Structure and Stratigraphy of the Jefferson Mountain Area Centennial Range, Idaho-Montana
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AN ABSTRACT OF THE THESIS OF CLYDE LEONARD MURRAY for the MASTER OF SCIENCE (Name of student) (Degree) in GEOLOGY presented on c3Of / 9 7;a, (Major) (Date) Title: STRUCTURE AND STRATIGRAPHY OF THE JEFFERSON MOUNTAIN AREA, CENTENNIAL RANGE, IDAHO- MON TANA Abstract approved:Redacted for privacy David Bostwick The Jefferson Mountain area, located at the eastern end of the Centennial Mountains in Fremont County, Idaho and Beaverhead County, Montana, is approximately 42 square miles in size.The area contains exposed metamorphic, sedimentary,and volcanic rocks ranging in age from Precambrian to Tertiary.Approximately 2,500 feet of Paleozoic and Mesozoic sedimentary rocks lie between a basal complex of Precambrian metamorphic rocksand overlying Tertiary volcanics and Quaternary alluvium. The Precambrian rocks in the area are regionally meta- morphosed sedimentary rocks of the Cherry Creek Group that include hornblende schists, granite gneisses, quartzites, and dolomites. Eleven Paleozoic formations occur in the area, representing all of the Paleozoic periods except Ordovician and Silurian.The dominant rock types representing the Paleozoic are dolomites and limestones, but sandstone and shales make up a significant part of the section.The Triassic Dinwoody Formation, a silty dolomite, is the only representative of Mesozoic deposition in the thesis area. The volcanics in the area are part of the late Tertiary Yellow- stone Tuff and Snake River volcanics of eastern Idaho and north- western Wyoming. Pliocene and Early Pleistocene downwarping of the Snake River Plain resulted in block-faulting along its northeastern margin, forming the Centennial Range, a large south-tilting cuesta. The sedimentary rocks were deposited by a series of trans- gressions and regressions of marine water during the evolution of the Cordilleran Geosyncline.The sedimentary rocks represent the shelf facies along the eastern margin of the miogeosyncline. Oil and gas possibilities in the area are negligible.Structural and stratigraphic relationships do not indicate any type of control for the entrapment of petroleum.The economic potentiality for phosphate mining in the area is low because of the thinness of the phosphate-bearing member of the Phosphoria Formation. Structure and Stratigraphy of the Jefferson Mountain Area Centennial Range, Idaho-Montana by Clyde Leonard Murray A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science June 1973 APPROVED: Redacted for privacy Prfessor of 'Geology in charge of major Redacted for privacy Head of Department of Geology Redacted for privacy Dean-orGraduate School Date thesis is presented Typed by Ilene Anderton for Kyde Leonard Murray ACKNOWLEDGEMENTS The author wishes to thank Dr. David A. Bostwick, his major professor, for his helpful criticisms, advice, and field visitation. I would also like to thank Drs. H. E. Enlows and K. F. Oles for their critical review of this manuscript. Appreciation is extended to Dr. William H. Taubeneck for his encouragement and his examination of several thin-sections. The writer wishes to thank his father, Charles Murray for companionship in the field, and I am gratefully indebted to my wife, Cheri, for her companionship and encouragement during the prepar- ation of the manuscript. TABLE OF CONTENTS Page IN TR ODUC TION Location and Accessibility 1 Previous Work 1 Purpose and Methods of Research 3 Relief and Drainage 4 Climate and Vegetation 4 STRA TIGRAPHY 6 Precambrian Rocks 6 Schists 10 Gneisses 11 Dolomite s 12 Quartzite s 13 Paleozoic Rocks 15 Flathead Formation 15 Distribution and Topographic Expression 15 Thickness and Lithology 16 Fossils and Age 20 Regional Correlation 20 Depositional Environment 21 Meagher Formation 21 Distribution and Topographic Expression 22 Thickness and Lithology 22 Fossils and Age 25 Regional Correlation 27 Depositional Environment 28 Park Formation 28 Distribution and Topographic Expression 28 Thickness and Lithology 29 Fossils and Age 30 Depositional Environment 30 Pilgrim Formation 31 Distribution and Topographic Expression 32 Thickness and Lithology 32 Fossils and Age 34 Regional Correlation 34 Depositional Environment 35 Page Jefferson Formation 36 Distribution and Topographic Expression 36 Thickness and Lithology 37 Fossils and Age 38 Regional Correlation 38 Depositional Environment 39 Three Forks Formation 39 Distribution and Topographic Expression 40 Thickness and Lithology 41 Fossils and Age 43 Regional Correlation 43 Depositional Environment 44 Madison Group 45 Regional Correlation of the Madison Group 46 Lodgepole Formation 47 Distribution and Topographic Expression 47 Thickness and Lithology 47 Fossils and Age 50 Depositional Environment 51 Mission Canyon Formation 52 Distribution and Topographic Expression 52 Thickness and Lithology 52 Fossils and Age 53 Depositional Environment 54 Amsden Formation 55 Distribution and Topographic Expression 56 Thickness and Lithology 56 Fossils and Age 56 Regional Distribution 57 Depositional Environment 58 Quadrant Formation 59 Distribution and Topographic Expression 61 Thickness and Lithology 62 Fossils and Age 63 Regional Correlation 63 Depositional Environment 64 Phosphoria Formation 64 Distribution and Topographic Expression 65 Thickness and Lithology 67 Fossils and Age 71 Regional Correlation 72 Depositional Environment 73 Page Mesozoic Rocks 74 Dinwoody Formation 74 Distribution and Topographic Expression 74 Thickness and Litho logy 75 Fossils and Age 77 Regional Correlation 77 Depositional Environment 78 Cenozoic Rocks 79 Basalt Flows 79 Distribution and Topographic Expression 79 Thickness and Litho logy 79 Age 81 Welded Tuff 82 Distribution and Topographic Expression 82 Thickness and Litho logy 83 Age 84 Quaternary Deposits 84 Glacial Till 84 Alluvium 86 Mass Gravity Deposits 86 STRUCTURAL GEOLOGY 88 Regional Structure 88 Thesis Area Structure 89 GEOMORPHOLOGY 91 Stratigraphic and Structural Control of Topography 91 Stream Erosion 92 Glacial Features 92 GEOLOGIC HISTORY 95 Precambrian 9 5 Paleozoic Era 95 Mesozoic Era 98 Cenozoic Era 99 ECONOMIC GEOLOGY 100 BIBLIOGRAPHY 102 LIST OF FIGURES Figure Page 1. Index map of southwestern Montana showing location of the Jefferson Mountain area. 2 2. Precambrian-Cambrian unconformity on the east canyon wall at the headwaters of the south fork of Duck Creek. 9 3. Precambrian to Mississippian rocks at the northwest face of Mt. Jefferson 14 4. Excellent exposure of the Cambrian Flathead Sandstone in the SE1/4 SW1/4 Sec. 35,T. 15 N., R. 42 E. 19 5. Mottled limestone which characteristically typifies the Meagher Formation. 23 6. Photomicrograph of glauconite-calcite structures believed to be algal bodies or dessicated glauconite pellets replaced by calcite. 26 7. Typical exposure of the thin-bedded Mississippian Lodgepole Limestone. 48 8. Planar fore set cross-bedding developed in the clean, white sandstones of the Pennsylvanian Quadrant Sandstone. 60 9. Looking south at Reas Peak, which is capped by the upper tongue of the Shedhorn Sandstone. 66 10. Excellent exposure of the Tosi Chert Member of the Phosphoria Formation located on the northeast face of Reas Peak. 70 Figure Page 11. Exposure of the Triassic Dinwoody Formation. 76 12. Exposure of Tertiary basalt and agglomerate flows. 80 13. Looking northwest at the south side of Mt_ Jefferson with Hellroaring Canyon in foreground, 85 LIST OF TABLES Table Page 1. Summary of rock units, Jefferson Mountain Area, Montana. 7 2. Regional correlation chart. 17 LIST OF PLATES Plate Page 1. Geologic map. folder STRUCTURE AND STRATIGRAPHY OF THE JEFFERSON MOUNTAIN AREA, CENTENNIAL RANGE, IDAHO - MONTANA IN TR OD UC TION Location and Accessibility The Jefferson Mountain Area is located at the extreme eastern end of the Centennial Range (Figure 1).The Centennial Range extends westward from the Henry's Fork of the Snake River to the southern part of the Beaverhead Mountains.Straddling the Continen- tal Divide, the area of this study is located in Beaverhead County, Montana and Fremont County, Idaho and lies approximately 20 miles southwest of West Yellowstone, Montana.The 42 square mile area includes parts of Tps. 14 and 15 S., Rs.1 and 2 E. in Montana and Tps. 14 and 15 N., Rs. 41 and 42 E. in Idaho. Two Forest Service roads, which enter the area from the south along Willow and Blue Creeks, can be reached by the Forest Development Road 030 which connects, with Highway 191 in Idaho. Another road constructed by the Federal Aviation Administration, connecting with Highway 191, gives access to the area from the east. Previous Work D. D. Condit, E. H. Finch, and J. T. Pardee (1927) were the Scale of Miles 10 0 10 20 30 40 SO Butte. ; Three Forks Bozeman MONTANA livingston Ennis Dillon BEAVERHEAD Hebgen COUNTY lake YELLOWSTONE NATIONALPARK*-) 1 West 9.0 Monida Yellowstone enry Lake ' Thesis Area I CLARK F REMONT r COUNTY COUNTY _1 IDAHO WYOMING Figure 1.Index map of southwestern Montana showing location of the Jefferson Mountain area. Scale of Miles 10 0 10 20 30 40 50 \*-- Butte -:. Thre e **,./ Forks Bozeman MONTANA Livingston Ennis Hebgeni lake YELLOWSTONE NATIONAL PARK West Yellowstone nervy Lake Thesis Area CLARK FREMONT COUNTY COUNTY L IDAHO WYOMING Figure 1,Index map of southwestern Montana showing location of the Jefferson Mountain area. 3 first to measure and describe several stratigraphic sections of Late Paleozoic and Early Mesozoic rocks in the Centennial Range.In 1947-48 G. C. Kennedy mapped and described the Lyon Quadrangle, which includes the central part of the Centennial Mountains.Other geologists who have worked in the Centennials are L. L. Sloss and C. A. Mortiz (1951), J. P. Jemmet (1955), R. W. Swanson (1953), F. S. Honkala (1953), and J. L. Moran (1970). Purpose and Methods of Research The purpose of this study was to describe the major rock units and to provide a detailed geologic map of the Jefferson Mountain Area. Field work was accomplished in approximately two and a half months during the summer of 1970.The geology was plotted in the field in low altitude (1 :15, 840) aerial photographs obtained from the U. S. Department of Agriculture, Denver, Colorado.Data were transferred from the photographs to U.