HISTORY OF CENOZOIC FRACTURE DEVELOPMENT AT SLIM BUTTES, SOUTH DAKOTA

Extensive and enigmatic normal faults occur within Oligocene White River Group strata in the northern Slim Buttes area of NW South Dakota. They are unconformably truncated by Miocene Arikaree Group strata, but are absent in the southern Slim Buttes area. Deformation bands, joints, and clastic dikes also occur. Field and thin section relationships allow a history of fracture development and diagenesis to be inferred. Uncompacted delicate volcanic glass shard morphologies in sandstones and large, angular collapse blocks along sub-vertical Brule Fm. channel margins (within the White River Group) indicate near surface diagenesis. Deformation bands in Brule Fm. sandstones (dilation, compaction and shear-enhanced compaction bands) display considerable strike variability, but have preferred orientations, including dominant fault-perpendicular and subordinate fault-parallel sets. Bands are absent in the equivalent but unfaulted strata to the south. In thin section, delicate zeolite crystals lining deformation band pore space indicate zeolite diagenesis post-dated band formation. An earlier, fine-grained, patchy calcite cement is absent or less developed within the deformation bands. These relationships suggest deformation band growth during early diagenesis, as is typical elsewhere. Conjugate and orthogonal joint set pairs in Brule Fm. strata are fault aligned. Only the orthogonal pair occurs to the south. Rotation of a conjugate joint pair and smaller normal faults within larger fault tilt blocks indicates pre- or syn- lithification faulting. Two joint sets in the overlying Arikaree Group strata are parallel to the fault-aligned orthogonal pair in the underlying Brule Fm. strata, but other sets are unique. Clastic dikes are more common in the southern Slim Buttes area, and are highly variable in their orientation. Dike absence in and possible truncation by Arikaree Group strata suggests dikes predate these strata. Larger normal faults are listric and portions of a detachment are exposed. The dominant trend is ≈120°, a fracture direction seen elsewhere in Cenozoic Great Plains strata. A corresponding Oligocene regional stress field can be inferred. Diagenetic processes, a migrating compaction front, and/or lateral spreading over weak surfaces may have driven deformation.