STRUCTURAL GEOLOGY—NORTHWEST MARGIN, BIGHORN BASIN: PARK COUNTY, WYOMING AND CARBON COUNTY, MONTANA

ABSTRACT The geologic structure of the northwestern Bighorn basin margin is controlled by four major reverse faults—Beartooth thrust, Line Creek thrust, Heart Mountain-Shoshone thrust and Oregon Basin thrust. The thrust faults dip westward, tectonic transport is eastward and displacement is on a scale of miles. Lesser faults, both imbricate splays and backthrusts, sole out on the ma|or fault planes. The axis of the Bighorn Basin is displaced by the major thrusts and is in part overridden by the hanging wall of the thrusts. Precambrian basement is involved in all cases and is both folded and faulted. Faults propagate upward from the basement into the overlying sediments that may be further deformed by secondary faults. Rattlesnake Mountain demonstrates folding of the basement with associated faulting. All structures are the result of response to a horizontally directed stress field, that shortened the crust. The Heart Mountain and South Fork detachment thrust are excluded from this discussion.