MAXIMUM DEPOSITIONAL AGE CONSTRAINTS AND PROVENANCE OF CENOZOIC DEPOSITS OF THE CENTRAL HIGH PLAINS IN WESTERN KANSAS, USA

An interdisciplinary team of scientists is now engaged in a scientific drilling program to study intact cores of Cenozoic strata in western Kansas to improve understanding of the chronostratigraphy and hydrostratigraphic architecture of sediments comprising the High Plains aquifer. To date, 26 long continuous cores totaling over 860 meters have been collected from 10 study areas. Geophysical and geochemical analyses include particle size, stable isotope geo- and hydro-chemistry of organic and inorganic mineral constituents and in-situ pore fluids, spectral gamma ray logging, magnetic susceptibility and magnetostratigraphy, optically-stimulated luminescence dating (OSL), and dating of detrital zircons via laser ablation inductively coupled mass spectrometry (LA-ICP-MS). HP1A in southwestern Kansas is the first and deepest intact core of the aquifer ever attempted with ~98 m of core retrieved. The core is divided into 4 sections based on lithofacies changes: 1) the lowest ~12 m is comprised of fine- to medium-grained sands and gleyed paleosols suggesting a suspended load fluvial system and poorly drained overbank conditions; 2) these are overlain by ~49 m of very coarse-grained sands and gravels suggesting a high energy, bed load dominated fluvial system; 3) above the gravels, ~25 m of interbedded fine-grained sands and silt suggest suspended load fluvial deposits and relatively mature paleosols. The upper 12 m of the core is Quaternary loess based on OSL.  Zircons from five fine-grained intervals were analyzed for U/Pb ages by LA-ICP-MS. Maximum depositional ages (MDA) at 16 m and 24 m were ~27.9 Ma, samples from 34 m and 52 m were ~35.4 Ma, while zircons from 87 m yielded ~36.2 Ma. These zircons likely originate from explosive volcanism associated with the Mid-Tertiary ignimbrite flare-up (36–18 Ma) which blanketed much of western North America in vast ash-flow tuffs. The absence of Middle-Late Miocene zircons from cores in southwestern Kansas is striking given that such grains, likely derived from the Snake River Plain volcanic province (16.1–0.6 Ma), are readily identified in the Ogallala Fm in northern and central Kansas and Nebraska. MDAs in southwestern Kansas suggest Eocene to Oligocene age deposits equivalent to the White River Group in Nebraska―ages previously unknown from Cenozoic strata in Kansas.