EVALUATION OF GEOMORPHIC FORCING BY THE MEDIEVAL CLIMATE ANOMALY UPON PEDIMENTS ACROSS THE WHITE RIVER BADLANDS, SOUTH DAKOTA

Sod tables throughout the White River Badlands are vestigial remnants of pediments, which have been subsequently dissected by streams. The pediments formed as detritus from the eroding highlands washed and tumbled upon the bedrock remnants, forming a sloping apron along the base of cliffs. As a result, sod tables have planar, gently dipping, turf covered surfaces that stand a part to many meters above the surrounding washes, with their greenery contrasting vividly against surrounding bedrock. The stratigraphy of colluvial/alluvial silts within the sod tables often contains one to six paleosols. We have studied these exposures at localities on the upper prairie, Badlands National Park. Radiocarbon ages of the paleosols reveal that intervals of soil formation occurred at approximately ca. 3600, 2400, 1800, 1200, and 900 RCYBP, several of which have also been reported by other investigators for ages of paleosols within proximal eolian deposits.

We take the uppermost paleosol (youngest age) to be the oldest date for the onset of incision that dissected the pediments into sod tables. This constraint exists because for a soil on a pediment to become a paleosol by burial under subsequent alluvium, the pediment surface must remain connected to the bluff. The incision that forms a sod table separates the sod table from the retreating cliff, precluding further alluvial burial of its surface. The timing of this incision is provocative. In addition to disclosing rapid vertical incision of 2-4 cm and lateral retreat of 1-2 cm annually, we now suspect that the causality of incision is the Medieval Climate Anomaly (MCA). A growing body of literature is linking various proxy records, including dune activation and lake sediments within the Great Plains, to the MCA. We intend to test our proposed linkage of the incision to the droughts associated with the MCA, and hope to further constrain its timing, by dating the uppermost paleosol at localities along a regional transect.