ANALYSIS OF A FOSSILIFEROUS SANDSTONE SEQUENCE OF THE PLIOCENE TIRABUZON FORMATION AT SANTA AGUEDA, SANTA ROSALIA BASIN, BAJA CALIFORNIA SUR, MEXICO

The Santa Rosalia Basin located along the western shore of the Gulf of California in central Baja California Sur, México is filled by a sequence of Miocene to Quaternary sediment deposited during the initial rifting of the gulf and subsequent incursion of marine waters. Wilson (1955) divided the basin sequence into the Boleo, Gloria, Infierno, and Santa Rosalia formations, each separated by a local angular unconformity, and capped by marine and fluvial terraces. We measured a 34-m-thick section of the Tirabuzon Formation (formerly known as Gloria Fm), along Estero Santa Agueda where the formation dips gently eastward and has an erosional contact with volcanic and lower Boleo Fm rocks that dip 55° east. We divided the section into 16 beds and collected samples for petrographic, mineralogical, and fossil analyses. The basal 0.3-m-thick sandy conglomerate with sub-rounded volcanic and Boleo Fm cobbles, a localized fossil organic debris, and shell fragments represents a transgressive lag. The lower five beds are mostly bioturbated, fine-grained sandstone with abundant disarticulated fossil pectins, oysters, forams and ostracods indicating marine subtidal conditions. A cross-bedded, bioturbated sand (Bed 6) with possible root casts suggest a backshore environment. Beds 7- 13 are an interbedded sequence of grey to brown, fine sandy silt with few fossils. A green, glauconite-rich sandstone (Bed 14) with abundant shells and whole sand dollars, a tan sandstone (Bed 15) with few shells, and uppermost 3-m-thick conglomerate (Bed 16) with abundant shells (coquina) are interpreted as the transition from the marine to beach facies. Petrographic analyses shows that the sandstones are predominantly calcite-cemented, felspathic litharenites or lithic feldspathic arenite, with angular grains and accessory minerals of isolated crystals of amphibole, biotite, plagioclase, and pyroxene. A pulse of tectonic tilting and erosion of most of the Boleo formation occurred prior to deposition of this section of the Tirabuzon Fm. The sea transgressed and rapidly brought marine shelf-depth waters into the basin and then regressed twice in the section. The section is capped by Quaternary marine terrace indicating the basin continued to subside and was uplifted during a second phase of tectonics. This research is part of the Baja Basins REU.