EARLY OLIGOCENE PALEOSOLS AND FLUVIAL CHANNELS AT TOADSTOOL GEOLOGIC PARK, NORTHWEST NEBRASKA: EVIDENCE FOR STATIC CLIMATE THROUGH CHANGING STYLE OF SEDIMENTATION

Understanding local and regional reactions to the global Eocene-Oligocene climate transition is a continuing challenge. The White River Group in the North American midcontinent preserves dynamic fluvial, volcaniclastic and lacustrine facies that yield to aeolian-dominated landscapes. To test whether this shift in sedimentation style is driven by aridification, paleosols and fluvial channels were analyzed from the earliest Oligocene Orella Member through the aeolian-dominated Whitney Member of the Brule Formation at Toadstool Geologic Park, NE. Representative paleosol profiles reveal the relative influence of fluvial to aeolian sedimentation up-section, with the following trends noted: 1) Lowland paleosols become more well-drained, 2) Upland paleosols record progressively greater aeolian influx, even in the lower Orella Mbr., 3) Fluvial influence on landscapes becomes laterally confined, 4) Paleosols generally decrease in development. Mature soils throughout the study interval exhibit accumulation of clay and calcite, reflective of a semi-arid (but not arid) climate. Results of climofunction calculations from paleosol Bw and Btk horizons show mean annual temperature (~9.4 °C) and precipitation (~668.7 mm/y) do not significantly vary across the transition, contrary to previous interpretations.

Destabilization of local base level in the lower Orella Mbr. caused periods of downcutting and formation of the nested Toadstool Park Channel Complex (TPCC). The TPCC is a sandy-braided, seasonal, semi-arid channel system that transitions to sandy sheetflow and backfill muds. Overland sheet-flow and mudstone deposits persist in the low-relief landscapes following paleovalley infilling. A channel preserved near the Orella-Whitney contact exhibits characteristics of a semi-arid, ephemeral, confined channel dominated by flashy, erosive pulses preceding sediment deposition. The increasing supply of aeolian sediments also affected fluvial channel style. Overloading of streams by aeolian sediment reduced stream capacity, thereby restricting fluvial control on landscape stability. Hiatuses in fully-aeolian sections may represent small-scale climate patterns and appear to be the main drivers in landscape stability in non-fluvial Brule Formation deposits.