SPATIAL AND TEMPORAL PATTERNS OF OGALLALA FORMATION DEPOSITION REVEALED BY U-PB ZIRCON GEOCHRONOLOGY

Difficulties in the correlation of 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 inform management of depleted groundwater resources, illuminate the causes of fluvial aggradation, enhance the western US climate record, and strengthen the temporal precision of the Neogene paleontological record. Such relationships can be derived from chronostratigraphic information provided by abundant volcanogenic zircon in volcanic ashes and fluvial sediments, which provide the first high-precision (ca. 1-5% uncertainty) radioisotopic dates for the Ogallala Formation. These zircon-bearing 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 perhaps under-appreciated potential to time-stamp terrestrial surfaces at great distances from contemporaneous magmatic centers. Volcanic ash depositional ages are consistent with maximum depositional ages from fluvial sediments, 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. An inferred Norton lobe in northern Kansas aggraded to near-modern levels in the Middle Miocene, measurably earlier than the Late Miocene deposition of an inferred Ellis lobe over a bedrock high in central Kansas. The observed temporal relationships predict aquifer anisotropy that may inform efforts to develop numerical groundwater models designed to forecast aquifer response to different conservation strategies.