Taphonomy of Holocene and Pliocene Molluscan Assemblages in Bahia Concepcion and the Loreto Basin, Baja California Sur, Mexico

Taphonomy is a useful tool for enhancing paleoenvironmental interpretation. Holocene molluscan shell assemblages of Bahia Concepcion - a transtensional rift basin forming a shallow marine bay in southern Baja California - exhibit distinct environmental variation in taphonomic features. Molluscan assemblages were sampled from mangrove swamp, pocket bay, fan-delta, nearshore shelf and offshore shelf environments. Shell concentration varies from 1-3% of sediment volume in all areas except intertidal mangroves and the shallowest (depth) areas of pocket bays, where shell concentration ranges up to 13% of sediment volume. Shells in deeper areas (\u3e20m depth) exhibit little taphonomic alteration, while shells in shallow areas (depth) are often highly altered. Shells from mangrove swamps and shallow pocket bays (areas of low clastic input) typically show high alteration. It appears that water depth and sedimentation (specifically clastic input) are the primary controls on taphonomic alteration. These data were applied to the interpretation of fossil shell concentrations in marine outcrops of the Loreto Basin, a Pliocene rift basin analogous to Bahia Concepcion. Taphonomic data in conjunction with sedimentologic, stratigraphic and faunal data, demonstrate that the Holocene of Bahia Concepcion provides a useful working model for paleoenvironmental interpretations in Neogene rift basin strata of Baja California