Modern Marine Sediments of Bahia Concepcion: Patterns, Processes, and Potential Analogues to Neogene Rift Basin Deposits of Baja California

Neogene strata in Baja California preserve a record of extensional basins produced during early phases of rifting in the Gulf of California. This actualistic study addresses the feasibility of using Holocene sediments of Bahia Concepcion - a rift basin forming a shallow marine bay In southern Baja California - as analogues for the interpretation of Neogene strata in the Pliocene Loreto Basin. Three major lithofacies occur in Bahia Concepcion: Mud sediments dominate most of the central portion of the bay. Moderately sorted sands occur adjacent to shorelines formed by coalesced alluvial fans and a broad alluvial plain, while carbonate sands dominate semi-restricted embayments and rocky shorelines. The distribution of these lithofacies is largely attributable to local geology and changes in base level. Eastern alluvial fans sourced in the foot-wall of a normal fault on the Concepcion Peninsula shed clastic sand far into the bay. In contrast, drainages on the west side of the bay appear to be backfilling in response to Holocene eustatic sea-level rise, resulting in clastic-starved conditions favorable to carbonate production. Comparisons of modern sediments of Bahia Concepcion with those of the Pliocene Loreto Basin suggest that modern lithofacies are most useful in interpreting sediments deposited during transgressive episodes