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Upper Cretaceous transitional fluvial to marine variegated shale (upper Nubia Formation) and the fully marine Duwi (phosphate) Formation occur as thin, widespread, shallow-marine deposits in an east-west-trending belt spanning the lower-middle latitudes of Egypt. These deposits consist of a heterogeneous suite of hemipelagic and shallow-water carbonate rocks that lie near the base of a generally transgressive marine sequence that was deposited on the fringes of the Arabo-Nubian craton in Cenomanian-Maestrichtian time. On a large scale, the phosphoritic rocks in Egypt represent but a small portion of a laterally extensive Middle Eastern-North African phosphogenic province of Upper Cretaceous-Lower Tertiary age that accounts for accumulation of minable marine phosphate in e cess of 70 billion tons.
Phosphorites, porcelanites/cherts, organic carbon-rich shales, glauconitic sandstones, and bioclastic and fine-grained carbonate rocks variously reflect major hemipelagic and shallow-water carbonate sedimentation. Biosiliceous hemipelagic deposits, now diagenetically altered to porcelanite and chert, reflect low energy depositional conditions that were periodically interrupted by high energy, possibly storm-induced currents and/or downslope redeposition. Both dark shales and porcelanites locally contain abundant organic matter and are commonly finely laminated. These strata probably reflect conditions of high biologic productivity and periodic anoxia in the water column. Porcelanites and black shales are phosphatic, containing phosphatic grains identical, morphologically and chemicall , to those found in associated phosphorites, and are probably the source from which the phosphorites were derived. Several lines of evidence suggest that the phosphorites of the Duwi Formation are clastic sedimentary deposits that have accumulated through mechanical winnowing, reworking, and concentration of preexisting phosphatic fine-grained sediment.
The organic carbon-rich shales of the Duwi Formation appear to be quite laterally extensive and may, depending on thermal maturity, represent potential hydrocarbon source rocks in other portions of the region (e.g., Western Desert, Gulf of Suez), where they are more deeply buried.
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