DEPOSITIONAL SETTING, PROVENANCE, AND TECTONIC IMPLICATIONS OF THE CARMEN FORMATION ON SAN MARCOS ISLAND, BAJA CALIFORNIA SUR, MEXICO

Exposures of the Carmen Fm were studied on San Marcos Island (SMI) offshore of NE Baja California Sur to determine their environments of deposition and provenance and to gain insight into the tectonic history of the Gulf of California oblique rift. The Carmen Fm on SMI is early to middle Pliocene in age based on a new Ar/Ar date from an interbedded lava (Sommer et al., this meeting), and correlation to the Tirabuzon Fm (Carreno, 1981) and Infierno Fm (Wilson and Rocha, 1955) that overlie the Boleo Fm in the Santa Rosalia Basin (SRB). We measured and described stratigraphic sections at three outcrops on southern SMI. Two sections lie above an angular unconformity with underlying San Marcos Fm, which is likely correlative with the upper Miocene Boleo Fm in the SRB based on interbedded gypsum deposits (Anderson, 1950). Gilbert-style low-angle foreset beds with rounded and imbricated pebbles and cobbles are interbedded with locally pebbly and fossiliferous marine sandstones. Petrographic, macrofossil, and microfossil analyses reveal a variety of marine organisms including gastropods and barnacles that suggest a rocky intertidal environment; bivalves found in sandier facies imply a more open shelf environment. Lithofacies analysis of the sections reveals 4–5 cycles of shallow marine sandstone and coarse marine fan-delta conglomerate facies. These cycles are interpreted to reflect the progradation of a fan-delta into accommodation generated by basin subsidence, followed by retrogradation of the delta as the basin aggraded and relative sea level rose. Petrographic analysis reveals an abundance of angular to subrounded lithic clasts supported by a fine-grained carbonate matrix. This textural immaturity suggests derivation from proximal sediment sources composed of volcanic and granitic basement rocks. An upsection increase in the abundance of granitic clasts also suggests that SMI was located near a granitic source during Pliocene time; the nearest such exposure is located ~40 km to the NW on the Baja peninsula near La Reforma Caldera. Growth structures and granitic clasts observed in the Pliocene Carmen Fm support syntectonic deposition within a transtensional pull-apart basin and up to ~40 km of post-Miocene dextral slip between SMI and Baja California after the onset of seafloor spreading in the Gulf of California rift.