Southwest Section AAPG Annual Convention

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Large, up to 1km long, sandstone injectites first described by Warner (1968) intrude the Cretaceous Mowry Formation in the vicinity of Sheep Mountain Anticline (Bighorn Basin, WY). These injectites were sourced by the Peay sand member of the overlying Cretaceous Frontier Formation, and represent a significant potential fluid pathway through impermeable shales. We present the characterization of the 3D geometry, internal structure, and intrusion mechanics of these injectites using high-precision GPS, traditional field measurements, and structural analysis. Sand injection was aided by pre-existing joints in the Mowry formation before the folding of Sheep Mountain Anticline. Most of the injectites restore to vertical (dike) and horizontal (sill) orientations when unfolded around bedding. Downward injection of the Peay sand is evidence of a highly stratified 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 minor cataclasis and 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. While the injectites represent potential fluid pathways for hydrocarbon migration, the deformation bands may act as barriers to fluid flow. Constraining the timing of injectite intrusion and deformation band formation will provide a better understanding of their petrophysical implications and how they are related to the kinematic development of the Sheep Mountain area.