UPPER OLIGOCENE VOLCANIC STRATIGRAPHY IN THE CASCADE-SISKIYOU NATIONAL MONUMENT IN NORTHERN CALIFORNIA

Oligocene volcanic stratigraphy in this section of the monument is comprised of two formations: the Colestin Formation including interbedded rhyolitic tuff and basaltic andesite lava and breccia, overlayed by extensive basaltic andesite lava flows and breccia of the Roxy Formation. Dikes and intrusions are found in both formations. The 28 to 25.5 Ma Colestin Formation’s vitric, crystal, and lithic-vitric airfall and welded tuffs contain common quartz crystals and broken plagioclase, lesser biotite and hornblende, and rare pyroxene. Volcanic sandstones and breccias are poorly sorted with angular fragments. Lava flows contain phenocrysts of augite, plagioclase and magnetite, as well as lesser olivine and rare hypersthene. In contrast, the Roxy Formation’s Aa and brecciated basaltic andesite lava flows contain more abundant olivine and hypersthene as well as plagioclase. Local unconformities within and between both formations suggest extensive periods of erosion. The dominance of lava flows in the younger Roxy Formation indicates a westward movement of volcanic centers through time. Steeply dipping, northwest-trending faults with normal and minor right lateral offsets affect both formations.