Molluscan Biofacies, Their Distributions and Current Erosion on the Nile Delta Shelf

Abstract

Mollusc species (141) have been identified off Egypt, on the Nile delta shelf between the coast and uppermost slope. To focus on recently living autochthonous faunas, poorly preserved specimens of older age and infralittoral species recovered at depths >25 m were removed from consideration. Multivariate analyses were performed using a data matrix consisting of 38 samples and 23 species, resulting in four clusters and three principal components (bottom instability, vegetal cover and water depth). Five molluscan biofacies on the mid- to outer shelf were identified from biocoenotic interpretation of the four clusters: Bathyarca pectunculoides biofacies, Goodallia triangularis biofacies, Bittium reticulatum biofacies, Corbula gibba biofacies and Bittium exiguum biofacies. Also distinguished were a more proximal inner shelf to coastal (Donax and Tellina biofacies) and a lagoonal biofacies.

The six molluscan biofacies on the shelf are distributed in distinct contour-parallel belts, and these record diverse effects of bottom currents that, to the present, have eroded extensive sectors of the seafloor. The coastal to inner shelf sector, characterized by the Donax and Tellina biofacies, records highest energy levels due to wave-driven scour and winter storm activity. The mid- to outer shelf sector, characterized by the Corbula gibba biofacies, comprises zones of bottom instability. Predominantly east-directed currents have winnowed fine-grained sediment now deposited on the NE delta shelf and on the lower energy outer shelf and upper slope. We predict that, as a consequence of sediment cut-off by the High Aswan Dam and structures in the delta below Cairo, the effects of scouring as recorded by molluscs will become more apparent on the Nile shelf with time.