N BSTRCT OF THE ThESIS OF in David 3. Byrne for the decree ofMaster of Science Geoloay oreserited on November 8.1S8 Title:Stratigraphy and DepositiorialHistory cf the Ucer Mississippian Big Snowy Formationin the Srowcrest Range. southwestern Montana. Redacted for privacy bstract approved Prof. Kjith F. Oles The Big Snowy Formation inthe Snowcrest Rangereflects in an middle Meramecian throughlate Chesteriari deposition intermittently si lied trough insouthwestern Montana.The name by the Kibbey Big Snowy Formation hasrecently been supplanted Formation and Lombard Limestone,which compose the lower two-thirds of the SnowcrestRange Group (new). The Kibbey is poorly exposed throughout theSnowcrest Range, but canbe in the subdivided into two informalunits based on measurements Blacktail Mountains, just northof the study area.The lower unit consists predominantly ofsilty and sandy dolostoneswhich represent deposition in an upperintertidal to lowersupratidal quartz arenites, environment.Calcite- and dolomite-cemented deposited on a lower shorefaceto lower intertidalenvironment, civerlies the compose the upper unit. The Kibbey unconforrnably the Mission Canyon Limestone arid isconformably averlairi by Lombard Limestone. Lombard carbonate sedimentationdenotes rapid basin shc'reface to the subsidence and consequentmigration of the Ribbey east in late tleramecian time. Four lithofacies are recognized in the Lombard, whichare, in ascending order of predominant stratigraphic occurrence,(1) lime mudstone,(2) fossiliferous wackestone-packstone,(3) calcareous shale, and (4) dolomiticlime inudetone lithofacies. The first two lithofacies displayan intertonquing relationship recordingepisodic differential subsidence in the trough. Rapid basinwide subsidence and consequent deposition In a stratified watercolumn are reflected in the last two 1 ithofacies. Seven major Late Mississippiantectono-sedimentary events are suggested by regional relationshipsof the Lombard lithofacies in the study area and correlativeunits in east-central Idaho and central Montana. Times of uplift or cessation of subsidenceon the outer cratonic platformseem to correlate to times of increased subsidence on the inner cratonicplatform, and vice versa. Lombard sedimentation responded to the combinedinfluences of episodic subsidence within the trough and to twoperiods of carbonate bank development on the outer cratonicplatform. The lower part of the Lombard is dominated by the limemudstone lithofacies, which was deposited in a slightly silled basinbehind the Scott Peak carbonate bank complex. Drowning of the bank in earlyChesterlan time corresponds with a major progradational eventdocumented in the upper part of the Lombard. Upbuilding of the Surrett Canyonbank in late Chesterian time occurred concomitantlywith rapid subsidence of the Snowcrest trough, creating a deep, silled,vertically stratified basin characterized by calcareous shaledeposits. Subsequent shoaling of the trough is recorded in the dolomitic lime mudstone lithofacies, which disconformably underlies the overstepping Conover Ranch Formation in the study area. Geochemical analyses of the lime mudstone and calcareous shale lithofacies of the Lombard indicate a moderate to good petroleum source rock potential. STRATIGRAPHY AND DEPOSITIONAL. HISTORY OF THE UPPER MISSISSIPPIAN BIG SNOWY FORMATION IN THE SNOWCREST RANGE, SOUTHWESTERN MONTANA by David Jerome Byrne A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed November 8, 1985 Commencement June 8,1986 APPROVED: Redacted for privacy Professor Geology in e of major Redacted for privacy of Depart ment /f eo logy Redacted for privacy Dean of Grad&i4e School Date thesis is presented November 8,1985 ACKNOWLEDGEMENTS To Dr. Keith F. Oles goes my appreciation for his critical review of this manuscript and for the many helpful suggestions made during his visit to the field area and during the past year. I am especially grateful to Dr. Tom 3. DeVries for hisinnumerable technical comments and criticisms, and for the many hours spent discussing the "final model" and reviewing this manuscript. I also wish to thank Dr. J.6. Johnson for his technical assistance and for reviewing this manuscript. To my colleague, Cohn Key,I wish to extend a very special thanks for his friendship, support, and suggestions during the field investigation and subsequent preparation of this thesis. I am indebted to U. J. Sando and 3. T. Dutro, Jr., of the U. S. Geological Survey, for their identification of the Mississippian corals and brachiopods collected in the thesis area, and for the constant encouragement extended to the author throughout this study. Additional appreciation for their helpful technical comments and advice goes to: Jack Cox, James Ruff in, and Pat M6raw (Tenneco Oil Company); Chip Goodhue and Dr. Alan R. Niem (Oregon State University); Pete Isaacson (University of Idaho); 3. K. Rigby (Brigham Young University); Chris Schmidt (Western Michigan University); and E. K. Maughan, Russ Tysdal, and Bruce Wardlaw (U.S. Geological Survey). A special acknowledgement goes to ray family; to ray fellow graduate students, especially Phil Rarey, Irene Jepsen, and John Chesley; and to my special friends, Linda Blythe, Patti VanDeCoevering, Gayin Nordman, and Clay Penhollow, whose unending support, love, and encouragement will always be remembered. The financial support provided by Tenneco Oil Company and Shell Oil Company is gratefully acknowledged. TABLE OF CONTENTS Page INTRODUCTION 1 Location 1 Purpose of study 1 Accessibility 3 Geomorphology and climate 5 Investigative methods 9 Geological setting 13 Snowcrest Range setting 18 PREVIOUS INVESTIGATIONS 22 Introduction 22 Historical development of nomenclature 22 The Quadrant Quartzite and Formation 22 Introduction of the Big Snowy Group 26 The Charles Formation conflict 30 The Mississippian-Pennsylvanian boundary problem 31 Relationship of the Big Snowy Formation to the underlying Mission Canyon Limestone 33 Big Snowy trough 35 Kibbey Formation 35 Otter Formation 36 Heath Formation 37 Snowcrest trough 40 New terminology 42 Kibbey Formation (revised) and Lombard Limestone 42 Conover Ranch Formation 43 Railroad Canyon Formation 43 Relationship of the Big Snowy Formation to the overlying Amsden Formation 44 RELATIONSHIP OF THE KIBBEY FORMATION TO THE UNDERLYING MISSION CANYON LIMESTONE 46 Description 46 Interpretation 57 KIBBEY FORMATION 64 Introduction 64 Description 64 Snowerest Range 64 Blacktail Mountains 72 Dolostone Lithofacies 74 Sandstone lithofacies 76 Deposit ional environment 82 Introduction 82 Dolostone lithofacies 84 Sandstone 1 ithofacies 90 Age and Stratigraphic interpretation 98 TABLE OF CONTENTS (Continued) Pao LOMBARD LIMESTONE: DESCRIPTION 107 Introduction 107 Lime mudstone lithofacies 110 Tongue ID 119 Tongue lID 121 Tongue hID 122 Tongue IVD 122 Tongue VD and VID 123 Fossi 1 iferous wackestone-packstone lithofacies 124 Tongue IS 128 Tongue 118 130 Tongue IllS 132 Tongues IVS-VIS: Introduction 138 Tongues IVS-VIS: Lower part 139 Tongue IVS 145 Tongues IVS-VIS: Upper part 147 Tongues IVS-VIS: Uppermost part 161 Tongue VS 166 Tongue VhS 167 Clover Meadow section 172 Summary 179 Calcareous shale lithofacies 181 Dolomitic lime muditone lithofacies 192 Unit 1 192 Unit 2: Lower part 195 Unit 2: Upper part 198 Unit 3 200 Unit 4 202 LOMBARD LIMESTONE: DEPOSITIONAL ENVIRONMENT 208 Introduction 208 Lime mudstone lithofacies 208 Fosi 1 iferous wackestone-packst one lithofacies 213 Calcareous shale and dolomitic lime mudstone lithofacies 216 AGE OF THE LOMBARD LIMESTONE 221 RELATIONSHIP OF THE LOMBARD LIMESTONE TO THE OVERLYING CONOVER RANCH FORMATION 230 LOMBARD LIMESTONE DEPOSITIONAL MODEL 235 Early late to late Meramecian 235 Early to middle late Chesterian 244 Middle late Chesterian 248 Latest late Chesterian 256 TABLE OF CONTENTS (Continued) PaQe PETROLEUM POTENTIAL 257 CONCLUSIONS 272 BIBLIOGRAPHY 277 APPENDICES 288 A: MEASURED SECTION DESCRIPTIONS 288 I. Red Reck River section 290 II. Clover Divide section 309 III. Sawtooth Mountain section 324 IV. Sunset Peak section 338 V. Hoqback Mountain section 343 VIA. Sliderock Mountain section I 361 VIB. Sliderock Mountain section II 373 VII. Snoworest Mountain section 378 VIII. Clover Meadow section 401 IX. Sheep Creek Canyon section 406 B: SOURCESFOR KIBBEY FORMATION AND LOMBARD LIMESTONE ISOPACH DATA 409 C: COLLECTION LOCATIONS OF SURFACE SAMPLES FOR GEOCHEMICAL ANALYSES (PETROLEUM SOURCE ROCK POTENTIAL) 410 D: GAS CHROMATOGRAPHY RESULTS 411 LIST OF FIGURES Figure Page 1 Location map of southwestern Montana and adjacent Idaho, showing mountains and specific locations mentioned in text. 2 2 Map of Snowcrest Range area, showing major peaks, access routes, and measured section localities (numbers correspond with general locations of measured sections listed in Appendix A). 4 3 View of Snowcrest Range, showing hogbacks which define the ridgeline. 6 4 View of typical geomorphic expression of upper Paleozoic units in Snowcrest Range, showing characteristic valley between two ridges or hills composed of Madison Group (Mine) carbonates and Quadrant Formation (M Pq) sandstones. 8 Recent landslide deposits involving failure within the Big Snowy Formation, on western flank of northern Snowcrest Range. 10 6 Mississippian paleotectonic elements in the western United States. 14 7 Isopach map of Late Mississippian rocks of southwestern Montana. 17 8 Geologic map of southwestern Montana. 20 9 Nomenclature, faunal zonat