Abstract

Recognition of the timing of differential crustal movements in the Rocky Mountain region should lead to a clearer understanding of (a) the types of forces involved in Laramide and post-Laramide folding and faulting, (b) the possible times of accumulation of oil and gas in traps that may later have been modified, and (c) the reasons for the absence of oil and gas in apparently good traps. The following is the writer’s interpretation of significant periods of folding and faulting that developed the known structures in Wyoming.

  1. Post-Lewis, pre-Lance time: broad regional uplift occurred along the western margin of Wyoming; a westward-trending gentle anticline formed in central Wyoming; the Medicine Bow and Uinta mountains began to rise.

  2. Close of Cretaceous time: sharply folded anticlines, overthrust toward the west, developed in Jackson Hole; local anticlines formed in south-central and southwestern Wyoming; some eastward thrusting may have occurred in westernmost Wyoming; broad mountain arches rose in approximate positions of the present Salt River, Wind River, Granite, Bighorn, Medicine Bow, and Sierra Madre mountains; regional uplift occurred in the southeast corner of Wyoming.

  3. Close of Paleocene time: Salt River and Wyoming ranges were thrust toward the east; southward thrusting occurred along the western part of the Owl Creek Mountains and at the south end of the Bighorn Mountains; the Wind River Mountains were thrust southwest; other mountain arches and major anticlines continued to rise.

  4. Close of earliest Eocene (Indian Meadows) time: rapid uplift accompanied by extensive erosion occurred in the Bighorn, Beartooth, Owl Creek, Wind River, Granite, Medicine Bow, and Laramie mountains; the east flank of the Medicine Bow Mountains was thrust east; southward thrusting occurred along the south flank of the Owl Creek, Washakie, and Granite mountains; southeastward movement emplaced the South Fork thrust block near Cody.

  5. Close of early Eocene time: southwestward thrusting occurred in the northwest part of the Wind River basin, eastward thrusting in the central Bighorn Mountains; southeastward movement emplaced the Heart Mountain thrust sheet; recurrent folding continued along many previously formed anticlines.

  6. Close of middle Eocene time: the Uinta Mountains were thrust northward.

  7. Close of Eocene time: southeastern Wyoming again was regionally uplifted and rugged topography developed on this upland; gentle folding and small-scale faulting occurred in southwestern and northwestern Wyoming.

  8. Close of Oligocene time: gentle northwestward-trending folds developed in central Wyoming; gentle warping occurred in the Absaroka region; the southeast rim of the Powder River basin may have been tilted southeast at this time.

  9. Close of early Pliocene time: extensive thrust sheets moved southwestward in southern part of Jackson Hole, and the western part of the Gros Ventre Mountains was uplifted.

  10. Post-middle Pliocene, pre-Pleistocene time: large-scale block faults developed in many parts of Wyoming; the floor of Jackson Hole dropped several thousand feet; the southern end of the Wind River Mountains collapsed; the central arch of the Granite Mountains dropped several thousand feet; local areas west of the east margins of the Sierra Madre, Medicine Bow, and Laramie mountains were downdropped; part of the Rawlins uplift collapsed and a broad westward-trending anticline formed south of Rawlins; a large area southeast of the Hartville uplift was down-faulted; the southern end of the Bighorn Mountains probably collapsed at this time.

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