University of Southern California Dissertations and Theses

Biotic recovery from the end-Permian mass extinction: Analysis of biofabric trends in the Lower Triassic Virgin Limestone, southern Nevada

(USC Thesis Other)

Biotic recovery from the end-Permian mass extinction: Analysis of biofabric trends in the Lower Triassic Virgin Limestone, southern Nevada

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. 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 bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UM I a complete manuscript and there are missing pages, these w ill be noted. Also, if unauthorized copyright material had to be removed, a note w ill indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. ProQuest Information and Learning 300 North Zeeb Road, Ann Arbor, M l 48106-1346 USA 800-521-0600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIOTIC RECOVERY FROM THE END-PERMIAN MASS EXTINCTION: ANALYSIS OF BIOFABRIC TRENDS IN THE LOWER TRIASSIC VIRGIN LIMESTONE, SOUTHERN NEVADA Copyright 2001 by Sara Brady Pruss A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Parial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (GEOLOGICAL SCIENCES) December 2001 Sara Brady Pruss Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U M I Number. 1411035 Copyright 2001 by Pruss, Sara Brady All rights reserved. __® UMI UM I Microform 1411035 Copyright 2002 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, M I 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U N IV E R SIT Y O F S O U T H E R N C A L IF O R N IA T H E GRADUATE S C H O O L UNIVERSITY PA R K COS A N G ELES. C A LIFO R N IA 9O0OT This thesis, written by _________ S A caJB xarix-B xiififi_______________________ under the direction of hex. Thesis Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of Master of Science in Geological Sciences D tm * r w 12-17-2001 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS I would like to thank the organizations responsible for the money for this project. These include The Paleontological Society, the American Museum of Natural History, The Wrigley Institute, and the USC Department of Earth Sciences. I would also like to extend a special thanks to my thesis committee including Robert Douglas and Frank Corsetti for help in the thought process and realization of this project. Their patience, support and input has been very much appreciated. The lab members during the time I have been at USC have been unfaltering in their help and support. Karina Hankins. Margaret Fraiser. Steve Dombos, Pedro Marenco, Nicole Fraser. Tran Huynh. Gerald Grellet-Tenner, Rich Twitchett. and Nicole Bonuso have been irreplaceable to me. I feel extremely grateful to have had the continual love and support of my family. My mother and father made my dream of graduate school a reality. My grandmothers taught me strength and endurance as a woman pursuing her goals. Finally, I would like to extend a special thanks to my advisor. He has helped me grow as both a scientist and a person through all of my experiences in graduate school. With unending patience and kindness, support and guidance, he has made possible my achievements. Thank you. ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS Ac know Iedgements List of Figures Abstract Chapter I: Introduction Chapter 2: The End-Permian Extinction Event Possible Causes Boundary Sections and Isotopes Chapter 3: Extinction of Organisms Following the End-Permian Extinction Event Chapter 4: Recovery from the End-Permian Extinction Event Chapter 5: Stratigraphy of the Virgin Limestone Member Methods Muddy Mountains Vigo Lost Cabin Springs Chapter 6: Onshore-Offshore Trends: Trace Fossils Methods Resuts Chapter 7: Onshore-Offshore Trends: Bivalves Methods Results Discussion Chapter 8: Onshore-Offshore Trends: Microbial Fabrics Previous Work Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 9: Stromatolites of the Virgin Limestone Member 204 Methods 204 Field Analysis 213 Thin-Section Analysis 233 Paleoenvironmental Implications 236 Early Triassic Microbial Reefs 252 Chapter 10: Conclusions 255 References 260 Appendices 273 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Figure Page L.Plot showing familial diversity versus time 2. Geologic time scale for Late Permian and Early Triassic 3. Model of oceanic stratification and global cooling 4. Graph of stomatal densities through time 5. Stratigraphic column showing Early Triassic superanoxic ocean 6. How chart showing volcanic winter 7. Paleogeography of Early Triassic continents 8. Stratigraphic column of Tesero Road Section. Italy 9. Stratigraphic column of Schuchert dal Formation, Greenland 10. Stratigraphic reconstruction of South China basins 11. Stratigraphic column showing both superanoxia, prolonged anoxia 12. Carbon isotope curve for Early Triassic 13. Oxygen isotope curve for Early Triassic 14. Sulfur isotope curve for Early Triassic 15. Strontium isotope curve for Late Permian-Early Triassic 4 11 15 18 21 25 28 31 34 36 39 43 45 48 16. Diagram showing reduction in burrow diameter in Early Triassic 55 17. Diagram showing precipitation of Early Triassic cements 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18. Map of tectonic activity of W. United States in Early Triassic 66 19. Photograph of outcrop of Virgin Limestone 68 20. Map of Lincoln and Clark Counties, Nevada 71 21. Stratigraphic column from Muddy Mountains 79 22. Photograph of outcrop of Virgin Limestone, Muddy Mountains 81 23. Photograph of ripple marks. Muddy Mountains 83 24. Photograph of Promyalina bed. Muddy Mountains 86 25. Photograph of trace fossils. Unit 9a. Muddy Mountains 88 26. Photograph of trace fossils. Unit 10. Muddy Mountains 90 27. Photograph of gutter casts. Unit 10. Muddy Mountains 92 28. Photograph of crinoid ossicles. Unit 13, Muddy Mountains 94 29. Photograph of outcrop, Vigo 97 30. Stratigraphic column from Vigo 99 3 1. Photograph of outcrop. Units I and 2, Vigo 102 32. Photograph of bioturbation, Unit I, Vigo 104 33. Photograph of bioturbation. Unit 2, Vigo 106 34. Photograph of outcrop. Unit 7, Vigo 108 35. Photograph of bioturbation. Unit 10a. Vigo 111 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36. Photograph of Promyalina bed, Unit LO b, Vigo 113 37. Photograph of bioturbation. Unit 11, Vigo 115 38. Stratigraphic column of Lost Cabin Springs 117 39. Photograph of outcrop of Virgin Limestone, Lost Cabin Springs 119 40. Photograph of stromatolite mound. Unit I, Lost Cabin Springs 122 41. Photograph of side-view of stromatolite mound. Lost Cabin Springs 124 42. Photograph of outcrop. Unit 7. Lost Cabin Springs 127 43. Photograph of bioturbation. Unit 11, Lost Cabin Springs 129 44. Photograph of bioturbation. Unit 15, Lost Cabin Springs 131 45. Photograph of large stromatolite mound. Lost Cabin Springs 134 46. Photograph of smaller domes within mound. Lost Cabin Springs 136 47. Diagram showing ichnofacies 141 48. Photo of Planolites and Arenicolites traces. Muddy Mountains 143 49. Photo of Paleophycus traces. Muddy Mountains 145 50. Photo of Rhizocorallium traces. Muddy Mountains 147 51. Photo of Rhizocorallium traces, close-up. Muddy Mountains 149 52. Photo of trace fossils. Unit 2. Vigo 152 53. Photo of Planolites traces. Vigo 154 54. Photo of Thallassinoides traces. (Unit 11) Lost Cabin Springs 157 vu Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55. Photo of Ttiallassinoides traces, (unit 15) Lost Cabin Springs 159 56. Photo of Planolites traces, Lost Cabin Springs 162 57. Photograph of beds underlying stromatolite mounds 164 58. Chart of ichnofabric trends 167 59. Sketch of 4 bivalves of the Early Triassic 170 60. Promyalina bed at Muddy Mountains 174 61. Photo of half of Promyalina valve. Muddy Mountains 176 62. Photo of disaggregated half of Promyalina, Muddy Mountains 178 63. Photo of slabbed sample of Promyalina, Muddy Mountains 180 64. Promyalina bed at Vigo 183 65. Diagram of onshore-offshore biofabric trends 190 66. Diagram of rise and fall of stromatolites 195 67. Stratigraphic column of South China 199 68. Diagram of Great Bank of Guizhou, S. China 202 69. Photograph of stromatolite mounds in outcrop. Lost Cabin Springs 205 70. Photo of stromatolite and Dr. David J. Bottjer 207 71. Photo of stromatolite mound (side view) and Dr. David J. Bottjer 209 72. Photograph of large stromatolite mound 211 viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73. Labeled sections of stromatolite mound 214 74. Photo of beds underlying stromatolite mounds 217 75. Photo of bioturbation under stromatolites 219 76. Photo showing Planolites burrows under stromatolites 221 77. Field photo of smaller domes 223 78. Photo of domes weathering in outcrop 225 79. Field photo of mound 227 80. Photo showing infilling beds around mounds 229 81. Field photo showing beds pinching out around mounds 231 82. Thin-section photo of stromatolitic laminations 234 83. Thin-section photo of microbial fabric 237 84. Thin-section photo of muddy micritic fabric 239 85. Thin-section photo of micritic and microbial fabrics 241 86. Diagram showing conditions favoring stromatolite growth 248 87. Diagram showing Union Wash and Moenkopi Formations 250 ix Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT The Virgin Limestone Member of the Moenkopi Formation, deposited in Southern Nevada, records a critical time in the history of life: the recovery interval from the end-Permian mass extinction. During the end-Permian extinctions, estimates have been made that between 90% (McKinney. L995) to as many as 96% of the marine species (Raup, L979) went extinct. The recovery interval from this extinction event lasted an unusually long 7-8 million years (Hallam. 1991). An onshore-offshore transect of the Lower Triassic Virgin Limestone Member was constructed to assess how biofabrics and fossil composition varied from intertidal paieoenvironments to the middle shelf. This was completed to better ascertain the environmental factors that played a role in prolonging the recovery from the end- Permian mass extinction. In the study of this onshore-offshore transect, biofabric trends emerged. Trace fossils followed the ichnofacies model of Seilacher (1964) that predicts domination of intertidal settings by vertical burrowing (Muddy Mountains), while subtidal to middle shelf environments tend to be dominated by horizontal bioturbation (Vigo and Lost Cabin Springs). Perhaps the most surprising trend was the development of stromatolites in the most offshore section with an absence of Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. carbonate microbial activity at the locality, (Muddy Mountains) representing shallowest depositional environments . Results from this research demonstrate that the development of stromatolites, which represent some of the earliest reef systems in the Mesozoic, may have been intimately linked to the prolonged recovery of metazoans from this mass extinction. Periodic incursion of water masses with deleterious offshore conditions such as carbon dioxide supersaturation or anoxia from deeper water on to the shelf may have reduced metazoan activity (such as burrowing) while simultaneously fostering microbial build-ups. A link must exist between the slowed recovery of metazoans in the Early Triassic. and the occurrence of only microbial reefs during this time. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION The end-Permian extinction event was the largest extinction in the history of life. Estimates have been made that as many as 95% of marine species (Raup, 1979) and 62% (McKinney, 1985) of marine families went extinct during this time interval. Sepkoski (1981) plotted familial diversity through time (Figure I), and showed that the end of the Permian marks a major faunal turnover of the so called “Paleozoic fauna” to the “Modem fauna”. As compared to the other major extinctions, the recovery interval from the end-Permian extinction was unusually long. Estimates by Hallam (1991) suggest that marine ecosystems did not recover for 7-8 million years after this event. This recovery interval persisted throughout the Scythian: recovery to pre-extinction diversity occurred in the Anisian (Hallam and Wignall, 1997) (Figure 2). Various mechanisms have been proposed to explain why this recovery interval was so protracted. Some have suggested that continued environmental stresses related to the extinction persisted throughout the Early Triassic, thus inhibiting organisms from recovering (Hallam, 1991). Others have suggested that preservational biases have played a role in the fossil record of the Early Triassic (Erwin and Hua-Zhang, 1996). 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure I: Plot showing familial diversity versus time and end-Permian reduction in families (modified from Sepkoski. 1981). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 400 200 0 GEOLOGIC TIME Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2: Geologic time scale showing Late Permian and Early Triassic stages (modified from Hallam and Wignall. 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 5 0 - Late Permian Extinction Interval 2 6 0 — u mm c a c n < e • 2 £ AC W CT. *5 5 T L S w - 5 Z < m m 5 * x M i e C 5 e s m X *afi * * 5 5 * Q C c 1 £ • • e a « « • Spathian e 5 s * Smithian a e m s « z Dienerian * 3 STAGES Anisian (Jricsbuciiiati Changxingian (Dorashamian) YVtijiupingiaii (Dzhuiflan) Maokuuan Chihsian Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Erwin and Hua-Zhang ( L996) suggest that perhaps organisms that were present after the extinction were not being preserved in the fossil record, and thus researchers get a skewed view of diversity. Some have suggested that the sheer severity of the event itself so devastated ecosystems, that the length of time for organisms to recover is most directly related to the time it takes for the advent of specialized species or for them to spread from refugia (Schubert and Bottjer. 1995). To test this hypothesis, an investigation was begun into the potential continued environmental stresses that may have played a role in prolonging the recovery interval from the end-Permian mass extinction. Evidence for deleterious conditions that originated in deep-water environments such as anoxia (Isozaki. 1994. 1997) or carbon dioxide poisoning (Woods et al.. 1999) may have prolonged the recovery of marine metazoans during the Early Triassic. To best test this hypothesis, an onshore-offshore transect of the Lower Triassic Virgin Limestone in Southern Nevada was constructed along which one could assess facies and biofabric trends. Three localities including Muddy Mountains. Vigo and Lost Cabin Springs were selected because they each represented a different paleoenvironment along the onshore-offshore transect. At each locality, trace fossil, microbial fabrics and faunal changes were studied to ascertain the differences between paleoenvironments from 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the onshore to the offshore. The biofabric trends were used as a way of assessing if deleterious conditions existed in offshore paleoenvironments. Using this transect, one could determine the changes in ichnofabrics, microbial build-ups and fauna! occurrences to best conclude if deleterious offshore conditions such could have played a role in hindering the recovery of organisms in the Early Triassic. In this thesis, a summary of the end-Permian extinction event will be given to explain the hypothesized causes and effects. This background will provide a framework to understand the relevance of the research conducted for this project. A summary of the methodology employed for this project will be presented followed by conclusions. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE END-PERMIAN EXTINCTION EVENT The end-Permian extinction event, dubbed the '‘mother of all extinctions” by Erwin (1994), was the largest extinction event in the history of life. Estimates have been made that as much as 62% of the marine invertebrate families (McKinney, 1985) and 96% of the marine species (Raup, 1979) went extinct during this time. While other extinction events have been recognized throughout the history of life, it is largely acknowledged that marine ecosystems were severely decimated during the end-Permian extinction, with subsequent restoration of pre-extinction diversity levels not achieved until the Cretaceous (Allison and Briggs. 1993: Benton. 1995). Two pulses of extinctions are generally recognized: the first occurred in the Late Maokouan during which Tethyan faunas were hit more severely than the Boreal faunas (Jin et al.. 1994). The Chanxingian extinction is the more severe of the two. however, the duration of this extinction event is still the subject of debate (Twitchett et al.. 2001: Hallam and Wignall. 1997: Holser and Morgantz. 1992). The second end-Permian extinction event occurred around 251.4 million years ago (Jin et al.. 2000) and marks a rapid decline in species and genera at this time. The event is believed by some to represent a marine collapse in as few as 10-30 k.y. (Twitchett et al.. 2001: Rampino and Adler, 1998) and is followed by a recovery 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. interval in the Early Triassic. Hallam (1991) has put forth an estimate that this recovery interval in the Early Triassic persisted for 7-8 million years THE END-PERMIAN EXTINCTION EVENT: POSSIBLE CAUSES Several scenarios have been invoked as possible mechanisms for the end- Permian extinction event. One of the greatest difficulties facing researchers is the fact that many extinction scenarios seem to contradict each other. It has also been noted by some that this extinction may be the result of several interactions that could have culminated in one great extinction (Erwin. 1994). Some of the hypotheses include regression, global cooling, global warming, marine anoxia, volcanic winter and extraterrestrial impacts. Marine regression has been cited as a cause by some (Newell, 1952. 1967: Valentine and Moores. 1973: Schopf. 1974: Sweet et al., I992)since sea level may have been at an all time low. This mechanism would be responsible for the loss of habitat of shallow marine seas and thus the annihilation of many of the organisms that live there. While there does seem to be a major regression associated with the close of the Permian, this may be responsible for the late Maokouan reef and tropical faunal extinctions and not the Changxingian extinctions (Hallam and Wignall. 1997). There seems to be little geological evidence (such as karst formation or incised 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. valleys) for a major regression in the sequence boundaries during the end of the Permian. Global cooling has also been put forward as a possible mechanism for the end-Permian mass extinction. Knoll et al. (1996) propose a decrease in global temperature as a result of decreased carbon dioxide in the atmosphere (and a concurrent increase in oceanic carbon dioxide) (Figure 3). The kill mechanism associated with a reduction of temperature and an increase in oceanic CO: would be hypercapnia. Knoll et al. (1996) suggests that large carbon isotopic excursions, continental glaciations and anomalous carbonate precipitation characterize the late Neoproterozoic Era and that reoccurrence of these in the Late Permian would suggest that a similar environmental framework must exist for both the late Neoproterozoic and the Late Permian. This environmental framework would include the overturn of an anoxic ocean, fostering the enrichment of surface waters with carbon dioxide that was previosuly sequestered in the deep ocean. This enrichment would lead to hypercapnia. or the poisoning of organisms by carbon dioxide. In this hypothesis by Knoll et al. (1996), deep-water enriched in carbon dioxide would periodically flood the shelf and poison the shallow water fauna. While 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3: Model showing how carbon dioxide draw down can lead to oceanic stratification and global cooling. Overturn following stratification releases carbon dioxide to the atmosphere, and warming ensues (modified from Knoll et al.. 1996). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A Stratified Ocean CO, * MCO,' Sulfate reduction «co3 ‘ M * s co, co, Glacier C ° 2 M C 03 ' Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. there does exist evidence for high levels of oceanic carbon dioxide during the Early Triassic (Woods, et al., 1999). the climatic trend seen in the Late Permian and Early Triassic is not consistent with a global drop in temperature. Most evidence during this time period indicates a warming trend (Dobrunski. 1987; Veevers et al.. 1994; Smith. 1995; Retallack. 1995; Retallack et al., 1996; Retallack, 1996). As discussed previously, there is much evidence for global warming during this time period, however, it is still widely debated as to whether or not this warming was detrimental enough to bring about a mass extinction of the magnitude seen during the end-Permian extinction event. Warm temperate paleosols are reported from Antarctica and Australia (Retallack, 1996) and the Karoo Basin shows a changeover from a humid temperate climate to a more arid climate in the Early Triassic (Smith, 1995). There is also little evidence for ice during this time, with an absence of dropstones noted by Worsely et al (1994). Major warming could have brought about the changes reflected in the land flora. It is unclear, however, how this would relate to the marine extinction. The floral evidence for a rise in temperature is compelling. The end-Permian extinction event represents a time when there is a sharp changeover in floras from the cold- adapted glossopterids to warmer temperate vegetation (Dobruskina. 1987; Veevers t 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. al.. 1994; Retallack et al.. 1996). The “coal gap” that characterizes the Early Triassic has been cited by Retallack et al., (1996) to reflect the extinction of peat-forming floras. This extinction may also be recorded in the drop in 8l3 Corg that characterizes the Early Triassic. Early Triassic vegetation was dominated by seed ferns, conifers and lycopsids and this is believed by some to illustrate the low productivity that followed the end-Permian extinction (Retallack et al.. 1996). The mechanism for global warming from the end of the Permian into the Triassic has been debated. It is now believed that elevated levels of CO: may have caused a global increase in temperature (Erwin, 1993). The eruption of the Siberian Traps was first cited by Brandner (1988) as a possible source of elevated carbon dioxide levels during the end of the Permian to the Early Triassic. The eruptions of the Siberian Traps may not be responsible for the negative shift in 5I3 C (Erwin. 1993) but may have significantly contributed to an increase in carbon dioxide in the atmosphere. The overall biotic effects of elevating global temperatures are still unknown, but the kill mechanism associated with the theory of global warming involves the destabilization of ecosystems in response to the warming. Retallack et al. (2001) have cited recent work on stomatal densities as evidence for low levels of atmospheric carbon dioxide during the Permian (Figure 4). 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4: Graph showing stomatal density indices through geologic time. Peak occurrences are in the Permian and Neogene, and correspond with cool global climates (modified from Retallack et al.. 2001). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Using stomatal data for the past 300 million years, the concentrations of atmospheric carbon dioxide can be estimated. With this data set, two intervals of low atmospheric carbon dioxide emerge: one in the Neogene that coincides with glaciations and one during the Early Permian. Retallack et al. (2001) warn against using carbon isotopes as a carbon dioxide proxy because this data can be compromised by the release of isotopically light methane. In addition, increases in methane, water vapor, and carbon dioxide to the atmosphere can increase temperatures, which may indicate why a warming trend followed into the Early Triassic after a cool global climate in the Permian. Evidence for marine anoxia has also been found in several sections globally during the transition from the end of the Permian into the Triassic. Isozaki (1997) reports that from sections in Japan and British Columbia, pelagic cherts record a “wide deep-sea anoxic event” across the Permo-Triassic boundary (Figure 5). The redox conditions seen in these sediments illustrate an anoxic, stratified ocean that existed for 20 million years. Carbonaceous and siliceous claystones contain abundant pyrite, and sulfur and rare-earth element levels support anoxic deposition during the latest Permian and Early Triassic. This stratified ocean may have brought about the demise of many marine organisms, and could ultimately account for the seemingly 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5: Stratigraphic column showing a superanoxic ocean occurring during the Early Triassic (modified from Isozaki. 1997). L 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. i ! i l l ii * ; s i ! 2 * 1Ml & « I c • U i I 2 5 1 M i i s , I o iModei Tor Anoxic Ocean Rea one cnen Grey * * * " * * ” i Silidom “ > 1 aaystorw 2 Si o C V A 2 ......... 1 o lA n a u c P T B U l 3 | CeiBoneceoue § oeysione -S £ JL rV) aeynone IM a t G rey a n o u c en e n Heooiuc enen I 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. preferential survival of dysaerobic benthic groups and nektic taxa (Hallam and Wignall. 1997). Volcanic winter associated with the Siberian Trap eruptions (the largest flood basalts ever erupted, Renne et al., 1995) could have played a role in the extinctions at the end of the Permian (Figure 6). Campbell et al. (1992) cited five reasons as to how the Siberian Traps could have prolonged darkness, and destabilized ecosystems. These include: I ) the Siberian Traps are the largest subaerial eruptions of the entire Phanerozoic: 2) they were likely sulfur-rich eruptions making them extremely noxious: 3) S 0 2 levels may have increased as the lava flowed over anhydrite-rich rocks: 4) they contain a high amount of tuff deposits implying large volumes of ash were being dispelled during the eruptions: and 5) the tuffs contain high amounts of rock fragments. For all of these reasons, Campbell et al., (1992) determined that these flood basalts were unusually explosive, and could have discharged large amounts of ash and debris into the atmosphere. This volcanic winter could have caused a crash in primary productivity that has been noted during the Permo-Triassic boundary interval. 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6: Row chart showing volcanic winter as a possible cause of the end-Permian extinction event. The eruption of the Siberian traps would release carbon dioxide which would explain rise in global temperature, as well as cause a productivity collapse. The occurrence of ensuing acid rain would raise the strontium isotope ratios (modified from Hallam and Wignall. 1997). 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Emission of CO 2 N tlia liv r swin|> o f A *Jl' v a lu e s l — “■ (ilu h a l u a r n i in ^ Eruption of Siberian Traps " 1 ~ lunksion of S()2 and dust | Acid rain | (VOLCANIC WINTER HYPOTHESIS) Global darkness, short glaciation Productivity collap.se Regression = Z I ----- . * * - * -1 | MASS EXTINCTION I Elevated rates of continental weathering Increased Sr isotope ratios 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Another hypothesis in this extinction study involves the evidence supporting extraterrestrial impacts during this time. Recent work by Becker et al. (2001) has determined that certain fullerenes found near Permian-Triassic boundary sections contained noble gas ratios that were consistent with an extraterrestrial origin. The fullerenes sampled by Becker et al. (2001) encapsulate extraterrestrial gases in their molecular structure, and these gases have ratios that are similar to meteorites and comets, but dissimilar to those found in Earth’s atmosphere. Because of these notable differences. Becker et al. (2001) hypothesized that these fullerenes were deposited during an extraterrestrial impact event that coincided with the end-Permian extinction event. Kaiho et al. (2001) found that a gigantic sulfur release may have coincided with the end-Permian extinction, and hypothesize that it may be related to a bolide impact. While the work by Becker et al. (2001) suggests that these fullerenes were sampled from Permian-Triassic boundary sections, recent work by Isozaki (2001) implies that these fullerenes were erroneously sampled, and thus do not accurately reflect a Permian-Triassic boundary phenomenon. Though the abrupt extinction of terrestrial flora and the rapid decline in phytoplankton productivity are consistent with an extraterrestrial impact, there are 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. other ways to bring about this collapse in the biosphere. A bolide impact may have coincided with the Permo-Triassic boundary, however, its direct relationship with the extinction has yet to be explained. A plethora of evidence exists for a variety of mechanisms that may have brought about the largest extinction in the history of life. Some have accepted that perhaps a combination of environmental factors best explains the widespread demise of oceanic and terrestrial ecosystems during the end of the Permian (Erwin, 1993) (Figure 7). Evidence for increases in global temperatures, marine anoxia, and elevated levels of carbon dioxide during this period are accepted by most Permo- Triassic workers, but what the relationship is with any one of these occurrences and the mass extinction is still unclear. It is possible that the increase in global temperature, the occurrence of global marine anoxia and the elevated levels of carbon dioxide might better explain the slowed recovery following this extinction than providing a single kill mechanism for the extinction event. Twitchett et al. (2001) recently tied the marine collapse with the terrestrial collapse during the end-Permian extinction event. Sections studied in Greenland show this synchronous collapse, with a decline in marine organisms and a coeval fungal spike that is thought to represent the mass mortality of arboraceous 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 7: Map showing global paleogeography of Early Triassic continents (modified from Hailam and Wignall, 1997). Arrow indicates general location of studied sections. 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. P A N G A E A 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vegetation. This research is the first to tie the occurrences on land and in the sea, and shows that declines in both systems occur over a similar stratigraphic interval. While no one extinction mechanism can yet be touted as the indisputable cause of the end- Permian extinction event, it is now clear the both marine and terrestrial sections were affected on similar time scales. THE END-PERMIAN EXTINCTION: BOUNDARY SECTIONS AND ISOTOPES Global Boundary Sections The timing and extent of this end-Permian extinction event has been studied at many sections around the world. Some of the most complete and best known sections come from Greenland. Italy (Figure 8), South China (type section) and Japan. These sections provide insight into different seaways and have produced a global picture of the world’s oceans from the end of the Permian and into the Triassic. The section in Greenland was deposited in a basinal area from the Boreal Ocean (Stemmerik et al., 1992). Because of its northerly extent, it has provided a unique look at high latitude conditions during this time interval. The stratigraphy of the Upper Permian and Lower Triassic consists of two formations. These include the 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 8: Lower Triassic stratigraphic column of the Tesero Road Section in Italy (modified from Wignall et al.. 1996). 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TESERO ROAD SECTION U m o m u s bedding planes finch laminated raicritc . m • »• C O MWPG K ey .•I uoids a * TonminXm 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7057 Foldvik Creek Formation and the Wordie Creek Formation (Grasmuck and Trumpy, 1969; Teichert and Kummel, 1976). Dating is poorly constrained in these units, but it has been recently reported based on the presence of the conodont Hindeodas parvus (Twitchett, et al.. 2001) that the boundary must exist within the first 24 m of the Wordie Creek Formation (Figure 9). The section in Greenland is unusual because it seems the Late Permian faunas outlived their counterparts in other oceans suggesting that the late Maokouan event may not have affected the Boreal Ocean fauna (Hallam and Wignall, 1997). In Italy, the Permian-Triassic interval is found in the Dolomites and includes the Permian Bellerophon Formation, the Tesero Oolite, and finally the overlying Mazzin Member. The Bellerophon Formation consists of silty dolomicrites. a supratidal flat facies and wackestones containing abundant Changxingian marine faunas (Hallam and Wignall. 1997). The Tesero Oolite represents a deepening sequence, in which the oolite beds thin into the Mazzin Member, which is dominated by micritic limestone (Hallam and Wignall, 1997). The Mazzin Member is dominated by such facies as flat-pebble conglomerates, microgastropod grainstones, and stromatolitic beds and is believed to represent a relatively shallow water dysoxic to anoxic environment of deposition (Hallam and Wignall 1997). 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 9: Stratigraphic column of the Upper Permian Schuchert dai Formation and Lower Triassic Wordie Creek Formation in Greenland (modified from Twitchett et al.. 2001). 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■ C o £ z 0 % Z x a sr x ut X o tu “ 7. 2 ; 2 a t 4 K n oij« M I K 1 0 1 4 IU ■ s 5 • * m cipM l « lOJ 36- 3 2 - 30- 26- 20- 1 6 - 10 - i s -30 -25 -20 5 ' W 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sections in South China have also been studied intensely, with the stratotype section located at Meishan in the Zhejiang Province (Figure 10). During the time of deposition. South China was located near the equator in the eastern region of the Tethys seaway. The stratigraphy of South China Permo-Triassic sections is dominated by carbonate deposition in a variety of environments including basinal, shelf, ramp and platform environments (Yang and Li. 1992). In the condensed Meishan section, abundant Changxingian faunas occur up to within 20 cm of smectitic white ash that is 5 cm thick, at which point this fauna abruptly disappears (Hallam and Wignall, 1997). The smectitic ash layer is bounded by laminae with pyrite and trace-metal enrichment suggesting anoxic condition during deposition (Hallam and Wignall, 1997). Beds deposited after the last bed containing Permian holdovers show the changeover from carbonates to a more marl- dominated succession (Hallam and Wignall. 1997). The sections in Japan (Figure 11) are part of an accretionary wedge melange representing pelagic deposition from the Carboniferous to the Early Jurassic (Musashino. 1993). These sections contain the deep-sea record of the Permian- Triassic boundary in the Tethys Sea. In this succession. Upper Permian red cherts turn to gray as they lose their hematite. This change is followed by deposition of 33 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 10: Stratigraphic reconstruction for the Permo-Triassic basins of South China (modified from Hallam and Wignall. L997). 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Stratigraphy o f South Chiua Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Figure 11: Stratigraphic column showing both superanoxia and prolonged deep- anoxia (modified from Isozaki, L994). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. a i . . . i Paleozoic Peintian & ... j Mesozoic Tiiassic ca, 5-6 m. y. < > Middle Late u. Eaily Middle Mil £ w w i C ap g ul I I W atli gl (*•'* ro Gi | Ol g | Sm j s p n £ Alls 2 Lad absense ol tads. Superanoxia absense ol cheil 13 m. y. ca. 20 m.y. deep-sea anoxia I _____ u > -4 ^ light gray siliceous shales with high pyrite content (Musashino, 1993). Finally, black laminated shales with high organic carbon content (Isozaki, 1994) overlie the siliceous light gray shales. The overlying succession is a mirror image of the underlying sections. This section has been interpreted to represent deep-sea anoxia (Isozaki, 1994. 1997). The red radiolarian chert deposition did not resume until into the Anisian, leading Isozaki (1994. 1997) to his conclusions about a superanoxic event. Geochemical Evidence Carbon, oxygen, sulphur and strontium isotopes have been studied as proxies for climate change, anoxia, and global weathering rates as well as chemostratigraphy. There has also been an effort to determine if there was an iridium spike associated with the end-Permian extinction (Sun et al.. 1984), which would lend support for the extraterrestrial impact theory. Carbon isotopes from the end of the Permian into the Triassic have been extremely well-studied (Figure 12). Marine carbonate and organic 8l3 C isotopes show a minor positive peak towards the end of the Permian followed by a prolonged negative shift into the Triassic (Baud, et al.. 1989: Holser et al.. 1991). This record has been duplicated in terrestrial sections (Thackeray et al.. 1990) from preserved 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 12: Carbon isotope curve from both carbonate and kerogen samples of the Late Permian and Early Triassic. Both show extreme negative shifts at the boundary between the Permian and Triassic (modified from Hallam and Wignail. 1997). Carbon isotopic values are taken from Holser et al. (199 L ) and Wang et al. (1994). 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A n i s l a n I S ^ C k e ro g e n G artnerkofel only / .3 2 - 3 0 & J 1 1 1 1 S c y t h i a n C h a n g x i n g W u j i a p i n g . M a o k o u a n 5 13C carbonate Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. organic carbon. Intense sampling across the Permian-Triassic boundary in many sections has elucidated a more complex carbon curve with two large negative spikes occurring at the Meishan section with a magnitude of 8%c (Holseret al., 1991; Xu and Yan 1993). While some of the spikes may be diagenetic in origin, some do represent a primary signal, and this reflects the complexity of the record during this time. Exposure of coals, collapse of primary productivity, methane release and oceanic stratification have each been described as causes for the carbon curve. One of the proposed causes of the shifts described here involve the exposure of coals during the Permian regression (Holser and Margaritz, 1987; Baud et al.. 1989; Holser et al.. 1991). However, some evidence has shown a discrepancy between the timing of the regression and the negative carbon shift. Collapse of primary productivity has also been touted as a possible cause of the shift seen in the carbon curve (Margaritz et al.. 1992) however this, too. has not yielded any conclusive results. Research by Retallack et al. (2001) suggests that the role of methane as a source of isotopicaily light carbon can also affect the interpretations of carbon isotope curves. The negative shift in carbon isotopic values followed by a positive return may even reflect a 41 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. transition from a mixed oxic ocean to a stratified ocean (Hallam and Wignail, 1997), so it is clear that the complexities of this record keep it from being entirely resolved. Oxygen isotopes have also been useful in determining what occurred from the transition of the Permian into the Triassic (Figure 13). The SlsO also shows a trend similar to the 8I3 C record. There is an initial positive excursion followed by a major negative peak in the earliest Triassic. Because oxygen isotopes tend to be less affected than carbon isotopes by fluctuations in the ocean-atmosphere system, a negative shift of l%c is viewed as quite dramatic (Hallam and Wignail, 1997). Some possible sources of this shift may be a major increase in global temperature (Hallam and Wignail. 1997) or decreased evaporation of seawater (Holser. et al.. 1991). Similar to the problems in the carbon isotopic record, this record too is unresolved. Sulphur isotopes have been used during the Permian-Triassic transition to determine if anoxic conditions could have led to widespread pyrite burial (Figure 14). The sulphate curve shows a negative shift during the Late Permian with a positive shift in the Early Triassic. This rapid rise in sulfate isotopes indicates precipitation and burial of massive amounts of pyrite, however, some point out that the magnitude of this event would call upon impossibly large amounts of pyrite to be buried during this time (Holser, 1977: Claypool et ai., 1980). This record has been 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 13: Oxygen isotope values of 8lsO from the Permian into the Triassic. Values follow a similar trend to the carbon isotopes, and become very negative near the boundary (modified from Hallam and Wignail, L997). Oxygen isotope values taken from Holser etal. (1991). 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -7 - 5 - 3 -1 5 1 8 0 carbonate Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 14: Sulfur isotope curve from the Permian to the Triassic. Positive shift occurs from the Late Permian to the Late Triassic (modified from Hallam and Wignail. 1997). Sulfur isotope values from Claypool et al. (1980) and Kramm and Wedepohl (1991). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27 25 23 13 15 17 5^Sjulph*lt Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. used as evidence for widespread oceanic anoxia because the measurements of both oxygen isotopes and sulphate isotopes are believed to support this (Claypool et al., 1980). Strontium isotopes have been studied (Figure 15) to determine the extent of global weathering during this time.. This curve has been widely accepted as being the result of increased global weathering during the end-Permian and into the Triassic (Holser and Margaritz, 1987,1992; Erwin, 1993; Martin and Macdougall. 1995). The strontium curve shows a decline in the Permian and a rapid rise in the Early Triassic (Martin and Macdougall, 1995). While most agree weathering had increased, the mechanism for this increase is still debated. Some have postulated that global regression and erosion of land area is the culprit (Holser and Margaritz, 1998. 1992) while others believe that enhanced chemical weathering caused by an increase in atmospheric carbon dioxide is the source of weathering (Erwin. 1993; Martin and Macdougall. 1995). The sea-level curve does not support the former theory, however, since the peak in strontium corresponds to a widespread transgression. 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 15: Strontium isotopes from the Late Permian to the Middle Triassic. Positive shift begins in the Late Permian (modified from Hallam and Wignail, 1997). Stronium isotope values from Martin and MacDougall (1995). 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Anisian Scythian Gnuxczytukl ttmi.l 1992) W ujiaping. M aokouan 0.7085 0.7075 0.7065 8 7 S r /8 6 S r Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. EXTINCTION OF ORGANISMS FOLLOWING THE END-PERMIAN EVENT Nearly ail marine ecosystems from nearshore to deeper sea environments were affected by this extinction event. The extinctions that occurred during this time interval eventually led to the restructuring of marine ecosystems, with the rise of dominance of bivalves, and the changeover in reef-building corals from rugose and tabulate corals to scleractinians. While the causes of this extinction clearly remain debated as discussed above, the effects are clear. Metazoan reefs and reef-building organisms were dominant parts of shelf environments in the Permian. Following the end-Permian event, reef systems were devastated. For 7-8 million years following this extinction, the interval has been called a 'reef gap' (Fagerstrom. 1987), marking the disappearance of reef building metazoans during the Early Triassic. Metazoan reef communities did not reappear until the Middle Triassic as patch reefs (Hallam and Wignail, 1997). The history of metazoan reefs during this time shows a total extinction during the end-Permian event, an absence for several million years, and then a rapid radiation in the Middle Triassic. 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Rugose and tabulate corals suffer such a devastating extinction that it marks their all time demise (Fedorowski. 1989; Ezaki, 1994). The disappearance of rugose corals has been suggested by some to represent a gradual decrease throughout the end of the Permian (Fedorowski. 1989). Tabulates similarly underwent a decline during the end of the Permian, and only a few survived until the end of the Changxingian (Fedorowski, 1989). Corals did not make a true come back until the middle of the Triassic (Stanley, 2001). Bryozoans suffered an extinction that was slightly less severe than the corals (Ross, 1995). The Cystoporata. Trepostomata, Fenestrata. and Cryptostomata suffered severe generic level extinctions during the Late Permian (Taylor and Larwood, 1988). The timing of these extinctions has been debated and the radiation of bryozoans following the end-Permian extinction was slow (Sakagami. 1985). The Early Triassic is a time of very low diversity of bryozoans, with a radiation in the Middle and Late Triassic (Sakagami, 1985). Echinoderms. too, suffered severe diversity decline during the end-Permian extinction. Only one genus of cladid crinoids survived into the Mesozoic (and radiated as the Subclass Articulata (Paul. 1988; Simms and Sevastopulo. 1993; Schubert et al., 1992)). The echinoid genus Miocidaris survived and radiated to form 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. all of the echinoids known from the Mesozoic to today (Kier, 1984; Erwin, 1994). Blastoids went entirely extinct at this time interval (Simms et al., 1993). Crinoids are a common fossil found in Lower Triassic strata, however, their size is reduced relative to their Permian predecessors (Schubert et al., 1992). Brachiopods were severely affected by this event, and the end-Permian extinctions represent a time of reorganization of ecosystems during which brachiopods diminish in dominance and bivalves become the most prevalent normal marine fauna in Early Triassic oceans (Hallam and Wignail, 1997). In the Changxingian. brachiopods suffered their greatest extinction with 90% of the families and 95% of the genera becoming extinct (Carlson, 1991; Erwin. 1994). In the Triassic. a radiation of brachiopods is not seen until the Anisian (Dagys. 1993). Bivalves are affected by this extinction, but overall they fare much better than most groups, and become the basis of the most cosmopolitan assemblage in the Early Triassic (Hallam and Wignail, 1997). Only three families went extinct at the Permian-Triassic boundary (Yin. 1985) and others suffered “pseudo-extinctions”. These resulted from taxonomic biases that led researchers to think some bivalves had gone extinct when in fact their names had simply been changed (Nakazawa and Runnegar, 1973). In the Early Triassic however there are four common genera seen 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in almost all paleoenvironmental settings. These include Claraia, Eiunorphotis, Unionites, and Promyalina (Hallam and Wignail, L997). These dominate in the Early Triassic until the Spathian, where they suffer a decline. Marine chordates seem to suffer the least during the end-Permian extinction event. Fish suffered little to no extinction (Schaeffer, 1973: Patterson and Smith. 1987). An apparent radiation occurred in the Scythian during a time when most groups were barely surviving much less radiating. Some conodont species disappear around the boundary, but this too is followed by a Scythian radiation (Clark. 1987). Similar to the chordates. nautiloids suffer little during the Permian-Triassic extinction (Teichert. 1990). although they do become rare fossils in the Scythian. Some have attributed the preferential survival of marine chordates and other nektonic organisms to their life modes and roles as marine predators (Hallam and Wignail. 1997). The higher trophic levels seem to better weather the end-Permian event. 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RECOVERY FROM THE END-PERMIAN EXTINCTION EVENT Some of the most dynamic periods of evolution in ecosystems occur during the recuperation from mass extinctions. The recovery interval following the end- Permian extinction event is unusual because it seems to take organisms an unusually long time to recuperate from this extinction. Estimates would suggest that ecosystems did not recover for 7-8 million years perhaps because of the persistence of environmental stresses throughout the Early Triassic (Hallam, 1991). Studies of the Early Triassic recovery interval have focused on the trace fossil record (Twitchett. 1999). paleoecological community changes (Schubert and Bottjer. 1995) opportunistic species (Schubert and Bottjer. 1994: Rodland and Bottjer 2001: Fraiser, 2000), disaster taxa that proliferated during the Early Triassic (Schubert and Bottjer, 1992) and environmental fluctuations potentially related to the end-Permian extinction event (Woods, et ai.,1999). Changes in the ichnology from the Permian into the Triassic have been documented from sections in Italy (Twitchett. 1999) (Figure 16) and show decreases 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 16: Diagram showing reduction in burrow diameter following the end- Permian extinction event from the Dolomites, Italy. X-axis shows members in the Werfen Formation, with n equal to the number of burrows measured (modified from Twitchett, 1999). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 1 Permian Triassic E ra Q 5 o 3 CQ C unptl V4I Buou Cciuxni^hc S. Lucaih> 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in diversity and ichnofabric index. Four stages of the “Repopulation Interval” are outlined by Twitchett (1999) and show a rapid decline in burrow diameter across the boundary, which has been attributed to low oxygen conditions. Small burrow diameter, low ichnofabric indices and low diversity of traces (presence of only Planolites) in the basal Griesbachian sections of the Werfen Formation in Italy thus reflect anoxic conditions that may be related to the extinction event (Twitchett, 1999). The second stage reviewed by Twitchett (1999) shows a general amelioration of environmental stresses reflected in the changes in ichnogenera from the basal Griesbachian. In the upper Griesbachian and lower Dienerian strata, such ichnogenera as Lockeia, Skolithos, Arentcolites and Diplocriterion reappear. The absence of common shallow water traces such as Rhizocorallium and Thalassinoides show that the environment has not achieved pre-extinction diversity, but does mark a recovery from early post-extinction diversity levels. The subsequent two stages first show a brief decrease in diversity of trace fossils, and then a resurgence to pre-extinction diversity levels and burrow diameter. This resurgence characterizes the fourth and final stage of the recovery as outlined by Twtichett (1999). The ichnology begins to achieve pre-extinction diversity, size 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and amount of bioturbation by the Spathian. These fluctuations seen throughout the Early Triassic support the idea that continued environmental stresses inhibited the full recovery of ecosystems for several million years after the end-Permian extinction. Work on the Early Triassic of the western United States by Schubert and Bottjer (1995). has shown that the recovery interval from the end-Permian extinction event is characterized by opportunistic species that proliferated during this recovery interval. They also determined that there was a change in community structure during the Early Triassic. Recent work by Rodland and Bottjer (2001) and Fraiser (2000) have buttressed this research by close examination of two opportunistic organisms that were common during the Early Triassic. This research found that both Lingula and microgastropods act as “disaster taxa” (Fischer and Arthur, 1977): organisms that can thrive following intervals of severe environmental stresses, or “disasters”. The assemblages of the Early Triassic have long been characterized as being low diversity and cosmopolitan (Hallam and Wignail, 1997) and this has also been attributed to the devastation of ecosystems during the end-Permian extinction. Schubert and Bottjer (1995) examined the marine invertebrate species preserved in Lower Triassic strata of the western United States and found that the 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. community structure was dominated by low-diversity, Iow-complexity opportunistic generalists. These organisms include such things as the inarticulate brachiopod Lingula and the paper pecten Claraia. The Nammalian is characterized by mostly molluscan faunas with the addition of several new faunas in the Spathian. However, a closer comparison of Spathian paleocommunities with those from the Permian still demonstrates a low guild diversity and low species richness. This may imply that ecosystems of the late Early Triassic in the western United States have still not attained pre-extinction levels of diversity and complexity. A closer look at the inarticulate brachiopod Lingula was done by Rodland and Bottjer (2001) who found that long-ranging opportunistic generalists have a window of opportunity that follows mass extinctions. The temporary removal of other organisms by extinction would allow newly available ecospace to be exploited by generalists. An investigation of the potential taphonomic bias reiterated that the widespread presence of Lingula in Griesbachian strata of the western United States reflects an actual dominance of this organism during the interval following the extinction event, not a preservational effect. Recent work by Fraiser (2000) has shown that microgastropods also act as opportunists during this time interval. Microgastropods and bivalves are the 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. dominant fauna in Nammaiian strata of the western United States. These organisms are commonly the dominant rock-forming clast, and are believed to represent generalists similar to Lingula. Such microgastropods satisfy a key requirement of biological opportunists in that they are not facies restricted. These organisms were able to thrive in a variety of environments in the Early Triassic while others were excluded entirely. The predominance of the generalist Lingula and of microgastropods in the Early Triassic has indicated to the authors that there must be a length of time needed for the origination or spread of specialized taxa following this extinction event. This again may be linked to the persistence of environmental stresses that precludes specialized taxa while creating ecospace for the proliferation of generalists. The very slow recovery of specialized organisms also supports this idea. The presence of disaster taxa during the Early Triassic was documented by Schubert and Bottjer (1992). The disaster form described by these workers were the stromatolites found in Spathian strata of the western United States. These stromatolites are found in the Virgin Limestone Member of the Moenkopi Formation and represent an unusual resurgence of stromatolites in normal marine environments. Because stromatolites have been all but excluded from normal marine environments 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. since the Ordovician, their occurrence during this time interval indicated that these represent disaster forms that could thrive in the face of mass extinctions. The occurrence of stromatolites in the latest Early Triassic indicates the extended length of time in which disaster forms were present. The authors attribute the occurrence of stromatolites during this time interval to be related to the absence of grazers following the devastating extinction of metazoans. Recent work by Pruss and Bottjer (2001) has shown that the persistence of disaster forms into the late Early Triassic would indicate that continued environmental stresses may have played a large role in inhibiting metazoans and would allow for the proliferation of stromatolites in normal marine environments. More evidence for continued environmental stresses throughout the Early Triassic has been put forth by Woods et al. (1999) in their study of basinal synsedimentary carbonate cements (Figure 17). These cements have been documented from the Union Wash Formation in east-central California, and are the outer shelf-basinal equivalents of the Virgin Limestone. The occurrence of these cements reflects direct precipitation of calcium carbonate on the seafloor. Cements of these types were last seen in the Proterozoic when ocean values of carbon dioxide were oversaturated relative to Phanerozoic levels (Knoll et al., 1996). 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 17: Diagram showing a mechanism for the precipitation of synsedimentary cements in Early Triassic oceans. Carbon dioxide degassing could be a source for hypercapnia (modified from Woods et al., 1999). 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Organic Matter CO^ degassing l Potential Source o f Carbon Dioxide Poisoining) «2S S u i t a t e R eduction S y n ssd im sn tary C em ents Organic Matter O aposition Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. These cements indicate high values of carbon dioxide in offshore environments of the Early Triassic. Such values could indicate offshore anoxic waters that are supersaturated in carbon dioxide, and therefore fostered the precipitation of these cements. Hallam (1991) first put forth the concept that anoxic waters periodically impinging on the shelf could have inhibited the recovery of metazoans and thus have been the driving force behind the slow recovery interval in the Early Triassic. These cements provide the first conclusive evidence that unusual ocean chemistry was present in the offshore even until the late Early Triassic, and that these environmental stresses may be intimately linked to the prolonged convalescence of marine metazoans. 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE STRATIGRAPHY OF THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION, SOUTHERN NEVADA The Moenkopi Formation of the Western United States is Triassic in age is thought to have been deposited on a westward dipping shelf (Blakely, 1974). This formation was deposited in a mixed siliciclastic-carbonate environment, and in Nevada and Utah, it lies unconformably on the Upper Permian Kaibab Formation (Shorb. 1983). The Moenkopi extends through Colorado, Nevada and into Utah, where the youngest transgressive tongue is called the Sinbad Limestone (Reif. 1978) The Spathian Virgin Limestone Member of the Moenkopi Formation, which crops out in Nevada and Utah, has been studied extensively (Figure 18). It represents a transgressive systems tract that was deposited during the Spathian, or late Early Triassic (Pauli et al.. 1989). In the field, the Virgin Limestone Member is composed largely of limestone and dolomite ledges, with intervals of sandstones and siltstones (Figure 19). This unit represents a period of subsidence in addition to local sea level rise (Pauli and Pauli. 1986), and thus the variations from section to section most directly reflect the balance between sea level, tectonism and clastic input. 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 18: Map of tectonic activity of the Western United States during the Early Triassic. Areas of Virgin Limestone deposition are starred (modified from Shorb. 1983). 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ID A H O UTAH *Mueow vmlimy Uswilwtft NEVADA Ca l if o r n ia lake Mud * S p r i n g ARIZONA Vugui Limcsuiuc present at these localities Leading edge or Early TrosscGoiconda Thrust Ajos of Lower Triassic □eposoonal Basin Southermost extent or Lower Triassic basmal facies of Carr and Pauli 11983) Eastern and Soutnem Limit of Moenkopi deposition Moenkopi Source area - Triassic outcrops m study area 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 19: Outcrop of Virgin Limestone at Muddy Mountains locality showin interbedded siliciclastics and carbonates. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Virgin Limestone Member was deposited as a transgressive tongue from the Panthailasa seaway on a westward dipping ramp during the Early Triassic (Blakely, 1974). This member was deposited in a mixed siliciciastic-carbonate setting (Shorb, 1983), and amount of carbonate increases to the north and west (Reif and Slatt. L979) (Figure 20). This may be related to an increased distance from the sediment supply from the southern Mogollon Highland and the western Uncompahgre Highland and Defiance Uplift (Blakely. 1974). It is largely composed of limestone, dolomitic limestone, calcareous mudstone, siltstone dolomite and minor chert (Shorb. 1983). The Virgin Limestone Member weathers a characteristic brownish- yellow. and tends to outcrop in ledges with covered intervals of clastic material. The Virgin Limestone is rich in fossil material that is mostly composed of echinoderm debris, bivalves, gastropods, ostracods. and brachiopods (Shorb. 1983). Reif and Slatt (1979) studied the clastic red beds of the Moenkopi Formation. The Lower, Middle and Upper Red Members represented times of increased sediment supply and were deposited along a muddy tidal flat. A clastic sediment source was determined to exist in the south and east. These members interfinger with the Virgin Limestone, and were most likely more prominent in areas close to the clastic sources. 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 20: Map of Nevada showing Lincoln and Clark counties. Meadow Valley Mountains was the location of the Vigo locality and the Lost Cabin Springs locality was in the Spring Mountains. 71 Reproduced with permission of ,he copyright owner Further reproduction prohibited without permission 1 Meadow Valley Mountains \ * 2 Muddy Mountains \ \ « 3 Lost Cabin Springs ' v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. These Red Bed Members of the Moenkopi Formation were deposited in a muddy tidal flat. The Moenkopi Formation represents a gradual transition from continental fluvial sedimentation to the east through intertidal and subtidal deposition in the west. The interfingering nature of the Red Beds and Virgin Limestone Member are believed by the authors to represent small changes in sea level and rate of subsidence and sedimentation along this tectonically stable gentle slope. Previous structural work on these sections have shown some displacement during tectonics at the close of the Triassic (Marzolf. 1994). Mesozoic thrust faults and Cenozoic strike slip faults also cause displacement of earlier deposited Early Triassic strata of the Moenkopi Formation, southern Nevada (Marzolf. 1984). Marzolf (1990) summarized the early Mesozoic sedimentary basins along the Colorado Plateau, and found a thickening of the Virgin Limestone Member from east to west, showing a location of paleoshoreline to the east. M ethods The primary purpose of this research was to establish an onshore-offshore transect along which biofabric and fauna! changes could be documented. A better understanding of the biofabric and faunal trends from the onshore to the offshore 73 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. could reveal information about conditions in the deep-sea and the impact this may have on shelf communities. To establish this onshore-offshore transect of the Virgin Limestone Member of the Moenkopi Formation, it was necessary to do a paleoenvironmental analysis of the three localities. Muddy Mountains, Vigo, and Lost Cabin Springs (see Appendix), using stratigraphic correlation, sedimentary structures and facies analysis. While geographically the sections at Muddy Mountains, Vigo, and Lost Cabin Springs have been displaced, their paleoenvironments can still be ascertained using stratigraphic correlation, sedimentary structures, and facies analysis. In accounting for the structural displacement previously described by Marzolf (1990. 1994), Muddy Mountains was closest to the paleoshoreline and was thus most influenced by siliciclastic input. Vigo and Lost Cabin Springs to the north and west of Muddy Mountains were deposited in more open marine environments away from the paleoshoreline. As a result, these localities contain more subtidal facies such as skeletal limestones interbedded with carbonate mudrocks whereas Muddy Mountains is consistent with a more intertidal facies. Tepee structures, a structure thought to originate by desiccation of carbonates results in preferential growth of sparry 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. cements within fractures (Demicco and Hardie, 1994), occur only at Muddy Mountains. Stratigraphic correlation provides a useful framework to compare the three localities. Two correlative surfaces were used for correlation between the three localities. The first was the basal flooding surface where the Virgin Limestone Member is in contact with the Lower Red Member. The second bed used for correlation is called the Promyalina bed. While this bed is not identical at all three localities, it was a unit that occurred at all three localities and was similar in thickness (-0.5 m), fossil content, and valve orientation. For the purpose of this research, both beds were used for correlation. The two beds provided a bracket with which to compare the three localities. In using stratigraphic correlation, some trends emerged. Muddy Mountains had four beds containing ooids and Vigo had three oolitic beds. No ooid beds were found at Lost Cabin Springs. Similarly, several grainstones were found at Muddy Mountains (-18% total section) and some were found at Vigo (-15% total section), however Lost Cabin Springs (Schubert. 1989) had fewer grainstones (<10% of the total section), and they were significantly thinner than at the other two localities. The average grainstone thickness at Lost Cabin Springs was 0.5 m while at Vigo and 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Muddy Mountains, ledges were 5-LO m in thickness. Lost Cabin Springs has the most muddy micritic units of the three localities, also indicating that it was deposited in a more offshore setting. Based on this trend, the grainstones, representing higher energy environments, point to Muddy Mountains as being deposited in the environment of highest energy. Vigo was slightly more subtidal, and Lost Cabin Springs was the most offshore section of the three. Sedimentary structures also provide useful tools for assessing paleoenvironmental conditions. Since the biota of the Early Triassic is characterized largely by cosmopolitan, opportunistic species, biological components have become a less reliable tool for ascertaining paleoenvironments during this time. In this recovery interval from the end-Permian mass extinction, more uniformitarianistic criteria such as physical sedimentary structures become important for elucidating depositional systems. Several sedimentary structures were documented from each of the three localities. Interference ripple marks were found in three beds at Muddy Mountains, and there were no beds with asymmetrical ripples. At Vigo and Lost Cabin Springs, interference ripple marks were far less common, and seen only in some siliciclastic 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. talus. At Lost Cabin Springs, asymmetrical ripple marks were preserved in the carbonates. Ripple marks are frequently used as indicators of paleoenvironments since certain ripples are more common in come snvironments than others(Klein, 1970). One such model shows the formation of interference ripple marks, similar to those found at Muddy Mountains, can result from the ebb and flow of tides. Because of this, interference ripples are generally associated with shallow intertidal deposition (Klein, 1970). Other types of ripples, such as the asymmetrical ripples of Lost Cabin Springs, can be used to reconstruct current direction. Such ripple marks may characterize deeper settings, and can show current action (Harms, 1979). Tidal cross-stratification was found in two beds at Muddy Mountains, while there was only rare cross-stratification in siliciclastic talus seen at Vigo and Lost Cabin Springs. At Lost Cabin Springs, one carbonate bed had lateral cross stratification, but was not tidal in origin. Tepee structures were only found at Muddy Mountains. Oolitic beds were also the most common at Muddy Mountains. Eight beds containing abundant oolites were found in the whole section at Muddy Mountains, while only four total beds were oolitic at Vigo. At Lost Cabin Springs, no beds were found to be oolitic (Schubert. 1989). 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Based on stratigraphic correlation, sedimentary structures and bed analysis. Muddy Mountains was found to be the shallowest locality with the most evidence for an intertidal paleoenvironment. While Vigo and Lost Cabin Springs were similar. Lost Cabin Springs was found to be the deepest of the three, representing a middle shelf subtidal paleoenvironment. It was the only locality with asymmetrical ripples showing below normal wave base current action, and had no significant oolitic component. Based on the sedimentary structures and analysis here, each locality represents a slightly different paleoenvironment. The model for the deposition Virgin Limestone Member of the Moenkopi Formation invokes a fluctuating trangressive tongue that is controlled by sea level and subsidence changes. During periods of high sedimentation, the Red Bed Members of the Moenkopi Formation were deposited (Reif and Slatt, 1979). The Virgin Limestone Member instead represents a subsidence event or a reduction in sedimentation that allows for the deposition of carbonates. Muddy Mountains The section at Muddy Mountains was measured and sampled using stratigraphy from Shorb (1983) (Figure 21 and Figure 22). This section was sampled between ShorbT s (1983) Unit 7 (Figure 23) to Unit 19, and totaled over 200 m in 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2I:Statigraphic column measured from the Muddy Mountains locality. Rock type, fossil content, and sedimentary structures are all noted on the column and described in the key. 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Middy Mountains 200 m r ~ 7 / / VMAAAAMAAM 3Fr Z r V n ? r v ^ \ / r \ 150 m / III 1 r',77r, * - m r m T \ r\/r\/r\/r\/r\ u IrJ rlrJ rJ T /7777fflI7]TW /7fM '\ 100 m olrynlr^VY^vrt A ■ A» lABflM A JA 1 5 0 , 1 U i i i i n r i fl n L 4 1 ( r 1 ? n L _ f ^ 1 ^ n 1 rv t 5 0 m V M A M A A A A A M * ^ * _ n r\ it r\ it ~ 5 r\ fTr\ B Om 2 L J 5 I J A ? M A ? l u m a K ey b ivalves b gastropods A crinoios v ostracods 4 * r cross-stratification ooids * u-diaped b urrow s b horizontal b urrow s rppis m arics — • u n fcssiiiferou s mudstone —1 - sly mudstone / / dolomite I 1 limestone • chert wavy kunnadxis j s . stromatolites o t h e r p o s s f c b mi r obi i l f a b r i c s Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 22: Outcrop of Virgin Limestone at Muddy Mountains. The Virgin Limestone is the thick First ledge above the talus slope on the far hills. Sections are overturned at this locality, so Upper Permian Kaibab Formation is seen above the Virgin Limestone here. 81 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 23: Ripple marks from Muddy Mountains Unit 7. These ripple marks are symmetrical, showing shallow water intereference ripple activity. Field knife is 9 cm long for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thickness. From each bed sampled, a 15 cm thick interval was sampled in five bulk sampling bags. An equivalent amount was taken from each bed. At times, it was necessary to sample from various beds within a larger unit. Oriented slabs were also taken for trace fossil and sedimentary structure analysis. The basalmost unit sampled was Unit 7. This was a gray-brown siltstone with interference ripple marks. Above that. Unit 8 (Figure 24) was a thick resistant ledge containing abundant bivalve fossils, identified as Promyalina. These fossils were oriented concave down, and are discussed in more detail in a subsequent chapter. Unit 9 (Figure 25)was a thick limestone ledge with abundant bioturbation. Some of the trace fossils have been identified as possible Planolites, Diplocraterion. and Arenicolites traces. Unit 10 is a thick limestone ledge with apparent flat-pebble conglomerate clasts at the base, and a bivalve and gastropod rich fossil assemblage at the top. Another section sampled within Unit 10 was a yellow silty limestone (Figure 26) that contained gutter casts (Figure 27) and abundant Rhyzocorallium traces. Unit 11 and 12 were covered, and thus not sampled. Unit 13 (Figure 28) was sampled as well because it was the next limestone ledge in outcrop. The section sampled within this unit was a thick fossiliferous limestone ledge. Unit 16 was similar in composition to Unit 13, but contained silicified crinoid and other 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 24: Muddy Mountains Unit 8 Promyalina bed showing disarticulated. concave-down oriented valves. Geopetal structures show that valves were deposited in the preferred hydrodynamical position. U.S. 25 cent piece (1.5 cm diameter) is used for scale. 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 25: Muddy Mountains Unit 9a is pictured here as the light colored unit (left) and Unit 9b is pictured as the darker unit (right). Both Unit 9a and Unit 9b show abundant Planolites. Arenicolites. and Diplocraterion burrows. Reid knife (9 cm) is used for scale. 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 26: Muddy Mountains Unit 10 is pictured here. Rhizocorallium traces are seen here as U-shaped burrows. Field knife (9 cm) is used for scale. 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 27: Muddy Mountains Unit 10 gutter cast is pictured here. Gutter casts accompanied the same horizons as the bioturbation in Unit 10, and indicate storm activity. U.S. 25 cent piece for scale ( L.5 cm in diameter). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 28: Muddy Mountains Unit 13 is pictured here with abundant crinoid ossicles from Holocrimts smithi. Crinoid ossicles are also found on bedding planes with disarticulated echinoid spines, and are more common in the upper half of this stratigraphic section. Reid knife (9 cm) is for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. echinoderm debris. Unit 19 was sampled last, and this unit was unique in that it contained the largest horizontal traces (>l cm thick) at this site, as well as the most abundant echinoderm debris. Both crinoid and echinoid fossils were found in this unit. Based on the stratigraphic correlation, sedimentary structures and facies analyses conducted at this site, it was determined that strata at Muddy Mountains were most likely deposited in a shallow subtidal to intertidal environment. The interference ripple marks and trace fossil assemblages also indicate a shallow water environment. Based on these sedimentary criteria. Muddy Mountains was found to contain the shallowest paleoenvironmental indications of the three localities. Vigo Vigo was the next locality that was sampled for this research (Figure 29 and Figure 30). Again Shorb’s (1983) stratigraphic framework was employed. Vigo was much thicker than Muddy Mountains, with up to 550 m of section exposed. At this locality, the Virgin Limestone Member grades into the lower portion of the Shnabkaib Member of the Moenkopi Formation, and so a delineation between these two members is not made. Because of this, the total thickness of the Virgin Limestone Member at this locality is closer to 250 meters in thickness (Shorb. 1983). 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 29: Field photo of the Vigo Locality, with the Virgin Limestone outcropping behind the central hill. 9 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Figure 30: Stratigraphic column measured from the Vigo locality. Rock type, fossil content, and sedimentary structures are noted on the column and described in the key. Dotted line shows location where Virgin Limestone grades into Shnaibkaib Member. Sections with question marks indicate covered intervals that were not sampled. 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vigo 400 m 5 .c M a 05 \ m m m T r _ L c 350 m 300 ra 250 m 200 in | • T I 150 m z f z 7 ~ ./, / & W S : 100 m 50 m 0 m ‘n i t w P h i ■ a a t S k m a Y r^ rx A * ^ C rZ l L , Key /'’x bivalves & gastropods ^ crinoids 9- ostracods 4M T cross-stratification — * ooids * u-shaped burrows b horizontal burrows vam ripple marks — - unfossffierous mudstone — * silty mudstone / / - dolomite I I limestone • chert r~\J~' wavy lam inations stromatolites o t h e r p o s s f c k m i u r u b i a l t a b r i a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The sampling at Vigo was similar to Muddy Mountains with both bulk and oriented samples taken. Sampling occurred from Shorb’s (1983) Unit I to Unit 11. Unit I (Figure 31) was the basal most bed overlying the exposed Lower Red Member. This limestone unit was unusual because it alternated between heavily bioturbated limestones (Figure 32) (ichnofabric index of 5; Droser and Bottjer. 1986) to limestones with microbial? wavy laminations. These laminations were examined carefully, and most likely represent the signature of microbial activity. Both beds were sampled. Unit 2 (Figure 33) was a brown resistant dolomite ledge with bioturbation (ichnofabric index 4-5) and crinoid debris. Unit 3 was a thick gray limestone bed in which an ammonite was found. This bed had thick calcite veins protruding through it. Unit 4 was covered and inaccessible. Unit 5 was a thick limestone ledge with abundant bivalves. Unit 6 was a thick limestone ledge with characteristic black staining. This had abundant crinoidal debris. Unit 7 (Figure 34) marks the part of the sequence where lithotypes become more consistent with the Shnabkaib Member, so sampling only proceeded to Unit 11. Unit 7 was the overlying thick limestone ledge above Unit 6 with abundant pods of chert. Unit 8 was a mixed carbonate-siliciclastic unit with some shell beds. 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 31: Field photo of Vigo Units 1 and 2 in outcrop. Unit I outcrops as the basal most ledge, and Unit 2 outcrops towards the top of the hill. Sections in between are talus slopes. 1 0 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 32: Bioturbation in Vigo Unit I. Bedding is almost entirely disturbed* showing an ichnofabric index of 4 or 5. Horizontal bioturbation is prominent although discrete trace fossils are difficult to elucidate. These sections are interbedded between the wavy lamination sections of Unit 1 . 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 33: Bioturbation pictured from Vigo Unit 2. Horizontal traces are the predominant traces as in Unit I. Field knife (9 cm) is for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 34: Vigo Unit 7 is pictured here. This unit outcrops as a thick limestone ledge. with pods of secondary black chert. Crinoids are the predominant fossils seen in outcrop. Field knife (9 cm) for scale. 1 0 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Unit 9 was largely covered, and not sampled. Unit 10 was comprised of two different sections. The underlying section was heavily bioturbated (Figure 35) (ichnofabric index of 4-5) with few identifiable traces. Some Planolites traces were observed. Overlying this unit was a 0.5 m thick densely packed Promyalina bed (Figure 36) that closely resembled Unit 8 from Muddy Mountains. Unit 11 (Figure 37) was the next thick gray limestone ledge that had slump structures and gastropod remains. Based on stratigraphic correlation, sedimentary structures facies composition, it was determined that Vigo was deposited in a more subtidal environment. It had fewer grainstones than Muddy Mountains and less siliciclastic deposition. While many trace fossils were difficult to identify, the ichnofabric indices tended to be higher than Muddy Mountains. The traces were largely horizontal, also indicating a more offshore paleoenvironment than Muddy Mountains (see Chapter 5: Methods). Lost Cabin Springs Lost Cabin Springs (Figure 38 and Figure 39) was the final section studied to form the previously discussed onshore-offshore transect. The upper part (50 m) of the section was measured and fit into a stratigraphic framework previously established by Schubert (1989). The units in this section were labeled for delineation but are not correlative with units at Muddy Mountains or Vigo. 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 35: Vigo Unit 10a showing horizontal bioturbation. Discrete trace fossils are difficult to identify in outcrop. Planolites burrows were seen in talus. Field knife (9 cm) for scale. I l l Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 36: Vigo Unit L O b pictured here with abundant Promyalina valves. Valves are oriented in the hydrodynamically stable position (concave down) evidenced by the abundant geopetal structures. Valves that are weathering out of the limestone are silicified. Field knife (9 cm) for scale. 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 37: Vigo Unit 11 showing bioturbation and wavy soft-sediment deformation. Upper part of picture shows horizontal bioturbation, but discrete traces were not seen in outcrop. Field knife (9 cm) for scale. LIS Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 38: Stratigraphic column measured at Lost Cabin Springs locality. This section is correlative to the upper 50 m of Schubert’s (1989) section. Rock type, fossil content and sedimentary structures are noted on the column and described in the key. Stromatolites are shown in the basalmost section (Unit I). 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lost Cabin Springs 40m r\ i r \ i t i A ■M~r\ A J L cl r\ [ _ r\ m OLA 1 1. f . l i i H S I I T I M S I L T I L S I 2 0 m b\ j r v ^ f i r u r v b * w * K ey b iv a lv es gastropods crinoids ostracods cross-stratification ooids u-sh aped b u r r o w s hnromial b u r r o w s tppie m ar ks unfcssifcxujs mudstone sky mudstone dolomite limestone chert O^Owavy la m in a ti o n s 4 = \ stromatolites o t h e r p o s d t e t n k x c b a l f a b r i c s -Birtial Stratigraphic section (upper portion) •Total Section thickness = 160m 118 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119 Figure 39: Virgin Limestone outcrop at Lost Cabin Springs. Limestone ledges outcrop in mid- to upper-left part of the picture, with covered intervals in between. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 119 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Unit 1 (Figure 40) was the basalmost exposed bed, and was a i m thick bed of stromatolites (Figure 41). This is the smaller of the two stromatolite beds with the larger bed exposed lower in the section. In research conducted later, the larger stromatolite bed was also reinvestigated. The stromatolites were domal in shape and at times coalesced to form bioherms. They are exposed at all places where the bed is exposed. Unit 2 was a thick limestone that weathered brown and was highly stylotitized. This unit outcropped as a thick ledge over the smaller stromatolite bed. Unit 3 was a thick gray limestone with cross-bedding and some fabrics that looked microbial in origin. Unit 4 was a thin resistant limestone ledge with some horizontal bioturbation preserved on the top of the bed. Unit 5 was a heavily bioturbated unit with some bivalve shell beds. This unit weathered such that identification of individual traces was difficult. Unit 6 was a thick resistant bivalve shell bed with some horizontal bioturbation preserved. Some siliciclastic units are present between units 4 and 6, but were not found in place. 121 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 40: Unit L at Lost Cabin Springs locality. Pictured here is a small stromatolite mound in the basal part of this unit. This mound has a clotted, thrombolitic fabric made more obvious by weathering. Rock hammer is 27.5 cm long for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 123 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 41: Picture of side of stromatolite mound from Lost Cabin Springs Unit 1 . Underlying fine micritic beds are seen here, and mound shows some layering. Rock hammer (27.5 cm) is for scale. 124 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Unit 7 (Figure 42) was a thick limestone ledge with some microbial features. These features include wavy laminations however there are no fully developed mounds. Units 8-10 made up a thick resistant ledge that outcropped at the top of the hill. They were bioturbated, and Unit 10 had pods of chert. There were also extensive Thalassinoides burrows at the top of Unit 10. Unit 11 (Figure 43) was a thick limestone ledge with bivalves that may be preserved in a storm bed. Unit 12 was a storm bed that graded from rip-up clasts to bivalve to crinoidal debris. Unit 13 was similar in composition to unit 12. Unit 14 was also a storm bed with abundant crinoids. Unit 15 had some basal horizontal bioturbation (Figure 44) with an overlying thick Promyalina bed with similar orientations to those found at Muddy Mountains and Vigo. At the top of Unit 15. the section is heavily bioturbated with an ichnofabric index of 5 or 6. Based on sedimentary structure analysis, stratigraphic correlation, and facies analyses (see Chapter 5). a middle shelf paleoenvironment was ascertained for the Virgin Limestone Member at Lost Cabin Springs. Abundant horizontal traces with high ichnofabric indices and lesser components of siliciclastic material relative to Muddy Mountains or Vigo implied that this section was the deepest of the 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 42: Lost Cabin Springs Unit 7 pictured here. Talus piece shows wavy microbial laminations. Laminations are present, but domes only occur in the basal most unit (Unit I) at this locality. Rock hammer (27.5 cm) is for scale. 127 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 2 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 43: Lost Cabin Springs Unit 11 showing bedding plane of Thallassinoides burrows. Burrows appear to be infilled, and branching, and cover entire bedding plane. Rock hammer (27.5 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 44: Lost Cabin Springs Unit 15 bedding plane showing Ttiallassinoides burrows. Burrows are infilled and branching, but are less densely packed than those pictured in Unit 11. Rock hammer (27.5 cm) used for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 132 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. transgressive tongue. Siliciclastics are more dominant up-section as an increase in covered intervals is seen in outcrop. The lowermost part of this section is composed of thick limestone units with very few covered intervals showing a predominance of limestone production relative to siliciclastic deposition. This section is correlative to the uppermost 50 m in Schubert (1989). Further work at Lost Cabin Springs has focused on a close location on beds lower than those sampled here. In addition to the research described, another study focusing on the Lost Cabin Springs stromatolites in a lower part of the Virgin was also part of this research (Figure 45). The stromatolites (Figure 46) were reinvestigated to determine their paleoenvironmentai setting, their composition, and whether they attained significant relief off of the seafloor and could be called true reefs. For this research, three stromatolite "mounds” were measured, photographed and sampled regularly. Stromatolites "mounds” in this study are defined as the large agglomerations of smaller “domes” (Schubert. 1989: Eliding, 2000) that weather in outcrop as individual units. These mounds were at times individual elements but also occasionally coalesced with other nearby mounds. The individual domess weathered out as subunits of the larger mounds, and ranged in size from about 10 cm to 0.5 m in thickness. A sketch of the mounds and the surrounding stratigraphy was done, and 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 45: Second Lost Cabin Locality first sampled by Schubert (1989). Pictured here is a large stromatolite dome sampled for the stromatolite study. Mound measures approximately 2.5 m from top to bottom. Underlying beds are deformed, showing cohesive nature of the mound upon compaction. Mound is an agglomeration of smaller subunits or domes. 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 135 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 46: Close-up photo of stromatolite mound. Smaller domes can be seen with wavy laminations. These smaller domes were sampled for slab and thin-section study. Rock hammer (27.5 cm) for scale. 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. both the underlying and overlying beds were sampled, described and photographed. Finally, measurements were made of the height and width of each of the mounds. Thin-sections were taken of the smaller domes sampled within the larger mounds. Three distinct fabrics were noted in thin-section. The first fabric was stromatolitic laminae. These laminae could be seen on some of the sections. The majority of the two sections were made of a micritic fabric and a microbial fabric. The muddy micritic fabric contained abundant metazoan debris. Identifiable pieces of ostracods, bivalves, and gastropods could be seen in thin-section. The microbial fabric made up the majority of the sections and contained drapes of the muddy micritic fabric that was washed in by current or storm activity. 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ONSHORE-OFFSHORE TRENDS DOCUMENTED FROM THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION: TRACE FOSSILS To present the trace fossil data in an accepted format, these traces were categorized by their “ichnofacies”. The ichnofacies concept was first introduced by Seilacher (1964) and later refined by Crimes (1975), Frey and Seilacher (1980), Ekdale et al„ (1984). and McKinney (1991). An “ichnofacies” means that different marine paleoenvironments have characteristic traces. A rocky shoreline paleoenvironment is called the “Trypanites Ichnofacies” because traces like Trypanites are often found there and would have very different traces than a “Skolithos Ichnofacies”. or semi-consolidated intertidal zone. In this way. traces can be useful paleoenvironmental indicators, especially when ichnofossils are treated as an assemblage. Using several different traces from a particular stratigraphic interval thus gives the best paleoenvironmental reconstruction. The trace fossil assemblages varied in assemblage content and ichnofabric index from the onshore Muddy Mountains section to the more offshore Vigo section to the middle shelfal section at Lost Cabin Springs. These variations reflect the 139 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. diversity and type of bioturbation expected at different paleoenvironments (Figure 47). In general, intertidal sections tend to be characterized by the Skolithos ichnofacies (Seilacher, 1964; Crimes, 1975), which means the majority of the traces are simple vertical burrows. The more offshore, or subtidal to middle shelf environments, tend to be characterized by the Cruziana ichnofacies (Crimes, 1975). implying most traces are horizontal, with fewer vertical burrows. Methods At Muddy Mountains, traces were collected from Units 8.9, 10 and 19 (Shorb. 1983). Unit 8 was heavily bioturbated at the base by horizontal burrows that were badly weathered. They could not be identified, but covered about 40% of the bedding plane. From Unit 9. two horizons were sampled (Figure 48 and Figure 49). The first bed (9 A), was dominated by Arerticolites and Paleophycus. The ichnofabric index of these beds ranged from 4-5. and covered about 25-30% of the bedding plane. The second horizon on Unit 9 (9B) contained Planolites, Rhizocorallium, and Arenicolites The ichnofabric index was 2 and the bedding plane coverage was <10%. Unit 10 (Figure 50) was also sampled, and the traces on these beds were Rhizocorallium (Figure 51) and Planolites. The ichnofabric index was 2-3 and the bedding plane coverage was 10%. 140 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 47: Diagram showing ichnofacies from onshore to offshore environments. Vertical traces tend to be dominant in more nearshore environments while horizontal traces are prevalent in sublittoral to abyssal environments (modified from McKinney, 1991). 141 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. m m l l « Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 48: Field photo showing Unit 9 at Muddy Mountains. Planolites burrows are the horizontal traces. The U-shaped burrows are typical of Arenicolites. which is also seen on this bedding plane. Bedding plane coverage for the darker beds is 25-30% while for the lighter beds is closer to 10%. U.S. 25 cent piece (1.5 cm) for scale. 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 144 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 49: Unit 9 at Muddy Mountains showing Paleophxcus burrows. Bedding plane coverage is 25-30% and ichnofabric index is close to 4. Weathering gives burrows a jagged look in taius. Field knife (9 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 146 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 50: Unit 10 from Muddy Mountains showing Rhizocoralliurn traces as the U- shaped burrows. A gutter cast is also running through the center of the photo. U.S. 25 cent piece (1.5 cm) for scale. 147 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 148 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 51: Rhizocorallium traces taken in talus from Unit 10 at Muddy Mountains. U-shaped burrows show infill typical of Rhizocorallium. U.S. 10 cent piece (0.5 cm in diameter) for scale. 149 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 150 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Unit 10 horizontal traces (Figure 52) had an ichnofabric index of 4. Traces in Unit 10 were identifiable (Figure 53) in talus as Planolites and these beds had an ichnofabric index of 3. The bedding plane coverage was <10%. Unit 11 was also sampled, but had unidentifiable horizontal traces. The ichnofabric index of this bed was 3-4. While the ichnofabric indices were clearly increasing from the more onshore section at Muddy Mountains, the vertical tracemakers were not abundant at Vigo. The only identifiable trace was Planolites. but all other traces were similarly Many of the traces found at Muddy Mountains were vertical traces such as Arenicolites and Rhizocorallium. These types of traces tend to occur in shallow water settings, and are less common in more offshore environments (McKinney. 1991). The majority of the beds had an ichnofabric index of 2. with only one interval with an ichnofabric index of 4-5. These low ichnofabric indices may be reflecting that vertical bioturbation at this time had not yet achieved pre-extinction levels. The trace fossil assemblage at the Vigo locality was also studied. Traces were sampled from 4 beds, and bedding plane percentages were only measured when possible. Units 1.2, 10 and 11 (Shorb. 1983) were sampled in this study. Unit 1 was heavily bioturbated in parts with an ichnofabric index of 4-5. The horizontal burrows resembled Planolites but were difficult to identify. In Unit 2. unidentifiable 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 52: Hand sample of Unit 2 from Vigo. Horizontal traces are seen as darker patches, and there is some echinoderm skeletal debris. Ichnofabric index of Unit 2 was 4. U.S. one cent piece (1 cm diameter) for scale. 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. E ~ 153 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 53: Talus slab taken from Unit 10 at Vigo. Traces are horizontal Planolites burrows, and are difficult to see in outcrop. Bedding plane coverage is <10%. U.S. 10 cent piece (0.5 cm) for scale. 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. horizontal in nature. This is consistent with the Cruziana ichnofacies (Crimes, 1975), which is found in subtidal environments. The trace fossil assemblages are consistent with the sedimentological data that indicates Vigo was deposited in a more offshore environment relative to the Muddy Mountains section. Trace fossils were also sampled from the Lost Cabin Springs locality. Identification of these traces was extremely difficult because of weathering. Six beds including Units 5,6,7,9.11 and 15 were sampled, and ichnofabric indices were determined in the field. Unit 5 was heavily bioturbated by horizontal burrows. No bedding planes were found, so both identification and bedding plane coverage analysis could not be completed. This unit had an ichnofabric index of 3-4. Unit 6 was burrowed at the top. The ichnofabric index of this unit ranged from 4-5, and the predominant trace was Planolites that covered 10% of the bedding plane. Units 7 and 9 are similar units that had been horizontally bioturbated with an ichnofabric index of 3-4. There were no bedding plane analyses, and no identification of the traces. Unit 11 contained a bedding plane that was dominated by Thalassinoides (Figure 54) burrows. These covered about 10-25% of the bedding plane, and the bed had an ichnofabric index of L-2. The last unit sampled was Unit 15 (Figure 55) that 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 54: Bedding plane of Thallassinoides burrows from Unit 11 at Lost Cabin Springs. Burrows are both branching and infilled. Bedding plane coverage is 25-30%. Rock hammer (27.5 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 158 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 55: Unit 15 from Lost Cabin Springs showing infilled Thallassinoides burrows. Burrows are branching, and beds contain and ichnofabric index of 5. Rock hammer (27.5 cm) for scale. 159 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. had two horizons of bioturbation. The lower horizon had an ichnofabric index of 1-2, and the Planolites trace covered <10% of the bedding plane. The upper horizon was heavily bioturbated, and had an ichnofabric index of 5. These traces were horizontal, but unidentifiable. Based on the trace fossil assemblage of Lost Cabin Springs, this locality is also consistent with the Cruziana ichnofacies (Crimes. 1975). Many of the traces reported were horizontal in nature, and many beds had high ichnofabric indices. The trace fossils indicate that Lost Cabin Springs was deposited in subtidal environments similar to Vigo, however, the previously discussed sedimentological data suggests Lost Cabin Springs is the most offshore relative to Muddy Mountains and Vigo. In conclusion. Muddy Mountains was dominated by a relatively high diversity of traces, vertical bioturbation and low ichnofabric indices and was characterized as the Skolithos ichnofacies (Crimes. 1975). Vigo had a trace fossil assemblage consistent with the Cruziana ichnofacies (Crimes, 1975), with mostly horizontal traces such as Planolites (Figure 56). Lost Cabin Springs was also consistent with the Cruziana ichnofacies (Crimes. 1975), and had horizontal traces such as Planolites and Thalassinoides. The distribution of these traces (Figure 57) has paleoenvironmental significance. 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 56: Trace fossils from Schubert’s ( L989) section of Lost Cabin Springs. Traces are located in beds below the stromatolite mounds, and are simple Planolites horizontal burrows. Field knife (9 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 57: Beds underlying stromatolite mounds at Lost Cabin Springs showing ichnofabric index of 5. Horizontal traces are present, and in talus appear to be Planolites burrows. Field knife (9 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Results Based on a study conducted in the Dolomites of Italy, trace fossils reflect a slow recovery through the Early Triassic (Twitchett, 1995). The first initial assemblages after the end-Permian extinction were low diversity and consisted largely of very reduced simple Planolites burrows (Twitchett, 1995). The vertical traces were not present immediately following the extinction, and gradually recovered later in the Early Triassic. Based on this research, vertical bioturbation was far less prominent at all sections than horizontal bioturbation. The Muddy Mountains section, being deposited in a high energy shallow subtidal to intertidal environment, had some vertical traces, but had very low ichnofabric indices relative to Vigo and Lost Cabin Springs. The only bed where an ichnofabric index was higher than 3 was in a bed dominated by horizontal traces. Bioturbated beds at Vigo and Lost Cabin Springs were dominated by ichnofabric indices of 3 or greater, with only one bed having an ichnofabric index of I. This evidence illustrates that the depth and magnitude of vertical bioturbation had not recovered to pre-extinction levels by the Spathian. but that horizontal bioturbation was significantly able to disturb sediment by this time (Figure 58). 166 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 58: Chart showing ichnofabric index trends from the more onshore Muddy Mountains section to the more offshore Lost Cabin Springs section. Ichnofabric indices increase in the more offshore direction, with both Vigo and Lost Cabin Springs having the majority of beds with ichnofabric indices of 4 or greater. Muddy Mountains has only one bed with an ichnofabric index greater than 3. X-axis shows ichnofabric indices and Y-axis shows number of beds measured. 167 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C 0 0)4 Q . § “ 0 ^ ^ C O ■ 5 “ n ° = §>s 1 f 3 □ ■ B 1 6 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ONSHORE-OFFSHORE TRENDS DOCUMENTED FROM THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION: BIVALVES The Early Triassic bivalves make up a group that is both low in diversity and very cosmopolitan (Figure 59). They were predominant components of most assemblages during this time (Schubert and Bottjer, 1995), and ultimately experienced a turning point at the end-Permian extinction. In the Early Triassic, bivalves became dominant fossils in many assemblages (Schubert and Bottjer, 1995: Hallam and Wignall, 1997). To retrace the steps of the rise of bivalves to dominance in the Mesozoic, it becomes beneficial to understand their early recovery following the end-Permian extinction. Understanding the environmental and ecological constraints that enabled bivalves to survive and radiate in the Early Triassic will yield much information both on the evolution of bivalves as a group and also on the unique marine environment of the Early Triassic. Three Early Triassic sections in southern Nevada contain outcrops of the Spathian Virgin Limestone of the Moenkopi Formation. The bivalves of the Virgin Limestone were selected for study to attain paieobiological information about the recovery faunas of the Early Triassic. Similar to other Lower Triassic sections worldwide (China. Japan and Italy), bivalves are an integral part of most post- 169 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 59: Four genera of bivalves that are the most cosmopolitan in the Early Triassic. Pictured are the following: a) Claraia b) Eumorphotis c) Unionites d) Promyalina. Scale bar is approximately 2 cm (modifed from Hallam and Wignall. 1997). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. extinction assemblages in the Virgin Limestone (Schubert and Bottjer, 1995). A unique feature of the Virgin Limestone is the occurrence of beds dominated by the bivalve Promyalina. Study of the bivalves in these beds has yielded taphonomic, paleobiological and paleoenvironmental information, and may provide a framework for global assemblage comparisons. Methods Three localities of Lower Triassic Virgin Limestone (Spathian) were studied to look at the biotic recovery following the end-Permian mass extinction. While the primary focus of this research was to see changes in biofabric along an onshore- offshore transect, other beds were sampled to see how metazoans. including bivalves, were recovering during this time. Bivalves provide an interesting subject for study since it is after the end-Permian extinction that bivalves radiate, and take over as the dominant shelly benthos for the remainder of the Phanerozoic (Hailam and Wignail. 1997). To attain a better understanding of the significance of bivalves during this recovery interval, beds were sampled at the three localities previously mentioned. The focus of this study was to review the ubiquitous ”Promyalina beds” which outcropped at all three localities. This bed was unique in that it was similar in 172 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thickness and composition at all three localities. Promyalina bivalves were the dominant fauna, and bed thicknesses were similar. These beds were sampled within 0.5m of strata, and both bulk samples and oriented samples were taken. Photographs were taken, and field observations were made as a means of comparing the "Promyalina beds” along the previously described onshore-offshore transect. Muddy Mountains was the first locality investigated, and is considered to represent a fairly shallow paleoenvironment. The Promyalina bed (0.5m) in Unit 8 (Figure 60) at this locality has many unique features. It contains several geopetal (Figure 61 and Figure 62) structures with the valves oriented primarily concave (Figure 63) down. These valves also appear to be imbricated when slabbed in the lab, and the top of the bed is stylotized. Clasts composed of lime mud and some terrigenous material (red and tan silt clasts) are also imbricated. While thin sectioning of these beds has yielded specimens of gastropods (Shorb. 1983), the bivalve Promyalina is the main component of this fossil assemblage. In outcrop, this Promyalina bed appears as a thick resistant purplish ledge, probably due to the color of the matrix. The matrix is a purplish limestone with tan and reddish colored clasts. Original shell material was found on one valve and 173 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 60: Promyalina bed outcropping at Muddy Mountains. Shells are generally oriented concave down, and at times appear stacked. Concave down orientation is the most hydrodynamically stable position, and is indicative of storm deposition. U.S. 25 cent piece (1.5 cm) for scale. 174 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 175 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 61: Disaggregated sample collected from Unit 8 at Muddy Mountains. Half of Promyalina shell is pictured with geopetal structure. Original shell material has been replaced by calcite. Shell is approximately 4 cm from hinge to commissure 176 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 177 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 62: Disaggregated sample from Muddy Mountains Unit 7 showing half of a Promyalina shell. Valve is thick, and has been replaced by calcite. Some internal ornamentation can be seen. Shell is approximately 4 cm from valve to commissure. 178 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 63: Slabbed sample from Muddy Mountains Unit 7. Valves show geopetal structure, and concave down orientation. Valves are all disarticulated and replaced by calcite. Slab is 10 cm wide at the top. 180 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. original ornamentation could be seen. In this section, however, most valves are replaced. Vigo was included in this study and the Promyalina bed (0.5m) in Unit 10 (Figure 64) found here has many similarities to the Muddy Mountains locality. Geopetal structures are also common, and show an overall orientation of valves concave down. There are many more broken shell components at Vigo than at the Muddy Mountains locality. There also seems to be fewer lime mud clasts than at Muddy Mountains. Shell debris weakly imbricated, and there are some calcium carbonate mud clasts (fewer brown or red clasts than Muddy Mountains, matrix is mostly gray). In outcrop, shells weather out orange and are silicified. There is also some grading within this bed. Again. Promyalina is the dominant, and most identifiable fossil, but thinner shell components can be seen. Lost Cabin Springs is different than the other two localities, but this may be a function of diagenesis. The Promyalina bed here (Unit 15: >lm) outcrops within a larger ledge towards the uppermost part of the section. Most shells are replaced, and are oriented in a concave-down position. Geopetal structures were identified under the valves. No identifiable clasts are associated with these shells at this locality. The bed is severely stylolitized. and has undergone more diagenesis than the other two 182 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 64: Promyalina bed in outcrop from Vigo locality. Shells are silicified around the exterior, and weather out as brownish-black valves. Geopetal structures show original orientation as concave down, due to storm deposition. 183 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. localities. There may be other shells aside from Promyalina., but they are smaller components of the overall beds than the Promyalina fossils.sectioned. The unit contains less lime mud. and the matrix is a very fine gray carbonate micrite. One of the better specimens from the Lost Cabin Springs locality is an internal mold of a shell where all of the original shell material has been dissolved. This Promyalina assemblage is different in that it is part of a larger unit (Unit 15 at Lost Cabin Springs). It is underlain by a bed that consists of laminated carbonate, with an ichnofabric index of I or 2. and overlain by a bed that is thoroughly bioturbated (ichnofabric index 5). This assemblage does not weather out to form a ledge, but rather represents sort of a "lens" in a larger carbonate unit. Results To summarize the taphonomy of the Promyalina beds at all three localities, it seems that the main mechanism of deposition on these localities is storm deposition. These beds most likely make up proximal tempestites (Kidwell, 1988) with the valves that are disarticulated without evidence of significant abrasion. The beds contain some grading, and are mostly wackestone or packstones. They show cross- stratification and orientation of single valves. 185 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Low amounts of abrasion of the Promyalina shells suggest little time spent at the sediment-water interface (Flessa et al. 1993) which is not particularly surprising since many studies have shown that larger shells tend to be buried quickly. Lighter shells tend to be abraded at the sediment-water interface for awhile before burial. This can been seen with the broken fragments of thinner shells at the Vigo locality. The types of beds at each locality can be summarized in the following way: Muddy Mountains Promyalina bed- carbonate bivalve wackestone: Vigo Promyalina bed- carbonate bivalve packstone; and Lost Cabin Springs Promyalina bed- bivalve micrite. lime mudstone with some spar and shell material. The species of Promyalina in these beds can be subdivided into Promyalina putiatensis and Promyalina spathi (Ciriacks. 1963). P. putiatensis contains bivalves that show variation in morphologies but cannot be grouped into distinct separate groups because the varying forms occur in close association with each other. These show a highly variable species (Ciriacks. 1963). Hinge angles vary and some shells have a high obliquity, although less then the Permian counterparts of the genus Myalina (Ciriacks. 1963). P. spathi varies from the former because of its relatively straight anterior margin and that these shells are also somewhat smaller (Ciriacks. 1963). P. spathi 186 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. may be characteristic of at least some of the Muddy Mountains bivalves, but all assemblages seem to have a mix of the two species. Other species may also be present, but have yet to be identified. Early Triassic myalinids, represented by Promyalina, are generally smaller than their Permian counterparts. They are considered to be a holdover taxon (Ciriacks, 1963). Myalinids have their likely origin in the Early Devonian or Early Mississippian and range to the Early Jurassic, with some possible occurrences in the Late Jurassic (Cox et al.. 1969). Marine mytilids byssally attach through most, if not all. of their ontogeny (Cox et al.. 1969). The Promyalina are believed to be semi- infaunal endobyssate organisms that require semi-consolidated sediment to affix to (Cox et al.. 1969). The Order Pterioida. which contains the Family Myalinidae, has modem bivalves, but the myalinids are extinct after the Jurassic (Cox et al.. 1969). The Family Pteriidae evolved during the Triassic and some modem organisms in this family share a similar life mode to the myalinids (Cox. et al.. 1969) A thick, silty laminated bed underlies the Muddy Mountains Promyalina bed with ripple marks preserved as well as bivalve resting traces. This unit outcrops below the bivalve ledge, and has a distinctly different lithology. Above the Muddy 187 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mountains Promyalina bed is a flat pebble conglomerate with some bivalve shells, although they are rarer than in the underlying ledge. Vigo stratigraphy of the Promyalina bed shows similar shallow deposition. It is underlain by a thick bioturbated ledge with an ichnofabric index of 2-3. There are also gutters present in these beds, and the bioturbation is mostly horizontal (simple Planolites burrows). This unit is about 5.5m thick, and consisted of gray micrite, and the Promyalina bed outcrops as a capping ledge above this unit. Above the bivalve bed is Unit 11 (see figure 64) (Shorb, 1983). This consists of 5 limestone ledges that are interbedded with pale mudstones, and dolomitic and limestone ooliths. There is an increase in siliciclastics above the Promyalina bed, that has been documented in the previous methods section. Fossils include crinoids, bivalves and gastropods. There are some load structures in the basal most ledge, as well as bioturbation. The Lost Cabin Springs Promyalina bed is near the top of the measured section in Unit 15 (see Figure 30), and consists of a thick accumulation of bivalves between two different lithologic sections. It is underlain by laminated gray limestone with an ichnofabric index of 1-2, and it is overlain by heavily bioturbated units (ichnofabric index of 5). The Promyalina bed here may represent another type of storm deposit (Kidwell, 1988). 188 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Discussion There are distinct differences between the three localities (Figure 65), but there seems to be more similarities between them. The Promyalina beds at all three localities seem to represent tempestite deposition. While each locality was deposited on the shelf, the Virgin Limestone reflects the waxing and waning of an epicontinental seaway. The condition of the shells also provides some clues to deposition. The disarticulation of the bivalves implies some transport with little abrasion. This suggests relatively rapid burial event beds. Also the Promyalina beds are not monotaxic. Promyalina do. however, represent the dominant fauna. This may be more the function of hydrodynamics than a true reflection of the original faunal composition 189 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 65: Diagram summarizing onshore-offshore biofabric trends for the Virgin Limestone at Muddy Mountains, Vigo and Lost Cabin Springs localities. Vertical bioturbation is isolated to the shallowest paleoenvironment (Muddy Mountains) while the more offshore environments have microbial fabrics (Vigo) and microbial build-ups (Lost Cabin Springs). 190 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Southern Nevada FhHv 'lViassic (Spathian) Uhshore-OBshore California | Neva da Key Muddy Mountains stromatolites I wavy laminations > Vigo other pussibb tnkiubiul labiis $ harnonial burrows 3 Lost Cabin Springs w vertical burrows 19 L Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ONSHORE-OFFSHORE TRENDS DOCUMENTED FROM THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION: MICROBIAL FABRICS An assessment of microbial fabrics was made across the onshore-offshore transect described previously. From the most onshore section at Muddy Mountains to the most offshore section at Lost Cabin Springs, the carbonate beds were examined, and a trend emerged in the course of this study. In the carbonates of Muddy Mountains, there were no microbial sedimentary structures found in the carbonates. At Vigo, in the basal most limestone, some microbial wavy laminations were documented. These were studied in outcrop, and are believed to be primary signatures of microbial activity, not the result of slumping or other deformation. This was determined based on the occurrence of these wavy lamination beds between heavily bioturbated sections that do not show any slumping or deformation. At Lost Cabin Springs, the most offshore section of the three, stromatolites as well as other microbial fabrics were noted. These stromatolites occur as mounds in two horizons; one in the lower half of the section in a 2 m thick bed and one in the upper half of the section which is typically less than 1 meter thick. Several mounds 192 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. were measured in each of the beds, and the lower bed averages a mound thickness of about 2.5 m whereas the upper bed mounds average around 1 m from base to top. Previous Work Stromatolites were first documented in the Virgin Limestone Member of the Moenkopi Formation by Schubert and Bottjer (1992) who described these structures as disaster forms. These stromatolites have been characterized as having both stromatolitic and thrombolitic textures apparent in outcrop (Schubert, 1989). In this study, the stromatolites were studied in depth both in outcrop and in thin-section to further characterize them. Riding (2000) has attempted to disentangle the complex terminology used to describe microbial carbonates. He uses the following terms to describe microbial fabrics : stromatolites, thrombolites. dendrolites and leioloites. The first two, stromatolites and thrombolites, are most commonly used to describe carbonate build ups that are primarily formed by the agglutination of particles. The distinguishing features between stromatolites and thrombolites are the ways in which morphologies vary: stromatolites tend to be more layered accumulations whereas thrombolites tend to appear more clotted. 193 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In outcrop and thin-section, the microbial fabric of the Virgin Limestone appears in places to be a “coarse agglutinated thrombolite” (Riding, 2000). This means that coarse bioclastic sediment is incorporated into the fabric, and fabrics vary between stromatolitic to crudely blotchy to somewhat structureless. All of these fabric types can be on the domes in outcrop and in thin-section. and will be discussed further in this section. Not only are the morphological characteristics of these stromatolites important to study, but their ecological significance is also necessary to elucidate. Recent work by Grotzinger and Knoll (1999) has attempted to determine what the presence of stromatolites can most directly reveal about paleoenvironmental conditions. Since the Ordovician, stromatolite occurrences in normal marine environments have been largely reduced (Figure 66) due to increases in grazing and competition for space by metazoans (Awramik. 1971: Garrett. 1970: Walter and Heys. 1985). It has also been suggested that the decline in stromatolites may be linked to a global decrease in ocean water carbonate saturation levels (Grotzinger. 1990). The general understanding of stromatolites in the Phanerozoic is that they L94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 66: Top diagram shows the rise and fall of stromatolite development from the Archean through the Phanerozoic. There is a steady decline after the Proterozoic. Bottom chart shows number of occurrences of stromaotlites through the Phanerozoic, and of note is the unusual resurgence of stromatolites in the Early Triassic. with more occurrences of stromatolites from that time period than for any other of the Phanerozoic (modifed from Bottjer et al.. 1997). 195 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. suttA dO SN O iTistianii * « t n w S£N3ttfftB040£fln(lN 196 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TIME P E R IO D “are unusual sedimentary features, commonly indicative of restricted environments or mass extinction” (Grotzinger and Knoll, 1999 p. 352). Based on this, the microbial fabric trends seen in the Virgin Limestone represent an unexpected reverse gradient of microbial development. Since the Ordovician, stromatolites have largely been excluded from normal marine environments (Awramik, 1991: Bottjer. 1997), and this sudden resurgence of normal marine stromatolites during the Early Triassic is highly unusual. Furthermore, it is also somewhat unexpected that there is an absence of microbial development in the most nearshore (restricted) section with the majority of the development being in the most offshore section. In addition to the microbial development described here, there are possible microbialite crusts (Kershaw et al.. 1991) and microbial biostromes and mounds (Lehrmann et al., 1997) described from two localities in South China. These occurrences in combination with the microbial build-ups of the Western United States indicates that the occurrence of microbial fabrics in the Early Triassic is a global and tropical phenomenon. These occurrences have also illustrated that microbial development was not simply a boundary phenomenon, but rather extended as far as the late Early Triassic. 197 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Early Triassic carbonate deposition is limited to a narrow equatorial belt between Paleotethys and Neotethys and to narrow platforms along convergent margins of Western Pangea (Golonka and Ford, 2001). Not only were metazoan reefs dampened, but carbonate deposition as a whole seemed to be reduced during this time (Stanley, 200 L ). The change from icehouse to greenhouse conditions following the Permian-Triassic boundary is concurrent with low amounts of carbonate deposition in the Early Triassic. (Golonka and Ford. 2001). Kershaw et al. (1999) studied in detail the presence of the possible microbial crust (Figure 67) that occurred in the upper Changxing Formation, just underlying the Feixianguan Formation. The Changxing Formation is Upper Permian in age, and the uppermost section is composed of crinoidal limestones, whereas the Lower Triassic Feixianguan Formation is composed of depauperate layered micrite. The location of this microbialite crust corresponds with the end-Permian extinction event, and led the authors to believe that unusual ocean chemistry associated with this extinction may have contributed to the formation of the crust. They feel that some scenarios represent possibilities as to how this crust formed: l)the microbiota took advantage of topographic highs on reef tops: 2) the microbiota were a disaster biota after the end-Permian extinction; 3) the tnicrobia took advantage of high CO, content 198 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 67: Stratigraphic column from Upper Permian and Lower Triassic of South China, showing the development of a possible microbialite crust occurring synchronously with the end-Permian mass extinction. Occurrence of crust has been attributed to unusual oceanic conditions during the extinction event (modified from Kershaw et al., 1999). 199 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 0 iaoH| 160- 120H fC o v ef^ j c 0 £ w 0 U L 0) c X 0) c < 0 £ HO Crinotaai grainstone S Reet 8.01 7.0 - 6.0 - CD ■ r ' . — Z C f c x * . 5.0 - B Lam inated micrite and day partings 4.0 - 3.0-1 F F Oucoids in laminated $-*j micrite ^ M icrogastropods, ostracods, ‘microspheres’ in = 3 micrite with clay = 5 partings *T O fip Layering hidden by cover (?microbiaiite crust) Crinoidai limestone and dolomite 2 0 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to calcify; and 4) the crust was an inorganic crust precipitated from carbon dioxide rich waters (Kershaw et al., 1999). All of these suggested to the authors that unusual oceanic conditions may have been the source of this crust development. Lehrmann (1999) found biostromes and mounds in the Nanpanjiang Basin of South China in two horizons of Lower Triassic strata (Figure 68). The first horizon was located in Griesbachian strata and was composed of microbial biostromes. These biostromes in places occurred through 15 m of strata. In the Spathian strata, conical and inverted microbial mounds were found, and these are believed to have achieved some topographic relief off of the seafloor. These occurrences on the Great Bank of Guizhou in South China were also thought to be related to unusual Early Triassic paleoenvironments, (perhaps related to a decrease in predation pressure by metazoans) and may represent reef systems in the Early Triassic. The stromatolites of the Virgin Limestone Member of the Moenkopi Formation at the Lost Cabin Springs locality were investigated closely to determine their paleoenvironmental significance and also to assess if these microbial build-ups are related to the prolonged recovery from the end-Permian mass extinction. The occurrence of microbial fabrics was not a boundary phenomenon and happened 201 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 68: Diagram showing the Great Bank of Guizhou. South China during the Early Triassic. Carbonate platform has unusual build-ups of microbial biostromes and mounds in both the earliest Triassic strata and the late Early Triassic. and are bound on either side with ollitic shoals (modified from Lehrmann. 1999). 2 0 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Peiagic. Utne Muostone. Deoris-now Breccia ana Turotaue Grams tone -» .# Calcunicrooial K aunas Miirun Cycbc Peritiaai Limestone v contains Lonc noama navaaensisi Hm ospminaaus tunoamais (E.-M. Triassic Boundary) 2Mm Skeletal Pacxstone to Grams tone witn P aJ ao tuS uim a (Lalast Permian) CalcimicroDiat B le a fram es witn isaracaua issracm (Earnest Triassic) Siliceous Lutita witn Rat oai scot mf ms ana MaogongoMla cnm n g M in g m n si s (Latest Permian) 203 Reproduced with permission of the copyright owner Fi.rthn ner' FUrther reproducti°n prohibited without permission. globally. It is also significant that these build-ups are found between 30°S and 30°N. indicating that these may be isolated to tropical areas where carbonate deposition was occurring during the Early Triassic (Golonka and Ford, 2001). A closer investigation of these stromatolites may have implications for the unusual environmental conditions necessary to foster their growth, and may be linked to the prolonged recovery of metazoans throughout the Early Triassic. STROMATOLITES OF THE VIRGIN LIMESTONE MEMBER Methods To conduct a study of these stromatolites, a new field and thin-section study was undertaken. Some goals of this project were to take new field measurements of these “mounds” (Figure 69) (the previously described large agglomeration of smaller 10-50 cm "domes” that formed cohesive units) and investigate the surrounding stratigraphy to potentially ascertain the paleoenvironments in which these mounds formed. The mounds were sampled regularly (Figure 70 and Figure 71) by taking oriented samples of the smaller domes within the mounds and of the surrounding beds. The mounds (Figure 72) were sketched to record specifically where samples were taken, and measurements of both the mounds and underlying and overlying strata were taken. 204 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 69: Field photo showing stromatolite mounds in outcrop. Mounds range in height from 2-2.5 m, and are an agglomeration of smaller domes. The stromatolite bed has moundss in all places where it is not covered, and the mounds at times coalesce as pictured here. 205 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 206 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 70: Field photo showing sampling of stromatolite via David J. Bottjer for scale(~2 m). The dome pictured here was sampled for slab and thin- section study. 2 0 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 71: Field photo of stromatolite dome. David J. Bottjer for scale (~2 m). 209 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 1 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 72: Field photo showing large stromatolite mound. This mound consists of smaller subunits, or domes, which weather out individually. Underlying beds are somewhat deformed, indicating this mound was a cohesive unit upon compaction. Mound is approximately 2.5 m in height. 211 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 1 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thin sections were also taken from samples of these stromatolites. The samples were taken to the lab and slabbed to look at internal structures. From the slabbed samples, thin-sections were taken in places where more than one fabric was noticeable or where laminations were obvious (Figure 73). These thin-sections were then observed and described in detail. Field Analysis During field analysis, the bed studied was the lowermost stromatolite bed (Schubert. 1989) that had mounds ranging in thickness from 2-2.5 m. These stromatolites occur in all places where the bed is uncovered. The stromatolite mounds are an agglomeration of smaller domes. In places the mounds coalesce to form tabular biostromes and in other sections of the bed they occur as individual mounds. In outcrop, the mounds have both stromatolitic and thrombolitic fabrics. Some of the smaller domes have laminated surfaces and in other places have a more thrombolitic fabric. The domes vary in size and weather out as individual units suggesting that they were smaller cohesive domes forming larger mounds. They are made of dark gray limestone that is largely devoid of metazoan fossils in outcrop. 213 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 73: Diagram showing the labeled sections that were sampled for slab and thin-section study. This dome was regularly sampled to get an accurate analysis of the types of fabrics preserved in thin-section. Oriented samples were taken from each mound labeled here, and then slabbed and thin-sectioned. Rock hammer is 27.5 cm for scale. 2 1 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 1 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The surrounding beds were also studied and sampled to ascertain the paleoenvironmental conditions under which these stromatolites were formed. The underlying units (Figure 74 and Figure 75) consist of 30 cm of a crinoidal grainstone with some cross-stratification and some horizontal bioturbation (Figure 76) preserved on the top of the bed. Next is an overlying unit of fine micrite that has some original bedding but has been largely horizontally bioturbated. This bed has an ichnofabric index of 4. Weathering in this unit makes identification of discrete traces difficult, but they have been preliminarily identified as Planolites. Overlying this unit is a thick resistant limestone ledge that the stromatolite mounds (Figure 77 and Figure 78) commonly deform. The layer must have been somewhat consolidated before being deformed by the overlying mounds. The beds that surround the stromatolite mounds were also examined. The limestone ledges surrounding the stromatolites (Figure 79) have microbial fabrics that contain some laminations. The sediment that tends to infill the mounds (Figure 80) and that commonly onlaps them (Figure 81) consists of a very fine green lime mudstone. It is also noticeable that bed thicknesses are frequently different on either side of the domes in outcrop. This may imply differential deposition due to current 216 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 74: Field photo showing beds underlying stromatolite domes. Lowermost bed pictured is a 30 cm thick crinoidal limestone. Overlying the crinoidai limestone are heavily bioturbated tine micrite units, with an ichnofabric index of 4. Bioturbation is mostly horizontal, but has significantly disturbed bedding. The stromatolite domes outcrop above these bioturbated beds. 217 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 218 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 75: Field photo of bioturbated beds underlying stromatolite domes. Horizontal burrows are seen here, and these units have an ichnofabric index of 4. Field knife (9 cm) for scale. 219 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 2 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 76: Field photo showing bioturbated beds underlying stromatolite mounds. Pictured here are discrete Planolites traces. Bioturbation mostly consists of these horizontal traces. Burrows are simple and unbranching. Field knife (9 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 2 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 77: Field photo showing close-up of smaller dome sampled within larger mound. Dome is approximately 60 cm across. Stromatolitic laminations can be seen. This dome contained sections 2 and 4 for sampling. 223 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 2 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 78: Field photo of a dome weathering in outcrop. As dome weathers. stromatolitic laminations are more obvious. Layering and laminations can be see here. Field knife (9 cm) for scale. 22 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 79: Field photo of second dome sampled for slab and thin-section study. This mound was also sampled regularly from various smaller domes. It occurred in the same bed (approximately 5 meters laterally) as the other mound. Rock hammer (27.5 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 228 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 80: Field photo showing infilling beds around stromatolite mound. These beds consist of a finer micrite than the microbial micrite forming the stromatolites. These beds tend to onlap the mounds on either side. Rock hammer (28 cm) for scale. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 230 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 81: Field photo showing stromatolite mound pinching out. with infilling beds surrounding it. Some mounds pinch out like this, and form individual units, and some mounds coalesce with other mounds. The infilling beds consist of a very fine micrite. Rock hammer is 27.5 cm for scale. 231 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 3 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. action, and may provide evidence that these units were wave-resistant. The stromatolite domes, however, show no evidence of current scour or current movement. A sedimentological and ichnological analysis of these beds suggest stromatolites formed in an offshore paleoenvironment (see also Chapter 5). The graded storm beds with apparent cross-bedding were most likely deposited as a storm event (Kidwell, 1988) while the heavily bioturbated fine muddy micrite, which shows no evidence of wave disturbance, may have been deposited in a lower energy environment than the storm bed. The stromatolites may have been smothered by overlying units, or the conditions that favor stromatolite development may have been eradicated. The overlying units do not contain intense bioturbation but have been reported by Schubert (1993) to contain fossil remains of bivalves and crinoidal debris. Thin-Section Analysis The second part of this study involved analysis of thin sections to ascertain the components of the stromatolites. In thin-section, three distinct fabrics emerged. The first fabric noted was the preservation of original stromatolitic laminations (Figure 82). These could be seen across some of the sections, but was a relatively 233 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 82: Thin-section photo showing stromatolitic laminations. These laminations were seen in some of the thin-sections, and are infilled with a drusy calcite. These laminations show that microbial activity was preserved both in the macro and micro scale (magnification of 10X). 234 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 235 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. minor component. The two most prominent fabrics that make up the majority of the thin sections include the microbial fabiic (Figure 83) that made up the background of the sections and the muddy micritic fabric (Figure 84 and Figure 85) that was often filled with metazoan debris. The two fabrics are most distinct from each other because the muddy micritic fabric contains metazoan debris. This fabric is often densely packed with the remains of ostracods. bivalves, and gastropods. The debris does not have a specific orientation, and may represent episodic deposition during current activity or storms. The microbial fabric tends to be devoid of metazoan debris, and has voids that have been infilled by drusy calcite. These voids may represent the remains of Renalcis. a sediment-binding calcified bacterium reported from sections in South China (Lehrmann. 1999) PALEOENVIRONMENTAL IMPLICATIONS The occurrence of normal marine stromatolites in the Early Triassic is unusual because they have largely been excluded from normal marine environments since the Ordovician (Awramik. 1991; Bottjer. 1997). Modem stromatolites are commonly found in environments where they are able to form without predation pressures from metazoans. In Shark Bay, Australia, stromatolites are able to form 236 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 83: Thin-section photo showing microbial fabric. This fabric made up the bulk of the thin-sections, and was devoid of metazoan debris. The cavities pictured here are infilled with drusy caicite.(magnification of IOX). 2 3 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 238 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 84: Thin-section photo showing muddy micritic fabric. This fabric contains the remains of metazoans such as ostracods, bivalves and gastropods. This fabric represents episodic deposition of fine fossiliferous micrite and skeletal debris that was washed in by current or storm activity, and incorporated into the microbial matrix (magnification of 10X). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T u n — 240 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 85: Thin-section photo showing both microbial fabric and muddy micritic fabric with metazoan debris. These fabrics have distinct interfaces, as seen here. This illustrates that the muddy micrite and debris was washed episodically into pre-existing cavities formed by the microbial fabric (magnification of 10X). 241 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 242 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. because they grow in hypersaline environments from which metazoans are excluded (Palyford, 1980). In studying the Lower Triassic stromatolites of the Virgin Limestone Member of the Moenkopi Formation, it is important to address why these stromatolites were able to form in “normal” marine environments. In recent work by Reid et al. (2000), study of modem marine stromatolites has yielded information as to the types of complex communities that form these build-ups and that are often lost in fossilization. From a study of modem marine stromatolites, it was found that these communities represent a balance between sedimentation and lithification of the cyanobacterial mats. These mat communities alternate between three types. The first of the three is considered the pioneer community. The second type exhibits development of calcified biofilms that appear as thin crusts on the surface of the mats. The third type is dominated by an endolithic bacterium that bores grains that are later fused together. The authors suggest that these communities represent responses to changes in microbial community structure and sedimentation. Type I is present during sediment accretion, or during periods of high sedimentation. Type 2 takes over during intermittent periods of quiescence, and is dominant when the mats begin to lithify. Type 3 is the maturest community, and the endolithic activity of certain bacteria 243 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. contributes to the construction of laterally cohesive carbonate crusts. The driving force behind early lithification of stromatolites is believed to be intimately linked to surface mat bacterial degradation (Reid, 2000). Other workers have suggested that the formation of stromatolites is more directly linked to fluctuations in seawater chemistry (Grotzinger and Knoll, 1999: Riding, 2000). While the mechanisms for lithification may be most directly related to the attributes of microbial communities, the determination as to whether or not stromatolites can form at all depends on environmental parameters. Ocean chemistry fluctuations related to the state of ambient seawater are believed to play a role in the lithification of stromatolites (Riding, 2000). Because of this, some feel that stromatolites represent “environmental dipsticks”, or a way to assess environmental factors that influence their growth (Grotzinger and Knoll. 1999). The Lower Triassic stromatolites described from the Virgin Limestone Member of the Moenkopi Formation have much paleoenvironmental significance. It has been proposed by other workers (Schubert and Bottjer, 1992) that these stromatolites represent periods of time where a decrease in metazoan predation pressure has enabled these build-ups to attain significant relief. Upon closer analysis of these stromatolites, further conclusions can be drawn. 244 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The reverse onshore-offshore gradient of microbial development seen in the Lower Triassic Virgin Limestone Member described earlier indicates that conditions that affected the middle shelf paleoenvironments were different than those that affected the intertidal regions. This is evidenced by the development of normal marine stromatolites in the most offshore section with an absence of microbial build ups in the carbonates of the shallowest section. No other stromatolites have been reported from more inland sections of Lower Triassic carbonates, which may indicate that the middle shelf paleoenvironment of the Virgin Limestone was in the best position to tap deeper water conditions. While the diversity of trace fossils was highest in the shallowest section, the ichnofabric indices were high in the most offshore section, indicating that metazoans were able to significantly disturb the sediment in all sections. There does seem to be a lack of grazing activity, with an absence of such typica1 grazing traces as Nereites, Scolicia and Phycosiphon (Bromley, 1996). Underlying the stromatolite bed at Lost Cabin Springs is a fine micrite with an ichnofabric index of 4. This means that in sediments deposited just previously to the stromatolitic beds, metazoans were actively bioturbating the sediment. The immediate development of stromatolites in the overlying beds implies that an 245 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. environmental change must have occurred to exclude bioturbation and foster the development of stromatolites. Ichnofabric indices of the Virgin Limestone Member show that metazoans had recovered significantly from the beginning of the Early Triassic when ichnofabric indices were closer to 1 or 2 (Twitchett, 1999). Because of this, it is not the absence of bioturbators that leads to the development of the late Early Triassic stromatolites, but rather some episodic environmental factors that periodically inhibited metazoans from destroying these microbial fabrics. Recent work by Woods et al. (1999) has suggested that deleterious conditions existed in the outer shelf-basinal settings during the late Early Triassic. The Union Wash Formation shows evidence for calcium carbonate supersaturation in the form of aragonite fans that were precipitated on the seafloor. The Union Wash Formation was deposited coevally with the Moenkopi Formation in more offshore settings, and it is perhaps these conditions that could have periodically affected the deposition of shelfal paleoenvironments of the Virgin Limestone Member. Ocean water that was anoxic (Isozaki. 1994. 1997) or supersaturated in carbon dioxide may have periodically flooded the shelf and inhibited metazoans while simultaneously creating a haven for stromatolites. This would account for why stromatolites are found in the most offshore section of the Virgin Limestone yet are absent in more nearshore 246 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. strata. Furthermore, these stromatolites, acting as “environmental dipsticks” (Grotzinger, and Knoll, 1999), may be providing evidence for ocean chemistry that could have inhibited the recovery of metazoans throughout the Early Triassic, and thus contributed to the prolonged recovery from the end-Permian mass extinction. The following model for stromatolite development at Lost Cabin Springs (Figure 86) is proposed: I) the end-Permian extinction event severely devastated ecosystems with a loss of as many as 96% of the marine species (Raup. 1979); 2) colonial metazoans are devastated by this event and so an ensuing metazoan “reef gap” (Flugel, 1994) follows throughout the Early Triassic; 3) the end-Permian extinction has been shown to affect the depth and intensity of bioturbation (Twitchett. 1997); 4) these factors created conditions more conducive to stromatolite formation in normal marine environments since metazoans had been inhibiting their growth since the Ordovician (Awramik, 1971; Garrett, 1970; Walter and Heys, 1985); 5) the Virgin Limestone stromatolites occur in the most offshore section coevally with the precipitates documented by Woods et al., (1999) that were thought to represent deleterious deep-water conditions (Figure 87); 6) the occurrence of normal marine stromatolites may indicate the periodic flooding of the shelf with the deleterious deep-water conditions (perhaps anoxia or hypercapnia) that could have 247 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 86: Diagram showing the progression of conditions from the end-Permian extinction to the development of Early Triassic stromatolites. The same conditions responsible for the growth of stromatolites (anoxia/calcium carbonate supersaturation) may also be responsible for the prolonged recovery of metazoans in the Early Triassic. 248 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. END-PERMIAN EXTINCTION Extinction ot colonial metazoans / \ Reduction in DioturDation \ Conditions are favorable for stromatolite growth ♦ Deleterious conditions exist in offshore Periodic flooding ot shelf with offshore water (Anoxia/Calcium carbonate supersatuiaeun; / \ Inhibits metazoans Fosters growth of (bioturbation) stromatolites 249 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 87: Diagram showing the Union Wash and Moenkopi Formations. The Middle Member of the Union Wash Formation contains seafloor calcium carbonate precipitates (Woods et al., 1999) and was deposited coevally with the Virgin Limestone Member of the Moenkopi Formation. Because of this relationship, the conclusion has been drawn that the stromatolites forming in shelfal environments of the Virgin Limestone may be linked to oceanic conditions (anoxia/calcium carbonate supersaturation) seen in the Union Wash Formation. The middle shelf paleoenvironment of the Lost Cabin Springs locality was in a better position than Vigo or Muddy Mountains to be affected by these offshore deleterious conditions. 2 5 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lower Triassic Union Wash and Moenkopi Formations Southern Nevada East-central California Upper CC C O CC JZ ui < U 'C of the copyright owner. Further reproduction prohibited without permission. inhibited metazoans while at the same time fostering the growth of stromatolites; and 7) this is further evidenced by the lack of microbial build-ups in more onshore sections (Muddy Mountains) that would have less communication with offshore conditions. STROMATOLITES OF THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION: EARLY TRIASSIC MICROBIAL REEFS Research conducted on these beds was to not only establish the paleoenvironmental conditions under which these stromatolites formed, but also to establish whether or not these stromatolites represent true reefs. Many definitions have been used to describe the characteristics intrinsic to reefs (Cornell and Karlson, 1996; Hubbell, 1997). and many have invoked wave-resistance as a possible criterion (Ladd. 1944; Lowenstam. 1950; Newell et al.. 1953. Dunham. 1970). For this research, a definition of reefs was used that emphasized topographic relief upon the seafloor as a primary criterion for determining reef structures. This definition, put forth by Rachel Wood (1999), describes a reef as a "discrete carbonate structure formed by in-situ or bound organic components that develops topographic relief upon the seafloor’’. Based on this definition and research conducted in this study, the stromatolites of the Virgin Limestone Member represent true reefs. 252 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Based on measurements and field analysis, these individual mounds attained a relief of 1-2 meters of off the seafloor. This was evidenced by the onlapping nature of surrounding beds and the deformation of underlying units, showing the stromatolite mounds had to be at least somewhat cohesive upon compaction. The mounds were an agglomeration of smaller domes that exhibited stromatolitic laminae both in outcrop and in thin-section. These microbial build-ups occurred at all places where the beds was exposed, and occasionally coalesced to form bioherms. The differences in bed thickness on either side of the mound also suggest topographic relief. These stromatolite domes in thin-section show that they are complex networks of both microbial fabrics and metazoan debris. The microbial fabrics make up the majority of the fabrics in thin-section. but the distinct micritic fabric with metazoan debris shows deposition by current or wave activity. The micritic fabric was most likely lithified in place after being washed into the microbial fabric. These domes thus represent a heterogeneous combination of both microbial networks and micritic fabrics that were able to lithify and form build-ups on the seafloor. Based on both the research of this study and the Wood (1999) criteria for establishing reefs, these microbial build-ups of the Virgin Limestone Member 253 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. represent Early Triassic patch reefs in deeper water on a ramp. Similar to the biostromes and mounds reported by Lehrmann (1999) of South China, these mounds were able to attain significant relief off of the seafloor. These structures also occurred globally in the Early Triassic, and suggest similar environmental factors must have been influencing these different environments. The occurrence of these microbial build-ups also suggests that both the carbonate platforms of South China and the shelfal Western United States were influenced by deeper water deleterious conditions. The previously accepted “reef gap" of the Early Triassic was used to describe the lack of colonial metazoan reefs due to their vast extinction at the end of the Permian (Flugel. 1994). Though colonial metazoans do not fully recover until the Middle Triassic. the Early Triassic “reef gap” must now account for the development of microbial build-ups during this time. These structures, which are likely related to unusual oceanic conditions of the Early Triassic, do represent normal marine patch reef systems. Because of this, a link must exist between the prolonged recovery of metazoans during the Early Triassic and the coeval occurrence of only microbial reef development. 25 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONCLUSIONS The onshore-offshore transect of the Virgin Limestone Member of the Moenkopi Formation has yielded much information on the Early Triassic recovery interval. From close analysis of sedimentary facies, trace fossils, bivalves and microbial fabrics, a variety of trends emerged. A closer look at the stromatolites of the Lost Cabin Springs locality provided insight into Early Triassic reef development and implications for the slowed recovery of the end-Permian extinction event. The three localities selected for this study showed a variety of sedimentary facies. The Muddy Mountains locality contained limestone beds with the largest percentage of grainstones. Based on sedimentary structures and trace fossil analysis, this locality was the shallowest of the three, being deposited primarily in an intertidal paleoenvironment. Vigo was deposited in a more subtidal environment and Lost Cabin Springs was the most offshore of the three, deposited mostly in a middle shelf setting. Using this established transect, trace fossils were studied in detail at all three localities. From this research, it was determined that vertical bioturbation had not fully recovered by the late Early Triassic. The only locality with significant vertical bioturbation. Muddy Mountains, had the lowest ichnofabric indices of the three. 255 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Horizontal bioturbation had, however, significantly recovered by this time. In beds at Muddy Mountains where horizontal traces were prominent, ichnofabric indices were often found to be 3 or greater, showing a destabilizing of the sediment due to horizontal bioturbation. At both Vigo and Lost Cabin Springs, horizontal bioturbation was found in beds with ichnofabric indices of 3 or higher. This transect illustrates that horizontal trace-makers recovered more quickly than vertical trace- makers from the end-Permian extinction event. The bivalve Promyalina also shows an interesting distribution along this transect. There are distinct differences between the three localities, but there seem to be many similarities between them. The Promyalina beds at all three localities occur in shallow paleoenvironments and represent tempestite deposition. Disarticulation of the bivalves implies some transport with little abrasion. The Virgin Limestone Member reflects the waxing and waning of an epicontinental seaway, and the Promyalina beds tend to occur in more shallow water deposits, which is also consistent with storm deposition. Though the Promyalina beds are not monotaxic. P rom yalinid bivalves, however, represent the dominant fauna. This may be more the function of hydrodynamics than a true reflection of the original fauna! composition. 256 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The microbial fabrics represent the most unusual trend along this onshore- offshore transect. The shallowest locality, Muddy Mountains, contains no microbial fabrics in the carbonates, but both Vigo and Lost Cabin Springs show evidence for microbial development. The wavy laminations at Vigo occur at the basalmost part of the section, and represent the primary signature of microbial activity. Lost Cabin Springs has other beds with laminations reminiscent of microbial fabrics (i.e. Unit 7), and two beds that contain fully developed normal marine stromatolites. This occurrence is extremely intriguing since stromatolites have been largely excluded from normal marine environments since the Ordovician (Awramik. 1991. Bottjer. 1997). Because of this unusual occurrence of stromatolites in the most offshore section, deeper water oceanic conditions have been invoked as a possible environmental parameter that fostered their growth and development. Recent work by Woods et al.. (1999) has shown precipitation of aragonite fans on the seafloor in the Union Wash Formation which was deposited coevally to the Virgin Limestone Member of the Moenkopi Formation. These precipitates have been suggested to represent unusual ocean chemistry supersaturated in calcium carbonate. It is perhaps the periodic impingement of these waters onto the shelf that may have created a unique environment in which these stromatolites could thrive. 257 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Waters rich in calcium carbonate could both exclude metazoans and foster rapid lithification to allow these stromatolites to form significant relief off of the seafloor. Because of the relief achieved by these stromatolites, it has been determined through the course of this research that these stromatolites represent patch reefs. This has implications for the previously accepted “reef gap” (Flugel, 1994) which is supposed to characterize the entirety of the Early Triassic. While colonial metazoans are not making reefs at this time, microbial build-ups are reported from both South China (Lehrmann. 1999) as well as these in the Western United States, and seem to be the only type of reef that was forming during this interval. The lack of colonial metazoan reefs and dominance of microbial reefs has implications for oceanic conditions during the Early Triassic. Microbial occurrences are documented throughout the Early Triassic (Kershaw et al, 1997: Lehrmann. 1999; Schubert and Bottjer. 1992) and are not only a boundary phenomenon. These occur both in earliest Triassic and in the late Early Triassic, suggesting that this was a prolonged happening. Oceanic conditions that may have inhibited the recovery of metazoans may have also fostered the growth of microbial build-ups during the Early Triassic. 258 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The periodic flooding of the shelf with deep-waters having unusual ocean chemistry may have limited the recovery of metazoans and simultaneously nurtured the growth of microbial build-ups. This is best evidenced by the occurrence of stromatolites in the most offshore section of the onshore-offshore transect. Based on the research conducted in this project, oceanic conditions played the largest role in the 7-8 million year recovery of metazoans (Hallam, 1991), and also account for the occurrence of only microbial reefs in the Early Triassic. 259 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES Allison, P.A. and D.E.G. Briggs. 1993. Paleolatitudinal sampling bias, Phanerozoic species diversity, and the end-Permian extinction. Geology, v. 21, p. 65-8. Awramik, 1971. Precambrian columnar stromatolite diversity; reflection of metazoan appearance. Science, v. 174, p. 825-27. Baud. A., Magaritz, M. and W.T. Holser. 1989. Permian-Triassicof the Tethys: carbon isotopes studies. Geologische Rundschau, v. 78, p. 649-77. Becker. L.. Poreda. R., Hunt, A.G., Bunch, T.E. Rampino, M. 2001. Impact event at the Permian-Triassic boundary; evidence from extraterrestrial noble gases in fullerenes. Sceicen. V. 291. p. 1530-33. Benton. M J. 1995. Diversification and extinction in the history of life. Science, v. 268. p. 52-8. Bottjer. 1997. Phanerozoic non-actualistic paleoecology. Geobios. v. 30. p. 885-93. Brandner. R. 1988. Plate tectonics and fluctuations of sea level and climate at the Permian-Triassic boundary. Berichte der Geologischen Bundesanstalt. Wien, v. 15. p. 3 Bromley. 1996. Trace fossils; biology, taphonomy and applications, second edition. Chapman and Hall. London. 361 p. Campbell, I.H.. Czamanske. G.K.. Fedorenko, V.A., Hill. R.I., and V. Stepanov. 1992. Synchronism of the Siberian Traps and the Permian-Triassic boundary. Science, v. 258, p. 1760-3. Carlson. SJ. 1991. A phylogenetic perspective on articulate brachiopod diversity and the Permo-Triassic extinction. In The Unity of evolutionary biology (ed. E. Dudley), p. 119-142. Dioscorides Press. Portland. Oregon. 260 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ciriacks, 1963. Permian and Eotriassic bivalves of the Middle Rockies. Bulletin of the American Museum of Natural History, v. 125, p. 1-lOClark, DL. 1987. Conodonts: the final fifty million years. In Paleobiology of conodonts, (ed. R J . Aldridge), p. 165-74. Ellis Horwood, Chichester Claypool, G.E.. Holser, W.T.. Kaplan, I.R., Sakai, H. and I. Zaki. 1980. The age curves of sulfur and oxygen isotopes in marine sulfate, and their mutual interpretation. Chemical Geology, v. 28, p. 199-259. Cox. L.R. and N.D. Newell. 1969. Treatise on invertebrate paleontology. Part N. Mollusca 6. Bivalvia. v. 1-2, Geological Society of America. New York, 489 P- Crimes. T.P. 1975. The stratigraphical significance of trace fossils. In: The study of trace fossils; a synthesis of principles, problems, and procedures in ichnology (ed. R.W. Frey),p. 109-135, Springer-Verlag, New York. Dagys. A.S. 1993. Geographic differentiation of Triassic brachiopods. Palaeogeography. Paiaeoclimatology, and Palaeoecology, v. 100. p. 79-87. Dobrunskina. I.A. 1987. Phytogeography of Eurasia during the Early Triassic. Palaeogeography. Paiaeoclimatology. Palaeoecology, v. 58. p. 75-86. Dunham. R J. 1970. Stratigraphic reefs versus ecologic reefs. The American Association of Petroleum Geologists Bulletin, v. 54. p. 1931-32. Ekdale, A.A.. Bromley, R.G. and S.G. Pemberton. 1984. Ichnology; the use of trace fossils in sedimentology and stratigraphy. SEPM Short Course, v. 15. Erwin. D.H. 1993. The great Paleozoic crisis: life and death in the Permian. Columbia University Press. New York. Erwin. D.H. 1994. The Permo-Triassic extinction. Nature, v. 367. p. 231-6. 2 6 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Erwin, D.H. and P. Hua-Zhang,1996. Recoveries and radiations; gastropods after the Permo-Triassic mass extinction. Biotic recovery from mass extinction events. Geological Society Special Publications, v. 102. p. 223-29. Ezaki. Y. 1994. Patterns and palaeoenvironmental implications of end-Permian extinction of Rugosa, South China. Palaeogeogrpahy, Paiaeoclimatology, and Palaeoecology, v. 107, p. 165-77. Fagerstrom, J.A. 1987. The evolution of reef communities. John Wiley and Sons, New York. Fedorowski, J. 1989. Extinction of Rugosa and Tabulata near the Permian/Triassic boundary. Acta Palaeontologica Polonica. v. 34, p. 47-70. Fischer. A.G. and M.A. Arthur. 1977. Secular variations in the pelagic realm. Society of Economic Paleontologists and Mineralogists. Special Publication, v. 25. p. 19-50. Ressa. K.W.. Cutler. A.H.. and K.M. Meldahl 1993. Time and taphonomy: quantitative estimates of time-averaging and stratigraphic disorder in a shallow marine habitat. Paleobiology, v.19. p. 266-86. Flugel. E. 1994. Pangean shelf carbonate: controls and paleoclimatic significance of Permian and Triassic reefs. Geological Society of American Special Papers, v. 288. p. 247-266. Fraiser. M.L. 2000. Paleoecology and Paleoenvironments of Early Triassic mass extinction biotic recovery faunas, Sinbad Limestone Member, Moenkopi Formation, South Central Utah, [Masters thesis], University of Southern California. 311 p. Frey.R.W.. and A. Seilacher. 1980. Uniformity in marine invertebrate ichnology. Lethaia, v.13. p. 183-207. 262 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Garrett, P. 1970. Phanerozoic stromatolites; noncompetitive ecologic restriction by grazing and burrowing animals. Science, v. 169, p. 171-3. Golonka, J and D. Ford. 2000. Pangean (Late Carboniferous-Middle Jurassic) paleoenvironment and lithofacies. Palaeogeography, Paiaeoclimatology, Palaeoecology, v. 161, p. 1-34. Grotzinger, J.P. and A.H. Knoll 1999. Stromatolites in Precambrian carbonates: evolutionary mileposts or environmental dipsticks? Annual Review of Earth and Planetary Sciences, v. 27, p. 313-358. Grotzinger, J.P. 1990. Geochemical model for Proterozoic stromatolite decline. In: Proterozoic evolution and environments (eds. Knoll, A.H. and J.H. Ostrom), p. 80-103 Yale University. New Haven. Hallam. A. 1991. Why was there a delayed radiation after the end-Palaeozoic extinction? Histroical biology, v. 5. p. 257-62. Hallam, A. and P.B. Wignall. 1997. Mass extinctions and their aftermath. Oxford University Press. New York. 320 p. Holser. W.T. and M. Magaritz. 1985. The Late Permian carbon isotope anomaly in the Bellerphon Basin. Camic and Dolomite Alps. Jahrbuch der Geotogischen Bundesanstalt, Wien. v. 128. p. 75-82. Holser. W.T. and M. Magaritz. 1987. Events near the Permian-Triassic boundary. Modem Geology, v. II. p. 155-80. Holser. W.T. and M. Magaritz. 1992. Cretaceous/Tertiary and Permian/Triassic boundary events compared. Geochimica and Cosmochimica Acta. V. 56. p. 3297-309. 263 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Holser, W.T. and M. Magaritz. 1985. The Late Permian carbon isotope anomaly in the Bellerphon Basin, Camic and Dolomite Alps. Jahrbuch der Geologischen Bundesanstalt, Wien, v. 128, p. 75-82. Holser, W.T., Schonlaub. H.-P.. Boeckelmann, K., Magaritz, M., and C J . Orth. 1991. The Permian-Triassic of the Gartnerkofel-1 core (Camic Alps. Austria): synthesis and conclusions. Abhandlungen der Geologischen Bundesanstalt, v. 45, p. 213-32. Isozaki, Y. 2001. Plume winter scenario for the Permo-Triasic boundary mass extinction. Paleobios. v. 21. p. 70. Isozaki. Y. 1997. Permo_triassic boundary superanoxia and stratified superocean: records from lost deep sea. Science, v. 276. p. 235-8. Isozaki. Y. 1994. Superanoxia across the Permo-Triassic boundary: recorded in accreted deep-sea pelagic chert in Japan. Canadian Society of Petroleum Geologists. Memoir, v. 17. p. 805-12. Jin. Y.. Zhu. Z.. and S. Mei. 1994. TheMaokouan-Lopingian sequences in South China. Paleoworld. v. 4. p. 138-52. Jin.Y. G.. Wang, Y.. Wang. W.. Shang, Q. H.. Cao. C. Q.. and D.H. Erwin. 2000. Pattern of marine mass extinction near the Permian-Triassic boundary in South China. Science, v. 289, p. 432-6. Kershaw, S., Zhang T., L. Guangzhi.1999. A ?microbialite carbonate crust at the Permian-Triassic boundary in South China, and its paleoenvironmental significance. Palaeogeography, Paiaeoclimatology, Palaeoecology. v. 146, p. 1-18. Kidwell, 1988. Taphonomic comparison of passive and active continental margins: Neogene shell beds of the Atlantic Coastal Plain and northern Gulf of California. Palaeogeography, Paiaeoclimatology, Palaeoecology, v. 63. p. 201-223. 264 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fCier, P.M. L984. Echinoids from the Triassic (St. Cassian) of Italy, their lantern supports and a revised phylogeny of Triassic echinoids. Smithsonian Contributions of Paleobiology, v. 56, p. 1-41. Knoll, A.H., Bambach, R.K., Canfield, D.E., and J.P. Grotzinger. 1996. Comparative Earth history and Late Permian mass extinction. Science, v. 273, p. 452-7. Ladd. 1944. Reefs and other bioherms. U.S. Geological Survey, v. 10. p. 26-29. Lehrmann. D.J. 1999. Early Triassic calcimicrobial mounds and biostromes of the Nanpanjiang Basin, South China. Geology (Boulder), v.27. p. 359-62. Lehrmann, D.J., 1998. Controls on facies architecture of a large Triassic carbonate platform: the Great Bank of Guizhou, Nanpanjiang Basin. South China, Journal of Sedimentary Research, v. 68. p. 311-26. Lowenstam, H.A. 1950. Niagaran reefs of the Great Lakes area. Journal of Geology, v. 58. p. 430-87. Magaritz, M.. Krishnamurthy. R.V.. and W.T. Holser. 1992. Parallel trends in organic and inorganic carbon isotopes across the permian-Triassic boundary. American Journal of Science, v. 292, p. 727-39. Martin. E.E. and J.D. Macdougall. 1995. Sr and Nd isotopes at the Permian/Triassic boundary: a record ofclimate change. Chemical Geology, v. 125. p. 73-100. McKinney, M.L.1991. Completeness of the fossil record; an overview. In: The process of fossilization (ed. S.K. Donovan), p. 66-83, Columbia University Press. New York. McKinney, M.L. 1985. Mass extinction patterns of marine invertebrate groups and some implications for a causal phenomenon. Paleobiology, v. 11, p. 227-33. 265 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Musashino, M. 1993. Chemical composition of the Toishi-type’ siliceous shale- Part 1. Bulletin of the Geological Survey of Japan, v. 44, p. 699-705. Nakazawa. K. and B. Runnegar. 1973. The Permian-Triassic boundary: acrsis for bivalves? In The Permian and Triassic systems and their mutual boundary (ed. A. Logan and L.V Hills. Newell, N.D. 1952. Periodicity in invertebrate evolution. Journal of Paleontology, v. 26, p. 371-85. Newell. J.D. 1967. Revolutions in the history of life. Geological Society of America Special Paper, v. 89, p. 63-91. Newell. J.D.. Bradley, J. S.. Whiteman. A. J., and A.G. Fischer. 1953. The Permian reef complex of the Guadalupe Mountains region. Texas and New Mexico: a study in paleoecoiogy. Geological Society Memoir, v. 67. p. 407-436. Patterson, C. and A.B. Smith. 1987. Is the periodicity of extinctions a taxonomic artefact? Nature, v. 330. p. 248-51. Paul. C.R.C. 1988. Extinction and survival in echinoderms. In: Extinction and survival in the fossil record (ed. G.P. Larwood), p. 155-70. Systematics Association Special Volume, v. 34. Pauli, R.A. and R.K Pauli. 1986. Depositional history of Lower Triassic Dinwoody Formation, Bighorn Basin, Wyoming and Montana. In: Geology of the Beartooth Uplift and adjacent basins (ed. Garrison, P. B.), p. 13-25, University of Wisconsin at Milwaukee, Milwaukee. Pauli, R.K., Pauli. R.A., Kraemer, R.A. and R. Bradley. 1989. Depositional history of Lower Triassic rocks in southwestern Montana and adjacent parts of Wyoming and Idaho. In: Montana Geological Society 1989 field conference guidebook: Montana centennial edition: Geologic resources of Montana, Volume I, p. 69-90. (ed. French. D. E. and R J 7 . Grabb), University of Wisconsin, Milwaukee. 266 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Playford, P.E. 1980. Australia’ s stromatolite stronghold. Natural History, v. 89, p. 58-61. Pruss. S.B. and D J . Bottjer 2001. Development of microbial fabrics in Early Triassic oceans, PaleoBios, v. 21, p. 106. Rampino, M.R. and A.C. Adler. 1998. Evidence for abrupt latest Permian mass extinction of Foraminifera; results of tests for the Signor-Lipps effect. Geology (Boulder), v. 26, p. 415-18. Raup, D.M. 1979. Size of the Permo-Triasic bottleneck and its evolutionary implications. Science, v. 206, p. 217-28. Reid. R.P., Visscher, P.T.. Decho. A.W. Stolz, J.F., Bebout, B.M., Dupraz. C.. Macintyre. I.G., Paerl, H., Pinckney, J.I— Prufert-Bebout. L.. Steppe, T.F.. and D J . DesMarais. 2000. The role of microbes in accretion, lamination and early lithification of modem marine stromatolites. Nature, v. 406. p. 989-92. Reif. DM . and R.M. Slatt. 1979. Red bed members of the Lower Triassic Moenkopi Formation, southern Nevada; sedimentology and paleogeography of a muddy tidal flat deposit. Journal of Sedimentary Petrology, v. 49, p. 869-89. Renne. P.R., Zhang, Z., Richardson, M.A., Black, M.T., and A.R. Basu. 1995. Synchrony and causal relations between Permo-Triassic boundary crises and Siberian flood volcanism. Science, v. 269. p. 1413-16. Retailack. G J . 1995. Permian-Triassic crisis on land. Science, v. 267. p. 77-80. Retallack. G J., Veevers. JJ.. and Morante. R. 1996. Global coal gap between Permian-Triassic extinction and Middle Triassic recovery of peat-forming plants. Bulletin of the Geological Society of America, v. 108. p. 195-207. Retallack. G J. 2001. A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles. Nature, v. 411, p. 287-290. 2 6 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Riding, R. 2000. Microbial carbonates; the geological record of calcified bacterial- algal mats and biofilms. Sedimentology, v. 47, p. 179-214. Rodland, D.L. and D J. Bottjer. 2001. Biotic Recovery from the end-Permian mass extinction: behavior of the inarticulate brachiopod Lingula as a disaster taxon, Palaios, v. 16, p. 95-101. Ross, J.R.P. 1995. Permian bryozoa. In: The Permian of northern Pangea l(ed. P.A. Schoile, T.M. Peryt. and D.S. Ulmer-SchoIIe), P. 98-123. Springer-Veriag, Berlin. Sakagami, S. 1985. Palegeographic distribution of Permian and Triassic Ectopocta (Bryozoa). In: The Tethys: her paleogeography and paleobiogeography from Paleozoic to Mesozoic (ed. K. Nakazawa and J.M. Dickins), p. 171- 83. Tokai University Press, Tokyo. Schaeffer, B. 1973. Fishes and the Permian-Triassic boundary. In: The Permian and Triassic systems and their mutual boundary (ed. A. Logan and L.V. Hills), p. 497-7. Canadian Society of Petroleum Geologists, Memoir 2 Schopf, TJ.M . 1974. Permo-Triassic extinctions: relation to sea-floor spreading. Journal of Geology, v. 82. p. 129-43. Schubert. J.K.1989. Paleoecology of the LowerTriassic Virgin Member [Master's thesis], University of Southern California. 234 p. Schubert. J.K., and D J. Bottjer. 1992. Early Triassic stromatolites as post-mass extinction disaster forms. Geology, v. 20, p. 883-6. Schubert, J.K. and D J. Bottjer. 1995. Aftermath of the Permian-Triassic mass extinction event: Paleoecology of LowerTriassic carbonates in the western USA: Palaeogeography, Paleoeclimatology, Palaeoecology, v. 116, p. 1-39 Schubert. J.K.. Bottjer. D J . and M J . Simms. 1992. Palebiology of the oldest articulate crinoid. Lethaia, v. 25, p. 97-110. 2 6 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sepkoski, J.J. Jr. 1981. A factor analytic description of the Phanerozoic marine fossil record. Paleobiology, v. 7, p.36-53. Shorb, W.M.1983. Stratigraphy, facies analysis and depositional environments of the Moenkopi Formation (Lower Triassic), Washington County, Utah, and Clark and Lincoln counties, Nevada. [Master's thesis], Duke University, 205 p. Simms. M.J. and G.D. Sevastopulo. 1993. The origin of articulate crinoids, Palaentology, v. 36, p. 91-110. Simms. M.J.. Gale, A.S.. Gilliland. P., Rose, E.P.F., and GD . Sevastopulo. 1993. Echinodermata. In: The fossil record 2 (ed. M.J. Benton), p. 491-528. Chapman and Hall, London. Smith. R.M.H. 1995. Changing fluvial environments across the Permian-Triassic boundary in the Karoo Basin, South Africa and possible causes of tetrapod extinctions. Palaeogeography. Paiaeoclimatology, Palaeoecology. v. 40. p. 183-98. Stanley. G.D. Jr.. and D.G. Fautin. 2001. The origins of modem corals. Science, v. 291. 1913-14. Stemmerik L. and P.A. Scholle. 1992. Karst-controlled facies mosaic in Late Permian carbonate platform sequence, Wegener Halvo Formation. Wegener Halvo. East Greenland. American Association of Petroleum Geologists. Annual Meeting Abstracts, v. 126, p. 46. Sun. Y.. Xu, D.. Zhang, Q.. Yang, Z., Sheng, J., Chen. C.. Rui, L., Liang, X., Zhao, J., and J. He. 1984. The discovery of an iridium anomaly in the Permian- Triassic boundary clay in Changxing, Zhejiang, China and its significance. In: Developments in Geoscience. Contributions, p. 235-45. 27Ih International Geological Congress. Sceince Press. Beijing. 269 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sweet. W.C.. Yang, Z., Dickins. J.M and Yin H. 1992. Permo-Triassic events in the eastern Tethys- an overview. In: Permo-Triassic boundary events in the eastern Tethys (ed. W.C. Sweet, Z. Yang, J.M. Dickins and H. Yin), p. 1-8. Cambridge University Press, Cambridge. Taylor, P.D. and G.P. Larwood. 1988. Mass extinctions and the pattern of bryozoan evolution. In: Extinction and survival in the fossil record (ed. G.P. Larwood), p. 99-119. Systematics Association Special Volume, v. 34. Teichert. C. and B. Kummel. 1976. Permian-Triassic boundaryin the Kap Stosch area, east Greenland. Meddelelser om Gronland, v. 197, p. 1-49. Teichert. C. 1990. The Permian-Triassic boundary revisited. In: Extinction events in Earth History (ed. E.G. Kaufmann and O.H. Walliser), p. 199-238. Springer- Verlag, Belin. Thackeray, J.F.. van der Merve. N.J., Lee-Thorpe, J.A.. Sillon. A.. Lanham. J.L.. Smith, R., Keyser, A., and P.M.S. Monteiro. 1990. Changes in the carbon isotopes ratios in the Late Perman recorded in therapsid tooth apatite. Nature, v. 347, p. 751-3. Twitchett, RJ., Looy. C.V.. Morante, R.. Visscher, H., and P.B. Wignall. 2001. Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis. Geology, v. 29, p. 351-4. Twitchett, R.J. 1999. Unusual intraclastic limestones in LowerTriassic carbonates and their bearing on the aftermath of the end-Permian mass extinction, Sedimentology, v. 46. p. 303-16. Twitchett. R J. 1999. Palaeoenvironments and faunal recovery after the end-Permian mass extinction. Palaeogeography, Paiaeoclimatology, Palaeoecology, v. 154. p. 27-37. 270 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Valentine, J.W. and E.M. Moores. 1973. Provinciality and diversity across the Permian-Triassic boundary. In: The Permian and triasic systems and their mutual boundary (ed. A. Logan and L.V. Hills), p. 759-66. Canadian Society of Petroleum Geology, Memoir 2. Veevers, JJ. Conaghan, PJ., and S.E. Shaw. 1994. Tuijing point in Pangean environmental histroy at the Permian/Triassic (P/Tr) boundary. Geologcail Society of America Special Paper, v. 288, p. 187-96. Walter, M.R. and G.R. Heys. 1985. Links between the rise of the Metazoa and the decline of stromatolites. In: Stratigraphic methods as applied to the Proterozoic record Precambrian Research (eds. Young, G.M., Chen, J.B.. and H. Zhang), p. 149-174. American Geological Institute. Wignall. P.B., Kozur. H.. and A. Hallam. 1996. The timing of palaeoenvironmental changes at the Permo-Triassic (P/Tr) boundary using conodont biostratigraphy. Historical Biology, v. 12, p. 39-62. Wood. R. 1995. The changing biology of reef-building, Palaios. v. 10, p. 517-29. Woods, AT)., Bottjer. D.J.. Mutti, M., and J. Morrison 1999. LowerTriassic large sea-floor carbonate cements; their origin and a mechanism for the prolonged biotic recovery from the end-Permian mass extinction. Geology, v. 27. p. 645-8. Worsely. T.R.. Moore, T.L., Fraticelli, CM ., and C.R. Scotese. 1994. Phanerozoic CO; levels and global temperatures inferred from changing paleogeography. Geological Society of America Special Paper, v. 288, p. 57-73. Xu. D. and Z. Yan. 1993. Carbon isotope and iridium event markers near the Permian/Triassic boundary in the Meishan section, Zhejiang Province. China. Palaeogeography, Paiaeoclimatology, Palaeoecology, v. 104. p. 171-5. 271 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Yang, Z. and Z. Li. 1992. Permo-Triassic boundary relations in South China. In: Permo-Triassic events in eastern Tethys (ed. W.C. Sweet, Z. Yang, J.M. Dickins and H. Yin), p. 9-20, Cambridge University Press, Cambridge. Yin. H. 1985. Bivalves near the Permian-Triassic boundary in South China. Journal of Paleontology, v. 59. p. 572-600. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix I: Topographic map showing Muddy Mountain locality in the Valley of Fire, near Logandale. NV. 273 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. • • M u a M j r M m m m U i i i » K m il 20 2 4 , 10 X ahvnr> » p |in u u i> i( lucaiiun u f Mu p lliit T I • • ^=r~i=t t-i i-=r~ IC*ii 1:2*.uuu a _________ H H H H H E i*> 2 7 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 2: Topographic map showing Vigo locality near Caliente, NV. 275 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V IQ O a V A O R A H t l f f S $ t i < v £■€ r. ^ \ D i ~ \ \ ) > i \ f l V 9 w M r J V u > 7 . . ^ '1 F v ^ ^ > A ] \ \ i k h i r % fH k v * h r( w ^ s i-5R c — / W f / k 2 s t - s T I X shows ip p aiiiu aic luotiiou of w a p iu i( E = i sCalE 1:24.Oitu o ____ Wh i h h h h Uiltf a* i«i 276 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 3: Map showing Lost Cabin Springs locality in southern Nevada. Following Blue Diamond Highway east, dirt road veers north at approximately 26 miles from Las Vegas, (modified from Schubert. 1989). 277 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Laft CdBvt • Spring 1 , 'J M A T ■ Lost CaDin Spring Quadrangle. Nevadftj ; •- I' 1 • • .1 • ! '< ■ ; . ! } u . / - M - r v= u ; > i ■ V I "n! •i r ■ t V - > - - r f A y 4 S ' . / I M ~ ~ W ' r ~ * r ~ jlr T 2 2 s 1 Shows areiafyf sampling !r 2 7 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 4: Table showing trace fossil distribution, ichnofabric indices, and % bedding plane coverage from Muddy Mountains. 279 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Muddy Mountains Ichnofabrics Unit Bad Thickness 1. Index T race ty p e D escriptions 8A .5 m N/A N/A This bed is heavily bioturbated a t the b ase ONLY and extends only a couple ot a n into the sedim ent. Beading p*40% T hese b ed s are dark gray, and show a weblike pattern ot bioturbation. They contain lighter gray b e d s intermittent m layers. The dark gray beds are extensitvely bioturbated. and w here weathering has affected the beds less, one can s e e the round nature ot m e traces. They weathei ta look m ore jagged. 9 A .5m (A*B) 7 arenicolites paleoptiycux 9B .5m (A-rB) ll-ii? pianolites T nese are tne lignt gray beds tnat are close to the o a se ot the uark gray beds. T nese lignter colored o eu s contain traces, but arc less neaviity bioturbated m an tne darker traces. BttOOing ps<tO% diDlocratarion? mizocoraiiium 1 0 5 .5 m ii-iii rnizocorailium This yellowish shaly unit contains gutter casts, a s well a s rnizocorailium traces. It is difficult to tell it m e traces are continuous tnrougn alt 5.5m ot m e bed, but m ere are som e m at appear in outcrop at m e o ase or the unit. BttddiOQ p*-iO% pianolites 1 9 0 7 .9 m ii pianolites T hese traces occurred in m e top ot m e Unit 19. and th ese traces are am ong m e largest seen at mis locality. T hese traces can be up to 1cm wide and 10 cm long. They do not appear to distubr bedding, and are mostly horizontal. Som e U -snapeo burrows were also collected from th e se beds. BtddifiQ p«<10% arenicolites 280 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 5: Table showing trace fossil distribution, ichnofabric indices, and % bedding plane coverage from Vigo. 28 L Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vigo Ichnofabrics Unit Bad Thickness 1 . Index T ra c e type D escriptions IB 32m (A*B) iv-y N /A This bed is heavily biotuioated horizontally with interm ittent layers ot wavy laminations. T he bioturbated b eo s mere sam pled in m e upper Sm ot Shorb's Unit i . T hese beds are monied, an a traces are dilticult to identity. 2B 41m (A+B) iv N/A T hese traces w ere found in the upper halt ot Shorb’ s Unit 2 and were chertified in som e places. The am ount ot bioturbation was less m an lower in m e section, how evei. m e traces were larger an a m ore distinct. Buirows m ay have a m ore vertical com ponent m an those lower in m e aecu..i> 10A Sm (A only) iii pianolites Tne Diotuioation in tnese oeos consists primarily ot horizontal buirows. The becom e is not completely destroyed, however, m e weatnering ot m e beds m akes collecting slabs dilticult. in talus, ourrows are curved and non-branching. The weathering pattern is similar to oeds sam pled at m e Lost Cabin locality. 11 4 4 .4 m (to la l) iii-iv N /A The Dioturoation in mis unit occurred in m e lirst tedge above Unit 10. This leoge snow ed odd wavy seo structures, wim som e evidence ot bioturbation. The Diotuioation w as difficult to collect u > ai«u=>. and createa a mottling ot parts ot m e ledge, (sam e a s S hore's Ledge 1) 2 8 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 6: Table showing trace fossil distribution, ichnofabric indices, and % bedding plane coverage from Lost Cabin Springs. 283 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lost Cabin Ichnolfabrics Unit Bed Thickness L Index T race type D escriptions 5 1 .5 -1 .7 m iii-iv N/A Unit 5 h a s a covered b ase, but appears to w eather similarly to Unit 10A a t Vigo. This Unit is heavily bioturbated horizontally, but does not destroy all original bedding. B ecause of the weatnering tn ese b ed s nave undergone, it is dilticult to s e e individual traces on bedding planes. 6 2 m iv-v N/A Unit 6 is primarily a limestone ledge which is neavily bioturbated at the top. Som e beds in tne upper section ol Unit 6 contain the sam e types ot weathering patterns seen in Unit 5. Again, m ost ot tne burrowing is norizontal. On tne top ol tne lower ledge ut Unit 5 are som e webluie patterns ot bioturbation. 7 3 .2 5 m liii-iv N/A Unit 7 is tne next m assive lim estone tedge, and contains intensely w eathered beds that again do not preserve much ot the original trace structure. The bedding is still som ew nat preserved since tne majority ut m e bioturbation is horizontal. Tnis unit ulsu contains odd stiuU utes m at may oe microbiai. 9 Sm iii-iv In /A Unit 9 consists mostly ot bioturbated beds mat resem oies Unit 7. There is som e original bedding, witn u. lot ot w eam enng mat m axes inoentitic&tion ot individual traces difficult. Tnis unit is very m assive and neavily bioturbated noiizontally. It is one ot m e largest outcrops a t mis section snowing bioturoation througnout m e beds. 11 1 .1 -1 .3m l-ii thaiassinoides This unit is neavily bioturbated at its oase, and nas limited exposures. W here exposed, it show s thick, brancning burrows, up to .5 ctn-l cm tnick. T hese burrows a re intilled and cover mucn ot m e bedding plane wnei c exposed. tttddm g p*1(h23% 15 low er 2 .7 -3 m (to tal) M S pianolites The bottom ot Unit IS contains branching burrows m at are inntleu. T hese branches ao not destroy uriginai bedding, and are mostly horizontal. They do not cover m e surface, but occur sporadically on bedding planes. The top ledge ot Unit 15 is thoroughly bioturbated and m e bioturbation d e s tiu ,s m ost original bedding so m at m e upper ledge looks lixe a m assive ditt ot bioturoated gray limestone. T hese traces are also difficult to identity du e to m e nature ot m e biotuibation. o cd o tn ff 0««1O% 15 u p p e r V N/A Dedfling M(MO% T S t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Evolutionary paleoecology and taphonomy of the earliest animals: Evidence from the Neoproterozoic and Cambrian of southwest China

PDF

Barnacles as mudstickers? The paleobiology, paleoecology, and stratigraphic significance of Tamiosoma gregaria in the Pancho Rico Formation, Salinas Valley, California

PDF

The unusual sedimentary rock record of the Early Triassic: Anachronistic facies in the western United States and southern Turkey

PDF

Helicoplacoid echinoderms: Paleoecology of Cambrian soft substrate immobile suspension feeders

PDF

A phylogenetic analysis of oological characters: A case study of saurischian dinosaur relationships and avian evolution

PDF

Paleoecology and paleoenvironments of early Triassic mass extinction biotic recovery faunas, Sinbad Limestone Member, Moenkopi Formation, south-central Utah

PDF

Early Jurassic reef eclipse: Paleoecology and sclerochronology of the "Lithiotis" facies bivalves

PDF

Grain-size and Fourier grain-shape sorting of ooids from the Lee Stocking Island area, Exuma Cays, Bahamas

PDF

Paleoenvironments and paleoecology of the disaster taxon Lingula in the aftermath of the end-Permian mass extinction: Evidence from the Dinwoody Formation (Griesbachian) of southwestern Montana...

PDF

Characterization of geochemical and lithologic variations in Milankovitch cycles: Green River Formation, Wyoming

PDF

Dynamic fluvial systems and gravel progradation in the Himalayan foreland

PDF

Holocene sedimentation in the southern Gulf of California and its climatic implications

PDF

Elemental analysis as a first step towards "following the nitrogen" on Mars

PDF

A proxy for reconstructing histories of carbon oxidation in the Northeast Pacific using the carbon isotopic composition of benthic foraminifera

PDF

Geology and structural evolution of the southern Shadow Mountains, San Bernardino County, California

PDF

Fourier grain-shape analysis of quartz sand from the Santa Monica Bay Littoral Cell, Southern California

PDF

Cyclostratigraphy and chronology of the Albian stage (Piobbico core, Italy)

PDF

Construction of a gabbro body in the Trinity Complex, northern California

PDF

A tectonic model for the formation of the gridded plains on Guinevere Planitia, Venus: Implications for the thickness of the elastic lithosphere

PDF

Preservation Of Fossil Fish In The Miocene Monterey Formation Of Southern California

Asset Metadata

Creator Pruss, Sara Brady (author)

Core Title Biotic recovery from the end-Permian mass extinction: Analysis of biofabric trends in the Lower Triassic Virgin Limestone, southern Nevada

School Graduate School

Degree Master of Science

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,paleoecology,paleontology

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Bottjer, David (committee chair), Corsetti, Frank (committee member), Douglas, Robert (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-292345

Unique identifier UC11342334

Identifier 1411035.pdf (filename),usctheses-c16-292345 (legacy record id)

Legacy Identifier 1411035.pdf

Dmrecord 292345

Document Type Thesis

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