OLYMPIA AGE PALEOTOPOGRAPHIC INFLUENCES ON VASHON GLACIOFLUVIAL SEDIMENTATION BENEATH THE EASTERN BEAR CREEK PLATEAU, KING COUNTY, WASHINGTON

Subsurface exploration data combined with outcrop information demonstrate the presence of persistent paleotopographic highs beneath the Bear Creek Plateau. Incipient drainage development on the Olympia-age depositional surface formed in response to a lowering of base level associated with a relative drop in sea level prior to the onset of the Fraser Glaciation. Thin interval thicknesses of Vashon glaciofluvial sediments reflect deposition on the paleotopographic highs. Thick sequences of glaciofluvial sediments are located in paleotopographic drainages. The Olympia-age surface ranges from elevation 380 to 650 feet over a nine square mile area. Localized maximum relief on the paleotopographic surface is 110 feet over a distance of 1,000 feet.

Carbon-14 dating indicates the paleotopographic surface is early- to mid-Olympia-age with C-14 ages ranging from 30,270 +/- 1,290 BP to 45,540 +/- 1,930 BP (10 samples) to radiocarbon infinite (16 samples).

Geochemical analyses performed on Vashon and Olympia sediments demonstrate the Vashon glacial sediments have a distinctly different geochemical signature compared to Olympia sediments. Relatively low values of Cesium, Lanthanum and Zircon distinguish glacial Vashon sediments from non-glacial Olympia sediments. Pre-Olympia glaciolacustrine sediments are not geochemically distinguishable from Olympia sediments with the current data set, suggesting similar provenance areas for the Olympia and pre-Olympia glaciolacustrine sediments.