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