COMPARISON OF BRULE FORMATION VS. ARIKAREE GROUP CHANNEL COMPLEXES, SOUTHERN SLIM BUTTES, SD

Slim Buttes in NW South Dakota (SE Harding County), is located some 100 km NE of the Black Hills uplift. Oligocene and Miocene White River and Arikaree group strata are part of a vast wedge of terrestrial sediments sourced in the Rocky Mountains that extends across the Great Plains. The White River Group in northern Slim Buttes is anomalously deformed by normal faults, which are truncated by an angular unconformity with the overlying Arikaree Group strata. Brule Fm. channels in the upper Arikaree Grp. are nested. Channel walls with > 12 meters of vertical relief, are locally subvertical, and often mantled by angular, more chaotic material interpreted as colluvial deposits. Locally derived basal conglomeratic and bouldery debris flow deposits fine upwards into fine sandstones with trough cross bedding and current ripples, overlain by thin bedded, planar fine-grained, bioturbated sandstones and occasional mudstones. Larger channels predominantly run NW -SE parallel to the expected regional slope, suggesting a structurally controlled dendritic drainage. The Arikaree Grp. channels are fining upward from a basal channel lag gravel to point-bar deposits from medium to fine sand, with large planar and curved cross-beds up to 15m high. In cliff sides, these ancient point bar deposits show well developed and laterally extensive accretion surfaces for 100s of m. Large cross-beds are consistent with eolian origin, but the grain size and variability indicate a fluvial origin. The cross-beds are interpreted as point bar deposits in a meandering river system. The dominant dipping direction of the large cross-beds is northeast. Feldspar, quart, chertz and often biotite are common clast types, indicating a distal source. Though channel size is similar, the Arikaree Grp. Channels differ significantly from those in the underlying Brule Formation. Sedimentary structures in the Brule Fm. indicate a semi-arid badlands environment, with a structurally controlled drainage (trellis pattern) with major channels predominantly run NW-SE. The Arikaree Grp. channel complex, with its distinctive large cross-beds is interpreted as a meandering river system with a semi-continuous flow to the NE. These differences are most easily explained by drainage pattern rearrangement and a change in climate from dryer to wetter across the unconformity.