Systematic changes in compositions of Eocene sandstones in the southern part of the Oregon Coast Range constrain proposed models of tectonic accretion and rotation of this crustal fragment. Three basic petrofacies of lithic, arkosic, and volcanolithic composition can be defined sequentially from the base to the top of the section. The lithic petrofacies was deposited prior to rotation and was derived from the nearby Klamath Mountains during collision of the subjacent seamount terrane. The transition to arkosic composition coincided with the end of collision and the initiation of forearc basin subsidence. The arkosic sands contain muscovite and potassium feldspar derived from a continental-plutonic source area; their presence in the forearc sequence suggests that the Coast Range block lay farther to the east and closer to interior parts of the Mesozoic batholith belt prior to tectonic rotation. Volcanolithic sandstones were deposited after rotation of the Coast Range had begun, and they mark the beginning of volcanic activity in the adjacent Cascade arc.

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