STRUCTURAL EVOLUTION OF BASEMENT-INVOLVED ANTICLINES ON THE WESTERN MARGIN OF BIGHORN BASIN, WYOMING

A series of basement-involved structures are exposed along the western margin of Bighorn basin, Wyoming. These anticlines are primarily formed on steep, easterly-dipping reverse faults and are characterized by short steeply dipping forelimbs and long gently dipping back limbs. The deformed units consist of alternating clastic and carbonate sediments of Cambrian to late Cretaceous age above granitic basement. Rattlesnake Mountain and Hamilton dome are examples of such structures. To understand the three-dimensional geometry of the structures, a series of six balanced cross sections were constructed through the Rattlesnake Mountain and adjacent structures. The cross-sections integrated data from published surface geologic maps, additional surface mapping, color infrared digital orthophotos, well tops from more than 200 wells, and dipmeter data from 22 wells. Over a length of 19 kilometers, the top of the basement is 4.5 kilometers deeper to the east, corresponding to a gradient of approximately 13 degrees between the undeformed footwalls of the Oregon basin master thrust and the back thrust. The gradient is identical to the dip of the back limb of the Rattlesnake Mountain anticline in the cover sediments. Although a number of explanations have been suggested for the back limb dip, we suggest that the dip of the basement surface and the back limb of the Rattlesnake Mountain anticline is a structural feature formed during the forelimb deformation on the larger-scale Oregon basin fault system.