CHRONOSTRATIGRAPHIC INTERPRETATIONS OF CENOZOIC PALEOSOLS IN NEBRASKA USING INTEGRATED U-PB DATING AND CARBON ISOTOPE CHEMOSTRATIGRAPHY

An integrated geochronologic and chemostratigraphic study to date pedogenically-modified Cenozoic mudrocks is currently under way in stratigraphic sections from the Niobrara River Basin in Boyd County, Nebraska. These 60 m of section represent part of the Ogallala Group, namely the Miocene Valentine Formation and possibly the Fort Randall Formation as well. Stratigraphically controlled samples were retrieved from two long cores (37 m and 23 m) and an outcrop section (5.3 m). U-Pb dating of populations of zircons (n=300) from each sample of these time-rich horizons are being carried out to extract maximum depositional ages. In addition, high-resolution organic stable carbon isotope (δ¹³C) profiles (0.3 m spacing) allow comparisons of geochronologic ages to inferred paleovegetation during the Neogene transition from C3 to C4 paleofloras in the Great Plains. Of the six paleosols sampled for zircon U-Pb dating, two units produce maximum depositional ages of 15.30±0.63 Ma and 15.64±0.33 Ma, confirming the middle Miocene ages of the sampled units. Organic carbon δ¹³C values range between -30 to -25 ‰ VPDB with a very gradual upward increase, indicating C3-dominated paleofloras. This is generally consistent with other paleoecologic reconstructions of the early to middle Miocene paleofloras of the North American Great Plains. Comparisons between geochronologic ages and the chemostratigraphic analyses of organic matter in these terrestrial strata are thus producing consistent results. The integration of these two techniques can significantly reduce the uncertainties in dating terrigenous sediments in continental sedimentary basins.