ON THE DISTRIBUTION AND HISTORY OF SOD TABLES IN THE EXPOSURE AREA OF THE WHITE RIVER GROUP, NORTHERN GREAT PLAINS, USA

We have studied sod tables across the exposure area of the White River Group (WRG). Our impetus is better understanding of Holocene landscape evolution - morphology and causality - in the western NB, SD, and ND, along a transect from Chimney Butte (CB) in the south to Theodore Roosevelt Park (TRNP) in the north. There are boundaries to the geographic distribution of sod tables, tied to outcrops of controlling stratigraphic units. We observe that pediments surrounding CB and Scotts Bluff, NB as well as TRNP, ND are minimally dissected, thus lacking sod tables, as the pediments remain continuous with the highlands, or are separated only by a fan-head trench. Between these extents, from Toadstool Geologic Park, NB to Slim Buttes, SD sod tables are generally present along exposures of the Brule Fm. This generality is interrupted by one critical stratigraphic constraint. If the Chadron Fm outcrops at the base of badlands, pediments do not form, for apparently the shrink-swell behavior of the clays in the unit stymies sufficient cohesion to preclude a stable surface upon which the detritus of adjoining cliffs can accumulate to construct a pediment. We have previously advanced a model for sod table development where pediments form along the base of retreating badlands castles. The fans exhibit punctuated aggradation followed by landscape stability with pedogenesis, then a terminal fluvial dissection event carves the slopes into remnant sod tables. Hence, without requisite pediments, no sod tables can exist. We have compiled dozens of radiocarbon paleosol ages from the “Wall”, south into Nebraska. Paleosol ages reveal that late in the Holocene Epoch pediments rapidly evolved. Recent reconnaissance identified pending field study sites that extend northward past Alligator Butte to Slim Buttes. We conclude with a statement of two problems that focus this ongoing research of slopes. (1) It is perplexing that no paleosols older than ~3,600 RCYBP have been located. What were the slopes like in the early Holocene and Pleistocene? (2) Fan-head incision of a pediment precludes subsequent burial by alluvium, hence, by cross-cutting relationships, the uppermost paleosol constrains the onset of incision to sometime in the last thousand years at several localities. Did drought during the Medieval Climate Anomaly trigger dissection of pediments?