Discovery of Paleogene Deposits of the Central High Plains Aquifer In the Western Great Plains, U.S.A.

Abstract

Abstract: Recent drilling in southwestern Kansas has produced intact cores to investigate the subsurface lithostratigraphy, sedimentary provenance, and chronostratigraphy of strata constituting the central High Plains Aquifer (HPA). In large portions of the HPA, groundwater withdrawals greatly exceed rates of recharge, leading to dramatic water-level declines and growing concerns for long-term sustainability. Two cores, HP1A (98 m) and CMC1 (50 m), are the first and deepest intact cores of the HPA ever collected. The cores show decameter-scale intercalations between suspended-load fluvial deposits composed of fine-grained sands with pedogenically modified overbank deposits, and very coarse-grained sands and gravels consistent with high-energy, bedload-dominated fluvial systems. Six intervals in HP1A and one from CMC1 were analyzed for detrital-zircon U-Pb ages by LA-ICP-MS. The HP1A samples show maximum depositional ages (MDAs) ranging from ∼ 27.5 to 36.4 Ma with depth. A MDA of ∼ 30 Ma was measured in the CMC1 core. These Paleogene zircons likely originated from explosive volcanism associated with the mid-Cenozoic ignimbrite flare-up (44–18 Ma), which blanketed much of western North America with high-volume air-fall tuffs. We propose that a large evaporite-dissolution basin in southwestern Kansas provided the accommodation space to preserve a record of Paleogene strata. The lack of younger 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. The MDAs suggest Eocene to Oligocene age deposits that are time-equivalent to the Arikaree and White River groups in Nebraska are also present in Kansas.