STRATIGRAPHY OF THE DUWAMISH RIVER VALLEY SOUTH OF SEATTLE, WASHINGTON

Within the Seattle uplift, the stratigraphy of the Duwamish River Valley and adjacent uplands consisted of Tertiary bedrock, Pleistocene glacial deposits, and thick deposits of Holocene sediments. Subsurface conditions were explored along an 8.8 km section of the proposed Sound Transit Light Rail alignment from South Seattle through Tukwila, Washington that included 108 soil/rock borings and 13 cone penetration test probes to depths ranging from 6 to 91 meters.

In the Duwamish River Valley, sediments encountered were Holocene strata that included marine, beach, estuary and alluvial deposits. These deposits reflect the early Holocene isostatic rebound followed by a rise in relative sea level, which occurred due to the melting of glacial ice sheets. A thin silty peat layer (10.3 to 10.9 Ka), encountered across much of the Duwamish River Valley approximately 48 meters deep, suggests that the peat was above relative sea level during the early Holocene. Approximately 8 meters of displacement was observed in the peat layer and several overlying strata along the east edge of the valley that may indicate faulting during the Holocene. At the top of the sequence thick deposits of ashy alluvial deposits ranging from 3 to 27 meters in thickness represent the rapid aggradation of lahar run-out from Mt. Rainier and alluvium from the Duwamish River during the late Holocene.

Deposits of the Vashon Stade of the Fraser Glaciation overlie older glacial deposits exposed in the southern half of the alignment. Pre-Vashon glacial deposits include till covered by indurated recessional outwash. Vashon deposits are discontinuous and include glacial lake and advance outwash capped by till.

Tertiary bedrock, the upper and middle members of the Tukwila Formation, consisted of andesite and volcaniclastic deposits interfingered with organic rich laminated sandstone and siltstone. These deposits mantle fossiliferous sandstone and siltstone identified to be 37 to 40 Ma. The bedrock topography depicts an undulating paleo-landscape that has been folded and faulted. Overturned beds and breccia containing slickensides, microfolds, fault gouge, and talc are observed throughout the alignment. Over 2,712 m of soil and rock was sampled within the Seattle uplift and has opened a small window into 40 million years of South Seattle’s geologic history.