Silicification and dolomitization of the Lower Eocene carbonates in the Eastern Desert between Sohag and Qena, Egypt

Abstract

The Lower Eocene carbonates in the Eastern Desert between Sohag and Qena show remarkable diagenetic silicification and dolomitization beside other minor features of solution, cementation, syntaxial rim cement, compaction, recrystallisation and authigenic gypsum, anhydrite and halite formation. Two stages of silicification are recorded in the studied sequence, namely the syngenetic stage and the replacement stage. The first one is confined to the studied sequence, namely the syngenetic stage and the replacement stage. The in the open shelf lagoon facies as hard, dark silicified limestone concretions. The very early diagenetic chert formation was interrupted by the introduction of dolomite suggesting that dolomization postdates silification. The mode of occurrence of the syngenetic chert in the study area seems to be controlled by: the volume of the influx of hydrous silica in the interstitial water and the rate of lime mud deposition. Dolomites which are recorded here for the first time, represent about 40–60% of the carbonates and are confined mainly to the lime muds of the tidal flats and oyster bioclastic nearshore facies. The dolomitization phenomenon is considered to be of late diagenetic stage. Small elongate evaporite laths are scattered in the dolomitic lime mudstones. These authigenic evaporates associating the dolomitic facies of the tidal flats may suggest that hypersaline brines, rich in magnesium, were probably adjacent to these carbonates.