Two communities of shelled benthic macrofauna are recognized south of the Mississippi delta by means of cluster analysis. The faunal pattern is correlated closely with water depth, pelecypod feeding type, and substrate texture. Correlation of faunal, lithologic, and environmental characteristics reflects joint sedimentation and biotic production in response to the present hydrologic regime.

East of the Mississippi delta, 8 communities of benthic shelled macrofauna are recognized by means of cluster analysis. Distribution patterns of these communities (biofacies) appear to reflect the primary environmental factors controlling the nature of the water mass: distance from the delta front; water depth on the shelf away from the influence of the delta; and subdivision of the shelf by the Chandeleur-Breton islands. Faunal and substrate patterns are poorly correlated; histograms of sediment texture for each of the biofacies are not significantly different from the histogram of sediment texture for the whole area. The poor correlation of fauna with substrate texture is the result of the formation of the substrate distribution pattern during deposition of the St. Bernard delta. Fa nal distribution patterns are determined primarily by the environmental factors controlling present water-mass characteristics and only secondarily by the relict substrate texture pattern. If preserved in the geologic record, the co-occurring fauna and sediments would represent 2 different periods of deposition.

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