WESTERN AND HIGH CASCADE SERIES VOLCANISM IN THE CASCADE-SISKIYOU NATIONAL MONUMENT, SOUTHWEST OREGON AND NORTHWEST CALIFORNIA

The Cascade-Siskiyou National Monument is dominated by volcanic rocks. These rocks overlie a sequence of Jurassic crystalline basement rocks, Upper Cretaceous marine rocks, and Upper Eocene continental river deposits. The oldest, and slightly altered, volcanic rocks are part of the Western Cascade Volcanic Series (WCVS; 35.9-19.5 Ma). From SW to NE, and older to younger, the WCVS is comprised of: distal to medial volcaniclastics with minor lava; medial to proximal facies shield and composite volcanic lavas and volcaniclastics; an interval of silicic ash falls, ash flows, and andesite to basalt lava flows; and younger basaltic andesite lava. All are tilted to the northeast with the tilt decreasing in progressively younger units. In the eastern portion of the Monument, a succession of High Cascade (5.77-0.33 Ma) shield volcanoes of basalt and basaltic andesite lava flows and vents unconformably overlie WCVS rocks. Steeply-dipping NE-trending faults affect rock units older than a 26.5 Ma tuff. One of the major faults, the Siskiyou Summit Fault, shows oblique movement including right lateral motion. Steep NW-trending normal faults affect all younger rocks, a few of which show right-lateral motion.