WHERE DID THEY COME FROM, WHERE DID THEY GO? VOLCANIC ASH LAYERS AT HAGERMAN FOSSIL BEDS NATIONAL MONUMENT, IDAHO, USA

Tephrochronology can be used to answer a variety of questions, including inquiries about the volcanoes from which tephras were erupted as well as about the landscapes upon which they were deposited. Identifying the sources and extents of ancient eruptions is useful to both categories of queries.

The Pliocene Glenns Ferry Formation consists of fluvial-lacustrine deposits from the former Lake Idaho interbedded with tephra and lava flows. The Glenns Ferry is exposed across approximately 40 kilometers of southern Idaho, crossing the boundary of the eastern and western Snake River Plain. The Formation contains a rich assemblage of fossils most notably preserved at Hagerman Fossil Beds National Monument (HAFO). Over 200 species have been discovered at HAFO, representing over one and one-half million years of evolution. Determining the precise ages and relationships of these species is critical to a full understanding of the Pliocene at Hagerman. Fortunately, numerous tephra layers are interspersed amongst the fossil-bearing sediments.

We have identified and correlated at least fifteen discrete tephra layers as well as several basaltic lava flows throughout HAFO. We have determined the ages and provenance of many of these volcanic deposits using a variety of techniques, including detailed tephra mapping, geochemical characterization of tephra via EMP and ICP-MS methods, updated paleomagnetic directions, detrital zircon analysis, and ⁴⁰Ar/³⁹Ar age determinations.

While the lava flows emanated exclusively from Snake River Plain sources, the tephra were derived from a mix of local sources as close as five kilometers away and distal sources several hundred kilometers away. The basaltic tephra within HAFO all came from local sources, while the rhyolite-to-dacite tephra at HAFO were erupted from a variety of sources including eruptive sources in the nearby Snake River Plain and in Cascadia and/or the Basin and Range provinces. The updated ages for the HAFO volcanic deposits and improved understanding of the geochemical variability of the tephra layers allows for more robust correlation of these tephra across the Glenns Ferry Formation and with other western U.S. tephra deposits.