THE FIRST U-PB ZIRCON AGES FOR THE ASHFALL FOSSIL BEDS AND GROVE LAKE VOLCANIC DEPOSITS IN NEBRASKA: TWO DIFFERENT YELLOWSTONE HOTSPOT TRACK SOURCES

Ashfall Fossil Beds State Historical Park is the site of a renowned, world-class Lagerstätte characterized by a mass-death assemblage of Miocene reptiles, birds and mammals in a ~3m thick, minimally reworked volcanic ash deposit. Although the ash deposit is a nearly pure vitric tuff, no radiometric age from this tephra has been generated due to an alleged deficiency in dateable minerals. Past studies have geochemically correlated the ash horizon at Ashfall to the Ibex Hollow Tuff, a pyroclastic fall deposit extensive throughout the western U.S. and associated with the Bruneau Jarbidge eruptive center. However, this geochemical data is unpublished, leaving the age of the ash and associated Lagerstätte uncertain. For this study, we collected samples from four superposed stratigraphic intervals at Ashfall, and also from four superposed intervals at an exposed ash deposit approximately 6 km southeast of the Ashfall park at Grove Lake. Zircons were recovered from the lowermost sampled interval at both locations, and subsequently analyzed via LA-ICP-MS (Laser Ablation Inductively Coupled Mass Spectrometry), generating U-Pb ages of 11.93 ± 0.13 Ma for the Ashfall ash and 6.02 ± 0.17 Ma for the Grove Lake ash. These results corroborate previous work correlating the Ashfall Fossil Beds to the Ibex Hollow Tuff, which has an ⁴⁰Ar/³⁹Ar sanidine age of 11.93 ± 0.03 Ma, determined on proximal deposits. The U-Pb age of ash from Grove Lake is coincident with reported ages for deposits from the Heise volcanic field in Idaho. Zircon U-Pb dating and geochemical analyses of primary ash fall deposits will help to better constrain the ages of important paleontological sites such as Ashfall Fossil Beds and to further refine chronostratigraphic interpretations in the Great Plains region.