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Structural Style of the Laramide Orogeny, Wyoming Foreland. (Volumes I and II) Order Number 8804154 Structural style of the Laramide orogeny, Wyoming foreland. (Volumes I and II) Brown, William Gregor, Ph.D. University of Alaska Fairbanks, 1987 UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLEASE NOTE: In all cases this material has been filmed in the best possible way from the available copy. Problems encountered with this document have been identified here with a check markV . 1. Glossy photographs or pages f 2. Colored illustrations, paper or print ______ 3. Photographs with dark background g / 4. Illustrations are poor copy ______ 5. Pages with black marks, not original copy ______ 6. Print shows through as there is text on both sides of page ______ 7. Indistinct, broken or small print on several pages _ 1/ _ 8. Print exceeds margin requirements______ 9. Tightly bound copy with print lost in spine _______ 10. Computer printout pages with indistinct print ______ 11. Page(s) ___________lacking when material received, and not available from school or author. 12. Page(s) seem to be missing in numbering only as text follows. 13. Two pages numbered . Text follows. 14. Curling and wrinkled pages / 15. Dissertation contains pages with print at aslant, filmed as received i / ' 16. Other________________________________________________________ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRUCTURAL STYLE OF THE LARAMIDE OROGENY, WYOMING FORELAND A THESIS Presented to the Faculty of the University of Alaska in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY By William G. Brown, B.S., M.S. Fairbanks, Alaska May 1987 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRUCTURAL STYLE OF THE LARAMIDE OROGENY, WYOMING FORELAND RECOMMENDED: Department Head APPROVED: Graduate Programs /£S2_ Date Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT This study documents the styles of laramide struc­ tures of the Wyoming foreland, and presents an integrated synthesis of foreland development. Theoretical data and experimental results are compared to field observations, subsurface well control, and geophysical data to judge their application to the foreland. The direction of collision (N40-50E) between the North American and Parallon plates represents the direc­ tion of maximum compressive stresses during the Laramide orogeny. A "tectonic front" progressed from west to east across the foreland during Campanian to middle Eocene time. No single model of uplift applies throughout the foreland. Dips of basement faults vary with changes in strike (i. e. northwest strike: 20-45 degrees; north to north-northwest strike: 45-60 degrees; east to northeast strike: 60 degrees to vertical). Northwest-trending uplifts are fold-thrust structures exhibiting dual fault systems and rotated flanks. North-northwest trending uplifts are thrust uplift structures bounded by single reverse faults. East- and northeast-trending uplifts are drape folds and upthrust structures along which displace­ ments were dominantly vertical. 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 The forms of Laramide structures were controlled by upward movement of basement forcing blocks, arrangement of lithology into mechanical packages, and deformational mechanisms which were determined by dominant lithology, applied stresses, and depth of burial. "Crowd" struc­ tures (i. e. back-limb, cross-crestal, and rabbit-ear folds) developed as volumetric adjustments in upward- tightening, parallel-folded synclines. There is no consistent relationship between basement fabric and northwest-trending Laramide structures; how­ ever, reactivated PreCambrian-age zones of weakness con­ trolled east- and northeast-trending compartmental faults, which intersect and segment other crends. As a result, northwest-trending thrust and fold-thrust structures may change directions of asymmetry, be offset, or even re­ placed by other structural styles, while maintaining overall structural balance. This study has documented approximately 50 miles of crustal shortening (representing 15$ strain), which was pervasive and uniform across the foreland. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OE CONTENTS P a g e ABSTRACT............................................. 3 LIST OF FIGURES..................................... 13 LIST OF TABLES...................................... 27 LIST OF APPENDICES.................................. 28 1 .0 Introduction................................... 29 1 .1 Purpose.............. 29 1.2 Definition of Wyoming foreland............ 30 1.3 Definition of Laramide orogeny............ 30 1.4 Methods of investigation.................. 31 1.5 Acknowledgments............ 33 1.6 Personal Dedication....................... 38 2.0 Historical review of Wyoming foreland structural interpretations..................... 39 2.1 Introduction.............................. 39 2.2 Early geologic studies (1867-1919)........ 39 2.3 Development of earliest foreland structural concepts (1920-1940).......... 42 2.4 Development of modern structural models (1941-1970)....................... 44 2.5 Continued debate of the various structural models (1971 to present)...... 48 2.6 Summary.................................... 50 3-0 General geology of the Wyoming foreland......... 52 3-1 Precambrian basement complex.............. 52 3.11 Introduction................... 52 3.12 Age of Precambrian rocks........... 52 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 Page 3.13 Precambrian rock types............. 53 3.14 Precambrian structural trends...... 54 3.15 Summary............................ 56 3.2 Phanerozoic sedimentary section........... 57 3.21 Introduction....................... 57 3.22 Paleozoic Erathem.................. 59 3.23 Mesozoic Erathem................... 63 3.24 Cenozoic Erathem................... 67 3-25 Summary............................ 69 3.3 Regional foreland structure............... 70 3.31 Introduction....................... 70 3-32 Orientations of major Laramide structures.......................... 70 3-33 Time of development of major structures.......................... 76 3-4 Summary................................... 78 4.0 Plate Tectonic Setting of the Laramide Orogeny......................................... 80 4.1 Introduction.............................. 80 4.2 Sevier orogeny............................ 80 4.3 Laramide orogeny.......................... 82 4-31 Plate convergence and direction of compression..................... 82 4-32 Rate of convergence and dip of subduction zone................. 85 4-4 Post-Laramide plate movements............. 88 4-5 Summary................................... 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5-0 Structural Characteristics of Basement Rocks. 91 5-1 Precambrian basement structures...... 91 5.11 Introduction................... 91 5.12 Relationship of Precambrian and Laramide structures............ 95 5.15 Summary......................... 99 5.2 Rock mechanical properties of Foreland Basement................................. 101 5.21 Characteristics of mechanical basement........................... 1ni'w' 1 5.22 Mechanical basement and theoretical/experimental models 105 5 -- 25 Summary........ 108 5-5 Concept of Folded Basement................ 110 5.31 Observations of basement surface.... 1 1 1 5.32 Possible "folding" mechanisms....... 112 Cataclastic deformation........ 112 Macro-fracturing............... 115 Rigid-body rotation............ 116 Flexural slip.................. 116 5-4 Geometry of Faulted Basement.............. 118 5.41 Upthrown fault blocks............... 119 5.42 Dovnthrown fault blocks............. 123 5.5 Orientations and attitudes of Basement Faults................................... 125 5-51 Fault dip angles.................... 126 Low angle reverse faults....... 126 High angle reverse faults...... 130 Synthesis...................... 131 5-52 Basement fault cut-offangles ......... 132 5.6 Summary..................................... 135 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 Page 6.0 Structural Characteristics of the Sedimentary Section......... 139 6.1 Introduction.............................. 139 6.2 Structural Stratigraphy................... 139 6.21 Mechanical stratigraphy............. 141 Nonwelded, nonthinning......... 143 Welded, nonthinning............ 145 Welded, ductile................ 1^-6 6.22 Mechanical stratigraphy applied to the foreland.................... 146 6.23 Summary............................. 152 6.3 Compressive Loading of the Sedimentary Section................................... 154 6.31 Introduction........................ 154 6.32 Foreland examples of flexural slip................................ 154 6.33 Structures related to synclinal adjustments........................ 157 Backlimb folds................. 157 Cross-crestal structures....... 161 RabMt-ear folds..............
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