Nature and genesis of silt-size carbonate sediments; Northern Red Sea, Egypt

Abstract

The nature and genesis of silt-size carbonate fractions in the Northern Red Sea is investigated via detailed SEM, XRD and ICP analyses of the sediments. Results show that inorganic aragonite represents only 25% of the fine silt and clay fractions, but up to 60% of the total coarse silt. In general there is a compositional trend of increasing proportion of Mg-calcite with decreasing grain size of the sediments. Petrographic examination demonstrates that the coaree silt size carbonates are mainly composed of comminuted pieces of unaltered carbonate or micritized marine skeletal debris. On the other hand, the fine and very fine silt fractions are mainly composed of authigenic carbonate, formedin situ, and replacing the comminuted pieces of carbonate debris. The argument for the inorganic origin of the fine carbonate fractions is based on the detectable increase in the Sr content of aragonite, and the increasing proportion of Mg-calcite with decreasing grain size of the sediments. It is concluded that the Mg-calcite component provides a more important clue to the origin of the silt fraction. Micritization of skeletal grains is an important source of carbonate mud. Thus Mg-calcite from the Red Sea reefal sediments, either as skeletal or cryptocrystalline lumps, is the principal source of this component in the carbonate mud, especially in the clay size fractions. The Carbonate mud in the study area is mineralogically similar to the Belize carbonate mud, and differs from the Bahamss and Arabian gulf carbonate sediments.