VERTICAL CHANGES IN PALEOSOLS, SEDIMENTOLOGY, AND VERTEBRATE TAPHONOMY WITHIN THE OLIGOCENE POLESLIDE MEMBER OF THE BRULE FORMATION, BADLANDS NATIONAL PARK, SD

The Poleslide Member of the Brule Formation is composed of 53 m of fossiliferous fluvial, lacustrine, and eolian deposits. Changes between these depositional environments is recognized as distinct facies associations, paleosols, and vertebrate fossil taphonomic modes ranging from isolated to articulated elements which likely reflect the influence of paleogeomorphic conditions. The bottom contact of the Poleslide Member is marked by the lithologic transition from mudstones of the underlying Scenic Member to muddy siltstones, as well as a newly recognized, regionally extensive volcanic tuff, the Cactus Flat Bentonite bed. The bottom of the Poleslide Member is dominated by fluvial deposits. Paleosols formed within fining upward overbank deposits, with vertebrate fossils primarily preserved as isolated elements, except for the very bottom of the Poleslide Member which preserves a relatively dense accumulation of vertebrate material in various stages of articulation. Fluvial conditions give way vertically to a period of relative landscape stability across the region marked by well developed lacustrine facies and an increase in soil maturity. Lacustrine facies are preserved as a springfed carbonate lake deposit with ostracodes, charophytes, aquatic snails, fish, and stromatolites. Lateral to these deposits, up to 5 km away, paleosols preserve evidence of wetter conditions, including hydromorphy and increased weathering. Environments shift to eolian dominated conditions up section and preserve a combination of vertical accretion episodes vs. periods of landscape stability. Periods of eolian vertical accretion are marked by massive volcaniclastic sitlstones with interspersed root traces and soil nodules, and bones that are randomly distributed throughout. Periods of stability are marked by well defined and resistant layers representing former A horizons. Both vertebrate and invertebrate fossils are concentrated within these resistant intervals, suggesting attritional accumulation during periods of increased soil formation. A brief period of fluvial deposition with rare fossils as lag replaces the eolian facies high in the section, but reverts back to eolian deposition before being capped by the Rockyford Ash Member of the overlying Sharps Formation.