Paper No. 33-8
Presentation Time: 11:00 AM
LOWER MIOCENE WESTERN CASCADE AND UPPER MIOCENE TO QUATERNARY HIGH CASCADE VOLCANISM IN SOUTHWESTERN OREGON
Diagenetically-altered Western Cascade (WC) volcanic rocks, tilted 10-15° to the NE, are unconformably overlain by High Cascade (HC) mafic lava flows. The earliest unit of the WC Heppsie Formation consists of basalt through andesite lava and breccia, most of which contain phenocrysts of abundant plagioclase, hypersthene and augite pyroxenes, and altered olivine. A distinct red paleosol separates them from overlying olivine (+/- augite) basaltic andesite shield volcano flows and lesser breccia. Transition to a younger but dissimilar unit of platy trachyandesite lava, andesite to rhyolite welded and air-fall tuff, and thick matrix-supported silicic breccia crops out prior to being overlain by HC lava flows. There is an abrupt change from WC composite and shield eruptive products to HC shield and fissure volcanism. As compared to preceding WC volcanism, HC compositions are dominated by tightly grouped olivine basalt to basaltic andesite lava flows and minor volcaniclastic deposits. Six separate HC units were identified, three of which erupted from fissures and three from vents with radiating dikes. WC rocks are transected by a NW-trending fracture and fault set that was initiated before or during a ~ 21-22 Ma dike intrusion event. Similarly-oriented faulting continued through the Early Miocene. All rock units and NW-trending fractures were subjected to clockwise rotation. Reactivation of some NW-trending faults affected overlying HC rocks. HC lavas are cut by a subsequent NNW-trending fault set, some of which were conduits for eruption. Both fault sets suggest normal-oblique or right-lateral movement.