Detachment folds basinward of Laramide Rocky Mountain arches are relatively poorly known, partially due to coverage by synorogenic strata that may conceal undiscovered anticlinal fields. This study documents the geometry and kinematics of the Beaver Creek Detachment system (BCD), which is located west of a series of NW-trending thrust faults and folds defining the Beaver Creek reentrant on the western edge of the Bighorn Arch. Possible origins for this proposed detachment include syn-Laramide detachment rooted in mountain-front faulting, syn-Laramide gravity sliding during mountain-front folding, and post-Laramide gravity sliding.

Over 470 minor faults and 1070 shear bands were analyzed at 34 stations in the Leavitt Reservoir Quadrangle area north of Shell, Wyoming. Kinematic analyses of these small strike-slip and thrust faults indicate slip in two distinct directions, with N45°E horizontal compression and shortening within detached strata and N65°E compression and shortening within Paleozoic strata in the mountain front. Balanced cross sections are consistent with detachment in Jurassic strata, most likely in the Gypsum Springs Formation. Continuous Triassic Chugwater contacts show that the detachment is not rooted in nearby basement-involved mountain-front thrusts. Variation in shortening directions indicates that different stratigraphic levels moved in different directions due to three-dimensional (3-D) space constraints within the Beaver Creek reentrant. An analog structure replicated the general slip patterns using wooden blocks (basement) and a flour and salt mixture (Phanerozoic strata). In conclusion, the BCD is interpreted as a syn-Laramide, unrooted detachment structure driven by the combination of regional ENE shortening and gravitational sliding in association with range-front constriction.