NEW APPROACH TO U-PB ZIRCON DATING AND CARBON ISOTOPE GEOCHEMISTRY OF PALEOSOLS IN THE MIOCENE OGALLALA GROUP, NORTH-CENTRAL NEBRASKA, USA

This integrated geochronologic and chemostratigraphic study of the paleosol-bearing lower Ogallala Group in Boyd County, Nebraska greatly expands on prevailing, local to continental concepts of vertebrate biostratigraphy. We propose that paleosols in such a setting gradually accumulate datable zircons from contemporaneous volcanic eruptions. Moreover, the isotopic geochemistry of the Boyd County paleosols may also preserve the transition from C3- to C4-dominated paleofloras on the ancient Great Plains, thereby offering an independent means of assessing geochronology. Paleosols were identified in three cores and an ~10-m-thick outcrop in north-central Nebraska. Approximately 300 zircons were separated from each of six paleosols for U-Pb analyses, with the intent of estimating maximum depositional ages of paleosol-bearing stratigraphic intervals. Five U-Pb zircon ages ranging from 14.9±0.8 Ma to 15.8±0.8 Ma (middle Miocene) resulted. Three cores and an outcrop section were also sampled at high resolution (30 cm spacing) for organic-carbon stable isotope (δ¹³C) analyses to infer the ratio of C3 to C4 paleofloras. The resulting chemostratigraphic profiles show an overall trend in organic δ¹³C values of approximately -29‰ VPDB, increasing upward to nearly -19‰ VPDB. This pattern suggests an initial dominance of C3 plants followed by significant increases in C4 grasses from ~5% to 30%; it is also consistent with prior interpretations of increasingly widespread C4 grasslands in the early to middle Miocene on the Great Plains. We conclude our integrated geochronologic and chemostratigraphic approach can date paleosol-bearing Cenozoic strata with constraints on the order of 1 m.y. Our results also portend a widely-applicable method of reducing the uncertainties in dating terrigenous sediments in continental basins.