A conceptual phosphogenesis model for the Red Sea phosphorites, Quseir area, Egypt

Along the Red Sea coast (at Quseir area), the Upper Cretaceous-Lower Paleogene phosphorite-bearing sediments were laid down under shallow marine conditions. There, thick economic phosphorite beds are intercalating the Quseir, Duwi, and Dakhla formations. Furthermore, thin phosphorite beds (< 20-cm thick) have been recorded as interbeds within the above-mentioned rock units. Each high-grade, economic phosphorite bed experienced many successive stages of phosphogenesis. The initial stage of phosphogenesis started with scattered phosphate peloids, which accumulated below fair-weather wave base and under calm conditions. Subsequently, the frequency and quantities of these peloids increased with time. The closing session of the phosphogenesis scenario was characterized by exhumation, fragmentation, and redeposition of the peloids by high-energy episodes (i.e., storms). Prolonged action of these high-energy events resulted in mixing of peloids with other phosphatic components in crudely fining upward sequences. Both one-event and amalgamated (multi-event) storm beds are common. The SEM investigations have indicated that algal blooms played a vital role in the origin of the Red Sea phosphorites. These algal blooms acted as important sites for P fixation and consequently, a release of P. Integrated field and laboratory studies have revealed that phosphorite rocks display noticeable variations from one locality to another. These variations include changes in thickness, P2O5 content and the hosting sediments (siliciclastics and/or carbonates). A plausible explanation for these variations may be related to the effect of synsedimentary local tectonics.