DEPOSITS, EROSIONAL FEATURES, AND FLOW CHARACTERISTICS OF THE LATE-GLACIAL TANWAX CREEK-OHOP CREEK VALLEY FLOOD-A LIKELY SOURCE FOR SEDIMENTS COMPOSING THE MIMA MOUNDS, PUGET LOWLAND, WASHINGTON

Lag boulders, boulder-cobble bars, local megaripples, scour features, and a network of erosional channels that cut the drumlinoid fabric of the Puget Lobe glacial drift plain are the trace of at least one, extraordinary late-Glacial flood (Pringle and others, 2000). This flood resulted from breaching of an ice-dammed lake or lakes in (and adjoining?) the Carbon River valley northwest of Mount Rainier. In contrast with the surrounding/underlying Puget Lobe outwash deposits, material of the Tanwax Creek-Ohop Creek valley flood is characterized by its highly andesitic character, much of which is likely derived from an earthflow of volcaniclastic debris that temporarily dammed the floodway. Some andesites in the flood deposits may have originated from Mount Rainier runoff (or an eruption?), because Mount Rainier's drainages were shunted around the toe of the Puget Lobe ice. Downstream extent of the flood can be traced at least as far as Elma, Washington in the Chehalis River valley, 125 km downstream of the Cascade Mountain Front. A backflood may have extended to the south more than 17 km upstream into the Chehalis River valley as far the City of Chehalis. Stratigraphic continuity, channel geography and morphology, grain-size sorting characteristics, and common andesitic character of clasts and of subangular matrix fragments for the Mima Mounds, suggest that the source material for the mounds at Mima, Rocky, and Violet Prairies was deposited by the same flood event. The Mima Mounds deposits were most likely a backflood of sediment-charged (hyperconcentrated?) floodwater into the Black River valley, which is reentrant to the main floodway.