EARLY PLEISTOCENE PALEOLANDSCAPES AT BORCHERS BADLANDS, MEADE BASIN, SOUTHWEST KANSAS

Early Pleistocene strata at the Borchers Badlands locality in the Meade Basin, SW Kansas, have produced a wealth of vertebrate fossils that reveal dramatic evolutionary and community change. Significant recent findings include: 1) changes in body size preceding the extinction of the cotton rat Sigmodon minor and the gopher Geomys quinni, 2) the disappearance of the tortoise Hesperotestudo sp., and 3) a 40% turnover of the entire rodent community, including the immigration of cold-adapted voles (Microtus sp.). These findings suggest that either climatic cooling or other environmental stressors may have driven shifts in the faunal community composition. To test this hypothesis, we used sedimentology and paleopedology to reconstruct the climate and environments of the Crooked Creek Fm. at Borchers Badlands. Stratigraphic sections were measured from the basal Stump Arroyo Member through the overlying Atwater Member at or near fossil localities. Age control is given by the Huckleberry Ridge Ash (HRA, 2.113-2.135 Ma) near the base of the Atwater Member and the Cerro Toledo B Ash (1.23-1.47 Ma) near the top of the section. Paleosols of the Stump Arroyo Member formed in broad, abandoned bedload channels. Abundant calcareous rhizoliths, as well as clay and carbonate coatings on gravel clasts, suggest that the paleosols were similar to modern calcic Inceptisols or Aridisols. An unconformity of unknown duration divides the Stump Arroyo and Atwater Members. Paleosols of the Atwater Member formed on a continuum of sandy to clayey floodplain sediments lateral to and above a paleochannel that contains the HRA. Paleosols similar to modern Inceptisols, Vertisols and Alfisols variably contain carbonate nodules, slickensides and argillans, and are interbedded with laminated floodplain pond sediments. These features suggest a dramatic change in hydrology coincided with the Stump Arroyo-Atwater Member transition. However, paleosol climofunctions show no change in mean annual precipitation and temperature throughout the section. These results suggest one of the following scenarios: 1) paleosol proxies were unable to capture climate changes that that drove geomorphic and faunal changes in the section, or 2) paleosol proxies are accurate and the observed geomorphic and faunal changes were not driven by climate change.