EXTENDING AND EXPANDING THE STRATIGRAPHY OF THE SOUTHERN PUGET LOWLAND

Previous mapping in the southern Puget Lowland in the area from Olympia to the Nisqually Delta on the east and Tenino to the south identified a stratigraphic sequence of three Cordilleran drift sheets and interbedded interglacial sediments representing Oxygen isotope stages (OIS) 2-6. We have mapped nonglacial sediments bearing abundant Sunset Amphitheater tephra from Mt. Rainier (ca. 192 k.y.a.) demonstrating the presence of OIS 7 sediments. A normally magnetized underlying Cordilleran drift may be an OIS 8 glaciation, although we have not directly dated it. We also mapped two glacial driftts of Cascade provenance with north-directed paleocurrents, suggesting a Mt. Rainier source probably equivalent to Hayden Creek and Wingate Hill Drifts of Crandell. The upper of these drifts is the prolific aquifer that supplies most of the city of Olympia water.

Farther south, between Tenino and Rainier, previous workers mapped an older drift, largely on the basis of greater erosion than the Vashon (OIS 2) ground moraine to the north. Lea mapped an older end moraine near Tenino and tentatively correlated it to Double Bluff Drift (OIS 6). The drift near Tenino is indeed pre-Vashon but the relatively eroded drift farther north is Vashon that was extensively modified by ice-marginal drainage shortly after glacial maximum. The steady, rapid retreat of the Vashon ice front resulted in construction of extensive unmatched terraces formed by streams bounded on one side by ice and cutting into moraine deposits on the other. The stream that deposited one of these terraces was probably fed by a glacial outburst flood (Tanwax flood of Pringle and Goldstein, 2002), that deposited abundant ice-rafted boulders as large as 2 to 3 ft in diameter in the Deschutes River valley and formed kettles near Offutt Lake. Mima Mounds in this area are confined to the terrace underlain by Tanwax Flood deposits. We have not yet traced these deposits farther west to the type locality of the Mima Mounds.