PALEOENVIRONMENTS ACROSS THE PLIO-PLEISTOCENE TRANSITION IN THE MEADE BASIN, SOUTHWEST KANSAS

The Plio-Pleistocene transition marks the final collapse of Cenozoic warm intervals and a shift to extensive northern hemisphere glaciation. Records from the North American continental interior across this transition, however, are nearly absent. The Meade Basin, southwest Kansas, not only preserves deposits that span this interval but also contains abundant rodent fossils to which geologic archives can be compared. Here, we report the first ²⁶Al/¹⁰Be isochron burial age of 3.63 ± 0.27 Ma for the informally-named Stump Arroyo member of the Crooked Creek formation, which was previously assumed to be early Pleistocene in age. Using sedimentology and paleopedology, we demonstrate a substantial change in fluvial and pedogenic processes between the Stump Arroyo member and the overlying early Pleistocene (ca. 2.1–1.2 Ma) Atwater member. The Stump Arroyo consists of three stratified sand and gravel units, indicative of a bedload-dominated fluvial environment. Each unit has been extensively modified by pedogenesis, including mineral weathering, clay translocation, and accumulation of pedogenic carbonate. In contrast, the Atwater member comprises fine-grained paleochannel fills or overbank deposits, indicative of a mostly suspended load system. Atwater paleosols have compound profiles that lack significant carbonate accumulation, show little mineral weathering, and experienced limited shrink-swell processes. Potential drivers of observed changes in sedimentology and paleosol morphology include (1) alternating warm/wet to warm/dry episodes in the late Pliocene, changing to cool/wet climates in the early Pleistocene, and/or (2) changes in the rate of subsidence. Turnover in fossil rodent communities across the Blancan-Irvingtonian boundary support a climatic driver. Two paleoprecipitation models based on paleosol bulk geochemistry show no difference in rainfall amount in either stratigraphic member relative to modern conditions. Previous oxygen isotope data were interpreted as reflecting summer cooling and increased winter precipitation in the early Pleistocene. Recent clumped isotope analyses, however, show no change in temperature across the Plio-Pleistocene boundary. Ongoing work, including clumped isotope and ostracod analyses, will provide further insights into forcing mechanisms.