DISCOVERY OF ZIRCONS IN THE ASHFALL FOSSIL BEDS AND GROVE LAKE VOLCANIC DEPOSITS OF NORTHEASTERN NEBRASKA: U-PB AGES AND SUCCESSFUL SAMPLING STRATEGIES

Ashfall Fossil Beds State Historical Park in northeastern Nebraska is the site of a renowned Konservat-Lagerstätte of Miocene mammals, birds, and reptiles preserved in a 3m thick and nearly pure volcanic ash deposit. While no absolute ages have been generated previously from samples taken directly from the exposure at Ashfall Fossil Beds, it is widely reported that the ash correlates with the tuff of Ibex Hollow, an ~11.93 Ma pyroclastic fall deposit originating from the Bruneau-Jarbidge volcanic field in southwest Idaho. While this correlation is well established in the literature, the methods and data supporting the Miocene age correlation remain unpublished, leaving the age of the ash and in situ Lagerstätte uncertain. Previous attempts to date Ashfall directly were unsuccessful due to an alleged deficiency in datable minerals (e.g. zircon). For this study, we collected samples from four superposed stratigraphic intervals at Ashfall and another four from an ash bed cropping out ~6 km to the southeast at Grove Lake. Abundant zircons were recovered from the lowermost intervals at both locations and analyzed via Laser Ablation Inductively Coupled Mass Spectrometry. Zircon U-Pb ages of 11.93 ± 0.13 Ma and 6.41 ± 0.06 Ma for Ashfall and Grove Lake, respectively, substantiate the previous correlation of Ashfall to the tuff of Ibex Hollow. Our research indicates that detrital- and air-fall zircons are not evenly distributed stratigraphically in ash beds, particularly reworked deposits, and finding such grains may require a higher-resolution sampling scheme than is typically utilized. Finally, these new U-Pb ages are a significant step in the development of a chronostratigraphic framework of Cenozoic deposits in the Great Plains of North America.