SANDSTONE INJECTITES RECORD PRE- AND SYN-FOLDING DEFORMATION AT SHEEP MOUNTAIN ANTICLINE, WYOMING

Large, km-scale sandstone injectites first described by Warner (1968) intrude the shale-dominated Cretaceous Mowry formation at regular ~1 km intervals at Sheep Mountain Anticline. The source material for the injectites is the lower sandstone member of the overlying Cretaceous Frontier formation, which is, in turn, overlain by a bentonite-bearing shale. These injectites represent significant potential fluid pathways through impermeable shales of the Mowry formation. We present results of high-resolution 3D mapping and structural analysis of the injectites using differential GPS and LiDAR data, integrated with traditional measurements of structural fabrics associated with the injectites. Sand intrusion occurred before folding. During intrusion, sand injected downward along pre-existing joints and bedding discontinuities in the cohesive sediments of the Mowry formation. Dikes and sills are segmented and appear to have enhanced the dike-parallel joint density near dike contacts, likely by the formation of new fractures within the process zone of the propagating dikes. Most dikes and sills restore to vertical and horizontal orientations, respectively, upon unfolding bedding, however some dike segments have been warped due to internal heterogeneous deformation within the Mowry shale. The interior structure of the sandstone dikes is dominated by two pervasive sets of deformation bands (DB1 and DB2) representing significant (~10%) porosity loss. DB1 bands are offset by shearing along DB2. Both deformation band sets were likely formed at different stages of folding during the development of Sheep Mountain Anticline. Ongoing study of the internal structure of the dikes will provide a better understanding of the hydraulic properties of the dikes, and will yield insights into the kinematic development of Sheep Mountain Anticline, complementing existing datasets based on fold-fracture relationships.