STRATIGRAPHIC ANALYSIS OF HALF GRABENS IN 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 normal 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 multiple shallow detachments. The faults occur in Paleocene-Oligocene sedimentary rocks, which are unconformably overlain by Miocene-aged rocks, constraining the time of deformation to the latest Oligocene or earliest Miocene. The Brule formation, consisting of channel sandstones, windblown silt, playa deposits, and mudstones, is the youngest unit disturbed by faulting.

Truncation of some of some faults by strata within the upper Brule formation suggests some deformation was synsedimentary. Typically during synsedimentary normal faulting, a wedge architecture develops as new sediments accumulate on the hanging wall side of the fault, filling in the space created by the rotating half-graben. Stratigraphic sections measured at five locations within fault-bounded half-grabens at Slim Buttes and one at the nearby West Short Pine Hills, totaling 470m logged, were used to determine the 3-D geometry of the sedimentary units in relation to the faults. Paleosol horizons within the silt of the upper Brule are particularly helpful in identifying a possible relationship between faulting and sedimentation. A distinctive series of petrocalcic, calcic, and light green, clay-rich horizons can be correlated between sections throughout Slim Buttes. Within one fault block, the horizons are thicker and more widely spaced near the fault, and become progressively thinner with condensed spacing away from the fault.

Within the lower Brule, playa lake deposits interbedded with sandstones and silt may be localized near faults. Additionally, limited paleocurrent data suggests that the local transport direction was sub-parallel to faulting, different from the expected regional slope. All of this evidence is consistent with depositional facies changes caused by half-graben rotation. Such a constraint on fault timing helps to inform models for this enigmatic type of deformation and guide future research in the area.