WHEN THE BADLANDS WENT DRY: A PALEOPEDOLOGIC INVESTIGATION OF CLIMATE CHANGE IN THE OLIGOCENE OF SOUTH DAKOTA

The Poleslide Member of the Brule Formation preserves evidence of climatic transitions that took place following the Eocene-Oligocene transition. At a level 23m above the Scenic/Poleslide Member contact, soil morphologies of the Poleslide Member change from those formed during a sub-humid climate to those of a semi-arid environment. Soils at the base of the transitional zone formed during a period of elevated water table levels. The soils display hydromorphic horizons that laterally correlate with a nearby lacustrine deposit. Overlying soils have well developed clay accumulation horizons associated with periods of leaching; suggesting a sub-humid environment. The soils progressively change to weakly horizonated soils, which do not appear to have been affected by elevated water levels. The uppermost soils have well developed epipedons that consist of extensive pedogenic carbonate and fossil accumulations. These soils appear to have resulted from a period of landscape stability in a much drier climate. Field descriptions of changes in soil structure are supported by micromorphology, clay mineralology and geochemical analyses. Micromorphology of the Poleslide Member demonstrates an increase in carbonate accumulation and decrease in clay translocation vertically. The proportion of kaolinite to smectite decreases vertically in the zone of hypothesized transition, as observed in x-ray diffraction relative peak intensity ratios. The vertical change in paleosol morphologies is key to determining the stratigraphic position of this transition. Identification and interpretation of this change is necessary to the understanding of the paleontology, sedimentology and stratigraphy of the White River Group in South Dakota and across the Northern Great Plains. Recognition of this transition regionally may provide insight into the nature of this environmental change, as it may be diachronous, and possibly the driving force behind the regional Orellan/Whitneyan land mammal transition. The recognition of paleopedologic conditions during land mammal transitions increases our understanding of seemingly independent data sets and provides a comprehensive view of ecological change during the Oligocene.