Stratigraphic Analysis of Synsedimentary Half Grabens in the White River Group of NW South Dakota

The northern portion of Slim Buttes in NW South Dakota is affected over a 15 by 20 km2 area by significant faulting of assumed nontectonic origin. The cause is not well understood, but gravitational collapse is a possible mechanism. The structure is characterized by rotated half-grabens, with the associated faults connecting in listric geometry to a shallow detachment at the base of the visible layers. The faults occur in Eocene-Oligocene sedimentary rocks, which are overlain by Miocene-aged rocks deposited after faulting ended, constraining the time of deformation to circa 26-27 Ma. The Brule formation, consisting of channel sandstones and loess, is the youngest unit disturbed by faulting. Evidence suggests that some of the faulting occurred as Brule sediments were still being deposited. Typically during synsedimentary faulting, a wedge architecture develops when the space at the top of a rotating half-graben accumulates new sediment. Stratigraphic sections measured within the fault-bounded half-grabens were used to determine the 3-D geometry of the sedimentary units in relation to the faults. Distinctive paleosol horizons within the Brule formation can be correlated between sections and show that sedimentary layers become progressively condensed with increasing distance from faults, consistent with depositional facies changes caused by half-graben rotation. Additionally, playa lake deposits found localized near faults are consistent with lakes forming in areas of lower relief. Such a constraint on fault timing helps to inform models for this enigmatic type of deformation and guide future research in the area