NEW MAPPING AND TEPHROCHRONOLOGY AT HAGERMAN FOSSIL BEDS NATIONAL MONUMENT, IDAHO, USA - A PROJECT DECADES IN THE MAKING (Invited Presentation)

Hagerman Fossil Beds National Monument (HAFO) is a rich Pliocene fossil site located in southern Idaho, USA. The monument contains three geologic formations but is dominated by the Pliocene Glenns Ferry Formation, which is comprised primarily of fluvial-lacustrine deposits with numerous interbedded silicic and basaltic tephra and two basaltic lava flow units.

New and updated tephrochronology provides additional data on the geochemical composition of the tephra units at HAFO, resulting in new and revised correlations and a high-resolution temporal framework for currently known and potential future fossil sites. Previous studies identified four silicic tephra (informally named the lower lapilli, Peters Gulch ash layer, Fossil Gulch ash layer, and Horse Quarry ash layer) and six basaltic tephra (informally named beds E, F, G, H, I, and J) within the monument. In addition, previous research either focused on broad regional mapping or on smaller, defined areas within or adjacent to HAFO. In comparison, we conducted detailed mapping and geochemical characterization of tephra deposits throughout nearly the entire eighteen-square-kilometer national monument.

Extension of the mapped areal distribution and correlation of tephra layers was achieved using standard field mapping techniques as well as new and updated major, minor, and trace element volcanic glass data obtained utilizing single-shard electron microprobe (EMP) and bulk purified glass inductively coupled plasma – mass spectrometry (ICP–MS) techniques. We have also identified and characterized four additional silicic tephra units (informally named the PPE ash layer, the rattlesnake ash layer, an unnamed silicic ash layer, and the Oregon Trail ash layer) and three additional basaltic tephra layers (informally named bed E2, bed F2, and bed F3). New Sr, Pb, and Nd isotopic data provide insight into the provenance of these tephra and revised dates provide refined age control, allowing us to better constrain the temporal context for associated stratigraphic units and fossil assemblages.