GEOCHRONOLOGY OF THE PLEISTOCENE TEPHRA SEQUENCE AT FOSSIL LAKE, OREGON

Fossil Lake represents the most important Quaternary vertebrate paleontological region in the Pacific Northwest. Fossils have been known since 1876, and the lacustrine deposition has been compared to that at Lake Bonneville. Recently, we have made very large collections from these Pleistocene deposits, culminating in thousands of stratigraphically collected vertebrates. Most age speculations have been based on fossils with the exception of a radiocarbon date of 29 Ka bp (Allison, 1966) using snails. This date and the described fossils suggested a late Pleistocene (Rancholabrean) age. Based upon careful vertically controlled collections of fossil remains, a composite stratigraphic section has been delineated, including rhythmically bedded packages, some containing tephras. These packages consist of upward fining sequences of gravel and sand grading into silt and clay, the lacustrine deposits. The discrete packages indicate the overall ephemeral nature of Fossil Lake. The compositions of the glasses within the tephras were compared to dated tephras from the western United States. The base of the exposed section is comprised of yellow and white clays with interbedded sand and gravel. Tephras from this portion of the section compare most reliably (.94-.95 similarity coefficient-s.c.) with the Dibekulewe Ash dated at 610 Ka. Above a gravel-punctuated disconformity, rhythmic beds predominate. In the middle of the section, a pumice compares most favorably (.98 s.c.) with the Tulelake Ash dated at 120-130 Ka. Another tephra found higher in the section associated with a tan silt and clay compares most favorably (.98 s.c.) with the Mount St. Helens set C tephra dated at 47 Ka. The uppermost tephra occurring at Fossil Lake compares most favorably (.98 s.c.) with the Trego Hot Springs Ash, dated at 23.2 Ka. These tephra analyses, funded by the Desert Research Institute and the first author and processed by Dr. F.F. Foit, Washington State University, indicate that most deposits exposed at Fossil Lake are significantly older than previously thought, extending back into the Irvingtonian NALMA.