AN ABSTRACT OF THE THESIS OF William V. Goodhue Jr. for the degree of Master of Science in Geology presented on May 9, 1986 Title: Bedrock Geology and Stratigraphy of the Northern Parts of the Arinstead and Madigan Gulch Anticlines, Beaverhead County, Montana. Redacted for Privacy Abstract approved: Approximately 18 square miles are included in the study area which lies 17 miles southwest of the town of Dillon, Montana. Strata exposed in the area include metamorphic, sedimentary, and volcanic rocks ranging in age from Precambrian to Recent. The Paleozoic and Mesozoic section is approximately 8000 feet thick and includes conglomerates, sandstones, siltstones, shales, cherts, carbonates, and andesite flows and tuffs. These rocks are divided into 18 formations. Cenozoic strata include basalt flows and volcaniclastic rocks, unconsolidated gravels, alluvium, colluvium, and landslide deposits. The study area, located on the western edge of the Paleozoic-Mesozoic Cordilleran cratonic shelf, was dominated by marine sedimentation in shallow to moderate water depths from Middle Cambrian to Late Jurassic time. Eustatic sea level changes, and epeirogenic movements on the shelf and in adjacent areas, acted alone or in concert to effect numerous episodes of subaerial exposure and erosion on the shelf during this period. Cretaceous rocks consist of thick accumulations of syorogenic conglomerates and sandstones of the Beaverhead Formation. These rocks reflect increasing diastrophism on the cratonic shelf and within the adjacent geosyncline to the west. Laramide deformation culminated within the study area in Late Cretaceous time, forming large, north- to northwest trending folds and the low angle, east-verging Tendoy and Indian Head thrust faults. These thrust faults are thought to involve both the Phanerozoic sedimentary strata and the underlying Precambrian metamorphic basement. Volcanic activity, in the form of andesite flows and tuffs, closely followed the period of maximum deformation. An extensional tectonic regime, which resulted in normal faulting and the formation of block-faulted ranges and subsiding basins, developed in the study area during Oligocene time and continues to the present. The faulting was accompanied, probably in Eocene time, by basaltic volcanic activity. Bedrock Geology and Stratigraphy of the Northern Parts of the Armstead and Madigan Gulch Anticlines, Beaverhead County, Montana by William V. Goodhue Jr. A THESIS submitted to Oregon State University in partial fullfillment of the requirements for the degree of Master of Science Completed May 9, 1986 Commencement June 1987 APPROVED: Redacted for Privacy Profes of Gega'ogy in charge of niajor Redacted for Privacy n of Depa nt of Geology Redacted for Privacy Dean of Gr te Schoo Date thesis is presented May 9, 1986 ACKNOWLEDGEMENTS To my parents, Bill and June, I want to express my appreciation for their support, encoragement, and patience without which this thesis would not have been possible. I would like to thank Dr. Keith F. Oles for his advice, encouragement, and prompt critical reading of this manuscript. My gratitude also goes out to the late Dr. Harold E. Enlows, for his assistance with petrographic analyses, and to Drs. J.G. Johnson and L.W. Snee for their critical reading of this manuscript and their advice and guidance. Discussions with Dr. R. Pearson, Dr. E. Ruppel, Dr. R. Scholten, Ed Maughan, Herb Saperstone, and Dr. B. Wardlaw contributed greatly to my understanding of the local structure and stratigraphy. My thanks go out to all of them. I would particularily like to thank my friend Ed Maughan for taking the time to accompany me for several days in the field. Assistance in identifying fossils by Drs. Charles Ross, Gary Lane, and Michael Hansen is also greatly appreciated. Discussions with fellow students and collegues, Charlie Clarke, Tom Dill, Gail Hildreth, Dave Byrne, and Cohn Key helped to reinforce or negate many hypotheses which evolved during the writing of this thesis. The financial support provided by Tenneco Oil Company is thankfully acknowledged. In addition, the assistance of the expert drafting staff of the Oil and Gas Division of Unocal, Bernie Kirsch, Lang Taylor, and Julio Soto, is greatly appreciated. Last but by no means least, I would like to thank my field assistant, fishing partner, and good friend Kathryn Munro for always being there. I couldn't have done it without you, Katy. TABLE OF CONTENTS Page INTRODUCTION Location andaccessibility 1 Geomorphology 4 Purposes 7 Investigativemethods 7 Previous work 9 THESIS AREA STRATIGRAPHY 16 Flathead Formation 16 Nomenclature 16 Distribution and TopographicExpression 16 Thickness and Lithology 18 Age and Correlation 23 Depositional Envircnment 24 Wolsey Formation 27 Nomenclature 27 Distribution and Topographic Expression 27 Thickness and Lithology 29 Age and Correlation 32 Depositional Environment 33 Pilgrim Formation 35 Nomenclature 35 Distribution and Topographic Expression 36 Thickness and Lithology 38 Age and Correlation 45 Depositional Environment 46 Jefferson Formation 49 Nomenclature 49 Distribution and Topographic Expression 49 Thickness and Lithology 51 Age and Correlation 58 Depositional Environment 59 Three Forks Formation 61 Nomenclature 61 Distribution and Topographic Expression 62 Thickness and Lithology 62 Age and Correlation 68 Depositional Environment 71 Lodgepole Formation 73 Nomenclature 73 Distribution and Topographic Expression 74 Thickness and Lithology 75 Age and Correlation 86 Depositional Environment 88 Mission Canyon Formation 93 Nomenclature 93 Distribution and TopographicExpression 93 Thickness and Lithology 95 Age and Correlation 109 Depositional Environment 111 Snowcrest Range Group 114 Nomenclature 114 Kibbey Sandstone 116 Nomenclature 116 Distribution and Topographic Expression 116 Thickness and Lithology 117 Age and Correlation 121 Depositional Environment 124 Lombard Limestone 126 Nomenclature 126 Distribution and Topographic Expression 126 Thickness and Lithology 127 Age and Correlation 138 Depositional Environment 140 Conover Ranch Formation 143 Nomenclature 143 Distribution and Topographic Expression 143 Thickness and Lithology 144 Age and Correlation 155 Depositional Environment 156 Quadrant Formation 158 Nomenclature 158 Distribution and Topographic Expression 159 Thickness and Lithology 160 Age and Correlation 165 Depositional Environment 166 Park City, Phosphoria, and Shedhorn Formations 16 Nomenclature 168 Distribution and Topographic Expression 173 Thickness and Lithology 176 Age and Correlation 194 Depositional Environment 196 Page Dinwoody Formation 204 Nomenclature 204 Distribution and Topographic Expression 205 Thickness and Lithology 206 Age and Correlation 213 Depositional Environment 215 Beaverhead Formation 219 Nomenclature 219 Distribution and Topographic Expression 221 Thickness and Lithology 223 Age and Correlation 231 Depositional Environment 234 Cretaceous and Tertiary Volcanic Rocks 237 STRUCTURAL GEOLOGY 243 Regional Structure 243 Thesis Area Structure 248 Folds 248 Faults 253 GEOLOGIC HISTORY 262 REFERENCES CITED 279 APPENDIX 293 LIST OF FIGURES Figure Page 1 Location map of thesis area. 2 2 Physiographic features of southwestern 3 Montana. 3 View of vertical beds of the Flathead 17 Formation. 4 Photomicrograph of superivature quartz 21 arenite, Flathead Formation. 5 Laminated quartz arenite, Flathead 25 Formation 6 View of Cambrian section on the east 28 flank of the Armstead anticline. 7 Photomicrographs of glauconitic 31 quartz arenite, Wolsey Formation. 8 View of steeply dipping Pilgrim and 37 Jefferson Formations. 9 Photomicrograph of dolomitized 41 oosparite, Pilgrim Formation. 10 Photomicrographs of dolomitized 43 oolite and intraclast, Pilgrim Formation. 1]. Photomicrograph of dolomite rhoinbs 44 filling vuggy porosity, Pilgrim Formation. 12 Tectonic breccia, Jefferson Formation. 54 13 Photomicrograph of laminated 56 dolomite, Jefferson Formation. 14 Photomicrograph of limestone breccia, 67 Three Forks Formation. 15 View of overturned crinoidal 79 biosparites, Woodhurst Member of the Lodgepole Formation. Figure Page 16 Photomicrograph of sparse biomicrite, 81 Paine Member of the Lodgepole Formation. 17 Photomicrograph of biopelsparite, 83 Woodhurst Member of the Lodgepole Formation. 18 Photoinicrograph of biopeisparite, 85 Woodhurst Member of the Lodgepole Formation. 19 View of transition zone between the 96 Lodgepole and Mission Canyon Formations. 20 Photoluicrograph of echinoderm 99 biosparite, Mission Canyon Formation. 21 Views of karst solution breccia zone, 104 Mission Canyon Formation. 22 Photomicrographs of silicified karst 106 solution breccia, Mission Canyon Formation. 23 View of steeply dipping strata of the 115 Snowcrest Range Group on the east flank of the Armstead anticline. 24 Photomicrograph of calcareous quartz 120 sandstone, Kibbey Sandstone. 25 Photomicrograph of Girvanella from 123 the Kibbey Sandstone. 26 Photomicrograph of biopelmicrite, 130 lower part of the Lombard Limestone. 27 Photomicrograph of biomicrite, upper 131 part of the Lombard Limestone. 28 Photomicrograph of poorly washed 133 biosparite, upper part of the Lombard Limestone. 29 Photomicrograph of poorly washed 134 biosparite, upper part of the Lombard Limestone. Figure Page 30 Photomicrograph of articulated 135 ostracode displaying geopetal fabric, upper part of the Lombard Limestone. 31 View of large burrows, upper part of 137 the Lombard Limestone. 32 View of steeply dipping biomicrites 145 and siltstones of the Conover Ranch Format