Abstract 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 slinding during mountain-front folding, and post-Laramide gravity sliding.