The region of western Oregon and Washington at the beginning of the Tertiary was the site of a north-trending eugeosyncline that is inferred to have occupied the site of the present Coast Range-Olympic Mountains uplift and the Puget-Willamette trough. The distribution of marine and continental facies suggests that the eastern margin of the eugeosyncline extended under the Tertiary volcanic rocks of the Cascade Range. Analysis of the distribution, thickness, facies changes, and sedimentary structures of a thick Eocene turbidite sequence indicates that the western margin of the eugeosyncline lay west of the present coast line.

Tholeiitic pillow lavas and breccias, as much as 20,000 feet thick, were erupted in places into the subsiding geosyncline during early to middle Eocene time and interfingered complexly with marine tuffaceous siltstone and sandstone. Uplift south of the geosyncline during middle Eocene time resulted in an influx of great quantities of arkosic sands which were swept generally northward along the axial part of the trough by turbidity currents. Concurrently, northeast of the geosyncline, a plutonic and metamorphic terrane supplied large quantities of arkosic detritus that accumulated on a broad coastal plain and intertongued westward with marine beds.

In post-middle Eocene time broad uplifts and thick volcanic accumulations divided the geosyncline into several separate basins. These basins were the sites of deposition of as much as 15,000 feet of upper Eocene to Pliocene marine sandstone and siltstone and associated pyroclastic and epiclastic volcanic debris. Upper Eocene and middle Miocene basalt flows from local centers interfinger in places with this sequence.

In western Oregon these Tertiary strata have been folded and faulted into structures that trend predominantly northeastward, parallel with the structure of the pre-Tertiary rocks of the Klamath Mountains. North of the Columbia River, the principal structures trend northwestward, approximately parallel with the structural grain of the pre-Tertiary rocks in northern Washington. This trend is interrupted by the more complex Olympic Mountains uplift.

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