ANALYSIS OF DEFORMATION BANDS AT SLIM BUTTES, SOUTH DAKOTA

Deformation bands at Slim Buttes occur in the lowermost sandstones of the Brule Formation. The Brule is a fluvial deposit within the White River Group (Oligocene) and hosts prominent normal faulting that is pre-Arikaree Group in age (Miocene) based on a truncating angular unconformity. The NW-SE trending faults have a listric geometry with significant tilting.

Deformation bands are typically mm-thick planar zones of localized strain that require grain reorganization to form. The host rock/sediment must be porous enough for movement to occur (forming before or during the early stages of lithification). The bands at Slim Buttes are previously undescribed in the literature and exhibit unusual characteristics, including consistent/close spacing and complex orientation distributions on an outcrop scale. A primary question behind this research was if the deformation bands have a preferred orientation and how the bands relate to the faults. Orientation plots show significant dispersion, but four preferred orientations. Two of the orientations are interpreted as a conjugate pair of sets with the fault strike bisecting the obtuse angle. There are also fault perpendicular and parallel sets. The deformation bands were absent in the same strata in the southern portion of Slim Buttes were no faulting has occurred. Four preferred orientations imply a complex stress field evolution and identifying the type of deformation bands is critical to understanding this evolution.

Thin sections revealed porosity reduction in many of the bands. Lithics and inequant grains help show grain reorganization and reorientation within the bands. The long axis of grains outside the deformation bands tend to be parallel to bedding, while within they are perpendicular to bedding. The primary classification system for these structures is based on kinematics and separates them into three categories: compaction, dilation, and/or shear. Our research indicates most are shear-enhanced compaction bands. Given coeval deformation bands and faulting, the deformation occurred pre-lithification and is consistent with syn-depositional faulting. This constraint may help us understand the enigmatic nature of the deformation in the area.