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Developing a Paleosol Geochronometer: Examples From Cenozoic Strata of the High Plains of the Western United States


A new approach targeting paleosols as a source of geochronologically useful detrital zircons is significantly revising our understanding of the chronostratigraphy of Cenozoic deposits in the High Plains of North America. Paleosols are condensed stratigraphic intervals in terrestrial stratigraphic successions, and offer the potential to produce stratigraphically-useful radiometric dates from volcanogenic zircons. As condensed terrestrial sections, mature paleosols are good target facies for preserving a record of tephra deposits that have largely been removed from terrestrial deposits, to the extent that discrete tephra layers are not preserved. Volcanogenic zircons are introduced into the paleosol matrix via pedoturbation, including piping through root channels, desiccation cracks, and animal burrowing activity. Thus, mature paleosols are likely to preserve a time-averaged record of zircon crystal populations that overlap with air fall tephra events. Initial work has concentrated on a 98 m-long core (HP1A) of deposits in western Kansas previously assumed to be the Miocene Ogallala Formation; the first of six anticipated cores. Optically-stimulated luminescence dating of selected intervals from Quaternary loess deposits in the uppermost 12 m of the core produce ages ranging from 76.8±13 Ka to 44.3±7.8 Ka. Core sections from below the loess sequence are comprised of thick fluvial sands interbedded with thin mature paleosols. Microscopic zircon crystals extracted from four (4) paleosol intervals were sorted to isolate only clear, prismatic, euhedral, grains lacking abraded margins hypothesizing that these grains are remnants of tephra deposits that were incorporated into the ancient soil profiles. Preliminary U/Pb dating of these sorted zircons via LA-ICP-MS from depth ranges of 89 m to 33.5 m in the HP1A core suggests that the contained paleosols have depositional ages ranging from 38±1.4 Ma to 33.4±0.7 Ma. These new geochronologic results are being tested further, but if true, they suggest Eocene to Oligocene aged deposits that correlate to the Paleogene White River Group in Nebraska, previously unknown from Cenozoic strata in Kansas. Additional analyses are underway on four (4) samples collected from between the 33m and 12 m interval as a further test of this approach to determine if any Neogene deposits actually are present in the HP1A core.