The Brule Formation at Slim Buttes, Harding County, South Dakota, is composed of 265 ft of Orellan (approximately medial Oligocene) and 175 ft of Whitneyan (approximately late Oligocene) claystone, siltstone, and sandstone beds. The formation is divided into eight lithologic units (A-H). Environmental facies vary from main channel to floodplain to dry plains. Approximately 3500 mammalian fossils were collected stratigraphically and identified. The vertebrate faunal list was increased to 43 genera. The section is as fossiliferous as that of the Big Badlands and has a particularly rich micro-fauna. No evidence for the presence of Manitsha (Rodentia, Ischyromyidae) in Orellan beds was found, and field relations suggest the possibility that the holotype was collected from Chadronian (approximately early Oligocene) or older rather than Orellan sediments as previously believed. The geologic range of the typically Chadronian genus Plesictis (Carnivora, Procyonidae) was extended into the Orellan.
Structural complexities in the latest Oligocene or earliest Miocene at Slim Buttes have previously been ascribed to slumping or landsliding of strata into stream-carved valleys or from along escarpment faces. Evidence against such an interpretation includes: (1) a parallel series of uninterrupted horst-graben sequences; (2) consistent southwest tilting during faulting of long (up to 3 miles), narrow, and uncontorted grabens composed of at least 550 ft of sediment; (3) the presence of two dominant joint sets with dips in opposite directions at about 60° from the bedding planes and with strikes parallel to the strike of the bedding; (4) the absence of evidence for a major river in latest Oligocene or earliest Miocene time—a river of sufficient size to carve canyons that allow slumping; (5) the inability of the local Oligocene sediments to act as a cap rock for deep valley walls or escarpments; (6) the presence of occasional transverse faults oriented at approximately 90° to the main fault trend. A more likely explanation is that superficial normal faulting with some rotation of the grabens occurred due to minor translatory movement along one or more low-angled, gravity-controlled detachment faults accompanied by lateral spreading of soft clay from beneath firmer material. The proposed detachment faults arc probably deep in the underlying Ludlow Member of the Fort Union Formation.