STRATIGRAPHY AND PALEONTOLOGY OF PRE-LATE WISCONSINAN OUTBURST FLOOD DEPOSITS, SOUTHEASTERN WASHINGTON

The well-documented late Pleistocene Missoula Floods are the most recent in a long history of glacial outburst floods spanning at least the past 780 ka . Evidence comprises rhythmic deposits showing internal stratigraphy and sedimentary structures similar to Missoula Flood deposits but which are demonstrably older; deposits that are unconformity bounded; and deposits that contain extra-basinal clasts. Analysis of caliche-coated flood gravel in the Pasco Basin northwest of Walla Walla indicates reverse magnetism (>780 ka). North of Walla Walla, in Washtucna Coulee, loess deposits include flood cut unconformities that truncate tephra assigned to the Mt. St. Helens set C (+/- 47 ka). In Walla Walla County, flood cut unconformities form the boundaries of four sediment sequences; they truncate caliche soils and tephra. Two tephras in loess from the third sequence match tephras from Newberry Volcano (Oregon) and Long Valley Caldera (California), and suggest an age of +/- 200 ka. Ash in the lower part of the outcrop has yet to be identified. In the oldest sediment sequence, a diamict associated with a soil zone contains clasts of extra-basinal origin, including both igneous and metamorphic lithologies; these clasts are significantly larger than the average grain size of sediment units in which they are contained. The basal unit in the section is quartz-mica sand showing large (eolian) and small (fluvial) scale cross beds and oxidation. A diverse vertebrate fauna has been discovered in the oldest unconformity bounded sequence. The assemblage includes a peccary (first record in Washington Pleistocene), a deer, a horse, a member of the weasel family, a coyote, and several rodents. In addition to the mammals, the fauna includes a single bone apparently representing a reptile. The fauna suggests a savannah/forest/riparian ecosystem for the sediment unit beneath the oldest unconformity.