OSTRACODES AS INDICATORS OF THE PALEOENVIRONMENT IN THE PLIOCENE GLENNS FERRY FORMATION, LAKE IDAHO, PRELIMINARY RESULTS

The Glenns Ferry Formation is exposed within the Hagerman Fossil Beds National Monument (HAFO), located in south-central Idaho. The Glenns Ferry Formation was deposited approximately 3.7-3.3 Ma as part of the Idaho Group, which is composed of upper Cenozoic sediments and volcanics located in the western Snake River Plain. In the depositional area of the Glenns Ferry Formation, evidence of a large and permanent lake (Lake Idaho) includes lacustrine sediments, fish fossils, and nonmarine fossil ostracodes. Ostracodes are excellent indicators of aquatic paleoenvironments because of their sensitivity to hydrochemical and climatic changes.

Preliminary research indicates the presence of three distinct ostracode fauna stratigraphically distributed within the HAFO exposures of the Glenns Ferry Formation. The lowest fauna indicates a deep, permanent, cold, freshwater lake with Cytherissa lacustris as its dominant species. The lowest fauna is found closest to the Snake River at around 853 m elevation asl. The intermediate fauna is indicated by the appearance and dominance of Candona rawsoni, suggesting the change to a more ephemeral system. The intermediate fauna appears at about 950 m elevation asl. The upper fauna, located in the Smithsonian Horse Quarry area (975 m elevation asl), also contains Candona rawsoni but includes several endemic Limnocythere species (previously noted in a study by Forester, 1991). The presence of endemic species suggests a more permanent system, because a stable hydrologic system must be present for endemic species to evolve in the lake. Further analysis of these fauna will include oxygen isotope analysis, which should shed light on the changing hydrologic and climatic conditions during this time.