Abstract

Composite reconstruction of isolated sedimentary rock sequences on San Clemente Island indicates that middle Miocene and younger strata overlie the main succession of volcanic rocks. Argillaceous rocks form the bulk of the sedimentary sequence; pebbly sandstone beds, flows, and pyroclastic zones are interbedded and intercalated at a few places. Benthonic foraminifers, together with diatoms and mollusks, suggest that most of these sandstone and shale units were deposited during parts of the Relizian and Luisian Stages of provincial middle Miocene age. Other, younger deposits of the same general rock types contain fossils representative of the Mohnian Stage of provincial late Miocene age. Calcareous sandstone beds in two limited areas of outcrop have yielded mollusks of Pliocene age.

In upward succession, the pectinid species Amussiopecten cf. A. vanvlecki (Arnold); Lyropecten orassicardo (Conrad) in association with Leptopecten discus (Conrad); and Patinopecten healeyi (Arnold) together with Pecten bellus (Conrad) suggest, respectively, provincial middle Miocene, late Miocene, and Pliocene ages. Paleoenvironmental indicators in the megafossil and microfossil assemblages, which have probable depth ranges of mid-bathyal to neritic, imply episodes of local subsidence and uplift of the island platform in middle and late Miocene time. Megainvertebrates from the Pliocene strata suggest sublittoral environments on a high-standing submarine ridge or near an island.