Field survey of Isla Cerralvo in Baja California Sur reveals the presence of invertebrate fossils weathering from an unnamed marine sedimentary formation presumed. The fossils occur in Pliocene (?) conglomerate, sandstone and fossiliferous calcarenite sedimentary rocks exposed in the hanging wall of the La Gringa normal fault along the west-central coast of the island (Montrella et al., 2003). Flat-lying Pliocene(?) fossiliferous limestone/sandstone terrace deposits are also located at the south end of the island at Punta Montana and Punta Sur. A preliminary paleontological survey of the fossils from the La Gringa fault locality reveals the presence of at least two different bivalves. These were identified by Scott Rugh from the San Diego Natural History Museum as Ostrea fisheri and Argopecten sp. The first fossil, Ostrea fisheri, has been described from Upper Pleistocene rocky shoreline fossil assemblages at Bahia Santa Ines farther north in the Gulf of California where they are overlain by deposits correlated to oxygen isotope substage 5e which is about 125,000 years ago (Libby and Johnson, 1997). The Argopecten sp. resemble abeietis abbotti which occurs in the Broadway faunal horizon in San Diego area where it is dated at around 500,000 years old (Rugh, personnel communication, 2005). Good preservation of the bivalves is also consistent with a Pleistocene age. These tentatively identified fossils indicate that the La Gringa fault on Isla Cerralvo may be an active fault. More work is necessary to check this possibility.
