Chronostratigraphic and geographic studies of Eocene arkosic sandstones suggest deposition during a "volcanically quiet" interval resulting from the westward jump of the Farrallon-Kula plate subduction zone in Oregon and Washington.

The Eocene arkosic sandstones were deposited as part of a broad fluvial plain-coastal plain-shelf margin basin complex extending throughout Oregon and Washington between uplands

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of Mesozoic rocks. Feldspathic-quartzose sediments were transported from the east by river systems draining granitic terrains perhaps as far away as the Idaho Batholith. Local volcanism within the drainage system, along the coastal plain, and on volcanic islands to the west, added variable amounts of volcanic rock fragments to the feldspathic-quartzose sediments.

Chronostratigraphic correlations suggest that the arkosic sandstones were deposited along the margins of the depositional system during the early Eocene, prograded westward during the middle Eocene, and then regressed during the latest Eocene and Oligocene simultaneously with the influx of abundant pyroclastic debris. The pyroclastic material was derived from ancestral Cascade Mountain Range volcanism. Onset of Cascade volcanism, and volcanic-lithic grain dilution of the feldspathic-quartzose sandstones, began in the south during the early middle Eocene and extended northward reaching Washington in the early late Eocene.

The Eocene tectonic history of western Oregon and Washington provides a framework for understanding the occurrence of the feldspathic-quartzose rocks in a dominantly "graywacke" fore-arc provinence.

During the early Eocene, a northwest-southeast seamount chain was extruded on the Farallon and Kula plates west of an eastward-dipping subduction zone. Subduction of the oceanic plates moved the seamount chain obliquely toward the subduction zone.

In middle Eocene time--49 to 40 m.y.b.p.--the seamount chain reached the subduction zone creating instability in the subduction system and resulting in the westward jump of the underthrust boundary between the Farallon-Kula and North American plates. The westward jump of the underthrust boundary resulted in both the accretion of the seamount chain as part of a newly formed fore-arc accretionary prism, and a decrease in magnetic arc volcanism. The relative decrease in arc volcanism occurred during the interval after the consumption of the detached eastern subduction plate and the onset of magma generation from the newly formed western subduction zone.

Coincident with and continuing after the subduction zone jump and seamount accretion, eastwardly derived arkosic sediments prograded across Oregon and Washington spilling into the new fore-arc basin and enveloping the seamounts. Basaltic intrusion within the fore-arc basin occurred along tensional fault systems within the accretionary prism.

As the western subduction zone developed, a new, more western magmatic arc formed along the axis of the modern Cascade Mountains. Beginning in the late Eocene, the fore-arc basin subsided and the sea transgressed eastward depositing fine-grained arc-derived tuffaceous sediment over the entire basin.

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