Sedimentary basins beneath the Puget Lowland of Washington State

A series of geophysical experiments carried out by the USGS and our university collaborators have mapped three large sedimentary basins beneath the Puget Lowland: The Everett, Seattle and Tacoma basins. The Everett basin is believed to be 6 to 9 km thick, and lies beneath Camano Island, Whidbey Island and the city of Everett. The south edge of the basin is formed by the Southern Whidbey Island fault (SWIF). The basin is described in a scientific paper by Johnson and others (1996).

The Seattle basin underlies the cities of Seattle, Bellevue, Kirkland and Redmond. The basin is 7 to 9 km deep, and its south edge is formed by the Seattle fault. The basin sediments thin to the north as they form the south edge of an anticline, the Kingston arch. The basin is described in Johnson and others (1994) and Pratt and others (1997). The Seattle fault is described in Pratt and others (1997), tenBrink and others (2003) and Blakely and others (2002).

The Tacoma basin is the least known of the three major basins. The Tacoma basin was estimated by Pratt and others (1997) to be 3.5 to 4 km thick, but Brocher and others (2001) argue that it is 6 km thick on its west end. The north edge of the basin is formed by a series of faults and folds, part of the Tacoma fault zone. The area between the Seattle and Tacoma basins is an area of shallow bedrock named the Seattle uplift by Pratt and others (1997). The south edge of the Tacoma basin is formed by either a fault or fold at the north edge of the Black Hills bedrock area.

To the east and west of the Puget Lowland, bedrock is at the surface in the Olympic Mountains and Cascade Range.

The geology of faults beneath the Puget Lowland is described in the following scientific papers:

Blakely, R.J., Wells, R.E., Weaver, C.S., and Johnson, S.Y., 2002, Location, structure, and seismicity of the Seattle fault zone, Washington: Evidence from aeromagnetic anomalies, geologic mapping, and seismic-reflection data: Geological Society of America Bulletin, v. 114, p. 169-177.

Brocher, T. M. and others 2001. Upper crustal structure in Puget Lowland, Washington: Results from the 1998 seismic hazards investigation in Puget Sound. Journal of Geophysical Research, volume 106, pages 13541-13564.

Johnson, S. Y., C. J. Potter, and J. M. Armentrout. 1994. Origin and evolution of the Seattle fault and Seattle basin, Washington. Geology, volume 22, pages 71-74.

Johnson, S. Y., C. J. Potter, J. M. Armentrout, J. J. Miller, C. Finn, and C. S. Weaver. 1996. The southern Whidbey Island fault: An active structure in the Puget Lowland, Washington. Geological Society of America Bulletin, volme 108, pages 334-354.

Johnson, S.Y., C.J. Potter and J.M. Armentrout, 1994. Origin and evolution of the Seattle fault and Seattle basin, Washington, Geology, 22, 71-74.

Ludwin, R. S., C. S. Weaver, and R. S. Crosson. 1991. Seismicity of Washington and Oregon, p. 77-98. In D. B. Slemmons, et al. [ed.], The Geology of North America, Neotectonics of North America. Geological Society of America.

Pratt, T. L., S. Johnson, C. Potter, W. Stephenson, and C. Finn. 1997. Seismic reflection images beneath Puget Sound, western Washington State: The Puget Lowland thrust sheet hypothesis. Journal of Geophysical Research, volume 102, pages 27469-27489.

ten Brink, U.S., Molzer, P.C., Fisher, M.A., Blakely, R.J., Parsons, T., Crosson, R.S., and Creager, K.C., 2002, Subsurface geometry and evolution of the Seattle fault zone and the Seattle basin, Washington: Bulletin of the Seismological Society of America, v. 92, p. 1737-1753.