AN ABSTRACT OF THE THESIS OF R. KUNBESADLER forthe degree of MASTER OF SCIENCE in GEOLOGY presentedon December 12, 1980 Title: STRUCTURE AND STRATIGRAPHY OF ThE LITTLE SHEEP CREEK AREA BEAVERREAD COUNTY, MONTANA Signature redactedfor privacy. Abstract approved: Keiih F. Oles The Middle and West Forks of Little Sheep Creek in the southern Tendoy Range have incised valleys across Cenozoic structural features exposing strata that range in age from the Mississippian to the Neoene. Paleozoic strata are 1,349 m thick and belong to the Mission Canyon Limestone, the Big Snowy, Amsden, Quadrant, and Priosphoria Formation' The Scott Peak Formation of the White Knob Group of Idaho is allochtno- nous and forms the upper plate of the Medicine Lodae thrust. Paleozoic and Triassic strata of the thesis area represent sedimentation across a transition zone between a stable craton to the east and the Cordilleran miogeosyncline to the west. Regional unconformities are recognized locally at the top of the Mission Canyon Limestone and the Phosphoria and Thaynes Formations, but not at the top o the Big Snowy, Amsden, or Quadrant Formations, Mesozoic strata have a total thickness of 1,404 m and belong to the Dinwoody, Woodside, and Thaynes Formations, thenewly recognized Gypsum Spring Tongue of the Twin Creek Formation, the Sawtooth and Pierdon For- mations of the Ellis Group, the Morrison and Kootenai Formations, the Colorado Shale, and the Beaverhead Formation. Cenozoic strata are represented by the newly named Round Timber limestone (informal) of the Medicine Lodge beds (Miocene) and the Edie School rhyolite (Pliocene). Detailed stratigraphic and petrographic analyses were made of the Triassic Diuwoody, Woodside, and Thaynes Formations, the Jurassic Gyp- sum Spring Tongue, and the Niocene Round Timber limestone in order to determine environments of deposition. The limestones, calcareous siltstones, and silty liinestones of the Dinwoody and Thaynes Formations were deposited in a shallow marine shelf environment as a result of two transgressiva pulses separated by an Early Traissic regression. The Triassic seas had transgressed eastward onto the craton from the miogeosyncline. The Dinwoody arid ThayrLes fauna indicate normal salinities and open marine conditions; the wide- spread regional distribution of the limestone-siltstone facies indi- cates broad equable conditions for sedimentation. Deposition was primarily a tractive processgenarated by storm-driven, tidal, and long- shore currents within a maximum depth of approximately 50 rn. The Early Triassic regressive phase is represented by the deposition of the variegated siltstone, sandstone, limestone, and dolomite of the Wood- side Formation in a tidal flat environment. The Gypsum Spring Tongue consists of interbedded variegated silt- stone, sandstone, limestone, dolomite, and limestone conglomerate that were deposited in a tidal flat and restricted marginal marine environ- ment extending east from southwestern Montana and eastern Idaho across Wyoming arid southern Montana. The Middle Jurassic sea transgressed South across the North American continent. The Miocene Round Timber limestone was deposited in a fresh-water lake in which calcite was being deposited as encrustations on the green algae Chara and as a precipitate directly from solution. The folding and faulting within the thesis area are the result of cratonic and miogeosynclinal responses to Cretaceous-Early Tertiary orogenesis. A southwestward-plunging anticline has been refolded into northeastward-yielding, overturned, doubly-plunging folds oriented northwest-southeast. High angle reverse faults of minor displacement have occurred along the southeastern limb of Garfield anticline and within the axis of the Seybold syncline.Post-Laramide relaxation of compressional forces has caused north- and northwest-oriented normal faults that transect earlier structures. Sandstone and limestone of the Scott Peak Formation of the White Knob Group were implaced along the Medicine Lodge thrust. Structure and Stratigraphy of the Little Sheep Creek Area, Beaverhead County, Montana by R. Kumbe Sadler A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed December 1980 Commencement June 1981 APPROVED: Signature redacted forprivacy. Prófe' in charge of major Signature redacted for privacy. Chairman of Departmff of Geology Signature redacted for privacy. Deanf Graduate choo1 Date thesis is presented December 12, 1980 ACKNOWLEDGEMENTS To my parents, Wesley and Roslyn Sadler, I wish to express my gratitude for once more providing a home and unending love, enthusiasm, and encouragement that supported me during the preparation of this thesis. To Dr. Keith F. Oles goes my appreciation for his critical reading of this manuscript and for his coents, criticisms, and suggestions that rose out of his visit to the field area and numerous conversations during the past year. I also wish to thank Drs. J. G. Johnson and A. R. Niem for reviewing this manuscript and for their guidance in the preparation of this thesis. The orientation to the stratigraphy of the Tendoy Range by Dr. Robert Scholten of Pennsylvania State University and his critique of my field interpretations during the summer of 1979 are greatly appreciated. I wish to extend a word of gratitude to my colleagues, Richard A. Kiecker and Gail D. Hildreth, for the numerous conversations during the preparation of our respective theses in which were developed the theories presented herein. A special acknowledgement goes to Rebekah Wozniak-Gelzer who typed this manuscript. Her role as grammarian has been priceless and will always be appreciated. I also wish to thank the cartographers, Mary Monette and Linda Donaldson, who prepared many of the figures, charts and maps in this thesis;. Karl Wozniak, who made the chemical analysis on the Edie School Rhyolites; and W, J. Sando of the United States Geological Society, who helped identify some Mississippian fossils from the thesis area. My thanks also to Bill and Sue Leitch, resi- dents of Little Sheep Creek, whose friendship and hospitality during the field season will always be remembered with gratitude. The financial support provided by Union Oil Company of Califor- nia is gratefully acknowledged. TABLE OF CONTENTS INTRODUCTION Location and Accessibility 1 Purpose 3 Investigative Methods 4 Previous investigations 6 Geomorphology 9 STRATIGRAPHY Carboniferous System 13 White Mnob Group 17 Scott Peak Formation 17 Lithology and Distribution 18 Fossils 20 Madison Group 22 Mission Canyon Limestone 23 Distribution and Lithology 24 Big SOvy Formation 26 Regional Distribution 27 Distribution 28 Lithology 29 Correlation of Mississippian Strata 29 Amsden Formation 31 Distribution and Topographic Expression 34 Lithology 35 MississippianPennsylvanian Boundary 36 Quadrant Formation 38 Regional Distribution and Correlation 40 Distribution and Topographic Expression 41 Thickness and Lithology 41 Fossils and Age 43 Perinian System 46 Phosphoria Formation 48 Park City Formation 48 Regional Distribution and Correlation 50 DistributionandTopographic Expression 51 Thickness and Lithology 52 Grandeur Tongue 52 Franson Tongue 54 Meade Peak Phosphatic Shale Member 54 Rex Chert Member 56 Retort Phosphatic Shale Member 56 Tosi Chert Member 57 Fossils and Age 57 Triassic System 63 Dinwoody Formation 63 Regional Distribution and Correlation 63 Distribution and Topographic Expression 64 Thickness and Lithology 67 Fossils and Age 74 TABLE OF CONTENTS (Continued) Page Environments of Deposition 76 Woodside Formation 84 Regional Distribution and Correlation 85 Thickness, Lithology, and Distribution 87 Fosiiand Age 101 Erivirontneuts of Deposition 101 Thaynes Formation 106 Regional Distribution and Correlation 107 Distribution and Topographic Expression 109 Thickness and Lithology ill Authigenic Shelf iinera1s 128 Phosphate 128 Glauconite 131 Fossils and Age 132 Environments of Deposition 133 Jurassic System 137 Twin Creek Formation:Gypsur9 Spring Tongue 137 Regional Distribution and Correlation 137 Distribution artd Topographic Expressiort 141 Lithology 141 Fossils and Age 150 Environments of Deposition 150 Ellis Group 154 Sawtooth Formation 155 Regional Distribution and Correlation 156 Distribution and Topographic Expression 157 Thickness and Lithology 157 Fossils and Age 159 Rierdon Formation 159 Regional Distribution and Correlation 160 Distribution and Topographic Expression 161 Thickness and Lithology 161 Fossils and Age 163 Swift Formation 164 Morrison Fortnation 166 Regional Distribution and Correlation 168 Distribution and Topographic Expressiort 169 Lithology 169 Age 170 Cretaceous System 171 Kootenai Formation 171 Regional Distribution and Correlation 173 Distribution and Topographic Expression 174 Thickness and Lithology 175 Fossils and Age 178 Colorado Shale 179 Distribution and Topographic Expression 183 Thickness and Lithology 184 Fossils and Age 186 Beaverhead Forinatiort 187 Age and Correlation 189 TAELE OF CONTENTS (Continued) Page Distribution and Topographic Expression 190 Thickness and Lithology 191 Tertiary System 194 Round Timber Limestone 194 Distribution and Topographic Expression 194 Lithology and Diagenesis 197 Fossils and Age 214 Environments of Deposition 216 Conglomerate 216 Algal-mat Boundstone 219 Charophyta and other Biotics 222 Composition and Texture of the Fenestral Limestone 226 Conclusion 232 Edie School Rhyolites 233 DistributIon and Topographic Expression 234 Lithology and Chemistry