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Structural Geology of the Wind River Basin, Wyoming

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Structural Geology of the Wind River Basin, Wyoming Structural Geology of the Wind River Basin, Wyoming GEOLOGICAL SURVEY PROFESSIONAL PAPER 495-D Prepared in cooperation with the Geological Survey of Wyoming and the Departtnent of Geology of the University of Wyoming as part of a program of the Department of the Interior for development of the Missouri River basin Structural Geology of the Wind River Basin, Wyoming By WILLIAM R. KEEFER GEOLOGY OF THE WIND RIVER BASIN, CENTRAL WYOMING G E 0 L 0 G I C A L S U R V E Y P R 0 F E. S S I 0 N A L P A P E R 4 9 5 - D Prepared in cooperation with the Geological Survey of Wyoming and the Department of Geology of the University of Wyoming as part of a program of the Department of the Interior for development of the Missouri River basin UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director ••• For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Structural geology-Continued Page Abstract___________________________________________ D1 Basin. structures________________________________ D22 Introduction ______________________________________ _ 2 West margin ______________________________ _ Method of study ___________________________________ _ 22 4 North margin _____________________________ _ 23 Summary of stratigraphy and geologic history _________ _ 6 East margin _______________________________ _ 25 :Meaning of term "Laramide" _______________________ _ 10 South margin _____________________________ _ Structural geology _________________________________ _ 25 11 Structural analysis _________________________________ _ Mountain structures ___________________________ _ 11 26 Foreland deformation __________________________ _ Wind River Range _________________________ _ 11 26 Washakie Range ___________________________ _ 14 Mechanics of deformation ______________________ _ 26 RE-gional uplift_ _______________________________ _ Owl Creek Mountains ______________________ _ 16 29 Volcanism ____________________________________ _ Southern Bighorn Mountains _________________ · 19 30 Casper arch _______________________________ _ 20 Crustal strnctme and t~e implications of isostasy __ _ 31 Granite Mountains ________________________ _ 20 References cited ___________________________________ _ 33 ILLUSTRATIONS [Plates are in pocket] PLA'l'J~ 1. Geologic map of the Wind River Basin. 2. Structure sections across the Wind River Basin. 3. Structure-contour map of the Wind River Basin. Page FIGURJ<:: 1. Physiographic diagram of the Wind River Basin and adjacent areas in central Wyoming------------------------~-------------------------------------- D3 2-6. Thickness maps of: 2. Paleozoic rocks _______________________________________ ---------------- 9 3. Mesozoic rocks below the Cody Shale _______________________ --- ____ -_--- 10 4. Cody, Mesaverde, Lewis, and Meeteetse Formations _____________________ _ 11 5. Uppermost Cretaceous and Paleocene rocks _____________________________ _ 12 6. Lower Eocene rocks _________________________________________________ _ 13 7. Chart showing trends of 92 structural features in the Wind River Basin ___________ _ 13 8-17. Photographs showing- S. Dip slopes of Paleozoic and lower Mesozoic rocks along east flank of the Wind River Range ______________________________________________________ _ 15 9. Exhumed Paleozoic rocks along Horse Creek, northwestern Wind River Basin ____________________________________________________________ _ 1o 10. Precambrian gneisses, schists, and pegmatite dikes overlain by Cambrian rocks, Wind River Canyon ________________________________________ -- 17 11. Vertical to overturned upper Paleozoic rocks in contact with Triassic rocks along south edge of Owl Creek Mountains ____________________________ _ 17 12. Exposure of Boysen normal fault in Wind River Canyon _________________ - 19 13. Structural features along Badwater Creek, southwest end of Bighorn Moun- tains-------------------------------------------------------------- 21 14. Bald knobs of granite projecting above level plain of Miocene rocks, central part of Granite Mountains _________________________________________ _ 22 15. Cretaceous rocks faulted against lower Eocene rocks along north side of Wind River------------------------------------------------------------- 23 16. Folds in Indian Meadows Formation at Shotgun Butte ___________________ _ 24 17. Intrusive mass of middle Eocene igneous rocks at Garfield Peak, central Rattlesnake Hills __________________________________________________ _ 25 18. Diagrams showing alternate interpretations regarding crustal movement in central Wyoming--------------------------------------------------------------- 27 III IV CONTENTS Pngc FIGURE 19. Diagram showing variations in density and crustal thickness required to maintain crustal equilibrium in the Wind River Basin and adjacent mountain ranges_____ D30 20. Map showing relations of Bouguer gravity anomalies to regional topography and areas of thickest sequences of low-density sedimentary rocks in the Wind River Basin and surrounding mountain ranges____________________________________ 31 21. Cross section showing method used in calculating amounts of Laramide uplift and subsidence-------------------------------------------------------------- 33 TABLES Page TABLE 1. Selected wells drilled for oil and gas in the Wind River Basin _____________________ _ D4 2. Stratigraphic units in the Wind River Basin ____________________________________ _ 7 t: 3. Major periods of sedimentation and deformation in central Wyoming ______________ _ 8 GEOLOGY OF THE WIND RIVER BASIN, CENTRAL WYOMING STRUCTURAL GEOLOGY OF THE WIND RIVER BASIN, WYOMING By WILLIAM R. KEEFER •• ABSTRACT Mountains on the north and the Casper arch on the east. Asym­ 'l'he Wind River Basin, which occupies 8,500 square miles in metry toward the southwest and a pronounced northwest aline­ central "'yoming, is typical of the large sedimentary and struc­ ment (N. 40° ,V.) of individual folds and faults dominate the tural basins that formed in the Rocky Mountain region during structural pattern across the entire region. A geologic map, 22 Laramide deformation. Broad 'belts of folded and faulted Pre­ structure sections, and a structure contour map drawn on top cambrian, Paleozoic, and Mesozoic rocks surround the basin, of the Paleozoic (Permian) sequence depict the princfpal char­ including the Wind River Range on the west, the 'Vashakie acteristics of most structural features in the basin and sur­ Range and Owl Creek and southern Bighorn Mountains on the rounding mountain ranges. north, the Casper nrch on the east, and the Granite Mountains Along the east flank of the Wind River Range, Paleozoic and on the south. Heln.tively uudeformed lower Eocene rocks occupy Mesozoic strata form a linear outcrop 1belt with uniform the central part of the ll.>asin. northeastward (basinward) dips of 12°-15°. The monoclinal Precambrian bnsement rocks exposed in the cores of moun­ continuity is interrupted by a series of sharply folded northwest­ tain ranges indicate n long and complex history of sedimen­ trending anticlines which extends along the west margin of the tation, plutonism, metamorphism, and deformation during Pre­ basin for 90 miles. T'he 13 individual folds that occur along this cambrian time. Hock types are chiefly granite, gneiss, and schist; trend have structural closures ranging from 500 to 4,000 feet. abundant .mafic dikes occur locally. The 'Vas}lakie Range, at the northwest corner of the 'Vind During Paleozoic and much of Mesozoic time, central Wyo­ River Basin, is a series of faulted folds in Precambrian, Paleo­ ming was part of the forclancl bordering the Cordilleran geo­ zoic, and lower Mesozoic rocks that were completely buried by syncline on the east. Sedimentary rocks representing all systems, volcanic debris of the Absarolm Range during post-early except possibly the Silurian, were deposited during repeated Eocene time. The structures have now been partly exhumed transgressions of the epicontinental seas. Tectonic activity was along the major drainages. A. continuous reverse fault probably limited to brond upwarping ancl downwarping of low amplitude separates the steep south flank of the range from the adjacent along trends which in many places did not coincide with the northwest margin of the basin. trends of Inter Laramide structures. In Cretaceous time the sea­ The Owl Creel{ Mountains along the north margin of the 'Vinci ways shifted eastward into eastern 'Vyoming in response to River Basin include a complex group of structures showing di­ uplift in southeastern Idaho, and a thick sequence of alternating verse trends and structural behavior. 'Vest of Mexican Pass the marine and nonmarine deposits accumulated across the Wind range is partitioned into several horstlike blocks of Precambrian .Hiver Basin area. rocks flanked by Paleozoic rocks. Individual blocks, trending Laramide deformation in central Wyoming began in latest northwest, are almost completely surrotmded by reverse faults or Cretaceous time with downwarping of the basin trough ancl monoclinal flexures. The adjacent basin margin is also intensely broad doming of parts of the peririheral areas. The intensity of deformed into sharply folded asymmetric anticlines and syn­ move1nent increased through the Paleocene and culminated in clines. East of Mexican Pass the structure of the range is earliest Eocene time in high mountains that were uplifted along virtually
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