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Cenozoic Stratigraphic Architecture of Oregon-Washington Cascades: A Sedimentologist's Perspective

Gary A. Smith

Continental margin volcano-tectonic processes have produced a diverse sedimentary record within and adjacent to the Cascade Range. Eocene to early Miocene lava flows, ignimbrites, and intercalated sediments accumulated to 3-6 km thick in a longitudinal basin coincident with the arc. Volcaniclastics include fine-grained fluvial or lacustrine facies, local accumulations of coarse-grained sediment adjacent to volcanic centers, and thick ignimbrite sequences. Large volumes of ash drifted eastward to produce thick tuffs intercalated with locally erupted rhyolites of the ignimbrite flare-up. Increasing constructional and tectonic relief caused loci of sedimentation to migrate away from the Cascade axis during the Neogene. Sedimentation in east flank basins was influenced by (1) style of volcanism in the adjacent segment of the Cascades; (2) drainage disruption by Columbia Plateau flood basalts; and (3) tectonism, which produced a transverse foldbelt on the north and graben on the south.

Since the late Miocene, the Washington Cascades have been influenced by compressive tectonics and the Oregon Cascades by extension. Hence, the Quaternary volcanics of Washington consist of widely spaced andesitic stratovolcanoes lying on uplifted Tertiary volcanics, plutons, and pre-Tertiary rocks, whereas those in Oregon form a broad platform of mafic lavas, locally confined to intra-arc grabens. Most sedimentation related to Quaternary volcanism occurs as terraces in glaciated valleys near the volcanoes. Forearc marine rocks record the onset of Cascade volcanism during the Eocene. However, shallow marine facies are primarily arkosic sands supplied by rivers flowing through the arc from the Rockies; pyroclastic sediment was selectively removed by nearshore processes leaving potential reservoir rocks.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.