PALEOGENE STRATA OF THE HIGH PLAINS IN WESTERN KANSAS? PRELIMINARY RADIOMETRIC DATES FROM VOLCANOGENIC ZIRCONS IN MATURE PALEOSOLS

Stable isotopic chemostratigraphy and preliminary U/Pb dates of volcanogenic zircons from Cenozoic strata in western Kansas suggest an older and more complex depositional history of the High Plains Aquifer System than previously thought. Initial work has concentrated a 98 m-long HP1A core drilled in NE Haskell County in SW Kansas; the first of six anticipated cores. Core sections were retrieved by hollow stem auger from the unsaturated zone and rotosonic drill with hydraulic piston core barrel in water-saturated zones. Drilling fluids were not employed enabling extraction of in-situ pore fluids from core. Geophysical measurements include laboratory scanning of spectral gamma-ray logs and magnetic susceptibility, and subsampling of selected intervals for thermal demagnetization and magnetic polarity measurements to identify core segments with normal and reverse magnetic polarity. 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. Chemostratigraphic records of the δ¹³C of pedogenic calcite and sedimentary organic matter do show the expected long-term Cenozoic temporal trend from lower values more characteristic of a C₃ paleoflora, to higher values more characteristic of C₄ paleoflora, although the timing of this transition in the core is uncertain at this time. Paleosols are condensed stratigraphic intervals in terrestrial stratigraphic successions, and offer the potential to produce stratigraphically-useful radiometric dates from volcanogenic zircons. Core sections containing mature paleosols were split to extract microscopic zircon crystals. Separated zircon populations were sorted to analyze clear, elongate euhedral grains that are indicative of primary air fall tephra that was subsequently worked into the paleosol matrix via pedoturbation. Preliminary U/Pb dating of 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 equivalent in age to the White River Group in Nebraska and previously unknown from Cenozoic strata in Kansas.