MOLLUSCAN FAUNAS AND PALEOBATHYMETRY OF HOLOCENE SEQUENCES IN THE NORTHEASTERN NILE DELTA, EGYPT

The fossil molluscan component serves as a valuable indicator of the paleobathymetric variations which occurred in the Nile delta, Egypt, during the past 7500 years. Paleobiocoenoses have been identified, on the basis of molluscan faunas, in three radiocarbon-dated cores as part of a long-term study to help interpret the Holocene evolution of the northeastern delta. One of these, Smithsonian boring S-21 near Port Said, has been selected as a type-core because it includes a near-complete Holocene coarsening-upward sequence of distinct open-marine to coastal lithofacies of this depocenter. The fossil molluscan faunas, comprising 59 species, all of which are presently living in the Mediterranean, serve to determine approximate changes in paleodepth at this site with time. In particular, the recognized paleocommunities show a good correspondence with several infralittoral biocoenoses of unconsolidated mud- and sand-rich substrates of the present Mediterranean (Terrigenous Mud, Fine Well-Sorted Sands, Fine Sands in Very Shallow Water and Superficial Muddy Sands in Sheltered Areas). The methodology was applied to two other cores of near-comparable age in this region (Smithsonian core S-31 and boring Fadda-1 earlier studied by others). These two were also selected because they are characterized by a Holocene sequence of marine to coastal lithologies similar to core S-21. The approach used serves to determine temporal and spatial differences in paleobathymetry at the three sites. This investigation shows that analysis of the faunas in subsurface sections serves to refine paleoenvironmental interpretations of the Nile delta. Moreover, the molluscan paleobiocoenoses are best evaluated in conjunction with the lithofacies in which they occur.