PN ABSTRACT OF THE THESIS OF Colin F. Ke7, for the degree of Master of Science in Geolo' presented on May 5. LW. Title: Strati_grffir and Detpositicmal History of the Amsden and lower Ouadrant Formations, Snawcrest Range, Beaverhead and Mason_ Counties. Montana Redacted for Privacy Abstract approved: 7 Keith F. Oles During the Late Mississippian-Early Pennsylvanian (latest Chesterian-early Morrowan), a thick sequence of shallow -water carbonate-clastic sediments, equivalent to the "Amsden" and lower Quadrant Formations, was deposited in the Snowcrest trough on the western margin of the northern Rocky Mountain Cordilleran platform in southwestern Montana. Today this late Chesterian sequence is eNposed primarily within the Snawcrest Range which trends in a northeasterly direction through Beaverhead and Madison Counties, Montana. Conforming to new Late Mississippian namenclature for rocks in southwestern Montana, the following stratigraphic units, listed in ascending order, were recognized; the Conover Ranch Formation of the Snowcrest Range Group and the Quadrant Formation. In addition, a relatively thick (325 foot average), unnamed, Shallow water carbonate sequence was recognized betwwen the Conover Ranch and Quadrant Formations. Thus, three stratigraphic units, the Conover Ranch Formation, an unnamed limestone unit, and the Quadrant Formation were recognized and comprise the latest Mississippian-earliest Pennsylvanian strata in the thesis area. The Conover Ranch Formation (late Chesterian) represents a law- energy, marginal marine depositional sequence. The formation consists primarily of red shales and siltstones with occasional quartz arenite -limestone conglomerate lenses. The quartz arenite- limestone conglomerate lenses are thought to represent higher energy, subtidal channel deposits. The Conover Ranch Formation rests unconfonnably upon the Lombard Limestone (Big Snowy equiva- lent) thoughout much of the thesis area. The unnamed limestone unit (latest Chesterian) was de1rsited in a shallow-water carbonate shelf environment. The unit consists primarily of wackestones, packstones, and mudstones. Occasionally, an organic-rich mudstone interval was recognized and samples were collected for hydrocarbon source-rock analyses. The unnamed limestone unit grades vertically into a lower dolomitic interval of the Quadrant Formation. Only the lacer dolomitic interval of the Quadrant Formation (latest Chesterian? -Morrowan) was studied in this thesis. The sequence consists primarily of dolostones and dolomite - cemented quartz arenites which were deposited in a littoral to neritic environment. The origin of the dolomite is considered to be secondary, resulting from post-depositional diagenetic events. In general, the Late Mississippian -Early Pennsylvanian stratigraphic units in the thesis area reflect a single, major transgressive-regressive depositional sequence. The transgression of the sea onto the Cordilleran platform began with the deposition of the Cbnover Ranch Formation, reached a maximum during deposi- tion of the unnnamed limestone unit, and began to retreat off the platform at the onset of clastic deposition dharacteriztic of the Quadrant Formation. The overall hydrocarbon source rock potential of the unnamed limestone unit( "Pmsden ") in southwestern Montana is poor. Stratigraphy.and Depositional History of the Pmsden and lower Quadrant Fonuations, Snawcrest Range, Beaverhead and Madison Cbunties, Montana. by Colin F. Rey A THESIS submitted to Oregon State University in partial fulfillment of the reguirenents for the degree cf Master of Science Campleted May 5, 1986 Camencement June 1987 APERCAIED: Redacted for Privacy ErofeAu.....of Geology in( charge of major Redacted for Privacy department oq' Geaogy Redacted for Privacy Dean of Gr dte School Date thesis presented jilay 1, 1986 ACKNOWLPnnENENIS I would like to thank Dr. Keith F. Cles for his continual guidance and support during the preparation of my thesis. I would also like to thank Drs. J. Granville Johnson and Edward M. Taylor for their critical reviews of my thesis. A special thanks is extended to William (Chip) Goodhue for suggesting tome this thesis topic. A special thanks is also extended to David J. Byrne whose friendship and expertise in the field were invaluable. I would also like to express my gratitude to Keith Mullen who kindly helped me out in the field. During the preparation of my thesis, conversations and correspondence with L. E. Davis, J. T. Dutro Jr.,E. K. Maughan, W. C. Pecora, W. J. Perry Jr., W. J. Sancti,H. I. Saperstone, M. K. Sheedlo, and I. J. Witkind proved to be enlightening and were greatly appreciated. A sincere thanks is extended to ARCO for providing me with financial aide and to Tenneco Oil Company for providing geochem- ical source-rock analyses. Finally, a very special thanks is offered to my parents and to my wife, Janell, whose patience, support, and concern for this project never waivered! TABLE OF CONTENTS INTROEUCTICN 1 DEVELOPMENT OF NOMENCLATURE 20 Introduction 20 Amsden Group: Montana 22 Amsden Formation: Wyaning 27 Combining the "Ansdens" 30 Quadrant Formation 35 Stratigrapbic Nomenclature used in thesis 38 PALEDTECTCNIC SEWING 41 SZRATIGRAPHIC INTERPRETATION 45 Conover Ranch Formation 45 Limestone unit (unnamed) 76 Quadrant Formation 91 STRATIGRAPHIC CORRELATIONS 101 Southwestern Montana: Thesis area 101 Regional Correlations 102 Central Montana 103 Wyoming 105 Conclusions 1C8 CAFECNIFERCUS GECLCGIC HISTORY 112 HYDROCAIBCN SCURCE ROCK ANALYSIS 118 REFERENCES 127 APPENDIX 136 Sheep Creek 136 Ruby Gap 142 Sncwcrest Mountain 145 Sliderock Mountain 149 Spur Mountain 156 Hogback Mountain 162 Sunset Peak 167 Sawtooth Mountain 173 Red Rock River 179 Big Sheep Creek 184 LIST OF FIGURES Figure Raga 1. Generalized paleogeography of the northern Rocky Mountain region during the late Paleozoic. 2. Late Mississippian-Early Pennsvlvanain trough system across the northern Rocky Mountain Cordilleran platform in Montana. 3 3. Outline map of basins and mountain ranges adjacent to the Snowcrest Range, southwestern Montana. 7 4. View of northern Snowcrest Range from Upper Ruby River Road. 8 5. View of poorly exposed, grass-covered section typical of exposure quality in southwestern part of the Snowcrest Range. 9 6. View of excellent, ledge- to cliff-forming exposures typical of exposure quality in northeastern part of the Snowcrest Range. 10 7. Outline map of towns and roadways adjacent to the Snowcrest Range, southwestern Montana. 12 8. Measured section localities within thesis area. 14 9. Present day structural setting, southwestern Montana. 16 10. Nomenclatural development of the "Amsden" in Montana. 21 11. Nomenclatural development of the "Armsden" in Wyoming. 28 12. Relationship of the "Amsden Group" to the "Amsden Formation" between Montana and Wyoming as it existed in the literature during the mid-1970's. 32 13. Combining the "Amsdens" across the MOntana- Wyoming border. 34 14. Nomenclatural development of the Quadrant Formation. 36 15. Strati graphic nomenclature used in thesis. 39 Figure Page 16. Late Mississippian-Early Pennsylvanian paleotectonic setting across Montana. 42 17. Type section of the Conover Ranch Formation along Sheep Creek Canyon in the Blacktail Mountains. 46 18. Exposure of the Conover Ranch-Lombard Limestone contact at Sawtooth Mountain. 49 19. Close-up of contact between Conover Ranch Formation and Lombard Limestone at Sawtooth Mountain. 50 20. Trench revealing both a pale reddish (10R 6/2) and light gray (N 6) color for a section of shales through the Conover Ranch Formation. 53 21. View of the contact between the Conover Ranch Formation and Lombard Limestone at Sawtooth Mountain. 55 22. View of the contact between the limestone conglomerate an quartz arenite units of the Conover Ranch Formation at Sawtooth Mountain. 56 23. Schematic composite stratigraphic column through the quartz arenite - limestone oonglanerate unit of the Conover Ranch Formation. 57 24. View of the quartz arenite-limestone conglomerate exposure on the eastern flank of Sliderock Mountain. 58 25. Photomicrograph of quartz arenite from the Conover Ranch Formation displaying syntaxial quartz overgrowths. 60 26. Photomicrographs of calcite-cemented quartz arenite from the Conover Ranch Formation. A) Uncrossed nicols, B) crossed nicols. 61 27. Photomicrograph of calcite-cemented quartz arenite from the Conover Ranch Formation displaying preferential replacement of the calcite cement by henatite along crystallographic cleavage traces. 63 28. Symmetrical wave-rippled surface, Conover Ranch quartz arenite, six inch ruler for scale. 64 Figure 29. Photograph of the limestone conglomerate lithology within the Conover Ranch Fonnation, six inch ruler for scale. 65 30. Photograph of Ziphonophylla sp. within the limestone conglanerate unit of the Conover Ranch Formation. 67 31. Photanicrograph of limestone conglomerate unit fran the Conover Ranch Formation. 68 32. Compositional plot of 18 quartz arenite-limestone conglanerate lithologies. 69 33. View of medium-bedded limestones Characteristic of the unnamed limestone unit; rock hammer for scale. 78 34. Photograph of dark gray (N 3) chert stringers within light olive gray (5Y 6/1) limestone of the unnamed limestone unit, six inch ruler for scale. 80 35. Photanicrograph of an articulated ostracod carapace 81 36. Photomicrograph displaying an optically continuous calcite overgrowth around a crinoid plate. 82 37. Photomicrographs of fossil fragments which have been partially silicified; A) bryozoan, B)