EVOLUTION OF PEDIMENTS ADJACENT TO THE WHITE RIVER GROUP IN NEBRASKA AND SOUTH DAKOTA, INCLUDING POTENTIAL INFLUENCE OF THE MEDIEVAL CLIMATE ANOMALY

Late Holocene pediment development in outcrop areas of the White River Group on the northern Great Plains demonstrates several stages of development that relate a story with linkages between geomorophology and paleoclimate. First, we note that pediments develop best where the relatively resistant Brule Formation outcrops at the base of badland ‘castles’. Where the underlying Chadron Formation outcrops along the ‘castle’ base, such as at the Limestone Buttes near Oelrichs SD, pediments are not present. Then, aggradation of the alluvial-colluvial fans capping the pediments has been punctuated by intervals of landscape stability, favoring pedogenesis. Subsequent deposition upon pediments has left a record of paleosol ages in Holocene alluvium, several of which are synchronous with paleosol ages reported for regional eolian deposits. We take the age of the uppermost paleosol as the constraint for the maximum age for the onset of the incision that subsequently dissected the pediments into the sod tables, which remain observable throughout the region today. This incision occurred sometime early in the last millennium, suggesting that the Medieval Climate Anomaly (MCA) may have provided the forcing that substantially altered pediment geomorphology. Dune field migration reported for the period of 1,000-1,300 AD on the Great Plains suggests a dominant wind blowing from the Great Basin to the northeast, across the Rocky Mountain front and adjacent plains. Perhaps, the resulting dessication of the plains led to a loss of vegetation or an increased flashiness of rainfall. Either, or both, of these linkages appear to have enabled mega droughts associated with the MCA to dissect pediments into sod tables through fluvial incision. Our recent geomorphic transect across Nebraska and South Dakota yielded additional observations to support these postulated linkages and pending radiocarbon dates on paleosols should help further constrain the timing of the terminal incision that formed sod tables.