IMPROVING CHRONOSTRATIGRAPHY OF CONTINENTAL DEPOSITS WITH INTEGRATED GEOCHRONOLOGY AND GEOCHEMISTRY: CASE STUDIES ON THE MID-CONTINENT US HIGH PLAINS

Continental clastic strata are important hosts for groundwater resources, but they have several characteristics that make them notoriously difficult to correlate and model. Many are lithologically monotonous vertically, and horizontally discontinuous, and at best they contain datable volcanic ash beds that are discontinuous. The Ogallala Group in the midcontinent US hosts the High Plains Aquifer, one of the most important sources for groundwater on the continent, but chronostratigraphic information in some areas has been relatively limited, and poorly constrained, with age uncertainties ranging up to 10 my. With a number of recent case studies we have started improving the chronostratigraphy of the Ogallala Group. First, by combining fast data acquisition by laser ablation using U-Pb zircon dating and geochemical fingerprinting of glass shards from volcanic ash deposits. Future high precision U-Pb dating by thermal ionization mass spectrometry has the potential to improve precision to better than 100 ka.

In the thick, secondary ash deposits in the Calvert Ash mine of Central Kansas and at Ashfall Fossil Beds, Nebraska, specific attention was given to sampling strategies for zircon dating. Prior studies had not been successful, but careful selection of the basal layers yielded abundant volcanogenic zircon from major Yellowstone hot spot track eruptions at ca. 11.9 Ma, and surprisingly from a less well-known eruption at 6.4 Ma.

An entirely new approach for clastic sediments was developed using zircons extracted from mature paleosols as close proxies for exposure/deposition ages. This is based on the hypothesis that during times of frequent (but possibly distal) explosive volcanism, paleosols may offer the best preservation potential for zircons in the terrestrial environment, unlike discrete ash layers or fluvial sands. Our paleosol results are consistent with the relative stratigraphic context of volcanic ash beds used to test the hypothesis.

Future workflow will be fast reconnaissance by geochemical fingerprinting, followed by laser ablation zircon dating, supplemented by paleosol zircon dating and high precision dating, to construct a comprehensive correlation of the Ogallala Group across the High Plains. This approach has the potential to be applied to many terrestrial clastic formations worldwide.