Structural Geology of the Swift Reservoir Culmination, Sawtooth
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Structural geology of the Swift Reservoir culmination, Sawtooth Range, Montana by Donald Scott Singdahlsen A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in Earth Sciences Montana State University © Copyright by Donald Scott Singdahlsen (1986) Abstract: Northwest trending, west dipping thrust—faulted and folded rocks of Paleozoic and Mesozoic age comprise the Sawtooth Range of northwestern Montana. The configuration of crystalline basement may have controlled the development and geometry of the Sawtooth Range, a part of the Northern Disturbed Belt. Primary influences include the Sweetgrass Arch and a series of northeast trending features which may reflect a fundamental structural grain in the Precambrian basement. Timing of Sevier-style deformation within the Northern Disturbed Belt is bracketed to have occurred primarily during the Paleocene. The Swift Reservoir culmination is defined by a doubly-plunging thrust sheet anticlinorium which is structurally higher than adjacent areas along strike. Fold axes within the culmination plunge away from the culmination center. Kinematic development of the culmination is a result of polyphase deformation of the Backbone thrust sheet. Uplift, rotation, and folding of the Backbone thrust by footwall imbrication and potential duplex formation is responsibe for development of the Swift Reservoir culmination. Major structures within the culmination include thrust faults, tear faults, and folds. Thrust faults are dominantly bedding-plane thrusts with their location strongly controlled by stratigraphic zones of weakness. Concentric folding by flexural-slip mechanisms is the dominant folding style. Folds are commonly asymmetric with vergence in the direction of tectonic transport. Deformational style and intensity within the culmination are controlled by lithologic variability and structural level of development. Ductility contrasts and mechanical behavior of different lithostratigraphic packages partition thrust sheet deformation within the Swift Reservoir area. Because of these factors, the Cambrian lithostratigraphic package exhibits the greatest deformational intensity as penetrative structures are developed. Orogenic shortening across the Swift Reservoir culmination due to structural thickening is approximately 64%. Development of penetrative fabrics such as boudinage, pencil structure, and axial-planar solution cleavage accounts for an estimated 10 to 25% internal shortening superimposed on the translational shortening observed for the culmination. STRUCTURAL GEOLOGY OF THE SHIFT RESERVOIR CULMIKATIOK1 SAHTOOTH RAKGE0 MONTANA by Donald Scott Singdahleen A thesis submitted in partial, fulfillment of the requirements of the degree of Master of Science in Earth Sciences MONTANA' STATE UNIVERSITY Bozeman, Montana January 1986 /V37? ii APPROVAL of a thesis submitted by Donald Scott Singdahlsen This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citations, bibliographic style, and consistency, and is ready for submission to the College of Graduate Studies. Date Approved for the Major Department Date Head Iajor Department Approved for the College of Graduate Studies Date Graduate Dean iii STATEMENT OF PERMISSION TO USE In presenting this thesis in partial fulfillment of the requirements for a master's degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Permission for extensive quotation from or reproduction of this thesis may be granted by my major professor, or in his/her absence, by the Director of Libraries when, in the opinion of either, the proposed use of the material is for scholarly purposes. Any copying or use of the material in this thesis for financial gain shall not be allowed without my written permission. Signature Date iv ACKNOWLEDGMENTS I would like to thank Dr. David R. Lageson (Committee Chairman), Dr. William W. Locke, Dr. David W. Mogk, and Dr. James G. Schmitt of the thesis committee for their suggestions and constructive criticism during the research and preparation of this thesis. Assistance from the numerous geologists and geophysicists involved in exploration within the area is greatly appreciated. Special thanks are extended to Mark W. Hills for his hard work and patience as a field assistant. Funding provided by the American Association of Petroleum Geologists Research Fund, ARCO Exploration Company, Champlin Petroleum Company, Marathon Oil Company, and Tenneco Oil Company supported field work and greatly facilitated the completion of this study. Well logs provided by Exxon Company, USA are also appreciated. Graduate study • at Montana State University was supported by a graduate teaching assistantship. V TABLE OF CONTENTS Page 1. LIST OF FIGURES....................... yiii 2. LIST OF PLATES............................................ x 3. ABSTRACT....................... xi 4. INTRODUCTION.......... I Purpose........................... I Study Area............... I Field Methods................................ 4 Previous Investigations.................... 4 5. STRATIGRAPHY.......... 6 Paleozoic................... on Cambrian................ Steamboat Limestone. Switchback Shale.... Devils Glen Dolomite Oo oo vo vo Devonian............................... 10 Maywood Formation........ 11 Jefferson Formation........ 11 Three Forks Formation................. 11 Mississippian............. 12 Allan Mountain Limestone....... 13 Castle Reef Dolomite........ 14 Mesozoic........... 16 Jurassic.......................................... 16 Sawtooth Formation........ 17 Rierdon Formation.................. 17 Swift Formation.............. 17 Morrison Formation............. 18 Cretaceous....................... 18 Mount Pablo Formation........................ 19 Kootenai Formation........................... 19 Blackleaf Formation.......................... 20 Marias River Shale............ 21 Telegraphic Creek Formation....... 21 Virgelle Sandstone................ 21 Two Medicine Formation........ 22 vi TABLE OF CONTENTS— Continued Page 6. REGIONAL TECTONIC SETTING 23 Montana Thrust Belt................................... 23 Northern Disturbed Belt..................... 24 Development of the Northern Disturbed Belt............ 27 Northeast Trending Structural Features................ 32 Timing of Deformation................................ 35 7. STRUCTURAL GEOLOGY.. „ ...................... ....... 42 Thrust Faults..... ..................... .............. 42 Fish Lake Thrust Fault....... .......... ........ 45 Eagle Creek Imbricate Zone....... 46 Volcano Reef Thrust Fault.... ............ 48 Old Man Thrust Fault..... .............. 49 Walling Reef Thrust Fault.............. 49 Peak Thrust Fault.............. 53 Backbone Thrust Fault.................. 56 Tear Faults........................................... 59 Spillway Tear Fault..................... 60 Split Mountain Tear Fault........ 62 Folds............ 63 Fold Type and Classification.................. 63 Structural Domains................................ 65 Domain I................................. 68 Domain II....... 70 Domain III................... 71 Domain IV........................ 74 Fold Development.................... 78 Thrust-Fold Relationships................. 80 Strain......................... 82 Penetrative Structures................................ 83 Boudinage.......................................... 83 Pencil Structure..................... 83 Cleavage.................... 84 8. SWIFT RESERVOIR CULMINATION................ 89 General Characteristics 89 Kinematic Development.. 90 Orogenic Shortening 93 9. PETROLEUM GEOLOGY 96 Exploration................ 96 Source Rock Evaluation................................ 99 Source Rock Geochemistry......... 100 Swift Reservoir Area................................ 102 vii TABLE OF CONTENTS— Continued Page Reservoir Evaluation....................... 102 Swift Reservoir Area............................ 104 Structural Traps................ 105 Swift Reservoir Area......... 108 Potential..... 109 10. CONCLUSIONS....... ............................... Ill 11. REFERENCES............. ............................. 115 viii LIST OF FIGORBS Figure Page 1. Generalized tectonic map of northwest Montana.. 2 • 2. Study area location map....................... 3 3. Generalized stratigraphic column.............. 7 4. Mississippian stratigraphic nomenclature chart. 13 5. Subbelt map of the Northern Disturbed Belt.... 26 6. Influence of pre-existing structures on thrust ramp formation........... ..................... 29 7. Structure map of the top.of the basement....... 30 8. Computer enhanced Landsat image of the Swift Reservoir area showing lineament sets......... 33 9. Timing of deformation in the Northern Disturbed Belt................... ....................... 37 . 10. Stratigraphic position of thrust faults....... 44 11. View of Old Man and Walling Reef thrusts...... 50 12. View of Walling Reef thrust................... 52 13. Intraformational thrust within hanging wall of Walling Reef thrust........................... 52 14. Lateral ramp in the Walling Reef thrust....... 54 15. Closeup view of lateral ramp and drag fold.... 54 16. View of Peak and Walling Reef thrusts......... 55 17. View of imbricate splay to the Backbone thrust. 58 18.