The concept of defining sedimentary environments in terms of energy levels is applied to interpreting ostracod distribution patterns in recent sediments from the E. Mississippi delta area. "Environmental energy level" represents the amount of energy (mechanical, chemical, thermal, etc.) used in varying the physical and chemical characteristics of the environment. High-energy environments are unstable, with variable physical-chemical conditions; low energy environments are relatively more stable, with less variable physical-chemical character. Analysis of ostracod distribution and abundance data shows distinct offshore and inshore biofacies, each recognized by a characteristic ostracod assemblage and different gross faunal characteristics. Offshore biofacies have greater numbers both of species and individuals and a higher number of mature carapaces; many species are restricted to offshore assemblages. Inshore biofacies have fewer species, generally fewer individuals, and mostly immature carapaces; some species are restricted to, and several are most abundant in, inshore assemblages. Offshore energy levels are generally low, inshore, generally high. The conclusions may be applied in stratigraphic work to interpretation of Tertiary (and possibly Cretaceous) environments and paleogeography.

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