Mineralogical, petrographical, geochemical, diageneses and provenance of the Cretaceous Black Shales, Duwi Formation at Quseir-Safaga, Red Sea, Egypt

AbstractThe Duwi Formation in the Quseir-Safaga area is conformably overlain by the Quseir variegated shales and underlain by the Dakhla shales. Detailed mineralogical and geochemical studies were carried out on the middle member of the black shales of the Duwi Formation in Quseir-Safaga province to infer their source rocks, paleo-weathering and provenance of the sources. The studied black shales are highly fossiliferous and fissile. They are texturally classified as phosphatic marly mudstone microfacies. Mineralogically, they are composed mainly of calcite, apatite, quartz, pyrite and clay minerals. The studied clay fraction reveals that the well-crystalline Na-montmorillonite is the dominating clay mineral with mixed layer montmorillonite-illite and minor kaolinite. These shales could be used as a source for the sodium bentonite. The studied black shales are highly mature, detrital and authigenic in origin. They are most probably derived from basic volcanic rocks by intensive chemical weathering. The provenance constituted a part of passive and active continental margins similar to those the Devonian/Mississippian shales in USA. The Cretaceous black shales of the Duwi Formation in Quseir-Safaga area are part of Mes-Neoarchean Shales, and are typically comparable to the Archean Shale Average.The average values of Cu, Ni, Zn, Cr, V, As and Se and, with the ratios Rb/Sr and Cr/Ni, are enriched with respect to those of the PAAS values. This could be attributed to the abundance of mafic components. The felsic source rocks were considered as an added source similar to Cretaceous and Cambrian black shales in India and China; respectively. The ratios V/Cr, V/(V+Ni) and Mo/Al and the high concentrations of the redox sensitive elements revealed that the studied black shales were deposited under relatively euxinic to anoxic reducing marine environments coupled with hydrothermal solutions. They were found comparable to the Cretaceous Gulf of California, Posidonia, Cretaceous/Tertiary black shales, and those in the Black Sea. The studied black shales can be classified as good to excellent source of oil and trace elements. The chemical composition of the studied black shales of the Duwi Formation (Cretaceous) was correlated to published data at different localities and ages