Spatial and Temporal Patterns of Ogallala Formation Deposition Revealed by U-Pb Zircon Geochronology

Abstract

Difficulties in correlation within continental clastic basins arise from the common shortage of reliable marker beds, which limits characterization of geological relationships. In the High Plains of the central United States, an improved understanding of the stratigraphic architecture of the terrestrial Ogallala Formation can be used to improve management of depleted groundwater resources, illuminate the causes of fluvial aggradation, enhance the western US climatic record, and strengthen the temporal precision of the North American Land Mammal Ages. Such relationships can be derived from chronostratigraphic information provided by abundant, dateable, volcanogenic zircon in volcanic ashes and fluvial beds, which provide the first highprecision (ca. 1-5% uncertainty) radioisotopic dates for the Ogallala Formation. These zirconbearing ashes appear to have travelled ~1350 km from their interpreted sources within the Bruneau-Jarbidge and Twin Falls volcanic centers in Idaho, suggesting that volcanogenic zircon has the previously unappreciated potential to time-stamp terrestrial surfaces at great distances from contemporaneous magmatic centers. Volcanic ash depositional ages are consistent with fluvial maximum depositional ages, indicating that deposition of common lithologies within the Ogallala Formation, including sands, paleosols, and volcanic ashes can be reliably dated with modern, high-precision techniques. Zircon U-Pb LA-ICP-MS results suggest diachronous aggradation of the Ogallala Formation in Kansas, particularly deposition of an inferred Norton lobe in northern Kansas that initiated prior to ~12.5 Ma and aggraded to near-modern levels by 11.7 Ma, measurably earlier than the ~8-9.5 Ma deposition of an inferred Ellis lobe over a bedrock high in central Kansas. The observed diachronous relationships predict aquifer anisotropy that could inform efforts to develop numerical groundwater models designed to forecast aquifer response to different conservation strategies. iv ACKNOWLEDGEMENTS