TEPHROCHRONOLOGIC MAPPING AS A TOOL FOR CONSTRAINING THE PLIOCENE FOSSIL STRATIGRAPHY OF HAGERMAN FOSSIL BEDS NATIONAL MONUMENT, IDAHO, USA

Within Hagerman Fossil Beds National Monument (HAFO), Idaho, Pliocene exposures of the Glenns Ferry Formation preserve lacustrine, fluvial, marsh, grassland, and woodland deposits interbedded with tephra and lava flows. The formation also contains a rich assemblage of fossils. Over 200 species of fauna and flora, including several vertebrate fossil holotypes, have been discovered at HAFO. Fossil localities are widespread across 17.6 km² of deeply incised gullies and span in age from ~4.5 to 3.0 Ma.

Past studies focused on two areas within the monument, Fossil Gulch and Peters Gulch, and identified four silicic tephra (informally named lower lapilli, Peters Gulch ash layer, Fossil Gulch ash layer, and Horse Quarry ash layer) and five basaltic tephra (informally named beds F, G, H, I, and J). Additionally, previous publications either focused on large-scale regional mapping or intensively discussed only one or two areas or volcanic units within the monument. In comparison, since 2016 we have conducted detailed tephrochronologic mapping and highly refined geochemical characterization of numerous newly and previously recognized volcanic ash deposits throughout the entire national monument.

Updated major, minor, and trace elemental volcanic glass data were obtained utilizing single-shard electron microprobe and inductively coupled plasma – mass spectrometry techniques, extending mapped areal distributions and correlations of tephra layers and associated fossil sites at HAFO. These new data also provide modern insight into the chemical variability within these tephra, some of which are quite heterogeneous. Moreover, we have identified three previously unknown tephra within the monument, herein informally named the Oregon Trail ash layer, the rattlesnake ash layer, and bed F2. A higher resolution tephrochronologic framework and refined stratigraphic correlations are herein provided for the monument. Age determinations for key HAFO taxa and the overall fossil assemblage are also better constrained.