PROVIDING TEPHROSTRATIGRAPHY AND PALEOENVIRONMENTAL CONTEXT FOR HAGERMAN FOSSIL BEDS NATIONAL MONUMENT, IDAHO: A CASE STUDY OF THE SAHARA COMPLEX

Hagerman Fossil Beds National Monument (HAFO) preserves a diverse Pliocene (~ 4.5 to 3.0 Ma) faunal and floral assemblage, which includes multiple vertebrate taxa. The fossils are widely distributed throughout 17.6 km². Exposures of sedimentary lithologies and fossil proxies indicate a range of lacustrine, wetland, shoreline, grassland, and woodland habitats. Interbedded within these lithologies are several mafic and silicic tephra, providing spatial and temporal links between paleontological sites and environmental contexts. Since 2016, we have identified and traced several otherwise discontinuous tephra across the monument with the aid of electron microprobe analysis, which in turn has expanded the lateral extent of previously-dated volcanic ash beds and refined age constraints for several important fossil localities.

The informally named Sahara Complex is one area of geological and paleontological interest. It consists of a series of fossil-rich localities extending across the eastern ridge and down the southern flank of a large gully. The Sahara Complex has yielded a diverse fossil fauna, but is of particular significance due to recent confirmation of Canis lepophagus, a coyote-sized canid, from these deposits. This increases the geographic range of C. lepophagus in the mid-Blancan. It joins the previously known smaller C. ferox, and a larger borophagine dog, Borophagus hilli, increasing the Hagerman carnivoran guild to nineteen.

Three tephra units have been mapped through the site (the Fossil Gulch ash layer, Bed G, and the rattlesnake ash layer), with a fourth and fifth (the Peters Gulch ash layer and Bed F) projected within a few tens of meters below the ground surface based on exposures in adjacent gullies. While not exposed here or dated directly, the Peters Gulch ash and Bed F overlie the Basalt of Oster Lakes (4.5 Ma) elsewhere. This basalt flow provides an oldest possible date for the fossil beds as a whole. The Fossil Gulch ash layer and Bed G (3.86+/-0.03 Ma) provide distinctive markers through the middle of the exposure and overlie the C. lepophagus locality. A previously unmapped tephra (the rattlesnake ash layer) caps the local section and stratigraphic information from elsewhere in the monument indicates that it is older than the Horse Quarry ash layer (3.07+/-0.23 Ma).