A RECORD OF LATE HOLOCENE PEDIMENT EVOLUTION IN THE WHITE RIVER BADLANDS, SOUTH DAKOTA

Pediments are gently sloping, coallescing fans of moderate relief that build along margins of mountains. Soil formation occurs on these surfaces during intervals where climate is relatively moist and the land surface is stable. These stable surfaces become paleosols when covered by additional sediments. Fluvial channels dissecting the pediments in the White River Badlands isolate sod tables (pediment remnants) from the Tertiary ‘castles' that shed the fans. Paleosols in sod tables have been radiocarbon dated to constrain the ages of late Holocene land surfaces, along with intervals of fan construction, stability, and dissection that contributed to their development. Field evidence for fluvial and colluvial deposition on modern fan surfaces is abundant. In profile CT 3-05, there are paleo channels in the sod table stratigraphy that provide evidence of similar processes in the past. Modern analogs from Roosevelt National Park, ND show a landscape that reflects what the White River Badlands looked like 600-500 years ago, as the fans remain attached to the adjacent source areas. We seek to constrain the timing and elucidate the causality of pediment incision.

We have 22 radiocarbon dates from over a dozen soil profiles, principally at two localities about 6 km apart, along strike on the upper prairie. Our data show clusters of ages occurring at ~900, 1100-1300, 1700-1900, 2300-2400, and ~3600 RCYBP.

These dates reveal intervals of land surface stability occurring during a relatively moist climate. The youngest of our dates, 570 RCYBP, is taken as the maximum age of onset for the fluvial incision that dissected the pediments into sod tables. The pediments in Roosevelt NP have not been dissected and separated from their source areas, and we take them as an analog for what the Badlands of South Dakota looked like before the onset of dissection that isolated the sod tables.

Ongoing investigations include organic carbon content and additional radiocarbon dating in order to assist in correlating paleosols among outcrops within the vicinity to better constrain intervals of land surface stability. Meanwhile, a literature review is underway to illuminate other paleoclimate records from the mid-continent that might help explain the onset of pediment incision around 600-500 years ago.