Neogene rift basin deposits in the Gulf of California region record early phases in the tectonic evolution of the modern gulf. Potential Holocene analogs to these deposits occur today in Bahía Concepción, a rift basin forming a shallow marine bay on the eastern coast of Baja California Sur at ~26.5°North latitude. Bedrock geology and geomorphology suggest Bahía Concepción formed by drowning of an asymmetric graben during the Holocene transgression. A large normal fault zone bounds the bay's east side, associated with a 30-km-long shoreline bajada backed by small, steep drainages. A smaller fault zone probably bounds the bay's west side. The west side has larger drainages with gentler gradients and is characterized by rocky shorelines, pocket bays, mangrove swamps, and few exposed alluvial fans. Holocene sediments of the basin accumulate in alluvial fan, coastal interfan flat, mangrove swamp, fandelta, pocket bay, nearshore shelf, and offshore shelf environments.Three major types of marine sediment dominate the bay. Green clastic mud (with variable mollusc shell content) dominates offshore below 20 m. Volcaniclastic sand (with variable mollusc shell and calcareous algae content) dominates the shallow fandeltas and nearshore shelf on the bay's east and south sides. Carbonate sand (both mollusc and calcalgal origin) dominates in shallow pocket bays and adjacent to rocky shorelines on the bay's west side. Comparison of drainage profiles from the east and west sides of the bay suggests that the restriction of carbonate sediments to the west side is due to trapping of terrigenous sediment in the large western drainages during Holocene sea-level rise.Variation in species composition and taphonomy of molluscan assemblages in the bay is correlated with water depth. Cluster analysis on molluscan shell samples identifies four overlapping, depth-related biofacies: mangrove swamp (intertidal), shallow nearshore (1-to 5-m depth), deeper nearshore (5-to 12-m depth), and offshore (below 20 m). Taphonomic analysis demonstrates depth zonation in both the abundance and condition of mollusc shells. Shell abundance is greatest in intertidal mangrove