SUBSURFACE GEOLOGY OF THE LAKE ALICE PLATEAU, KING COUNTY, WASHINGTON

Gravity data, seismic refraction surveys and subsequent exploratory well drilling document the presence of a paleovalley incised into Eocene bedrock beneath the Lake Alice Plateau in eastern King County, Washington. The paleovalley was filled with pre-Olympia and Olympia-age sediments, and was ultimately covered by Vashon age glacial deposits. The Pleistocene sediments onlap a bedrock high along the plateau's southern margin.

Subsurface exploration data and outcrop information document the thickness and facies distribution of Olympia nonglacial and Vashon advance outwash deposits beneath the Lake Alice Plateau. Maximum Olympia-age thicknesses exceed 400 feet, and consist predominately of lacustrine silts/clays, although thick fluvial deposits are also present. Outcrop and subsurface correlation, confirmed with radiocarbon age dating, indicate significant paleotopographic relief at the top and base of the Olympia nonglacial sequence. The top of Olympia-age lacustrine deposits is near elevation 550 feet along the plateau's northern margin where multiple age dates indicate deposition occurred approximately 24 to 29 ka. Older dates were obtained from higher elevation locations including one date of 41.2 ka at approximate elevation 900 feet.

Vashon advance deltaic and ice marginal fluvial sediments are widespread beneath the plateau. South to southeast flowing meltwater streams deposited up to 150 feet of deltaic sediment in a proglacial lake during Vashon ice sheet advance. The proglacial lake surface elevation is estimated at 780 feet along the plateau's northern margin. Southerly dipping foreset beds exposed in deep excavations near the plateau's northwestern margin record proglacial lake elevations of about 870 feet. One radiocarbon date indicates the proglacial lake was younger than 14.5 ka.

Groundwater flow beneath the plateau is controlled by a combination of paleotopography and distribution of coarse and fine-grained facies. Perennial spring flow occurs near elevation 550 feet at the contact between Vashon advance deltaic sediments and underlying Olympia-age lacustrine deposits in several ravines along the plateau's northern margin. Springs along the plateau's northwestern margin emerge near elevation 750 feet, reflecting paleotopographic relief at the top of the Olympia sequence.