DEVELOPMENT OF THE SEATTLE BASIN, TECTONICS OF THE SOUTHERN WHIDBEY ISLAND FAULT, AND PUGET LOWLAND LANDFORM DEVELOPMENT - FINDINGS FROM A DEEP TUNNEL PROJECT

Boreholes for exploration of 30 kilometers of wastewater tunnels and pipelines between Maltby, Washington, and Puget Sound have provided an unprecedented opportunity to observe the three-dimensional subsurface geology of the northern portion of the Seattle basin, an area that has been multiply glaciated during the Quaternary. It also facilitates study of deformation associated with the southern Whidbey Island fault zone, an active fold and thrust belt.

Radiocarbon and luminescence dating of sediments and amino-acid dating of marine shell provide sequence stratigraphic control. Older depositional sequences are present at or near ground surface in the eastern portion of the Seattle basin, and only the youngest sequences are exposed in the central portion. The thickness of the deposits generally increases to the west, into the interior of the basin, geometry consistent with the majority of the subsidence having occurred along the axis of the basin. The rate of subsidence at the axis of the basin during the last 80 ka has averaged about 0.8 mm/yr, decreasing to 0.4 mm/yr averaged over the last 300 ka (late Pleistocene), and based on the few deep borehole data for the area, much lower rates of during the middle and early Pleistocene.

The study area has regional, northwesterly trending topographic features that do not appear to result from glacial ice flow or recessional glacial processes. Stratigraphic discontinuities or vertical offsets of strata and indications of post-depositional fracturing and shearing of sediment at many locations co-occur with linear magnetic anomalies and/or lidar lineaments. These co-locations suggest that the discontinuities are caused by faults that are expressed at the ground surface and extend into Tertiary bedrock. Patterns of stratigraphic disruption over time show that many of these faults remained active over tens of thousands of years and have influenced patterns of deposition and erosion over multiple glacial and interglacial cycles. The best constrained examples indicate that the rate of vertical offset along these faults has averaged about 0.8 mm/yr.