Diagenetic evolution of the Eocene ramp carbonates (a paradigm from the Nile Valley, Egypt): Petrographical and geochemical attributes

Abstract

Carbonate rocks of east Nile Valley in Egypt have been investigated as a paradigm for revealing the diagenetic evolution of the Eocene ramp carbonates. The integration of petrographical, elemental geochemical and stable isotopic data revealed that the studied carbonates have been influenced by a sequence of diagenetic processes which are included in three diagenetic episodes (i.e. eogenesis, mesogenesis and telogenesis) and are related to four diagenetic environments (i.e. marine-phreatic, meteoric-phreatic, burial and uplifting environments). The paragenetic sequence of the studied carbonates commences with the eodiagenetic episode, which includes the marine- and meteoric-phreatic diagenetic environments. The marine-phreatic diagenetic environment comprises micritization, glauconitization, pyritization, marine cementation (fibrous calcite and syntaxial overgrowth rim cements), early dolomitization (formation of fine-crystalline matrix-replacive dolomites) and early stages of mechanical compaction before significant burial. The meteoric-phreatic diagenetic environment includes the neomorphism, dissolution, meteoric cementation (earlier phases of sparry calcite cements) and precipitation of megaquartz in the void spaces of the peritidal carbonates. During the mesogenesis episode, the studied carbonates were exposed to burial diagenesis. The shallow burial diagenesis resulted in the creation of point-tangenetial contacts between the allochemical constituents and the precipitation of a medium-coarse crystalline dolomite cement filling the dissolved fossils and dissolution pores. The deep burial diagenetic processes embody the chemical compaction (condensed-fitted interpeneteration and pressure-dissolution fabrics), fracturing and burial cementation. The telodiagenesis represents the last episode in the diagenetic history of the studied carbonates. It witnessed the fracturing, meteoric-vadose cementation and hematitization processes.