Two hundred ten species of mollusks from 84 core and grab samples are grouped into five biofacies, each of which characterizes a particular environmental facies within and adjacent to the Gulf of Batabano, Cuba. The biofacies within the Gulf are the Anomalocardia brasiliana-Corbula swiftiana Subassemblage, the Tegula fasciata-Arcopsis adamsi Subassemblage, the Tellina candeana-Ervilia nitens Subassemblage, and the Cerithium litteratum Subassemblage. A fifth biofacies of pelagic mollusks, mainly pteropods, occurs in deep water south of the Gulf platform margin.

The distribution patterns of the biofacies are related to those of calcareous sediments, salinity, median grain size, sorting, and certain other ecological factors. Salinity, amount of carbonate mud, and water turbulence primarily control the distribution of the mollusks. The biofacies tend to occur in concentric patterns and in particular sequences relative to the land margins of the Gulf and the reef tract. The A. brasiliana-C. swiftiana biofacies characterizes areas of variable salinity, quiet water, and predominantly muddy bottom that usually lie near the Cuban mainland. Areas in which the T. fasciata-A. adamsi biofacies dominates are centrally located within the Gulf and are similar to those for the foregoing biofacies except that salinity remains practically constant at normal m rine values. The T. candeana-E. nitens biofacies favors areas of carbonate sand sediment and relatively high water turbulence located in the southeastern part of the Gulf and along the oceanic margin. The C. litteratum biofacies also exists behind the Gulf reef tracts in areas of carbonate sand sediment and high turbulence.

The results are used to illustrate some useful techniques of paleoecological analysis that may help geologists to interpret basin configuration and the depositional environments for certain types of limestones, and to assess proximity to reefs or reef-prone areas.