PRELIMINARY GEOLOGIC MAPPING, STRATIGRAPHIC FRAMEWORK, AND AR/AR GEOCHRONOLOGY OF SAN MARCOS ISLAND, BAJA CALIFORNIA SUR, MÉXICO

The Gulf of California is a world-class example of an active oblique rift margin. The Baja Basins IRES Program is a U.S.-México collaborative research project funded by NSF that aims to provide field research experiences to undergraduate geology students through investigations into the geologic history of the Santa Rosalia Basin (SRB) in Baja California Sur, where a rare and accessible stratigraphic record sheds light on the tectonic and magmatic evolution of the rift margin. San Marcos Island (SMI) hosts an active gypsum mining operation and shares a similar stratigraphy to the SRB, yet the island has never been mapped geologically, and its potential correlation with the SRB is uncertain. The high-purity economic gypsum deposits on SMI may correlate with similar deposits within the SRB, but would require significant displacement on undocumented strike-slip faults offshore.

We conducted detailed geologic mapping at 1:10,000 scale and collected samples for petrographic analysis, geochemistry, and ⁴⁰Ar/³⁹Ar geochronology to establish a stratigraphic framework for SMI. The base of the stratigraphic sequence on SMI consists of sub-alkaline to alkaline lavas, tuffs, breccias, and interbedded volcaniclastic rocks. New ⁴⁰Ar/³⁹Ar ages from the base and top of this sequence indicate that the entire volcanic sequence was deposited between 8.25 ± 1.08 and 7.40 ± 0.70 Ma. This basal sequence is unconformably overlain by the San Marcos Fm, which consists of conglomerate, sandstone, and a ~60–80m-thick gypsum sequence that pinches out toward the north. The San Marcos Fm is in turn unconformably overlain by marine sandstone, conglomerate, and limestone of the Carmen Fm. We report a ⁴⁰Ar/³⁹Ar age of 3.90 ± 0.70 Ma from a distinct high-Mg basaltic trachyandesite lava within the Carmen Fm.

The age span and compositions of the basal volcanic sequence on SMI are very similar to those of the Rift Transition Volcanic sequence in the SRB (Busby et al., 2020), and the gypsum-dominated facies of the San Marcos Fm on SMI likely correlates with Boleo Fm, in particular the thick gypsum deposits in the northern SRB. If so, this correlation would imply ~20–40 km of undocumented dextral slip on offshore faults between the SRB and SMI and provide an important new constraint on the tectonic evolution of the Gulf of California.