The influence of explosive volcanism on fluvial sedimentation; the Deschutes Formation (Neogene) in central Oregon

The upper Miocene to lower Pliocene Deschutes Formation of central Oregon is a sequence of interstratified volcanic and sedimentary rocks, as much as 700 m thick, largely derived from the adjacent Cascade Range. The formation is correlative with the early High Cascade eruptive episode that included voluminous pyroclastic eruptions and culminated in the development of an intra-arc graben in the central Oregon High Cascades. Facies associations and paleocurrent data define three depositional settings. 1) The eastward-sloping arc-adjacent alluvial plain was the depositional site by sheetfloods, debris flows, and hyperconcentrated flood flows. Their deposits from 10- to 70-m-thick sequences separated by erosion surfaces with up to 60 m of relief. Widespread superimposed paleosols overlie these facies and constitute the upper 10 to 50 m of the section. Sedimentation was episodic with pulses of rapid aggradation, corresponding to periods of explosive volcanism and abundant sediment supply, separated by periods of nondeposition when streams incised to reestablish grade as sediment loads diminished. 2) A gravel-bedload river flowed northward through the basin at the foot of the alluvial plain and is represented by channel-fill conglomerates and overbank sandstones and siltstones. 3) A thin sequence derived from older Tertiary highlands was deposited on the east side of the basin and was little affected by Cascade volcanism. Extensive development of thick, superimposed paleosols reflects the inactive nature of the eastern basin margin as a sediment source. Deschutes Formation deposition records the response of a fluvial system to large, volcanism-induced sediment loads. Popular facies models do not adequately allow for the influence of volcanism on fluvial systems because sediment is mobilized episodically and on a scale unparalleled in nonvolcanic settings.