Sandstone Injectites Record Pre-, Syn-, And Post-folding Deformation At Sheep Mountain Anticline, Wyoming

Abstract

Sandstone injectites ranging from 30 m to over 1 km in outcrop length intrude the Cretaceous Mowry Formation in the vicinity of Sheep Mountain Anticline (Bighorn Basin, WY). These injectites were sourced by the Peay Member of the overlying Cretaceous Frontier Formation, and represent a significant potential fluid pathway through impermeable shales. Sand injection was aided by pre-existing joints in the Mowry Formation during the early folding of Sheep Mountain Anticline. Upon unfolding around bedding orientations, most of the injectites restore to vertical (dike) and horizontal (sill) orientations. Downward injection of the Peay sand was likely made possible by a highly stratified horizontal stress field resulting from the deposition, burial, and lithification history of the rock units in the area. The internal structure of the injectites is dominated by two sets of mutually offsetting deformation bands. The deformation bands have shear and compaction components, exhibiting significant porosity loss, as well as cataclasis and minor pressure solution. After formation of the deformation bands, subsequent faulting occurred along planes parallel to the deformation bands, evidenced in the field by slickensided surfaces. A detailed kinematic analysis of slickenline lineations yield shortening and extension axes consistent with deformation band formation associated with the initiation of Laramide-oriented shortening, and continuing through the folding of Sheep Mountain Anticline. Beyond the formation and deformation of these sandstone injectites, this study highlights the importance of rock mechanical properties in the containment of hydraulic fractures.