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The unusual sedimentary rock record of the Early Triassic: Anachronistic facies in the western United States and southern Turkey
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The unusual sedimentary rock record of the Early Triassic: Anachronistic facies in the western United States and southern Turkey
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THE UNUSUAL SEDIMENTARY ROCK RECORD OF THE EARLY TRIASSIC: ANACHRONISTIC FACIES IN THE WESTERN UNITED STATES AND SOUTHERN TURKEY Copyright 2004 by Sara Brady Pruss A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (GEOLOGICAL SCIENCES) August 2004 Sara Brady Pruss R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. UMI Number: 3145267 Copyright 2004 by Pruss, Sara Brady All rights reserved. INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 3145267 Copyright 2004 by ProQuest Information and Learning Company. 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, Ml 48106-1346 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Acknowledgements I have so many people to thank for all of the support and encouragement that has culminated in this dissertation and the list here is by no means all inclusive. Firstly, I want to sincerely thank my advisor, David Bottjer, for his unfailing guidance and for believing in me when I didn’t always believe in myself. Frank Corsetti has also been both an invaluable mentor and dear friend, and I am so grateful he arrived at USC during my time here. Additionally, I want to thank the other members of my committee, Donn Gorsline, Doug Capone, and A 1 Fischer, who helped shape this project into what it has become. I owe a debt of gratitude to my labmates, who have offered advice and support during the trying times and joy and laughter during the happy times. I am grateful to have shared my graduate experience with Nicole Bonuso, Matthew Clapham, Steve Dornbos, Margaret Fraiser, Karina Hankins, Catherine Jamet, Pedro Marenco and Katherine Nicholson. I could not have accomplished any of my goals without people to help me carry my rocks, and for that I would like to acknowledge many of my labmates, as well as Craig Herbold and Andrew Turner. Last, but certainly not least, I want to thank my mother, father and brother for their unconditional love and pride in my accomplishments and my husband, David DeSwert, for making every day worthwhile. perm ission o f the copyright owner. Further reproduction prohibited without perm ission. iii Table of Contents Acknowledgements ii List of Figures vi Abstract xi Chapter 1: Introduction 1 The end-Permian mass extinction 1 Possible Causes 3 The Early Triassic: On the Road to Recovery 5 Chapter 2: Anachronistic facies in the Early Triassic: Background 12 Previous Work 14 Microbial Reefs 14 Carbonate Seafloor Fans 18 Flat-pebble Conglomerates 18 Ribbon Rock 20 Wrinkle Structures 20 Hypothesis 21 Methodology 22 Field Study 22 Thin-Section Analysis 26 Stratigraphy and Paleoenvironments 26 Southwestern United States 26 Southern T urkey 3 7 Chapter 3: Early Triassic Microbial reefs: western United States and southern Turkey 45 Introduction 45 Geologic Description 51 Methods 57 Results 58 Field Analysis 58 Thin-Section Analysis 65 Discussion 82 Depositional Model 85 Early Triassic stromatolites as disaster forms 86 Summary 87 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Chapter 4: Recovery of Reefs from the end-Permian mass extinction: An Overview 89 Introduction 89 The end-Permian mass extinction: The effects on reefs 90 The Early Triassic: A Survival Phase from the Biotic Recovery 92 Early Triassic microbial reefs 95 Middle Triassic: Recovery of metazoan reefs 99 Discussion 101 Summary 102 Chapter 5: Carbonate Seafloor Fans 104 Introduction 104 Geologic Setting 105 Methods 108 Results 110 Field Analysis 110 Thin-Section Analysis 120 Discussion 126 Summary 130 Chapter 6: Flat-pebble conglomerates 132 Introduction 132 Methods 133 Results 133 Field Analysis 133 Thin-section Analysis 137 Discussion 138 Summary 139 Chapter 7: Wrinkle Structures 140 Introduction 140 Study locations and methods 143 Results 145 Depositional and Taphonomic Conditions 147 Summary 151 Chapter 8: Other Unusual Facies 153 Introduction 153 Thinly-bedded limestone-silty limestone facies 153 Chip facies 156 Thin bioturbated beds 160 Summary 165 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. V Chapter 9: Sequence Stratigraphic Framework 167 Introduction 167 Muddy Mountains Overton locality 168 Muddy Mountains Ute locality 168 Mountain Pass locality 173 Lost Cabin Springs locality 176 Middle and Upper Members of Union Wash Formation, Darwin Hills locality 179 Implications for Anachronistic facies 182 Summary 184 Chapter 10: Early Triassic Trace Fossils: Case Study from the Virgin Limestone Member, southwestern United States 186 Introduction 186 Stratigraphy 188 Methods 189 Description of Virgin Limestone Trace Fossils 199 Size Distribution 209 Ichnofabric Index and Bedding Plane Coverage 213 Trace Fossils and Paleoenvironments 214 Trace Fossils as Indicators of Biotic Recovery 217 Trace Fossil Size 219 Ichnofabric Indices and Bedding Plane Coverage 222 Trace Fossil Complexity 224 Tiering 225 Trace Fossils and Environmental Stress 226 Summary 228 Chapter 11: Early Triassic Paleooceanography 231 Permian-T riassic Boundary 231 Early Triassic Oceanography 233 Chapter 12: Conclusions 240 Summary 246 References 249 Appendix 269 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. vi List of Figures Figure 1-1: Phanerozoic marine familial diversity 2 Figure 1-2: Early Triassic lag phase diagram 6 Figure 1-3: Stages of the Early Triassic B Figure 1-4: Paleogeographic map from the Early Triassic 11 Figure 2-1: Locality map from southwestern United States 15 Figure 2-2: Locality map from southern Turkey 16 Figure 2-3: Diagram showing Great Bank of Guizhou during the Early Triassic 17 Figure 2-4: Diagram showing effects of anoxic ocean overturn 19 Figure 2-5: Stratigraphic column of studied sections 28 Figure 2-6: Stratigraphic column of the Virgin Limestone Member, Muddy Mountains Overton locality 29 Figure 2-7: Stratigraphic column of the Virgin Limestone Member, Muddy Mountains Ute locality 31 Figure 2-8: Stratigraphic column of the Virgin Limestone Member, Mountain Pass locality 33 Figure 2-9: Stratigraphic column of the Virgin Limestone Member, Lost Cabin Springs locality 35 Figure 2-10: Stratigraphic column of the Middle and Upper Members of the Union Wash Formation, Darwin Hills locality 38 Figure 2-11: Stratigraphic column of the Katarsi Formation 40 Figure 2-12: Stratigraphic column of the Kokarkuyu Formation 41 Figure 2-13: Stratigraphic column of the Sapadere Formation 44 Figure 3-1: Reef components of the Late Permian through Early Jurassic 47 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 3-2: Photograph of a smaller dome within a larger mound, southwestern US 52 Figure 3-3: Photograph of a large microbial mound, southwestern US 54 Figure 3-4: Photograph of small domes within large mound, southwestern US 56 Figure 3-5: Photograph and sketch of microbial mound, southwestern US 59 Figure 3-6: Photograph of layered microbialites, southern Turkey 61 Figure 3-7: Photograph of small thrombolite mound, southern Turkey 62 Figure 3-8: Photograph of domes within thrombolites 63 Figure 3-9: Photograph of large thrombolite mounds 64 Figure 3-10: Photograph of small domes within thrombolites 66 Figure 3-11: Thin-section photograph of microbial laminations 67 Figure 3-12: Thin-section photograph of bladed cements atop laminations 68 Figure 3-13: Thin-section photograph of metazoan fragment wackestone 69 Figure 3-14: Thin-section photograph of void with bladed cements 70 Figure 3-15: Thin-section photograph of peloidal fabrics 73 Figure 3-16: Thin-section photograph of fossil debris with cements 74 Figure 3-17: Thin-section photograph showing cements on mounds 75 Figure 3-18: Thin-section photograph of pyrite 76 Figure 3-19: Thin-section photograph of biserimminid foraminifera 77 Figure 3-20: Thin-section photograph of Rectocornuspira kalhori 78 Figure 3-21: Thin-section photograph of many foraminifera 80 Figure 3-22: Thin-section photograph of Girvanella 81 perm ission of the copyright owner. Further reproduction prohibited without perm ission. Figure 4-1: Diagram showing effects of end-Permian extinction on reefs Figure 4-2: Paleogeographic map showing locations of microbial reefs Figure 5-1: Photograph of underside of stromatolite bed, southern Turkey Figure 5-2: Photograph of cross-bedded oolite, southern Turkey Figure 5-3: Photograph of calcium carbonate fan, southwestern US Figure 5-4: Photograph of cross-cutting fans, southwestern US Figure 5-5: Photograph of large crystal mound, southern Turkey Figure 5-6: Photograph of thin crystal beds, southern Turkey Figure 5-7: Photograph of calcium carbonate fans, southern Turkey Figure 5-8: Photograph of limestone beds between mounds, southern Turkey Figure 5-9: Photograph of beds of crystals, southern Turkey Figure 5-10: Thin-section photograph of carbonate crystals Figure 5-11: Thin-section photograph showing cross-section of crystal Figure 5-12: Thin-section photograph showing twinning of crystals Figure 5-13: Thin-section photograph showing crystal spindles Figure 5-14: Thin-section photograph showing laminations in crystals Figure 5-15: Thin-section photograph showing crystal nucleation site Figure 6-1: Photograph of a bedding plane of flat-pebble conglomerates, southwestern US Figure 6-2: Photograph of vertical section of flat-pebble conglomerate, southern Turkey Figure 6-3: Photograph of same flat-pebble bed pictured in Figure 6-2 Figure 7-1: Paleogeographic map showing location of wrinkle structures perm ission o f the copyright owner. Further reproduction prohibited without perm ission. ix Figure 7-2: Photograph of Lower Triassic strata showing wrinkle structures, southwestern US 146 Figure 7-3: Photograph of Cambrian wrinkle structures, western US 149 Figure 8-1: Photograph of thinly-bedded limestone-silty limestone facies, Mountain Pass locality, southwestern US 155 Figure 8-2: Photograph of thinly bedded limestone-silty limestone facies, Darwin Hills locality, southwestern US 157 Figure 8-3: Photograph of chip facies, southwestern US 158 Figure 8-4: Photograph of a talus piece of chip facies, southern Turkey 159 Figure 8-5: Photograph of thin bioturbated beds, southwestern US 162 Figure 8-6: Photograph of thin bioturbated beds within a limestone ledge, southwestern US 163 Figure 8-7: Photograph of Planolites, southwestern US 164 Figure 9-1: Measured stratigraphic column showing trends in depth, Muddy Mountains Overton locality, southwestern US 169 Figure 9-2: Measured stratigraphic column showing trends in depth, Muddy Mountains Ute locality, southwestern US 171 Figure 9-3: Measured stratigraphic column showing trends in depth, Mountain Pass locality, southwestern US 174 Figure 9-4: Measured stratigraphic column showing trends in depth, Lost Cabin Springs locality, southwestern US 177 Figure 9-5: Measured stratigraphic column showing trends in depth, Darwin Hills locality, southwestern US 180 Figure 10-1: Stratigraphic column from Muddy Mountains Overton locality showing distribution of trace fossils 190 Figure 10-2: Stratigraphic column from Muddy Mountains Ute locality showing distribution of trace fossils 192 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. X Figure 10-3: Stratigraphic column from Mountain Pass locality showing distribution of trace fossils 194 Figure 10-4: Stratigraphic column from Lost Cabin Springs locality showing distribution of trace fossils 196 Figure 10-5: Photograph of Arenicolites and Rhizocorallium 200 Figure 10-6: Photograph of Asteriacites 202 Figure 10-7: Photograph of Gyrochorte 203 Figure 10-8: Photograph of Laevicyclus 205 Figure 10-9: Photograph of Planolites 206 Figure 10-10: Histogram of Planolites and Rhiocorallium burrow diameters 207 Figure 10-11: Photograph of Thalassinoides 210 Figure 10-12: Histograms of Thalassinoides burrow diameters 211 Figure 10-13: Schematic diagram showing paleoenvironments of traces 216 Figure 11-1: Diagram showing ocean overturn mechanism to explain delayed recovery 238 Figure 11-2: Diagram showing fluctuations in ocean chemistry that changed substrates during the Early Triassic 239 R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. xi Abstract The end-Permian mass extinction occurred ~251 million years ago and represents the largest extinction in the history of life. Following end-Permian mass extinction, a large-scale reorganization of marine communities occurred after a delay of 4 —7 million years. To better understand why this recovery was delayed, the Early Triassic has been widely studied to understand the mechanisms responsible for the suppression of metazoans during this time. The widespread occurrence of anachronistic facies has been documented globally from Lower Triassic sections. Anachronistic facies are facies that commonly formed in marine environments much earlier in time (Proterozoic-Ordovician) and thereafter became geologically rare for hundreds of millions of years. Anachronistic facies include microbial reefs, carbonate seafloor fans, flat-pebble conglomerates, and wrinkle structures. In addition to true anachronistic facies, a variety of other unusual facies are reported and described here. The unusual resurgence of anachronistic facies in the Early Triassic has been documented in this research from the Spathian Virgin Limestone of the Moenkopi Formation and Union Wash Formation, southwestern United States and the Griesbachian Katarsi, Kokarkuyu, and Sapdere Formations from southern Turkey. Anachronistic facies occur at two intervals of time that bracket the Early Triassic, and this indicates that prolonged stressful environmental conditions such as low oxygen levels and carbon dioxide poisoning persisted through much of the Early Triassic. These conditions would have suppressed the recovery of metazoans, and this suppression is reflected in the low diversity of organisms as well as low levels of R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. bioturbation. As part of this study, trends in bioturbation were documented from Lower Triassic sections in the southwestern United States. The results of this study illustrated that equatorial regions likely recovered more slowly from the end-Permian mass extinction than mid-high latitudes. Through study of anachronistic facies and the unique bioturbation record of the latest Early Triassic, it can be concluded that deleterious environmental conditions played a role both in the formation of the anachronistic facies and the suppression of marine organisms. This suppression likely delayed the biotic recovery from the end-Permian mass extinction. R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 1 Introduction The end-Permian Mass Extinction The end-Permian mass extinction event occurred 251.4 million years ago (Bowring et al., 1998) and brought about a near annihilation of marine life. Estimates suggest that 80% of marine invertebrate species became extinct at this time (Stanley and Yang, 1994), bringing about the demise of the “Paleozoic fauna” (Sepkoski, 1981; Sepkoski, 1984) (Figure 1-1). This extinction occurred in two pulses separated by approximately 5 million years (Stanley and Yang, 1994). The first pulse devastated Tethyan faunas and occurred in the Late Maokouan (Jin et al., 1994). The second pulse occurred at the close of the Permian and was the more severe of the two extinctions. The duration of this extinction event is still the subject of debate (e.g. Hallam and Wignall, 1997; Twitchett et al., 2001). The devastation of the end-Permian extinction is not limited to marine ecosystems; evidence suggests that terrestrial ecosystems were also distressed by this event. A changeover in river systems across the Permian-Triassic boundary has been recorded from the Karoo Basin, South Africa (Ward et al., 2000), suggesting a rapid turnover in land plants. An extinction of terrestrial vertebrates (e.g. Benton, 1985; Retallack et al., 2003) and peat-forming floras (Retallack et al., 1996) have also been documented. A large-scale degradation of organic material is also thought to have caused a fungal spike at the Permian-Triassic boundary (Twitchett et al., 2001). perm ission of the copyright owner. Further reproduction prohibited without perm ission. Number o f Families 800- 600- 400- M odem Fauna Paleozoic F auna 200- C am brian Fauna 200 400 Geologic Time (m illions o f years) Figure 1-1: Phanerozoic marine familial diversity curve showing three evolutionary faunas (modified from Sepkoski, 1981). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 3 Possible Causes Several mechanisms have been proposed as causes for the end-Permian extinction event; however, study of this extinction has so far failed to yield a “smoking gun”. Some of the proposed mechanisms include global warming, marine anoxia, extraterrestrial impacts, catastrophic methane release, and widespread volcanism. Some of these mechanisms may be inter-related (i.e. widespread volcanism may have caused global warming by releasing large volumes of carbon dioxide into the atmosphere. This in turn could have led to catastrophic methane release), and it is also possible that this extinction was multi-causal (Erwin, 1994). There is much evidence for global warming during the transition from Permian to Triassic time, however, it is still debated whether or not this warming could bring about a mass extinction. Evidence for warming includes, but is not limited to, the occurrences of warm temperate paleosols from Antarctica and Australia (Retallack et al., 1996) and, as previously mentioned, the changeover in the Karoo Basin, South Africa, from a humid temperate climate to an arid climate in the Early Triassic (Ward et al., 2000). A variety of mechanisms have been cited to explain global warming from the end of the Permian into the Triassic. It is thought that elevated levels of C 02 possibly related to the eruption of the Siberian Traps may have caused a global increase in temperature (e.g. Martin and Macdougall, 1995; Wignall, 2001). Warming could have also brought about a destabilization of methane clathrates, and release of methane may have fed back on global warming (e.g. de Wit et al., 2002; Ryskin, 2003; White, 2002). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 5 The Early Triassic: On the Road to Recovery In addition to the research that has been conducted on the causes of the end- Permian mass extinction, much work has focused on the recovery interval from this event. True biotic recovery from the end-Permian extinction, determined by the re appearance of organisms not present since the Permian and an increase in marine ecosystem diversity, did not occur until the Middle Triassic (Hallam, 1991). The Early Triassic is therefore a lag phase from the biotic recovery during which diversity of organisms was very low (Erwin, 1993) (Figure 1-2). The possible reasons for this delayed recovery include severity of the mass extinction (e.g. Schubert and Bottjer, 1992), taphonomic biases (e.g. Erwin and Hua-Zhang, 1996) and prolonged environmental stresses (e.g. Hallam, 1991; Woods et al., 1999). The Early Triassic, encompassing the lag phase of the biotic recovery, is characterized by unusual marine faunas (e.g. Hallam and Wignall, 1997) and an anomalous sedimentary rock record (e.g. Wignall and Twitchett, 1999). Four genera of bivalves dominated most marine benthic communities (Hallam and Wignall, 1997), and biotic recovery opportunists flourished in the aftermath of the end- Permian extinction (Fraiser and Bottjer, 2004; Rodland and Bottjer, 2001). Many organisms such as corals and sponges that are known from most time intervals of the Phanerozoic were also conspicuously absent from marine benthic and reef ecosystems (e.g. Fliigel, 2002). The sedimentary rock record reflects the changes in the biota during the Early Triassic . A chert gap has been noted from Permian to R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 4 A bolide impact has been suggested as a potential cause of the end-Permian mass extinction. Evidence for this includes the presence of chondritic meteorite fragments found in Permian-Triassic boundary sediments (Basu et al., 2003) and the presence of fullerenes containing extraterretrial gases (Becker et al., 2001). A model of bolide-induced catastrophic sulfur release from the mantle has also been proposed (Kaiho et al., 2001). Although some research does suggest that the timing of a bolide impact corresponds to the Permian-Triassic boundary, no study has unequivocally linked the cause of the end-Permian mass extinction with a bolide impact. Widespread anoxia occurred during Late Permian-Early Triassic time, and this has been suggested as the leading cause of the extinction. Anoxic deposits have been reported from many locations around the world (e. g. Isozaki, 1994; Isozaki, 1997; Wignall and Hallam, 1992), and recent modeling efforts have shown that ocean stagnation may have caused this anoxia (Hotinski et al., 2001). Oceanic anoxia has been evoked as a kill mechanism for the end-Permian mass extinction (e.g. Wignall and Hallam, 1992), but other consequences of anoxia such as hypercapnia (carbon dioxide poisoning) have also been suggested as the dominant mechanisms for the end-Permian mass extinction (Knoll et al., 1996). The interrelated nature of many of these proposed mechanisms supports the hypothesis that an intersection of many conditions may have ultimately brought about the end-Permian mass extinction (Erwin, 1993). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 6 End o f Perm ian Early Triassic M iddle Triassic survival extinction recovery D I V E R S I T Y TIME Figure 1-2: Early Triassic lag phase diagram showing end-Permian extinction (extinction of organisms), followed by the survival phase (dominated by survivors and biotic recovery opportunists) in the Early Triassic and recovery phase (radiation and re-appearance of organisms) in the Middle Triassic (modified from Hallam and W ignall, 1997). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 7 Middle Triassic time, and this has been attributed to the extinction of siliceous organisms such as radiolarians and siliceous sponges (Racki, 1999). A coal gap also spans the Early Triassic due to the demise of peat-forming floras at the end of the Permian (Retallack et al., 1996). Because of the extinction of many colonial and reef-dwelling organisms, metazoan reefs are absent globally from Lower Triassic strata (e.g. Fagerstrom, 1987). In addition to the various features missing from the sedimentary rock record, there is a proliferation of anachronistic facies in Lower Triassic strata (e.g. Wignall and Twitchett, 1999), and this will be discussed in detail below. For these reasons, the Early Triassic has garnered attention because the biota and sedimentary rock record are anomalous when compared to the rest of the Phanerozoic. The delayed recovery of organisms did not occur until Middle Triassic time, and this is reflected in the unusual biota and sedimentary rock record of the Early Triassic that spans from the Griesbachian to the Spathian (Figure 1-3). The exact timing of the biotic recovery has been debated. Some have suggested that the biotic recovery was delayed for as long as 7 million years after the end-Permian mass extinction (Hallam, 1991). Other workers in South China have dated the Spathian—Anisian boundary at ~247 Ma (Martin et al., 2001), suggesting the recovery was closer to 5 million years. Fine-scale resolution of the timing of the Early Triassic and the biotic recovery is, therefore, still debated. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 8 247 Ma 251. 4 Ma Spathian o • i—H tz ) CZ3 03 • Smithian C 3 w Dienerian Griesbachian Figure 1-3: Stages of the Early Triassic with dates from Bowring et al. (1998) and Martin et al. (2001). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 9 As mentioned above, various explanations have been put forth to explain the Early Triassic biota and sedimentary rock record including the severity of the end- Permian mass extinction, taphonomic biases, and prolonged environmental stresses. When describing the proliferation of disaster forms in Early Triassic time, Schubert and Bottjer (1992) suggested that the devastation of marine organisms was so severe that their recovery was slow as a result. Subsequent work suggested that the sedimentary rock record was poorly preserved during the Early Triassic, and that this could account for the apparent low diversity of organisms (Erwin and Hua-Zhang, 1996). Other work has suggested that prolonged environmental stresses related to the end-Permian mass extinction persisted through the Early Triassic and inhibited the recovery of organisms (e.g. Hallam, 1991; Lehrmann, 1999; Pruss et al., 2003; Woods et al., 1999). All of these factors may have played a role in the delayed recovery; however, the prolonged environmental stress hypothesis is supported by the research presented here. Environmental stress has been touted as a mechanism to explain the delayed recovery, but the source of this stress is still questioned. An increase in oceanic C 02 , as has been suggested by Knoll et al. (1996), would cause hypercapnia, or carbon dioxde poisoning. The mechanism to bring about this poisoning would include the overturn of an anoxic ocean that fostered the enrichment of surface waters with carbon dioxide. Deep-water enriched in carbon dioxide could periodically flood the shelf and poison benthic organisms. Another source of environmental stress may be oceanic anoxia. Evidence for marine anoxia has also been found in several sections R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. 10 globally during Permian-Triassic time (Isozaki, 1994; Isozaki, 1997; Wignall and Hallam, 1992). Some evidence suggests that deep-sea anoxia could have persisted for a long as 20 million years (Isozaki, 1997). Recent work has suggested that the decimation of reefs at the close of the Permian is consistent with oxygen-related stress (Weidlich et al., 2003). Widespread anoxia may have also fostered the build ups of hydrogen sulfide, and this, too, may have contributed to deleterious environmental conditions (e.g. Kajiwara et al., 1994; Keith, 1982; Marenco et al., 2003). Another factor that likely played a role in the prolonged environmental stress was the configuration of the continents during Late Permian-Early Triassic time (e.g. Hotinski et al., 2001). The continents were assembled into a supercontinent, Pangea, with the largest ocean being Panthalassa and a small, predominately equatorial ocean called Paleotethys (Figure 1-4). The preponderance of oceanic anoxia during Late Permian through Early Triassic time discussed above has been linked to this stagnation (e.g. Isozaki, 1994; Isozaki, 1997; Wignall and Twitchett, 1996). R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. 11 Paleotethys rE A Panthalassa Panthalassa Figure 1-4: Paleogeographic map from the Early Triassic showing supercontinent Pangea, and Panthalassa and Paleotethys oceans (modified from Pruss et al., 2004; after Erwin, 1993). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. CHAPTER 2 Anachronistic Facies in the Early Triassic: Background Sepkoski et al. (1991) first described anachronistic facies after noting the restriction of certain facies such as flat-pebble conglomerates to Precambrian—Early Paleozoic time. Their hypothesis was that these facies commonly formed in the Precambrian—Early Paleozoic because of low levels of bioturbation, therefore, when these facies are found later in geologic time, they indicate environmental conditions that restrict metazoan activity such that bedding resembles that of the Cambro- Ordovician or earlier times. In the Precambrian-Early Paleozoic, early in the evolution of life, metazoans had not developed the ability to significantly disturb bedding through bioturbation (e.g Bottjer et al., 2000). Bioturbation in shelf environments was minimal, so the mixed layer that commonly develops in modern shelfal settings was reduced. Because of this, facies such as flat-pebble conglomerates commonly formed in shelfal settings. The rarity of these facies in post-Cambrian environments was hypothesized to reflect an increase in metazoan activity. Thus, where these facies do occur in post-Cambrian settings, it is thought to reflect unusual environmental conditions that suppress metazoan activity such as bioturbation. In the case of the carbonate seafloor precipitate anachronistic facies, this proliferation is not thought to be linked to reduced bioturbation as proposed by Sepkoski et al. (1991); it instead suggests that ocean chemistry typical of the Archean and Paleoproterozoic, when precipitates like these commonly formed (e.g. perm ission of the copyright owner. Further reproduction prohibited without perm ission. 13 Sumner and Grotzinger, 1996a; Sumner and Grotzinger, 1996b; Sumner and Grotzinger, 2000), was transiently restored in the Early Triassic. The Early Triassic oceans were therefore likely oversaturated with respect to calcium carbonate and may have contained inhibitors to micrite production such as dissolved Fe+ 2 (Sumner and Grotzinger, 1996a; Sumner and Grotzinger, 1996b; Sumner and Grotzinger, 2000). The anachronistic facies that have been documented globally from Lower Triassic strata include microbial reefs (e.g. Lehrmann, 1999; Pruss and Bottjer, 2004), carbonate seafloor fans (Heydari et al., 2003; Woods et al., 1999), flat-pebble conglomerates (Wignall and Twitchett, 1999), and siliciclastics containing wrinkle structures (Pruss et al., 2004). Other unusual facies have also been documented from Lower Triassic strata including thin-bedded limestone-mudstone facies, unusual chip facies, and thin bioturbated beds, and these will also be discussed in detail. Anachronistic facies such as microbial reefs, flat-pebble conglomerates and siliciclastics containing wrinkle structures form in the absence of deep bioturbation and therefore indicate that bioturbation did not fully recover until the Middle Triassic. The presence of carbonate seafloor fans, however, points to anomalous ocean chemistry and suggests that Early Triassic oceans may have been more similar to those of the Precambrian than to the rest of the Phanerozoic. In this research, anachronistic facies were documented and described from Lower Triassic strata of the southwestern United States and southern Turkey. The Spathian Virgin Limestone Member of the Moenkopi Formation and R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. 14 Smithian—Anisian Middle and Upper Members of the Union Wash Formation were studied as part of this research in the southwestern United States (Figure 2-1). In southern Turkey, the Griesbachian Katarasi, Kokarkuyu and Sapadere Formation were studied at three localities (Figure 2-2). In the following chapters, the occurrence of anachronistic facies in the southwestern United States and southern Turkey will be documented in a detailed analysis. Previous Work As mentioned above, Sepkoski et al. (1991) first described anachronistic facies after noticing the restriction of flat-pebble conglomerates to Cambrian deposits. After Cambro-Ordovician time, many of these facies disappear from the geologic record. Recent work on Lower Triassic strata has determined that many anachronistic facies undergo a resurgence (e.g. Wignall and Twitchett, 1999). Microbial reefs, carbonate seafloor fans, flat-pebble conglomerates, and ribbon rock have all been documented from Lower Triassic sections around the world. Microbial Reefs Microbial build-ups were first noted by Schubert and Bottjer (1992) and subsequently Lehrmann (1999) detailed the first microbial reefs in Lower Triassic strata of South China (Figure 2-3). Subsequent work on Lower Triassic strata yielded microbial reefs from sections in southern Turkey, Armenia and Oman (Baud et al., 1996; Baud et al., 2002), Greenland (Wignall and Twitchett, 2002), and the southwestern United States (Pruss and Bottjer, 2004). These occurrences suggest that R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 15 NV 37° N Muddy Mountain O v e rto n ^ - CA Las Vega: Dar win Muddy Mountains Darwin Hills LosSCabin Hencfenton fountain Pass_ _ 30 mi '35°N 114°W 117°W Figure 2-1: Southwestern United States locality map showing all localities where Lower Triassic sections were measured and sampled. R eproduced with perm ission o f the copyright owner. Further reproduction prohibited without perm ission. TURKEY AntiiLa,____ ___ Demirta/y-Ku^davul * *-*-4— Kopuk dag NvAlanya * « 4 -.(^iiriik dag Kemer \ s. J MEDITERRANEAN SEA \ 30°E Figure 2-2: Southern Turkey locality map showing all places where Lower Triassic strata were measured and sampled, indicated by asterisks. R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 17 Low-Relief Bank Stage Early Triassic Calcimicrobial Mounds within Peritidal Cyclic Limestone / Oolite / m I Lim e M udstone Debris-flow Breccia and Turbidite Grainstone Earliest Triassic Calcimicrobial Framestone Siliceous Lutite 25° M Skeletal Packstone to Grainstone 1 KM Figure 2-3: Diagram of the Early Triassic Great Bank of Guizhou in South China (modified from Lehrmann et al., 2001). R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. 18 microbial reefs proliferated during the Early Triassic when metazoan reefs were globally absent from the geologic record. These reefs typically occur as 1-2 m patch reef mounds, and in some cases contain preserved remains of calcimicrobes such as Renalcis (Lehrmann, 1999) or microbial laminations (Pruss and Bottjer, 2004). Carbonate Seafloor Fans The unusual resurgence of carbonate seafloor fans was first documented by Woods et al. (1999), who attributed their formation to the overturn of an anoxic basin and concomitant degassing of carbon dioxide (Figure 2-4). They noted the occurrence of these fans in the Union Wash Formation of eastern California and attributed their formation to unusual ocean chemistry. These have now also been described from Lower Triassic strata in Iran (Heydari et al., 2003). Microbialite crusts that may consist of carbonate fans have also been reported from boundary sections in South China (Kershaw et al., 2002; Kershaw et al., 1999). Flat-Pebble Conglomerates Flat-pebble conglomerates have been reported from Lower Triassic sections in the southwestern United States (Schubert and Bottjer, 1995), south China and Italy (Wignall and Twitchett, 1999). These are thought to represent the temporary restoration of Cambrian-style shelf conditions including low levels of vertical bioturbation and microbial mat development. This reduction is likely related to the broad-scale demise of deep bioturbators at the end of the Permian and their subsequent delayed recovery (Twitchett, 1999; Twitchett and Wignall, 1996). Under Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 4 4 4 Organic Matter C 0 2 Degassing (P o te n tia l S o u rc e o f C a rb o n D io x id e P o iso in in g ) r - 2- HCO3- e HC03- C°3 H2S CO. Sulfate Reduction Synsedimentary Cements Organic Matter Deposition Figure 2-4: Diagram showing overturn mechanism as a means of producing carbonate seafloor (modified from Woods et al., 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 reduced infaunalization conditions, early lithification of the seafloor would occur, and storm or wave activity would exhume clasts of carbonate and redeposit them as intraclastic conglomerates (Sepkoski, 1982). Whereas intraclasts are common components of carbonate rocks throughout time, the occurrences of conglomerates made entirely of intraclasts are typically restricted to Precambrian—Cambrian shelf deposits. Ribbon Rock Ribbon rock is another type of anachronistic facies described from Lower Triassic strata. This facies forms in tidal environments, and consists of interbedded layers of peloidal grainstones and micrite. This facies was common during greenhouse times in the early Phanerozoic when small scale sea-level fluctuations brought about the deposition of these layers in tidal environments (Lehrmann et al., 2001). In Early Triassic sections, the same mechanisms are thought to have played a role in the formation of ribbon rock, including low levels of bioturbation. During the post-Ordovician Phanerozoic, ribbon rock is not preserved in tidal areas because of increased infaunalization. The ribbon rock occurrences in the Early Triassic are therefore thought to represent the restoration of the Early Paleozoic type of deposition (Lehrmann et al., 2001). Wrinkle Structures Wrinkle structures have been documented from Lower Triassic strata in the southwestern United States and southern Italy (Pruss et al., 2004). In the southwestern United States, these have been documented from two formations: the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21 Moenkopi Formation and the Thaynes Formation. In southern Italy, these have been found in the Campil Member of the Werfen Formation (Pruss et al., 2004). These occur in siliciclastic strata, and are preserved on bedding planes. Wrinkle structures, sedimentary structures that form because of the cohesiveness of microbially-bound sediment (e.g. Hagadorn and Bottjer, 1997; Noffke et al., 2002), have been found in association with hummocky cross-stratification, and trace fossils such as Asteriacites, Rhizocorallium, and Arenicolites, indicating that microbial mats formed in shallow subtidal paleoenvironments (Pruss et al., 2004), a depositional environment in which these had not been preserved since the Cambrian (Hagadorn and Bottjer, 1999). Hypothesis The hypothesis that was tested in this research is that the prolonged biotic recovery in the Early Triassic following the end-Permian mass extinction is directly related to environmental stresses existing throughout this time period. Oceanic conditions such as anoxic or COz -rich deep waters, that have been shown to exist during the Early Triassic, (Isozaki, 1994; Isozaki, 1997; Wignall and Hallam, 1992; Woods et al., 1999) may have periodically impinged on the shelf and poisoned metazoans. This research tested this hypothesis by documenting the occurrence of anachronistic facies, which have been shown to demonstrate unusual environmental conditions (Sepkoski et al., 1991). Sequence stratigraphy was utilized as one tool to determine the evolution of systems tracts which tracks the changes in water depth on a shelf (Mitchum, 1977; Van Wagoner et al., 1988). Some of these anachronistic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22 facies, which represent stressed environmental conditions, were shown to occur during transgressive systems tracts, or time periods when deeper water is transgressing onto the shelf. This indicates that these facies do in fact record deeper water stressed conditions encroaching on to the shelf. Additional trends were noted when tracking anachronistic facies and bioturbation across an onshore-offshore transect. These results will be discussed at length below. A test as to what environmental stresses were responsible for inhibiting metazoans was also undertaken as part of this research. Unusual carbonate synsedimentary cements found in the Union Wash Formation serve as indicators of this elevated alkalinity (Woods et al., 1999). The presence of early marine cements in thin-sections of the anachronistic facies also provided evidence for early lithification, another condition necessary for the preservation of many of these facies. Additionally, a detailed study of bioturbation was undertaken as part of this study to elucidate what trends could be seen in metazoan activity and to determine what role bioturbation played in the formation of anachronistic facies. Coupling the occurrences of anachronistic facies with the bioturbation results enabled some conclusions to be drawn about the lag phase from the biotic recovery interval that spanned the Early Triassic. Methodology Field Study To test the hypothesis that anachronistic facies undergo an unusual resurgence in the Early Triassic, Lower Triassic sections were studied in the southwestern United Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 States and southern Turkey. Detailed stratigraphic sections were measured through the uppermost Lower Triassic Virgin Limestone Member of the Moenkopi Formation in which anachronistic facies such as flat-pebble conglomerates and normal marine microbial reefs had previously been documented (Schubert and Bottjer, 1992; Schubert and Bottjer, 1995)(See Figure 2-1). To better understand the occurrences of anachronistic facies throughout the Early Triassic and from other regions, the Griesbachian Katarasi, Kokarkuyu, and Sapadere Formations from southwestern Turkey were studied (See Figure 2-2). These formations were especially important because they were deposited in the Tethyan realm; a completely different seaway than the Panthalassa seaway which deposited the sections in the southwestern United States (see Figure 1-4). Anachronistic facies were recorded in a bed-by-bed analysis and detailed field descriptions were made. Samples for thin- sections were taken to determine grain size, fossil content, and sorting to establish environmental energy of deposition. One of the goals of this research was to perform a facies analysis of Lower Triassic strata in the southwestern United States and southern Turkey. This was accomplished by measuring multiple stratigraphic sections of Lower Triassic strata at both locations to formulate a sequence stratigraphic framework. Sequence stratigraphy, first developed as a tool in Exxon oil exploration (Payton, 1977) has been adopted by most researchers as the most accurate means of tracking changing water depth at a given locality. This method employs analyses of grain size, sedimentary structures, and fossil content to determine trends in water depth. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24 Genetically-related depositional packages, called parasequences, show either deepening or shallowing trends that can ultimately reveal if the ocean was transgressing onto the shelf (deepening) or regressing to the basin (shallowing) (Mitchum et al., 1977; Vail and Mitchum, 1977; Van Wagoner et al., 1988). Although exposure at many of these localities made elucidating parasequences difficult, it was possible to detect overall trends. At each locality, measurements were taken from the base of the exposed section or, where possible, at a sequence boundary. Each bed, including those with anachronistic facies, was measured for thickness, and covered intervals were noted. Fossil content and abundance, ichnofabric indices (Droser and Bottjer, 1986), and sedimentary structures were documented. Thin-section work provided additional information about fossil content and identification, grain size, and sorting, as well as other features that are not readily visible in the field (see below). All field observations were incorporated into sequence stratigraphic interpretations. Tracking trends in grain size, sorting, and sedimentary structures from bed to bed helped to determine if the anachronistic facies occurred in transgressive or regressive sequences. Where possible it was also determined whether these anachronistic facies formed during the initial flooding event or during shallowing intervals (Posamentier and Vail, 1988). Once these trends in anachronistic facies occurrences were determined, results were compared between the southwestern United States and southern Turkey. This was important to determine if anachronistic facies reflected different depositional regimes. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 5 In addition to placing these anachronistic facies into a sequence stratigraphic framework, a paleoenvironmental analysis was performed. This involved ascertaining the specific environmental conditions that were conducive to the formation of anachronistic facies. To accomplish this, observations were made to elucidate environmental energy, ocean chemistry (when possible), and diagenesis. Environmental energy was determined by examining beds for evidence of storm deposits, tidal currents, current scour or winnowing, and bioturbation. It was also important to determine if a specific environmental energy regime was responsible for the deposition of anachronistic facies. Thin-sections were studied to look for the presence of minerals and cements to determine ocean chemistry and diagenesis. To test the hypothesis that anachronistic facies reflect environmental stresses related to deleterious deep-water conditions such as anoxia, abundant hydrogen sulfide, and/or hypercapnia, it was necessary to compare the occurrences of anachronistic facies across an onshore-offshore transect. In each measured section, the beds containing anachronistic facies were plotted and then compared to each other to see which section contained the largest portion of anachronistic facies. This provided a semi-quantitative means of testing this hypothesis because the expected result from this study would be that larger percentages of sections in offshore environments would consist of anachronistic facies. Nearshore environments would have low percentages because they would be less affected by deeper water conditions. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 26 Thin-Section Analysis A thin-section study was also performed to augment field data. Study of thin- sections allowed for a more detailed analysis of the fossil content, grain size, sorting, and diagenetic history of sampled beds. In thin-section, fossils that were not visible to the naked eye were identified (e.g. ostracods). Additionally, the grain size and sorting of beds on a smaller scale were also investigated. Evidence for diagenesis was also recorded. Early marine cements were noted in carbonate samples as indicators of early lithification. In the case of microbial features such as microbial reefs and wrinkle structures, thin sections were used to search for the evidence of microbial activity and the former presence of microbial mats. Thin-sections were also used to describe the microstructure of the anachronistic facies. In the case of flat-pebble conglomerates, features of the clasts and matrices could be described. The carbonate seafloor fans were sectioned and studied to determine the original mineralogy of the fans. Finally, trace fossils were thin-sectioned to search for evidence of infilling or burrow lining. Thin-sections were extremely useful in providing date not gleaned by field observations alone. Stratigraphy and Paleoenvironments Southwestern United States During the Early Triassic, the western margin of North America was a passive margin characterized by a broad shallow epicontinental shelf that extended from southern Idaho as far south as southern Arizona (Marzolf, 1993). The Spathian Moenkopi Formation was deposited on the Colorado Plateau in the following Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 7 sequence from stratigraphically lowest to highest: Timpoweap, Lower Red, Virgin Limestone, Middle Red, Shnabkaib, and Upper Red members (Marzolf, 1993) (Figure 2-5). The Moenkopi reflects changes in the Panthallasa seaway, with the marine incursions represented by the Virgin Limestone and Shnabkaib members, and the various Red members signifying non-marine deposition (Reif and Slatt, 1979). The Smithian-Spathian Union Wash Formation was deposited west of the Moenkopi and rests unconformably on folded Permian rocks (Stone et al., 1991) (See Figure 2- 5). This formation crops out primarily in eastern California. The Middle and Upper members are correlative with the shelfal Moenkopi Formation, but were deposited in a slope-basinal setting (Stone et al., 1991). The Virgin Limestone Member of the Moenkopi Formation typically crops out as limestone ledges that weather a characteristic brownish-yellow. This mixed carbonate-siliciclastic unit is composed of limestone, as well as dolomitic limestone, calcareous mudstone, and siltstone (Reif and Slatt, 1979) and contains fossil material such as echinoderm debris, bivalves, and gastropods (Schubert and Bottjer, 1995). It was measured at the Muddy Mountains Overton locality (Figure 2-6), Muddy Mountains Ute locality (Figure 2-7), Mountain Pass locality (Figure 2-8), and Lost Cabin Springs locality (Figure 2-9) (Appendices 1-4). The Union Wash Formation crops out as dark, thick limestone ledges with intervals of siltstone (Stone et al., 1991). Some of the common body fossils include Eumorphotis bivalves and ammonoids (Woods, 1998). Middle and Upper Members of the Union Wash Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 8 East-central 1 California | Southern Nevada o > — 1 G O G O < i — i 0 4 Spathian c .2 Upper R s ......... U o 4S Middle R £ C # © * s s Lower s Virgin ® _________ £ Lower Red © _ _ ____ _ 'a, jo s Timpoweap © Smithian H 0 4 G w G £ b x G o Q G .2 - G o X ) o Figure 2-5: Stratigraphic column showing studied sections of Lower Triassic Moenkopi Formation and Union Wash Formations (modified from Pruss and Bottjer, 2004, Woods et al., 1998, F. Corsetti, 2003, pers. commun., Marenco et al., 2004). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 9 Figure 2-6: Stratigraphic column of the Virgin Limestone Member of the Moenkopi Formation, Muddy Mountains Overton locality, southwestern United States, with anachronistic facies indicated. Numbers on columns refer to stratigraphic descriptions in appendix. See Appendix 1 for detailed locality information Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 0 48 47 46 3 OOO s a g 27-29 OOO [ uiu»w m usn 20-21 12-13 10-11 OOO 4-6 _________KEY flat-pebble conglom erates ooo oolites radiating m icrobialites m icrobial m ounds w rinkle structures 1 0 = 1 thin-bedded lim estone-m udstone carbonate precipitates I'U -J lim estone siltstone m ixed lim estone/siltstone partially covered covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31 Figure 2-7: Stratigraphic column of the Virgin Limestone Member of the Moenkopi Formation, Muddy Mountains Ute Locality, southwestern United States, with anachronistic facies indicated. Numbers on columns refer to stratigraphic descriptions in appendix. See Appendix 2 for detailed locality information Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 2 43 36 35 32-32 28-3( B 3 3 B : ooo 26 25 24 22~23 20 19 6-7 2-5 KEY flat-pebble conglom erates ooo oolites radiating m icrobialites m icrobial m ounds w rinkle structures m thin-bedded lim estone-m udstone carbonate precipitates lim estone siltstone m ixed lim estone/siltstone partially covered X covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 3 Figure 2-8: Stratigraphic column of the Virgin Limestone Member of the Moenkopi Formation, Mountain Pass Locality, southwestern United States, with anachronistic facies indicated. Numbers on columns refer to stratigraphic descriptions in appendix. See Appendix 3 for detailed locality information Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 4 24 23 21-22 20 18-10 16-17 14-15 ooo 1-12 0 0 0 KEY flat-pebble conglom erates ooo oolites radiating m icrobialites m icrobial m ounds w rinkle structures m thin-bedded lim estone-m udstone carbonate precipitates lim estone siltstone ib id m ixed lim estone/siltstone partially covered X covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 5 Figure 2-9: Stratigraphic column of the Virgin Limestone Member of the Moenkopi Formation, Lost Cabin Springs Locality, southwestern United States, with anachronistic facies indicated. Numbers on columns refer to stratigraphic descriptions in appendix. See Appendix 4 for detailed locality information Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 6 41-42 2 X 38-39 37 35-36 32-34 30 27-29 25 24 19-23 14-17 10-11 6-9 10m 2-4 KEY flat-pebble conglom erates ooo oolites radiating m icrobialites m icrobial m ounds w rinkle structures m thin-bedded lim estone-m udstone \y/ carbonate precipitates lim estone ph siltstone m ixed lim estone/siltstone f^j><3 partially covered X covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 7 Formation at the Darwin Hills locality (Woods, 1998) (Figure 2-10) (Appendix 5). For detailed descriptions of all measured beds, see Appendices (1-5). Southern Turkey In southern Turkey, the Upper Permian Pamucak and Yligliiktepe Formations and Lower Triassic Katarasi, Kokarkuyu, and Sapadere Formations are exposed. During Early Triassic time, the Tethyan seaway covered much of Turkey, and carbonate deposition occurred in an equatorial region. The Pamucak and Yiigliiktepe Formations, which are Middle-Late Permian in age, consist primarily of nodular limestones. The Pamucak Formation varies with exposure, but at some outcrops, there are very few obvious fossils found in the nodular limestones in hand sample. Although exposure varies, the Pamucak has been measured with a thickness from 400-600 m (Baud et al., 1996; Baud et al., 2002). The facies are hypothesized to represent inner to outer carbonate platform facies. The upper part of the Permian Pamucak Formation’s nodular limestones contain pods of chert. At the Ciiriik dag locality, there are horizons that are rich in calcareous algae (Dasyclaadacea) and small foraminifera (Milliolidae), as well as brachiopods, echinoderms and crinoids (Baud et al., 1996; Baud et al., 2002). These beds are also rich in ostracodes that typify the Tethyan region (Bairdiacea and Cypridacea) with a notable absence of Palaeocopidae (attributed to the deep paleoenvironment) (Baud et al., 1996). The Pamucak formation is overlain by the Lower Triassic Katarasi and Kokarkuyu Formations (Figure 2-11 and 2-12), which contain Induan foraminifera Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 8 Figure 2-10: Stratigraphic column of the Middle and Upper Members of the Union Wash Formation, Darwin Hills Locality, southwestern United States with anachronistic facies indicated. Numbers on columns refer to stratigraphic descriptions in appendix. See Appendix 5 for detailed locality information. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 9 section continues but is covered 26-27 c 1-2 KEY flat-pebble conglom erates ooo oolites radiating m icrobialites m icrobial m ounds /©■S' w rinkle structures m thin-bedded lim estone-m udstone carbonate precipitates lim estone siltstone B m ixed lim estone/siltstone partially covered X covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 0 limestone thrombolites thrombolites massive limestone wavy microbialite Permian-Triassic boundary Figure 2-11: Stratigraphic column of the Lower Triassic Katarsi Formation, Ciiriik dag locality, southern Turkey (modified from Baud, 2003). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41 c o o •a cd .5 o H U - i s- 5 _ _ . o cd 2 4 O 30 m « 25 m. 20 m« 15 m« 10 m. 5 m« i = = e BE •r -t T ~ r t ~~~r~ limestone limestone with crystal mounds wavy microbialite Permian-Triassic boundary Figure 2-12: Stratigraphic column of the Lower Triassic Kokarkuyu Formation, Kopuk dag locality, southern Turkey (modified from Baud, 2003). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42 (Ammodiscus, Rectocornuspira, Cyclogira, Earlandia) and conodonts (Isarcicella isarcica and Hindeodus parvus) (Baud et al., 1996; Baud et al., 2002). The basal part of these formations consists of about 2 m of laminated microbial limestones (with obvious laminations, and a stratiform morphology). This microbial limestone contains stromatolites that occasionally dome and form small heads, but are largely stratiform. When the base of this bed is exposed, small round bases of stromatolite domes are visible. The microbial laminations alternate between dark and light laminae, and the light-colored micrite contains peloids. Overlying the stratiform stromatolites in the Katarsi Formation is a calcilutite that also contains microbialites. Approximately 3.7 m of thinly-bedded and massive limestone caps the laminated microbial limestone. The changes from thinly-bedded to massively-bedded limestone vary laterally with exposure. Above this is the bedded limestone that occasionally contains thrombolite mounds. These mounds are 0.5 m thick, and show some draping of overlying beds, but relief was fairly minimal (< 0.5 m). Atop these thinly-bedded limestones were large thrombolite mounds (2 m thick). These likely attained significant relief off of the seafloor. In beds lapping against the sides of the mounds, there are seafloor-precipitated crystals (A. Baud, 2003, pers. comm.). The Kokarkuyu Formation differs from the Katarsi Formation in that it contains large carbonate mounds instead of thrombolites. These carbonate mounds are exposed in at least three horizons, and various other carbonate seafloor precipitates are found in talus. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43 The Upper Permian Yligliiktepe Formation was exposed at only the Demirtaz^-Ku^davut locality (Figure 2-13). At this locality, the Yiigluktepe Formation is overlain by the Griesbachian Sapadere Formation. Although the Yiigluktepe Formation is similar to the Pamucak Formation, at the Demirtaz?- Ku§davut locality it is more fossiliferous than the Pamucak. Abundant bellerophontid gastropods, calcareous algae, and other macrofossils are visible in hand sample. The Sapadere Formation is similar to the Kokarkuyu and Katarasi Formations, however, a large 8 m-thick cross-bedded oolite is preserved at this locality. Additionally, carbonate precipitates are found, but do not attain the height of the precipitate mounds of the Kokarkuyu Formation at the Kopuk dag locality. There is a regional unconformity that causes a local absence of Dienerian strata after the Griesbachian microbial sediments. The overlying beds that are occasionally found in contact with the carbonates of the Griesbachian consist of siltstones and sandstones that are thought to be Smithian-Spathian in age (A. Baud, 2003, pers. comm.). These are thinly-bedded pink, tan and green siliciclastics that contain tool marks and trace fossils. Also, abundant rip-up beds including some flat- pebble conglomerate beds were noted and traces such as Arenicolites as well as other U-shaped traces, Planolites, and possibly Rhizocorallium were found in talus pieces. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 4 limestone and calcareous siltstone cross-bedded oolite laminated limestone wavy laminations a f ° 1 ^0 oolite Permian-Triassic boundary Figure 2-13: Stratigraphic column of the Lower Triassic Sapadere Formation, Dcmirtazg-Kugdavut locality, southern Turkey (modified from Baud, 2003). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68 45 CHAPTER 3 Early Triassic Microbial Reefs: western United States and southern Turkey Introduction Research on microbial build-ups of the Spathian Virgin Limestone Member of the Moenkopi Formation in the western United States and the Griebachian Katarasi Formation of southern Turkey has shown that these microbial build-ups formed patch reef mounds. Detailed field analysis of these reef mounds and the surrounding beds has established that they attained 1-2 m of topographic relief above the seafloor. A petrographic study of these microbial mounds illustrates that these consisted of microbial fabrics, voids with early marine cements, and formed in the absence of in situ metazoans. These reef mounds formed during the aftermath of the end-Permian mass extinction that spanned the Early Triassic. Due to the devastation of marine metazoans at this extinction, the Early Triassic has often been deemed a reef gap because no metazoan reefs are found at this time. Whereas colonial metazoan reefs are globally absent from Lower Triassic strata, microbial build-ups similar to these described from the western United States and southern Turkey have been found in south China (Lehrmann, 1999), Armenia and Oman (Baud et al., 1996; Baud et al., 2002), and Greenland (Wignall and Twitchett, 2002). A regional unconformity spanned from the Early Permian to late Early Triassic in this southwestern United States study region; however, the occurrence of these microbial reefs suggests that conditions favoring microbial reef development must have existed as long as 4 —8 million years after the end-Permian mass extinction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46 Reefs have been important components of normal marine ecosystems through much of Earth history (Heckel, 1974). Whereas reef communities have changed through time, they are common ecosystems found throughout the Proterozoic and much of the Phanerozoic. Following several major mass extinction events metazoan reefs have suffered declines, thus creating reef gaps or reef eclipses, as has been documented for the Early Triassic (Fagerstrom, 1987; Fliigel, 1994) and Early Jurassic (Stanley, 1988) (Figure 3-1). These reef gaps represent periods of time when colonial metazoans have suffered extinctions, so that metazoan reefs became globally rare or absent. The end-Permian mass extinction caused the elimination of metazoan reef systems (Hallam and Wignall, 1997) and a reef gap (Fagerstrom, 1987; Fliigel, 1994) during the Early Triassic. Colonial metazoan reef communities did not reappear until the Middle Triassic as patch reefs (Hallam and Wignall, 1997) (see Figure 3-1). The reef record mirrors the recovery of most marine metazoans, with an unusually delayed recovery occurring 4 —8 million years after the end- Permian mass extinction (Hallam, 1991; Martin et al., 2001). The marine metazoans that were present in the Early Triassic tend to be characterized by low diversity communities dominated by opportunists and generalists (Rodland and Bottjer, 2001; Schubert and Bottjer, 1995). Whereas the absence of metazoans from reef ecosystems in the Early Triassic is significant, recent studies have shown that describing this time period as a reef gap may be inaccurate (Lehrmann, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47 Figure 3-1: Reef components for the Late Permian, Triassic and Early Jurassic, including the reef gap of the Early Triassic. Modified from (Calvet and Tucker, 1995) incorporating data from Wood (1999) as well as results from this paper. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 8 Prim ary Reef ComDonents y c /5 o n 5 "Reef eclipse" Interval < a t , Bivalves a ! . D < u Scleractinian Corals e g o o c - l Scleractinian Corals Foraminifera U J Calcisponges Tabulozoans -J V c / ) oo "Tubiphytes" < Calcisponges H H a! L d J Microbial Crusts H Q Q Red and Green Algae i Scleractinian Corals > 3 "Reef gap" Interval U - 4 < Microbial ites U J 2 £ £ < " Calcified Sponges t — t JJ "Tubiphytes" c d 5 : -J Bryozoans w e u Phylloid Algae Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49 For example, prior to the delayed biotic recovery from the end-Permian mass extinction event, microbial build-ups have been documented from several locations globally (Baud et al., 1996; Baud et al., 2002; Lehrmann, 1999; Lehrmann et al., 2003; Schubert and Bottjer, 1992; Wignall and Twitchett, 2002). These build-ups in the western United States were first identified as disaster forms; forms that were able to flourish during the aftermath of the end-Permian mass extinction (Schubert and Bottjer, 1992). In recent research conducted in Lower Triassic strata of south China (Lehrmann, 1999; Lehrmann et al., 2003), Armenia and southern Turkey (Baud et al., 1996; Baud et al., 2002), and Greenland (Wignall and Twitchett, 2002), normal marine microbial build-ups have also been documented. Other areas containing microbialites in Lower Triassic strata include boundary sections from Japan (Sano and Nakashima, 1997) and Iran (Heydari et al., 2001), as well as possible microbial crusts from China (Ezaki et al., 2003; Kershaw et al., 2002; Kershaw et al., 1999)}. The occurrence of these supports the observation that there is a global proliferation of microbialites during the Early Triassic. Lehrmann (1999) found biostromes and mounds in the Nanpanjiang Basin of South China in two horizons of Lower Triassic strata and has interpreted these to represent Early Triassic patch reefs. The first horizon is located in Griesbachian strata and is composed of microbial biostromes. These biostromes commonly occur through 15 m of strata. Microbial mounds are found in the Spathian section, and these are believed to have achieved some topographic relief above the seafloor. In sections of Armenia and southern Turkey, microbial build-ups from Griesbachian Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 strata have also been documented (Baud et al., 19%; Baud et al., 2002). There are a variety of stromatolite and thrombolite facies that have been described from these sections including large domal stromatolites and thrombolites that measure up to 2 m high (Baud et al., 1996; Baud et al., 2002). These are similar to those documented from south China (Lehrmann, 1999; Lehrmann et al., 2003). In Greenland, Griesbachian microbial mounds have also been reported from Jameson land {Wignall and Twitchett, 2002). In the western United States, Schubert and Bottjer (1992) first described microbial build-ups from the Moenkopi Formation as subtidal level-bottom stromatolites. The purpose of this study was to reinvestigate these stromatolites to determine if they instead were part of a patch reef system like that of south China and, if so, to integrate this into a larger picture of the distribution of microbial reefs in the Early Triassic. In the study area, Griesbachian and Dienerian strata are not preserved because of a regional unconformity that extended from Early Permian to late Early Triassic time, however, globally only microbial build-ups are known from these times {Baud, 1996 #87;Baud, 2002 #88;Lehrmann, 1999 #114;Sano, 1997 #156;Wignall, 2002 #172}. The western United States microbial reefs formed in the latest part of the Early Triassic (Spathian) and signify that reefs forming without reef-building metazoans persisted for millions of years after the end-Permian mass extinction. This has significant implications for understanding the slow recovery of metazoans, most especially the colonial metazoans, and the presence of a global reef gap in the Early Triassic. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51 Geologic Description The Lower Triassic Moenkopi Formation of the western United States crops out in Nevada and Utah where it is the more proximal equivalent of the Union Wash Formation (see Figure 2-5). The Virgin Limestone Member of the Moenkopi Formation was deposited as a transgressive marine tongue from the Panthalassa seaway on a westward dipping ramp (Blakely, 1972). It is composed largely of interbedded limestone and siliciclastic beds. The limestone beds are composed of micrite, dolomite, and calcareous mudstone (Reif and Slatt, 1979) and are rich in fossil material such as echinoderm debris, bivalves, and gastropods (Schubert and Bottjer, 1995). At the Lost Cabin Springs locality (Figure 2-9; Appendix 4), limestones of the Virgin Limestone Member (~175 m thick) were deposited in a variety of shelf palaeoenvironments (Schubert and Bottjer, 1992). The newly discovered microbial reef mounds crop out in one bed at this locality and are laterally extensive. The mounds are carbonate bodies consisting of smaller 10—50 cm domes (Figure 3-2) that form cohesive units (e.g. James, 1980) (Figs. 3-3 to 3- 4). At the £iiriik dag locality, the Upper Permian Pamucak Formation and the Lower Triassic Katarasi Formation are exposed. The Pamucak Formation consists of a nodular limestone that is overlain by an oolite that is thought to represent a mixed zone that may have been deposited during latest Permian time (Baud et al., 2002). The basalmost Triassic Katarasi Formation is defined by the first occurrence of Hindeodus parvus, an Early Triassic conodont (Baud et al., 1996; Baud et al., 2002). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 Figure 3-2: A. Photograph showing close-up of a small dome found within a larger mound exposed in the Virgin Limestone Member of the Moenkopi Formation, southwestern United States. Arrow indicates one of the numerous coarse microbial laminations that are visible on this and many of the other domes. B. Photograph showing dome with microbial laminations enhanced by weathering. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 54 Figure 3-3: A) Photograph of microbial mound in outcrop, showing the height of this mound to be ~2 m exposed in the Virgin Limestone Member of the Moenkopi Formation, southwestern United States. Arrows indicate the edge of the mound. This also shows deformation of underlying beds in the mound’s shadow, under the lower left arrow, implying that the mound was cohesive upon compaction. Bracket A indicates ~1.75 m of underlying bioturbated beds, also pictured in Figure 4. Ladder is ~2.5 m high. B) Close-up of the same microbial mound showing location of twelve domes that were sampled (white numbers) for thin- section analysis. Rock hammer above 1 is 28 cm long. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 56 Figure 3-4: Photograph showing small domes that coalesce to form a larger, cohesive microbial mound (~2 m high) within the Virgin Limestone Member of the Moenkopi Formation, southwestern United State. Four large white arrows indicate perimeter of larger mound. Two smaller white arrows indicate examples of smaller domes within the mound. Bracket A indicates ~1.75 m of underlying bioturbated beds. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 7 At this locality, approximately 17 m of Katarasi Formation is exposed (Baud et al., 2002), and Early Triassic forams such as Ammodiscus, Rectocornuspira, Cyclogira, and Earlandia have been reported from these beds (Baud et al., 2002). Aside from these, much of this formation is depauperate of faunas. The majority of the Katarasi Formation is dominated by microbialites. The basalmost beds consist of laminated microbialites and the overlying beds contain two intervals of thrombolites. Methods In this analysis, several newly discovered microbial build-ups were studied at the Lost Cabin Springs locality in the western United States and the £uriik dag locality in southern Turkey. These mounds were measured in detail in outcrop, and the underlying and overlying beds were mapped, measured, sampled and described. The goals of this study were to describe the microbial fabrics seen in outcrop and in thin-section, and to re-assess whether these microbial build-ups occurred as part of level-bottom communities, or if they were part of a patch reef system. The mounds and surrounding strata were sampled extensively as part of this study. The sedimentary beds surrounding these mounds were investigated to ascertain the paleoenvironments in which they formed. The mounds were sampled by taking oriented and located samples of the smaller microbial domes within the mounds and of the surrounding beds (e.g. Figure 3-3). These samples were slabbed to examine internal structures, and thin-sections were made from these. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 8 Results Field Analysis Field analysis of the Virgin Limestone Member, southwestern United States at the Lost Cabin Springs locality shows that the microbial mounds occur in one bed in all places where the bed is exposed, and they range in thickness from 2 to 2.5 m (Figures 3-3 and 3-4). The microbial mounds are an agglomeration of smaller domes that range in diameter from 0.3 to 0.5 m (see Figures 3-2 to 3-4 for examples). The mounds themselves are typically 1—2 m in diameter and exhibit both stromatolitic and thrombolitic fabrics. The stromatolitic fabrics are characterized by distinct laminations (see Figure 3-2). These laminations are commonly made more visible by weathering (see Figure 3-2). The thrombolitic fabrics differ from the stromatolitic fabrics in that they exhibit a clotted appearance and lack distinct visible laminations in outcrop. No geopetal structures have been found in any of these mounds, however, the onlapping nature of the surrounding beds does imply 1-2 m of topographic relief of the mounds above the seafloor (Figure 3-5). The mounds consist of dark gray micritic limestone and are completely devoid of macrofossils in outcrop. The underlying units include a 30-cm-thick graded crinoidal packstone that is cross-stratified. Above this is a 1.75 m thick bioturbated micrite bed (ichnofabric index of 4 —5 (Droser and Bottjer, 1986)) (see Figures 3-3 and 3-4) with abundant Planolites. Overlying this unit is a 0.5 m thick limestone unit that directly underlies the microbial mounds. The mounds sometimes deform this bed (see Figure 3-3) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A 3 0 .5 m B Figure 3-5: Photograph and sketch of the same microbial mound exposed in the Virgin Limestone Member, southwestern United States. A) Photograph of the edge of a mound indicated by white arrow. Infilling beds are located to the left of the mound. B) Sketch of the mound (1), underlying beds (2) and beds that lap against the side of the mound (3). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60 indicating that the microbial build-ups were somewhat cohesive when compacted by overlying units. No other deformation is noticeable in the limestone bed with the exception of where it has been affected by some of the overlying mounds. In some cases, the mounds do not deform the underlying beds (see Figure 3-4) indicating that the aforementioned localized deformation was caused by compaction rather than tectonic deformation. The limestone beds that lap out against the sides of the mounds consist of a very fine micrite (Figure 3-5). The mounds do not show any evidence of desiccation and most likely formed fully submerged. The underlying graded crinoidal packstone has been interpreted to represent a storm deposit, and this would place deposition of these units below normal but above storm wave base (Kidwell et al., 1991) in an inner to middle shelf environment. Following on the previous work of Baud et al., (1996,2002) in southern Turkey, a field analysis of these reef mounds was conducted to compare and contrast these to the microbial mounds of the western United States. The base of the Katarasi Formation consists of 3.7 m of laterally extensive layererd microbialites that likely attained little relief above the seafloor (Figure 3-6). These microbialites were overlain by a unit consisting of small thrombolite mounds (0.5 m thick) (Figure 3-7). The mounds do not show laminations in outcrop, but consist of smaller domes (Figure 3-8). There is some draping of overlying thinly bedded limestone beds but relief was limited to < 0.5 m. (Figure 3-9). Above this are the large 2 m thrombolites mounds. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 1 Figure 3-6: Layered microbialites from the basal 3.7 m of the Katarasi Formation. Small mounds attain relief of <10 cm above the seafloor and consist of alternating dark and light layers of micrite (arrow). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62 Figure 3-7: Small thrombolite mounds in Katarsi Formation (bracket), in beds overlying stratiform stromatolites. Thrombolites consist of small microbial domes (arrow) that do not exhibit laminations. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3-8: Close-up of domes (arrow) that make up small thrombolites in the Katarsi Formation pictured in Figure 3-7. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64 Figure 3-9: Large thrombolite mounds from the Katarsi Formation. These mounds likely attained significant relief above the seafloor, and make up small microbial patch reefs. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65 These thrombolites mounds also consist of smaller domes (Figure 3-10), although relief of these mounds ranged from 1-2 m (Baud et al., 2002). No obvious stromatolitic laminations were noted on any of the exposed thrombolites. In beds that locally lap out against the sides of the mounds, small crystal precipitates are found (A. Baud, 2003, pers. comm.). Beds overlying these mounds were not preserved at this locality. Thin-Section Analysis Thin-sections from the microbial reefs of the Virgin Limestone Member, southwestern United States revealed several different fabrics. The microbial fabric shows the preservation of original microbial laminations (Grotzinger and Knoll, 1999) (Figure 3-11). Atop some of these microbial laminations are bladed cements (Figure 3-12). Clotted textures are sometimes found near these laminations (Figure 3-11). The fabric that dominated much of the thin-sections is a metazoan fragment wackestone (Figure 3-13). This fabric consists of featureless micrite with sparse metazoan fragments and detrital quartz grains. Also preserved in thin-sections are the remnants of open framework crypts lined with bladed cements (Figure 3-14). The stromatolitic laminations (Figure 3-11) in the microbial fabric are each approximately 10 jim thick, which is consistent with the size of mat-building filamentous cyanobacteria (the producers of Archaeolithoporella) (Grotzinger and Knoll, 1995; Newell et al., 1953). The laminations noted in these thin-sections are wavy, frequently subparallel, and can fold back on themselves (Figure 3-11), Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 6 Figure 3-10: Photograph of the small domes that make up some of the larger thrombolites in the Katarsi Formation. Micrite beds drape the tops of the domes (arrows). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67 Figure 3-11: Thin-section photographs of samples from the Virgin Limestone microbial reefs, southwestern United States, showing microbial laminations with clotted fabrics below laminations on the lower right side of the photograph. Laminated layers show cohesiveness, with one lamina folding back upon itself (arrow). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 8 200um Figure 3-12: Photomicrograph of samples of microbial reefs from the Virgin Limestone Member, southwestern United States, showing (1) bladed cements growing on top of (2) microbial laminations (photographed in cross-polarized light). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 Figure 3-13: Thin-section photograph of a samples of the microbial reefs of the Virgin Limestone Member, southwestern United States, of metazoan fragment wackestone fabric including: a possible shell fragment (1), an unidentified foram (2) and a stylolite (3). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70 Figure 3-14: Thin-section photograph of a samples of microbial reefs of the Virgin Limestone Member, southwestern United States, of internal void with fringing bladed cements (arrow). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 71 indicating some plasticity of the layers. The laminations likely formed by a pioneering microbial community followed by growth of a calcium carbonate crust (Figure 3-12). These laminations are morphologically similar to microbial laminations that have been observed in the Satonda Crater Lake of Indonesia, although the microbial communities responsible for the genesis of these similar structures are likely very different (Arp et al., 2003). In addition to the laminations found in this fabric, a clotted texture is also common in the microbial fabric (see Figure 3-11), which likely had microbial genesis (Bosence and Bridges, 1995; Monty et al., 1995, and references therein). This texture contains peloids that are common features in the microstructure of carbonate microbial mud mounds (e.g. Monty et al., 1995, and references therein). The metazoan fragment wackestone (Figure 3-13) contains shell debris and quartz grains that were likely transported by currents into the spaces created by the irregular topography of the microbial mounds. Finally, the open framework crypts with fringing bladed cements (Figure 3-14) indicate that early lithification occurred in the voids of these microbial reefs (Wood, 1999) Samples from the southern Turkey Katarasi Formation were taken from the basal stratiform microbialite and the overlying thrombolite reef mounds and thin- sections were made from these. A variety of fabrics were visible in thin-section, as well as abundant microfossils. Peloidal fabrics, the possible remains of filamentous cyanobacteria, and early marine cements were visible in many of the thin-sections. The stratiform microbialites consisted of small, microbialite domes (< 5 cm) with fine-grained inter-dome fill. Alternating dark and light laminations were visible Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 2 macroscopically; however, in thin-section, these were less obvious. The fabrics of the domes are largely peloidal (Figure 3-15). The inter-dome fill consists of a fine grained micrite that contains fossil debris encased in bladed cements (Figure 3-16). Bladed cements also grew on the edges of the domes (Figure 3-17). Abundant microfossils were also found within the domes and interspaces, and these were commonly preserved with dark cubic minerals inside of them. The cubic minerals were also found scattered throughout the thin-sections (Figure 3-18). The microfossils include a variety of foraminifera such as a biseriamminid foraminifer that was likely reworked from underlying Permian sediments (J. Groves, pers, comm., 2004) (Figure 3-19), and the disaster foraminifer Rectocornuspira kalhori (J. Groves, pers. comm,, 2004) (Figure 3-20 and 3-21). The overlying thrombolites also preserved a variety of fabrics. These include the preservation of filaments that resemble the cyanobacteria Girvanella (Figure 3-22). These filaments were preserved with cubic pyrite inside their shafts, as were the foraminiefera. Foraminifera were also preserved in the thrombolites and no laminations were noted. The microbialites from the Katarsi Formation represent a complex microbial network. The basal stratiform microbialites consist of a peloidal fabric, a common fabric in microbialites (e.g. Monty et al., 1995, and references therein). The fossil debris and the sides of the domes themselves exhibit abundant bladed cements suggesting that both the fossils and the domes acted as nucleation sites for carbonate precipitation. This suggests that these domes formed in a highly-alkaline environment, characterized by supersaturation of calcium carbonate. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 3 Figure 3-15: Photomicrograph of samples of basalmost layered stromatolites from the Katarsi Formation, southern Turkey. Peloidal fabric is very common. Alternating dark and light laminations visible in outcrop are less obvious in thin-section. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 74 Figure 3-16: Photomicrograph of a sample of the basalmost layered microbialite from the Katarsi Formation, southern Turkey showing fossil debris preserved in mud that infilled the small microbial domes. Bladed cements grow off of the individual fossil clasts. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 5 Figure 3-17: Thin-section photograph of a sample of the small microbial domes from the Katarsi Formation, southern Turkey showing bladed cements that grow off of the edges of the domes. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 76 Figure 3-18: Photomicrograph of samples of the thrombolites from the Katarsi Formation, southern Turkey, showing cubic pyrite minerals scattered in the thin-section. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 7 Figure 3-19: A possible reworked Permian biserimminid foraminifera found in a thrombolite sample taken from the Katarsi Formation, southern Turkey (J. Groves, pers. comm., 2004) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 8 Figure 3-20: Photomicrograph of the foraminifer Rectocornuspira kalhori taken from a thrombolites sample of the Katarsi Formation, southern Turkey. A) A top view of the foram B) Side view of the foram chambers. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79 £ 100 feW*? SKJTfc* * • 200 (im Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 0 Figure 3-21: Another example of Rectocornuspira kalhori from a microbialite sample from the Katarsi Formation, southern Turkey. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81 Figure 3-22: A photomicrograph of the microstructure of a microbialite from the Katarsi Formation, southern Turkey showing a network of filaments interpreted to represent the remains of the cyanobacteria Girvanella. This meshwork makes up a large portion of the thin-section fabrics. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 2 (e.g. Sumner and Grotzinger, 2000). The cubic minerals that impregnated many of the microfossils likely formed as pyrite, but have now been oxidized. This suggests that organic decay within the fossils created an anoxic microenvironment (e.g.Schippers and Jorgensen, 2002). The presence of oxidized pyrite in other areas of the thin-section suggests that the microbialites may have created a partially anoxic environment to allow for pyrite to form (Schieber, 1999) Discussion Many definitions have been used to describe the characteristics intrinsic to reefs invoking wave-resistance as a possible criterion (Dunham, 1970; Ladd, 1944; Lowenstam, 1950; Newell et al., 1953). The definition used in this research describes a reef as a “discrete carbonate structure formed by in situ or bound organic components that develops topographic relief upon the seafloor” (Wood, 1999, p. 5). This definition emphasizes topographic relief upon the seafloor as a primary criterion for determining reef structures, and does not include any criteria for framework reef metazoans. Based on this definition and research conducted in this study, the microbial build-ups of the Virgin Limestone Member and the Katarasi Formation represent patch reef mounds. Individual mounds consist of an agglomeration of smaller domes that exhibit stromatolitic laminae and clotted fabrics in outcrop and in thin-section. The deformation of units underlying the mounds shows that these were cohesive units upon compaction. Additionally, the presence of abundant bladed cements indicates that these mounds were lithified prior to burial. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 3 The occurrence of Early Triassic normal marine microbial reefs that lack any framework or baffling metazoans is unusual because microbial reefs of this type have largely been excluded from these environments since the Ordovician (Awramik, 1991; Bottjer, 1997; Bottjer et al., 1997). Whereas reef systems throughout the Phanerozoic incorporate microbial fabrics, microbial reefs occurring without any in situ or associated reef metazoans are unusual. Even the famous Mississippian mud mounds were commonly covered with bryozoans acting as bafflers (e.g. (Brown and Dodd, 1990). It has previously been proposed that the build-ups at this locality indicated a decrease in metazoan predation pressure that enabled these microbialites to form (Schubert and Bottjer, 1992). These microbial build-ups had been dubbed “disaster forms” because they proliferated in the aftermath of the end-Permian extinction. This study has revealed that these reef mounds may in fact serve as indicators of stressful environmental conditions related to the end-Permian mass extinction, and that these represent “disaster forms” that were able to flourish in the face of environmental stress. The formation of these microbial build-ups may not be directly linked to suppressed bioturbation, but likely reflect the same stresses that suppressed metazoans and delayed the biotic recovery for the entirety of the Early Triassic. Deposition of microbial mounds in the inner to middle shelf in Lower Triassic strata of the western United States as well as southern Turkey indicates that conditions that affected these environments were linked to deleterious conditions in outer-shelf to basinal settings (Woods et al., 1999). Normally, the middle to outer Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 84 shelf reef paleoenvironment was suitable for sponge-microbial build-ups during other times in the Phanerozoic (Leinfelder et al., 2002; Weidlich, 2002). The absence of these types of reefs from shelf environments for the entirety of the Early Triassic points to suppression of reef-building metazoans. The occurrence of the southern Turkey microbialites forming just after the extinction during earliest Triassic time suggests that deleterious conditions related to the extinction event brought about a resurgence in microbial development in shelf environments. The occurrence of the western United States microbial build-ups deposited during the latest Early Triassic indicates that suppression of metazoans lasted as long as 4 —8 million years. Recent work has suggested that deleterious conditions (such as anoxic and/or C 02 -rich waters) existed in outer shelf to basinal settings during the late Early Triassic (Isozaki, 1994; Isozaki, 1997; Woods et al., 1999). The Union Wash Formation, which was deposited coevally with the Moenkopi Formation in more offshore settings, shows evidence for unusual ocean chemistry in the form of large aragonite fans precipitated on the seafloor. Additionally, carbonate seafloor precipitates are also interbedded with microbialites in the Katarasi Formation of southern Turkey. Carbonate seafloor precipitates are extremely rare after the Paleoproterozoic, and their formation has been linked to supersaturation of CaC03 in the presence of micrite inhibitors and low oxygen (Sumner and Grotzinger, 1996a). It is perhaps these conditions that could have periodically affected the deposition of shelfal paleoenvironments of the Lower Triassic sections in the western United States and southern Turkey. Ocean water in basinal settings may have periodically Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 85 flooded the shelf and inhibited metazoans while simultaneously creating a haven for development of microbial structures. The previously mentioned carbonate seafloor fans do indicate calcium carbonate supersaturation that is likely related to upwelling events (Grotzinger and Knoll, 1995) delivering deleterious ocean water to the shelf that contributed to the delayed recovery from the end-Permian mass extinction (Woods et al., 1999). Depositional Model The following interpretation for microbial reef mound development during the Early Triassic is proposed: 1) the end-Permian extinction event severely devastated ecosystems with a loss of as many as 80% of the marine species (Stanley and Yang, 1994); 2) colonial metazoans were devastated by this event and so an ensuing metazoan reef gap followed throughout the Early Triassic (Fliigel, 1994); 3) immediately following the mass extinction, microbialites flourished in earliest Triassic time (southern Turkey); 4) the Virgin Limestone microbial mounds formed several million years later in an inner—middle shelf palaeoenvironment coevally with the precipitates documented by Woods et al. (1999) that have been interpreted to represent deleterious deep-water conditions (Marenco et al., 2003); 4) the occurrence of these microbial reef mounds may indicate flooding of the shelf with stressful deep-water conditions at multiple times during the Early Triassic (perhaps anoxic or C 02 -rich waters) that could have inhibited metazoans while at the same time fostering the growth of microbialites. The global occurrence of only microbial build-ups throughout the Early Triassic (4—8 million years) implies that the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 6 conditions favoring their growth may be linked to those that hindered the recovery of colonial and other metazoans. Early Triassic Stromatolites as Disaster Forms Schubert and Bottjer (1992), in their study of these Virgin microbial build ups, were the first to note the common occurrence of stromatolites in normal marine Lower Triassic strata, and proposed that they represent disaster forms, which they defined as “opportunistic taxa, typically of long stratigraphic range, which normally occur in marginal and environmentally unstable settings but become abundant and environmentally widespread during times of biotic crisis (Fischer and Arthur, 1977)”. Schubert and Bottjer (1992) further hypothesized that environmental stress, which caused the end-Permian mass extinction, led to partial relaxation of ecological constraints that typically exclude post-Middle Ordovician stromatolites from normal marine level-bottom environments, allowing stromatolites such as those found in the western United States and southern Turkey to form. Much work on the Early Triassic and its microbialites has been done since the study by Schubert and Bottjer (1992). Relaxation of ecological constraints that affect the formation of stromatolites, such as the disappearance of deep bioturbation from subtidal environments, has been documented from throughout the Early Triassic (e.g. Ausich and Bottjer, 2002). The proliferation of microbially-mediated wrinkle structures in Lower Triassic siliciclastic strata has also been interpreted to indicate a reduction of infaunal bioturbation throughout this time (Pruss et al., 2004). As documented for studied beds in the Virgin Member and Katarsi Formations, and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 7 elsewhere, many occurrences of Early Triassic stromatolites represent microbial reefs that lack in situ metazoans. However, as occurs in the stratiform bed of the Katarasi Formation and the second stromatolite bed in the Virgin, isolated individual stromatolite domes also formed in level-bottom subtidal settings. These biotic features of the Early Triassic were very likely strongly influenced by continued environmental stress, related to the stress which caused the end-Permian mass extinction (e.g. Hallam, 1991; Woods et al., 1999). Thus, the emerging picture of stromatolites as disaster forms in the Early Triassic is a complex one. Early Triassic stromatolites formed as reef mounds and level-bottom individual domes, with an overlying template of ecological relaxation. As postulated herein, these conditions of general ecological relaxation may have been interrupted by repeated periods of additional environmental stress, such as incursions of anoxic and/or COa-rich deep-water into shelf environments. These incursions could have caused increased inhibition of metazoans, resulting in increased ecological relaxation, further enhancing the potential for stromatolites and other microbialites to form. At times, such as documented in this study, these microbial structures developed into patch reef mounds that, because metazoan reef- builders were absent during this time, represent the only reefs found during the Early Triassic. Summary Based on this research and the criteria for defining reefs of Wood (1999), these microbial build-ups described from the western United States and southern Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 8 Turkey represent Early Triassic reef mounds in an inner to middle shelf environment on a ramp. Similar to the biostromes and mounds reported by Lehrmann (1999) of south China and Wignall and Twitchett (2002) of Greenland, these mounds were able to attain significant relief above the seafloor. The global occurrence of these structures in the Early Triassic suggests similar environmental factors must have been influencing these different regions. The occurrence of these microbial build-ups also suggests that deposition in all of these areas may have been influenced by deeper water stressful conditions (anoxic or C 02 -rich waters). Another notable condition of the microbial reef mounds in the western United States is the lack of evidence for in situ metazoans in their framework. Whereas microbial fabrics have been important in reef systems throughout the Phanerozoic in a variety of environmental settings (Wood, 1999), these Early Triassic reef systems are quite unusual for their lack of framework or baffling metazoans. The only metazoan remnants were found as washed-in debris. Whereas colonial metazoans do not fully recover until the Middle Triassic, the Early Triassic metazoan reef gap is characterized by the development of microbial reef mounds during this time. These structures, which are likely related to unusual oceanic conditions of the Early Triassic, represent patch reef mounds that formed millions of years after the end- Permian mass extinction. Because of this, a link must exist between the environmental conditions that simultaneously prolonged the recovery of metazoans during the Early Triassic and fostered the global occurrence of microbial reefs. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 89 CHAPTER 4 Recovery of Reefs from the end-Permian mass extinction: An Overview Introduction The end-Permian mass extinction brought about a near annihilation of reef- building organisms at the close of the Paleozoic. There is an abrupt extinction of many groups of reef-builders at the end of the Permian followed by an absence in the Early Triassic and a rapid recoveiy in the Middle Triassic. Tabulate and rugose corals disappeared forever from reef ecosystems (e.g. Hallam and Wignall, 1997) and sponges did not recover until the Anisian (e.g. Fagerstrom, 1987). Because of the paucity of reef-building metazoans during the 7-8 million years following the end- Permian mass extinction, the Early Triassic has been dubbed a reef gap (Fagerstrom, 1987). This view has been subsequently modified because of the discovery of microbial reefs in Lower Triassic strata (Baud et al., 1996; Baud et al., 2002; Lehrmann, 1999; Pruss and Bottjer, 2004; Wignall and Twitchett, 2002). In the Middle Triassic, sponge-algal patch reefs formed by ‘Tubiphytes’, Girtycoelia, and various trepostrome bryozoans became re-established, and scleractinians radiated rapidly (Fliigel and Stanley, 1984; Fois and Gaetani, 1984). The absence of metazoan reef builders from the Early Triassic has been well- documented (e.g. Fagerstrom, 1987); however, the proliferation of microbial reefs in their absence has only recently been noted (e.g. Lehrmann, 1999). Early Triassic microbial reefs have been documented from a variety of locations globally including South China (Lehrmann, 1999; Lehrmann et al., 2003), southern Turkey, Armenia, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 90 Iran, and Oman (Baud et al., 1996; Baud et al., 2002), Greenland (Wignall and Twitchett, 2002), and western North America (Pruss and Bottjer, 2004). These reefs formed in the absence of metazoans acting as framework builders, bafflers, or binders. The widespread occurrence of microbial reefs from earliest to latest Early Triassic time suggests that the suppression of reef-building metazoans may be linked to environmental conditions that favored microbial growth (e.g. Kershaw et al., 1999; Lehrmann, 1999; Pruss and Bottjer, 2004). Because the end-Permian mass extinction so severely devastated reef organisms, many of those that appear in the Middle Triassic differ from their Permian predecessors. There are some Lazarus taxa that reappear in the Norian, and this has been attributed to the survival of organisms in unknown refugia (Stanley, 1994). Middle Triassic sponge genera are new despite morphologic similarities to their ancestors (Fliigel, 1994; Senowbari-Daryan et al., 1993). Specimens of ‘Tubiphytes’, a putative calcimicrobe, are different from Permian examples, and Girtycoelia is likely a homeomorph of earlier forms. Interestingly, scleractinian corals appear as a diverse fauna when first documented in the Middle Triassic (Fliigel and Stanley, 1984; Fois and Gaetani, 1984). The End-Permian Mass Extinction: The Effects on Reefs The largest extinction in the history of life occurred ~250 million years ago and brought about a reorganization of almost every marine ecosystem; the reefs were no exception to this. A variety of mechanisms have been put forth as a possible cause of this extinction including, but not limited to, widespread volcanism that may have Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 91 brought about catastrophic methane release (e.g. Ryskin, 2003), global anoxia (Isozaki, 1994; Isozaki, 1997; Wignall and Twitchett, 1996), a runaway greenhouse effect (Erwin, 1995), and a bolide impact (Basu et al., 2003; Becker et al., 2001). None of these has been unilaterally accepted as the mechanism for the end-Permian mass extinction. Many reef building-metazoans became extinct during the end-Permian mass extinction. Rugose and tabulate corals suffer such a devastating extinction that it marks their all time demise (e.g. Fedorowski, 1989). The disappearance of rugose corals may be attributed to a gradual decrease throughout the Late Permian (Fedorowski, 1989), although other work suggests that rugose corals thrived until the end of the Permian (Ezaki, 1994). Tabulates similarly underwent a decline during the Late Permian, and only a few survived until the end of the Changxingian (Fedorowski, 1989). Corals did not recover until the Middle Triassic when scleractinians emerged. Bryozoans suffered major extinctions at the generic level, but only one order, the fenestrates, disappeared entirely (Taylor and Larwood, 1988). Diversity is low for bryozoans throughout the Early Triassic; a radiation follows in the Middle and Late Triassic (Sakagami, 1985). In addition to the dominant reef- building organisms discussed above, many ancillary members of reef ecosystems were also devastated by the end-Permian mass extinction. These include crinoids, brachiopods, and foraminifers, which attributed significantly to Permian reef diversity (Fan et al., 1982; Fan et al., 1990). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 92 Another area of growing interest is the delayed recovery from the end- Permian extinction event (e.g. Erwin, 1993). It has long been recognized that marine ecosystems did not attain pre-extinction diversity levels until the Middle Triassic (e.g. Hallam, 1991); however, this too has to date been unsatisfactorily explained. Reef-building metazoans exhibit the same trend as many other metazoans; they re appear synchronously, and at a variety of locations globally, after an absence from the world’s oceans for 7-8 million years. The Early Triassic experiences a brief resurgence of Cambrian-like microbial reefs that have been documented from many locations, and the replacement of microbial reefs by metazoan reefs in the Middle Triassic has been attributed to the dissipation of environmental stress. The Early Triassic: A Survival Phase from the Biotic Recovery The Early Triassic follows the end-Permian mass extinction and is characterized by low diversity marina faunas and a dearth of marine organisms common during most of the Phanerozoic such as sponges and corals. Because the true biotic recovery did not begin until the Middle Triassic, the Early Triassic has been called a “survival phase”(Hallam and Wignall, 1997) (Figure 4-1). This phase was dominated by 4 genera of bivalves that were cosmopolitan, and biotic recovery opportunists such as microgastropods and lingulid brachiopods were also prominent members of marine communities (Fraiser and Bottjer, 2004; Rodland and Bottjer, 2001); these organisms were able to thrive in the aftermath of the end-Permian mass extinction when other organisms were suppressed. The depauperate Early Triassic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 3 M iddle Triassic Early Triassic End o f Perm ian extinction recovery surviva ’ Tubiphytes" coralline sp o n g e s siliceou s sp o n g e s scleractinian corals b ryozoan s m icrobialites D I V E R S I T Y coralline sp o n g e s ru g o se corals 'Tubiphytes" ✓ ta b u la te corals a lg a e b ryozoan s TIME Figure 4-1: Diagram showing effects of the end-Permian mass extinction on reef organisms. Dominant metazoan reef builders become extinct during the extinction phase. The survival phase encompasses disaster forms that build reefs in the aftermath of the extinction. The subsequent radiation of reef organisms begins the recovery phase. This phase also encomppasses the re-appearance of taxa that had been absent from the geologic record since the extinction event (modified from Hallam and Wignall, 1997, with data from Flugel, 2002). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 94 biota is considered highly unusual when compared to assemblages from other times of the Phanerozoic (e.g. Hallam and Wignall, 1997). The marine communities are far from the only unusual features of the Early Triassic. The sedimentary rock record has garnered much attention because it also reflects unusual environmental conditions following the end-Permian mass extinction. A notable increase in anachronistic facies (sensu Sepkoski et al., 1991) including flat-pebble conglomerates (Wignall and Twitchett, 1999) and ribbon rock (Lehrmann et al., 2001) has been well-documented and is thought to represent a return to Early Paleozoic-style carbonate deposition (Pruss et al., 2003). The Early Triassic has been deemed a chert gap because few siliceous deposits are known from this time (Racki, 1999). Coal deposits are also absent from the Early Triassic rock record, creating a “coal gap” from the Permian to the Middle Triassic (e.g. Retallack et al., 19%). As previously discussed, the global absence of metazoan reef builders from the Early Triassic has garnered the title “reef gap” (Fagerstrom, 1987); subsequent work on the proliferation of microbial reefs from this time has modified this concept (Lehrmann, 1999; Pruss and Bottjer, 2004) (See Figure 16). The occurrences of Early Triassic microbial reefs from a variety of locations have signified that this time period is not a true reef gap. The Early Triassic instead shows a resurgence of a facies not commonly seen since the Cambrian: microbial reefs forming without metazoans. During most other times in the post-Cambrian Phanerozoic, microbial fabrics co-occur with reef-building metazoans. The famous Waulsortian mounds of Carboniferous time are no exception; the baffling activity of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95 fenestrate bryozoans commonly played a significant role in their formation (e.g. Lees and Miller, 1995; Pray, 1958; Wilson, 1975). Because the Early Triassic microbial reefs resemble those from much earlier in the Phanerozoic, these represent another type of anachronistic facies. Despite the resurgence of microbial reefs in Early Triassic time, microbial fabrics have been significant components of reef systems since the Archean. In the Archean and Proterozoic, platforms show environmental zonation that is linked to the occurrence and diversity of microbial reef systems. Microbial reefs thrived during this time, and may have reached their peak in diversity and abundance during the Palaeoproterozoic (Grotzinger, 1990; Hoffman, 1974)). Even after the onset of metazoan reef development that began with archaeocyathids (Bowring et al., 1993), microbial communities continued to play key roles in reef systems throughout the Phanerozoic, but generally occurred with reef-building metazoans (e.g. Bertling and Insalaco, 1998; Camoin et al., 1998; Macintyre et al., 1995; Soja and Antoshkina, 1997; Wood, 1999). Early Triassic Microbial Reefs In recent research conducted in Lower Triassic strata of South China (Lehrmann, 1999; Lehrmann et al., 2003), southern Turkey, Armenia, Iran, and Oman (Baud et al., 1996; Baud et al., 2002), Greenland (Wignall and Twitchett, 2002), and the southwestern United States (Pruss and Bottjer, 2004), normal marine microbial build-ups have been documented. In addition to the proliferation of build ups, other microbialites also occur in boundary sections from Japan (Sano and Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 96 Nakashima, 1997), Iran (Heydari et al., 2001), South China (Kershaw et al., 2002; Kershaw et al., 1999) and South Tibet (Garzanti et al., 1998). One of the most fascinating aspects of these microbial reef occurrences is that some occurred as many as 4-7 million years after the end-Permian mass extinction (Lehrmann, 1999; Pruss and Bottjer, 2004). This means that microbial reefs, although also present in the earliest Triassic, were not isolated to the interval immediately following the mass extinction. For this and other reasons, the occurrence of Early Triassic microbial reefs has been linked to long-term stressful environmental conditions related to the end-Permian mass extinction event (Lehrmann, 1999; Pruss and Bottjer, 2004). Early Triassic microbial reefs have now been described from many regions including eastern Panthalassa, eastern, central, and western Tethys, and the Boreal ocean (Figure 4-2). These reefs occur primarily as reef mounds that attained a relief of about 2 m above the seafloor. These are generally described as patch reef systems and are not thought to have been as regionally extensive as Permian reefs (e.g. Weidlich, 2002). The microbial reefs tend to crop out as individual mounds, and exhibit both stromatolitic and thrombolitic features. Some microbial build-ups contain the preserved remains of microbes such as Renalcis (Lehrmann, 1999), and others contain only preserved microbial laminations (Pruss and Bottjer, 2004). In Lower Triassic strata of South China, microbial build-ups occur as calcimicrobial mounds and biostromes (Lehrmann, 1999). The Smithian-Spathian calcimicrobial mounds attained the most significant relief of all the microbialites Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 7 Figure 4-2: Early Triassic paleogeographic map showing the approximate locations of microbial reefs. A) western United States, deposited on the eastern margin of Panthalassa. B) Jamesonland Greenland, deposited in the Boreal Ocean. C) southern Turkey, deposited in the western Tethys. D) Iran, deposited in central Tethys. E) South China, deposited in the eastern Panthalassa (modified from Pruss et al., 2004, after Erwin, 1993). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 98 described from the Great Bank of Guizhou. These formed as domal or inverted conical mounds, and range in size from 0.1 m to 1.5 m (Lehrmann, 1999). Because of their topographic relief, rigid organic framework, and presence of microorganisms such as Renalcis, these have been interpreted to represent microbial patch reefs (Lehrmann, 1999). Other examples of microbial reefs have been described from Lower Triassic strata of southern Turkey (Baud et al., 1996; Baud et al., 2002) (see Figures 3-8 and 3-9). A variety of microbialites have been described including, but not limited to, columnar, domal, and conical stromatolites, and thrombolites (Baud et al., 2002). The giant domal stromatolites attained a relief of ~2 m above the seafloor. Some examples of these giant stromatolites extend laterally for 10 meters. Thrombolites consisting of massive mounds of clotted micrite measure up to 2 m in height and 10- 20 m laterally (Baud et al., 2002). Stromatolitic bioherms have been documented from Lower Triassic strata of Greenland (Wignall and Twitchett, 2002). These occur in Lower Griesbachian strata, and consist of small, laterally extensive build-ups (< 1 m) that formed within laminated silty shales (Wignall and Twitchett, 2002). These build-ups formed on a thin bed of broken microbialite clasts and thick-shelled bivalve debris (Promyalina). In thin-section, the bioherms consist of alternating dark and light laminae of micrite. These carbonate build-ups stand in stark contrast to the surrounding siliciclastics (Wignall, P. B., 2004, pers. commun.). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 99 Early Triassic microbial build-ups have been described from the western United States. Schubert and Bottjer (1992) first noted that these stromatolites represent disaster forms that were able to flourish in the aftermath of the end- Permian mass extinction. Subsequent work on these microbial build-ups has suggested that they attained significant relief above the seafloor and therefore formed patch reefs (Pruss and Bottjer, 2004) (see Figures 3-3 and 3-4). The build-ups occur in one bed in which they are laterally extensive. Thin limestone beds lap out against the sides of the individual mounds suggested a topographic relief of 1 m or more. In thin-section, microbial laminations, clotted fabrics, open framework crypts with bladed cements, and disarticulated metazoan debris are common features (Pruss and Bottjer, 2004). In addition to the various reports of microbial build-ups, other microbialites have been noted. Microbialites from Japan (Sano and Nakashima, 1997), Iran (Heydari et al., 2001), and South Tibet (Garzanti et al., 1998) have been reported from lowermost Triassic sections. Additionally, a possible microbial crust has been described from Permian-Triassic boundary sections of South China (Kershaw et al., 2002; Kershaw et al., 1999). These occurrences reflect widespread microbialite deposition immediately following the end-Permian mass extinction. Middle Triassic: Recovery of Metazoan Reefs The diversification of metazoan reefs took place in the Middle and Late Anisian and lasted about 2 million years (Fliigel, 2002). In addition to the diversification of reef-building metazoans, reef abundance also increased during this Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 0 0 time. The oldest Tethyan reefs have been reported from the Dont Formation in the Dolomites of Italy (e.g. Fois and Gaetani, 1984). These reefs exhibit abundant ‘ Tubiphytes’, although the ‘ Tubiphytes’ are considered to represent different forms than those of the Permian (Fois and Gaetani, 1984). A similar increase in 'Tubiphytes’-bearing reefs occurs in Middle Triassic strata of the Nanpanjiang Basin of South China (Payne, J. L., 2003, pers. commun.). Middle Triassic reef occurrences represent the recovery of metazoan reefs, a facies that had been absent from the rock record for ~8 million years (Fliigel, 2002). The metazoans reefs that re-appeared during the Anisian are composed of microbes and calcimicrobes, calcareous and siliceous sponges, bryozoans, and corals, with other organisms being locally important (Fliigel, 2002). Low diversity communities dominate many reefs of this time; however, a few examples of high diversity sponge-coral reefs have been described from southern Spain, the Dolomites of Italy, and Austria (e.g. Fliigel, 2002, and references therein). Reef proliferation continued into the Ladinian and Early Carnian, and many of those reefs share characteristics with their Anisian predecessors. Examples of Ladinian and Early Carnian reefs include bivalve build-ups in Germany, algal and microbial mounds in Spain, and microbial-calcareous sponge mounds in the Alps (Fliigel, 2002). Ladinian and Early Carnian reefs are comprised of the same constructional reef types as Anisian reefs; however, the late Middle Triassic reefs are more widely distributed and abundant than those of the Anisian (Fliigel, 2002). Additionally, the taxonomic composition of the Ladinian-Early Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 101 Carnian reefs differs markedly from Anisian reefs; many Anisian sponges and corals became extinct prior to the Ladinian (Fliigel, 2002). The establishment of large reef complexes occurred later in the Triassic during the Norian—Rhaetian reef bloom, and at this time, scleractinian corals replaced calcareous sponges in many reef successions, illustrating the initial rise to dominance of scleractinian reefs (Pruss, in review?). The Norian also marks the appearance of Lazarus taxa that had been absent since the Permian, suggesting a long-term existence in refugia (Fliigel, 2002). Discussion The decimation of reef ecosystems at the end of the Permian brought about a long-term absence of metazoan reefs in the Early Triassic. For as long as 8 million years after the end-Permian mass extinction, microbial reefs proliferated in their absence. As discussed above, the re-appearance of metazoan reefs took place at various locations in the Anisian. The long-term replacement of metazoans reefs by microbial reefs has been interpreted to represent prolonged environmental stress (Baud et al., 1996; Baud et al., 2002; Lehrmann, 1999; Pruss and Bottjer, 2004). A variety of criteria must be assessed when discussing the causal mechanisms of the absence of metazoan reefs from Lower Triassic strata. The first consideration is ecospace availability because without environments conducive to reef development, reefs could not form. During the Early Triassic, however, a widespread transgression facilitated the development of shelves on continental margins of northern and western Tethys, South China, and eastern Panthalassa (e.g. Hallam and Wignall, 1997). Therefore, reef ecospace was available during the Early Triassic. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 0 2 Another consideration is the time it took for metazoans to recover from the devastating end-Permian extinction. In examining the timing of recovery for other mass extinctions like the Cretaceous-Tertiary event, biotic recovery occurred between 10,000 and 100,000 years after the extinction (e.g. Hallam and Wignall, 1997, and references therein). In comparison, the biotic recovery from the end- Permian mass extinction was ~80 times as long. Additionally, the re-appearance of Lazarus taxa in Late Triassic time suggests that some organisms survived the end- Permian event but either existed in low numbers (Erwin, 1996) or in refugia (Fliigel, 2002) for most of the Triassic. All of these findings suggest that environmental parameters were the dominant control on the biotic recovery from the end-Permian mass extinction because ecospace was available, biotic recovery from mass extinction can occur in a much shorter time frame than 8 million years, and some Permian reef-dwellers had passed through the end-Permian extinction event but likely persisted in refugia for millions of years. Summary The end-Permian mass extinction brought about one of the greatest reorganizations of metazoan reef ecosystems since their advent in the Cambrian. After a near annihilation of reef building organisms such as sponges and corals at the close of the Permian, an ~8 million year metazoan reef gap ensued (Fagerstrom, 1987). This reef gap encompassed the entirety of the Early Triassic; however, in the absence of metazoan reefs, microbial reefs proliferated. Microbial build-ups have now been reported from Griesbachian (earliest Triassic) (Baud et al., 1996; Baud et Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 103 al., 2002; Lehrmann, 1999; Wignall and Twitchett, 2002) and Spathian strata (late Early Triassic)(Lehrmann, 1999; Pruss and Bottjer, 2004), and these occurrences have been linked to the presence of stressful environmental conditions in the aftermath of the end-Permian mass extinction. Following an 8 million year hiatus, metazoan reefs re-appear at a variety of locations globally in the Anisian. These reefs were initially dominated by microbes, calcimicrobes, calcareous and siliceous sponges, bryozoans, and corals; however, in Late Triassic time, corals and sponges take over as the dominant reef builders, establishing in some aspects the “modern reef’ ecosystem (Fliigel, 2002). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 104 CHAPTER 5 Carbonate Seafloor Fans Introduction Carbonate seafloor fans, ranging in size from centimeters to meters, were common features of carbonate shelf environments of the Archean and Paleoproterozoic but are thereafter only rarely found in the geologic record (Sumner and Grotzinger, 1996a; Sumner and Grotzinger, 1996b). Their anachronistic presence in younger strata suggests anomalous oceanic conditions conducive to the formation of seafloor precipitates. Their prevalence in Archean and Paleoproterozoic sediments may have been related to an oversaturation of calcium carbonate, as well as the presence of abundant inhibitors preventing the formation of micrite (such as Fe ) (Sumner and Grotzinger, 1996a). Another time period during which seafloor carbonate precipitates undergo an unusual resurgence is during Neoproterozoic time; seafloor precipitates are found in some carbonates that cap enigmatic low latitude glacial deposits worldwide during “Snowball Earth” (e.g. Hoffman et al., 1998; Hoffman and Schrag, 2002; Kennedy et al., 1998). In this instance, their presence has been interpreted to represent unusually alkaline oceanic conditions (Higgins and Schrag, 2003; Hoffman et al., 1998; Kennedy et al., 1998) and a transient restoration of Archean—Paleoproterozoic-type ocean chemistry. Following the Neoproterozoic, carbonate seafloor fans disappear from normal marine environments until Permian- Triassic time, nearly 250 million years later. Grotzinger and Knoll (1995) first documented the resurgence of carbonate precipitates in Permian reefs. In Lower Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 105 Triassic strata, carbonate seafloor fans were first described from the Union Wash Formation in the western United States (Woods et al., 1999). Further research has revealed that carbonate seafloor fans occur in a variety of regions during the Early Triassic, including Iran (Heydari et al., 2000; Heydari et al., 2003), southern Turkey (Baud et al., 2002), and possible crusts from South China (Kershaw et al., 2002; Kershaw et al., 1999). It has therefore been determined that carbonate seafloor fans recur globally during the Early Triassic. In this study, carbonate seafloor fans were documented and described from the Spathian Union Wash Formation of the western United States and the Griesbachian Kokarkuyu and Sapadere Formations of southern Turkey. These occur as individual fans, carbonate seams, and carbonate mounds made entirely of precipitates. To understand the significance of these occurrences, both a field and thin section study were conducted to ascertain the stratigraphic relationship of the seafloor fans and to determine their mineralogy. Once these factors were elucidated, it was possible to formulate conclusions about the mechanisms responsible for their formation. Additionally, the occurrence of these seafloor fans at different times during the Early Triassic signified that conditions conducive to precipitate formation occurred at different times around the world. Geologic Setting The Middle and Upper Members of the Union Wash Formation consist largely of dark gray to black micrite with intervals of siltstones. The Union Wash Formation was deposited between the Smithian to late Spathian based on the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 106 conodonts Parachirognathus ethingtoni and Neogondolella timorensis respectively (Stone et al., 1991). The Union Wash Formation was deposited in an outer shelf- basinal setting based on the presence of slumping (Stone et al., 1991). It is divided into three members based on stratigraphic criteria, and at the Darwin Hills locality, the upper portion of the Middle and the entire Upper Member are exposed. Previous studies suggest that much of the Union Wash Formation was deposited under a variety of oxygenation conditions ranging from oxic to anoxic (Woods, 1998; Woods and Bottjer, 2000). At the Darwin Hills locality, the Union Wash crops out as large limestone units (-130 m thick), limestones interbedded with siltstones, and calcareous siltstones (See Figure 2-10 and Appendix 5). The Lower Triassic Kokarkuyu Formation overlies the Permian Pamucak Formation at the Kopuk dag locality (see Figure 2-12). At the Demirtaz^-Ku^davut locality, the Lower Triassic Sapadere Formation overlies the Yugliiktepe Formation (see Figure 2-13). The base of the Kokarkuyu Formation at the Kopuk dag locality consists of a -2.5 m thick limestone with laterally extensive low-relief stromatolites similar to those exposed at the Ciiriik dag locality (Figure 5-1). Overlying the stromatolites is a -5 m thick bedded limestone containing carbonate precipitate mounds. Overlying the mounds is a limestone that alternates between being massively bedded and thinly bedded and is 20 m thick (A. Baud, 2003, pers. comm.). At the Demirtazg-Kusdavut locality, the Lower Triassic Sapadere Formation also contains carbonate precipitates, although no carbonate mounds were found. The base of the Sapadere Formation consists of a -2.5 m thick interval of stratiform Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 107 Figure 5-1: Photograph of the underside of the laterally extensive stratiform stromatolite bed of the Kokarkuyu Formation exposed at the Kopuk dag locality, southern Turkey. Visible from the underside are the bases of the small domal stromatolites that form this bed. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 108 stromatolites that are overlain by a ~7.5 m thick limestone containing the carbonate precipitates. These are exposed in only one area at this locality, and therefore cannot be traced laterally. These limestones are overlain by an 8 m thick cross-bedded oolite (Figure 5-2). The top contact of this bed is erosive, and Smithian-Spathian units cap the exposed beds of the Sapadere Formation at the Demirtaz9-Ku5davut locality (A. Baud, 2003, pers. comm.). Methods Lower Triassic sections containing seafloor precipitates were examined at 3 localities. In the southwestern United States, these were investigated at two outcrops of the Spathian-Anisian Middle and Upper Members of the Union Wash Formation at Darwin Hills, California (See Figure 2-1). In southern Turkey, the Griesbachian Kokarkuyu Formation was studied at the Kopuk dag locality and the Sapadere Formation was examined at the Demirtaz^-K^davut locality (See Figure 2-2). In the southwestern United States, stratigraphic columns were measured and samples of seafloor precipitates were noted. In southern Turkey, samples were taken and noted on previously measured stratigraphic columns (A. Baud, pers. comm., 2003). Underlying and overlying beds were noted, and in the case of the crystal mounds, surrounding beds were also described. Seafloor precipitate samples were thin- sectioned to study in detail the microstructure of the precipitates. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 109 Figure 5-2: Massive cross-bedded oolite from the Sapadere Formation, Demirtaz?- Ku?davut locality, southern Turkey. Although oolites were preserved at all localities, this unit was the largest. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 110 Results Field Analysis In the western United States, the carbonate seafloor precipitates are found in two beds in the Upper Member of the Union Wash Formation at the Darwin Hills locality (See Figure 2-10). In the first precipitate-bearing bed, precipitates occur continuously through ~130 m of strata. The second interval is a 20 m thick bed found near the top of the section. The beds in which the carbonate precipitates are found consist of dark gray limestones that show evidence for slumping. The carbonate precipitates occur as radiating fans that range in size from <1 cm to ~5 cm thick (Figure 5-3). They are found in a variety of orientations and appear to sometimes crosscut each other (Figure 5-4). The precipitates are commonly found in dark bands within the gray limestones, and due to differential weathering, they are sometimes only visible in thin-section. The precipitates are occasionally draped by thin siliciclastic siltstone layers, indicating that crystal growth may have been terminated by deposition of overlying sediments. The growth forms of the carbonate precipitates are variable and likely reflect a complex interaction between sediment and water column. The precipitates are at times upwardly-oriented, but are also found oriented upside down, suggesting they grew from the tops of cavities. In other cases, precipitate fans occur as clasts within pockets of collapse breccias. The various orientations indicate that these precipitates did not only form directly on the seafloor, but may also have formed within cavities. Wave or current action may have caused fans to be scoured and redeposited in the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I l l Figure 5-3: Photograph of radiating calcium carbonate fans from the Union Wash Formation, California, southwestern United States. Scale shows centimeters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 1 2 Figure 5-4: Photograph of calcium carbonate fans from the Union Wash Formation, California, southwestern United States, showing they occasionally cross-cut each other. Scale shows centimeters. Arrows indicate fans but also point in the direction of fan orientation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 113 sediment. The morphological similarities between the fans themselves do suggest that they precipitated from seawater and were not produced by later diagenetic processes. In southern Turkey, the carbonate seafloor fans are found at two localities, the Kopuk dag locality and the Demirtaz^-Ku^davut locality (See Figure 2-2). At the Kopuk dag locality, these occur in the Kokarkuyu Formation as individual seafloor precipitates but also as large carbonate precipitate mounds (Figure 5-5). The carbonate mounds were found in at least three horizons. The morphology of precipitates varies; they occur both as thin beds of upward-oriented crystals (2-3 cm in length) (Figure 5-6) but also as spherical crystal bundles (~10 cm in length) (Figure 5-7). The mounds contain both types of crystal morphologies, and external surfaces of the mounds weather in such a way that they obscure some fabrics. Beds surrounding the crystal mounds consist of nonresistant limestone or calcareous siltstone. Bedding above and below the crystal mounds is commonly wavy and consists of thin 5-10 cm thick beds of limestone with yellow siltstone drapes (Figure 5-8). Carbonate mound dimensions vary; the two large mounds measured at the Kopuk dag locality have dimensions of 2.1 m in height and 1.6 m in width, and 0.7 m in height and 1.3 m in width. These attained heights similar to the thrombolite mounds reported from the Ciiriik dag locality. The Sapadere Formation at the Demirtaz^-Ku^davut locality contains crystals precipitates that are exposed in one bed, and because of exposure, are not laterally extensive (Figure 5-9). This formation did not contain large carbonate mounds like Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 114 Figure 5-5: Large crystal mound from the Kokarkuyu Formation, Kopuk dag locality, southern Turkey. Man is ~1.75 m for scale. Entire mound consists of calcium carbonate precipitates including beds of crystals. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 115 Figure 5-6: Photograph of thin crystal beds collected from talus of the Kokarkuyu Formation, Kopuk dag locality, southern Turkey. A) Talus piece showing at least two crystal horizons (brackets) indicating that the crystals sometime grew in beds. B) Talus piece showing fan-like nature of crystal beds. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 116 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 117 Figure 5-7: Calcium carbonate fans from the Sapadere Formation, Demirtaz?- Ku?davut locality, southern Turkey. Crystals show unusual spherical growth form, radiating outwards in all directions from a center. Scale shows centimeters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 1 8 Figure 5-8: Photograph of limestone beds between crystal mounds, Kokarkuyu Formaiton, Kopuk dag locality, southern Turkey. The limestone beds were fine-grained and contained thin drapes of silt (arrow). Hammer is 27.5 cm for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119 Figure 5-9: Photograph of crystals from the Sapadere Formation, Demirtazg- Ku?davut locality, southern Turkey. These crystals grew in beds and did not form mounds at this locality. Left part of scale bar shows centimeters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 120 Thin-Section Analysis In thin-section, the carbonate fans of the Union Wash Formation consist of radiating crystals. These expand outward from their base, and have square terminations (Figure 5-10). In cross-section, these individual crystals are hexagonal (Figure 5-11). The crystals are composed of an equant interlocking mosaic of calcite, those of the Kokarkuyu Formation. The carbonate precipitates at this locality consisted of small outwardly radiating fans that were mostly found in talus. These are thought to have radiated within a fissure that cut down through Permian strata because they were found to nucleate on Permian limestone beds that were deposited much lower in the section stratigraphically (A. Baud, 2003, pers. comm.) and at times exhibit twinning (Figure 5-12). In the field, carbonate precipitates are only visible where they have been weathered. In thin-section, many of the black seams that do not exhibit crystals macroscopically prove to consist entirely of precipitates. Thin layers of silt commonly overlay the precipitates and may have terminated crystal growth. Because these precipitates have been entirely recrystallized, many primary features have been destroyed. The carbonate precipitates of the Kokarkuyu and Sapdere Formation have undergone less diagenetic alteration than those of the Union Wash Formation. In thin-section, the precipitates appear pristine, and thin crystals that are rarely seen in formerly aragonite precipitates are preserved (Figure 5-13). Dark laminations commonly found in Archean-type stromatolites (e.g. Grotzinger and Knoll, 1999) are Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 121 Figure 5-10: Thin-section photograph of calcium carbonate crystals from the Union Wash Formation, California, southwestern United States. Note square terminations from plan view of thin-section. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 2 2 Figure 5-11: Thin-section cross-sectional photograph of calcium carbonate crystals from Union Wash Formation, California, southwestern United States. A) and B) show pseudohexagonal cross-sectional shape of the crystals. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 123 500 pm 500 pm Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 124 Figure 5-12: Thin-section photograph of calcium carbonate crystals from the Union Wash Formation, California, southwestern United States. Note twinning of crystals under cross-polarized light. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 125 Figure 5-13: Thin-section photograph of calcium carbonate crystals from mounds of the Kokarkuyu Formation at the Kopuk dag locality, southern Turkey. Plan view of the thin-section reveals thin crystal spindles that are typically destroyed during diagenesis (arrows). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 2 6 also preserved (Figure 5-14). The crystals appear clear with laminations when viewed in normal light, but show different extinctions when viewed under cross polarized light (Figure 5-15). In cross-section, these crystals exhibit a more diffuse appearance than those of the Union Wash Formation (Figure 5-16), and this is likely related to the angle at which these were cut for thin-section. Discussion The carbonate precipitate fans at both locations are currently composed of calcite, but the form would suggest that the original composition was aragonite. The crystal units have a hexagonal cross-section and an acicular growth form, supporting a primary aragonite composition ((Loucks and Folk, 1976; Mazzullo, 1980), and have blocky to square terminations, also consistent with aragonite precipitation ((James and Choquette, 1990; Sandberg, 1985; Sumner and Grotzinger, 2000). The crystal units of the Union Wash Formation consist of an equant interlocking mosaic of calcite, which is a common fabric that replaces aragonite (Sandberg, 1985); however, the crystals of the Kokarkuyu and Sapadere Formations preserve the individual formerly aragonite rays. Finally, in comparing these precipitates with gypsum that nucleated on the seafloor, it is clear that the spear-like terminations typically exhibited by gypsum crystals are not seen in these beds (e.g. Klein and Hurlbut, 1993). The carbonate precipitates of the Union Wash Formation of the western United States and Kokarkuyu and Sapadere Formations of southern Turkey suggest Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 127 500 um a yisam m t- - Figure 5-14: Thin-section photograph of calcium carbonate crystals from talus of the Kokarkuyuy Formation at the Kopuk dag locality. Note fine laminations preserved in the crystals as they grew upward. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 128 Figure 5-15: Thin-section photograph showing site of nucleation of calcium carbonate crystals, Kokarkuyu Formation, Kopuk dag locality. Both pictures show same view. A) View of nucleation surface under normal light. Note laminations. B) View of nucleation surface under cross-polarized light. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 500 Him Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 130 that a carbonate depositional regime not seen since Neoproterozoic Snowball earth time (e.g. Hoffman and Schrag, 2002; Kennedy, 1996) was restored during the Early Triassic. Carbonate seafloor fans were common during earlier times in Earth history when ocean chemistry is thought to have been very different than today. These differences consist of an oversaturation of calcium carbonate and the presence of micrite inhibitors such as Fe+ 2 (Sumner and Grotzinger, 1996b; Sumner and Grotzinger, 2000). The carbonate fans of the Union Wash Formation have been linked to elevated levels of alkalinity and an oversaturation of calcium carbonate that formed as a result of the periodic upwelling of an anoxic basin (Woods et al., 1999). Similar conditions may have also affected the Tethyan seaway during earliest Triassic time and fostered the formation of similar carbonate precipitates in southern Turkey. The occurrence of seafloor fans during two intervals of the Early Triassic suggests that these conditions were not isolated to the Permo-Triassic boundary, but persisted for millions of years after the end-Permian mass extinction. Summary The deposition of carbonate seafloor fans during two intervals of the Early Triassic suggests that anomalous ocean conditions related to the end-Permian extinction persisted for millions of years. Carbonate seafloor fans were common during Archean—Paleoproterozoic time, but thereafter have only rarely been reported. This temporal restriction has been cited as evidence that Archean- Paleoproterozoic ocean chemistry differed from that of subsequent time periods (Sumner and Grotzinger, 1996b; Sumner and Grotzinger, 2000). The presence of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131 carbonate precipitates from Lower Triassic strata suggests a resurgence of unusual ocean conditions such as oversaturation of calcium carbonate and the presence of micrite inhibitors. This resurgence did not occur at only one time period during the Early Triassic but rather existed for as long as 4-7 million years after the end- Permian extinction. This implies that unusual ocean chemistry may have played a role in inhibiting the recovery of metazoans (e.g. Hallam, 1991; Woods et al., 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 132 CHAPTER 6 Flat-pebble Conglomerates Introduction Sepkoski (1984) first noted that flat-pebble conglomerates represented a facies that was common to Precambrian and Cambrian shelf deposits, but thereafter occurred rarely in the geologic record. The idea of anachronistic facies was borne out of this discovery (Sepkoski et al., 1991). Flat-pebble conglomerates are intraclastic limestones consisting of long, tabular micritic clasts with a long axis >1 cm. These likely formed because early lithification of the seafloor occurred in subtidal environments, and subsequent storm events ripped up clasts of lithified micrite and redeposited them (e.g. Droser, 1987; Sepkoski, 1982). Another condition linked to the formation of flat-pebble conglomerates is the absence of deep bioturbation. Without deep bioturbation, the redox boundary of the sediment shallows and a substantial mixed layer does not develop (Sepkoski et al., 1991). As a result, carbonate sediment is able to lithify early and subsequent storm events would exhume these layers as long, micritic clasts. As bioturbation increases, these facies become less prevalent (Sepkoski et al., 1991). In Lower Triassic strata, researchers have noticed an increase in flat-pebble conglomerate occurrences. In the Lower Triassic Moenkopi Formation of the southwestern United States, Schubert and Bottjer (1995) documented the presence of flat-pebble conglomerates. In Lower Triassic sections of southern Italy and South China, flat-pebble conglomerates have been documented from a variety of shelf Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 133 settings (Wignall and Twitchett, 1999). The resurgence of this facies after a near absence from carbonate shelf environments for millions of years suggested that conditions like the Cambrian may have existed in the Early Triassic. This means that once again bioturbation must have been reduced such that these facies could again form in storm-dominated carbonate environments. Methods In this study, flat-pebble conglomerates were studied from Lower Triassic sections in the southwestern United States and southern Turkey. The Spathian Virgin Limestone Member of the Moenkopi Formation was studied at Lost Cabin Springs and Mountain Pass localities, and Smithian-Spathian strata of southern Turkey was studied at the Ciiriik dag locality. At these localities, flat-pebble conglomerates were sampled and noted. Some samples were later slabbed and thin-sectioned to look at the internal textures. Results Field Analysis In the southwestern United States, flat-pebble conglomerates are present in the Virgin Limestone Member (Figure 6-1) in three units of at the Lost Cabin Springs locality and two units at the Mountain Pass locality. Flat-pebble conglomerates are also present in Smithian-Spathian strata of southern Turkey at the Ciiriik dag locality (Figure 6-2 and 6-3). The flat pebble conglomerates commonly occur through more than 1 m of strata, although exposure often limits measurements. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 134 Figure 6-1: Photograph of a bedding plane view of flat-pebble conglomerate bed from Virgin Limestone Member, Lost Cabin Springs locality, southwestern United States. Clasts are as large as several cm in diameter. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 135 Figure 6-2: Photograph of a vertical section of a flat-pebble conglomerate bed in Smithian-Spathian strata, £Uriik dag locality, southern Turkey. Clasts are imbricated, some with a long axis of several centimeters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 136 Figure 6-3: Photograph of same flat-pebble conglomerate bed pictured in Figure 6-2. Bed was preserved in Smithian-Spathian strata, £iiriik dag locality, southern Turkey. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137 At the Lost Cabin Springs locality, the flat-pebble beds consist of long, thin dark gray micritic intraclasts in a light gray matrix. Some portions of the conglomerates exhibit imbrication of clasts whereas others look randomly oriented. These beds commonly form cliff-forming units, and two of the three beds occur at the top of the section at Lost Cabin Springs. At the Muddy Mountains locality, these are found in two beds and exhibit similar characteristics to those at Lost Cabin Springs. In southern Turkey, the Smithian-Spathian strata unconformably overlie the Katarsi Formation at the Ciiriik dag locality. At this locality, the Smithian-Spathian strata are composed predominately of siliciclastic units with a few intervals of flat- pebble conglomerates. Most of the section is covered by a talus slope and is difficult to find in place. Because of this, the specific numbers of beds containing flat-pebble conglomerates could not be ascertained. Where these were found, flat-pebble conglomerate beds contained long, tabular intraclasts that were imbricated. The clasts looked yellow in color and may represent the exhumation of a silty limestone. Some of the clasts in these beds had long axes of several centimeters. Thin-Section Analysis Thin-sections of flat-pebble conglomerates from the southwestern United States were made as part of this study. In thin-section, these flat pebble clasts are rounded tabular intraclasts occurring in a coarse-grained matrix. The clasts are devoid of fossil debris, but debris is preserved in the surrounding matrix. Although larger clasts make up much of the bed in outcrop (see Figure 4A and B), smaller clasts can be seen in thin-section. There does not seem to be a dominant size of clasts Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 138 in thin-section, and they do not show any orientation. The edge of the clasts is often smooth and occasionally stylotized. Discussion After an absence from the geologic record for millions of years, flat-pebble conglomerates undergo a unique resurgence during Early Triassic time. These have now been documented from Lower Triassic strata in Italy and South China (Wignall and Twitchett, 1999), the southwestern United States and southern Turkey. This indicates that these were forming around the world during Early Triassic time. Additionally, flat-pebble conglomerates of Italy and South China were found primarily in Griesbachian strata (Wignall and Twitchett, 1999). Those found in the southwestern United States and southern Turkey formed during the late Early Triassic. This indicates that the formation of flat-pebble conglomerates was not isolated to boundary sections but formed as long as 4 —7 million years after the extinction event. From field and thin-section analysis of the flat-pebble conglomerates from Lower Triassic strata of the southwestern United States and southern Turkey, some conclusions can be drawn. These consist of micritic clasts preserved in a carbonate matrix indicating that these formed in carbonate environments. The clasts are found to be devoid of fossils including any traces of bioturbation indicating that the initial lithification process likely took place in the absence of metazoans. The coarse grained matrix supports the hypothesis that these formed during storm deposits that initially ripped up the clasts of carbonates and re-deposited them as beds of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 139 conglomerates. The thickness of the conglomerate beds (> 1 m) suggests that these do not represent small episodes of lithification but likely indicate a large-scale lithification of the seafloor. Finally, these indicate that deep bioturbation had not fully recovered by latest Triassic time. This is substantiated by the occurrence of only shallow Thalassinoides burrows from the same localities (see Chapter 10). Summary Flat-pebble conglomerates are common features of Precambrian-Cambrian shelf deposits because deep bioturbation had not yet evolved. The Early Triassic is a return to such conditions because deep infaunalization of metazoans was repressed during the entirety of the lag phase from the end-Permian mass extinction (e.g. Twitchett, 1999). It is important to note that intraclastic limestones are found throughout the geologic record; however large-scale conglomerates like those described herein are extremely unusual. The flat-pebble conglomerates now reported from Lower Triassic sections in the southwestern United States and southern Turkey substantiate earlier trends documented from Italy and South China (Wignall and Twitchett, 1999). Flat-pebble conglomerates undergo a global resurgence during Early Triassic time, and are not isolated to boundary beds. Rather, these are found as long as 4 —7 million years after the end-Permian extinction event. This indicates that conditions responsible for the suppression of metazoans persisted for millions of years after the extinction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 140 CHAPTER 7 Wrinkle Structures Introduction Recent research on Lower Triassic strata in the southwestern United States and in northern Italy has yielded the first reported occurrence of microbially- mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the Cambrian (Pruss et al., 2004). The hypothesis is that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end-Permian mass extinction. These wrinkle structures occur in siliciclastic sediments in association with hummocky cross stratification and, in some cases, trace fossils such as Asteriacites, Rhizocorallium, Planolites and Gyrochorte that indicate a subtidal paleoenvironment. The research presented here focuses on wrinkle structures that were found in the Virgin Limestone Member of the Moenkopi Formation in the southwestern United States, although wrinkle structure have also been found in the Campil Member of the Werfen Formation of southern Italy and the upper limestone of the Thaynes Formation in the southwestern United States (Pruss et al., 2004). The presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm-dominated siliciclastic shelves. Wrinkle structures commonly formed in subtidal paleoenvironments during the Proterozoic-Cambrian, but thereafter became restricted to intertidal-supratidal and deep-sea environments (Hagadorn and Bottjer, 1997; Hagadorn and Bottjer, 1999). This restriction has been attributed to the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 141 increase in infaunalization of metazoans following the Cambrian radiation. The proliferation of wrinkle structures in subtidal settings during the Early Triassic suggests that infaunal bioturbation was reduced after the end-Permian mass extinction, and that this reduction lasted for millions of years. Wrinkle structures are a type of microbially mediated sedimentary structure found preserved in siliciclastic deposits, and include runzelmarken, micro-ripples, and Kinneyia ripples (Hagadorn and Bottjer, 1997). The formation of these structures has been attributed to the stabilization of the substrate by microbial mats (Hagadorn and Bottjer, 1997; Hagadorn and Bottjer, 1999; Noffke, 2000; Noffke et al., 2003; Noffke et al., 2002; Noffke and Krumbein, 1999). Wrinkle structures are common sedimentary features in Proterozoic-Cambrian strata (e.g. Gehling, 1986; Hagadorn and Bottjer, 1997; Hagadorn and Bottjer, 1999; Kopaska-Merkel and Grannis, 1990), although their record has been found to extend back to the Mesoarchean (Noffke et al., 2003). In the Proterozoic-Cambrian, wrinkle structures formed in intertidal to deep-sea marine settings (Hagadorn and Bottjer, 1999). In post-Cambrian time, wrinkle structures are restricted to tidal and deep-sea environments, and this restriction has been attributed to the increase in levels of bioturbation and consequent mixed layer development during the Ordovician (Hagadorn and Bottjer, 1999). Wrinkle structures were initially interpreted as sedimentary structures produced by physical current waning, wind-induced shear, and sediment loading (Allen, 1985; Dzulynski and Simpson, 1966; Reineck, 1969; Teichert, 1970). In Hagadorn and Bottjer’s (1997) pioneering study, wrinkle structures were determined Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 142 to be the preserved remnants of an irregular surface of a microbial mat community. Other work has suggested that the formation of wrinkle structures occurs when overlying sediment is deposited on a microbial mat, and this loading forms molds and casts (Noffke et al., 2002). The presence of a microbial mat is inherent to most accepted models of wrinkle structure formation (Hagadorn and Bottjer, 1997; Hagadorn and Bottjer, 1999; Noffke et al., 2002; Noffke and Krumbein, 1999). Microbial mats in siliciclastic environments consist of cyanobacterial cells and extracellular polymeric substances (Noffke et al., 2003). These mats do not form stromatolites because little to no mineral formation has been found to take place within these communities (Noffke et al., 2003). To resist the dynamic processes of water motion in subtidal environments, mats must form cohesive fabrics. Biofilms (an initial phase of cyanobacterial colonization) have been found to contribute little to the stabilization of sediment whereas mats (mature stages) provide much stabilization to siliciclastic sediment (Noffke, 1998). The wave and current interaction with these mats is recorded as microbially induced sedimentary structures (MISS) (Noffke and Krumbein, 1999). The end-Permian mass extinction was the largest extinction in the history of life and “brought the Palaeozoic great experiment in marine life to a close”(Erwin, 1994). A delayed biotic recovery occurred 4 (Martin et al., 2001) to 8 Ma (Hallam, 1991) after the extinction. Whereas microbialites have been recorded globally from carbonate sediments from this time (Baud et al., 1996; Baud et al., 2002; Kershaw et al., 1999; Lehrmann, 1999; Pruss and Bottjer, 2002; Schubert and Bottjer, 1992), this Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 143 study is the first to record coeval siliciclastic microbial structures. Wrinkle structures are found in the Spathian Virgin Limestone within the Moenkopi Formation of the southwestern United States (See Figures 2-5 and 2-9) Wrinkle structures in Lower Triassic strata can be described as components of “anachronistic facies”(Sepkoski et al., 1991): a facies which includes sediments and sedimentary structures that were more common during earlier times, and has since become rare. Wrinkle structures formed in many marine paleoenvironments of the Proterozoic-Cambrian, but thereafter have been restricted to deep sea and marginal marine settings (Hagadorn and Bottjer, 1999). The abundance of wrinkle structures in Early Triassic subtidal paleoenvironments is therefore anachronistic (Pruss et al., 2004). This occurrence suggests that the Early Triassic is unusual when compared to the rest of the Phanerozoic, and the conditions that allowed for the proliferation of wrinkle structures persisted for millions of years after the end- Permian mass extinction. Study Locations and Methods In the southwestern United States, wrinkle structures have been found in the Spathian Virgin Limestone Member of the Moenkopi Formation, which was deposited along the eastern margin of Panthallasa (Figure 7-1). The Virgin Limestone Member (~150-200 m thick) consists of a mixed carbonate-siliciclastic succession deposited in a subtidal paleoenvironment. Because many of the siliciclastic beds in the Virgin Limestone Member are weathered or covered, bedding plane exposure of wrinkle structures is limited. At each of the localities, bedding Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 44 Paleotethys Panthalassa Panthalassa Figure 7-1. Map of paleogeographic reconstruction of the continents during Early Triassic time with the localities at which wrinkle structures have been found indicated (modified from Pruss et al., 2004). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 145 planes containing wrinkle structures were described, and samples were collected. To further constrain the paleoenvironments in which the wrinkle structures formed, detailed observations were made on the sedimentary structures and trace and body fossils of the wrinkle structure-encompassing strata. Because the wrinkle structures exhibited a variety of geometries, crests and distances between crests (troughs) were measured for comparison (Hagadorn and Bottjer, 1999). Samples of wrinkle structures were slabbed in cross-section to study changes in grain size. In addition, thin-sections were prepared to search for evidence of the former presence of microbial mats. Results In the Virgin Limestone Member, wrinkle structures are preserved at two localities. At the Lost Cabin Springs locality (Schubert and Bottjer, 1992), these have been found in dark red siltstone talus. The crests of the wrinkle structures range in thickness from 1 to 5 mm, and the distances between crests are 2-3 mm. From side view, hummocky cross-stratification is preserved. There are also grains of mica concentrated in the wrinkle structure troughs between crests. At the Muddy Mountains Overton locality (Shorb, 1983), wrinkle structures are found in more than one horizon; however only one bedding plane is fully exposed (Fig. 3D). The bedding plane is exposed on the top of an 11.6 m thick calcareous siltstone unit. Wrinkle structures on this bedding plane cover much of the exposed area, with the conspicuous exception of several unidirectional scour marks (Figure 7-2). These Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 146 Figure 7-2: Photograph of a bedding plane containing wrinkle structures. Also visible are unidirectional scour marks. These wrinkle structures indicate the former presence of a cohesive microbial mat (modified from Pruss et al., 2004). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 147 wrinkle structures have smaller crests (1-2 mm) and smaller distances between crests (1-2 mm) than those reported from the Lost Cabin Springs locality. Ripple marks and cross-bedding are the sedimentary features found in close association with the wrinkle structures. Abundant trace fossils have also been found in the same unit as the wrinkle structures, and these include Rhizocorallium, Arenicolites, Planolites, Gyrochorte, and Asteriacites. Samples from the Virgin Limestone Member of the Moenkopi Formation were thin-sectioned to search for evidence of the former presence of microbial mats. In thin section, dark clay-rich bands that represent possible mat fabrics similar to those reported by Noffke et al. (2002) are evident. Additionally, aligned grains of mica were also visible, and these also indicate the former presence of a microbial mat (Schieber, 1999). Depositional and Taphonomic Conditions The Lower Triassic wrinkle structures not only represent the former presence of a microbial mat, but also the existence of biological and environmental conditions conducive to their preservation (Hagadorn and Bottjer, 1997; Hagadorn and Bottjer, 1999; Noffke et al., 2002; Noffke and Krumbein, 1999). Wrinkle structures form in siliciclastic settings dominated by microbially-bound sediment. In addition to the presence of microbial mats, a certain set of characteristics must exist such that these wrinkle structures form and are then preserved in the rock record. This necessitates the presence of a taphonomic window necessary for wrinkle structure preservation (Hagadorn and Bottjer, 1997; Noffke et al., 2002). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148 In recent work on the Upper Neoproterozoic Nama Group, Namibia, wrinkle structures were found to form in storm-dominated siliclielastic settings where hydrodynamic flow was strong enough to sweep away mud yet did not erode the mat surface (Noffke et al., 2002). The microbial mat colonized a surface and trapped and bound quartz grains during periods of reduced water agitation. The preservation of these mats took place thereafter when burial occurred without erosional destruction. The actual formation of wrinkle structures occurred when pressure from overlying sediment formed molds and casts before lithification (Noffke et al., 2002). In the Lower Triassic siliciclastic sediments where wrinkle structures are preserved, evidence suggests that depositional conditions, similar to those described by Noffke et al. (2002), were present. The presence of hummocky cross-stratification in the siliciclastic sediments provides evidence for a storm-dominated paleoenvironment. The fine grain size noted in the samples was optimal for colonization by cyanobacteria that ultimately formed mats (Noffke and Krumbein, 1999). The enrichment of mica on wrinkle structure-bearing bedding planes points to the trapping and binding activity of microbial mats (Hagadorn and Bottjer, 1997; Schieber, 1999). The preservation of these structures then occurred when obrution deposits were deposited on top of microbially-bound sediment, but did not cause complete erosional disruption (Noffke et al., 2002). The wrinkle structures at all localities exhibit a variety of geometries, and this is also consistent with what is seen in Proterozoic-Cambrian wrinkle structures (Figure 7-3) (Hagadorn and Bottjer, 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 149 Figure 7-3: Photograph of Cambrian wrinkle structures from the Lower Cambrian Poleta Formation, Westgard Pass, central California. Arrow shows various geometries exhibited by the wrinkle structures across a bedding plane (modified from Pruss et al., 2004). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 150 The suite of trace fossils found in the sediments associated with the wrinkle structures corroborates the sedimentological evidence; the wrinkle structures found in Lower Triassic strata occurred in shallow subtidal environments below normal wave base but above storm wave base (Hamblin et al., 1979). The trace fossil Asteriacites, an ophiuroid resting trace commonly found in Lower Triassic siliciclastic strata interpreted to represent subtidal paleoenvironments (Broglio Loriga et al., 1990; Twitchett and Wignall, 1996), is found at all localities where wrinkle structures are preserved (Broglio Loriga et al., 1990; Twitchett and Wignall, 1996). Other traces found with the wrinkle structures, including Rhizocorallium, Asteriacites, and Gyrochorte, are also indicative of a subtidal marine paleoenvironment (e.g. Bromley, 1996, and references therein). Work on trace fossils of the Early Triassic has shown that a vast reduction in the depth and extent of bioturbation occurred after the end-Permian mass extinction (Ausich and Bottjer, 2002; Bottjer et al., 1988; Hallam and Wignall, 1997; Twitchett, 1999; Twitchett and Wignall, 1996). By the late Early Triassic vertical traces such as Rhizocorallium had reappeared (Twitchett, 1999; Twitchett and Wignall, 1996), and this trace has been found in association with the wrinkle structures reported here. Despite the presence of vertical trace-makers, their ability to significantly disturb the sediment must have been reduced so that a taphonomic window was opened to the extent that global preservation potential for wrinkle structures increased. This means that Early Triassic siliciclastic environments consisted of a Proterozoic-style soft substrate with minimal bioturbation (Bottjer et al., 2000; Dombos et al., in press; Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 151 Hagadorn et al., 1999). This differs notably from the soft-substrates of most of the Phanerozoic, which are characterized by significant mixed layers produced by infaunal bioturbators (Bottjer et al., 2000; Dombos et al., in press; Hagadorn et al., 1999). Summary The global occurrence of wrinkle structures in Lower Triassic shallow subtidal siliciclastic strata is anomalous for the Phanerozoic. Whereas these structures were extremely common in subtidal paleoenvironments of the Proterozoic- Cambrian, thereafter they became restricted to deep sea or stressed environments (Hagadorn and Bottjer, 1999). This restriction is attributed to an increase in infaunalization after the Cambrian (e.g. Bottjer and Ausich, 1986; Crimes et al., 1992; Droser and Bottjer, 1988; Droser and Bottjer, 1989; Seilacher and Pflueger, 1994). The anachronistic resurgence of wrinkle structures in the Early Triassic is thus indicative of a return to Proterozoic-style soft substrates (Bottjer et al., 2000; Dombos et al., in press; Hagadorn et al., 1999). Unlike the Proterozoic-Cambrian, the reduction of vertical bioturbation in the Early Triassic (Twitchett, 1999; Twitchett and Wignall, 1996) does not occur because vertical bioturbators had not yet evolved, but because infaunal organisms were suppressed for several million years after the end-Permian mass extinction. The proliferation of microbialites in Lower Triassic carbonates has been well documented (Baud et al., 1996; Baud et al., 2002; Kershaw et al., 1999; Lehrmann, 1999; Pruss and Bottjer, 2002; Schubert and Bottjer, 1992), and these occurrences Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 152 have been linked to stressful oceanic conditions such as anoxia (Isozaki, 1994; Isozaki, 1997) and/or hypercapnia (Woods et al., 1999) following the end-Permian mass extinction. The prevalence of Lower Triassic subtidal wrinkle structures has great significance because it illustrates that siliciclastic paleoenvironments also show signs of environmental stress. The wrinkle structures reported here occurred in the Smithian-Spathian time interval, millions of years after the end-Permian mass extinction, meaning that Phanerozoic levels of bioturbation had still not recovered by this time (Ausich and Bottjer, 2002). This delayed recovery, in addition to the widespread occurrences of carbonate and siliciclastic microbialites in normal marine environments, is likely related to environmental stress that persisted for millions of years after the end-Permian mass extinction event (e.g. Hallam, 1991; Lehrmann, 1999; Lehrmann et al., 2003; Lehrmann et al., 2001; Woods et al., 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 153 CHAPTER 8 Other Unusual Facies Introduction In addition to the various anachronistic facies found in Lower Triassic strata of the southwestern United States and southern Turkey, there is also a prevalence of other unusual facies. These facies are not classified as anachronistic because although they may be present in strata from other time periods, little is currently known about their temporal distribution. They may also represent facies that are only found in Lower Triassic strata, but because they are thought to represent unusual environmental conditions, they are discussed here as part of this research. Some of the unusual facies reported here include thinly bedded limestone- mudstone facies, the chip facies, and thin bioturbated beds. These occur in a variety of beds in the Virgin Limestone Member of the Moenkopi Formation and the Middle and Upper Members of the Union Wash Formation in the southwestern United States. These facies have not yet been reported in the literature but it is possible that these occur in other Lower Triassic sections around the world. Thinly-bedded Limestone-Silty Limestone Facies A variety of unusual facies have been documented from the Virgin Limestone Member of the Moenkopi Formation that may not represent true anachronistic facies. One example is the thin-bedded limestone and mudstone facies: a facies that consists of thin layers of alternating lime mudstone and calcareous limestone. Droser (1987) first recognized this as a facies typifying Middle to Late Cambrian middle shelf Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 154 paleoenvironments. The preservation of these beds is largely attributed to low levels of bioturbation typical of Cambrian paleoenvironments. In Lower Triassic strata, a facies that resembles this has been documented; however, the Triassic examples consist of alternating beds of lime mudstone and silty limestone containing enriched in detrital quartz grains. The genesis of both the Cambrian and Triassic examples is thought to reflect low levels of bioturbation, but because they consist of different lithologies, the Triassic example is not a true anachronistic facies. This facies was first discussed in depth by Woods (1998) who postulated that these facies formed as part of a cyclic package deposited in open marine deep-water conditions. The preservation of small-scale laminations within some of the cm-scale beds is hypothesized as indicating deposition under low-oxygen conditions (Woods, 1998). The light gray beds are composed of fine-grained micrite whereas the tan beds contain some quartz silt. The variations in the composition of the couplets suggest that different depositional regimes were responsible for their formation (See Woods, 1998, Figure 6-18). For instance, the micrite beds may have formed during increased periods of upwelling whereas the beds rich in quartz silt indicate a moist continental interior that resulted in riverine input (Woods, 1998) These beds also differ in their isotopic composition, and this is thought to reflect differences in diagenetic effects (Woods, 1998). The thin-bedded limestone-mudstone facies occurs in one bed of the Virgin Limestone Member of the Moenkopi Formation at the Mountain Pass locality (Figure 8-1) and at two beds in the Union Wash Formation at the Darwin Hills Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 155 Figure 8-1: Photograph of thinly-bedded limestone-silty limestone facies in the Virgin Limestone Member, Mountain Pass locality, southwestern United States. All beds are limestones; however, the dark beds are micrite and the light colored beds have a coarser grain size and contain dolomite. These thin beds become increasingly bioturbated upsection. Field knife is 7.5 cm for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 156 locality (Figure 8-2). The thin-bedded limestone/mudstone facies found in the southwestern United States occurs as cm-scale alternating beds of silty limestone with lime mudstone. These can occur continuously through as much as 6 m of strata. The thin beds that make up this facies pinch and swell and are sometimes discontinuous. In thin section, the limestone and silty limestone beds alternate. The micritic beds are fine-grained and are devoid of fossils. The silty limestone mudstone facies is slightly coarser than the micrite and is also devoid of fossils. The surfaces between the layers are erosive. Chip Facies Another form of the flat-pebble conglomerate facies is the micritic “chip” facies. This facies is different from the flat-pebble conglomerates in that these clasts have a more irregular shape, and do not contain a long axis >lcm. These likely form when semi-lithified micrite is ripped up and redeposited. In thin sections of the flat-pebble conglomerate beds, smaller chip-like clasts are also found, indicating that not all of the clasts are tabular. Additionally, Lower Triassic flat-pebble conglomerate beds grade into chip facies in upper half of the Virgin Limestone at the Lost Cabin Springs locality, and this adds further support to the hypothesis that these are formed by the same mechanism. This chip facies has been found in the Virgin Limestone Member in two beds at the Lost Cabin Spring locality (Figure 8-3) and one bed at Mountain Pass. It has also been found in Smithian-Spathian talus of the CiirUk dag locality; however at this locality the chip facies is found only in talus pieces (Figure 8-4). The Turkish Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 157 Figure 8-2: Photograph of thinly-bedded limestone-silty limestone facies from the Union W ash Formation, Darwin H ills locality, southwestern United States. These thin beds are also limestone and no bioturbation is visible. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 158 Figure 8-3: Photograph of chip facies preserved in the Virgin Limestone Member, Cabin Springs locality, southwestern United States. Clasts are irregularly shaped and vary in size. Scale bar shows centimeters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 5 9 Figure 8-4: Photograph of a talus piece of the chip facies found in Smithian-Spathian strata, £uriik dag locality, southern Turkey. This photograph is a top view of a bedding plane, but the irregular edges of the clasts of micrite are visible. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160 examples are viewed from a bedding plane view, which is why the chip facies are pictured differently than those of the southwestern United States (See Figure 8-4). The chip facies is found at the same localities where flat-pebble conglomerates have been reported. The occurrence of the chip facies may indicate that some micrite clasts that are ripped up during storms events are well-lithified whereas others are partially lithified, and perhaps break differently or are subject to deformation. Thin Bioturbated Beds Thin heterolithic beds, an anachronistic facies first described by Sepkoski et al. (1991), consist of thin layered beds (mm partings to cm-sized beds) that were typical in Cambrian and earlier strata. These thin beds were described as “storm- dominated heterolithic facies” that consisted of coarse siltstone, fine sandstone and pelleted limestone (Sepkoski et al., 1991). Like ribbon rock, this facies was hypothesized as having formed in a depositional system devoid of intense bioturbation that would have mixed layers and created mottled massive beds rather than thin beds. In Lower Triassic strata of the southwestern United States, thin bioturbated beds have been documented (Schubert, 1993)(Figure 8-4). These beds differ from those described by Sepkoski et al. (1991) because they are made entirely of fine-grained micrite, not of coarse siltstone, sandstone and pelleted limestone; however, the genesis of both facies is likely linked to suppressed levels of vertical bioturbation. In Lower Triassic strata of the southwestern United States, thin beds are also a striking feature of the Virgin Limestone Member of the Moenkopi Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 161 These beds were first described in detail by Schubert (1993) who noted the abundance of this facies in the Virgin Limestone Member, Lost Cabin Springs locality. Schubert (1993) noted that there was a wide variety in the disruption of bedding at the Lost Cabin Springs locality in the thin-bedded facies (See Chapter 10 for a detailed discussion) and related this variance to differences in bioturbation, physical reworking processes, and diagenesis (specifically the effects of post- depositional stylolites). The thin-bedded facies is a dominant feature of the Virgin Limestone at the Lost Cabin Springs locality but is also present at the mountains pass locality. An unusual feature of these thin beds is that they crop out typically as cliff-forming beds, and range from being completely unbioturbated (ichnofabric index of 1) to exhibiting almost a complete loss of bedding due to horizontal bioturbation (ichnofabric index of 5 or 6) (Figure 8-5). The highly bioturbated ledges have been nicknamed “spaghetti rock” because of their appearance in the field (Figure 8-6). These thin-bedded ledges frequently show changes in bioturbation from unbioturbated sediment to almost homogenized sediment within a single ledge. The bed thickness remains constant (~1 cm) and no other distinct changes are noted in the field suggesting that environmental fluctuations other than sedimentation rate may be affecting bioturbation. Also of note is that these beds exhibit only one type of trace: the simple horizontal burrow Planolites (Figure 8-7). The occurrence of only simple horizontal traces is consistent with a low-oxygen paleoenvironment (e.g. Savrda and Bottjer, 1986), and therefore may suggest that these beds experienced fluctuations in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 162 .H O l^ Figure 8-5: Photograph of bioturbated beds in the Virgin Limestone Member, Muddy Mountains Ute locality, southwestern United States. Note these beds show intervals of high bioturbation (Bracket A) as well as intervals with no bioturbation (Bracket B). Scale bar shows centimeters. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 163 Figure 8-6: Photograph of bioturbated beds within a limestone ledge of the Virgin Limestone, Mountain Pass locality, southwestern United States. This entire ledge exhibits an ichnofabric index of 5 (Droser and Bottjer, 1986). and is representative of many of the limestone ledges of the Virgin Limestone Member at the Lost Cabin Springs and Mountain Pass localities. Field knife is 7.5 cm for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 164 Figure 8-7: A photograph of Planolites trace fossils on a bedding plane in the Virgin Limestone Member, Mountain Pass locality, southwestern United States. This trace is the most common form of horizontal bioturbation preserved in the bioturbated bed facies. Field knife is 7.5 cm long for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 165 pore water oxygen content. It is also important to note that other trace fossils are found in different facies at Lost Cabin Springs (e. g. Thalassinoides, Gyrochorte, Asteriacites, etc; see Chapter 10), so not all beds were deposited under low oxygen conditions. Thin beds (both unbioturbated and bioturbated) are found in thirteen units at Lost Cabin Springs locality, one unit at the Muddy Mountains Ute locality, and five units at Mountain Pass locality. Summary The occurrence of these facies suggests that not all unusual facies in Lower Triassic strata can be currently classified as anachronistic facies. Although the facies described here have unknown temporal distributions, it is also possible that these facies may be isolated to the Early Triassic. The presence of facies such as the thinly-bedded limestone mudstone facies and thin bioturbated beds suggests that low levels of bioturbation create a different preservational window. In the presence of deep bioturbators, these facies would be homogenized and therefore would not be preserved. The thinly-bedded limestone mudstone facies, although different than the facies described by Droser (1987), likely indicates a similar depositional regime. The chip facies indicates that flat-pebble conglomerates are only one manifestation of the exhumation of lithified micrite. The thin bioturbated beds reported from various localities suggest that oxygen-stress may have played a role in the distribution of trace fossils from Lower Triassic strata. These thin beds change abruptly from low levels of bioturbation (ii of 1) to high levels of bioturbation (ii of 5-6), suggesting that small-scale environmental fluctuations must have played a role in the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 6 6 development of this facies. The occurrence of only simple horizontal traces like Planolites also suggests the presence of environmental stress, similar to what has been noted from strata deposited immediately after the end-Permian mass extinction (Bottjer et al., 1988; Twitchett, 1999; Twitchett and Wignall, 1996). Detailed facies analysis of other Lower Triassic sections may yield similar unusual facies that have not yet been recognized. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 167 CHAPTER 9 Sequence Stratigraphic Framework Introduction The Virgin Limestone Member of the Moenkopi Formation was deposited as a mixed carbonate-siliciclastic succession. At some localities, the Virgin Limestone Member unconformably overlies the Lower Red Member of the Moenkopi Formation, and this transition has been marked by subaerial exposure surfaces (McKee, 1954). At other localities, the Virgin Limestone Member directly overlies the Kaibab Formation (Lower Permian). This unit represents a flooding event of the Panthalassa seaway onto a westward dipping ramp (Blakely, 1972; Marzolf, 1993). The Virgin Limestone Member can be divided into an initial transgressive phase followed by a highstand systems tract. Because much of the section is covered, the individual parasequences are difficult to elucidate, however, overall depositional trends can be determined. The transgressive systems tract is represented by a decrease in siliciclastic grain size (from siltstones to mudstones) and a concomitant decrease in carbonate grain size (from abundant skeletal grains to lime mud). These changes represent a decrease in environmental energy that is linked to an overall deepening event. The highstand systems tract represents the infilling of the basin after maximum flooding and is reflected in the facies as an increase in graded storm beds and other high-energy deposits. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 168 Muddy Mountains Overton locality Previous work has determined that the base of the Virgin Limestone Member unconformably overlies the Lower Red Member at this locality (Shorb, 1983); however this contact was never located during this study. The basal part of the measured section here consists of thin parasequences (Figure 9-1). The lower half of this section contains abundant siliciclastic deposits and an oolite. These parasequences thicken upwards, as the overall section becomes increasingly dominated by carbonate. The highstand systems tract encompasses about the last third of the measured section, and continues into the evaporitic deposits of the Shnabkaib Member (not measured). The Virgin Limestone Member at this locality contains one bed of flat-pebble conglomerate facies as the only carbonate anachronistic facies; however, there are abundant wrinkle structures reported from siltstones (Pruss et al., 2004) although these are only preserved in place on one bedding plane. This represents the shallowest section in this study because much of the section was deposited in a shallow subtidal paleoeovironment (Shorb, 1983). Muddy Mountains Ute Locality At the Muddy Mountains Ute locality, the Virgin Limestone unconformably overlies the Lower Red Member and is overlain by the evaporitic Shnabkaib Member. The base of the Virgin Limestone is dominated by shallow water siliciclastics and oolites as well as thin parasequences (Figure 9-2). This section also shows a change from cross-bedded siltstones to mudstones with gutter casts, as well Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 169 Figure 9-1: Stratigraphic column of the Virgin Limestone Member at the Muddy Mountains Overton locality, western United States. Grain sizes are noted in black and white dots. Grain size trends show an initial fining (indicative of deepening) followed by a coarsening (shallowing) near the top of the section. Numbers on side of column correspond to units in Appendix 1. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 170 KEY flat-pebble conglomerates o o o oolites radiating inicrobialites microbial mounds wrinkle structures E 3 thin-bedded limestone-mudstone carbonate precipitates H limestone B siltstone S H mixed limestone/siitstone partially covered El covered M/M mudstone/mudstone W/S wackestone/siltstone P/Sa packstone/sandstone G/CSa graimtone/coarse sandstone • carbonate sediments O siliciclastic sediments 48 .— I .— I — 39 35 i ^lig * M 20-21 ”i— i — i — r M /M W /S P /S a G /C S a A y H s i t g e h m s s t a T n r d a c > 10 m B a se is c o v e red S y s t e m s T r a c t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 171 Figure 9-2: Stratigraphic column measured of the Virgin Limestone Member measured at the Muddy Mountains Ute locality. Grain sizes are noted in black and white dots. Like the previous section, grain size data at this locality show an initial fining (indicative of deepening) during the transgressive phase followed by a coarsening (shallowing) near the top of the section. Numbers on side of column correspond to units in Appendix 2. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 172 KEY * 0 0 * flat-pebble conglomerates 000 oolites radiating microbialites microbial mounds wrinkle structures m thin-bedded limestone-mudstone carbonate precipitates S limestone g siltstone f r |r ] mixed limestone/siltstone partially covered ISI covered M/M m udstone/m udstone w /s wackestone/siltstone P/Sa packstone/sandstone G/CSa grainstone/coarse sandstone • carbonate sediments O siliciclastic sediments Section continues 43 E 5 E 5 5 S 42 39-41 36 35 32-3 27 26 25 24 22-23 j±i:5£r::r:: V I ttU W U W U S I 20 8- 1( : 6-7 : 10 m 2-5 M 7M W /S P /S a G /C Sa H ig h sta n d S y stem s T ra c t T r a n g r e s s i v e S y s t e m s T r a c t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 173 as a replacement of oolites and skeletal grainstones and packstones near the base to fine-grained micrite up-section (See Appendix 2). These parasequences thicken upwards, and the systems tract changes from a transgressive systems tract to a highstand systems tract approximately 130 m from the base. This is reflected by an increase in siltstones, fossil packstones, and graded storm deposits. This highstand systems tract continues into the evaporitic Shnabkaib Member, which conformably overlies the Virgin Limestone Member at this locality. The separation of these two members is difficult because of their similar lithologies (Shorb, 1983). A flat-pebble conglomerate bed is located near the top of the section in association with hummocky cross-stratified siltstones. Mountain Pass Locality Sequence strati graphic analysis of the Virgin Limestone Member at the Mountain Pass locality was difficult due to the many covered intervals. Some trends can, however, be ascertained. The base of the Virgin Limestone Member is in contact with the Timpoweap Member, a conglomeratic unit consisting of clasts of the Kaibab Formation. This conglomerate is overlain by an oolite, which represents the initial flooding surface (Figure 9-3). The transgressive systems tract shows a slight decrease in the amount of siltstones upsection and then changes into a highstand systems tract. Carbonates change from grainstones and packstones of fossil debris to fine-grained limestones up-section. The deepest part of the section occurs about 50 m from the base. The highstand systems tract replaces the transgressive systems tract, and this is reflected in a change from fine-grained limestones to graded storm beds Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 174 Figure 9-3: Strati graphic column of the Virgin Limestone member measured at the Mountain Pass locality. Grain sizes are noted in black and white dots The trends in grain size shows an initial fining (indicative of deepening) followed by a coarsening (shallowing). Numbers on side of column correspond to units in Appendix 3. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. KEY flat-pebble conglomerates o o o oolites radiating microbialites H M H microbial mounds wrinkle structures S thin-bedded limestone-mudstone carbonate precipitates S limestone Q siltstone m i mixed limestone/siltstone partially covered E l covered M/M m udstone/m udstone W/S wackestone/siltstone P/Sa packstone/sandstone G/CSa grainstone/coarse sandstone • carbonate sediments O siliciclastic sediments Top is covered i— i — i — r M/M W/S P/Sa G/CSa 25 24 23 21-22 20 18-19 16-17 14-15 13 1-12 H i 8 h s t a n d n s 8 r e ooo S S i v e 10 m Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 176 and grainstones. This continues for the rest of the measured section; however, the top is covered. The Upper Red Member overlies the Virgin Limestone Member at this locality, and the Shnabkaib Member has been removed entirely by an erosional event (Marzolf, 1993). Flat-pebble conglomerate and thin-bedded limestone and mudstone facies are both present at Mountain Pass. The flat-pebble conglomerate bed was formed as part of a storm deposit. The thin-bedded limestone and mudstone unit was deposited in a middle shelf paleoevironment in the absence of significant bioturbation and formed at the base of a parasequence. Lost Cabin Springs Locality At the measured locality, the base of the Virgin Limestone Member is not preserved; however, in nearby sections it unconformably overlies the Lower Red Member. The basal part of this section is dominated by silciclastics and skeletal packstones and grainstones that occur in thin parasequences (Figure 9-4). The parasequences thicken upwards, and the section becomes dominated by fine-grained carbonate. The highstand systems tract occurs after the maximum flooding surface near the middle of the measured section. Above this, abundant graded storm beds are preserved, and siliciclastic intervals become increasingly abundant and thick. The top of the section has been truncated by erosion and is not in contact at this locality with the Shnabkaib, Middle or Upper Red Members. Three beds of microbialite facies and three beds of flat-pebble conglomerates are preserved at this locality. The microbialite facies formed during flooding events at the base of parasequences. The Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 177 Figure 9-4: Strati graphic column of the Virgin Limestone Member measured at the Lost Cabin Springs locality. Grain sizes are noted in black and white dots. Trends in grain size show an initial fining (indicative of deepening) followed by a coarsening (shallowing). Maximum flooding surface corresponds with microbial reef development. Numbers on side of column correspond to units in Appendix 4. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 178 KEY flat-pebble conglomerates o o o oolites radiating microbialites M M microbial mounds w rinkle structures S3 thin-bedded limestone-mudstone carbonate precipitates ES limestone siltstone m ixed limestone/siltstone partially covered ISI covered M/M m udstone/m udstone w/s wackestone/siltstone P/Sa packstone/sandstone G/CSa grainstone/coarse sandstone • carbonate sediments O siliciclastic sediments 43 41-42 32-34 30 27-29 19-23 IPS 14-17 f r - 1 IW W K U W W U 10 m ■ 4 ~ -4 ~ 4 ~ h- 2-4 M /M W /S P /S a G /CSa T r a n s g r e s s i v e S y s t e m s T r a c t H i g h s t a n d S y s t e m s T r a c t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 179 microbial reef mounds represent maximum flooding surface after which the highstand systems tract replaced the transgressive systems tract. The flat-pebble conglomerates occur after maximum flooding, and represent the storm-dominated upper portion of the Virgin Limestone at this locality. Middle-Upper Members of the Union Wash Formation Darwin Hills Locality The Union Wash Formation represents the outer shelf-basinal equivalent of the Moenkopi Formation. The Middle-Upper Members are loosely correlative to the Virgin Limestone Member (Stone et al., 1991). The basal part of the measured section at Darwin Hills likely represents a maximum flooding surface, with ~130 m of continuous micrite deposition (Figure 9-5). Near the top of this bed, there is an increase in siliciclastic deposits that have been interpreted as representing a highstand systems tract consisting of parasequences that thin upward. The top of this unit is covered, but the section continues. The anachronistic facies present at this locality co-occur. The thinly-bedded limestone and mudstone facies underlies both carbonate seafloor precipitate units. The thinly-bedded limestone and mudstone facies was deposited in an outer shelf paleoenvironment. Atop this facies are the precipitate-bearing limestones, which represent flooding surfaces. Because the Middle and Upper Members of the Union Wash Formation are only loosely dated as Smithian/Spathian, it is difficult to precisely correlate these with their onshore equivalents in the Moenkopi Formation; however it is likely that the same deepening Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 180 Figure 9-5: Stratigraphic column of portions of the Middle and Upper Members of Union Wash Formation measured at the Darwin Hills locality. Grain sizes are noted in black and white dots. Grain sizes consist of micrite and siltstone, so no trends could be determined. Basal portion represents maximum flooding phases, followed by a highstand systems tract phase, and then a transgressive phase begins again near the top. Numbers on side of column correspond to units in Appendix 5. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 181 KEY flat-pebble conglomerates o o o oolites radiating microbialites microbial mounds wrinkle structures s thin-bedded limestone-mudstone carbonate precipitates b limestone E3 siltstone Hi mixed limestone/siltstone f l x l l partially covered B covered M/M m udstone/m udstone W/S wackestone/siltstone P/Sa packstone/sandstone G/CSa grainstone/coarse sandstone • carbonate sediments O siliciclastic sediments section continues but is covered 26-27 ^ M a x im u m F lo o d in g S u rfac e 20 m eters M /M W /S P /S a G /C Sa Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 182 event that brought about the widespread deposition of carbonate seafloor precipitate facies in the Union Wash can be linked to the microbialite occurrence(s) in the Virgin Limestone Member at Lost Cabin Springs. Geochemical analyses confirm these observations (Marenco et al., 2003). Implications for Anachronistic Facies Anachronistic facies are preserved at localities of the Middle and Upper Members of the Union Wash Formation and the Virgin Limestone Member of the Moenkopi Formation. These reflect an initial transgressive phase of the Panthalassa seaways followed by a highstand systems tract. The microbialite and carbonate seafloor precipitate facies occur at the base of parasequences during flooding (or deepening) events. Flat-pebble conglomerates are not isolated to flooding events within parasequences. The flat-pebble conglomerates at Lost Cabin Springs represent a subtidal storm-dominated depositional system that may be linked to flooding events, whereas at the Muddy Mountain localities, these were deposited in the initial phase of flooding events. Wrinkle structures at the Lost Cabin Springs and Muddy Mountains Overton localities form in storm-dominated siliciclastic deposits that do not appear to be linked to flooding events; however, the microbialite and carbonate seafloor precipitate facies are linked to deepening events. The flat-pebble conglomerate facies formed in a carbonate depositional regime that was periodically influenced by storms. The occurrence of anachronistic facies in Lower Triassic strata of the western United States indicates that deposition in Early Triassic seaways may more closely Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 183 resemble that of the Precambrian/Early Paleozoic than of the rest of the Phanerozoic. The genesis of anachronistic facies such as flat-pebble conglomerates and wrinkle structures requires the suppression of bioturbation to form (e.g. Droser, 1987; Sepkoski et al., 1991; Sepkoski, 1982). These facies commonly formed in Precambrian/Early Paleozoic time because deep bioturbation had not yet involved; their resurgence in the Early Triassic suggests that bioturbation was again occasionally suppressed. High ichnofabric indices have been reported from the Lost Cabin Springs and Mountain Pass localities (Schubert, 1993); these localities also contain anachronistic facies. This suggests that the suppression of bioturbation during this time was periodic rather than continual. Deleterious environmental conditions may have occasionally influenced deposition on the shelf causing a reduction in bioturbation and the formation of anachronistic facies. In addition to the flat-pebble conglomerates and wrinkle structures, microbialite and carbonate seafloor precipitate facies are also noted. The formation of these is linked to deepening events suggesting that flooding of deleterious ocean waters onto the shelf influenced the deposition of these. The microbialites occur in three beds; one in which the microbial mounds form reefs, a smaller microbial mound bed, and a bed in which microbialites are radiating off of micritic clasts. These represent different depositional regimes from low energy environments below storm wave base (reef mounds) to high-energy subtidal environments (radiating microbialites) (e.g. Riding, 2000), and this means that the formation of these microbialites is linked to oceanic conditions and is not restricted to one particular Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 184 environment. The carbonate seafloor precipitate facies is also linked to ocean chemistry. Carbonate seafloor precipitates have not formed abundantly in normal marine environments since the Archean and Paleoproterozoic (e.g. Sumner and Grotzinger, 1996a; Sumner and Grotzinger, 1996b); an exception of this is the cap- carbonates of Neoproterozoic snowball earth time (Hoffman et al., 2002; Hoffman et al., 1998; James et al., 1999; James et al., 2001; Kennedy, 1996). This means that oceanic conditions that caused their formation had disappeared by the Phanerozoic. The resurgence of carbonate seafloor precipitate facies during the Early Triassic suggests that oceanic conditions similar to those that existed in Precambrian time were present again. Summary The occurrence of anachronistic facies in Lower Triassic strata of the western United States indicates that a carbonate depositional regime akin to that of the Precambrian-Early Paleozoic was restored. Linking the microbialite and carbonate seafloor precipitate facies to flooding events suggests that basinal water flooding onto the shelf played a role in their formation. Because evidence exists that basinal water of the Early Triassic was anoxic, it can be hypothesized that the flooding of anoxic waters, perhaps enriched in hydrogen sulfate and carbon dioxide, would have periodically suppressed metazoans. The increased alkalinity generated by sulfate reduction would have aided in the early lithification of such features as the flat- pebble and thin-bedded limestone-mudstone facies. Increased alkalinity would also have played a role in the lithification of microbialites and the precipitation of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 185 carbonate seafloor fans. The link between oceanic conditions and the unusual anachronistic facies in the Early Triassic suggests that deleterious conditions existed for millions of years after the end-Permian mass extinction and caused a delayed recovery for the ocean’s biota. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 186 CHAPTER 10 Early Triassic Trace Fossils: Case Study from the Virgin Limestone Member, southwestern United States Introduction The end-Permian mass extinction left an indelible mark on trace fossil assemblages. This is evident during the lag phase of the biotic recovery interval that occurred in the Early Triassic. Research on the Spathian Virgin Limestone Member of the Moenkopi Formation, southwestern United States, has yielded a mixed carbonate-siliciclastic trace fossil assemblage. The presence of such traces as Thalassinoid.es, Laevicyclus, and Gyrochorte indicate that previously unreported metazoan behaviors had re-appeared in equatorial regions by the close of the Early Triassic. Whereas diversity of trace fossil assemblages had increased from earliest to late Early Triassic time, persistent small size, low average ichnofabric index, low bedding plane coverage, and reduced tiering point to prolonged stressful environmental conditions following the end-Permian mass extinction, conditions for which there is an abundance of global sedimentological evidence. This trace fossil assemblage provides a record of soft-bodied organisms that might not otherwise be detected from the study of body fossils alone; it therefore acts as a constraint on the timing of the biotic recovery. This assemblage of the Virgin Limestone Member also serves as an indicator of environmental conditions that might not otherwise be gleaned, illustrating the utility of using trace fossils as environmental proxies during the lag phase of a biotic recovery. The presence of some of these traces (i.e. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 187 Thalassinoides) in lowermost Triassic strata of western Canada suggests that the recovery of trace-makers after the end-Permian mass extinction was asynchronous, and that northerly latitudes may have experienced a less protracted biotic recovery than equatorial regions. The end-Permian mass extinction, the largest extinction in the history of life, changed marine ecosystems forever. The lag phase of the biotic recovery from this extinction took place during the Early Triassic, and estimates suggest full recovery was delayed for as long as 4 to 8 million years (Hallam, 1991; Martin et al., 2001). Because this recovery spans the Early Triassic interval, research has largely focused on characterizing the depauperate marine body fossil fauna from this time as a means of understanding what factors contributed to this slow recovery (e.g. Fraiser and Bottjer, 2004; Hallam and Wignall, 1997; Rodland and Bottjer, 2001; Schubert and Bottjer, 1995). Using trace fossils as a proxy for biotic recovery after a mass extinction has provided behavioral information that would not be gleaned from the study of body fossils alone (Twitchett, 1999; Twitchett and Barras, 2004; Twitchett and Wignall, 1996). Most studies of Early Triassic marine trace fossils consist of localized studies (e.g. De, 1998; MacNaughton et al., 2002; Srivastava and Kumar, 1992; Wang, 1997; Worsley and Mork, 2001; Yang, 1988; Zonneveld et al., 2002) and there have only been a few reports on trace fossils from nonmarine and marginal marine deposits (e.g. Miller, 1984). The trace fossil assemblages of the Spathian Virgin Limestone Member of the Moenkopi Formation, southwestern United States, described herein, contains trace fossils not previously reported from Early Triassic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 188 equatorial regions. These trace fossils include Thalassinoides, Laevicyclus, and Gyrochorte. Some of these traces have been reported from older Griesbachian strata of western Canada suggesting that trace makers in northerly latitudes may have experienced a faster recovery from the end-Permian mass extinction than those living in equatorial regions (Twitchett and Barras, 2004). Characteristics of the Virgin Limestone Member assemblages such as persistent small burrow size, low ichnofabric indices, and low bedding plane coverage also suggest that these trace- makers were living in a stressed environment, and prolonged stress likely played a role in inhibiting their recovery. Stratigraphy The Virgin Limestone Member of the Moenkopi Formation, which crops out in California, Nevada, and Utah, was deposited during the end of the Early Triassic as part of a transgressive marine tongue from the Panthalassa seaway to the west (Blakely, 1972) at a latitude of about 15° N (Carr and Pauli, 1983). The Moenkopi Formation in southern Nevada and eastern California consists of both marine and terrestrial deposits (Reif and Slatt, 1979). The Virgin Limestone Member of the Moenkopi Formation was deposited in a mixed carbonate-siliciclastic paleoenvironment. Where it is exposed, it has a characteristic yellow to buff color and typically crops out as a series of limestone ledges (Shorb, 1983). In addition to trace fossils, body fossils include bivalves, gastropods, crinoid and echinoid debris, and ammonoids (Schubert and Bottjer, 1995). The thickness of the Virgin Limestone varies from ~100 to 200 m at the four localities studied here. Due to complex post- Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 189 depositional faulting in this region (Marzolf, 1990), three of the four sections studied are structurally overturned. In northwestern North America, an epicontinental seaway covered much of western Canada and the northwestern part of the United States during the earliest Triassic (Griesbachian). Some Griesbachian deposits include the Toad and Montney Formations exposed in western Canada (e.g. MacNaughton et al., 2002; Zonneveld et al., 2002) and the Dinwoody Formation exposed in the northwestern United States (e.g. Pauli et al., 1985). No deposits of this age are found in the study area (southern Nevada and eastern California) because there was a hiatus in deposition in the southwestern United States. The first Early Triassic marine incursion preserved in the southwestern United States is represented by the Nammalian Sinbad Limestone (Blakely, 1972), which was deposited on the northern margin of the Moenkopi Formation. Methods Strati graphic sections at four localities of the Virgin Limestone Member were measured in eastern California and southern Nevada (see Figure 2-1). In some cases, the basal portion of the Virgin Limestone Member was covered or poorly exposed (See Figures 10-1 to 10-4). For each measured bed, lithology, sedimentary structures, trace and body fossils were described, and ichnofabric indices were assessed (see Figures 10-1 to 10-4). On bedding planes with exposed trace fossils, diameters and widths of traces were measured within delineated 20 X 20 cm quadrants. Because bedding plane exposure was rare, only 7 total quadrants containing Thalassinoides, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 190 Figure 10-1: Stratigraphic column of the Virgin Limestone Member measured at the Muddy Mountains Overton locality, southwestern United States, showing trace fossils and numbers to the right of trace fossils indicate ichnofabric indices. Numbers to the left of the column correspond to units in Appendix 1. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 191 KEY ^ Gyrochorte ^ Planolites & Asteriacites A Thalassinoides & Rhizocorallium U Arenicolites Laevicyclus ooo Oolites Limestone Partial Exposure Siltstone Covered Calcareous Siltstone 48 1-2 47 46 43 42 41 40 1 39 38 37 36 35 tl to 27-29 20-21 14 12-13 10-11 9 8 7 ooo 10 m U 1 4-6 3 2 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 192 Figure 10-2: Stratigraphic column of the Virgin Limeston Member, measured at the Muddy Mountains Ute locality, southwestern United States, showing trace fossils and numbers to the right of traces indicate ichnofabric indices. Numbers to the left of column correspond to units in Appendix 2. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 193 43 39-41 36 32-33 KEY t j R hizocorallium y A renicolites ft L aevicyclus ooo O olites ^ G yrochorte P lanolites & A steriacites T h a la ssin o id es 28-30 27 Partial E xposu re ^ C overed L im estone 26 S iltstone 25 24 C a lca reo u s Siltston e 22-23 20 O O O t f 15-16 0 0 0 8 -1 0 6-7 10 m 2-5 2 2 5 3 5 1 4 1 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 194 Figure 10-3: Strati graphic column of the Virgin Limestone Member measured at the Mountain Pass locality, southwestern United States, showing trace fossils, and numbers to the right of traces indicate ichnofabric indices. Numbers to the left of the column correspond to units in Appendix 3. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 195 KEY & Rhizocorallium U J Arenicolites ^ Gyrochorte ^ Planolites _ & Asteriacites " g Laevicyclus Thalassinoides o o o Oolites Limestone Siltstone Partial Exposure Covered -•Hv* Calcareous Siltstone 2 5 24 23 21-22 20 18-19 16-17 14-15 13 1-1 2 10 m a m s E l X S; 1111 X X A 3 ~ 3 A 3 A 3 A A 3 A 3 OOO A 3 ~ 2 4 ~ 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. co cn 196 Figure 10-4: Stratigraphic column of the Virgin Limestone Member measured at the Lost Cabin Springs, southwestern United States, showing trace fossils, and numbers to the right of traces indicate ichnofabric indices. Numbers to the left of column correspond to units in Appendix 4. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 197 4 3 41-42 38-39 35-36 32-34 30 27-29 KEY & Rhizocorallium y Arenicolites ft Laevicyclus ^ Gyrochorte ^ Planolites ^ Asteriacites Thalassinoides ooo Oolites S S Limestone 25 24 Partial Exposure 19-23 Covered Siltstone Calcareous Siltstone 14-17 10-11 6-9 2-4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 5 3 5 1 5 3 3 1 3 3 3 4 3 3 5 5 5 3 10 m C O 4 0 C O 4 0 CO 198 Planolites, and Rhizocorallium were measured. Lab analyses included thin-section description of lithologic characteristics of traces and surrounding matrices. Thin- sections were also used to search for evidence of burrow-lining in the vertical traces Laevicyclus and Thalassinoides. Trace fossil complexity was also assessed as part of this study. Recent advances in determining trace fossil complexity have involved construction of cladograms (Ekdale and Lamond, 2003) and determining behavioral complexity represented by traces (Miller, 2003). In this study, changes in trace fossil form were recorded throughout the Early Triassic to assess complexity. For example, as for the Precambrian-Cambrian boundary, a change from simple horizontal traces to vertical forms represents an increase in complexity (Corsetti and Hagadom, 2000). Variations in trace fossil form were also noted as reflecting changes in complexity. For example, Permian Thalassinoides reported from the southwestern United States consist of deeply-penetrating three-dimensional boxworks (commonly 1 m). The Thalassinoides traces reported from the Virgin Limestone Member of the southwestern United States were simple, shallow mazeworks penetrating a maximum of 5 cm into the sediment. This would indicate that the re-appearance of Thalassinoides in the latest Early Triassic represents a less complex version than those of the Permian in the southwestern United States. The earliest Triassic Thalassinoides reported from western Canada, however, represent a more complex form than those of the Virgin Limestone Member (J.-P. Zonneveld, pers. comm., 2004), and this has implications for the asynchronous timing of the biotic recovery. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 199 Trace fossil complexity was tracked through the Early Triassic by comparing this latest Early Triassic assemblage of the southwestern United States to previously documented assemblages of Early Triassic trace fossils in western Canada (e.g. MacNaughton et al., 2002; Zonneveld et al., 2002), the northwestern United States (Twitchett and Barras, 2004), and the southwestern United States (Fraiser, 2000), as well as southern Italy (Twitchett, 1999; Twitchett and Wignall, 1996). These previous studies were particularly important because they contained information on Griesbachian—Smithian trace fossil assemblages, time periods not preserved in the study area. The changes in trace fossil complexity were also placed in a global framework to understand the temporal and spatial constraints of the recovery of trace fossils from the end-Permian mass extinction. The tiering complexity of this assemblage was also assessed as part of this study (Bottjer and Ausich, 1986). Description of Virgin Limestone Trace Fossils Arenicolites: The trace fossil Arenicolites is a simple U-shaped burrow with no spreite that is preserved in the Virgin Limestone perpendicular to bedding. On bedding planes, these traces occur as pairs of holes (Figure 10-5). Arenicolites is most commonly preserved in siliciclastic sediments that exhibit ripple marks and cross-bedding and is often found in association with Rhizocorallium, and sometimes with Gyrochorte, and Asteriacites. Depth of penetration of burrows is difficult to measure, but the tube diameters are very small, suggesting small burrow size. This trace is believed to be a dwelling trace, made by the tidal flat shrimp Corophium Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 200 Figure 10-5: Photograph of a bedding plane from Virgin Limestone Member, Muddy Mountains Overton locality, southwestern United States, containing paired holes of Arenicolites (indicated by double arrows A l and A2), and Rhizocorallium (shown by arrow B). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 201 arenarium (e.g., Bromley 1996), and detritus-feeding lugworms like Abarenicola pacifica (Swinbanks, 1981). Asteriacites: This trace can exhibit a variety of shapes depending on the type of sediment in which it is preserved (Twitchett and Wignall, 1996). Asteriacites is only found in siliciclastic units and is associated with such sedimentary structures as ripple marks and cross-bedding. These traces are sometimes found associated with Rhizocorallium jenense, Arenicolites, and Gyrochorte, although comparatively, the occurrence of Asteriacites is rare. Traces range in size from 10-20 mm in diameter (including arms) (Figure 10-6). Asteriacites is the only resting trace found in the Virgin Limestone Member, and is interpreted to have been formed by a small ophiuroid (e.g. Twitchett and Wignall, 1996). Gyrochorte: This is a horizontal trace that is commonly found in siliciclastic units of the Virgin Limestone Member. It occurs as a bifurcated trail, and when it is well- preserved, there is a subtle braided pattern exhibited by the two parallel tubes (Figure 10-7). The Gyrochorte traces cut across each other and other traces on bedding planes. This trace is sometimes found in association with Arenicolites, Rhizocorallium, and Asteriacites. It has a width of <lcm, and its length varies greatly with exposure. The formation of this trace has been attributed to the activity of gastropods, as well as the tunneling of a polychaete-like worm through sediment (Heinberg, 1973). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 202 Figure 10-6: Photograph of a bedding plane from Virgin Limestone Member, Muddy Mountains Overton locality, southwestern United States showing three Asteriacites traces. These are preserved in epirelief. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 203 Figure 10-7: Photograph of a bedding plane from the Virgin Limestone Member, Lost Cabin Springs locality, southwestern United States, containing the bifurcated trace Gyrochorte indicated by white arrows. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 204 Laevicyclus: These are vertical traces that penetrate to a depth of < 5 cm perpendicular to bedding. In the Virgin Limestone Member, they are preserved in oolitic sediments and have an exterior lined wall surrounding an internal hole (Figure 10-8). Although preservation of this trace varies, the central hole is typically infilled with light colored sediment that is finer than the surrounding oolite. From side view, these traces are visible as straight tubes. From top view, they are preserved as two concentric circles; the smaller central circle consists of the light colored fine-grained sediment, and the exterior circle is a black halo in the oolite. Trace diameters are typically ~1 cm. Fossil burrows assigned to Laevicyclus resemble the dwelling trace of the modern annelid Scolecolepis (Seilacher, 1953). Planolites: This is a simple, unlined horizontal burrow that does not branch. It is the most abundant trace fossil found in the Virgin Limestone. Planolites occurs primarily in limestones, although this trace is sometimes found in siliciclastic units and is darker in color than the surrounding sediment (Figure 10-9). Burrow diameters range from 1-6 mm (Figure 10-10). Planolites is a feeding trace (e.g., Bromley 1996, and references therein), likely formed by a vagile deposit feeder. Rhizocorallium: In the Virgin Limestone, these traces are U-shaped burrows with spreite that occur oblique to bedding (see Figure 10-5). Rhizocorallium is commonly preserved in siliciclastic units, and is typically found in the same beds as Arenicolites, and sometimes with Gyrochorte and Asteriacites. These traces are also Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 205 Figure 10-8: Photograph showing a close-up of a bedding plane from the Virgin Limestone Member, Muddy Mountains Ute locality, southwestern United States, containing the trace fossil Laevicyclus. Arrow A indicates the light sediment infilling the central tube, and arrow B indicates the darker halo that surrounds each tube. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 206 Figure 10-9: Photograph of bedding plane in Virgin Limestone Member, Lost Cabin Springs locality, southwestern United States, showing Planolites burrows in lime mudstone. Burrows weather darker than surrounding sediment. Ichnofabric index of bed is 5. Knife for scale is 7.5 cm. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 207 Figure 10-10: A) Histogram showing the distribution of Planolites burrow diameters from two bedding planes. MM-O is Muddy Mountains Overton locality. Bottom histogram shows the compiled data from both bedding planes. B) Histogram showing the distribution of Rhizocorallium burrow widths from one bedding plane. MM-O is Muddy Mountains Overton locality. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 208 A. Planolites 20 r i * 20 O w i5 < u § .o £ io E 5 3 to 5 0 MM-O Unit 38D 0 * 1 5 » § . __ a !. 10 + m b ■Frequency — I— | — J — |— i 4 5 6 More Diameter MM-O Unit 38L — H — j l — i—1 ^ — I Frequency 4 5 6 More Diameter Total (n=73) 30 r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 V I s i 20 £ 1 1 0 1 “ iFrequency 1 2 3 4 5 6 More Diameter B. Rhizocorallium MM-O Unit 38 20 ° « ,r L » 15 2 5 J t 10 i s s L IFrequency 10 15 20 25 More W idth Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 209 found in calcareous siltstones and limestones. Burrow widths, measured from the outside edges of the U-shaped structure, range from 10-24 mm (see Figure 10-10). This trace is a dwelling and feeding trace and was formed by a suspension-feeder of unknown origins (e.g., Bromley 1996 and references therein). Thalassinoides: These occur as branching burrows with characteristic Y-shaped junctions in the Virgin Limestone Member. The burrow depth extends as deep as 5 cm into the sediment. These burrows occur in calcareous siltstones and limestones and are visible in outcrop as both intricate networks (from top view) (Figure 10-11) and vertical branching burrows (from side view). These traces are preserved in fine grained limestones and are commonly infilled with coarse sediment such as crinoid columnals or other echinoderm debris. In thin-section, some of these burrows contain rhombs of dolomite. The diameters of these burrows range from 2-15 mm (Figure 10-12). The formation of Thalassinoides has been attributed to the behavior of many organisms including anemones, enteropneusts, fish, and decapod crustaceans (Myrow 1995). Size Distribution Bedding plane measurements of the diameters or widths of Rhizocorallium, Planolites, and Thalassinoides were conducted as part of this study. Measurements of burrow diameters and widths were made in delineated 20 X 20 cm quadrants on exposed bedding planes. The Rhizocorallium traces were measured from the outer edges of the U-shaped burrow. The infilled part of the burrow was commmonly Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 210 Figure 10-11: Photograph of a bedding plane of the Virgin Limestone Member, Lost Cabin Springs locality, southwestern United States, showing Thalassinoides burrows in lime mudstone. White arrows indicate characteristic Y-shaped junction. Traces weather a lighter color than the surrounding sediment. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 211 Figure 10-12: Histograms that show the distribution of Thalassinoides burrow diameters from four bedding planes. LCS is Lost Cabin Springs locality and MM-O is Muddy Mountains Overton locality. Bottom histogram shows the compiled data from all of the bedding planes. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 212 A . Thalassinoides 25 o U 20 | o 15 XI L . | S 10 5 “ 5 0 LC Unit 11 j ——III—.—BB—,— — 2 4 6 8 10 12 More Diameter (m m ) 25 o u 20 - 1 5 2 1 5 J £ t 10 3 C O 5 Z LC Unit 15 . 1 _ . 2 4 6 8 10 12 More Diameter (mm) 25 T ° « 20 i i j s - E a 10 - 5 MM-O Unit 29 IX. ________________ 2 4 6 8 10 12 More Diameter (mm ) Number of Burrows M N J U) O O O O LCS Unit 41 . . . M l — __ , __ , 2 4 6 8 10 12 More Diameter (m m ) 60 | g 50 - o | 40 u *5 30 5 20 E 2 io Total (n = 1 1 6 ) i f . 2 4 6 8 10 12 More Diameter (mm ) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 213 mottled and difficult to see. The Thalassinoides and Planolites burrow diameters were measured from the part of the burrow that was most representative of the average width. Seven total bedding planes were measured to determine the size distribution of trace width and diameters of Rhizocorallium, Planolites, and Thalassinoides. The average burrow diameter for the Thalassinoides burrows varied between 4 bedding planes (see Figure 10-12). The average Thalassinoides burrow diameter of all of the bedding planes was 6.2 mm. The Planolites measurements were taken from two different bedding planes (see Figure 10-10); traces were preserved in a dark gray packstone and a light gray fine-grained limestone. The average burrow diameter for the dark bed was about 3.7 mm (n=35) whereas the lighter bed had an average diameter of 3 mm (n=38). The average burrow width of the Rhizocorallium burrows was 14 mm (n= 25), measured from a single bedding plane (see Figure 10-10). Bedding plane exposure was very limited for most of the other traces. Where bedding planes containing Asteriacites were exposed, at most, three individual traces were preserved. Depth of penetration of vertical burrows was difficult to measure because of lack of exposure in the field as well as an abundance of stylolites that cut through burrows. Ichnofabric Index and Bedding Plane Coverage Recording the ichnofabric index provides a semi-quantitative way of determining the amount bioturbation has affected bedding (Droser and Bottjer, 1986). Where possible, ichnofabric indices were recorded from beds. These are Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 214 noted on the measured stratigraphic columns (see Figures 10-1 to 10-4) dominated by the horizontal trace Planolites. The siliciclastic beds, containing vertical traces such as Arenicolites and Rhizocorallium, had low ichnofabric indices (ii=l to 3) as did limestones containing Thalassinoides (ii=3), and oolites with Laevicyclus (ii=3). On exposed bedding planes, the approximate percentage of the bedding plane covered by burrows was measured, and bedding plane bioturbation indices were assigned (Miller and Smail, 1997). For the 4 bedding planes with Thalassinoides, the coverage was approximately 10-25%, resulting in a bedding plane bioturbation index of 2-3. For the two bedding planes containing Planolites, the coverage was about 25% for the dark gray packstone (bedding plane bioturbation index of 3), and < 10% for the light gray fine-grained limestone (bedding plane bioturbation index of 1). The bedding plane coverage of Rhizocorallium was also about 10% (bedding plane bioturbation index of 1), and this was localized to one exposed area. Trace Fossils and Paleoenvironments The trace fossil assemblage described from the Lower Triassic Virgin Limestone Member represents the behavior of various organisms in many paleoenvironments. Traces are preserved in both siliciclastic and carbonate deposits (see Figures 10-1 to 10-4) that represent intertidal to middle shelf paleoenvironments. The traces Arenicolites, Gyrochorte, Rhizocorallium, and Asteriacites are predominantly found in siltstones and less commonly in limestones (see Figures 10-1 to 10-4). These traces are preserved in beds with trough cross bedding and interference ripple marks and likely formed in a shallow, intertidal to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 215 subtidal environment (Figure 10-13). The vertical trace, Laevicyclus, is unique because it is the only trace preserved in oolitic sediments (see Figures 10-2 and Figure 10-13) indicating that it formed in a subtidal environment. The Thalassinoides traces are preserved in massively-bedded limestones and calcareous siltstones and are frequently infilled with coarse debris. The Thalassinoides trace-makers are thought to have burrowed into a firmground, and these open burrows were subsequently infilled with coarse grains from storm events or other wave activity. Since the Ordovician, Thalassinoides traces have formed in firmground and/or hardground environments (Myrow, 1995) as part of a Glossifungites ichnofacies (e.g. Pemberton and Frey, 1985). In Lower Triassic strata, the beds containing Thalassinoides represent the same paleoenvironment. They are overlain by a bioclastic grainstone and the burrows are commonly infilled with crinoid and other echinoderm debris. For this reason, the burrows are believed to mark a transition in environments from a quiet-water middle shelf micritic firmground (Figure 10-13) to a high-energy more nearshore setting. The Planolites traces are preserved predominantly in fine-grained limestones that have been interpreted as having been deposited in a middle shelf paleoenvironment (Schubert and Bottjer, 1995). These traces sometimes occur in nearshore settings, and therefore show the widest range in paleoenvironments. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 216 KEY W Rhizocorallium ^ Gyrochorte Thalassinoides ooo ooo Arenicolites Planolites Oolites IT Intertidal SBT Subtidal OS Oolite shoal MS Middle Shelf Laevicyclus O Asteriacites Limestone Siltstone /Calcareous Siltstone MMO Muddy Mountains Overton MMU Muddy Mountains Ute MP Mountain Pass LCS Lost Cabin Springs MMO MMU MP LCS 'O O O *000 000 >000 000 >000000 " > 0 0 0 OOO- OO OO O O O O IT SBT OS MS Figure 10-13. Schematic diagram showing approximate paleoenvironments of the measured sections and the trace fossils that characterize each locality, with data from Shorb (1983), Schubert and Bottjer (1995), and Pruss (2001). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 1 7 Trace Fossils as Indicators of Biotic Recovery Studies of trace fossil diversity at a number of individual sites suggest that there is an initial decrease in trace fossil diversity at the close of the Permian followed by a slow recovery in the Early Triassic (Twitchett, 1999; Twitchett and Barras, 2004; Twitchett and Wignall, 1996). Compilations that integrate data from all stages of the Permian and the Triassic do not show this trend (Crimes, 1992), and it is therefore important to examine Early Triassic trace fossil diversity separately from the rest of the Triassic (Bottjer and Droser, 1994). The decrease in diversity across the Permian-Triassic boundary has been particularly well-documented from Lower Triassic sections in Italy where the only noted trace fossil found in sediments deposited just after the extinction was the simple trace Planolites (Hallam and Wignall, 1997; Twitchett, 1999). This suggests a disappearance of suspension- feeding domichnia (Twitchett and Barras, 2004). By the latest Early Triassic (Spathian), the Italian Dolomite sections show a recovery of the traces Rhizocorallium and Asteriacites among others (Twitchett, 1999). This indicates a re establishment of trace-makers after a local absence for 4-7 million years. Similarly, in the western United States the Griesbachian Dinwoody Formation records trace fossils that formed in the immediate aftermath of the end- Permian mass extinction. The Dinwoody Formation contains only simple Planolites traces (Twitchett and Barras, 2004). A gradual increase in trace fossil diversity is recorded in the Nammalian Sinbad Limestone Member of the Moenkopi Formation, which preserves the re-appearance of Rhizocorallium and Arenicolites (Fraiser, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 1 8 2000). This suggests an increase in trace fossil complexity occurs from Griesbachian to Dienerian/Smithian time in the western United States with the first reported vertical traces in the Sinbad Limestone. The Spathian Virgin Limestone Member contains the first reported Thalassinoides, Asteriacites, Laevicyclus, and Gyrochorte from this region, which include traces not previously reported from the Italian sections. The results from this research suggest that the Early Triassic trace fossil record from the western United States shows a gradual increase in trace fossil diversity and complexity, and that some traces preserved here have not been previously reported from other equatorial Early Triassic assemblages (Twitchett, 1999; Twitchett and Barras, 2004; Twitchett and Wignall, 1996). It is also important to note that some of the traces, such as Thalassinoides, had not yet attained the size and complexity of those found during the Permian or Middle Triassic. These traces are markedly smaller (~6 mm compared to 5 cm) and less complex (simple mazeworks rather than complex boxworks) than those known from Permian strata in the same region (e.g. Whidden, 1990). Some earliest Triassic strata of western Canada, however, contain Thalassinoides burrows that are more complex than those reported here (J.-P. Zonneveld, pers. comm., 2004). This corroborates previous findings that the recovery from the end-Permian mass extinction was asynchronous, and that higher latitudes may have experienced a biotic recovery prior to low latitudes (Twitchett and Barras, 2004; Wignall et al., 1998). Characteristics of the Spathian assemblage from the western United States suggest that environmental Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 219 conditions had not fully returned to normal in equatorial regions by the close of the Early Triassic. This corroborates other sedimentological indicators of environmental stress in the Virgin Limestone Member (Pruss et al., 2004), however, the first occurrence of Thalassinoides, implies that environmental conditions had begun to improve by the latest Early Triassic. Trace Fossil Size Using trace fossil size as a proxy for paleoenvironmental conditions has proved a useful tool (Ekdale, 1985; Savrda and Bottjer, 1987a; Savrda and Bottjer, 1987b; Seilacher, 1978). In studies of oxygen content of sediments, small size has been found to correlate with low oxygen levels (Savrda and Bottjer, 1987b). In brackish environments, traces are also commonly small in size (Pemberton et al., 1982) as well as in areas with low nutrient supply (Jumars and Wheatcroft, 1989). Small size thus typifies “stressed” environments, or those that deviate from normal marine conditions. For this reason, the size of the trace fossils in this Lower Triassic interval is indicative of prolonged environmental stress related to the end-Permian mass extinction. Permian trace fossils provide a control against which Early Triassic traces can be compared to determine the impact the end-Permian mass extinction had on trace diameter and width. Although Permian trace fossil research is voluminous, few studies focus on size. Some size characteristics can, however, be gleaned from the literature. Photographs of silicified Thalassinoides burrows of the Lower Permian Kaibab Formation, western United States, show burrows with diameters as large as 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 220 cm (Whidden, 1990). These formed in subtidal paleoenvironments below normal wave base (Whidden, 1990) similar to those preserved in the Lower Triassic Virgin Limestone. Thalassinoides diameters have also been reported from the Upper Permian Wasp Head Formation of Australia (McCarthy, 1979). These burrows range in diameter from 4-6 mm (similar to the average 6.2 mm diameters of the Lower Triassic Thalassinoides), but because these formed in a sandy tidal flat paleoenvironment, they are not thought to represent the same depositional system in which Thalassinoides of the western United States are found. Rhizocorallium burrows from the Wasp Head Formation formed in shallow subtidal paleoenvironments (like Rhizocorallium of the Virgin Limestone), and burrow widths are as large as 14 cm (McCarthy, 1979). These are much larger than the average diameter for those of the Lower Triassic (2.4 cm). Planolites burrows from the same formation have diameters of 2.5 mm (McCarthy, 1979), consistent with what is found in the Lower Triassic Virgin Limestone Member. Size comparisons with trace fossils of the Middle Triassic, when infaunal tiering returned to late Paleozoic levels (Ausich and Bottjer, 2002), indicate that diameters and widths of traces increased substantially after the lag phase of the biotic recovery. Recent work on the Middle Triassic Liard Formation in British Columbia found that the average burrow diameter for Thalassinoides is 10-20 mm (Zonneveld et al., 2001), although some are as large as > 8 cm (J.-P. Zonneveld, pers. comm., 2004). The Early Triassic Thalassinoides burrows have an average burrow diameter of ~6.2 mm. Most of the Early Triassic burrows fall out of this range altogether with Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 221 only 25 out of 115 burrows having a diameter of 10 mm or more. Early Triassic Rhizocorallium traces have a small size relative to those reported from the Middle Triassic. Although there are no measurements listed from the work done on the Liard Formation on Rhizocorallium, a figured specimen indicates that burrows have widths as large as nearly 5 cm in this formation (Zonneveld et al., 2001). A photograph from the Bravaisberget Formation, Middle Triassic of Svalbard (Bromley 1996, p. 194) also shows a much larger burrow width than those from the Lower Triassic Virgin Limestone (greater than the diameter of a lens cap, ~5 cm). Without actual measurements, it is difficult to state the magnitude of the size difference between those from the Early and Middle Triassic; however, the largest Rhizocorallium burrow width from the Virgin Limestone is 24 mm, which is substantially smaller than those pictured from the Middle Triassic. Asteriacites traces measure as large as 10 cm in Middle Triassic strata of the Liard Formation, however, the average is around 7 cm in width (J.-P. Zonneveld, pers. comm., 2004). These are significantly larger than the Asteriacites traces from the Lower Triassic Virgin Limestone Member. The Early Triassic Planolites measurements are consistent with measurements reported from the Permian Wasp Head Formation and Lower Triassic Werfen Formation in the Dolomites of Italy (Twitchett, 1999). This indicates that Planolites underwent the least amount of burrow diameter change across the Permian-Triassic boundary. Because this simple trace can be made by a variety of organisms, it is expected that it would be the least susceptible to a mass extinction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 222 Planolites is also the first trace reported after the end-Permian mass extinction in Griesbachian deposits (Hallam and Wignall, 1997; Twitchett, 1999), suggesting that the organism(s) making this trace may have been resistant to mass extinction-related stresses. Ichnofabric Indices and Bedding Plane Coverage The ichnofabric indices and bedding plane coverage data are important in determining paleoenvironmental conditions. In the Lower Triassic Virgin Limestone, the highest ichnofabric indices are isolated to limestones burrowed by Planolites (see Figure 10-10). It is important to note that ichnofabric indices in these beds do not attain the elevated levels of Upper Permian strata recorded from the Gartnerkofel core, Austrian Alps (ii of 6) (Fig. 7 in Twitchett and Wignall, 1996). Virgin limestones containing Thalassinoides only reach a maximum ichnofabric index of 3. The siliciclastic units (dominated by traces such as Arenicolites, Rhizocorallium, and Gyrochorte) also have low ichnofabric indices that reach a maximum of 3. The variations in ichnofabric indices may be attributable to different sedimentation rates. It is possible that the thinly-bedded limestones containing Planolites were deposited at a low sedimentation rate. This may have enabled burrowers to significantly bioturbate the sediment before overlying beds were deposited. This does not explain, however, why other thinly-bedded limestones were not bioturbated at all. This variance suggests that environmental conditions other than sedimentation rate may have affected similar paleoenvironments. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 2 3 In the siliciclastic units in the study interval, traces such as Gyrochorte, Arenicolites, and Rhizocorallium were found associated with hummocky cross- stratification and siltstone intraclasts indicating that these trace-makers were living in a storm-dominated environment. The rapid deposition of sediment in these environments may have left trace-makers little time to significantly bioturbate the sediment before overlying beds were deposited. In modem environments, trace-makers of Thalassinoides suevicus can rework the uppermost 75 cm of the intertidal zone in under a year (MacGinitie, 1934; Warme, 1967). This is significant because ichnofabric indices of the Thalassinoides beds in the Virgin Limestone reach a maximum ii of 3, indicating that there was never total destruction of bedding like what is commonly seen in modern environments. Although some of the variation seen in the Virgin Limestone may have been caused by different rates of sedimentation, it is also apparent that the Early Triassic traces were relatively small in size and not abundant when compared to similar assemblages from the Permian and Middle Triassic. Low diversity and/or low abundance of traces, as evident in the Virgin Limestone Member, most likely played a role in inhibiting the extent of ichnofabric development during the Early Triassic (Twitchett, 1999). Unlike ichnofabric indices, which at times reached high levels (ii of 5), bedding plane coverage was always low on exposed bedding planes. The approximate percentage of an exposed bioturbated bedding plane was typically 25% or less. Bedding plane bioturbation indices ranged from 1-3 (Miller and Smail, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 224 1997), indicating that organisms never fully colonized a bedding plane (bedding plane bioturbation index of 5). These observations may be biased because bedding plane bioturbation calculations were far more limited by exposure than ichnofabric indices; however, this observation supports the contention that stressed conditions limited bioturbation. Trace Fossil Complexity The trace fossil assemblage preserved in the Lower Triassic Virgin Limestone Member of the Moenkopi Formation has implications for the biotic recovery of organisms during this time. A better understanding of the recovery from the end-Permian mass extinction can be studied by tracking the re-appearance of vertical traces in strata deposited after the Permian-Triassic boundary. In this study, this change from trace fossil assemblages dominated by only simple horizontal traces to assemblages containing vertical forms is interpreted as representing an increase in behavioral complexity. Trace fossil assemblages of the western United States show an increase in trace fossil complexity from simple, horizontal Planolites burrows of the Dinwoody Formation (Twitchett and Barras, 2004) to more complex vertical traces of the Sinbad Limestone (Fraiser, 2000) and finally to the re-appearance of Thalassinoides in the Virgin Limestone. The re-appearance of Thalassinoides in the Virgin Limestone Member is interpreted as representing an increase in behavioral complexity that typically indicates the beginning of the biotic recovery elsewhere (Twitchett and Barras, 2004). The Thalassinoides traces in this assemblage are, however, less complex than Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 2 5 Thalassinoides reported from Permian strata of the same region (Whidden, 1990) and lowermost Triassic strata of western Canada (e.g. MacNaughton et al., 2002; Zonneveld et al., 2002). The re-appearance of Thalassinoides in the western United States suggests that environmental stresses may have started to dissipate by the latest Early Triassic in equatorial regions. The differences in complexity reflected in Thalassinoides preserved in Lowermost Triassic strata of western Canada and uppermost Lower Triassic strata of the western United States (J.-P. Zonneveld, pers. comm., 2004) demonstrate that the biotic recovery of trace-making organisms differed between low and high latitudes (Twitchett and Barras, 2004). Assemblages from the northerly Lower Triassic Canadian strata indicate that the biotic recovery may have begun at mid-high latitudes first, and that the equatorial western United States and Tethyan regions experienced a more protracted recovery (Twitchett and Barras, 2004). Tiering The levels of tiering above and below the sediment were greatly reduced after the end-Permian mass extinction (Ausich and Bottjer, 1982; Ausich and Bottjer, 2002; Twitchett, 1999). In Griesbachian strata deposited in equatorial regions, there is a loss of infaunal tiering with the preservation of only Planolites burrows (Wignall and Hallam, 1992; Wignall and Hallam, 1993). Trace fossils from the western United States indicate that infaunal tiering had started to recover by the latest Early Triassic in equatorial regions. With the recovery of more vertical traces such as Thalassinoides, the infaunal realm was beginning to be exploited; however most of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 226 these traces formed simple mazeworks that extended a maximum of only 5 cm into the sediment. This indicates that only shallow tiers were occupied by latest Early Triassic time in the western United States. In western Canada, tiering recovered more quickly after the end-Permian mass extinction with the occurrence of more complex Thalassinoides in Griesbachian sediments. This development of tiering in the earliest Triassic suggests an asynchronous biotic recovery, and points to a more rapid improvement of environmental conditions at mid-high latitudes than at equatorial regions. Despite this early occurrence of Thalassinoides in western Canada, complex infaunalization did not recover until the Middle Triassic (Ausich and Bottjer, 2002; Thayer, 1979). Trace Fossils and Environmental Stress Using trace fossils as a proxy for paleoenvironmental conditions has proved a useful tool (Ekdale, 1985; Savrda and Bottjer, 1987a; Savrda and Bottjer, 1987b; Seilacher, 1978). Trace fossil size, diversity, and complexity can reveal much about the paleoenvironment. These data, in conjunction with sedimentological studies, provide a useful way of assessing the environment of deposition. In studies of oxygen content of sediments, small size has been found to correlate with low oxygen levels (Savrda and Bottjer, 1987b). Traces are also typically small in size in brackish environments (Bromley 1996, p. 280). Small size thus typifies “stressed” environments; environments that deviate from normal marine conditions. Low diversity of trace fossils is also common in stressed, brackish environments (Pemberton and Wightman, 1987; Pemberton and Wightman, 1992). Whereas Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 227 complexity in equatorial regions had increased from the simple Planolites burrows of the earliest Triassic of Italy (Twitchett, 1999; Twitchett and Wignall, 1996) to more complex traces such as Thalassinoides in the latest Early Triassic of the western United States, the Thalassinoides burrows preserved in the Virgin Limestone Member still represent simple mazework forms of Thalassinoides. This indicates that trace complexity had not fully returned to normal in equatorial regions, and that trace fossils found in the Virgin Limestone Member of the Moenkopi Formation may be indicators of environmental stress. Various other studies of Lower Triassic strata have also suggested that the Early Triassic is a time of environmental stress (e.g. Lehrmann, 1999; Pruss et al., 2004; Woods et al., 1999). These studies have focused on sedimentological indicators of anomalous oceanic conditions such as open marine microbial mounds and carbonate seafloor fans (e.g. Lehrmann, 1999; Woods et al., 1999). In Lower Triassic sections in South China and Italy, the small Planolites burrows occur in sediments that were deposited under low-oxygen conditions (Hallam and Wignall, 1997; Twitchett, 1999; Twitchett and Wignall, 1996) during a time when the ocean may have experienced widespread anoxia (Isozaki, 1994; Isozaki, 1997). There is an increase in trace fossil diversity from the Griesbachian to the Spathian in examined Lower Triassic sections (Twitchett, 1999; Twitchett and Wignall, 1996); however, the persistent small size, low percent bedding plane coverage, and low ichnofabric indices demonstrated by this trace fossil assemblage suggest that these organisms were living in a stressed environment (e.g., Bromley 19%). This assemblage was Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 2 8 deposited concurrently with other sedimentological indicators of global environmental stress such as large microbial mounds (Kershaw et al., 1999; Lehrmann, 1999; Pruss and Bottjer, 2004; Sano and Nakashima, 1997; Schubert and Bottjer, 1992), subtidal wrinkle structures (Pruss et al., 2004), and seafloor- precipitated carbonate fans (Woods et al., 1999), indicating that normal oceanic conditions were present some of the time in the Early Triassic, but that their occurrence may have been punctuated by episodes of environmental stress caused by deleterious conditions delivered to the shelf from the basin during transgression events (Pruss and Bottjer, 2004; Woods et al., 1999). Summary Trace fossils are extremely useful in assessing the biotic recovery interval from the end-Permian mass extinction. Increased study of trace fossils during other biotic recovery intervals from mass extinctions may provide a quantitative way of assessing the length of the recovery or the duration of stress following a mass extinction. The Spathian Virgin Limestone contains one of the best-preserved trace fossil assemblages from this region. Study of this trace fossil assemblage has revealed that the late Early Triassic has a record of more complex metazoan behaviors than previously reported from equatorial regions. Whereas advances in complexity and the reappearance of such traces as Thalassinoides, Gyrochorte and Laevicyclus had occurred by the close of the Early Triassic in equatorial regions, the persistent small burrow size, low ichnofabric indices, and low bedding plane coverage indicates that these trace-makers were living in a stressed environment. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 229 This assemblage does not simply reflect a locally stressed environment because it formed concurrently with other global indicators of environmental stress (Lehrmann, 1999; Lehrmann et al., 2003; Lehrmann et al., 2001; Pruss et al., 2004; Woods et al., 1999). The following model can now be proposed to describe the trace fossil pattern observed during the Early Triassic: 1) Trace fossils undergo a dramatic reduction in size, diversity, and tiering as a result of the end-Permian mass extinction (Ausich and Bottjer, 2002; Twitchett, 1999; Twitchett and Wignall, 1996); 2) In equatorial regions, trace fossils re-appear in a stepwise fashion during the Early Triassic, with the recovery of traces such as Thalassinoides, Gyrochorte and Laevicyclus delayed until latest Early Triassic or Middle Triassic (Twitchett, 1999; Twitchett and Barras, 2004; Twitchett and Wignall, 1996); 3) At mid-high latitudes, there is a rapid re appearance of trace fossil ichnotaxa, with diverse assemblages containing Thalassinoides and Cruziana appearing in the Griesbachian of western Canada; 4) Sedimentological indicators of unusual oceanic conditions in equatorial regions persist until the late Early Triassic, and include carbonate seafloor fans (Woods et al., 1999), subtidal wrinkle structures (Pruss et al., 2004), and open marine microbial mounds (Lehrmann, 1999; Pruss and Bottjer, 2004), the latter two occurring at the same localities where this trace fossil assemblage has been documented; 5) The recovery of Thalassinoides, Gyrochorte, and Laevicyclus in the Virgin Limestone Member of the western United States indicates that environmental conditions may have begun to return to normal by the close of the Early Triassic in equatorial Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 230 regions; however, persistent small burrow size, low ichnofabric indices, low bedding plane coverage, and reduced tiering suggest that these normal conditions were punctuated by episodes of environmental stress likely related to the flooding of the shelf with anomalous oceanic conditions from more offshore environments (Woods et al., 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 231 CHAPTER 11 Early Triassic Paleoceanography Permian-Triassic Boundary Because many marine organisms were devastated by the end-Permian extinction, it has long been thought that a link must exist between this extinction and oceanographic conditions (e.g. Isozaki, 1994; Isozaki, 1997; Knoll et al., 1996; Wignall and Hallam, 1992). Following the end-Permian mass extinction, the Early Triassic is characterized as a lag phase from the biotic recovery, with the true recovery delayed until the Middle Triassic. This delay has also been attributed to unusual conditions in the ocean, as has the resurgence in anachronistic facies during the Early Triassic postulated in earlier chapters. To better understand the possible relationship between anomalous oceans and the occurrence of anachronistic facies, the current research concerning Early Triassic paleoceanography will be reviewed. Much of the pioneering work on the paleoceanography of the Late-Permian Early Triassic has focused on evidence that supports the hypothesis that ocean basins were anoxic. Work on sections in northern Italy yielded data indicating that sediments were likely deposited under anoxic conditions (Wignall and Hallam, 1992). Further, the authors postulated that widespread shelfal anoxia likely caused the end-Permian mass extinction. The delayed biotic recovery may also reflect long term environmental stress (e.g. Hallam, 1991; Pruss et al., 2004; Woods et al., 1999) persisting for millions of years after the end-Permian extinction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 2 Subsequent work corroborated that anoxia was extremely widespread during Permian-Triassic time (Isozaki, 1994; Isozaki, 1997). This work suggested the pelagic deposits from Japan and British Columbia were deposited under very low oxygen conditions, and these conditions persisted for millions of years. This work was important because it substantiated that Permian-Triassic anoxia was not limited to one region in the world. Woods and Bottjer (2000) found that anoxia was also present in the western United States and persisted until as late as the end of the Early Triassic. The data largely support the occurrence of widespread anoxia during the transition from the Permian to the Triassic; however, oceanographers have long debated whether or not ocean basins could sustain long-term anoxia (Hotinski et al., 2001). To test this hypothesis, modeling exercises were performed based on a GCM (general circulation model) and MOM (modular ocean model) to see if ocean stagnation could result in widespread anoxia (Hotinski et al., 2001). It was found that polar warming and tropical cooling of sea-surface temperatures could bring about anoxia in the deep ocean as a result of lower dissolved oxygen in source waters as well as because of increased nutrient utilization (Hotinski et al., 2001). It was also determined that anoxia sufficient to bring about a mass extinction would require a change in the whole ocean nutrient inventory. Other studies have focused on evidence interpreted as representing a reduction in primary productivity in the surface ocean (Twitchett et al., 2001). This model is used to explain the rapid decline in S1 3 C across the Permian-Triassic Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 3 boundary, as well as an apparent fungal spike recorded in earliest Triassic sediments (Twitchett et al., 2001). This model would also require a large-scale reorganization of oceanographic processes to sustain long-term productivity collapse. Recent work on the termination of the Permian Chert Event suggests that a shift occurred in thermohaline circulation from the Permian to the Triassic that resulted in a reduction in chert production (Beauchamp and Baud, 2002). This shift may have resulted from a slow down of ocean circulation from the Permian into the Triassic and an overall global warming trend that would have brought about the demise of thermohaline circulation (Beauchamp and Baud, 2002). This sluggish circulation may have caused widespread anoxia in the Early Triassic for which evidence exists at a variety of locations globally (e.g. Isozaki, 1994; Isozaki, 1997; Wignall and Hallam, 1992; Woods, 1998). Early Triassic Oceanography In addition to the ever-burgeoning database on the Permian-Triassic boundary, research has also focused on the paleoceanography of the Early Triassic as a way of understanding the delayed biotic recovery and occurrence of unusual sedimentary facies. It was initially suggested that prolonged environmental stress played a role in the delayed recovery (Hallam, 1991). Work on the extinction of reef biota and their slow recovery suggested that low oxygen played a role in inhibiting their radiation (Weidlich et al., 2003). This indicates that low oxygen levels were not isolated to the boundary. This research was also one of the first studies to link the pattern of extinction of organism to potential environmental stresses. Another similar Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 234 notable study linked the extinction of organisms at the end of the Permian to high levels of carbon dioxide (Knoll et al., 19%). Additional sedimentological work suggests that this, too, was not isolated to the boundary (Woods et al., 1999). In addition to the unusually depauperate faunas of the Early Triassic, this was also a time period with an extremely unusual sedimentary rock record. The widespread occurrence of only microbial reefs during this time has been globally documented (e.g. Baud et al., 19%; Baud et al., 2002; Lehrmann, 1999; Pruss and Bottjer, 2004). These occurrences have been linked to long-term stressful oceanic conditions that both inhibited the recovery of metazoans and also allowed for the proliferation of microbialites. Unusual carbonate seafloor fans have been documented from various regions globally (e.g. Heydari et al., 2000; Heydari et al., 2003; Woods et al., 1999), and these also suggest that oceanic conditions were unlike most of the Phanerozoic. Recent work on the abundance of Early Triassic oolites, or “disaster oolites”, indicates that abiotic calcium carbonate precipitates were favored during Early Triassic time in nutrient-poor waters that did not favor skeletonization (Groves et al., 2003). Wrinkle structures, occurring in the latest Early Triassic, also point to suppressed bioturbation for 4 —8 million years after the end-Permian mass extinction, and this suppression is indicative of environmental stress. The sedimentological evidence strongly supports the hypothesis that environmental stress, likely related to the end-Permian mass extinction, persisted throughout the Early Triassic. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 5 Detailed geochemical studies have also been performed to ascertain ocean chemistry during Early Triassic time. Recent work by Marenco et al. (2003) has determined that shifts in sulfur isotopes record the flooding of the shelf with waters enriched in S3 4 S. This indicates deep-water anoxia and increased bacterial sulfate reduction (BSR) that may have helped remove light sulfur in the form of pyrite. An 6M S excursion in carbonate associated sulfate (CAS) shifts from +5 per mil below to +25 per mil (Marenco et al., 2003). Trace sulfate concentrations from the same beds decrease with the rise of 53 4 S. The 53 4 S excursion reported from these localities is the largest sulfur isotope exclusion of the Phanerozoic (Marenco et al., 2003). Research on the Lower Triassic Siusi Section in northern Italy has yielded similar results showing an increase in S^S (Newton et al., 2004). This research determined that shifts in sulfur isotopes is also related to global anoxia, but suggests that shifts later in the Triassic record a “fossil” signal derived from earlier anoxia, not an original signal (Newton et al., 2004). Evidence for unusual oceanography indicates that these conditions may have originated at or near the Permo-Triassic boundary (e.g. Wignall and Hallam, 1992), but likely persisted until Middle Triassic time (e.g. Hallam, 1991). Work on Permian-Triassic facies (Isozaki, 1994; Isozaki, 1997), models for ocean circulation (Hotinski et al., 2001), and patterns exhibited by the extinction of organisms (e.g. Knoll et al., 1996; Weidlich et al., 2003) all suggest that anoxia and/or excess amounts of carbon dioxide existed in the ocean during the transition from the Permian-Triassic. Facies and geochemical analyses of Lower Triassic strata point to Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 236 environmental stresses persisting 4— 8 million years after the extinction (e.g. Marenco et al., 2003; Pruss et al., 2004; Woods et al., 1999). Increased Alkalinity, Environmental Stress, and Anachronistic Facies The resurgence of anachronistic facies from Lower Triassic strata has been documented from a variety of locations globally (e.g. Baud et al., 1996; Baud et al., 2002; Heydari et al., 2003; Heydari et al„ 2001; Lehrmann, 1999; Lehrmann et al., 2001; Wignall and Twitchett, 1999). The mechanisms responsible for their formation have not, however, been discussed at length. Suppressed bioturbation is necessary for the formation of such facies as flat-pebble conglomerates and wrinkle structures (Pruss et al., 2004; Sepkoski et al., 1991). Conditions favoring early lithification are also necessary for the formation of facies such as flat-pebble conglomerates and microbial reefs. Additionally, increased alkalinity is hypothesized as being essential to the formation of the carbonate seafloor precipitate facies (Woods et al., 1999). Grotzinger and Knoll (1995) first described the upwelling of alkaline waters onto the shelf as a mechanism to explain similar unusual facies (e.g. carbonate precipitates) in the Permian and Precambrian. Analogous mechanisms likely played a role in the deposition of the facies in Lower Triassic strata of the western United States. The upwelling of alkaline waters may not have acted as a source of environmental stress; however, byproducts associated with increased alkalinity may have created deleterious oceanic conditions. It has been shown that oceanic basins of the Early Triassic were likely anoxic (Isozaki, 1994; Isozaki, 1997; Wignall and Hallam, 1992; Wignall and Twitchett, 2002; Woods, 1998). In these anoxic basins, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 7 increased sulfate reduction would have brought about a build-up of H2 S and C 02 (Grotzinger and Knoll, 1995; Marenco et al., 2003). The periodic upwelling of basinal waters that were rich in hydrogen sulfide and carbon dioxide onto the shelf would essentially poison metazoans (Figure 11-1). A byproduct of sulfate reduction is bicarbonate, and this would cause an increase in the alkalinity of ocean waters. The simultaneous suppression of metazoans and increased alkalinity would bring about the deposition of anachronistic facies (Figure 11-2). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 8 metazoa o organic matter O organic matter Figure 11-1: Diagram showing possible mechanism to explain facies in the Early Triassic and delayed recovery of metazoans. Diagram shows build-up of anoxia and hydrogen sulfide in anoxic bottom waters followed by episodes of water flooding on the shelf and poisoning of metazoans (modified from Corsetti, 2003). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. KEY horizontal bioturbation iO v com plex burrows fl vertical bioturbation \ J wrinkle structures microbial reefs microbialites flat-pebble conglomerates PERMIAN w ell-developed mixgrounds/ high levels o f bioturbation EARLY TRIASSIC laminated sediments/ low levels o f bioturbation End-Permian mass extinction i anoxia H nS HCO Figure 11-2: Diagram showing how flucatuations in ocean chemistry could have brought about a change in substrates and fostered the formation of anachronistic facies Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 240 CHAPTER 12 Conclusions 1) Based on the research presented here and the criteria for defining reefs of Wood (1999), the microbial build-ups described from the western United States and southern Turkey represent Early Triassic reef mounds in a carbonate shelf environment on a ramp. 2) The global occurrence of these structures in the Early Triassic suggests similar environmental factors must have been influencing these different regions. The occurrence of these microbial build-ups also suggests that deposition in all of these areas may have been influenced by deeper water stressful conditions (anoxic or C 02 -rich waters). 3) Whereas microbial fabrics have been important in reef systems throughout the Phanerozoic in a variety of environmental settings (Wood, 1999), these Early Triassic reef systems are quite unusual for their lack of framework or baffling metazoans. These structures, which are likely related to unusual oceanic conditions of the Early Triassic, represent patch reef mounds that formed millions of years after the end-Permian mass extinction. Because of this, a link must exist between the environmental conditions that simultaneously prolonged the recovery of metazoans during the Early Triassic and fostered the global occurrence of microbial reefs. 4) After a near annihilation of reef building organisms such as sponges and corals at the close of the Permian, a 4 —8 million year metazoan reef gap Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 241 ensued (Fagerstrom, 1987). Following this 4 —8 million year hiatus, metazoan reefs re-appear at a variety of locations globally in the Anisian. These reefs were initially dominated by microbes, calcimicrobes, calcareous and siliceous sponges, bryozoans, and corals; however, in Late Triassic time, corals and sponges take over as the dominant reef builders, establishing in some aspects the “modern reef’ ecosystem (Fliigel, 2002). 5) Carbonate seafloor fans were common during Archean—Paleoproterozoic time, but thereafter have only rarely been reported. This temporal restriction has been cited as evidence that Archean-Paleoproterozoic ocean chemistry differed from that of subsequent time periods (Sumner and Grotzinger, 1996b; Sumner and Grotzinger, 2000). The presence of carbonate precipitates from Lower Triassic strata suggests a resurgence of unusual ocean conditions such as oversaturation of calcium carbonate and the presence of micrite inhibitors. 6) The resurgence of unusual oceanic conditions that fostered deposition of carbonate seafloor fans in the Griesbachian Kokarkuyu Formation and Sapdere Formation, southern Turkey and Spathian Union Wash Formation, western United States, did not occur at only one time period during the Early Triassic but rather existed for as long as 4 —8 million years after the end- Permian extinction. 7) Flat-pebble conglomerates are common features of Precambrian-Cambrian shelf deposits because deep bioturbation had not yet evolved. The Early Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 242 Triassic is a return to such conditions because deep infaunalization of metazoans was repressed during the entirety of the lag phase from the end- Permian mass extinction (e.g. Ausich and Bottjer, 2002; Twitchett, 1999). 8) The flat-pebble conglomerates now reported from Lower Triassic sections in the western United States and southern Turkey substantiate earlier trends documented from Italy and South China (Wignall and Twitchett, 1999). Flat- pebble conglomerates undergo a global resurgence during Early Triassic time, and are not isolated to boundary beds. Rather, these are found as long as 4 —8 million years after the end-Permian extinction event. 9) The global occurrence of wrinkle structures in Lower Triassic shallow subtidal siliciclastic strata is anomalous for the Phanerozoic. Whereas these structures were extremely common in subtidal paleoenvironments of the Proterozoic-Cambrian, thereafter they became restricted to deep sea or stressed environments (Hagadorn and Bottjer, 1999). This restriction is attributed to an increase in infaunalization after the Cambrian (e.g. Bottjer and Ausich, 1986; Crimes et al., 1992; Droser and Bottjer, 1988; Droser and Bottjer, 1989; Seilacher and Pflueger, 1994). 10) Unlike the Proterozoic-Cambrian, the reduction of vertical bioturbation in the Early Triassic (Ausich and Bottjer, 2002; Twitchett, 1999; Twitchett and Wignall, 1996) does not occur because vertical bioturbators had not yet evolved, but because infaunal organisms were suppressed for several million years after the end-Permian mass extinction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 243 11) The prevalence of Lower Triassic subtidal wrinkle structures has great significance because it illustrates that siliciclastic paleoenvironments show signs of environmental stress that had previously only been reported from carbonate environments (e.g. Baud et al., 1996; Baud et al., 2002; Lehrmann, 1999; Pruss and Bottjer, 2004). 12) The occurrence of unusual facies such as thin-bedded limestone-mudstone facies, chip facies, and bioturbated bed facies suggests that not all unusual facies in the Early Triassic are anachronistic. Although the facies described here are not known to occur earlier in time, these may be isolated to the Early Triassic. 13) Facies such as the thinly-bedded limestone mudstone facies and thin bioturbated beds suggest that low levels of bioturbation creates a different preservational window. In the presence of deep bioturbators, these facies would be homogenized and therefore would not be preserved. 14) The chip facies indicates that flat-pebble conglomerates are only one manifestation of the exhumation of lithified micrite. The thin bioturbated beds reported from various localities suggest that oxygen-stress may have played a role in the distribution of trace fossils from Lower Triassic strata. These thin beds change abruptly from low levels of bioturbation (ii of 1) to high levels of bioturbation (ii of 5-6), suggesting that small-scale environmental fluctuations must have played a role in the development of this facies. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 244 15) Using sequence stratigraphic analysis, a link was found to exist between the Virgin Limestone Member microbialites and Union Wash Formation carbonate seafloor precipitate facies and flooding events, suggesting that basinal water flooding onto the shelf played a role in their formation. 16) Because evidence exists that basinal water of the Early Triassic was anoxic, it can be hypothesized that the flooding of anoxic waters, perhaps enriched in hydrogen sulfate and carbon dioxide, would have periodically suppressed metazoans. The increased alkalinity generated by sulfate reduction would have aided in the early lithification of such features as the flat-pebble and thin-bedded limestone-mudstone facies. Increased alkalinity would also have played a role in the lithification of microbialites and the precipitation of carbonate seafloor fans. 17) Trace fossils are extremely useful in assessing the biotic recovery interval from the end-Permian mass extinction. The Smithian/Spathian Virgin Limestone contains one of the best-preserved trace fossil assemblages from the southwestern United States. Study of this trace fossil assemblage has revealed that the late Early Triassic has a record of more complex metazoan behaviors than previously reported from equatorial regions. Whereas advances in complexity and the reappearance of such traces as Thalassinoides, Gyrochorte and Laevicyclus had occurred by the close of the Early Triassic in equatorial regions, the persistent small burrow size, low Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 245 ichnofabric indices, and low bedding plane coverage indicates that these trace-makers were living in a stressed environment. 18) This assemblage does not simply reflect a locally stressed environment because it formed concurrently with other global indicators of environmental stress (Lehrmann, 1999; Lehrmann et al., 2003; Lehrmann et al., 2001; Pruss et al., 2004; Woods et al., 1999). 19) Grotzinger and Knoll (1995) first described the upwelling of alkaline waters onto the shelf as a mechanism to explain similar unusual facies (e.g. carbonate precipitates) in the Permian and Precambrian. We hypothesize that analogous mechanisms likely played a role in the deposition of the facies in Lower Triassic strata of the western United States. 20) The upwelling of alkaline waters may not have acted as a source of environmental stress; however, byproducts associated with increased alkalinity may have created deleterious oceanic conditions. It has been shown that oceanic basins of the Early Triassic were likely anoxic (Isozaki, 1994; Isozaki, 1997; Wignall and Hallam, 1992; Wignall and Twitchett, 2002; Woods, 1998). In these anoxic basins, increased sulfate reduction would have brought about a build-up of H2 S and C 02 (Grotzinger and Knoll, 1995; Marenco et al., 2003). The periodic upwelling of basinal waters that were rich in hydrogen sulfide and carbon dioxide onto the shelf would have poisoned metazoans. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 246 21) A byproduct of sulfate reduction is bicarbonate, and this would cause an increase in the alkalinity of ocean waters. The simultaneous suppression of metazoans and increased alkalinity would bring about the deposition of anachronistic facies, thus linking their occurrence with the delayed recovery of the Early Triassic. Summary The devastation of marine ecosystems following the end-Permian mass extinction has been well-established. The research presented here postulates that the effects of the extinction event lasted for as long as 4 —7 million years after the event. The documented occurrences of anachronistic facies during the earliest Triassic (Griesbachian) of southern Turkey and the latest early Triassic (Spathian) of the western United States indicates the anachronistic facies were not a boundary phenomenon, but rather persisted for millions of years after the extinction. In this research, anachronistic facies were documented and described from Lower Triassic strata of the southwestern United States and southern Turkey. The Spathian Virgin Limestone Member of the Moenkopi Formation and Smithian—Anisian Middle and Upper Members of the Union Wash Formation were studied as part of this research in the southwestern United States (see Figure 2-1). In southern Turkey, the Griesbachian Katarasi, Kokarkuyu and Sapadere Formation were studied at three localities (see Figure 2-2). The documentation of these facies has shown that their occurrence is linked to long-term environmental stress following the end-Permian mass extinction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 247 Anachronistic facies have been documented globally from Lower Triassic strata and include microbial reefs (e.g. Lehrmann, 1999; Pruss and Bottjer, 2004), carbonate seafloor fans (Heydari et al., 2003; Woods et al., 1999), flat-pebble conglomerates (Wignall and Twitchett, 1999), and siliciclastics containing wrinkle structures (Pruss et al., 2004). Other unusual facies now have been described from Lower Triassic strata including thin-bedded limestone-mudstone facies, unusual chip facies, and thin bioturbated beds. Anachronistic facies such as microbial reefs, flat- pebble conglomerates and siliciclastics containing wrinkle structures form in the absence of deep bioturbation and therefore indicate that bioturbation did not fully recover until the Middle Triassic. Detailed analysis of trace fossils and their distribution in Lower Triassic strata suggests that bioturbation in equatorial regions may have recovered from the extinction more slowly than mid-high latitudes. The presence of carbonate seafloor fans points to anomalous ocean chemistry and suggests that Early Triassic oceans may have been more similar to those of the Precambrian than to the rest of the Phanerozoic. This research has focused on identifying and documenting anachronistic facies in Lower Triassic strata, elucidating trends in bioturbation, and relating the results to the delayed biotic recovery from the end-Permian mass extinction. It has now been demonstrated that the Early Triassic represents a return to substrate conditions of the Precambrian-Cambrian such that microbial mats, microbial reefs, and other microbialites became prevalent in normal marine environments, and bioturbators were suppressed for millions of years after the end-Permian extinction. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 248 This substrate relapse is hypothesized as resulting from long-term environmental stress that inhibited the recovery of metazoans and fundamentally changed both carbonate and siliciclastic substrates during Early Triassic time. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 249 References Allen, J.R.L., 1985. Wrinkle-marks: An intertidal sedimentary structure due to aseismic soft-sediment loading. Sedimentary Geology, 41: 75-95. Arp, G., Reimer, A. and Reitner, J., 2003. 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Lower Triassic large sea-floor carbonate cements; their origin and a mechanism for the prolonged biotic recovery from the end-Permian mass extinction. Geology, 27(7): 645- 648. Worsley, D. and Mork, A., 2001. The environmental significance of the trace fossil Rhizocorallium jenense in the Lower Triassic of western Spitsbergen. Polar Research, 20(1): 37-48. Yang, S., 1988. Permian-Triassic flysch trace fossils from the Guoluo and Yushu regions, Qinghai. Acta Sedimentologica Sinica, 6(1): 1-11. Zonneveld, J.P., Gingras, M.K. and Pemberton, S.G., 2001. Trace fossil assemblages in a Middle Triassic mixed siliciclastic-carbonate marginal marine depositional system, British Columbia. Palaeogeography, Palaeoclimatology, Palaeoecology, 166(3-4): 249-276. Zonneveld, J.P., Pemberton, S.G. and MacNaughton, R.B., 2002. Ichnology and sedimentology of the Lower Montney Formation (Lower Triassic), Kahntah River and Ring Border fields, Alberta and British Columbia, CSPG Abstracts with Programs, p. 355. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 269 Appendix 1: Locality information of the Muddy Mountains Overton locality and stratigraphic description of all measured units of the Virgin Limestone Member, Moenkopi Formation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 270 Virgin Limestone Member, Muddy Mountains Overton Locality (182 m); located at 36°36’30” N, 114°29’ W on Weiser Ridge topographic map west of Logandale, NV. Unit Meters Description 1 0.22 gastropod packstone 2 3.0 covered 3 2.26 gastropod packstone grainstone 4 4.7 covered 5 1.6 calcareous siltstone, gastropod grainstone, Promyalina and gastropods, Arenicolites, Planolites 6 2.6 cross-bedded siltstone, no fossils 7 0.65 limestone, oolitic, horizons of gastropods, bivalves, and echinoid spines 8 3.9 cross-bedded red calcareous sandstone with few gastropods 9 0.5 packstone/grainstone, gastropods and bivalves 10 1.7 covered 11 1.7 calcareous siltstone, cross-bedded, few disarticulated bivalves 12 0.6 cross-bedded gastropod packstone/grainstone, thin bivalves, silicified fossils, large gastropods 13 0.2 massive fossil grainstone with bivalves and gastropods 14 3.1 fissile calcareous siltstone 15 0.2 fossil packstone 16 1.8 covered 17 0.35 flat-pebble limestone with bivalves and gastropods, Promyalina concave down, disarticulated 18 2.0 covered 19 0.4 limestone with Planolites, flat-pebbles, small gastropodsand bivalves 20 6.0 covered 21 9.1 calcareous siltstone 22 1.9 limestone with ooids (look silicified), gastropods and bivalves 23 1.1 covered 24 1.2 bivalve packstone/grainstone, disarticulated shells concave down 25 3.2 covered 26 0.4 limestone with bivalve packstone, disarticulated 27 3.6 covered 28 6.2 calcareous siltstone, cross-bedded, wrinkle structures, Rhizocorallium, Arenicolites, Planolites 29 5.4 calcareous siltstone, corss-bedded, interference ripples, scours on base of bed in association with wrinkle structures, Asteriacites, Gyrochorte, Arenicolites, Planolites, Thalassinoides, Rhizocorallium Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 271 Appendix 1 (cont.) 30 0.5 Promyalina bed, shells concave down, base is bioturbated 31 4.0 covered 32 0.3 bivalves packstone/grainstone 33 1.1 covered 34 0.7 wackestone, few bivalves 35 10 covered 36 0.35 oolite, bivalve packstone 37 5.4 covered 38 1.2 calcareous siltstone and limestone, Rhizocorallium, Arenicolites, Planolites, bivalve/gastropod hash with flat- pebbles 39 9.2 greenish laminated micrite 40 2.4 limestone with gastropods, flat-pebbles, bivalves, Arenicolites and Rhizocorallium 41 3.4 silty limestone interbedded with packstone with bivalves 42 4.6 ledge-forming limestone, huge gastropods, flat-pebbles, bivalves, silicified fossils, capped by oolite that laps out aginst several unusual mounds, mounds are composed of flaky, perhaps algal, material 43 6.1 thinly bedded limestone, intervals/channels of debris resembling material that makes the mounds, Planolites in lenses, similar to thinly bedded limestone of Muddy Mountains Ute locality 44 1 wackestone with bivalves and gastropods 45 3 covered 46 7.0 limestone with flat-pebbles, bivalves, gastropods, ooids, crinoids, Planolites and Arenicolites and echinoids spines 47 25 calcareous siltstone, Rhizocorallium, Arenicolites, Planolites 48 24.1 fissile limestone, crinoid ossicles, large Planolites, some silicified bivalves and gastropods top is covered, overlain by siliciclastics Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 272 Appendix 2: Locality information of the Muddy Mountains Ute locality and stratigraphic descriptions of all measured beds, Virgin Limestone Member of the Moenkopi Formation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 7 3 Virgin Limestone Member, Muddy Mountains Ute Locality (1% m); located at 36°37’N, 114°37’ W; Exit 80 Ute off of Interstate 15N, north of Las Vegas, NV Unit Meters Description 1 4.8 oolitic limestone, gastropod-lingulid bed, massive gastropod ledge 1.2 fissile siltstone to mudstone, laminated 2 0.2 grainy massively bedded limestone, small fossil fragments .13 largely weathered yellow mud or siltstone 3 0.8 packstone with abundant shell debris, small gastropods and bivalves (< 1cm), somewhat muddy matrix partially oolitic 0.7 fissile yellow mudstone 4 1 muddy limestone with rip-ups, shell debris, at top alternating micrite and silt layers, oolitic .25 siltstone 5 1.6 limestone with silty layers, base is oolitic, lower half seems devoid of fossils, upper half is oolitic/grainy with shell debris 5 fissile mudstones and siltstones, covered and badly weathered 6 1.12 mixed limestone/siltstone unit; base has abundant bivalves concave down and infilled with mud, moves into oolitic bed, moves into another bivalve bed (Promyalina?), unusual burrows cap this in oolitic bed 0.5 red siltstone 7 1 Pink oolite, no shells at base, overlain by pink limestone ledge 1.2 siltstone with horizontal bioturbation and ripple marks 8 0.5 oolitic limestone intercalated with siltstone, small gastropods and bivalves at base 9 1.4 silty limestone, cross-bedding, unfossiliferous 0.6 red/green siltstone layers 10 1.35 green silty limestone shells near base, overlain by unfossiliferous limestone 0.4 red siltstone 11 3.6 limestone, laminated in parts, some gutter casts, some horizontal bioturbation (Planolites), rip-ups associated with the gutter casts 0.8 fissile purple limestones 12 0.4 massively bedded limestone with shell debris, some Planolites at base 1.1 pink weathered siltstone 13 0.4 purple packstone of shell debris 5 covered siltstones 14 2.3 oolitic ledges, small shells 3.4 weathered green/pink limestone Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 274 Appendix 2 (cont.) 15 0.35 grainy limestone, rip-ups at base 0.8 weathered siltstones alternating with small 10 cm limestone ledges 16 2.3 micrite, cross-bedding at base, overlain by shell debris and rip-ups, overlain by stylotized oolite with gastropods 3.8 weathered siltstone 17 1.1 oolitic ledge, vertical traces like Unit 6 2.7 covered siltstone 18 0.65 packstone of bivalves, grainy, some shells preserve geopetals 3.2 covered 19 1.5 packstone of shells, large concave down Eumorphotisl near top 1.8 weathered siltstone 20 1.8 bivalve packstone, Eumorphotisl and some Promyalina (3-4 cm), concave down and infilled with mud 2.4 weathered red siltstone 21 1.3 packstone, shells have concave down orientation, geopetals 6.2 weathered siltstones 22 1.8 limestone with a few non-resistant layers, bioturbation, burrows look infilled with coarse material, small 0.5 cm in diameter 0.5 weathered siltstones 23 2.7 silty micrite, Asteriacites traces found on bases of beds 2.7 purple siltstones 24 0.9 pink limestone at base, overlain by gray limestone, shell debris and rip-ups 1.2 resistant purple siltstone 25 1 packstone, some shell hash with cavities of spar and mud 1.9 purple siltstone 26 3 limestone, odd orange weathering at base (may contain Arenicolites), unfossiliferous at the top 6.5 covered purple and green siltstones, may contain gutter casts 27 2.1 limestone ledge with shell debris and flat-pebbles 5.5 purple and green fissile siltstone 28 2.2 purple silty resistant limestone with shells (paper pectens?) 29 7 fissile silty limestone Planolites, mottling near base, laminated near top 30 1 oolite with large ooids, contains shell hash and rip-ups, gastropods and bivalve debris 31 0.8 weathered tan clays, rip-ups, shell hash near top 32 2.25 packstone with shell debris, lenses of bioturbation, shell hash Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 275 Appendix 2 (cont.) 33 2.5 silty limestone, base contains rip-ups, maybe shell debris and grainy/muddy interval that separates ledges, mottled beds (ii of 4 or 5) then change into laminated beds; limestone with odd horizontal structure, capped by laminated beds, beds alternate with laminated limestone and bioturbated intervals, with bioturbation again sustaining an ii of 4-5 near top 4 covered, possibly grainy limestone 34 0.4 grainy limestone with shell hash 0.3 covered 35 6 grainy limestone with bioturbation near base, some rip-ups like Unit 34 3.2 covered, possible calcareous mud 36 5.25 grainy limestones, similar unit to Unit 29, bivalves, fissile laminated beds with lenses of bioturbation; beds with ii of 3 moves into 4-5, then laminated again 3.6 covered intervals 37 6.4 calcareous mudstone, has rip-ups near base, moves into siltstone with rip-ups 5 covered unit, with interval of cross-bedded dark orange siltstone with hummocky cross-stratification, some flat- pebbles 38 0.35 thin limestone beds, oolite with bivalves, muddier near top 7.4 covered intervals, may be secondarily brecciated 39 1.4 thin-bedded dark micrite, ii of 4/5, similar to Lost Cabin Springs thin beds 40 1 packstone with first appearance of crinoids, bivalves, fossils look silicified, bioturbation at base 0.7 covered 41 2.3 limestone ledges with covered intervals in between; looks grainy but fairly devoid of fossils, next ledge is grainy with crinoid debris, some appear silicified, top is bioturbated 8 covered 42 1 grainy limestone, some bioturbated horizons 5.1 covered, green and purple siltstone 43 5 basal limestone ledge grainy with bivalve and crinoid debris, overlain by limestone with horizontal bioturbation, crinoid debris in top beds interbedded with fissile units 44 ~40 calcareous siltstone, white, muddy matrix, laminated beds, no real change in lithologies throughout unit, eventually and it is covered by reds alluvium; some odd wavy features in unit, appears to be unfossiliferous, may be Shnabkaib Member Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 276 Appendix 3: Locality information for the Mountain Pass locality and stratigraphic description of all measured beds, Virgin Limestone Member, Moenkopi Formation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 277 Virgin Limestone Member, Mountain Pass Locality (104 m); located at 36°10’30”N, 115°08’ 11”W; Bailey Road Exit off of 15N, ~60 miles north of Baker, CA Unit Meters Description 1 2.44 oolitic grainstone, oolites are < 1mm in diameter 2 1.6 limestone, base has ichnofabric 4/5, Planolites, overlain by less bioturbated beds 3 0.85 laminated micrite 4 1.6 limestone, ichnofabric of 3/4, Planolites, pods of chert, beds fluctuate between being heavily bioturbated to laminated (similar to Lost Cabin Springs beds) 5 2.0 yellow silty limestone, pods of chert 6 2.2 grainstone, lower 20 cm contains flat-pebbles and rip-ups, channelized peloids, Thalassinoides infilled with grainstone, cross-bedded shelll hash with gastropods and bivalves 7 6 interbedded yellow silty limestone with grainy limestone, rip- ups 0.8 covered 8 3.2 limestone ledge with calcitic veins, becomes grainier up- section with small shells and rip-ups, upper ledge has bioturbation preserved on top 9 3.4 limestone, fine-grained bioturbation visible on bedding planes 10 3.2 massive limestone ledge, looks faulted, featureless 11 1.8 limestone, Thalassinoides burrows infilled with grainstone and coarse sediments 12 0.9 coarse-grained, cross-bedded limestone, rip-ups that look siliciclastic, shelly debirs that infills underlying Thalassinoides 5 covered, talus looks like burrowed siltstone 13 1.4 grainstone, cross-bedded, rip-ups, oolitic, overlain by micrite that is burrowed with Thalassinoides, bivalves and gastropods 3.4 covered 14 1,1 graisnstone, weathers orange, rip-ups of siltstone, base looks burrowed 15 2.8 burrowed limestone, base is micrite, burrowed by Thalassinoides, and infilled with overlying grainstone, rip- ups, cross-bedding, first visible occurrence of crinoids 6.7 covered 16 2.5 limestone with orange weathering, bivalves that are concave down, silt rip-ups, base is burrowed by Thalassinoides, infilled with grainstone 17 2.5 thin-bedded dark micrite, ii of 4/5, similar to LCS thin beds Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 278 Appendix 3 (cont.) 5 covered 18 2.8 orange grainstone, Thalassinoides at base of bed, overlain by grainstone with rip-ups at the top 19 2.8 micrite with shell debris, base is burrowed with Planolites and Thalassinoides, overlain by heavily bioturbated beds, bivalves and gastropods, ii of 4/5, looks like cemented ripples in carbonate that may be infilled by mud, overlain by “chip” layer similar to Lost Cabin Springs 3.34 covered 20 2.4 graisntone with rip-ups, overlain by bioturbated beds with Thalassinoides at base, overlain by grainstone with large crinoid debris, large rip-ups 3.8 covered 21 1 grainstone with Thalassinoides, overlain by micrite with flat- pebbles and shells, grainy, some Planolites 22 2 limestone, ii of 3, laminated in places, thin-bedded limestone mudstone facies 7 covered 23 1.1 limestone, bivalves and/or brachiopods, base is coarse with small rip-ups, overlain by purple mudstone with bivalves, top is burrowed and weathers orange 8.3 covered 24 2.8 fissile limestone, silt clasts, abundant bivalves, flat-pebbles, weathers orange 3.4 covered 25 7.8 cliff-forming limestone, grainy, some silt rip-ups, small shells, pods of chert, top is covered, overlain by red and yellow siltstones Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 279 Appendix 4: Locality information for Lost Cabin Springs locality and stratigraphic descriptions of all measured beds, Virgin Limestone Member, Moenkopi Formation Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 280 Virgin Limestone Member, Lost Cabin Springs Locality (~175 m); located at 36°05’30”N, 115°41’00”W located on Lost Cabin Springs topographic quadrangle, ~26 miles west of Las Vegas, north of Blue Diamond Highway Unit Member Description 1 1.4 base is a purple/green siltstone (pencil weathering) with calcareous lenses, overlain by resistant limestone that is laminated 2 1.1 resistant limestone ledge, massively bedded, some shell debris, horizontal bioturbation at base, grainy crinoid debris ledges alternates with muddy micrite near top of unit 0.3 covered 3 1 grainy limestone ledge, may contain Thalassinoides, cross bedded siltstone exposed near base 4 0.6 weathered yellow non-resistant calcareous grainy siltstone with fossil debris 2.15 covered, siltstone, red/orange in talus, possible cross-bedding 5 7.5 mostly covered siltstone with alternating limestone ledges, looks like grainy limestone 6 2 resistant siltstone, some cross-bedding, small limestone intercalations 7 1.5 calcareous siltstones with limestone lenses, abundant gastropods near top, cross-bedding 2 covered 8 1.8 red siltstone (containing petroglyphs), limestone at the top, but mostly cross-bedded, resistant siltstone ledge 9 0.5 grainy limestone unit overlies siltstone, crinoids and shell debris 7 covered, limestone in talus 10 1 base is covered, alternating crinoidal grainstone with micrite, grainstone lenses are about 10-20 cm thick with bioturbation at the top, moves into bivalve rich unit with mud underneath the concave down shells 11 1.45 thinly bedded limestone, fissile, seems to have muddy intercalations, ammonite bed with Planolites traces 1.5 covered 12 5.6 thick limestone ledges consisting of thin beds, most are laminated or wavy, moves into bioturbated beds with ii 4/5, horizontal bioturbation, base has rip-ups and is grainy, top contains Thalassinoides, laminated micrite, then more rip-ups, capped by heavily bioturbated beds 3 covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 281 Appendix 4 (cont.) 13 1.85 limestone with ii of 4/5; looks laminated, even cross-bedded at base, then into bioturbated overlying units, become laminated again at the top 1.5 covered, maybe limestone 14 6.5 bioturbated limestone ledges, ii of 4 or 5, Planolites traces, muddy limestone with weathered base, channelized peloids 15 0.12 Thalassinoides occurring in a very muddy unit above the bioturbated beds of Unit 14, pellets occurring on top of mudflat surface just below deformed (slumped) units of Unit 16, may by parasequence boundary 16 4.5 thick limestone ledge, many traces and deformed (slumped) units, ii of 4 or 5, sometimes nearly laminated 17 0.6 limestone ledge, heavily bioturbated by Thalassinoides, infilled with grainstone, some vertical bioturbation, muddy at base and grainy near the top 3.2 covered, maybe siltstone 18 4.6 limestone, basal meter to meter and a half has silty intercalations, moves into grainstone with Thalassinoides marking the boundary, grainstone is packed with crinoidal debris, moves into ledge with more Thalassinoides and then muddy near the top, oolite exposed in areas, in channelized deposits associated with large flat-pebbles 4.5 covered 19 0.3 crinoidal packstone with horizontal bioturbation preserved on top 20 2 bioturbated beds (ii of 3/4), Planolites, and unusual microbial ledge underlying the microbial mounds 21 1.5-2 limestone bed containing the microbial mounds 22 7.1 limestone ledge overlying microbial mounds, grainy in places, contains echinoid spines, muddier at base, grainer near the top, silicified fossils 23 2 limestone, base is covered, grainy near the base, rip-ups, cross-bedding of fossil grains, into muddy mottled unit the grainy into bivalve unit, Thalassinoides near top 2 covered 24 3.7 limestone, covered, muddy base, overlain by packstone with fossil debris, horizontally bioturbated near top 1.5 covered, may be siltstone Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 282 Appendix 4 (cont.) 25 1.7 limestone, base covered, thick series of ledges with abundant crinoidal packstones, mottled in places, bioturbation at top, and bivalves with geopetals, Planolites 1.5 covered 26 3 fairly nonresistant orange-yellow siltstone, some thin limestone ledges with abundant shell debris (geopetals), Gyrochorte traces on top of siltstone beds, lots of traces in talus 3.2 covered, siltstone in talus 27 2 small microbial mounds, some dome features (~1 m), some bivalves concave down in overlying beds, stylolites 28 0.8 grainy limestone ledge, packstone unit with abundant crinoid and other debris 29 3.3 limestone ledge, base is fissile, unfossiliferous laminated base overlain by packstone with abundant bivalves near top 1.2 covered, some red siltstone 30 1.1 limestone ledge, grainy and muddy beds alternate, very grainy top with rip-ups, some bioturbation preserved on top, Planolites 1.4 covered 31 3.2 limestone, base is fissile, ii of 3, Planolites, muddy changes over to grainy ledges, ledges have ii as high as 4 or 5; packstone ledges as well with some bivalves and crinoids, maybe some Thalassinoides near top 3.6 covered 32 1.5 stromatolitic, microbial limestone, radiating from micritic clasts, not laterally continuous, seem to radiate off of light colored micritic clasts 33 4.4 limestone with abundant crinoid debris right above stromatolites, heavily bioturbated unit with ii 3 or 4, lots of horizontal traces, then moves into more laminated beds with ii 1 or 2, overlain by another grainstone/packstone, then an increase ii of 1 or 2, and then into unusual “rip-up bed” with lots of micrite clasts, then top of unit has ii of 4 or 5 before becoming covered (or laminated) in some places 0.2 covered 34 0.4 thick packstone of crinoidal debris, some rip-ups, some bioturbation preserved on top of muddy surfaces 3.9 covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 283 Appendix 4 (cont.) 35 6.8 limestone, very fissile base, ii of 3, increasing bioturbation up section, mottled by Planolites, upper half of section has alternating intervals of packstones followed by bioturbated muddy limestone, this is capped by a limestone ledge with rip-up clasts, upper part of the unit is a massive cliff-forming ledge with an ii of 5 or higher 36 0.6 limestone, base is covered, grainy limestone with bivalves concave down and crinoids, some Thalassinoides also 3 covered 37 1.3 limestone, base looks like a storm bed with silty rip-ups and fossils, lots of crinoids 6 covered, wrinkle structures found in talus 38 3.1 limestone, base is grainstone, moves into micrite with horizontal bioturbation ii of 1 or 2, then ledge with no bioturbation (but maybe Promyalina) and into beds with ii 5 or 6 39 3.5 limestone ledges, base is covered, muddy and bioturbated, moves into flat-pebble bed, ii of 3 or 4 in places, and upper part of bed contains Thalassinoides 3 covered 40 0.5 packstone with bivalves, and Thalassinoides, very grainy 3 covered 41 2.2 limestone, base looks grainy, moves into ii of 4 or 5, large resistant, mostly muddy limestone 42 1.5 limestone ledge, flat-pebble bed, muddy 4.5 covered 43 2.5 thick limestone ledge, muddy base, laminated, moves into heavily bioturbated section with ii of 5 or 6, top ledge is last resistant unit exposed at this locality Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 284 Appendix 5: Locality information for Darwin Hills locality and stratigraphic descriptions of all measured beds, Middle and Upper Members, Union Wash Formation, Darwin Hills Locality. Note that some measurements incorporate information from Stone et al. (1991) because bedding was sometimes difficult to elucidate at this locality. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 285 Union Wash Formation, Darwin Hill Locality (~300 m); located at 36°17’30”N and 117°37’30”W on Darwin topographic quadrangle, ~2 miles NE of Darwin, CA Unit Meter Description 1 6 siltstone and limestone intercalated beds, thin bedded limestone-mudstone facies 2 1.1 thick limestone breccia, varies laterally 3 4.5 calcareous siltstone with small thin limestone beds 4 134 thick precipitate-bearing limestone, large unit with large-scale (>1 cm) laminations in places, base of unit contains more slumping features, beds become more laminated up section, laminations show pods of black limestone and layers of gray micrite, the black limestone may represent aragonite precipitates, but only where weathering is sufficient are these obvious, there are pockets of breccias that looks like collapse breccias, and there are also more laterally extensive breccias that look like fault breccias, there are tiny pockets of silt that are found throughout section that in places look silicified, 2n d ledge below top of section contains the most obvious well- preserved crystal fans 5 23 mostly covered, measurement based on report from Stone et al (1991), in places greenish calcareous siltstone beds are exposed with Eumorphotis bivalves 6 2.07 gray limestone, top is covered 7 4.83 covered 8 3.57 mottled limestone with some possible silicified fossils 9 13.44 thinly bedded limestone with pink silty layers, no obvious fossils 10 6 covered 11 0.3 limestone ledge, similar to Unit 9 12 26-30 siliciclastic unit, beds are rarely in place, slightly metamorphosed, may be axis of syncline, become more calcareous upsection 13 4.3 limestone, lighter in color than precipitate-bearing limestone, thin siltstone stringers 14 3.6 covered 15 2.5 intercalated limestone and siltstone, top is covered 16 5 limestone, base is devoid of siltstone, siltstone stringers become more prominent near the top of the bed 17 20 intercalated limestone and siltstone beds, poorly exposed, siltstone not in distinct layers as before, measurements based on Stone et al., (1991) because bedding was destroyed Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 286 Appendix 5 (cont.) 18 3.5 micrite, devoid of siltstone 19 10 limestone, intercalated with siltstone, some Planolites traces, ii of 5, some burrows appear outlined in chert 20 27 limestone intercalated with siltstone 21 4.5 covered interval 22 15 limestone, less siltstone 23 9 covered interval 24 10 limestone with silty intercalations 25 7.5 covered 26 5 thin bedded limestone-mudstone facies 27 20 dark limestone, similar to Unit 4, precipitates are present, come laminated beds, affected by slumping 28 30 covered section and limestone ledges; limestones are bedded with siltstones, unit continues beyond 30 m but is poorly exposed, and looks to be capped by siltstone, this appears as the last cluster of limestone units exposed here, top is covered Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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Pruss, Sara Brady
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The unusual sedimentary rock record of the Early Triassic: Anachronistic facies in the western United States and southern Turkey
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Geological Sciences
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University of Southern California
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Identifier
3145267.pdf (filename),usctheses-c16-414274 (legacy record id)
Legacy Identifier
3145267.pdf
Dmrecord
414274
Document Type
Dissertation
Rights
Pruss, Sara Brady
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Access Conditions
The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...
Repository Name
University of Southern California Digital Library
Repository Location
USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA
Tags
paleontology