Neogene Rift Basins of the Gulf of California: Insights from Holocene Sediments and Molluscan Faunas of Bahia Concepcion, Baja California Sur
K. H. Meldahl, O. Gonzalez-Yajimovich., G. Avila-Serrano.,
C. Empedocles, C. Gustafson, M. Motolinea-Hidalgo., and T. Reardon
Neogene rift basin deposits in the Gulf of California region record early phases of Gulf evolution. Useful modern analogs to these deposits occur today in Bahia Concepcion, a shallow marine bay in southern Baja California. The bay occupies an asymmetric graben that was drowned during Holocene transgression. A major normal fault bounds the basin's east side, associated with a 30 kilometer long unincised shoreline bajada backed by small, high-gradient catchments. The basin's west side has large, low-gradient catchments, and is characterized by rocky shorelines, pocket bays, mangrove swamps, and few exposed alluvial fans.
Variation in species composition and taphonomy of molluscan assemblages in the bay is correlated with water depth. We recognize four overlapping, depth-related biofacies, and depth-related variation in both the concentration and condition of mollusc shells. Molluscs should thus provide useful bathymetric data for Neogene rift basin strata. Three major marine lithofacies dominate the bay. Clastic mud occurs offshore below 20 meters. Clastic sand dominates the fan-deltas and nearshore shelf on the bay's eastern and southern margins. Carbonate sand dominates the pocket bays and rocky shorelines on the western margin. The occurrence of carbonates is attributed to clastic starvation caused by back-filling of the large western drainages during Holocene base level rise. Modern lithofacies ma thus be most useful for interpreting transgressive episodes in Neogene rift basin evolution.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California