PLEISTOCENE STRATIGRAPHY OF THE SOUTH PUGET SOUND BASIN AND ADJACENT UPLANDS NEAR OLYMPIA, WASHINGTON

Our detailed mapping in the south Puget Sound Basin has demonstrated that the upper Pleistocene stratigraphy differs significantly from that exposed farther north in the Seattle area. Distal lacustrine advance outwash of the Vashon Drift (latest Wisconsin age) equivalent to the Lawton Clay in the Seattle area is conspicuously absent in most of the area, although anomalously young (<13,500 radiocarbon years) underlie Vashon Drift from the Nisqually Delta north to Tacoma Narrows. Fluvial and lacustrine sediments ("Olympia beds") immediately underlying the Vashon Drift range in age from ~33kya to radiocarbon-infinite. Pre-Olympia sands bearing tephras correlated to tephras of about 100kya and 200kya are interbedded with and underlain by glacial gravels (conspicuously lacking till) of both mixed and volcanic provenance. This suggests that glacial and nonglacial sediments of at least the last seven oxygen isotope stages are exposed above sea level. A pre-Olympia silt in this section that is reversely magnetized probably represents an excursion (Blake Reversed Subchron?) rather than Matuyama.

Reconnaissance mapping along the bluffs of the lower Nisqually River, identified a sequence of gravel, sand, silt, clay, and diamicton dominated by volcanolithic clasts of Mt. Rainier or other Cascade Range source that can be traced to within one mile of Puget Sound. Paleocurrent indicators suggest a southerly source. Some boulders are glacially striated, suggesting that these sediments are, at least in part, Cascade (Mount Rainier) source alpine drift, and are interbedded with lahar deposits, lake sediments and alluvium. Large boulders (up to 8 ft. in diameter) near the base of the section are deeply weathered (notches with chisel end of hammer to a depth of at least several inches), although most cobbles and boulders have weathering rinds <1mm; this suggest both Late Pleistocene and early Pleistocene drifts (Colman and Pierce, 1981, U.S.G.S. Professional Paper 1210). We tentatively suggest correlations to Hayden Creek (stage 6?) and the apparently much older Wingate Hill Drifts, which have not previously been recognized within 15 miles of here but which are widespread in the upper Nisqually. The upper part of this sequence is the prolific aquifer that supplies most of the municipal water in the Olympia area.