SCIENTIFIC DRILLING IN CENOZOIC STRATA OF THE CENTRAL HIGH PLAINS AQUIFER: A DEVELOPING U-PB ZIRCON CHRONOSTRATIGRAPHY IN WESTERN KANSAS

In large portions of the central High Plains aquifer (HPA), groundwater withdrawals greatly exceed rates of recharge―leading to dramatic water level declines and growing concerns for long-term sustainability. Significant geographic variance in water availability and aquifer response to pumping is likely derived from the formative processes and depositional histories of water-bearing and confining units comprising the HPA. Recent scientific drilling in southwestern Kansas has produced intact cores to investigate the subsurface lithostratigraphy of these sediments and their chronostratigraphy. To date, 26 long continuous cores totaling over 860 meters have been collected from 10 study areas. High-precision U-Pb zircon dates of volcanogenic and detrital zircons from the cores were generated to advance the goal of long-distance correlation of heterolithic sedimentary units and to delineate significant hydrostratigraphic subunits on a regional scale. HP1A and CMC1, in southwestern Kansas are the first and deepest intact cores of the HPA collected. Lithologically, the cores show decameter scale intercalations between suspended load fluvial deposits composed of fine-sands with pedogenically modified overbank deposits, and very coarse-grained sands and gravels suggesting a high energy, bed load dominated fluvial systems. Five fine-grained intervals in HP1A and one from CMC1 were analyzed for U-Pb ages by LA-ICP-MS. The six HP1A samples show superposed, progressively older maximum depositional ages (MDAs) ranging from 27.9±1.3 Ma to 35.7±0.7 Ma at depth. A MDA of 29.9±1.6 Ma was measured in the CMC1 core. These zircons likely originated from explosive volcanism associated with the Mid-Tertiary ignimbrite flare-up (36–18 Ma) which blanketed much of western North America in vast air-fall 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-Yellowstone hotspot volcanic provinces (16.1–0.6 Ma), are readily identified in the Ogallala Formation in north-central Kansas and Nebraska. MDAs in southwestern Kansas suggest Eocene to Oligocene age deposits time-equivalent to the Arikaree and White River Groups in Nebraska―ages previously unknown from Cenozoic strata in Kansas.