The Geologic History of the Chehalis Forearc Basin, Washington State, USA

The Chehalis Basin, in southwest Washington, is one of a series of forearc basins between the Cascade Arc and the Coast Range in the Pacific Northwest. While the Seattle basin to the north, and the Portland-Tualatin basins to the south are both relatively well studied, the geologic evolution of basins in between is poorly constrained. The Chehalis basin is underlain by accreted Eocene oceanic basalt of the Crescent Formation. The basin fill consists of sedimentary rocks and volcanic rocks deposited throughout the Cenozoic. It has undergone N-S shortening driven by transpressional stress from oblique subduction of the Juan de Fuca plate under North America and clockwise rotation of the Oregon forearc block. Local subsidence is driven by deformation on W and NW striking faults that bound and intersect the basin. Quaternary activity on these faults is possible but unconfirmed. To better constrain the development of the Chehalis Basin, I model three key subsurface horizons: the top of the Eocene Basement, the top of the Oligocene Skookumchuck Formation, and the top of the Neogene Lincoln Creek Formation. Isochore (vertical thickness) maps constrain the Chehalis Basin depocenter (the location of maximum sediment accumulation) during the Eocene (47.6 - 36.8 Ma), the Oligocene (36.8 - 20 Ma), and the Neogene (20 - 0 Ma). Results show a northward migration of the depocenter, which I interpret as a shift in basin deformation from the NW striking Cedar Creek Fault to the E-W striking Doty Fault. Clockwise rotation data in the area indicate the Cedar Creek Fault was preferentially aligned with N-S shortening, through the Eocene, but was deprioritized as the Doty Fault rotated to an E-W orientation by the Neogene. Sediment accumulation rates in the basin decreased drastically over time as late Eocene uplift of the Cascade Arc and Miocene uplift of the Coast Range limited accommodation space and shifted the depositional environment from deep marine to fluvial. Paleogene (~40 - 20 Ma) sediment accumulation rates averaged 196 m/Myr, comparable to the Seattle Basin to the north and Portland-Tualatin Basin to the south. Neogene (~20 - 0 Ma) rates averaged 27 m/Myr, similar to the Portland-Tualatin Basin, but an order of magnitude slower than the Seattle Basin. Due to a relatively steep subduction angle under Oregon and southern Washington, the Coast Range emerged (post ~20 Ma) in proximity to the Cascade magmatic arc, leaving minimal space for basin formation in the forearc. Comparatively, shallow subduction to the north resulted in more separation between the Olympic Mountains and Cascade magmatic arc leading to relatively high sediment accumulation rates in the Seattle Basin during Neogene time.