RHYTHMICALLY STRATIFIED PLEISTOCENE SEDIMENTS IN UPPER GRAND COULEE, WASHINGTON: A MORAINE DAMMED LAKE?

Stratified sediments exposed for about 1.6 km along the shore of Banks Lake south of Steamboat Rock, upper Grand Coulee, Washington were previously mapped as part of the late Quaternary loess cover. Recent investigations have shown them to be physically distinct from and older than the loess, and they appear to lie stratigraphically beneath Missoula Flood gravel exposed in pits adjacent to Steamboat Rock. Sediments comprise a 15 meter thick sequence of rhythmically graded beds ranging from about 5 to 30 cm in thickness. Bedding contacts are distinctly erosional and suggest variable energy input into still water. Sedimentary structures consist of ripples preserved on upper bedding surfaces and cross bedding with amplitudes ranging from about 5 to 20 cm. Absolute age control is lacking, however the stratigraphic position of the beds beneath the late Quaternary loess and Missoula Flood deposits, and above the basalt bedrock suggests a late Pleistocene age. Investigations of the upper surface of Steamboat Rock, just north of the study site, indicate the presence of glacial ice in upper Grand Coulee just prior to the late Wisconsinan Missoula Floods. We interpret the deposits of the study section to represent seasonal input of sediment into an ephemeral lake formed by the damming of Grand Coulee south of the study site, possibly by a terminal moraine. Sediment was transported to the site by glacial meltwater as the ice margin receeded northward. The hypothesized moraine dam was eventually breached, allowing the lake to drain, and stranding the sediments at their present elevation. Subsequent scour by the Missoula Floods removed most of the stratified sediments except at the study site, where they were protected in the lee of Steamboat Rock.