STRATIGRAPHY BENEATH 10.7 KILOMETERS OF THE SOUND TRANSIT LIGHT RAIL, FROM SEATAC INTERNATIONAL AIRPORT TO SOUTH SEATTLE, WASHINGTON

A preliminary geologic cross-section of the original 121 subsurface explorations was presented at the GSA Annual Meeting in 2003. Since then, construction has begun and the alignment has been extended 2.7 km south to SeaTac International Airport. An additional 189 borings were completed along the entire 10.7 km of the alignment providing more than 300 explorations ranging in depth from 6 to 91 m at roughly 40 m spacing, to “confirm” the subsurface conditions previously interpreted. These explorations revealed localized subsurface features such as fractures, valleys, and zones of artesian groundwater flow that painted a more detailed geologic history of the Seattle uplift.

The alignment traverses three depositional environments related to the formation of the Puget Lowland: Tertiary bedrock, Pleistocene glacial deposits, and thick deposits of Holocene sediments. Tertiary bedrock (the upper and middle members of the Tukwila Formation) consisted of andesite and volcaniclastic deposits interfingered with carbonaceous and fossiliferous sandstones and siltstones identified to be 37 to 40 Ma. The bedrock topography depicts an undulating paleo-landscape that has been folded and faulted, that correlates well with the structural geology presented by Brocher in 2003.

Deposits of the Vashon stade of the Fraser glaciation overlie older glacial deposits exposed in the southern half of the alignment. Vashon deposits are discontinuous and include glacial lake and advance outwash capped by till. In portions of the upland areas, recessional deposits mantle the glacial till.

Holocene sediments concentrated in the Duwamish River Valley included marine, beach, estuary and alluvial deposits. These deposits reflect the early Holocene isostatic rebound that was followed by a rise in relative sea level that occurred due to the melting of glacial ice sheets. A thin silty peat layer encountered across much of the Duwamish river valley approximately 10.5 Ka and 48 m deep suggests that the peat was above relative sea level during the early Holocene. At the top of the sequence, thick deposits of ashy alluvial sediment were encountered ranging from 3 to 27 m in thickness. This represents three distinct lahar run-out deposits from Mt. Rainier that rapidly aggraded with alluvium from the Duwamish River during the late Holocene (Zehfuss, 2005).