AGE, CORRELATION, AND ECOLOGY OF EOLIAN AND NON-EOLIAN FACIES OF THE PALOUSE SILT, SOUTHEASTERN WASHINGTON

The late Pleistocene Palouse silt of southeastern Washington has long been considered to represent a prolonged interval of loess deposition interrupted by formation of carbonate soils and catastrophic glacial outburst flooding. Stratigraphy and correlation in the loess sequence is based on pedostratigraphic units and tephra geochemistry. Outcrops of the Palouse silt in the southern Palouse region show distinctive facies, including fluvial and rhythmically bedded lacustrine deposits, and iron-rich soils, which are associated in outcrop with Cascade-sourced tephras. Outcrops also preserve a fauna consisting of rare rodent fossils and gastropod and bivalve mollusk fossils. AMS radiocarbon dating of shells from one outcrop establishes the age of the fossiliferous interval as 12,480 +/-60 14C yr B.P. Radiocarbon ages from two other outcrops are pending. The age of the mollusk fauna is consistent with the geochemical identity of the associated tephra (Mt. St. Helens set S) and places the fossiliferous interval near the end of the final event in the Missoula Flood sequence. Stratigraphy, tephra chronology and radiocarbon dating allows placement of these deposits into loess units L-1 and L-2 in the soil stratigraphic framework developed for the Palouse silt to the north. Interpretation of stratified sediments at the base of one section, with the radiocarbon age of fossil-bearing horizons, suggests that they may be associated with the latest events of flooding during the late Pleistocene Missoula Floods. Their position near the maximum recognized flood elevation in valleys tributary to main drainages, which were backflooded during the Missoula Floods, is consistent with this interpretation. The fossil mollusks are similar to species living in the Cascade and Rocky Mountains of Washington and British Columbia, and to species described from mid-late Pleistocene periglacial sediments in the Midwestern United States.