Structural Controls from Detachment Folds Associated with Foreland Arches: Beaver Creek Anticline, Wyoming

Sara M. Smaltz and Eric A. Erslev
Department of Geosciences, Colorado State University, Fort Collins, CO

Of the smaller, second-order anticlines adjacent to Rocky Mountain basement arches, detachment folds basinward of fault-propagation folds bounding the arches are mostly covered by synorogenic strata and are thus the least well known. These anticlines host important hydrocarbon reserves and provide prospects for undiscovered fields. This study documents the geometry and kinematics of the Beaver Creek Anticline (BCA) which is located basinward of a complex series of northwest-trending thrust faults and folds defining the Beaver Creek reentrant on the western edge of the Bighorn basement arch. Possible origins for this detachment fold system include syn-Laramide detachment rooted in mountain front faulting, syn-Laramide gravity sliding during folding of the mountain front, and post-Laramide gravity sliding down and away from the mountain front folds.

To test these hypotheses, 450 minor faults and 1,050 shear bands were analyzed from 34 stations in the Leavitt Reservoir Quadrangle and surrounding area. Both datasets record mostly strike-slip and thrust movements. Initial kinematic analyses indicate N45°E horizontal compression and shortening along the BCA within Mesozoic strata and N65°E compression and shortening within Paleozoic strata in the mountain front.

The presence of indurated fault planes and slickenlines indicate that if the BCA formed due to gravity sliding, the sliding was not a near-surface mass movement but occurred slowly under significant overburden. The variation in shortening directions suggests that different stratigraphic levels may have been moving in different directions due to 3D space constraints within the Beaver Creek reentrant. Independent movement at various levels in the strata suggests that local kinematics may not parallel regional deformation, resulting in discordant orientations for fracture systems at different stratigraphic levels.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado