UPPER MIOCENE TO PLEISTOCENE HIGH CASCADE VOLCANISM OF THE NORTHEAST SECTION OF THE CASCADE-SISKIYOU NATIONAL MONUMENT, SOUTHWEST OREGON

Overlapping High Cascade shield volcanoes with extensive lava flows and minor volcanic breccia that are affected by northwest-trending normal faults comprises the geology of the northeast portion of the Cascade-Siskiyou National Monument. This study builds on the geologic work of Mertzman and others who differentiated High Cascade rock units based on commonly similar lithology, topographic differences, geochemistry, and radiometric ages. The current work refined geologic contacts, delineated vent and dike complexes, identified major faults and fault zones, and determined different geologic relations based on detailed mapping. New features include collapse structures, spatter, dikes and lava erupted along faults, and multiple vents. Compositions of the lavas that range from porphyritic andesite to basalt lava flows with variable phenocrysts percentages of plagioclase, olivine, and pyroxenes. Dominant Northwest-trending normal faults affected all units through the Late Miocene to Pleistocene, localizing dikes and cinder cones, especially along the major Basin and Range Surveyor Mountain fault zone.