SUBSURFACE GEOLOGY OF THE BRIGHTWATER TUNNEL CORRIDOR, WESTERN KING AND SNOHOMISH COUNTIES, WASHINGTON

Age dating, geochemical provenance studies, and pollen and macrofossil identification were used in stratigraphic analysis to support design of the proposed King County Brightwater Conveyance System tunnels. The project will require construction of about 15 miles of wastewater tunnels extending generally east to west from Little Bear Creek to Puget Sound near the county line, in northwestern King County and southwestern Snohomish Counties, Washington. Analysis completed to date has identified deposits of the Vashon stade of the Fraser Glaciation, and Olympia bed non-glacial cycles. Older undated deposits appear to correlate with the Possession Drift, non-glacial Whidbey Formation.

The older non-glacial (Whidbey?) deposits form the sedimentary base of the majority of the alignment corridor. The older non-glacial deposits are characterized by laterally extensive sand and gravel beds, and peat and organic-rich silts. In the central portion of the alignment, the older non-glacial deposits have been deeply eroded and infilled with pre-Fraser glacial deposits. The pre-Fraser glacial deposits are characterized by extensive glacio-lacustrine and/or glacio-marine deposits and relatively minor till and outwash facies. The stratigraphic relationship and depositional facies suggest that the pre-Fraser glacial deposits are predominantly Possession Drift. The older (Whidbey?) non-glacial and pre-Fraser (Possession?) glacial deposits are overlain by a discontinuous blanket of radiocarbon-dated Olympia Bed non-glacial deposits that generally decrease in elevation to the west, toward Puget Sound. Vashon glacial deposits are present in troughs deeply incised into the older deposits in areas that typically occur as modern drainages and low areas.