STRATA-BOUND VEIN ARRAY IN THE BASAL PIERRE SHALE, FRANCIS CASE LAKE, SOUTH DAKOTA

There is renewed interest in the fracture mechanics and history of mudrocks. A distinctive stratabound vein array occurs in basal Pierre Shale of Francis Case Lake in South Dakota. Typically 2-4 meters in thickness, the vein array outcrops from Snake Creek Recreation Area in the south up to Chamberlain ( >50 km distance). North of Chamberlain it is less well developed to absent. Ash layers define the upper and lower bounds of the vein array. Two, suborthogonal, preferred directions of vertical veins (NE and SE strike) exist throughout the study area. By volume veins comprise 1-2% of the rock. Thinner, more discontinuous, and irregular horizontal veins also occur. Orientations are distinctly different from the joint/vein pattern in the immediately underlying marls of the top of the Niobrara Chalks, with strike preferred directions about 30 degrees counterclockwise. Traverse data suggests uniform horizontal extension in the vein array. Vertical veins are typically 1-2 cms thick, and contain massive jarosite, selenite, and fibrous gypsum. The abundance of jarosite and fibrous gypsum correlates with position in the weathering profile, and these phases are interpreted as replacement of original selenite due to deep modern weathering. In large, recently developed, shoreline cliff outcrops of the Niobrara Chalks, fibrous gypsum veins, often utilizing previously existing fractures, are clearly related to the degree of weathering. However, for the vein array the following suggests they are not related to modern weathering and formed at depth: a) a lack of correlation of vein width/frequency with position in the weathering profile, b) the regional extent, c) the very consistent preferred orientations, d) the uniform horizontal extension, and e) the coarse grained character of the selenite. The consistent strike pattern suggests influence of a regional stress field. The mechanism/timing of vein array formation is unclear, but formation due to diagenetic processes that are especially significant in mudrocks would explain the stratabound character and isotropic horizontal strain. Localization of the vein array may be related to the chalk/shale contact and related pore water geochemistry.