Some major and trace constituents of Phanerozoic Egyptian mudrocks and marls

Abstract

One hundred shale, mudstone and marl samples from diverse important Phanerozoic depositional basins in Egypt were analysed for their Si, Al, Ca, Fe, Mg, Ti, carbonate, acid-insoluble residue and total organic matter contents. The following 13 trace elemenys were also spectrographically determined: Ag, B, Co, Cr, Cu, Ga, Mn, Ni, Pb, Sc, Sr, V and Zr. The chemical data are statistically treated and discussed in conjunction with the regional geologic setting. For most aerated shelf basins, Si, Al, Ti, Zr and Sc and the insoluble residue all increase together. They represent the resistate detrital fraction of the sediments which appreciablyy increases in the shallower nearshore facies of these stable shelves. Ca, Mg and Sr generally tally with the carbonate content. In the black shales, Co and to a mucch lesser extent Ni, V and Cr crudely increase with increase in total organic matter content. For all marine samples boron contents are generally below that of the ‘average shale’ of Turekian and Wedepohl (1961) and this may be related in part to the dominance of montmorillonite, mixed-layer clays and kaolinite rather than true illite in the clay mineralogy of the majority of Egyptian mudrocks and marls. The Cretaceous black shales associated with the phosphorite beds are not particularly enriched in trace metals and their content fluctuetes in response to their much varied and changing depositional environments. The Carboniferous black shales of west central Sinai are, on the other hand, distinctly enrichedn Cu, Pb, Co, Cr and V. They most probably reflect true anoxic conditions od sedimentation. The absence of true orogenies throughout the Phanerozoic in Egypt and the gradual upliftin of the stable craton of southern Egypt caused reworking of older shelf sediments and depositio of reworked sediments in youngerasins. This may explain the observed progressive decrease in ‘total trace element enrichment index’ in progressively younger aerated shelf basins of Egypt.