PLEISTOCENE TEPHROSTRATIGRAPHY AND PALEOGEOGRAPHY OF THE SOUTH PUGET SOUND BASIN NEAR OLYMPIA, WA

Our detailed mapping in the south Puget Sound Basin has identified two tephras which are tentatively correlated to tephras from Mt. St. Helens and Mt. Rainier dated at about 50 kya and 200 kya, respectively. This, plus the observation that fluvial and lacustrine sediments immediately underlying the Vashon Drift of latest Wisconsin age are nearly everywhere radiocarbon-infinite suggests that glacial and nonglacial sediments of more than the last five oxygen isotope stages are exposed above sea level. Distal lacustrine advance outwash equivalent to the Lawton Clay in the Seattle area is conspicuously absent. Instead, a thick (>120') glaciolacustrine silt below the Vashon sediments contains dropstones and is radiocarbon infinite. Elsewhere, coarse-grained advance Vashon outwash rests unconformably? on radiocarbon-infinite nonglacial sediments. These relationships may imply that late Pleistocene tectonic activity has modified the paleotopography and stratigraphy of the south Puget Sound area.