
depositional environments and contain abundant horizontal and limited vertical
bioturbation. Although macroscopic microbially-mediated sedimentary structures are
common on bedding-plane surfaces, x-radiography and petrography of studied
samples revealed no microscopic evidence for microbial influence. Planolites, a
simple horizontal trace fossil, is the dominant type of bioturbation throughout,
although x-rays and thin sections of samples reveal the presence of rare vertical
bioturbation. Subtle patterns in bedding-plane bioturbation indices and diameters of
Planolites traces primarily reflect slight variations in environmental conditions rather
than large-scale evolutionary trends. However, an overall trend toward increasing
quantities of horizontal bioturbation up-section may reflect a combination of
environmental variation and evolutionary innovation.
1
CHAPTER I
Introduction and Previous Work
The Cambrian Period was an important time of transition in geological and
evolutionary history. Mineralized skeletons and skeletal elements, such as small
shelly fossils and sponge spicules, appeared in the earliest Cambrian and quickly
became common and diverse (e.g., Brasier and Hewitt, 1979; Brasier et al., 1997).
Biomineralizing organisms, with predator- and pressure-resistant skeletons, were
capable of occupying a greater range of niches than their soft-bodied counterparts
and therefore enjoyed a competitive advantage (e.g., Vermeij, 1989). Paralleling the
trend toward biomineralization among metazoans was the rapid diversification of
metazoan body plans known as the “Cambrian Explosion.” Metazoan body plans in
the Early Cambrian were dominantly simple and limited to a few types, whereas by
the latest Cambrian, most biological architecture considered characteristic of the
“Paleozoic Fauna” had already become established (Sepkoski, 1979).
Lower Cambrian shallow subtidal siliciclastic strata typically contain limited
vertical disruption of bedding and common microbially-mediated sedimentary
structures (e.g., wrinkle structures, Hagadorn and Bottjer, 1997), features that are
considered characteristic of Neoproterozoic rocks (Hagadorn and Bottjer, 1999).
Phanerozoic rocks tend to contain vertical bioturbation, whereas microbially-
mediated sedimentary structures typically occur in rocks deposited in restricted
environments (Hagadorn and Bottjer, 1999). The absence of vertical bioturbation
2
during the Neoproterozoic and Early Cambrian could imply that conditions beneath
the sediment-water interface were unfavorable for metazoan activity. Although
limited food resources beneath the substrate surface may have discouraged
organisms from burrowing infaunally, considerable evidence suggests that physical
and evolutionary limitations were primarily responsible for restricting benthic
organisms to epifaunal habitats (e.g., Bottjer et al., 2000).
The Neoproterozoic-Lower Cambrian abundance of microbially-mediated
sedimentary structures, sedimentary features thought to have formed in association
with seafloor microbial mats, implies that substrates of the Neoproterozoic and Early
Cambrian were bound together by microbial filaments, making them firmer and
more cohesive than those of the modern oceans (e.g., Hagadorn and Bottjer, 1997;
Gehling, 1999; Hagadorn and Bottjer, 1999). These “matgrounds” would have been
difficult or nearly impossible for benthic metazoans to penetrate, and the
combination of ubiquitous mats and a lack of infaunal bioturbation would have
generated an oxic-anoxic boundary in the substrate close to the sediment-water
interface (McIlroy and Logan, 1999). Instead, all activity took place on the surface of
the mat layer, within the mat, or immediately beneath it. Seilacher (1999) proposed
four guilds or “lifestyles” to characterize the range of activities that took place in late
Neoproterozoic subtidal benthic communities (Fig. 1). These are “mat encrusters,”
sessile organisms that lived permanently attached to the mat surface; “mat
scratchers,” which scavenged or hunted for food on the surface of the mat without
damaging the mat; “mat stickers,” suspension feeders that used conical shells to
FIGURE 1—Schematic representation of the agronomic revolution, in which
microbially-bound “matgrounds” were replaced by well-hydrated “mixgrounds” as a
result of increasingly-deep bioturbation by animals. Modified from Seilacher (1999).
3
4
maintain an upright orientation in the surface of the mat; and “undermat miners,”
which 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 non-restricted environments along with the microbial
mats themselves (Dornbos and Bottjer, 2001). Mat scratchers and undermat miners
were better equipped than the other two guilds for adjusting to the new substrate
conditions because their mobile lifestyles allowed them to reposition themselves in
response to changes in oxygen and substrate consistency (Bottjer et al., 2000).
However, their reliance on a consolidated substrate surface for locomotion and
feeding, through scraping, 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 an even greater problem
because of their specialized sessile lifestyles. Lacking a means of migrating to more
suitable environments, these organisms developed stems or direct attachment
mechanisms that allowed them to utilize the limited hard substrates that were
available in subtidal settings (Bottjer et al., 2000). Not all such groups were
successful, however. The mat-sticking helicoplacoid echinoderms, for example,
failed to adapt to the new substrate conditions and became extinct before the end of
the Cambrian (Bottjer et al., 2000; Dornbos and Bottjer, 2001).
How benthic metazoans overcame the barrier to infaunalization presented by
microbial mat-bound substrates remains unclear. Seilacher and Pflüger (1994)
proposed a scenario in which benthic metazoans acquired evolutionary adaptations
5
during the Cambrian explosion that allowed them to burrow vertically through
matgrounds. Bioturbation depth and intensity increased, disrupting the layered
structure of the microbial mats and increasing the hydration and oxygenation of the
substrate, which in turn made the substrate more suitable for colonization at greater
depths. This transition from two-dimensional lifestyles dictated by dominant
microbial mats to three-dimensional exploitation of the infaunal ecospace has been
termed the “agronomic revolution” (Seilacher and Pflüger, 1994) (Fig. 1). In the
wake of the agronomic revolution, mat development was relegated to marginal
environments, and the substrate took on characteristics more typical of the
Phanerozoic, notably improved nutrient distribution and an indistinct sediment-water
interface. Organisms unsuited for the new substrate conditions either adapted or
became extinct (e.g., helicoplacoid echinoderms, Dornbos and Bottjer, 2001). These
and other 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).
The agronomic and Cambrian substrate revolutions represent the earliest-
known instance of metazoan ecosystem engineering in the history of life. Ecosystem
engineering is habitat modification that results from the activities of organisms
(Jones et al., 1994). With increased depth and intensity of bioturbation, benthic
metazoans engineered a dramatic change in shallow subtidal substrates of the Early
Cambrian, making available a variety of previously-inaccessible resources and
6
ecological niches. Evolutionary adaptations made this change possible, and the
change itself resulted in the potential for developing a new category of adaptations.
Understanding the dynamics of ecosystem engineering in the Early Cambrian
is difficult due to the limitations of the Lower Cambrian body fossil record.
Mineralized skeletons had not yet become common among benthic metazoans in the
Early Cambrian, the result being a preservational bias toward mineralized groups in
most deposits, with the exception of Lagerstätten such as the Chengjiang Biota in
southern China. A wide variety of soft-bodied fossils have been described from the
Chengjiang, suggesting that unmineralized metazoans constituted a substantial
component of Early Cambrian benthic communities. Because relatively few Lower
Cambrian Lagerstätten have been recognized around the world, ichnofossils provide
the best source of information on benthic community composition in most Lower
Cambrian deposits.
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.
Trace fossils are most commonly studied through examination of bedding-plane
exposures, upon which cross cutting relationships and trace ornamentation are often
easy to observe. This is also the most straightforward way of noting the distribution
of microbially-mediated sedimentary structures and their relationship to trace fossils.
However, collecting such data from bedding planes alone cannot result in a complete
understanding of the degree and diversity of tracemaker activity, as well as microbial
7
mat distribution, within a given stratigraphic horizon. Instead, a method that provides
a sense of trace orientation in three dimensions is a necessary supplement to bedding
plane observations. Views perpendicular to bedding can be obtained at the outcrop, if
levels of weathering and diagenesis are minimal, or by cutting and polishing rocks in
hand sample. However, some traces are either too small or too indistinct to be
recognized and identified using this method, especially when the trace fill resembles
the surrounding sediment. Petrography is useful for identifying the mineralogical
components of trace fill and for recognizing mat-associated features, such as
concentrations of heavy mineral grains and the alignment of elongated grains such as
mica (Schieber, 1999). However, when larger hand samples and traces are involved,
the field of view becomes too small to be useful for recognizing trace fossil patterns.
X-radiography, on the other hand, provides a sense of depth (three-dimensionality)
and density contrast at the macroscopic level.
The Lower Cambrian has been examined extensively for its trace fossil
content (Alpert, 1974). Studies of Lower Cambrian trace fossils (e.g., Alpert, 1974;
Langille, 1974) have focused primarily on assessing ichnofossil diversity by
documenting as many different kinds of traces as possible. Such contributions are
important, but they do not necessarily provide accurate reflections of the
composition of the benthic community. Taking a different approach, Droser and
Bottjer (1986) developed ichnofabric indices as a semi-quantitative method of
estimating the amount of disruption by vertical bioturbation in outcrops (Fig. 2). This
method is useful for field scoring and for generating reproducible assessments of
FIGURE 2—Illustrations of representative ichnofabrics for the five ichnofabric
indices, showing increasing disruption of bedding. Modified from Droser and Bottjer
(1986).
8
9
bioturbation intensity. However, bioturbation in Lower Cambrian siliciclastic rocks
is primarily horizontal and, consequently, virtually imperceptible in outcrop view.
Thus, ichnofabric indices are not an effective method of assessing bioturbation
intensity in Lower Cambrian rocks. Recognizing this problem, Miller and Smail
(1997) developed bedding-plane bioturbation indices to perform nearly the same
function as ichnofabric indices but in two-dimensional bedding-plane view,
specifically in siliciclastic rocks (Fig. 3). This method was employed extensively
during the project to make accurate estimations of the quantity of bioturbation
present on each bedding plane that was sampled. BPBIs 1-5 correspond to increasing
levels of bioturbation intensity, with 1 indicating undisturbed bedding and 5 the
complete disruption of bedding by organisms.
Macroscopic microbially mediated sedimentary structures have been reported
from the Lower Cambrian of the White-Inyo Mountains (Hagadorn and Bottjer,
1997) and elsewhere (e.g., Noffke et al., 1996). Hagadorn and Bottjer (1997) used
petrography, in addition to field observations, to identify microscopic-scale evidence
for microbial mats in Lower Cambrian rocks. According to Schieber (1999), such
evidence includes wavy-crinkly laminations, cohesive behavior of laminations,
concentrations of heavy mineral grains such as pyrite, and concentrations of aligned
platy minerals such as mica. Noffke et al. (2001) also recognized the value of
petrography for recognizing mat-associated features and for identifying the
mineralogical components of trace fill. Others have used x-rays, of vertically
sectioned slabs only (Droser et al., 1999) or of both vertically and horizontally
FIGURE 3—Illustrated key for use in determining bedding-plane bioturbation
indices in the field. Scale at left contains examples of different sizes of trace fossils
that are distributed evenly; scale at right shows uneven distributions of different-
sized traces. Modified from Miller and Smail (1997).
10
11
sectioned slabs (Britt et al., 1992), to obtain information concerning ichnofabric and
bedding plane bioturbation.
Although, as discussed above, much work has addressed both ichnofossil
diversity and the challenges of estimating bioturbation intensity in Lower Cambrian
rocks, few studies have focused on the broader ichnofossil-sediment record in the
context of the agronomic and Cambrian substrate revolutions. This project 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 substrate engineers
of the Early Cambrian. Results from fine-grained units indicate that soft-bodied
worm-like organisms, represented by the horizontal trace fossil Planolites (Fig. 4),
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 fine-grained bedding planes reflect 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. The common occurrence in these fine-grained
units of the small, enigmatic Cambrian fossil Volborthella, interpreted to be the
skeleton of a matground-adapted animal (e.g., Seilacher, 1999; Bailey et al., 2006),
also supports the notion that microbial activity was the dominant factor influencing
substrate conditions in these environments. In contrast, analysis of coarser-grained
FIGURE 4—Field photographs of Planolites traces on bedding plane surfaces.
(Left) Bedding surface from the Poleta Formation, view is 24cm wide. (Right)
Bedding surface from the Campito Formation, view is 22cm wide.
12
13
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 environments.
Microbially Mediated Sedimentary Structures
A variety of microbially mediated structures are found preserved on bedding
planes in the interbedded siltstones and quartzites of the Campito, Poleta, and
Harkless Formations. These structures are thought to represent the effects of
sediment binding by microbial mats (Hagadorn and Bottjer, 1997; 1999; Pruss and
Bottjer, 2004) or “cohesive 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 features preserved in Lower Cambrian rocks, such as “wrinkle
structures” (Hagadorn and Bottjer, 1997).
Despite the seemingly low odds that impressions of non-mineralized
microbial mats and mat-induced structures would be preserved in rocks as old as the
Lower Cambrian, a considerable variety of such features appear to have survived
diagenetic and deformational processes. The following is a selection of microbially
14
mediated structures that are found preserved in the Lower Cambrian of the White-
Inyo Mountains:
Wrinkle structures (Fig. 5): Hagadorn and Bottjer (1997) consider “wrinkle
structures” to be a general category of features that includes runzelmarken, Kinneyia
ripples, micro-ripples, and elephant skin. Although these features differ noticeably,
all are “characterized by oddly contorted, wrinkled, irregularly pustulose, quasi-
polygonal, commonly oversteepened surface[s]” (Hagadorn and Bottjer, 1999).
Wrinkle structures are thought to be original impressions of the mat surface,
preserved due to excessive mat thickness and microbial sediment binding rather than
loading (Hagadorn and Bottjer, 1999).
Elephant skin: Gehling (1999) classifies elephant skin as a “sole-face
bedding-surface structure” and describes the surface as a delicate pattern of grooves
that resembles “patterns in animal skin.” These surfaces are associated with the
presence of intact Ediacaran fossils and, therefore, do not appear to have been
produced through sediment loading (Gehling, 1999).
Domal structures: Schieber (1999) describes domal structures as “low-
amplitude hummocks or domes” or “small hemispherical features,” which appear to
be rare in terrigenous clastics. These may represent competition within a microbial
mat for height above the substrate (Garlick, 1988) or pockets of gas trapped under
the mat surface (Noffke et al., 2001).
Syneresis cracks: Pflüger (1999) describes “spindle-shaped” syneresis cracks
from the Silurian of southwest Libya and hypothesizes that they represent shrinkage
FIGURE 5—Wrinkle structures. (Top) Field photograph of wrinkle structures on a
Harkless Formation bedding plane. (Bottom) Close-up view of wrinkle structures on
a sample from the Poleta Formation.
15
16
on a flat, continuous, microbially bound sediment surface. Organic matter with a
high water content is thought to comprise up to 75% of the sediment volume of
microbial mats, giving mats the potential for significant shrinkage (Wachendörfer et
al., 1994).
17
CHAPTER II
Geological Setting
Introduction
The Lower Cambrian succession in the White-Inyo Mountains, eastern
California, is approximately 1500 meters in thickness (Corsetti and Hagadorn, 2003)
and is comprised of the Upper Member of the Deep Spring Formation and the
Campito, Poleta, Harkless, Saline Valley, and Mule Spring formations (Nelson,
1962) (Fig. 6, 7). This succession represents alternating terrigenous clastic and
carbonate deposition in a shallow subtidal shelf setting (Stewart, 1970), which
experienced periodic minor fluctuations in sea level (Mount, 1980). The Campito,
Poleta, and Harkless formations are the focus of this project and will be discussed in
some detail below.
The Campito and Harkless Formations are predominantly comprised of
interbedded greenish micaceous siltstones and cross-bedded quartzites. The Poleta
Formation consists of two carbonate units that bracket a unit of interbedded
micaceous siltstones and quartzites (Corsetti and Hagadorn, 2003). Fine pyrite grains
and abundant microbially mediated structures and trace fossils can be found along
bedding planes in the interbedded siltstones and fine-grained quartzites of the
Campito, Poleta and Harkless Formations. Thicker layers of quartzite in the Poleta
and Harkless Formations contain the vertical burrow Skolithos. Microfossils,
including the enigmatic cone-shaped fossil Volborthella, have been reported from
FIGURE 6—Map showing the extent of Lower Cambrian outcrops (gray shading) in
the White-Inyo Mountains. Studied localities are indicated by dots; more information
on these localities can be found in the Appendix. Map after Mount (1982), Nelson
(1966), and Stewart (1970).
18
FIGURE 7—Generalized stratigraphy of the Lower Cambrian succession in the
White-Inyo Mountains, eastern California. Montenegro Member of Campito
Formation abbreviated as “Mont.” After Hagadorn et al. (2000a) with data from
Mount (1982) and Moore (1976).
19
20
both siliciclastic and carbonate units throughout the section (e.g. Lipps and
Sylvester, 1968).
Paleogeography and Tectonic Reconstruction of the Lower Cambrian Succession in
the Western United States
The Lower Cambrian succession in the White-Inyo Mountains was deposited
within the western portion of the Cordilleran miogeocline (Mount, 1982), 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
21
terrigenous clastic sequence along the continental margin, until the continental crust
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). The paleocoastline was
oriented roughly northeast-southwest at this time (McKerrow et al., 1992). Using
modern climate patterns, Rowland (1978) suggested that western North America in
the Early Cambrian would have had tropical environmental conditions with a narrow
range of annual temperatures.
Previous Work on the Lower Cambrian Succession in the White-Inyo Mountains
Walcott (1908) was the first to describe the Lower Cambrian succession in
the White-Inyo Mountains and begin to work out its stratigraphy. An exposure near
Waucoba Spring in Saline Valley was used as the basis for Walcott’s (1908) Silver
Peak Group, which included the uppermost portion of the Montenegro Member of
the Campito Formation through the top of the Mule Spring Formation (Nelson,
1962). Walcott (1912) chose the Waucoba Spring locality to be the Lower Cambrian
type section (Waucoban Series) for North America. Kirk (1918) continued Walcott’s
22
work and described the strata below the Silver Peak Group. Nelson (1962) completed
description of the Lower Cambrian succession in the White-Inyo Mountains and
replaced the pre-existing stratigraphic names with those that are currently in use.
Considerable work has been devoted to the sedimentology and stratigraphy of
the Lower Cambrian White-Inyo Mountain succession, although much of it exists in
the form of unpublished theses. Stewart (1970) was the first to develop a general
model for the deposition of the Lower Cambrian succession in the Great Basin based
on regional lithological trends. The Campito Formation has been investigated in
detail by Mount (1980; 1982) and by Gevirtzman and Mount (1986). In addition,
Robigou (1984) studied the metamorphic petrology of the Campito Formation. The
Poleta Formation has been examined in its entirety by Moore (1976), and in part
(Lower Member) by Rowland (1978). Schmidt (1977) documented the
sedimentology, stratigraphy, and petrology of the Quartz Arenite Member of the
Harkless Formation. Crews (1980) examined the Saline Valley Formation as well as
Harkless Formation-equivalent strata in the Death Valley succession.
Paleontological data has been documented for the Lower Cambrian of the
White-Inyo Mountains by a number of workers. Nelson (1976; 1978) summarized
the stratigraphic paleontology of the White-Inyo Mountains. Trilobite distributions
have been summarized by Palmer (1971; 1977), Nelson (1976) and others. The
paleoecology and stratigraphic distribution of archaeocyathids has been examined by
McKee and Gangloff (1969), Gangloff (1975; 1976), Fuller (1976), Morgan (1976),
and Rowland (1984). The distribution of brachiopods has been summarized by
23
Rowell (1977). Trace fossils in the White-Inyo Lower Cambrian succession have
been investigated by Alpert (e.g., 1976), Langille (1974), Hagadorn and colleagues
(2000b), and Wiggett (1975). Vertical bioturbation intensity and its impact on the
substrate, termed ichnofabric, has been evaluated primarily in carbonate rocks of the
White-Inyo Lower Cambrian section by Droser and Bottjer (1986). Miller and Smail
(1997) analyzed bedding-plane bioturbation intensity in the siliciclastic units of the
White-Inyo Lower Cambrian section. Macroscopic microbially mediated
sedimentary structures were first documented from the Lower Cambrian of the
White-Inyo Mountains by Hagadorn and Bottjer (1997).
Campito Formation
The Campito Formation was named by Kirk (1918) for exposures near
Campito Mountain in Mono County, California. Later work by Nelson (1962)
included subdividing the Campito Formation into two members, the lower Andrews
Mountain Member and the upper Montenegro Member. The Early Cambrian age of
the Campito Formation is corroborated by trilobite biostratigraphy. The base of the
Fallotaspis trilobite zone occurs in the middle of the Andrews Mountain Member,
and the Montenegro Member contains the conclusion of the Fallotaspis and the
lower half of the Nevadella zones (Fritz, 1972; Nelson, 1976).
An erosional surface, interpreted by Mount (1980) to be the result of
subaerial exposure, marks the contact between the uppermost Deep Spring
Formation and the base of the Andrews Mountain Member of the Campito
24
Formation. The Andrews Mountain Member is approximately 760-850m thick in the
White-Inyo Mountains (Nelson, 1962) and consists of interbedded feldspathic
quartzites, silty quartzites, and mudstones, all dark in color and representing
deposition in subtidal settings (Mount, 1982). Mount (1980; 1982) described two
distinctive facies within the Andrews Mountain Member based on overall lithology
and bedding features. These two facies, the sand-dominated and heterolithic facies,
were further subdivided into subfacies according to sedimentary structures and
textures, vertical lithological sequences, and lateral continuity.
The sand-dominated facies can readily be recognized by high sand content
(greater than 90%) and thick bedding (Mount, 1982). This facies consists of
consolidated units that occur in both laterally-continuous “sand sheets” and
discontinuous tabular bodies (Mount, 1982). The sand-dominated facies can be
divided into three subfacies: the thick-bedded, hummocky cross-stratified subfacies
(S1), the cross-bedded subfacies (S2), and the parallel-laminated subfacies (S3)
(Mount, 1982).
The heterolithic facies consists of resistant, “complexly interbedded” fine-
grained quartzites and chloritic siltstones and mudstones and makes up a more
significant portion of the volume of the Andrews Mountain Member than the sand-
dominated facies (Mount, 1982). Bedding is much thinner in the heterolithic facies,
and the sand content is below 90% (Mount, 1982). Mount (1982) identified two
subfacies within the heterolithic facies: the medium-bedded H1 and thin-bedded H2
subfacies, both of which are characterized by hummocky cross-stratification.
25
Mount (1982) grouped the S1, H1, and H2 subfacies together because they
grade into one another and collectively reflect lateral variations in deposition during
a storm event. This subfacies group is characterized by massive silty quartzites and
siltstones that alternate with laminated or hummocky cross-stratified fine sands
(Mount, 1982). The S1 and H1 subfacies are thicker sand units with erosional bases
that represent high-energy depositional events generated by strong, scouring currents
that eroded lower-energy deposits (Mount, 1982). The H2 subfacies represents
deposition during similar, but lower-energy, erosive events (Mount, 1982). Mount
(1982) reported abundant trace fossils within the S1-H1-H2 subfacies grouping, with
Planolites, Cruziana, and Rusophycus being the dominant forms. The presence of
this trace fossil assemblage, in addition to rare olenellid trilobite occurrences,
indicates that the Andrews Mountain Member was deposited in a shallow marine
environment (Mount, 1982). In addition, high concentrations of trace fossils
generally occur on upper bedding surfaces of the S1-H1-H2 subfacies grouping, in
some cases with ripple marks. Well-established epi- and shallow-infaunal benthic
communities reflect a stable substrate, and rippled bedding surfaces indicate a return
to fair-weather current patterns (Mount, 1982).
The S2 and S3 subfacies are found in between occurrences of the S1-H1-H2
subfacies grouping. Both subfacies are thought to represent offshore shoals from
which material was eroded, transported, and re-deposited by strong storm currents
(Mount, 1982). The S2 subfacies is well-sorted, homogeneous, discontinuous, and
lacking in trace fossils, reflecting a high-energy environment in which nearly-
26
continuous sand transport precluded the establishment of a benthic community
(Mount, 1982). The S3 subfacies is more complex, consisting not only of broad,
high-energy shoal deposits but also of thin intervals of fine-grained material
(siltstones to mudstones) that locally contain ripple marks and abundant trace fossils
and have been interpreted to represent sheltered, well-oxygenated intershoal areas in
which benthic communities could become established (Mount, 1982).
The upper portion of the Campito Formation, the Montenegro Member, is
approximately 350m thick (Mount, 1982) and consists of light-colored siltstones and
mudstones with local archaeocyathid “reefs” (Rowland, 1984) that occur in
uppermost portion of the member (Mount, 1980). The contact between this member
and the Andrews Mountain Member is gradational, and in places the Montenegro
Member appears to be absent (Mount, 1980). Similarly, a gradational contact
between the uppermost Montenegro Member and the overlying Poleta Formation has
been observed (Mount, 1980) and has been interpreted as a time-transgressive
contact (Rowland, 1978). According to the work of Mount (1980), the Montenegro
Member is dominated by a mud-dominated facies and an additional heterolithic
subfacies, the tidal subfacies (H3), which together represent shallow-water through
locally-exposed muddy shelf environments that formed shoreward of Andrews
Mountain Member deposition. An additional facies, the archaeocyathid bioherm
facies (Ar), constitutes a minor component of the Montenegro Member lithology
(Mount, 1980). According to Mount (1980), the shift from storm-dominated
deposition in the Andrews Mountain Member to current-dominated deposition in the
27
Montenegro Member resulted from the formation of an offshore barrier, which
altered the circulation patterns and bathymetry of the shelf and made the
environment more suitable for benthic life.
The mud-dominated facies of the Montenegro Member, named for its muddy
appearance in outcrop, consists of thickly-bedded siltstones, in association with
lenticular quartzites and micaceous siltstones, and rare calcarenite beds (Mount,
1980). The units within this facies are less than 50% sand and commonly are more
thinly-bedded and lighter in color than the units of the Andrews Mountain Member
(Mount, 1980). Mount (1980) described three subfacies within the mud-dominated
facies: the lenticular-bedded subfacies (M1), the fossiliferous mudstone subfacies
(M2), and the quartz siltstone subfacies (M3). The M1 subfacies is interpreted to
represent deposition of muds and silts in a relatively calm subtidal setting that
harbored a low-diversity benthic community represented by uncommon occurrences
of Planolites traces (Mount, 1980). The M2 subfacies is well-bioturbated and
fossiliferous, and bedding surfaces contain localized patches of wrinkle structures
(“runzelmarks”), possible mudcracks, and interference ripples (Mount, 1980). Trace
fossils include Skolithos and Monocraterion, and body fossils include trilobites,
brachiopods, “salterellids,” and hyolithids (Mount, 1980). Mount (1980) interpreted
the depositional environment of the M2 subfacies as a low-energy subtidal to
intertidal setting. The M3 subfacies is interpreted to represent a low-energy tidally-
dominated setting with variably-diverse benthic communities (Mount, 1980).
28
The heterolithic tidal subfacies (H3) consists of interbedded fine-grained
quartzites, silty quartzites, siltstones to mudstones, and channelized quartz arenites in
which a variety of sedimentary structures are preserved, including rare wrinkle
structures and mudcracks (Mount, 1980). The trace fossils Skolithos and
Monocraterion occur within the fine-grained and silty quartzites of this subfacies
(Mount, 1980). Mount (1980) interpreted this subfacies to represent deposition in a
range of subtidal to intertidal environments.
The archaeocyathid bioherm facies (Ar) consists of carbonate mounds that
vary from 1-6m in thickness and up to 30m in diameter, in which archaeocyathids
and Renalcis are poorly preserved (Mount, 1980). Where this facies occurs, the
carbonate mounds are bounded by the M2 and M3 subfacies, indicating that the
mounds formed in a shallow-water, low-energy, tidally-dominated setting. Mount
(1980) supported the conclusion made by Fuller (1976) that the carbonate mounds of
the archaeocyathid bioherm facies represent wave-resistant archaeocyathid
boundstones.
Regional greenschist-facies metamorphism is reflected by the rocks of the
Campito Formation (Mount, 1982). In addition, the Campito lithologies have been
altered by multiple stages of diagenesis (Mount, 1980). According to Mount (1980),
initial amounts of feldspar and ferromagnesium silicates have been reduced
significantly due to intrastratal solution and phyllosilicate replacement. Silica
cementation occurred early in diagenesis, and considerable recrystallization of
various mineral phases followed. The heavy mineral assemblage in Campito
29
Formation rocks reflects post-depositional mobilization of iron ions from original
unstable iron-bearing minerals (Mount, 1980).
Poleta Formation
Named by Nelson (1962) for exposures in Poleta Canyon in the western
White Mountains, the Poleta Formation was originally divided into two members, a
lower limestone member and an upper member comprised of siltstone, sandstone,
and limestone (Nelson, 1966). Later, McKee and Moiola (1962) and Stewart (1970)
subdivided the upper member into a siltstone and sandstone Middle Member and a
limestone Upper Member, divisions which remain in use. The Poleta Formation
varies in thickness from approximately 180m to 330m in the vicinity of the White-
Inyo Mountains (Moore, 1976). In contrast to the Campito Formation, the Poleta
Formation contains abundant archaeocyathids (McKee and Gangloff, 1969) and
trilobites (Nelson, 1975) as well as distinctive helicoplacoid echinoderms (originally
reported by Durham and Caster, 1963). The boundary between the Nevadella and
Bonnia-Olenellus trilobite zones occurs within the Middle Member of the Poleta
Formation (Nelson, 1975).
The Lower Member of the Poleta Formation is approximately 140m thick in
the White-Inyo Mountains, is limestone-dominated, and contains abundant
archaeocyathids (Moore, 1976). The Lower Member is comprised of five major
lithologies: archaeocyathid-bearing bioclastic limestone, archaeocyathid-bearing
biohermal limestone, oolitic limestone, bioclastic-oolitic limestone, and shale
30
(Moore, 1976). A range of carbonate-bank depositional environments, from back-
shoal to offshore marine, are represented by these Lower Member lithologies
(Moore, 1976). Moore (1976) showed that the major lithofacies of the Lower
Member of the Poleta Formation are distributed geographically according to their
proximity to the shoreline, from “more shoreward” in the southeast to “more
offshore” in the northwest. In the White-Inyo Mountains, the Lower Member
represents deposition in a carbonate bank-margin to subtidal open-marine
environment and consists primarily of bioclastic-oolitic limestone that contains
isolated “wedges” of shale (Moore, 1976). Bioclasts are dominantly composed of
archaeocyathid fragments with a minor component of pellets. Vertical to subvertical
tubes are common in the primarily-oolitic limestones, vary in diameter from 1mm to
2cm, and bear either sharply-defined or diffuse boundaries (Moore, 1976). In the
shale wedges, trilobite fragments and molds of brachiopods are common, reflecting
deposition in a shallow subtidal environment (Moore, 1976). In addition, thin
limestone lenses containing pebble-sized limestone and siltstone intraclasts occur
within the shale, indicating that strong currents occurred locally (Moore, 1976).
The Middle Member of the Poleta Formation is approximately 180m thick in
the White-Inyo Mountains and reflects terrigenous deposition following the
significant interval of carbonate-bank deposition represented by the Lower Member
(Moore, 1976). Helicoplacoid echinoderms occur within the Middle Member, as do
trilobites, brachiopods, and abundant trace fossils (Moore, 1976). The Middle
Member represents deposition in environments that range from tidally-dominated to
31
subtidal open marine (Moore, 1976). As in the Lower Member, these environments
are distributed along an onshore-offshore gradient from southeast to northwest.
Sandstone is a more significant component in the upper portion of the member, while
siltstone and shale dominate the lower portion (Moore, 1976). In the White-Inyo
Mountains, two thirds of the Middle Member’s thickness is comprised of the lower
siltstone unit, which consists of dark-colored siltstones and shales with thin
sandstone and limestone interbeds (Moore, 1976). The remaining third is comprised
of the lower sandstone-siltstone unit, the middle limestone-siltstone unit, and the
upper sandstone unit, which together represent a combination of changing
environmental conditions within the subtidal-to-foreshore setting and increasing sand
influx (Moore, 1976). Skolithos is abundant in the sandy upper portion of the upper
sandstone unit, and Planolites, Cruziana, and Rusophycus are common in siltier
lithologies (Moore, 1976).
The Upper Member of the Poleta Formation represents a carbonate-bank
depositional system similar to that of the Lower Member except that the former was
more restricted (Moore, 1976). The Upper Member is absent from the southern
White-Inyo Mountains, but to the north the member consists of five distinct
lithofacies representing deposition in the following environments: terrigenous-to-
carbonate transition (limestones, limestone breccia, interbedded siltstones), shoaling
bank (oolitic-bioclastic trough cross-laminated limestone), back-bank to shoal
transition (mottled oolitic-bioclastic limestone), back-bank (mottled micritic-pelletal
limestone), and bank exposure (massive crystalline limestone) (Moore, 1976).
32
Following carbonate bank exposure, another pulse of fine-grained terrigenous
material occurred, which is represented in the base of the Harkless Formation
(Moore, 1976).
Harkless Formation
The Harkless Formation was named by Nelson (1962) for outcrops near
Harkless Flat in the southern portion of the White-Inyo Mountains, where it is
approximately 600m thick. Trilobite body fossils have substantiated a Lower
Cambrian age for the formation (Nelson and Durham, 1966). Also present to locally
abundant are brachiopods and archaeocyathids (McKee and Gangloff, 1969) and the
enigmatic conical fossils Salterella (Stewart, 1970) and Volborthella (Lipps and
Sylvester, 1968). The dominant lithologies in this formation are siltstone, shale, and
fine-grained sandstone, with minor amounts of limestone (Stewart, 1970). In the area
of the type section, the Harkless Formation consists of greenish-gray siltstone and
isolated intervals of tan to gray fine-grained quartzite (Stewart, 1970). The quartzite
component diminishes toward the northwest but becomes more significant toward
the southeast (Stewart, 1970).
The siltstones of the Harkless Formation are generally thinly-laminated and
metamorphosed to phyllite or hornfels, in which muscovite, chlorite, biotite, and
quartz are the dominant minerals (Stewart, 1970). Trace fossils, such as Planolites
and Cruziana, are common in the siltstones (Stewart, 1970; Schmidt, 1977), and the
conical enigmatic fossil Salterella is also present, most commonly in the upper
33
siltstones of the formation (Stewart, 1970; Schmidt, 1977). Moore (1976) interpreted
the depositional environment of the lower Harkless Formation as a shallow-subtidal
to tidal-flat setting based on the presence of irregular interference ripple marks,
mudcracks, and wrinkle structures in the siltstones. Schmidt (1977) likened this
portion of the formation to the Middle Member of the Poleta Formation. Both Moore
(1976) and Schmidt (1977) interpreted the upper siltstone of the Harkless Formation
to be representative of deposition in a slightly deeper-water subtidal setting.
A number of marker beds, which consist predominantly of fine-grained
sandstones and represent tidal bars and channels, are present in Harkless Formation
outcrops in the Poleta Folds area of the White-Inyo Mountains (Moore, 1976). A few
of these marker beds, near the base of the formation, consist of archaeocyathid- and
ooid-bearing limestones. In the thicker quartzite units of the Harkless Formation,
notably at the section near Andrews Mountain, Skolithos burrows are common to
abundant (Stewart, 1970; Schmidt, 1977). Stewart (1970) also noted that trilobite
trace fossils such as Cruziana are often preserved at the “contact” between the base
of a quartzite unit and the upper surface of an underlying siltstone.
Trace fossils are highly abundant and diverse in the Harkless Formation,
particularly in the siltstone units (Alpert, 1974). Common ichnogenera include
Planolites, Bergaueria, Rusophycus, Cruziana, Diplichnites, Skolithos,
Monocraterion, and Scolicia (Alpert, 1974).
34
CHAPTER III
Bedding Plane Studies
Methods
Introduction: A range of field- and laboratory-based methods were employed
over the course of the bedding plane studies portion of this project. One of these, an
image-analysis technique, proved successful but inefficient. Although its use was
abandoned for this project, a description of the technique is included in this section
in the hope that it may be modified for use in future analyses.
Field Methods: All primary data for this project were collected from outcrops
of Lower Cambrian rocks in the White-Inyo Mountains, eastern California (Fig. 6,
8). Information on localities was obtained from Alpert (1974) (see Appendix for
detailed locality information). Stratigraphic sections were measured at the following
localities: Goat Spring, Folded Creek, and Andrews Mountain. At the remaining
localities, detailed stratigraphic information was recorded from meter-thick intervals
only.
Localities were chosen based on the presence of one or more bedding-plane
exposures of at least 600cm
2
in size. Bedding planes with smaller proportions supply
too little information to make proper assessment of bedding-plane bioturbation
indices and trace fossil diversity possible. Descriptive locality information was
FIGURE 8—Map of studied localities. Taphrhelminthopsis Canyon locality is here
abbreviated as “T. Canyon.” Gray-shaded areas indicate extent of Lower Cambrian
outcrops in this area. More detailed locality information can be found in the
Appendix.
35
36
compared with formation thicknesses and dip-slope relationships using maps of the
area (Nelson, 1966; Ernst et al., 1993) to determine which localities had the potential
to contain extensive bedding-plane exposures and, therefore, warranted further
examination.
To arrive at a three-dimensional perspective of Lower Cambrian bioturbation
in White-Inyo-Mountain strata, 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 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 the bedding surface
(Fig. 9). A 24 X 25cm form was used to evaluate broad bedding planes; another, 10
X 60cm, was used on narrow bedding planes having a shorter dimension smaller
than 24cm; more than one random selection was made on larger bedding planes in
order to obtain a maximum number of replicate samples.
Several types of data were recorded from each 600cm
2
portion of each
bedding plane. First, a brief description of the sedimentary characteristics of the
surface was generated, including an estimate of the average grain size and
composition. Next, when ichnofossils were present, these were identified to the
ichnogeneric level if possible. Ichnofossils that could not be readily identified were
described in detail for later identification. The dominant type of ichnofossil within
the 600cm
2
area was also noted. Last, an estimate was made of the intensity of
FIGURE 9—600cm
2
forms used to select portions of bedding planes at random for
analysis. (Top) 25 X 24 cm form. (Bottom) 60 X 10 cm form. Both bedding planes
from the Campito Formation.
37
38
bioturbation within the selected bedding-plane area. The bedding plane bioturbation
index (BPBI) method of Miller and Smail (1997) was used to produce a “score” of
zero to five to reflect the intensity of bioturbation present, from zero (none) to five
(most intense). In order to standardize the subjective assignment of scores as much
as possible, the bedding-plane area in question was visually compared against a card
containing graphical representations of each bedding-plane bioturbation index. Once
the BPBI had been estimated, a more precise determination was made of the
percentage of the surface area that had been bioturbated. This level of precision was
deemed necessary because each BPBI encompasses a range of bioturbation
percentages and, therefore, must not be used as a stand-alone estimate. The final step
in the data-collection process was photographing the 600cm
2
area framed by the
form. Each photograph was taken as close to the bedding surface as possible while
keeping the entire frame in view.
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 (Fig. 10). Vertical bioturbation intensity was evaluated
using the ichnofabric index method of Droser and Bottjer (1986). The position of
each studied 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.
FIGURE 10—Sedimentary characteristics and ichnofabric were logged at a
centimeter scale from each meter of vertical exposure.
39
40
Laboratory Methods: A subset of the samples collected from the 14 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 of the samples that were
selected for detailed analysis. Each thin section was cut, perpendicularly to bedding,
to the following approximate dimensions: 24 X 40mm for smaller samples and 45 X
70mm for larger samples. The sample number and orientation of bedding were noted
on each thin section. When the samples permitted, thin section blanks were prepared
so that the thin sections would bisect one or more surface traces.
Each thin section was examined using a Zeiss 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
alignments, that suggest 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 25.4 X 30.5 cm Kodak Industrex M
41
x-ray film. Film was exposed at 96kVA/8mA for 9, 12, or 21 minutes depending on
sample thickness. 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. Due to
their similar thicknesses (all ~1 cm), most of the billets required 12 minutes of
exposure for x-rays perpendicular to bedding. A few samples required slightly less or
more time (9 or 15 or minutes). A number of the samples were also x-rayed parallel
to bedding. However, the lengthy exposures required for generating adequate x-
radiographs of thicknesses of rock exceeding 40mm (18 minutes of exposure or
more) precluded the use of this type of analysis for the remaining samples.
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.
Data Analysis: The bedding plane bioturbation indices recorded from all
studied bedding planes were compiled and grouped according to their frequency of
occurrence within each formation. In addition, the results from all three formations
42
were graphed together to reveal any trends in bioturbation intensity that might be
present. The mean BPBI from each formation was also calculated.
A selection of well-defined Planolites-type trace fossils from each of the
three studied formations (89 from the Campito, 83 from the Poleta, and 93 from the
Harkless) were identified in bedding plane photographs. The diameter of each trace
was estimated by measuring the width of the trace as it appeared in the photograph
(Fig. 11). These diameters were then compiled and grouped into 1mm bins according
to their frequency of occurrence. As with the BPBI dataset, the burrow diameters
from all three formations were graphed together so that the complete dataset could be
evaluated for patterns in burrowing diameter. Also, the mean burrow diameter was
calculated for each formation.
An image analysis technique was developed to determine the precise
percentage of each bedding plane that was bioturbated using the bedding-plane
photographs taken in the field. The percentage obtained using this method was
intended for comparison with the more subjective estimate recorded in the field for
each bedding plane. The Adobe Illustrator program was used to trace the outlines of
discrete ichnofossils and patches of heavily-bioturbated sediment. Each outlined
region was then filled in with an opaque color using the fill tool in Illustrator.
Different colors were used to denote different types of traces in some images. Once
all bioturbated areas had been treated in this way, the inner edges of the 600cm
2
frame were traced to denote the precise boundaries of the region to be analyzed.
Then, all layers present in the Illustrator file were hidden from view except those
FIGURE 11—Diameters of Planolites-type burrows were measured from this
photograph of a bedding plane from the Poleta Formation. Measured burrow
diameters indicated by black lines. Inner horizontal dimension of frame is 24cm.
43
44
containing the trace of the frame boundaries and the color-filled outlines. The
resulting image was saved as a TIFF file and then opened with the Adobe Photoshop
program. The area within the trace of the frame was selected, and then the
“histogram” tool was used to display the colored (versus colorless) percentage of the
selection, which is equivalent to the percentage area of bioturbation on the bedding
plane. Although it accomplished the goal for which it was intended, this technique
was abandoned because of the amount of time required to trace ichnofossils one-by-
one. However, it can be useful for comparing the percentage area covered by two
types of trace fossils on a single bedding plane (Fig. 12).
Results
Introduction: Detailed information was recorded from vertical and bedding-
plane exposures within each studied one-meter-thick section, and these data are
reproduced below. Although every effort was made to record complete and detailed
sedimentological information from each vertical exposure, it should be noted that
large portions of many of the studied exposures, particularly within the Campito
Formation, appeared featureless when examined in the field. Mount (1980, p. 101)
expressed similar frustration with Campito Formation exposures: “…the majority of
the rocks examined…were structureless and yielded little to no information about
depositional processes.” Due to this limitation, many of the descriptions of vertical
exposures given below do not discuss sedimentary features on a centimeter-by-
FIGURE 12—Two distinctive types of trace fossils occur on this bedding plane,
also shown in Fig. 10: vertical Skolithos burrows, indicated by circular cross-sections
(black); and horizontal Planolites burrows (white). Not all burrows have been traced
and filled in. Inner horizontal dimension of frame is 24cm.
45
46
centimeter basis but rather highlight those intervals or horizons where definitive
identifications could be made.
Three of the studied meters, PC02 (Campito Formation), HT01 (Poleta
Formation), and FC01 (Harkless Formation), were chosen for additional analyses
using petrography and x-radiography. Different but complimentary results were
obtained through these two methods. X-radiography provided a three-dimensional
perspective, revealing structures that could not be seen by examining either the hand
samples or the thin sections, yet the resolution of the x-radiographs was poor overall
and limited by the need for density contrast. Petrography, on the other hand, afforded
highly detailed but localized information about a sample. By combining these two
types of analysis with the data obtained from bedding planes, it was possible to
assemble “three-dimensional” datasets for portions of these three studied meters
(Figs. 13, 14, 15).
The bedding-plane bioturbation indices recorded from bedding planes within
each formation were compiled and are shown graphically in Figures 16 (Campito),
17 (Poleta), and 18 (Harkless). A histogram displaying the combined data is shown
in Figure 19.
Meter Sections: Below are data recorded from each of the studied vertical
meter sections, which are grouped by locality.
47
FIGURE 13—Meter PC02
48
FIGURE 13—Meter PC02 (continued)
49
FIGURE 14—Meter FC01
50
FIGURE 14—Meter FC01 (continued)
51
FIGURE 15—Meter HT01
52
FIGURE 15—Meter HT01 (continued)
FIGURE 16—Bedding plane bioturbation index data from the Campito Formation.
Total number of bedding planes = 36.
53
FIGURE 17—Bedding-plane bioturbation index data from the Poleta Formation.
Total number of bedding planes = 27.
54
FIGURE 18—Bedding-plane bioturbation index data from the Harkless Formation.
Total number of bedding planes = 16.
55
FIGURE 19—Combined bedding-plane bioturbation index data from the Campito,
Poleta, and Harkless formations. Total number of bedding planes = 79.
56
57
Goat Spring Locality, Campito Formation – Meter GS01
GS01 represents meter 42 in the stratigraphic section measured at the Goat
Spring locality. An ichnofabric index of one was recorded consistently throughout
this meter. The vertical section consists mostly of tan-brown fine-grained sandstone
with a few narrow (1-5 cm-thick) intervals of siltier material that appears laminated.
Cross bedding was recorded sporadically throughout the meter, ranging from very
small crossbeds to larger (>1cm) crossbeds.
Four bedding planes were described within this meter, at 20cm, 33cm, 93cm,
and 100cm above the base. The 20-cm bedding plane consists of silty fine sandstone
with a very thin muddier layer on top. 30% bioturbation and a BPBI of three were
recorded for this bedding plane. Discrete trace fossils present included Planolites-
type traces and small semicircular traces that resemble entrances to vertical burrows.
No additional evidence for the presence of vertical burrows was found, however. The
33-cm bedding plane consists of fine-grained sandstone with a high concentration of
mica. No bioturbation was observed, and a BPBI of one was recorded. The 93-cm
bedding plane consists of silty fine sandstone that is indistinctly bioturbated.
Planolites-type traces may be present on the 10% of the bedding plane that is
bioturbated. A BPBI of two was recorded for this bedding plane. The 100-cm
bedding plane is large, making possible the evaluation of two 600cm
2
regions of the
surface. Both regions consist of silty fine sandstone with a thin surficial muddy layer
that appears to have been stripped away by tracemaker activity. The first region
contains medium-sized Planolites with a few smaller horizontal burrows and circular
58
traces. The surface is 50% bioturbated. The second region contains small Planolites,
with a few of these appearing to radiate outward from a single point, and small
circular traces. The surface is 45% bioturbated. A BPBI of four was recorded for
both regions of the bedding plane.
Goat Spring Locality, Campito Formation – Meter GS02
This meter represents meter 45 in the stratigraphic section measured for the
Goat Spring locality. An ichnofabric index of one was recorded consistently
throughout this meter. Brown to greenish-brown fine sandstones predominate,
although finer-grained material (silty to muddy fine sandstones) is quite common.
From centimeter 65 through the top of the meter, cross-bedding is extensive,
occasionally interbedded with thin intervals of laminated siltier fine sandstone.
Lower in the meter section, laminated silty intervals commonly contain some muddy
interbeds, although the lowermost 30cm also contain extensive cross-bedding.
Six bedding planes were described within this meter, at 52cm, 55cm, 91cm,
96cm, 99cm, and 100cm. The 52-cm “bedding plane” is the underside of the
overlying bed, on which trace fossils are preserved in hyporelief. The surface is
muddy and iron-rich, and the traces predominantly consist of large Planolites-type
burrows. 55% of the surface is bioturbated, and a BPBI of four was recorded. The
55-cm “bedding plane” is also the underside of the overlying bed, and the surface is
again iron-rich and muddy. The surface appears to be well-bioturbated (45%
bioturbation, BPBI four), but few well-defined traces are present except for medium-
59
sized and large Planolites. The 91-cm bedding plane is very muddy, with few traces
on the surface appearing to dip far enough downward to reach the fine sandstone
beneath. Small to medium-sized Planolites predominate, and the surface is 40%
bioturbated (BPBI four). The 96-cm bedding plane is an irregular surface in which a
top layer of mud is stripped away in places to reveal a lower layer of siltstone, which
lies above micaceous fine sandstone. Few traces are visible except for very small
horizontal burrows. However, some structures resembling linguliform brachiopods
are present. The surface is 10% bioturbated (BPBI two). The 99-cm bedding plane
consists of reddish fine sandstone that contains possible mud rip-up clasts. A BPBI
of one was recorded. The 100-cm bedding plane consists of silty fine sandstone with
an iron-rich muddy layer on top. Bioturbation, which covers 25% of the bedding
plane (BPBI three) consists mostly of small Planolites.
Goat Spring Locality, Campito Formation – Meter GS03
This meter represents meter 50.3 in the stratigraphic section measured at the
Goat Spring locality. An ichnofabric index of one was recorded throughout. The
lowermost ten centimeters contain no apparent sedimentary structures and are
composed of tan fine sandstone with dark brown weathering. An interval of
extensive cross-bedding occurs between 13 and 24cm. Above this is a massive-to-
laminated shaly interval with possible very thin muddy interbeds. The rest of the
meter section is predominantly cross-bedded fine sandstone, with two thin intervals
60
(62-67cm and 77-83cm) of finer-grained laminated material containing muddy
interbeds.
Six bedding planes were described within this meter section, at 20cm, 22cm,
66cm, 77.5cm, 80cm, and 100cm above the base. The 20-cm bedding plane is
composed of greenish micaceous silty fine sandstone beneath a very muddy iron-rich
top layer. The surface contains interference ripples. Bioturbation covers 70% of the
bedding plane (BPBI five) and consists of small and medium Planolites, many small
circular to oblong traces, and small horizontal burrows. The 22-cm bedding plane
consists of green silty fine sandstone that is micaceous beneath the surface. No
bioturbation was recorded except for possible circular traces that are five millimeters
in diameter. These are indistinct, however. The 66-cm bedding plane consists of
greenish to reddish silty fine sandstone, possibly with a thin veneer of mud on the
surface, and interference ripples. Because of its size, two regions of the bedding
plane were examined. The first is 30% bioturbated (BPBI three) with small, medium,
and large Planolites present. The second is 35% bioturbated (BPBI three), with
Planolites and smaller indistinct traces present. The 77.5-cm bedding plane has the
same lithology as the 66-cm bedding plane except that some traces penetrate the
muddy surface layer. Five different regions of this exceptionally large bedding plane
were evaluated. All of these had significant amounts of bioturbation (60-65%, BPBIs
four and five), with medium and large Planolites and small indistinct traces
predominating. The 80-cm bedding plane has the same lithology as the 66-cm
bedding plane except that interference ripples, if present, are not distinct.
61
Bioturbation, exclusively medium-size Planolites burrows, covers 55% of this
bedding plane. The 100-cm bedding plane is composed of greenish to tan silty fine
sandstone with a more micaceous underlying layer and a patchy muddier layer on top
that contains slickensides. The surface contains no apparent trace fossils.
Goat Spring Locality, Campito Formation – Meter GS04
This meter is thought to represent meter 155 in the stratigraphic section
measured at the Goat Spring locality. Its position was extrapolated across a creek bed
from the location of the measured section. The lowermost ten centimeters are made
up of tan-brown fine sandstone that is cross-bedded on a large scale. A thin muddy
layer at ten centimeters separates this interval from 20 centimeters of hummocky
cross-stratification above. The remainder of the meter consists of alternating
intervals of cross-bedded fine sandstone and laminated silty-muddy fine sandstone.
An ichnofabric index of one was recorded throughout the meter except for an
interval between 60 and 65cm in which vertical burrows occur. These burrows
include meniscate burrows that resemble escape structures and possible Spreite
burrows. Spreite burrows generally originate two centimeters between the escape
burrows, but both types extend up to five centimeters in depth.
Three bedding planes were examined in this meter, at 75cm, 76.5cm, and
100cm. The 75-cm bedding plane consists of dark tan to brownish-green fine
sandstone. Bioturbation covers only 5% of the surface (BPBI 1-2), with sparse traces
including indistinct Planolites, a few possible scratchmarks, and two indistinct
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oblong raised structures. The 76.5-cm bedding plane consists of tan fine sandstone
that is covered by a dark muddy layer. Some traces either penetrate the muddy layer
or have fill of fine sandstone. 45% of the surface is bioturbated (BPBI four). Most of
this bioturbation is indistinct except for a few small circular traces and small scratch-
like traces. The 100-cm bedding plane consists of tan fine sandstone with no muddy
layer on the surface. The surface is 8-10% bioturbated (BPBI two). The bioturbation
consists of small scratches, clusters of small horizontal burrows, and semicircular
traces.
Payson Canyon Locality, Campito Formation – Meter PC01
This meter consists predominantly of pale gray fine sandstone (dark gray
weathering) with small-scale cross-bedding and concentrations of iron along
bedding. Occasional very thin silty interbeds occur. A single laminated interval
occurs between 80 and 88cm.
Six bedding planes were examined in this meter, at 15cm, 49cm, 55cm,
80cm, 91cm, and 100cm. The 15-cm bedding plane consists of silty to muddy iron-
rich fine sandstone that contains large ripples. The surface is 25% bioturbated (BPBI
three) with small, medium, and large Planolites and smaller indistinct traces. The 49-
cm bedding plane consists of a silty-muddy surface with fine sandstone below it.
30% of the surface is bioturbated (BPBI three), and small circular traces that
resemble raindrop impressions are present in addition to Planolites. The 55-cm
bedding plane consists of fine sandstone that is somewhat silty. Very few Planolites
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are present, and only 5% of the surface is bioturbated (BPBI two). The 80-cm
bedding plane consists of silty fine sandstone topped by a muddy layer. Two regions
of the bedding plane were examined. Both are 35% bioturbated (BPBI three). Region
one contains small and medium Planolites, small scratch marks, and Bergaueria-like
semicircular traces. Region two contains medium and large Planolites, small scratch
marks, and indistinct bioturbation. The 91-cm bedding plane consists of silty fine
sandstone with a muddy surface layer and current ripples. Two regions of this
bedding plane were examined. The muddy surface layer is absent from region one,
which is 5-10% bioturbated (BPBI two) by small and medium Planolites. Region
two is 25% bioturbated (BPBI three) by Planolites, scratch-like traces, and circular
structures resembling raindrop impressions. The 100-cm bedding plane consists of a
silty to muddy surface that appears to contain large ripples. 5-10% of the surface is
bioturbated (BPBI two). The bioturbation consists predominantly of small Planolites
and scratch-like traces. Outside of the studied 600cm
2
on this bedding plane is a
large bulbous structure (c. 10cm in width).
Payson Canyon Locality, Campito Formation – Meter PC02
This meter is located immediately below PC01 stratigraphically, and it
consists primarily of the same material as PC01 (pale gray fine sandstone that
weathers to dark gray). This meter is characterized by alternating intervals of cross-
bedded fine sandstone and thinly-bedded to laminated siltier fine sandstone.
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Occasionally, a very thin muddy layer interrupts this pattern. No evidence of vertical
bioturbation was observed in this one-meter section (ichnofabric index one).
Twelve bedding planes were described within this meter, at 3cm, 10cm,
17cm, 20cm, 51cm, 53cm, 70cm, 74cm, 79cm, 80cm, 82cm, and 100cm from the
base. The 3-cm bedding plane consists of silty gray fine sandstone with a muddy
iron-rich layer on the surface. 20% of the surface is bioturbated (BPBI three), and the
bioturbation consists primarily of medium and large Planolites and a few smaller
indistinct traces. The 10-cm bedding plane consists of silty fine sandstone with an
iron-rich surface containing only a trace of mud. The surface is 10% bioturbated
(BPBI two), and the bioturbation consists of medium and large Planolites. A small
bilobed trail was also observed outside the 600cm
2
study area. The bedding plane at
17cm consists of gray silty fine sandstone with an iron-rich muddy surface layer.
15% of the bedding plane is bioturbated (BPBI three), and medium and large
Planolites are again common. A few small bulbous traces were observed elsewhere
on the bedding plane. The 20-cm bedding plane consists primarily of gray silty fine
sandstone with limited iron staining. The surface is irregular, and Planolites
bioturbation covers no more than 5% of the surface (BPBI two). The 51-cm bedding
plane has a dark gray, silty to muddy surface. 30% of the bedding surface is
bioturbated (BPBI three), and a range of sizes of Planolites predominate. A few
small semicircular and scratch-like traces were also observed. The bedding plane at
53cm consists of a muddy mottled surface above gray silty fine sandstone with
brownish iron stains. Large Planolites account for most of the bioturbation on this
65
surface (15% is bioturbated, BPBI three). The 70-cm bedding plane is very similar to
the 53-cm bedding plane except that it contains more mud. The surface is 50%
bioturbated (BPBI four), and Planolites is the most common type of trace present.
Many small indistinct traces give the surface a mottled appearance. The bedding
plane at 74cm consists of dark gray silty fine sandstone with a muddy iron-rich
surface layer. The surface is 45% bioturbated (BPBI four), and Planolites,
semicircular traces, and smaller indistinct traces are present. The 80-cm bedding
plane consists of silty fine sandstone with a dark greenish muddy layer on the
surface. 8-10% of the surface is bioturbated (BPBI two), predominantly by
Planolites. The bedding plane at 82cm has the same lithology as the 80-cm bedding
plane. The surface is 20% bioturbated (BPBI two) with small Planolites and
semicircular traces. The 100-cm bedding plane is composed of silty fine sandstone
with a very thin layer of iron-rich mud on its surface. 5% of the surface is
bioturbated (BPBI two), and the bioturbation consists of one large Planolites and a
few small Planolites.
Horse Thief Canyon Locality, Poleta Formation – Meter HT01
This meter is located four meters to the northwest of the road that leads into
Horse Thief Canyon, at the base of a hillside. Dark gray siltstones predominate and
are interrupted periodically by thin intervals of finer-grained material. The bottom
five centimeters of the meter section are composed primarily of mudstone. Lateral
variations in bed thicknesses are common. Brief (1-2cm thick) intervals of cross-
66
bedding are present in the upper half of the meter section. An ichnofabric index of
two was recorded throughout due to the presence of shallow vertical bioturbation and
distinct horizontal burrows.
Twelve bedding planes were described within this meter, at 19.5cm, 42cm,
44cm, 69cm, 70cm, 72.5cm, 83cm, 86cm, 88cm, 96cm, 98.5cm, and 100cm above
the base. The 19.5-cm bedding plane consists of muddy siltstone with scattered mica
grains. Approximately five percent of the surface is bioturbated (BPBI two), and this
bioturbation consists primarily of small to medium Planolites. The bedding plane at
42cm consists of silty fine sandstone with scattered mica grains. Less than 10% of
the surface is bioturbated (BPBI two), and the bioturbation consists of small to
medium Planolites and structures that resemble entrances to vertical burrows. The
44-cm bedding plane consists of a silty micaceous surface that contains ripple marks.
The bedding surface is 15% bioturbated (BPBI three), with medium Planolites
common and one bilobed trace present. Wrinkle structures were observed in patches
outside the studied region of the bedding plane. The 69-cm bedding plane consists of
a silty micaceous surface with a layer of mud on top. Less than 10% of the surface is
bioturbated (BPBI two), and the bioturbation consists of small to medium Planolites,
small indistinct traces, round traces resembling vertical burrow entrances, and scoop-
like traces attributed to trilobites. The 70-cm bedding plane consists of a surface
similar in composition to the 69-cm bedding plane. Bioturbation was observed on
less than 10% of the surface (BPBI two), and this consists mainly of small and
medium Planolites. Possible linguliform brachiopods were also observed. The
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bedding plane at 72.5cm consists of silty fine sandstone that contains micaceous
patches and irregular ripples. Very little bioturbation was observed (BPBI two), in
the form of possible Diplichnites. Wrinkle structures are common on this bedding
plane. The 83-cm bedding plane consists of silty fine sandstone with some mica. The
surface is 15% bioturbated (BPBI three), and the bioturbation consists of small and
medium Planolites. The bedding plane at 86cm consists of a silty micaceous surface
with mud appearing to fill in the depressions around traces. Bioturbation is unevenly
distributed over the surface, with about 40% of the total area being bioturbated
(BPBI three-four). Bioturbation consists primarily of small and medium Planolites.
Possible trilobite fragments and skeletal fragments resembling Volborthella were
also observed. The 88-cm bedding plane closely resembles that at 86cm.
Approximately 35% of the surface is bioturbated (BPBI three), with small to medium
Planolites dominating. The bedding plane at 96cm consists of a silty micaceous
surface that is 15% bioturbated (BPBI three). Small and medium Planolites are
common, and semicircular structures resembling vertical burrow entrances are also
present. The 98.5-cm bedding plane consists of muddy siltstone that contains ripple
marks. The surface is 45% bioturbated (BPBI four), and traces consist of small to
medium Planolites, small scratch marks, and shallow burrows that repeatedly
intersect the surface. The 100-cm bedding plane consists of a silty micaceous surface
that contains large ripple marks. No more than 10% of the surface is bioturbated
(BPBI two), and the bioturbation consists only of medium Planolites.
68
Horse Thief Canyon Locality, Poleta Formation – Meter HT02
This meter is located northeast of meter HT01, northwest of the Horse Thief
Canyon road, in between two shallow gullies. Greenish-tan muddy siltstone is the
predominant lithology, and thin mudstone interbeds are common. A sandier interval,
approximately five centimeters in thickness, is present low in the meter section, but
no cross-bedding was observed. Ripple marks are present on some bedding planes.
An ichnofabric index of one was recorded throughout the meter section.
Six bedding planes were examined within this meter, at 44.5cm, 94.5cm,
95.5cm, 98.5cm, 99.5cm, and 100cm. The 44.5-cm bedding plane consists of a tan-
colored irregular muddy surface. Fifteen percent of the surface is bioturbated (BPBI
three), and the bioturbation consists of small and medium Planolites and small
semicircular burrow entrances. The bedding plane at 94.5cm consists of a tan muddy
surface layer with an iron-rich layer beneath it. The surface is 20% bioturbated
(BPBI three). Bioturbation includes Planolites, small circular traces, and indistinct
traces. The 95.5-cm bedding plane consists of an undulating, iron-rich silty
micaceous surface. Twenty percent of the bedding surface is bioturbated (BPBI
three), and traces consist of medium Planolites and a few small circular traces. The
bedding plane at 98.5cm consists of a thin layer of tan mudstone overlying a layer of
grayish micaceous siltstone. The surface is somewhat irregular and lumpy.
Approximately 25% of the bedding plane is bioturbated (BPBI three), and this
bioturbation includes small and medium Planolites, a structure that resembles
Bergaueria, and a few small circular traces. Surface characteristics of the 99.5-cm
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bedding plane are very similar to those of the 98.5-cm bedding plane. Fifteen percent
of the 99.5-cm bedding plane is bioturbated (BPBI three), and this bioturbation is
dominated by small Planolites and small circular traces. The 100-cm bedding plane
consists of an irregular silty surface containing micaceous patches. Less than 10% of
the surface is bioturbated (BPBI two), and small Planolites dominate.
Horse Thief Canyon Locality, Poleta Formation – Meter HT03
This meter section is located on the southeast side of the Horse Thief Canyon
road at the crest of a southwest-trending ridge. The primary lithology is pinkish
medium-grained quartzite with some cross-bedding, although the uppermost 22cm
are considerably finer-grained and less massive. Much of the meter section contains
dense associations of Skolithos trace fossils, which qualify as “Skolithos piperock”
with an ichnofabric index of three. The finer-grained remainder of the section
contains a mixture of Skolithos and horizontal trace fossils, including Planolites. An
ichnofabric index of two was recorded for this portion of the section.
Two bedding planes were described within this meter, at 60cm and 100cm
above the base. The 60-cm bedding plane consists of medium-grained quartzite. The
surface is 15% bioturbated (BPBI three), and Skolithos is the only trace present. The
100-cm bedding plane occurs at the top of a fine-grained interval and is covered by
black desert varnish. Two regions of this bedding plane were examined. The first is
approximately 25% bioturbated (BPBI three) and contains Skolithos and medium to
large Planolites. The second region is also dominated by Skolithos and Planolites but
70
is 50% bioturbated (BPBI four). In addition, more varnish was observed within the
second region of the 100-cm bedding plane.
Horse Thief Canyon Locality, Poleta Formation – Meter HT04
This meter is located downslope from meter HT03, between HT03 and the
Horse Thief Canyon road, and consists primarily of dark purple to brownish-green
medium-grained quartzite. Occasional iron-rich intervals are visible. Cross-bedding
was observed from 0-3cm and 7-12cm above the base of the meter and occasionally
above 12cm. An ichnofabric index of one was recorded consistently throughout the
meter section.
A single bedding plane, representing the upper boundary of the meter section
(100cm), was described. This bedding plane consists of a grayish-green muddy
surface that is somewhat irregular, possibly due to the presence of large ripple marks.
Approximately 35% of the surface is bioturbated (BPBI three), and this bioturbation
consists of small indistinct Planolites, occasional circular traces (possible entrances
to vertical burrows), and possible trilobite scratch marks.
Taphrhelminthopsis Canyon Locality, Poleta Formation – Meter TC01
This meter encompasses the best bedding-plane exposures of
Taphrhelminthopsis at this locality and is situated approximately three meters below
a thick bluish-tan limestone unit. Sedimentary and biogenic structures (aside from
the large Taphrhelminthopsis trails) are difficult to discern in this outcrop due to
71
metamorphic alteration. Overall, the meter consists of pale green to tan fine-grained
sandstone with thin layers of silty mudstone at 21cm, 31cm, 35cm, and 47cm.
Possible cross-bedding was observed in the uppermost three centimeters of the
meter. An ichnofabric index of one was recorded throughout this meter.
Six bedding planes were described from this meter, at 33.5cm, 47.5cm, 55cm,
76cm, 78cm, and 100cm above the base. The 33.5-cm bedding plane consists of an
irregular iron-rich grayish-tan silty fine sandstone surface. The surface is estimated
to be five percent bioturbated (BPBI two), with indistinct Planolites and possible
wrinkle structures present. The 47.5-cm bedding plane consists of a light gray
micaceous silty fine sandstone surface that appears to contain ripple marks. The
surface is 5-10% bioturbated (BPBI two), and the bioturbation consists of indistinct
Planolites. The bedding plane at 55cm consists of mottled brownish-tan fine-grained
siltstone. Two regions of this bedding surface were examined. The bioturbation
intensity of the first region was estimated at 40% (BPBI four), with small and
medium Planolites predominating. The second region is approximately 45%
bioturbated (BPBI four), and the bioturbation consists of Planolites and indistinct
traces. The 76-cm bedding plane consists of an irregular brownish-tan siltstone
surface. No bioturbation was observed on this bedding plane (BPBI one). The
bedding plane at 78cm consists of a rippled brownish-tan siltstone surface. No
bioturbation was observed on this bedding plane (BPBI one). The 100-cm bedding
plane consists of an irregular brownish-tan silty fine sandstone surface.
72
Approximately 10% of the surface is bioturbated (BPBI three), and the only type of
ichnofossil present is Taphrhelminthopsis.
Folded Creek Locality, Harkless Formation – Meter FC01
This meter is equivalent to meter 24 in the stratigraphic section measured at
the Folded Creek locality. (Measurement of the section began in the creek bed.) The
meter consists primarily of tan fine sandstone that weathers to black. Cross-bedding
is abundant throughout the fine-sandstone portions of the meter. Centimeter-thick
silty to muddy interbeds are scattered throughout, and a thicker layer of fine-grained
material occurs from centimeters 55-57. These finer-grained interbeds are typically
exposed as partial bedding planes and are moderately-bioturbated. An ichnofabric
index of one was recorded throughout this meter.
Four bedding planes were described, at 19cm, 45cm, 75cm, and 100cm above
the base of the meter. The 19-cm bedding plane consists of greenish-tan silty fine
sandstone with large ripple marks. Approximately five percent of the surface is
bioturbated (BPBI two), and small to medium Planolites dominate. The 45-cm
bedding plane consists of tan to dark brown fine sandstone with interference ripples.
Silty to muddy material appears to have collected around some of the trace fossils
present. The surface is 15% bioturbated (BPBI three). Small, medium, and large
Planolites are present, as well as small segmented branching traces, a few of which
appear to radiate outward from a point. The bedding plane at 75cm consists of a silty
fine sandstone surface that contains indications of ripples. Approximately 45% of the
73
surface is bioturbated (BPBI four), and traces include small and medium radiating
Planolites-like traces and a possible bilobed horizontal burrow. The 100-cm bedding
plane consists of brownish-green silty fine sandstone with muddy patches. The
surface is 45% bioturbated (BPBI four), and traces include Planolites, shallow
depressions that resemble Bergaueria or Rusophycus, and indistinct bioturbation.
Folded Creek Locality, Harkless Formation – Meter FC02
This meter is located less than two meters above meter FC01. From the base
of the meter to 60cm, greenish-tan fine sandstone predominates, with rare thin silty
to muddy interbeds. The fine sandstone appears to be cross-bedded throughout. The
interval between 60cm and the top of the meter consists of much finer-grained
material (siltstone) with four centimeter-thick interbeds of fine sandstone
interspersed. An ichnofabric index of one was observed throughout the meter.
Six bedding planes were described within this meter, at 0cm, 11cm, 14cm,
20cm, 29cm, and 100cm above the base. The bedding plane at zero centimeters
consists of greenish-tan silty fine sandstone with interference ripples. The surface is
20% bioturbated (BPBI three), and small to medium Planolites dominate, although
radiating horizontal traces were observed elsewhere on the bedding plane. The 11-
cm bedding plane consists of greenish-tan silty fine sandstone with possible patches
of mud. The surface is approximately 50% bioturbated (BPBI four), with many small
to medium Planolites and indistinct bioturbation present. Bergaueria was observed
elsewhere on this bedding plane. The 14-cm bedding plane closely resembles the 11-
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cm bedding plane. This surface is approximately 40% bioturbated (BPBI four).
Small Planolites are present, as well as circular traces that are approximately one
centimeter in diameter. The 20-cm bedding plane consists of greenish-brown to tan
silty fine sandstone with a rippled surface and patches of mud. The surface is 25%
bioturbated (BPBI three), and bioturbation consists of small to medium Planolites,
small bilobed traces, and apparent radiating traces. The 29-cm bedding plane consists
of an irregular silty fine sandstone surface that appears to have been coherent,
allowing it to be “ripped up.” Slightly less than 10% of the surface is bioturbated
(BPBI two), with small Planolites and bilobed traces predominating. The bedding
plane at 100cm consists of a slightly coarser-grained brown to tan fine sandstone
with a micaceous surface. The surface is 25% bioturbated (BPBI three), again with
bilobed traces (small to medium in size) and a few medium Planolites.
Harkless Point Locality, Harkless Formation – Meter HP01
This meter is located to the east of Cedar Flat at the top of a cliff made up of
exposed Harkless Formation material. The dominant lithology is brownish-tan fine-
to medium-grained quartz-rich sandstone, usually cross-bedded. Thin silty layers
occur throughout, and fine-grained sandstone dominates from 71cm to the top. An
ichnofabric index of one was observed throughout this meter.
Six bedding planes were described, at 80.5cm, 86cm, 88.5cm, 91cm, 94.5cm,
and 100cm above the base of the meter. The 80.5-cm bedding plane consists of a
greenish-tan and reddish-brown silty surface topped by a thin layer of mud.
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Approximately 45% of the surface is bioturbated (BPBI four). Most of this
bioturbation is indistinct, although much of it resembles scratch marks. The bedding
plane at 86cm consists of similar material to that of the 80.5-cm bedding surface.
This bedding plane is 40% bioturbated (BPBI four), with large Planolites and
indistinct traces present. The 88.5-cm bedding plane consists of a similar surface to
those described above. 50% of the bedding plane is bioturbated (BPBI four), and the
bioturbation consists of small to large Planolites, scratchmarks, and other indistinct
traces. The 91-cm bedding plane consists of reddish-brown silty fine sandstone with
a thin layer of mud on top. Two regions of this bedding surface were examined. The
first is 35% bioturbated (BPBI three), with bioturbation consisting of medium
Planolites and small circular traces. The second region is 40% bioturbated (BPBI
four), with mostly Planolites traces present. The bedding plane at 94.5cm consists of
an irregular greenish-tan silty-to-muddy surface. The surface is 45% bioturbated
(BPBI four), but most of the traces are indistinct. The 100-cm bedding plane consists
of a greenish-tan to reddish-brown silty surface on which some mica is present. The
surface is 55% bioturbated (BPBI four), and the bioturbation consists of Planolites,
scratch marks, and indistinct traces.
Petrography: One to two thin sections were made from each of the samples
collected from meters PC02, HT01, and FC01. Examination of these thin sections
revealed that the samples resemble one another in terms of sedimentary
characteristics. Quartz is the predominant mineral throughout the samples, although
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the grains vary considerably in size. A layer of fine-grained material up to .25 cm
thick is found at the bedding-plane surface in many of the samples from the Poleta
and Harkless formations (Fig. 20). Campito Formation samples can generally be
distinguished by the presence of high quantities of opaque minerals, which are often
concentrated in one or two layers near the bedding plane or along cross beds (Fig.
21). Elongated mineral grains, predominantly mica, are present in fine-grained layers
within many of the samples and appear to have been aligned by post-depositional
alteration.
Sedimentary structures are visible in a number of thin sections and in some
instances are defined by concentrations of heavy mineral grains or layers of very
fine-grained material. Cross-bedding is conspicuous in one thin section from the
Campito Formation in which heavy mineral grains are abundant (Fig. 21). In general,
however, bedding is subtle in appearance due to the relative compositional
homogeneity of grains within the samples.
Evidence of bioturbation is very limited in these samples overall. In many
cases, areas of potential disturbance can only be recognized by comparing the thin
sections with their corresponding x-rays. In other samples, slight grain size contrasts
signal the presence of traces. Often, trace fill consists of quartz grains that are
coarser than the material surrounding the trace (Fig. 22).
Ovate to conical or globular structures comprised primarily of opaque
mineral grains occur in thin sections from all three formations, although these
structures are most common in thin sections of Harkless Formation samples (Fig.
FIGURE 20—Photomicrograph (crossed polars) of a thin layer of fine-grained
material at the bedding-plane surface of a sample from the Harkless Formation.
Apparent surface topography is an effect generated by the thin-section epoxy.
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FIGURE 21—Cross beds are accentuated by opaque mineral grains in this
photomicrograph. Thin-sectioned sample from the Campito Formation.
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FIGURE 22—Photomicrograph showing a possible horizontal burrow oriented
roughly perpendicular to the field of view (oval). Note grain-size contrast between
trace fill (predominantly quartz grains) and surrounding material. Sample from the
Poleta Formation.
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23). Some of these structures have ordered internal grain arrangements that will be
discussed further in the next chapter. In almost all cases, the boundaries of these
opaque-filled structures appear well-defined in thin section.
X-radiography: X-radiographs were made of all of the thin section billets
from meters PC02, HT01, and FC01. A number of sedimentary structures are visible
in the x-radiographs, including herringbone cross-stratification, thin laminations, and
alternating layers of contrasting grain sizes (Fig. 24). Bedding is highly disrupted to
nonexistent in some samples. However, there appears to be no correlation between
the intensity of bioturbation on bedding-plane surfaces and the degree of internal
disruption present within the samples.
Many of the concentrations of opaque minerals observed in thin section are
also visible in the corresponding x-ray images (Figs. 21 and 24A). As expected, most
of these grains are considerably denser than the material surrounding them. Density
contrasts also highlight features that are present but less noticeable in thin section,
such as boundaries between beds of contrasting grain size and localized disturbances
in the arrangement of grains. Discrete burrows are clearly visible in x-ray (Fig. 25),
and diffuse bioturbation is more apparent in the x-radiographs than in the thin section
but still difficult to interpret overall. Due to the depth afforded by the x-ray method,
many more discrete traces are visible in x-radiographs than in the thin sections.
Similarly, many more opaque-“filled” structures are visible in x-ray than in
thin section. Combining vertical and horizontal x-rays of the samples shows that
some of the structures that appear ovate in thin section are in fact conical (Fig. 26).
FIGURE 23—Four photomicrographs of opaque-grain-filled structures found in
samples from the Harkless Formation.
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A B
E
C D
FIGURE 24—Positive x-radiographs displaying sedimentary structures. Denser
regions appear darker, and notches in the upper edges of B, C, and D are artifacts of
the thin-sectioning process. (A, C, and D) Cross-bedding. (B) Finely-laminated
material with some possible bioturbation. (E) Undisturbed bedding with variable
heavy mineral grain component. Approximate horizontal dimension of A and B =
4cm; C, D, and E = 4.5cm. B and D from the Poleta Formation; A, C, and E from the
Campito Formation.
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A
B
C
FIGURE 25—Positive x-radiographs showing examples of discrete trace fossils.
Note density contrast between trace fill and surrounding material in B and C. Notch
in upper edge of C is an artifact of the thin-sectioning process. Approximate
horizontal dimension of A = 4.5cm; B and C = 4cm. A and B from the Campito
Formation; C from the Poleta Formation.
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A B C
E
D
FIGURE 26—X-radiographs of opaque structures in samples from the Harkless
Formation. All x-radiographs are positive except for E, which is negative. Note
concentrations of multiple opaque structures along single horizons in A and D, in
clusters in B and C, and on a bedding plane surface in E. Also note prominent
density zonation in some opaque structures in C and D. Approximate horizontal
dimension of A, B, and C = 4.5cm; D and E = 4cm.
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The ovate to conical structures seen in the x-radiographs range in length from ~1-5
mm. In some of these structures, grains appear to be grouped internally according to
density, creating a “banded” appearance when viewed in the x-radiograph (Fig. 26).
The x-radiographs of several samples from the Harkless Formation show multiple
opaque-filled conical structures clustered on what appears to be a single bedding
surface (Fig. 26D). All of these structures are oriented horizontally, with the long
axis of the structure lying parallel to the bedding surface.
Planolites Diameters: Diameters of Planolites-type burrows were measured
from images taken of the following bedding planes: 100-cm bedding plane (GS01,
Campito Formation), 100-cm bedding plane (HT03, Poleta Formation), and 0-cm
and 14-cm bedding planes (FC02, Harkless Formation). These four bedding planes
were selected as representative samples because they contain Planolites burrows
with particularly well-defined edges. Burrows that have comparatively indistinct
boundaries are difficult to measure precisely.
Results of this analysis for the Campito Formation showed a broad range of
Planolites burrow diameters (1-6mm) and peak in occurrences of burrow diameters
between 2 and 3mm (Fig. 27). The range of diameters for the Poleta Formation was
slightly narrower, with the 0-1mm size bin excluded, and roughly equal high
numbers of burrows (26, 28) fell within the 2-3mm and 3-4mm size bins (Fig. 28).
Data for the Harkless Formation showed a drastic reduction in larger-size burrows,
with the 4-5mm and 5-6mm bins remaining completely empty (Fig. 29). In addition,
FIGURE 27—Planolites burrow diameter data from the Campito Formation. Each
bin is labeled according to its upper limit (e.g. bin 0.1 includes burrow diameters
between 0 and 0.1cm). Total number of burrows = 89.
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FIGURE 28— Planolites burrow diameter data from the Poleta Formation. Each bin
is labeled according to its upper limit (e.g. bin 0.1 includes burrow diameters
between 0 and 0.1cm). Total number of burrows = 83.
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FIGURE 29— Planolites burrow diameter data from the Harkless Formation. Each
bin is labeled according to its upper limit (e.g. bin 0.1 includes burrow diameters
between 0 and 0.1cm). Total number of burrows = 93.
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the 1-2mm size bin contains a large peak (53 specimens). The next-highest peak is
32 specimens in the 2-3mm size bin. Combined data from the three formations are
shown in Figure 30. An interpretation of these results will be offered in the next
chapter.
FIGURE 30—Combined Planolites burrow diameter data from the Campito, Poleta,
and Harkless formations. Each bin is labeled according to its upper limit (e.g. bin 0.1
includes burrow diameters between 0 and 0.1cm). Total number of burrows = 265.
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CHAPTER IV
Piperock Studies
Introduction
The broad purpose of this portion of the study was to investigate the
relationship between substrate consistency and deep-burrowing behavior, and the
evolutionary implications thereof, by examining samples of Skolithos piperock from
the Poleta Formation. Samples were collected from quartzites within the upper
sandstone unit of the Middle Member of the Poleta Formation and were analyzed for
sedimentary and ichnofaunal characteristics using x-radiography, petrography, and
hand sample examination. Due to the nature of the material and the objectives of this
portion of the project, the methods described herein are different from those applied
in the remainder of the study.
Background
During the earliest Cambrian, deep vertical burrowing appeared within
coarse-grained substrates that consisted predominantly of quartz sand. Skolithos, the
dominant ichnogenus in these low-diversity ichnofaunal assemblages, is a “vertical
unlined dwelling shaft” (Bromley, 1996) 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 (2003) (Fig. 31). Piperock is
considered a typical feature of Cambrian high-energy nearshore clastic deposits
FIGURE 31—Field photographs of Skolithos piperock. (Left) Harkless Formation.
(Right) Poleta Formation.
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(e.g., Francis and Hallam, 2003) 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 and Mángano, 2003).
Previous Work
Schmidt (1977) assessed the sedimentology, stratigraphy, and petrology of
the Skolithos piperock-bearing quartz arenite portion of the Harkless Formation.
Droser and Bottjer (1989) presented guidelines for measuring and evaluating the
ichnofabric of high-energy nearshore sandstone deposits, which typically contain
Skolithos or Ophiomorpha. Droser (1991) reviewed Paleozoic occurrences of
Skolithos piperock in the literature and found that Cambrian occurrences of piperock
outnumber those recorded for any other period. Droser et al. (1994) described a
Cambrian to Ordovician nearshore assemblage in which Skolithos occurs as part of a
“postdepositional ichnocoenosis” that cross-cuts earlier traces and is preserved as a
“frozen tiered profile.” Reasons for the rare co-occurrences of
Skolithos/Monocraterion and Diplocraterion in Cambrian tidal deposits were
investigated by Cornish (1986). Sundberg (1983) reconstructed the Skolithos
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tracemaker as a phoronid-like organism. Ekdale and Lewis (1993) and Skoog et al.
(1994) described potential modern analogues for the Skolithos organism (Skoog et
al., 1994) and Skolithos piperock (Ekdale and Lewis, 1993).
Methods
Samples were collected primarily from quartzite float from the upper
sandstone unit of the Middle Member of the Poleta Formation along the eastern slope
of Horse Thief Canyon, where the quartzite is exposed sporadically. Five samples
were selected for analysis because they appear to be representative of the portions of
the slope that were sampled.
In order to obtain detailed information concerning the fine-scale sedimentary
structures and mineralogy of each sample, a total of six thin sections were made from
the five samples discussed in the previous subsection. Each thin section was cut,
perpendicular to bedding, to the following approximate dimensions: 40 X 24 mm
(smaller samples) and 45 X 70 mm (larger samples). The sample number and
orientation of bedding were noted on each thin section. When samples permitted,
thin section blanks were prepared so that the resulting thin sections would bisect one
or more vertical or horizontal traces present in the sample.
The following information was recorded for each thin section, if 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 alignments, for the presence of a microbial mat;
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evidence of mineral concentrations and/or grain alignments due to tracemaker
activity (e.g., burrow fill); and the presence/absence and description of bioturbation-
induced structures. Photomicrographs were taken of a selection of features that were
noted in the thin sections.
All of the thin section billets were x-rayed perpendicular to bedding in order
to obtain information concerning the internal structural complexity of the samples,
particularly the distribution and morphology of vertical trace fossils. Billets were not
x-rayed parallel to bedding due to the lengthy exposure times required to produce x-
radiographs from thicknesses of rock greater than one centimeter. Due to the
relatively uniform thickness of the billets (all ~1 cm), a satisfactory x-radiograph was
obtained for each billet by exposing the entire set of billets for six, nine, and 12
minutes. Features observed in x-radiographs were recorded and compared with those
documented from the thin sections and hand samples.
Although the emphasis of this project was obtaining data using x-radiography
and petrography, a limited amount of data was also compiled from hand samples. A
brief description of the sedimentary characteristics and trace fossil content of each
sample was prepared based on examination of the hand samples. These data were
then compared with the corresponding x-radiographic and petrographic data to
determine whether characteristics observed at the surface of a sample reflected the
internal structure of the sample as revealed by thin sections and x-radiographs. In
addition, an attempt was made to compare samples collected from different portions
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of the Poleta quartzite slope at Horse Thief Canyon using ichnofabric and
sedimentary characteristics.
Results
All of the studied samples are dominated by medium to coarse quartz sand
grains that are subangular to well-rounded. The grains are poorly sorted such that
few larger grains are scattered among many smaller grains. Aside from the quartz
sand, only one other type of material is present in the thin sections: very fine-grained
sediment of undetermined composition. Very few sedimentary structures can be
discerned in the thin sections, and no evidence of microbial influence was observed.
A number of bioturbation-induced structures, including recognizable Skolithos
burrows, are present (Fig. 32). Many of these are macroscopic and thus proved
difficult to capture in photomicrographs due to a limited field of view. Using a flat-
bed scanner to create digital images of the thin sections helped to alleviate this
problem (Fig. 33).
Many of the Skolithos burrows viewed in thin section share a similar grain
size pattern in their burrow fill. Although in all cases the burrow fill is virtually
identical to the surrounding sediment in terms of grain composition and size
variation, the grains tend to get smaller away from the middle of the burrow. The
largest grains are found toward the middle of the burrow, surrounded by smaller
grains. Both the range of grain sizes and the median grain size decrease outward
from the middle of the burrow. At the outer edges of each burrow are thin layers, at
FIGURE 32—Photomicrographs of two Skolithos burrows in thin sections of
samples from the Poleta Formation. Note the very fine-grained material that defines
the edges of the burrows.
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FIGURE 33—Scanned image of a thin section from the Poleta Formation in which a
Skolithos burrow is visible on the right-hand side. A possible second Skolithos
burrow is present near the left edge of the thin section. Horizontal dimension of the
thin section is 4cm.
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most tens of microns in thickness, of the very fine-grained sediment described above
(Fig. 32). The implications of this pattern will be discussed in the next section.
Primary bedding was observed in all of the x-radiographs examined. At the
same time, all of the images contain evidence of bioturbation, whether in the form of
discrete traces such as Skolithos or less distinct disturbances of the sediment. Low
concentrations of heavy mineral grains outline some bedding surfaces within the x-
radiographs, and larger individual heavy mineral grains (c. 500µm in diameter) were
found scattered throughout a few of the samples. None of the attributes of these x-
radiographs indicates the presence or influence of microbial activity on the substrate.
Where Skolithos burrows are recognizable in the x-radiographs, the trace is
outlined by a hollow “cylinder” (three dimensions extrapolated) of denser material
surrounding the burrow fill, which appears to be roughly the same density as the
surrounding material (Fig. 34). The “cylinders” may be equivalent to the thin layers
of very fine-grained sediment that were noted in the thin sections.
Based on examination of the five hand samples, it appears that finer-grained
layers containing horizontal trace fossils become more and more common up-section
within the Poleta quartzite at Horse Thief Canyon. Most of the samples contain a
mixture of vertical Skolithos-type traces and horizontal traces. One sample from the
top of the section appears to contain only horizontal traces on the surface of a finer-
grained bedding plane. Another sample from a lower portion of the slope contains no
horizontal trace fossils but has abundant Skolithos-type traces that are roughly half
the diameter of those observed higher on the slope.
FIGURE 34—Positive x-radiograph of a sample of Skolithos piperock from the
Poleta Formation showing a “halo” of denser material surrounding one portion of a
vertical Skolithos burrow (arrow). Two to three additional structures, which may be
Skolithos burrows, are present but are less distinct. Approximate horizontal
dimension of the x-radiograph is 4.5cm.
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The x-radiographs of the Poleta quartzite samples are similar in many respects to x-
radiographs of samples collected from units lower in the Poleta Formation and
above, in the Harkless Formation. Such similarities do not necessarily indicate that
environmental conditions were similar at the time of deposition in these settings but
rather that both finer- and coarser-grained settings experienced some degree of
vertical bioturbation.
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CHAPTER V
Discussion and Future Work
Introduction
The results presented in the preceding chapters were generated through study
of strata that represent a small sampling of shallow marine environments within a
single region. Therefore, these data cannot be used as the basis for broad hypotheses
concerning the evolution of burrowing behavior in benthic invertebrates. Rather, any
conclusions drawn from these results must be stated as being specific to these
particular environments and region. However, given that this study employed a novel
combination of techniques, it is useful to begin exploring some of the possible
broader implications of this type of dataset. A few such implications will be
discussed in the sections below.
Microbial Influence on the Substrate
Microbial Trapping and Binding: As noted previously, Hagadorn and Bottjer
(1997) documented evidence, in the form of aligned mica grains and concentrations
of pyrite on wrinkle-structure-bearing surfaces, that microbial mats had a distinct
influence on the consistency of the marine substrates represented by the Lower
Cambrian strata of the White-Inyo Mountains. However, no definitive microscopic
evidence of microbial trapping and binding of sediment grains was identified in thin
sections or x-radiographs of the samples examined in this study. Although mica
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grains were found in fine-grained layers beneath sampled bedding planes, these
grains did not appear to be aligned by any means other than post-depositional
alteration. Mica can also be generated by diagenetic alteration, and examples of
diagenetic mica have been reported from each of the studied formations (Stewart,
1970; Mount, 1980). Therefore, it cannot be stated with confidence that the mica
grains observed in these samples were present prior to lithification, given that these
rocks have experienced a moderate degree of metamorphic alteration. Pyrite grains
are present on many bedding-plane surfaces and, based on x-radiographic and
petrographic evidence, within samples in association with other heavy mineral
grains. However, there is no conclusive evidence to suggest that these concentrations
of pyrite grains, particularly those within samples and not on exposed bedding plane
surfaces, represent the effects of an overlying microbial mat.
Characteristics of Bedding Plane Surfaces vs. Underlying Bedding: Schieber
(1999) tied the presence of “wavy-crinkly laminations” in Mid-Proterozoic rocks of
the Belt Supergroup to the influence of microbial mats on the substrate. However,
when analyzed using x-radiography and petrography, most of the wrinkle-structure-
bearing samples examined in this study were found to contain little to no evidence
for microbial influence beneath the exposed bedding plane surface. Very few
wrinkle-structure-bearing samples contain thin laminations beneath the wrinkled
surface layer. In fact, x-radiographs of several wrinkle-structure-bearing samples
either indicate the presence of cross-bedding, which reflects moderate- to high-
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energy conditions, or homogeneous material, which may have resulted from
sediment mixing due to bioturbation. Similar observations have been made by
Hagadorn and Bottjer (1999) in Vendian to Lower Cambrian rocks of the
southwestern U.S.
Presumably, Early Cambrian microbial mats “colonized” bedding surfaces
during gaps in sedimentation and then quickly became established (Hagadorn and
Bottjer, 1997). Modern mats cannot survive in settings where the rate of
sedimentation is near-constant because they require sunlight for photoautotrophy.
Some species of mat bacteria are phototactic and can adjust their position on the
sediment relative to light, yet even these cannot survive when the deposition rate
exceeds the top speed at which they can grow upward through the sediment (Noffke
et al., 2001). Thus, microbial mats could not have colonized a substrate that was
characterized by high-energy cross-stratified sand unless a hiatus in sedimentation
occurred. A subsequent return to high sedimentation rates would have caused the
demise of the mat and, if conditions were favorable, the preservation of microbially
mediated sedimentary structures.
The lack of a correlation between surficial microbial features and underlying
sedimentary structures seems to suggest that the chief criterion for determining
whether wrinkle structures will be preserved on a bedding plane surface is not the
preceding sedimentation pattern but the degree to which microbial binding stabilizes
the surface of a bed. Presumably, the thicker a mat grows prior to a rise in
sedimentation rate, the more likely its impression in the sediment will be preserved
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despite subsequent compaction from the accumulation of additional sediment layers
above it. Because microbial mats have high components of both water and organic
matter, compaction should reduce the thickness of a mat by several orders of
magnitude, increasing the probability that the organic matter within the mat will
decay before preservation can occur. Therefore, this study supports Hagadorn and
Bottjer’s (1997) hypothesis that wrinkle structures were produced when pulses of
sediment rapidly buried a thick microbial mat. In addition, whether a sample
represents a microbially-influenced substrate should not be inferred solely on the
basis of the presence or absence of wavy-crinkly laminations when viewed in thin
section or x-radiograph.
Opaque-grain-filled Structures—Volborthella?
Many of the specimens collected from the Harkless Formation, and some
from the Poleta and Campito formations, contain one or more small (<1cm), dense
(near-white in x-radiograph), conical structures that resemble tiny scaphopods (Fig.
26). Some are slightly curved, and they vary in length from one to five millimeters.
X-radiographs show slight density variations along the lengths of some of the larger,
more visible specimens. In a sample containing surface features interpreted as
elephant skin, the fossils are oriented parallel to bedding and are concentrated in the
upper few layers of the slab, at and just below the exposed bedding surface. An x-
radiograph “looking down” on the elephant skin bedding plane reveals that the
conical structures are scattered across the bedding surface (Fig. 35). The conical
FIGURE 35—Negative (left) and positive (right) x-radiographs of an “elephant
skin” slab from the Harkless Formation showing abundant, dense conical structures,
some of which appear to be broken. Slab is 9.3cm wide at its widest point.
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fossils appear to be associated with microbially mediated structures in the other
specimens in which they occur at or near the bedding plane surface. These fossils are
interpreted to be examples of Volborthella, enigmatic mineralized structures that
appear 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 animal to which the tubes belong. Volborthella specimens from the White-Inyo
Mountains are composed predominantly of aligned zircon, magnetite, and pyrite
grains (Hagadorn and Waggoner, 2002) (Fig. 36). Thus, it appears that the
Volborthella animal preferentially selected heavy mineral grains for the construction
of its test (Hagadorn and Waggoner, 2002). This is reflected by the sharp density
contrast between the possible Volborthella fossils and the surrounding sample
material. 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) point out, only one specimen has been found to date
that indicates such an arrangement. A more widely accepted interpretation is that
Volborthella was a “mat sticker” (Seilacher, 1999), and a very recent interpretation
(Bailey et al., 2006) depicts Volborthella as a mat scratcher. The latter two
interpretations, which suggest that Volborthella was a part of the matground
community proposed by Seilacher (1999), make sense in light of the apparent
restriction of Volborthella to the Early Cambrian.
FIGURE 36—Photomicrograph of a conical fossil interpreted to be Volborthella in
a thin section from the Harkless Formation. Fragment of a second possible specimen
is at left. Note ordered, “V”-shaped arrangement of grains within the fossil.
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Interestingly, the specimens examined in this study do not discount any of the
interpretations of Volborthella discussed above. Many of the Volborthella specimens
are associated with microbially mediated structures, yet none appear to have been
preserved in an upright position relative to the surrounding sediment layers. Modern
microbial mats have the ability to trap and bind sediment grains that pass over their
surfaces (Noffke et al., 2001), so it seems reasonable that microbial mats of the Early
Cambrian could have done the same. Presumably, the Volborthella specimens
arrived at their final locations by one of three means: (1) as sclerites, being shed
from their parent organism to become detrital grains and then being trapped by a
mat, (2) dying in place as mat stickers and then being toppled over by strong currents
and/or rapid sediment influx, or (3) dying as free-living mat scratchers upon burial
by storm-deposited sediment. Bailey et al. (2006) argue that the Volborthella
organism was not a mat sticker because no specimens of Volborthella have been
found in which the fossil is preserved in life position, upright relative to bedding.
The findings of this study support the Bailey et al. (2006) argument, although
insufficient evidence exists to eliminate the possibility of a sclerite origin for
Volborthella.
Trace Fossils and Bioturbation
Dominance of Planolites: The primary hypothesis tested in this study was that
trilobite trace fossils, such as Cruziana and Rusophycus, represented the most
common and destructive types of bioturbation present on Early Cambrian shallow
110
subtidal siliciclastic marine substrates and that trilobites were thus responsible for
initiating the agronomic revolution in the environments represented by the Lower
Cambrian succession in the White-Inyo Mountains. Specimens of both Cruziana and
Rusophycus typically exhibit two parallel furrows that are comprised of successive
scratchmarks, which have been interpreted as the result of repetitive, directional,
shallow- to moderate-depth digging behavior by trilobites (e.g., Wenndorf, 1990).
Cruziana has been interpreted to represent sediment deposit-feeding behavior
(Seilacher, 1985) or a combination of grazing and hunting behavior (Wenndorf,
1990), while Rusophycus, long interpreted as a resting or hiding trace (e.g.,
Seilacher, 1955), has recently been re-interpreted as a hunting trace (Jensen, 1990)
based on the discovery of a number of specimens that each contain a Planolites-type
burrow that is truncated by the scratchmarks of Rusophycus (Jensen, 1990).
Although Cruziana and Rusophycus both reflect behaviors that were
potentially quite disruptive to the substrate and to any microbial buildup that may
have been present, Rusophycus is one of the earliest trace fossils to have had a
vertical component and thus figures prominently among “behaviors” that may have
contributed to the beginning of the agronomic revolution. However, it became clear
after examining many exposed bedding plane surfaces in the White-Inyo Mountains
that Rusophycus is not common or abundant enough in the environments represented
to have had more than a local influence on substrate consistency. In fact, Planolites,
a simple horizontal trace fossil (Fig. 4), was found to represent by far the most
common and abundant type of bioturbation activity in the facies studied.
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Planolites is a simple, directional, non-branching horizontal trace fossil that
contains little to no ornamentation, which if present would reflect either indentations
left by body structures or marks produced through the movement of appendages.
Because it is a directional trace fossil, Planolites was clearly produced by a
bilaterally-symmetrical animal. The lack of ornamentation on Planolites burrows,
particularly marks left by appendages, supports the inference that the primary
Planolites tracemaker was a worm-like organism. However, more than one type of
tracemaker may have generated Planolites.
Although Planolites does not typically have a vertical component, the shear
abundance of individual Planolites traces on many of the studied bedding planes
indicates that the activities of Planolites tracemakers had a greater collective impact
on the substrate in the environments studied than the activities of any other category
of tracemaker. In addition, the x-radiographic and petrographic evidence indicate
that some of this horizontal bioturbation may have occurred at shallow depths
beneath the sediment-water interface, possibly as the work of undermat miners.
Thus, it appears that, despite their behavioral limitations, the Planolites tracemakers
were the primary ecosystem engineers on the Early Cambrian substrates represented
by the particular material studied in the White-Inyo Mountains. Whether the
Planolites tracemakers caused significant damage to microbial mats through their
activities, and thus qualify as early “agents” of the agronomic revolution, remains
unclear based on the evidence available. It also appears that the agronomic
revolution may have progressed differently in different shallow marine
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environments. Most of the samples analyzed in this study were taken from outcrops
of fine-grained material representing offshore deposition. A contrasting example
from a coarser-grained, and potentially more nearshore, interval is discussed below.
Planolites Burrow Diameters: A range of sizes among individual Planolites
trace fossils was readily apparent during field examination of bedding planes (Fig.
37), and this variation is corroborated by the results of the Planolites burrow-
diameter study (Figs. 27-30). However, these results are difficult to interpret due to
limited sample sizes and the array of variables that must be taken into account. While
the burrow-diameter datasets from the Campito and Poleta formations are broadly
similar to each other, the dataset from the Harkless Formation is smaller overall,
although it overlaps the other two samples: the quantity of Harkless burrows in the 2-
3mm diameter bin falls in between quantities for the same bin in the other two
datasets. Because the three datasets are not completely distinct from one another, and
the three studied formations do not represent significantly different depositional
settings as shown earlier, it is unlikely that the environments of the Campito, Poleta,
and Harkless formations supported three taxonomically distinct populations of
Planolites tracemakers. Instead, it is more likely that a slight difference in
conditions, such as shallower water and lower current energy in the Harkless
environment, may have favored those particular Planolites tracemakers that relied
more heavily on nutrients abundant in the photic zone. Some, but not all, of the
generalists or deeper-water forms most likely survived in the shallower setting, and
FIGURE 37—Field photographs showing contrasting sizes of Planolites traces.
(Left) Larger-diameter Planolites trace from the Campito Formation. (Right)
Smaller-diameter Planolites trace from the Poleta Formation. Inner horizontal
dimension of frames is 24cm.
113
114
these could be indicated by the overlap in the 2-3mm bin. It is unlikely that these
data reflect the evolution of more specialized shallow-water forms because, as
mentioned earlier, most of the shallow marine shelf environments represented in the
Lower Cambrian succession in the White-Inyo Mountains are thought to have been
present continuously, if in different areas, throughout the period of deposition (e.g.,
Moore, 1976; Schmidt, 1977; Mount, 1980). A larger dataset representing a broader
range of shallow marine settings could help to clarify these potential patterns.
Bedding-plane Bioturbation Indices: As in the case of the burrow diameter
results, both sample size limitations and a large quantity of variables make it difficult
to interpret the data on bedding-plane bioturbation indices (Figs. 16-19). A
noticeable pattern among these data, which may or may not be significant, is that a
one-bin reduction in the range of bedding plane bioturbation indices occurs upsection
from the Campito to the Poleta and from the Poleta to the Harkless. It is possible that
this pattern reflects the smaller sizes of the Poleta and Harkless datasets relative to
the Campito dataset. A larger dataset would be more likely to capture uncommon
occurrences of bedding planes that represent the extreme conditions (bedding plane
bioturbation indices 1 and 5).
Keeping in mind the limitations of this dataset, if only the largest peak for
each formation in the number of bedding planes with a certain bedding plane index is
considered, then there is a trend toward larger bioturbation indices (increasing
quantities of bioturbation) upsection. If this trend in the data reflects a real
115
phenomenon, then its occurrence can be explained in two ways. Similarly to the
trend in burrow diameter results, the trend in bedding plane bioturbation indices may
reflect a response to slight differences in environmental conditions. Although the
three formations from which data were collected do not represent disparate
environments, it appears that the Harkless Formation data, both for burrow diameters
and bedding plane bioturbation indices, reflect a set of conditions that is distinct
from those represented by the Campito and Poleta formations. Unfortunately, the
sample sizes presented in this study are too small to generate meaningful results from
a statistical test of this observation. If this observation were correct, however, then
the larger quantities of primarily small-diameter burrows in the Harkless Formation
might reflect a shallow-water environment with abundant nutrient sources that
supported a large population of photic-zone-adapted tracemakers. A second possible
explanation for the observed pattern in the bedding-plane bioturbation index dataset
is that these data indicate an evolutionary trend toward increasing quantities of
horizontal bioturbation with time, across a broad range of shallow marine
environments. Although it is likely to a degree that increasing quantities of bedding-
plane bioturbation resulted from the evolutionary appearances of new forms adapted
to benthic lifestyles that involved the production of Planolites-like traces, it is
impossible to falsify this hypothesis using the available dataset. In addition, these
data represent only a small sampling of Early Cambrian shallow marine
environments from a single region. If a regional survey of a broader range of
environments were to be conducted, an entirely different trend might manifest itself.
116
The most reasonable interpretation of this limited dataset is that any visible
trends in the data are primarily the product of subtle changes or variations in
environmental conditions in the region coupled with an overarching evolutionary
trend toward increasing quantities of bedding-plane bioturbation over time. An
important consideration for future studies is that the emergence and adaptive
radiation of different groups of shallow-marine benthic organisms may have been
strongly influenced by environmental factors such as grain size, current energy, and
microbial activity. Mount (1980, p. 181-182) recognized the possible significance of
the sudden “debut,” in the White-Inyo Mountain Lower Cambrian succession, of
most non-trilobite phyla in the Montenegro Member of the Campito Formation:
A dramatic change in the number and type of fauna coincident with a change
in environmental conditions strongly suggests that the order of appearance of
organisms in the White-Inyo Mountains sections is facies-controlled (Stanley,
1976) rather than a product of simple phyletic evolution or punctuated
speciation events.
X-radiography and Petrography—Vertical Bioturbation: X-radiographs of
most of the studied samples display few trace fossils, although largely homogeneous
layers in several samples suggest that the sediment underwent periods of extensive
bioturbation. No sample appears to be completely disrupted by bioturbation,
however. Such intermittent periods of bioturbation as these may represent local
fluctuations in conditions at the sediment-water interface. More specifically, these
well-bioturbated intervals may represent the activities of organisms that were
burrowing underneath a microbial mat, which was periodically inundated by
sediment during storm events. Each successive bioturbated interval could represent
117
the re-establishment of the matground community. However, determining the
validity of this hypothesis is impossible because no microbially-mediated
sedimentary structures are visible in x-radiograph beneath exposed bedding surfaces.
X-radiographs of several samples reveal well-defined burrows, of which a
few are deeply vertical (Fig. 25). One burrow in particular from the Campito
Formation resembles an escape structure when viewed in x-radiograph (Fig. 25B).
Thus, deeply vertical bioturbation was not completely absent from the shallow
subtidal siliciclastic environments represented by the Campito, Poleta, and Harkless
formations. However, it appears to have been too small a component of the overall
pattern of bioturbation in these environments to have had much effect on the
substrate.
Implications for the Relationship between Substrate Consistency, Microbial
Activity, and Deep-burrowing Behavior—Skolithos Piperock: The grain-size pattern
observed within thin-section views of Skolithos burrows may reflect the physical
process of burrow infilling that would have occurred after the Skolithos tracemaker
abandoned its burrow. Larger grains would have settled toward the center of the
burrow, and smaller grains would gradually have filled the spaces around them until
only the smallest spaces remained to be filled by very fine-grained sediment. If this
were the case, it might be said that Skolithos burrows were free of sediment fill, and
potentially open to the seafloor, while the burrowing organism was alive. One
problem with this interpretation is the lack of evidence to suggest that the Skolithos
118
tracemaker stabilized its burrow in any way. In a substrate composed almost entirely
of coarse-grained sand that potentially was acted upon by nearshore processes, it
appears unlikely that a burrow could have remained open to the sediment-water
interface for any length of time without collapsing. An alternative interpretation is
that the very fine-grained sediment in these burrows may be composed of a
metamorphic mineral that preferentially replaced small quartz grains at the edges of
the Skolithos burrows. The burrow boundaries may thus have served as fluid
conduits during and/or after burial, which would have facilitated metamorphic
processes. Further investigation is needed to determine whether these interpretations
have merit.
The presence, in x-radiographs, of denser “cylinders” surrounding less dense
Skolithos burrows (Fig. 34) lends support to the hypothesis that metamorphic
minerals (denser than quartz) replaced the quartz grains that originally lined the
burrows. It is also a possibility, however, that the very fine-grained sediment is
primary and in its un-metamorphosed state is denser than the surrounding quartz
grains. Whatever their origin, these cylinders of dense material make Skolithos
burrows easily recognizable in x-ray.
Based on this very limited Skolithos piperock dataset from the Poleta
quartzite, it appears that physical rather than microbial processes were dominant in
the depositional environment of this unit. The relative insignificance of microbial
activity in this setting is most likely the product of a combination of factors, with
grain size and energy level being chief among them. Although the water depth
119
represented by the Poleta quartzite cannot be determined based on the data presented
in this study, coarse-grained sands commonly reflect deposition in moderate- to
high-energy environments, which typically occur close to shore. Shallow water is
ideal for cyanobacterial mats, which rely on photosynthesis for energy. However,
such mats cannot become established on substrates that are constantly shifting due to
wave activity. It is likely that microbes were active in the depositional environment
of the Poleta quartzite given that they are clearly dominant in environments of finer-
grained sedimentation at roughly the same time. This activity did not involve the
construction of mats, however.
Although it appears that microbial mats did not present a significant
limitation to burrowing organisms in the environment of the Poleta quartzite, their
mere absence is unlikely to be the only reason for the success of the Skolithos
tracemaker. Another important difference between this environment and other
siliciclastic settings represented in the Lower Cambrian succession of the White-Inyo
Mountains is grain size. The average grain size of the Poleta quartzite is significantly
larger than that of the other units. Coarse-grained substrates contain more porespace
than fine-grained substrates, and greater water content leads to greater oxygenation.
In addition, a coarse-grained substrate that is hydrated well below the sediment-
water interface can be penetrated more easily by vertical burrowers (Jumars et al., in
press). Thus, grain size may have been the key factor that allowed benthic organisms
to burrow deeply within the sands of the Poleta quartzite but contemporaneously
restricted their activities in other settings.
120
If Skolithos piperock appeared abruptly at the start of the Cambrian as the
literature and field evidence seem to suggest, how does this deeply vertical
bioturbation factor in to the agronomic revolution, if at all? Unless evidence comes
to light for a gradual transition toward Skolithos-type burrowing in coarse-grained
substrates, the answer to this question must be that the agronomic revolution
occurred very differently in settings that gave rise to Skolithos piperock in the
earliest Cambrian. An interesting possibility to consider is that the agronomic
revolution may have affected different marine environments at different times,
resulting in a kind of onshore-offshore transition toward greater infaunal activity.
More research is needed to determine the true nature of the agronomic revolution in
the full range of Cambrian marine environments.
Future Work
In addition to placing the data collected in this study in a broader temporal
and geographical context and determining whether the trends observed represent real
patterns, the goals of future studies will be to determine when the transition from
matground to mixground substrates occurred in a range of shallow marine
environments and whether this transition occurred differently in different
environments. In order to accomplish these goals, strata that range in age from Late
Neoproterozoic to Latest Cambrian will be examined using the techniques described
in this study. Strata will be selected that represent a broad range of generally
shallow-water depositional environments within the continental shelf. In addition,
121
the studied strata will be chosen from localities around the world so that a
cosmopolitan dataset may be developed. Such a dataset will reveal global trends in
bedding plane bioturbation and ecosystem engineering through a critical period in
the history of life.
122
CHAPTER VI
Conclusions
Although macroscopic microbially-mediated sedimentary structures,
particularly wrinkle structures, were observed on the surfaces of many sampled
bedding planes, no definitive evidence for microscopic microbially-mediated
sedimentary structures was found in either the thin sections or the x-radiographs of
the studied samples. Identification of microbial features in Cambrian rocks is
hampered by the often-pronounced effects of diagenetic processes and
metamorphism on the composition and orientation of grains within samples. Because
microscopic microbially-mediated sedimentary structures have been identified in
other Lower Cambrian rocks that contain macroscopic wrinkle structures, it is likely
that the studied samples once contained microscopic evidence of microbial influence
on the substrate.
Contrary to the hypothesis of Schieber (1999) that wrinkle-structure-bearing
bedding planes typically are underlain by strata that contain distinctive wavy-crinkly
laminations, no relationship was observed in the studied samples between the
presence of microbially-mediated sedimentary structures on a bedding-plane surface
and characteristics of the underlying bedding. In fact, x-radiographs of some samples
containing wrinkle-structure-bearing bedding surfaces indicate that the wrinkle
structures capped intervals of cross-bedding or bioturbation-homogenized material.
Based on these results, it appears that microbial stabilization of the substrate may
123
have occurred rapidly enough at times, following an abrupt change in energy
conditions, that very little evidence, if any, of the microbial activity was preserved.
In such cases, it may be impossible to identify any features that reflect microbial
influence in thin sections and x-radiographs.
Opaque-grain-filled conical structures resembling the enigmatic Cambrian
fossil Volborthella are present in a considerable number of samples collected from
the Campito, Poleta, and Harkless formations and commonly are associated with
microbially-mediated sedimentary structures. Volborthella appears to have been
restricted to the Early Cambrian, which indicates that the Volborthella animal may
have been specially adapted to life on Early Cambrian matgrounds and unable to
tolerate the changes in substrate conditions brought about by the agronomic
revolution. The dominantly horizontal orientation of specimens of Volborthella in
the studied samples supports the hypothesis, put forth by Bailey and colleagues (in
press), that the Volborthella animal was not a sessile “mat sticker” but a mobile “mat
scratcher.” However, no evidence was found to refute Signor and Ryan’s (1993)
hypothesis that Volborthella is the sclerite of a larger animal.
The horizontal trace fossil Planolites is numerically dominant on the bedding
planes examined in this study. The trilobite trace fossils Cruziana and Rusophycus
represent behaviors that were considerably more disruptive to the substrate than
Planolites, but these traces are uncommon on the studied bedding planes and thus
appear not to have had a significant impact on substrate conditions. Although
Planolites does not typically have a vertical component, its sheer abundance
124
indicates that the Planolites tracemakers were most likely the primary ecosystem
engineers on the substrates represented by the studied bedding planes. Whether the
Planolites tracemakers caused significant damage to microbial mats through their
activities, and thus qualify as early “agents” of the agronomic revolution, remains
unclear based on the evidence available.
Results of the Planolites burrow-diameter study reflect an overall decrease in
burrow diameters upsection from the Poleta to the Harkless Formation, although the
Campito, Poleta, and Harkless datasets overlap one another. These results indicate
that environmental rather than evolutionary factors were of primary importance in
determining the distribution of tracemakers of different sizes throughout these
formations. It is unlikely that the environments of the Campito, Poleta, and Harkless
formations supported three taxonomically distinct populations of Planolites
tracemakers. Instead, a slight difference in conditions, such as shallower water and
lower current energy in the Harkless environment, may have favored those particular
Planolites tracemakers that relied more heavily on nutrients abundant in the photic
zone. A larger dataset taken from a broader range of shallow marine environments
should help to establish whether the observed patterns indeed reflect subtle
environmental variations rather than an evolutionary trend.
Results of the study of bedding-plane bioturbation indices indicate both a
reduction in the range of data and an increase in the quantity of bioturbation
(reflected by larger bedding-plane bioturbation indices) up-section. Similarly to the
trend in burrow diameter results, the observed patterns in the bedding plane
125
bioturbation index dataset may reflect slight differences in environmental conditions.
The three formations from which data were collected do not represent drastically
different environments. However, it appears that the Harkless Formation data, both
for burrow diameters and bedding plane bioturbation indices, reflect a set of
conditions that is distinct from those represented by the Campito and Poleta
formations. At the same time, it is possible that these data indicate an evolutionary
trend toward increasing quantities of horizontal bioturbation over time, across a
broad range of shallow marine environments. Keeping in mind the limitations of the
dataset, the observed patterns most likely reflect subtle changes or variations in
environmental conditions in the region coupled with an overarching evolutionary
trend toward increasing quantities of bedding-plane bioturbation over time. An
important consideration for future studies is that the emergence and adaptive
radiation of different groups of benthic organisms, represented by different types of
trace fossils, may have been strongly influenced by environmental factors such as
grain size, current energy, and microbial activity.
X-radiographs and thin sections of the studied samples revealed intervals of
intensely-bioturbated material and rare examples of deeply-vertical bioturbation.
Most of these features were not apparent during examination of outcrops and hand
samples. Well-bioturbated intervals within the samples may reflect the activities of
organisms (“under-mat miners”) that were burrowing underneath a microbial mat
that was periodically inundated by sediment during storm events. Each bioturbated
interval might then represent the re-establishment of the matground community
126
following storm burial. Because only isolated examples of deeply-vertical
bioturbation were identified, such behavior most likely did not constitute a
significant component of the overall pattern of bioturbation in these environments
and thus had little impact on the substrate.
Skolithos piperock represents a very different set of conditions from those of
the meter-thick vertical sections that were examined in this study. Coarse-grained
quartzose sands, which are characteristic of Skolithos piperock, commonly reflect
deposition in moderate- to high-energy environments, which typically occur close to
shore. The comparatively smaller grain sizes of most of the studied samples reflect
deposition in lower-energy settings further from shore, although still in shallow
water. Shallow water is ideal for cyanobacterial mats, which rely on photosynthesis.
However, such mats cannot become established on substrates that are constantly
shifting due to wave activity. It is likely that the depositional environment of the
Skolithos-bearing Poleta quartzite was influenced by microbial activity, although to a
much lesser degree than lower-energy shallow-water settings. The relative
insignificance of microbial activity in this setting is most likely the product of a
combination of factors, with grain size and energy level being chief among them.
The contrast between the environment of Skolithos piperock, in which the absence of
thick microbial mats permitted deep burrowing behavior, and the environments of
Planolites-dominated bedding-plane bioturbation, in which microbial mats appear to
have prevented all but shallow-burrowing behavior, raises the possibility that the
agronomic revolution may have affected different marine environments at different
127
times, resulting in a kind of onshore-offshore transition toward increasing infaunal
activity.
Because the results presented here were generated through study of strata that
represent a small sampling of shallow marine environments within a single region,
these data cannot be used as the basis for broad hypotheses concerning the evolution
of burrowing behavior in benthic invertebrates. Additional work is needed to test the
validity of the observed patterns and to place these patterns within a broader
ecological and evolutionary context. Future studies will expand the temporal,
environmental, and geographical range of the data in order to arrive at a better
understanding of the dynamics of the transition from matground to mixground
substrates in shallow marine environments.
128
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APPENDIX
Locality Information
Campito Formation Localities
Goat Spring – Andrews Mountain Member
From Big Pine, take Westgard Pass Road and turn left onto the Ancient Bristlecone
Scenic Byway. Continue past Schulman Grove, where the road becomes dirt. At the
intersection with Silver Canyon Road, turn right. Park at the crest of the first hill (37°
25′ 34″ N, 118° 11 ′ 0″ W). Section is east of Silver Canyon Road and extends down
along the north and south sides of the dry creek bed.
Base of section: 37° 25 ′ 15 ″ N, 118° 10 ′ 43″ W, in creek bed near unconformable(?)
Campito-Deep Spring boundary
Section thickness (base to Silver Canyon Road): ~295m
Meters measured:
North side of creek bed:
GS01 – 43m above base
GS02 – 45m above base
GS03 – 50.3m above base
South side of creek bed:
GS04 - ~155m above base (extrapolated across creek bed)
Poorly exposed; predominant lithology is greenish-brown silty fine sandstone;
occasional muddy interbeds; intervals of massive fine sandstone interspersed; trace
fossils on scattered bedding planes throughout section.
Figure A1—Field photograph of
Goat Spring locality taken from the
south, at prominent curve in
Bristlecone Byway road. Outcrop
in the foreground contains meter
GS04. Largest outcrop to the north
of the creek bed (indicated by the
arrow) contains meters GS01,
GS02, and GS03.
138
Figure A2—Field photograph
of a prominent bedding plane
exposure north of the creek bed
at the Goat Spring locality. 60
X 10cm frame for scale. This
bedding plane occurs 77.5cm
above the base of meter GS03
and is heavily bioturbated
(BPBI 5).
Payson Canyon – Andrews Mountain Member
From Big Pine, drive toward Deep Springs Valley on Westgard Pass Road, past the
turnoff for the Bristlecone Scenic Byway and the Cedar Flat group campgrounds.
Section is on the left side of the road just beyond the eastern boundary of Inyo
National Forest and can be recognized by prominent bedding planes that dip steeply
toward the ESE.
Location of measured meters: 37° 18 ′ 46″ N, 118° 6 ′ 50″ W, near base of hillside
Meters measured: PC01, PC02 (stratigraphically below PC01)
Excellent bedding plane exposures; predominant lithology is gray silty fine
sandstone with iron staining; occasional thin silty to muddy interbeds, laminated
intervals; trace fossils present but not abundant on most bedding plane surfaces.
Figure A3—Field
photograph of Payson
Canyon locality taken
from the southeast on
Westgard Pass Road.
Several prominent
bedding planes, which
dip steeply toward the
east southeast, are
exposed at the base of
the hillside.
139
Poleta Formation Localities
Horse Thief Canyon – Middle Member
From Big Pine, take Westgard Pass Road to Fish Lake Valley. At the entrance to the
valley, the road forks. Take the right (unpaved) fork. This road curves around toward
the southeast and follows the strike of the ridge, eventually entering Willow Wash.
Shortly before the entrance to the wash is a turnoff to the right marked by a
wilderness sign. Take this road into Horse Thief Canyon. Locality is at a wide place
in the road where a vehicle can easily turn around (37° 22 ′ 2 ″ N, 117° 49 ′ 52″ W).
Meters measured:
West of Horse Thief Canyon road:
HT01 – base of hillside
HT02 – north of HT01, on a rise between two gullies
East of Horse Thief Canyon road:
HT03 – crest of the ridge (37° 21 ′ 48 ″ N, 117° 49 ′ 44″ W)
HT04 – down the hill toward the NW from HT03
Meters HT01 and HT02 occur within small shaley outcrops that are predominantly
composed of siltstone in which thin interbeds of cross-bedded fine sandstone and
mudstone occur. Wrinkle structures and trace fossils are common to abundant on
bedding planes. Mica grains are common on bedding plane surfaces. Meters HT03
and HT04 occur within a quartzite interval that crops out sporadically and poorly
along the hillside but is well-exposed at the crest of the ridge. Horizontal trace fossils
become more common toward the crest of the ridge; vertical trace fossils Skolithos
and Diplocraterion common to abundant throughout.
Figure A4—Field
photograph of the
Horse Thief Canyon
locality taken from
the east on the
hillside where meters
HT03 and HT04
occur. Vehicles are
parked near meter
HT01, at the base of
the northern slope.
Meter HT02 is
located outside the
frame of the photo,
on the northern side.
140
Taphrhelminthopsis Canyon – Middle Member
From Big Pine, take Westgard Pass Road into Deep Springs Valley. Turn right onto
the first dirt road that intersects Westgard Pass Road. Follow this road down into a
wash and then up and around the north side of the first of two isolated hills. To find
and enter the canyon, it is best to hike up the hillside and approach from above. A
prominent bedding plane containing Taphrhelminthopsis is located at the upper end
of the canyon and can be spotted from the hillside above the entrance to the canyon.
Location: 37° 18 ′ 45″ N, 118° 05 ′ 15″ W
Meter measured: TC01 (includes large Taphrhelminthopsis bedding plane)
Predominant lithology is gray to tan fine sandstone with occasional finer-grained
interbeds; very few sedimentary structures are visible due to metamorphism.
Bedding planes not extensively bioturbated, with exception of Taphrhelminthopsis.
Figure A5—Two field photographs of the Taphrhelminthopsis Canyon locality.
Photo at left taken looking down into the canyon from the southern (upper) entrance.
Students are sitting next to the prominent Taphrhelminthopsis bedding plane. Photo
at right taken looking into the canyon from the western side, showing entry route.
Large partially-shaded bedding plane exposure contains Taphrhelminthopsis.
141
Harkless Formation Localities
Folded Creek
From Big Pine, take Westgard Pass Road to the Cedar Flat group campgrounds. Turn
right into the first entrance, for the Piñon Group campgrounds. Turn right at the first
opportunity and park within one hundred meters of the turnoff. Hike south along the
west side of the creek bed; all outcrops of interest are on the western side.
Location: 37° 15 ′ 57″ N, 118° 8 ′ 51″ W
Meters measured:
FC01 – western side of creek bed
FC02 – directly above FC01
Predominant lithology is tan to greenish fine sandstone to silty fine sandstone with
occasional thin silty to muddy interbeds; ripple marks (both current and interference)
present on bedding planes; cross-bedding common; trace fossils common to
abundant on bedding plane surfaces.
Figure A6—Field photograph, taken toward the southwest, of the creek bed at the
Folded Creek locality, where Harkless Formation strata are moderately deformed.
Measured meters are located on the hillside above and to the right (west) of the creek
bed, outside the frame of the photograph.
142
143
Harkless Point
From Big Pine, take Westgard Pass Road to the Cedar Flat group campgrounds. Turn
right into the second entrance, for the Juniper and Poleta Group campgrounds. Turn
onto the second road that splits off to the left, near the radio telescopes. Follow this
road across the flat onto a ridge that overlooks Deep Springs Valley. Hike along the
strike of this ridge and then follow the ridge as it curves toward Deep Springs
Valley. The locality is a large knob of Harkless Formation material that juts out from
the ridge.
Location: 37° 17 ′ 6″ N, 118° 7 ′ 57″ W
Meter measured: HP01 – includes the uppermost bedding surface of the knob
Large outcrop with extensive bedding plane exposures although preservation is
poorer than at Folded Creek locality; predominant lithology is cross-bedded fine-
grained quartzose sandstone with occasional silty to shaley interbeds; trace fossils
mostly indistinct but common to abundant on bedding plane surfaces.
Andrews Mountain
From Big Pine, take Westgard Pass Road and, within a mile, turn right onto Saline
Valley Road. Continue on Saline Valley Road for ~10.5 miles. The turnoff for
Harkless Flat (a dirt road to the right) is ~2 miles before the turnoff for Andrews
Mountain. Turn right onto the dirt road leading to Andrews Mountain and Papoose
Flat (37 7 22 N, 118 4 33 W). The locality is ~4 miles from Saline Valley Road,
WNW of Andrews Mountain.
Top of section: 37° 4 ′ 47 ″ N, 118° 6 ′ 44″ W – Salterella shellbed
Skolithos-bearing quartzite unit: 37° 4′ 43″ N, 118° 6 ′ 29″ W
Thickness of Harkless Formation strata: ~220m (measured from top of uppermost
Poleta Formation limestone unit)
Base of section (near Poleta-Harkless contact) is highly metamorphosed; bluish-
green siltstone is predominant lithology in lowest 28 meters, some bedding planes
contain abundant trace fossils; thin Volborthella shell bed between meters 23 and 24
within thin quartzite interbeds; quartzite outcrops alternate with covered intervals
beginning at meter 51 through meter 77, become larger up-section; larger quartzite
outcrops contain abundant Skolithos; reddish-tan to greenish muddy siltstones occur
mostly in small outcrops between meter 77 and the top of the section; Salterella
common to abundant in siltstones between meter 200 and the top of the section; few
bioturbated bedding planes between 200-220m.
Figure A7—Field
photograph taken
toward the west
(Sierra Nevada in
background) of the
top of the Harkless
Formation section at
Andrews Mountain.
Small outcrops of
muddy reddish-tan
siltstone are shown,
which contain
abundant external
molds of Salterella.
Figure A8—Field photographs of a Salterella shellbed near the top of the Harkless
Formation section at Andrews Mountain in which Salterella is commonly preserved
as an external mold. Image at right is a close-up of the lower-right corner of the
photo at left. Pen, for scale, is 13.5cm in length.
144
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