PALEOSOLS AND VOLCANIC ASHES AS MARKER BEDS IN THE OGALLALA FORMATION, WESTERN KANSAS

Stratigraphic correlation of continental clastic basins is often hindered by secondary mineralization that makes lithologic correlation impractical; however, paleosols and volcanic ash beds can serve as temporal markers that allow the correlation of stratigraphically problematic sequences. In contrast to fluvial deposits that are composed of remobilized grains whose ages may have no bearing on timing of deposition, paleosols likely capture and preserve the dense, chemically resilient fraction of volcanic ash that includes zircon. These units thus contain information that can be used to construct detailed depositional and landscape evolution histories.

The Ogallala Formation is an ~100 m thick accumulation of fluvial sediments shed from the Rocky Mountains onto the Great Plains in the Late Miocene. Its inclusion of paleosols and lenticular beds of zircon-bearing volcanic ash from magmatic centers in the western United States makes it ideally suited to the exploration of dating methods for continental strata. These units contain detailed chronostratigraphic information, yet none have been previously dated with modern, high-precision techniques. New and in-progress LA-ICP-MS and TIMS U-Pb dates from a volcanic ash and overlying paleosol in Norton County, KS indicate that the paleosol contains large, dateable zircons derived from the underlying ash and surrounding catchments, but also potentially younger, non-remobilized grains that are contemporaneous with and date progressive soil formation.

The Ogallala Formation and its associated Ogallala-High Plains aquifer are crucial to agricultural production in the Great Plains and the United States, but its depositional processes and timelines are poorly understood. An increased understanding of High Plains stratigraphy will serve the development of more accurate groundwater models designed to assess the water resources of the High Plains, including stratigraphically informed predictions about variations in porosity and permeability across its extent. High precision U-Pb geochronology of pyroclastic zircons in paleosols and ash beds may thus provide powerful age constraints for solving longstanding and economically pertinent continental stratigraphic problems.