SPATIAL VARIATIONS IN HEAVY MINERALS AND PATTERNS OF SEDIMENT SORTING ALONG THE NILE DELTA, EGYPT

Sand samples were collected along 34 beach profiles spanning the 240-km length of the Nile Delta in order to examine the sorting patterns of heavy minerals that develop during cross-shore and alongshore sediment transport. Factor analysis of the heavy-mineral contents shows that two mineral factors or groups result from sorting due to the contrasting densities and sizes of the grains. One factor is dominated by augite, hornblende and epidote, while the second factor consists of opaques, garnet, zircon, rutile and monazite. Minerals of factor 1 are of lower densities and coarser sizes, and represent the grains that are selectively entrained by waves and currents in areas of beach erosion and preferentially transported to zones of beach accretion where they are deposited. The higher-density minerals of factor 2 tend to become concentrated in areas of beach erosion, accounting for the formation of black-sand placers where the erosion has been greatest. The spatial distributions of these two factors reflect the grain-sorting patterns of the heavy minerals which coincide with the general trends of shoreline erosion versus accretion that exist along the length of the delta. There is also a distinctive pattern of cross-shore sorting, with factor 2 containing the denser minerals being best developed within the inner surf zone, generally with a maximum at the shoreline. Its development corresponds to the formation of black-sand concentrates on the beach face where erosion has been greatest, implying that grain sorting has taken place mainly within the swash zone. There is a general decrease in factor 2 relative to factor 1 toward the offshore, often with oscillations that are congruent with the beach topography; factor 1 with the lower-density minerals is greatest over offshore bars where deposition has prevailed.