Abstract

Santa Catalina, San Clemente, and Santa Barbara Islands each contain limited outcrops of Miocene sedimentary rocks, chiefly shale, that are interbedded with or overlie volcanic rocks. On Santa Catalina, a schist and quartz diorite breccia unit of probable middle Miocene age, an upper bathyal shale unit of middle and late Miocene age (Luisian and lower Mohnian Stages), and a shallow-water calcarenite unit of probable late Miocene age occur as noncontiguous stratigraphic sections within a sequence of andesite and dacite flows. At San Clemente Island, volcaniclastic sandstone beds containing sublittoral mollusks and tuffaceous shale beds containing bathyal foraminifers accumulated independently in pockets in andesite and dacite flows and at places are intercalated with flows and lapilli tuff beds. These sedimentary rocks include both middle and late Miocene fossils (Relizian, Luisian, and Mohnian Stages) and locally are overlain by fossiliferous shallow-water calcarenite beds of Pliocene age. The sequence of volcanic flows rocks on Santa Barbard Island is broken by one, or possibly two thin zones of tuffaceous siltstone and shale beds that contain upper bathyal foraminifers of middle Miocene age (Luisian Stage). Although Neogene strata do not occur on San Nicolas Island and Begg Rock, the wave-cut platform on which they are situated is flanked by middle and late Miocene claystone and shale beds.