MARINE INVERTEBRATES PALEONTOLOGY OF THE SAN JUAN MEMBER (EL CIEN FORMATION), BAJA CALIFORNIA SUR, MEXICO

During late Oligocene, the Baja California Peninsula was part of a continental shelf, attached to the Mexico mainland, with a shoreline near the present Gulf of California coast. The sedimentary sequence started with a transgression over the shelf about 30 million years (Ma) ago. Upon this platform the phosphorite-bearing, clastic sequence of the San Juan Member of the El Cien Formation was deposited. The San Juan Member crops out in the San Juan de la Costa area, at La Paz Bay, Gulf of California, and in El Cien and El Aguajito areas, in southern Baja California Sur, Mexico. It´s mainly composed of laminated, partly tuffaceous, siliceous, diatomaceous of phosphatic mudstones, partly phosphatic silt and sandstone, granular phosphorite, tuff and conglomerates. Paleontologic, biostratigraphic and radiometric data provide a range of deposition from 30 to 23 Ma (Early-late Oligocene to Late Oligocene). Depositional environments shallow through the sequence, from mid-shelf to shoreface depths. Sedimentation occurred in a multiple transgressive and regressive sedimentary cycles, including seep and whale-fall deposits containing chemosymbiotic faunas. Preliminary work indicates 31 invertebrates species, representing 21 families, 8 classes and 5 phyla. Bivalvia are the most diverse, followed by Gastropoda. We report 20 new records of invertebrates species from the San Juan Member, and in doing so, extends the geographic and/or geologic range of some species. The paleobiogeography of decapods from North America west coast during the Oligocene presents a slight influence on their taxa from the Tethys, Southern Hemisphere, as well as the Pacific Northwest. Meanwhile, the recorded molluscan fauna, presents similarity in some of its taxa with fossil faunas known from the North American west coast (California, Oregon and Washington), and even those of Japan, reaffirming the Pacific Northwest influence on the fauna of this area during the Oligocene.