Sheep Mountain Anticline: Backlimb Tightening and Sequential Deformation

in the Bighorn Basin, Wyoming

By

Heather I. Stanton and Eric A. Erslev

Colorado State University, Fort Collins, CO

The southwestern flank of the Bighorn Mountains in Wyoming is marked by basement-involved, second-order structures exemplified by the well-exposed Sheep Mountain anticline. Their existence can be explained by modeling basement-involved arches, like the Bighorn arch, using block rotation over curved faults that transition to planar, subhorizontal detachments at depth. Shortening above the transition between translation and rotation on the underlying thrust localizes anticlines, often forming productive hydrocarbon traps. We call these folds backlimb-tightening structures (BLTs).

Previous work in the Sheep Mountain area showed that faults dip both northeast and southwest, but did not resolve the details of their three-dimensional geometries or provide rigorous balancing. Data from wells, seismic lines, and surface mapping have been used to generate balanced serial cross sections and a 4D structural interpretation for the Sheep Mountain area, providing a general model for the development of BLTs.

The intersection of NE- and SW-dipping thrust faults in the Sheep Mountain area complicates restorations, necessitating sequential slip on these faults. Cross-cutting relationships indicate that anticlines over out-of-the-basin thrusts formed first. Subsequent into-the-basin thrusting may have been favored by rotation of earlier thrusts to higher, less favorable dips and/or gravitational spreading of the rising Bighorn arch. The Sheep Mountain area provides a possible analog for other BLTs related to basement-involved uplifts in the Rocky Mountains and elsewhere.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming