USING A STRATIGRAPHICALLY CONSTRAINED BASIN MODEL TO UNDERSTAND THE GEOLOGIC HISTORY OF THE CHEHALIS FOREARC BASIN, WASHINGTON STATE, USA

The Chehalis basin is one of a series forearc basins located along the Puget Lowland, a regional depression between the Cascade volcanic arc and the Coast Range in the Pacific Northwest, USA. While the Seattle basin to the north, and the Portland-Tualatin basin to the south are both relatively well studied, the geologic evolution of basins in between is poorly constrained. The Chehalis basin is thought to be underlain, at least partially, by Eocene oceanic crust. The basin itself consists of a mix of sedimentary rocks and volcanic rocks deposited throughout the Cenozoic. It has been deformed by transpressional stress from oblique subduction of the Juan de Fuca plate under North America and clockwise rotation of the Oregon forearc block. In this study, we map the depocenter (the location of maximum sediment accumulation) of the Chehalis basin through time. Well logs, gravity, seismic, magnetic, and outcrop data are combined to model key surfaces and create isochore (thickness) maps of three stratigraphic units. Mapped surfaces include the Astoria and Lincoln Creek / Toutle Formations, Skookumchuck / Cowlitz and McIntosh Formations, and Eocene basement. Our basin model provides new constraints on the Cenozoic evolution of the Cascadia forearc, including the timing of deformation on basin-bounding structures like the Doty fault zone.