thumbnail

The geologic history of the Chehalis Forearc Basin, Washington State, USA

Tectonics
By: , , , and 
https://doi.org/10.1029/2025TC009259

Links

Abstract

The Chehalis basin is located between the Cascade arc and the Coast Range in southwest Washington State. It consists of sedimentary and volcanic rocks deposited throughout the Cenozoic and is underlain by the Siletzia terrane, a thick, large igneous province accreted in the Eocene. Here, we constrain evolution of the Chehalis basin depocenter using isochore maps derived from isostatic gravity anomalies, borehole data, and stratigraphy for several time periods: the Eocene (47.6–36.8 Ma), the Oligocene (36.8–20 Ma), and the Neogene (20–0 Ma). Our results suggest that local subsidence is driven by deformation on faults that bound and intersect the basin. We see northward depocenter migration, interpreted as a shift in deformation from the northwest striking Cedar Creek fault to the west striking Doty fault. We interpret these data in terms of the long-term north-south shortening and clockwise rotation of the Cascadia forearc. During the Eocene, the Cedar Creek fault was preferentially aligned with north-south shortening, but became less active when clockwise rotation brought the Doty fault into east-west alignment with the stress field. Sediment accumulation rates decreased from 196 m/Myr in the Paleogene (∼40–20 Ma) to 27 m/Myr in the Neogene (∼20–0 Ma) as Miocene uplift of the Coast Range limited accommodation space and shifted the depositional environment from marine to fluvial. Our results are consistent with the geologic evolution of the Portland and Tualatin basins to the south and reveal a depositional and structural history uniquely shaped by clockwise rotation of the Cascadia forearc.

Suggested Citation

Moe, R.L., Bershaw, J., Staisch, L.M., and Streig, A.R., 2026, The geologic history of the Chehalis Forearc Basin, Washington State, USA: Tectonics, v. 45, no. 7, e2025TC009259, 20 p., https://doi.org/10.1029/2025TC009259.

Study Area

Publication type Article
Publication Subtype Journal Article
Title The geologic history of the Chehalis Forearc Basin, Washington State, USA
Series title Tectonics
DOI 10.1029/2025TC009259
Volume 45
Issue 7
Publication Date July 04, 2026
Year Published 2026
Language English
Publisher American Geophysical Union
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description e2025TC009259, 20 p.
Country United States
State Washington
Other Geospatial Chehalis basin study area
Additional publication details