RECONSTRUCTION OF HOLOCENE PALEOFAN SURFACES, WHITE RIVER BADLANDS, BADLANDS NATIONAL PARK,SD

The morphology and distribution of sod tables adjacent to Badlands ‘castles’ suggest they were dissected from larger complexes of alluvial fans. Heffron et al. (2003) modeled a potential paleofan sequence based upon digital elevation models and aerial photography. This model suggested that two paleofans were the source of the current sod tables in the Keyhole Table study site. Field mapping was then completed to validate the model. The Keyhole Table site exists adjacent and west of the Old Northeast Road, north of the junction with the Badlands Loop Road, SD 240. Seventeen sod tables and the drainage channels between them were mapped using a Trimble XT GPS unit and converted into a three-dimensional TIN surface in ARCGIS. A total of eighty-four soil profiles and cores were sampled across the sod tables. The depth to horizons and their thicknesses were used to recreate each paleosurface encountered in the survey. The results of the field study indicate two paleosurfaces, with thickening paleosols trending away from the proximal castles. The lower paleosurface was consistent with the lower paleofan of Heffron et al. (2003) and was found only in the most distal sod tables. The upper paleosol surface was found in the lower portions of all proximal tables and was seen to decrease in thickness and pinch-out toward the source area of the ‘castle’ walls. We also identified a sequence of “modern” fans that the model did not distinguish, but were characterized by a sharp break in topography close to the castle walls. The results of the model indicate at least three distinct periods of deposition followed by periods of stability sufficient for soil formation. Further research is necessary to date the intervals between depositional events and the changes in climate that led to the incision of paleofans and isolation of the vestigial sod tables.