HOLOCENE PALEOSOL RADIOCARBON AGES AND SOD TABLE MORPHOLOGY IN THE WHITE RIVER BADLANDS REVEAL EROSION RATES OF 1-4 CM/YR, OR MORE, IN THE LAST 800 YEARS

Sod table morphology, coupled to radiocarbon dating of the bulk humate fractions of paleosols, help portray the sedimentary dynamic of the White River Badlands during the late Holocene. Throughout the interval from approximately 3,500 to 800 RCYBP, erosion of the Badlands ‘castles’ built pediments, within which the Oligocene Brule Formation widely exists as the bedrock root. During the last millennium, these pediments have been incised, leaving sod tables as vestiges of the pediments. The Holocene fans capping the pediments contain between one and six visible paleosols, from which we measured radiocarbon dates. The uppermost paleosol constrains the maximum age for the onset of incision because the burial required to create a relict soil requires that the pediment remains attached to the eroding highlands. Indeed, dissection of the pediment into sod tables ends fluvial-colluvial deposition upon the sod table. Erosion rates can be determined by surveying the maximum depth of the incised channel between two sod tables, as well as the width of that intervening arroyo. In this manner, the calculated erosion rates represent a minimum, in accord with the uppermost paleosol constraining the maximum age for onset of incision. We report two locations, where rates of stream incision and lateral retreat of ravine walls were calculated. For the pediment present along the northern fringe of Norbeck Ridge, we calculated a minimum incision rate of 0.8 cm/yr, and a minimum rate of lateral retreat of arroyo wall equaling 1.9 cm/yr. From the pediment adjacent to the ‘castles’ located to the east of the Loop Rd by the Fossil Trail exhibit, the minimum incision rate is 0.8 cm/yr and the minimum rate of lateral retreat of the arroyo wall is 4.9 cm/yr. These rates reveal very rapid erosion during the last millenium. The larger quandary concerning the geomorphic evolution of Holocene pediments and sod tables in the White River Badlands, however, lies in determining the environmental circumstance initiating incision of the pediments, which commenced sometime around 900 to 600 years ago.