CHRONOSTRATIGRAPHIC INVESTIGATIONS OF CENOZOIC UNITS IN THE NIOBRARA RIVER BASIN IN BOYD COUNTY, NORTHERN NEBRASKA

Cenozoic deposits exposed in the Niobrara River Basin in Boyd County are approximately 60 m thick, and include an overlying coarse-grained Miocene sandy unit referred to the Ogallala Group, and underlying more fine-grained deposits that have been variously referred to the Fort Randall, Turtle Butte, and Rosebud formations, deposits spanning from Oligocene to middle Miocene in age. Our study materials for these units come from two long cores drilled by hollow-stem auger that penetrated (1) the upper 36.6 m of section, (2) the lower 24.4 m of section, and (3) a middle interval of exposed strata spanning about 21.3 m in thickness. We are assembling a composite stratigraphic profile whereby lithologic characters are calibrated for elapsed geologic time using two laboratory-based techniques. Geochronologic calibration is being accomplished using paleosol units as independent geochrometers, by separating and sorting volcanogenic zircon grains for LA-ICP-MS dating using U/Pb analysis. The rationale for this approach is that chemically-resistant zircon phenocrysts from contemporary volcanic ash falls likely are concentrated in time-rich paleosol horizons. In sorting heavy mineral residues for clear, euhedral, prismatic zircon grains that lack evidence for abrasion by sedimentary transport, we can identify populations of zircon grains whose U/Pb ages closely approximate depositional ages, with an analytical uncertainty of about 2 %. Additional chronostratigraphic information is being assembled via organic carbon δ¹³C chemostratigraphy, sampled at stratigraphic frequencies of 30 cm. This approach is being used to independently calibrate the temporal shift from C3-dominated paleofloras to C4-dominated paleofloras, a phenomenon that has been well documented from Neogene terrestrial strata in the Great Plains, and elsewhere in the world. By documenting the timing of these paleoecologic shifts in Boyd County, Nebraska, we hope to develop a dataset that will permit comparisons to other highly-resolved profiles that currently are under development in the McPherson Channel deposits in central Kansas, and units from the Cimarron River Valley in southwest Kansas. We hope to learn whether δ¹³C chemostratigraphy has more generalized chronostratigraphic implications in the Great Plains area.