PALEOSOL-BASED INTERPRETATIONS OF BASIN HISTORY AND PALEOCLIMATIC CHANGE: THE EOCENE-OLIGOCENE WHITE RIVER GROUP OF NEBRASKA AND SOUTH DAKOTA

The late Eocene-Oligocene White River Group of the northern Great Plains is composed of volcaniclastic and siliciclastic claystones and siltstones, sandstones, and limestones deposited within fluvial, lacustrine, and eolian environments. Although the sediments are similar across the region, tectonic influences have promoted differences in sediment accumulation, preservation, and stratigraphy. These differences are reflected in paleosols within the basins through this region, and when combined with lithostratigraphic and geochronologic data, provide a detailed record of inter- and intrabasinal tectonic history and associated climate change. On a local scale, the vertical change in paleosols is a manifestation of both a change to progressively arid conditions throughout the Oligocene and changes in the rate and type of sediment accumulation. Within individual basins, lateral changes in paleosols are a manifestation of tectonics. Paleosol morphologies change as a function of basin asymmetry and associated facies. Paleosols in high subsidence areas are multistory, weakly developed profiles, whereas in areas of lower subsidence, soils are well developed and commonly overprinted. Periods of basin stability are marked by laterally extensive and very well developed paleosols. On the regional scale, tectonics, paleoclimate, and rates of sediment accumulation still influence paleosol morphology, but when viewed within a lithostratigraphic and geochronologic framework, the paleosols record the diachronous and episodic nature of basin subsidence and help explain the regional variability of lithofacies within the White River Group.