DEFORMATION BANDS IN LARGE SANDSTONE DIKES NEAR SHEEP MOUNTAIN ANTICLINE, NORTH-CENTRAL WYOMING

Clastic dikes can be an important conduit for fluid flow, and understanding the mechanics of their formation and internal structure is important for assessing their hydraulic characteristics. Deformation bands are can also exert an important influence on directional flow in sandstones. Most deformation bands act as fluid barriers; however, some may act as fluid pathways. The hydraulic behavior of individual bands depends on geometric properties such as thickness and connectivity, as well as petrophysical properties such as permeability and porosity. Both clastic dikes and deformation bands create anisotropy of fluid flow through the rock, however they are rarely, if ever, described as occurring together. Sandstone dikes near Sheep Mountain Anticline, WY, represent an injection of Cretaceous Frontier formation into the underlying Mowry shale, and contain abundant, anastamosing deformation bands striking subparallel to the dike walls. A previous investigation of the bands apparently missinterpreted them for sedimentary “flow structures” in the dikes, rather than post-lithification deformation features. The dikes appear to pre-date folding of the anticline, as they are parallel to pre-folding joint sets. We examine microstructures associated with the deformation bands to understand the exact nature of the deformation that occurred during their formation, and to estimate some petrophysical properties of the deformation bands. The dikes are composed of almost entirely silica based components: quartz grains, siltstone fragments, and silica cement. The deformation bands are predominantly compaction bands, characterized by almost complete loss of porosity with extensive pressure solution and microcracking at grain contacts. Most of the bands also have a small component of shear offset as evidenced by slickensides in the field.