University of Nevada Reno Paleoecology of Upper Triassic Bioherms in the Pilot Mountains, Mineral County, West-Central Nevada A tiles is submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by Diane Elinor Cornwall May 1979 University of Nevada Reno ACKNOWLEDGEMENTS I would like to thank Dr. James R. Firby for his supervision, support and for his guidance through those difficult "rough spots." Dan Howe was a constant source of interesting ideas and enthusiasm, especially during the early stages of the thesis project. Throughout my research Fred Gustafson and I worked together, he always being an interested and understanding coworker and friend throughout the lonely weeks of research. Dr. Joseph Lintz, Jr. was invaluable in his assistance in procurement of materials and equipment and in his unselfish desire to solve any problem. I thank Dr. Mead for joining my committee. I greatly appreciate those hearty souls who ventured out into the desert with me and braved the hazards and extremes; these include Barbara Foster, Mickie Dunn, Micheal Judge and my parents who spent all their vacations in my field area. I would also like to acknowledge the moral support given by my parents, special and other friends. i n ABSTRAC In the Pilot Mountains, Mineral County, Nevada, up to five horizons of bioherms are present within the top 76 meters of the lower member of the Upper Triassic (Karnian) Luning Formation. These bio­ herms are located in the carbonate portions of small terrigenous- carbonate rhythms. The biohermal mounds, less than 15 meters high^exhibit four of Wilson's (1975) seven facies. Mound formation was mainly autogenic. Four stages of Walker and Alberstadt's (1975) biological succession are recognized. The dominate organisms are sphinctozoans, corals and spongiomorphids, supported by pelecypods, brachiopods, crinoids, echinoids and gastropods. The mounds formed in moderately shallow, quiet warm euhaline waters in a slowly subsiding area of slow sedimentation and changing terrigenous sediment sources. TABLE OF CONTENTS Page ACKNOWLEDGEMENTS .............................................. iii ABSTRACT...................................................... iv LIST OF FIGURES, TABLES, AND P L A T E S .......................... viii INTRODUCTION .................................................. 1 Introduction to Reefs of the Triassic Period ................ 1 Location .................................................... 3 Purpose, Scope, and Limitations ............................ 5 Methods of Investigation .................................... 5 Previous Investigations .................................... 6 GENERAL GEOLOGY OF THE PILOT MOUNTAINS ........................ 9 Geologic Setting ............................................ 9 Structure .................................................. Stratigraphy ................................................ 1° Luning Formation .......................................... 14 Lower Member............................................ 14 Regional Geologic History . ............................ 15 CINNABAR AND DUNLAP CANYON BIOHERMS .......................... 19 FACIES........................................................ 23 Lopha Basal Pile............................................ 24 Bindstone Facies .......................................... • Sponge-Trichites Bindstone Subfacies ...................... Coral-Spongiomorphid Bindstone Subfacies .................. 26 Thecosmilia Bafflestone .................................... 26 Coral Framestone............................................ 27 Interreef Facies ............................................ 28 VI Page Crinoid Flanking Facies ...................................... 29 Gryphaea Facies .............................................. 30 Other Pelecypod B e d s ........................................ 30 Cavity and Fissure Fillings .................................. 31 Black Calcilutite............................................ 32 Lithoclastic Breccia ........................................ 32 Shale and Argillite B e d s .................................... 33 PALEOAUTECOLOGY . .............................................. 34 Porifera.................................................... 34 Sclerosponges ................................................ 37 C o r a l s ...................................................... 39 Superfamily Thamnasteriodidea Alloiteau, 1952 ........ .... 40 "Thamnasteria" ............................................ 40 Thecosmilia................................................ 42 Other Colonial Corals...................................... 4 3 Montlivaltia .............................................. 43 Brachiopods.................................................. 44 Mollusca.................................................... 46 Bivalvia.................................................. 46 Oysters ..... ........................................ 46 Gryphaea.......................... 46 Lopha.................................................. 47 Pinnidae................................................ 48 Pinna.................................................. 48 T r i c h i t e s ...................... '....................... 48 Cephalopoas................................................ 49 V l l Fage Gastropods................................................ 51 Echinoderms................................................ 51 Echinoids................................................ 51 Cidaroids.............................................. 51 Crinoids.................................................. 53 Vertebrates................................................ 53 PHYSICAL PALEOSYNECOLOGY ...................................... 56 Water D e p t h ................................................ 56 Light Conditions............................................ 58 Radiation.................................................. 58 Temperature of Water ........................................ 59 Shape of Water Body and Geomorphology of the Land Surface and Sea F l o o r ................................................ 59 Water Movement.............................................. 59 Bottom Sedimentalogical Conditions and Sedimentation Rates . 60 MOUND SYNECOLOGY.............................................. 62 Morphology.................................... ............. 65 Diversity.................................................. 69 Maturity.................................................... 69 Size........................................................ 69 Associations ................................................ 71 Predation................................................ 71 CONCLUSION.................................................... 73 SYSTEMATIC PALEONTOLOGY ...................................... 75 SELECTED REFERENCES .......................................... 118 APPENDIX I 133 v ii i Page APPENDIX ! I ..................... ...................................................................... 135 PLATES ............................................................................................... 136 LIST OF FIGURES, TABLES, AND PLATES Figure Page 1. Location Map of Dunlap and Cinnabar Canyons .............. 4 2. Structure Map of Dunlap and Cinnabar Canyons ............ 12 3. Cross Section Across Dunlap and Cinnabar Canyons ........ 13 4. Stratigraphic Section of Balloon Canyon .................. 16 5. Comparison of Triassic and Holocene Coral Borings ........ 41 6- Biological Diagrams of Mounds ............................ 63 7. Mound Locality M a p ...................................... 64 8 . Coral and Spongiomorphid Growth Forms .................... 66 9. Comparison of Triassic and Jurassic Mound Morphology . 67 10. Continuation of Figure 9 ................................ 68 11. Bahaman Reef Morphology .................................. 70 12. Fossil Localities ........................................ Table 1. Bivalve Living Habit and Trophic Level .................. 50 2. Distribution and Abundance of Organisms in Dunlap and Cinnabar Canyons ...................................... 55 Plate 1. Pamiroseris.............................................. 136 2. Margarastraea............................................ 136 3. Astrocoenia, Montlivaltia, Elyastraea .................... 136 4. Pelecypods ........................ ..................... 136 5. Pelecypods.............................................. 137 6. Brachiopods.............................................. I37 7. Juvavites, Cidarid spines, Sclerosponges ................ ] 38 ix INTRODUCTION INTRODUCTION TO REEFS OF THE TRIASSIC PERIOD Carbonate buildups of the Triassic, show some of the characteris­ tics of the Permian paleoreefs and evolve other characteristics which are still prevalent in modern reefs. Knowledge of the characteristics of the Triassic carbonate buildups has increased greatly in the last few years. The majority of Triassic bioherms did not form into true wave resistant reefs but formed on shallow protected platforms (Bosellini. and Rossi, 1974, p. 209). The latitudinal extent of Upper Triassic (Norian) reefoid bodies is between 10 S to 60 N (according to modern continental configuration). Though these buildups are widely distributed, they display a remarkable similarity to each other, more so than other build­ ups do in any later period. The oldest hermatypic scleratinian corals, already diversified into six families, appeared in banks of Middle Triassic Anisian