AN ABSTRACT OF THE THESIS OF GAIL DARICE HILDRETH for the MASTER OF SCIENCE (Name) (Degree) in GEOLOGY presented on December 18, 1980 (Major) (Date) Title: THE BEDROCK GEOLOGY AND STRATIGRAPHY OF THE MISSISSIPPIAN AND EARLY PENNSYLVANIAN ROCKS OF THE SOUTHEAST FLANK, ARMSTEAD ANTICLINE, BEAVERHEAD COUNTY, MONTANA. Abstract approved: Redacted for privacy Fourteen square miles of the southeast flank of the Arnstead anti- dine are included in the thesis area. The anticline lies 18 miles southwest of Dillon, in Beaverhead County, Montana. The thesis area was mapped in detail to define formation distribution and structural trends. The rocks exposed in the area range inage from Precambrian to Recent. The Precambrian basement rocksare overlain by 6,000 feet of Paleozoic strata which are divided into ten formations. These formations encom- pass the Cambrian, Devonian, Mississippian, and Pennsylvanian Periods. The only Mesozoic unit present in the thesis area is a part of the Cretaceous-Tertiary Beaverhead Formation. Cenozoic units include Beaverhead conglomerates, unconsolidated gravels, a basalt flow, river terrace gravels and alluvium. Descriptions of the stratigraphy and biostratigraphy of the Mississippian and Early Pennsylvanian rockswere the major areas of concentration. Specific attention was directed toward the petrography and biostratigraphy of the Big Snowy Formation units; this ultimately led to interpretation of the environments of deposition. This bio- stratographic and petrographic study is the first of its kind on the Big Snowy Formation west of the Montana cratonic platform.Approximate thickness determinations of Mississippian strata in the Arnistead anti- dine have furthered the understanding of the pattern of sedimentation in the area between the cratonic platform and the miogeosyricline. Both cratonic and geosynclinal styles of structural deformation are present in the thesis area. The Armstead anticline is a large, northwest-trending fold, which is flanked on the east by the Grayling syncline. The Tendöy thrust fault, which marks the eastern edge of the Grayling syncline, has juxtaposed Mississippian limestones on Cretaceous-Tertiary conglomerates. The economic potential for petroleum discovery in the Armstead anticline area does not appear encouraging. Besides Tertiary folding and faulting which may have exposed existing hydrocarbons, the inter- pretation of the Tendoy thrust as a high angle reverse fault with a lesser lateral displacement suggests that a petroleum trap does not exist within, or beneath, the main structure of the anticline. The Bedrock Geology and Stratigraphy of the Mississippian and Early Pennsylvanian Rocks of the Southeast Flank, Armstead Anticljne, Beaverhead County, Montana. by Gail Dance Hildreth A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science December 1980 Commencement June 1981 APPROVED: Redacted for privacy Professbrof Geol in charge of major Redacted for privacy Chairman of Departr1't of Geology Redacted for privacy Dean of Gkaduate Schoo Date thesis is presented December 18, 1980 Typed by Frances Gallivan for Gail Dance Hildreth ACKNOWLEDGMENTS I would like to express my appreciation for the financial assist- ance provided by Amoco Production Company. The guidance, advice and encouragement offered by Dr. Keith F. Oles during the field visit and preparation of the manuscriptwas deeply appreciated. To Drs. Johnson and Enlows go my appreciation for their critical reading of the manuscript and their advice and guidance. I am very grateful to Dr. Robert Scholten of Pennsylvania State University for the encouragement offered during thesummers of 1979 and 1980. Special gratitude is due to Drs. C. A. Sandberg, Mackenzie Gordon, and William Sando of the U. S. Geological Survey, whose assistance in the form of identification of fossils, information regarding the regional Paleozoic stratigraphy, and constructive criticism was very important to my work. To Kumbe Sadler and Rich Kiecker, thanks formany hours 0f discussion and support throughout the field season and during the preparation of the thesis. A special thanks to Jeff Coryell, my field assistant, for his assistance in the field and participation fn numerous geologic discussions regarding the fieldarea. Finally, my deepest gratitude to Mark Bartlett, my parents, and my brother for their unwavering support throughout the acquisition of my Master's degree. TABLE OF CONTEWVS INTRODUCTION........................................................1 Purposes.......................................................1 Investigative Methods..........................................1 Field....... ..... 1 Laboratory............. 2 GeographicSetting.............................................2 LocationandAccessibility................................3 Topography........ 5 PreviousWork.............. ................................... REGIONAL PALEOZOICSTRATIGRAPHY ...........................10 StratigraphicSuccession .................................12 Cambrian ................................................. 12 FlatheadFcrmation..................................12 WolseyFormation....................................13 Meagher Formation............ 13 ParkFormation......................................14 Pilgrim Formation...................................14 DryCreek Formation................................15 RedLion Formation..................................15 Devonian ................................................. 16 MaywoodFormation...................................16 Jefferson Formation.................................17 Three Forks Formation...............................17 Mississippian .............................................. 18 MadisonGroup.......................................19 Lodgepole Formation............................19 Mission Canyon Formation.......................20 BigSnowy Group .................................... 20 KibbeyFormation...............................21 Otter Formation................................21 HeathFormation................................21 AmsdenGroup........................................22 Tyler Formation................................22 AlaskaBenchLimestone.........................23 Devil's Pocket Formation ....................... 23 Pennsylvanian........... 24 Quadrant Formation........ ......................24 Permian........ ...........................25 Phosphoria............. 25 THESISAREASTRATIGRAPHY........ ..................................26 FlatheadFornation............................................26 Distribution and Topographic Expression..................26 Thickness and Litholoqy............ .....................28 Ageand Correlation ...................................29 EnvironmentofDeposition................................29 Page WolseyFormation.............................................. 30 Distribution and Topographic Expression..................30 Thickness and Litholoqy..................................31 AgeandCorrelation .................................31 EnvironmentofDeposition................................32 P ii grim Formation...... 32 Distribution and Topographic Expression..................32 ThicknessandLitholoqy..................................33 AgeandCorrelation ........ 34 Environmentof Deposition................................35 JeffersonFormation ..................................37 Distribution and Expression..............................37 ThicknessandLithology..................................37 AgeandCorrelation......................................38 Environment of Deposition................................ ThreeForksFormation.........................................40 Distribution and Topographic Expression..................40 Thickness andLithology................ 40 AgeandCorrelation......................................44 Environmentof Deposition................................48 LodgepoleFormation........................................... 48 Distribution and Topographic Expression..................48 ThicknessandLitholoqy..................................50 AgeandCorrelation................................... 54 Environmentof Deposition................................58 MissionCanyonFormation............... 59 Distribution and Topographic Expression..................59 ThicknessandLitholoqy..................................60 Ageand Correlation ................................... 70 Environmentof Deposition................................72 BigSnowy Formation...........................................73 Distribution and Topographic Expression..................73 Thickness andLitholoqy..................................75 AgeandCorrelation................................... 87 Environmentof Deposition.................................89 ArnsdenFormation..............................................93 Distribution and Topographic Expression..................93 Thickness and Litholoqy....... .......................... 94 AgeandCorrelation ....................................96 Environment of Deposition...... 98 QuadrantFormation............. ..........................99 Distribution and Topographic Expression..................99 ThicknessandLitholoqy..................................99 AgeandCorrelation ........ 102 Environment of Deposition............................... 104 BeaverheadFormation.................. ......................105 Distribution and Topographic Expression.................106 Thickness and Lithology.................................106 Age