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Geology of Seattle, a field trip

Ralph A. Haugerud, Kathy Goetz Troost and William T. Laprade
Geology of Seattle, a field trip (in From the Puget Lowland to east of the Cascade Range; geologic excursions in the Pacific Northwest, Ralph A. Haugerud (editor) and Harvey M. Kelsey (editor))
Field Guide (Geological Society of America) (January 2017) 49: 1-24


Seattle's geologic record begins with Eocene deposition of fluvial arkosic sandstone and associated volcanic rocks of the Puget Group, perhaps during a time of regional strike-slip faulting, followed by late Eocene and Oligocene marine deposition of the Blakeley Formation in the Cascadia forearc. Older Quaternary deposits are locally exposed. Most of the city is underlain by up to 100 m of glacial drift deposited during the Vashon stade of Fraser glaciation, 18-15 cal k.y. B.P. Vashon Drift includes lacustrine clay and silt of the Lawton Clay, lacustrine and fluvial sand of the Esperance Sand, and concrete-like Vashon till. Mappable till is absent over much of the area of the Vashon Drift. Peak local ice thickness was 900 m. Isostatic response to this brief ice loading was significant. Upon deglaciation, global ice-equivalent sea level was about -100 m and local RSL (relative sea level) was 15-20 m, suggesting a total isostatic depression of approximately 115-120 m at Seattle. Subsequent rapid rebound outstripped global sea-level rise to result in a newly recognized marine low-stand shoreline at -50 m. The Seattle fault is a north-verging thrust or reverse fault with approximately 7.5 km of throw. Conglomeratic Miocene strata may record initiation of shortening. Field relations indicate that fault geometry has evolved through three phases. At present, the north-verging master fault is blind, whereas several surface-rupturing faults above the master fault are south verging. The 900-930 A.D. Restoration Point earthquake raised a 5 km X 35 km (or larger) area as much as 7 m. The marine low-stand shoreline is offset by a similar amount, thus there has been only one such earthquake in the last approximately 11 k.y. Geomorphology is largely glacial: an outwash plain decorated with ice-molded flutes and large, anastomosing tunnel valleys carved by water flowing beneath the ice sheet. Euro-Americans initially settled here because of landscape features formed by uplift in the Restoration Point earthquake. But steep slopes and tide flats were not conducive to commerce: starting in the 1890s and ending in the 1920s, extensive regrading removed hills, decreased slopes, and filled low areas. In steep slopes the glacial stratigraphy is prone to landslides when saturated by unusually wet winters. Seismic hazards comprise moderately large (M7) earthquakes in the Benioff zone 50 km and more beneath the city, demi-millennial M 9 events on the subduction zone to the west, and infrequent local crustal earthquakes (M7) that are likely to be devastating because of their proximity. Seismic shaking and consequent liquefaction are of particular concern in Pioneer Square, SoDo, and lower Duwamish neighborhoods, which are largely built on unengineered fill that was placed over estuarine mud. Debris from past Mount Rainier lahars has reached the lower Duwamish valley and a future large lahar could pose a sedimentation hazard.

ISSN: 2333-0937
EISSN: 2333-0945
Serial Title: Field Guide (Geological Society of America)
Serial Volume: 49
Title: Geology of Seattle, a field trip
Title: From the Puget Lowland to east of the Cascade Range; geologic excursions in the Pacific Northwest
Author(s): Haugerud, Ralph A.Troost, Kathy GoetzLaprade, William T.
Author(s): Haugerud, Ralph A.editor
Author(s): Kelsey, Harvey M.editor
Affiliation: U. S. Geological Survey, Seattle, WA, United States
Affiliation: U. S. Geological Survey, Seattle, WA, United States
Pages: 1-24
Published: 201701
Text Language: English
Publisher: Geological Society of America, Boulder, CO, United States
ISBN: 978-0-8137-5649-3
ISBN: 978-0-8137-0049-6
References: 126
Accession Number: 2018-014474
Categories: Areal geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. col., geol. sketch maps
N47°34'60" - N47°34'60", W122°19'60" - W122°19'60"
Secondary Affiliation: University of Washington, USA, United StatesShannon & Wilson, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 201808
Program Name: USGSOPNon-USGS publications with USGS authors
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