The Fabric of Carbonate Cement and Matrix and its Dependence on the Salinity of Water

Abstract

Lithification of carbonate sediments occurs under both marine and freshwater conditions. Mineralogically, texturally, and genetically four different types of cement and matrix are recognized. These are: 1. marine aragonite as fibrous cement or micrite, 2. marine highmagnesian calcite micrite, 3. freshwater low-magnesian calcite drusy mosaic or wedge-shaped crystals or as fibrous cement or rods, and 4. freshwater low-magnesian calcite micrite. Aragonite and high-magnesian calcite are the minerals which form cement or matrix in the marine environment, whereas low-magnesian calcite makes up cement or matrix in the freshwater environment. Aragonite forms fibrous cements under hypersaline or very shallow marine conditions, such as in deep-sea carbonate sediments of the Red Sea, or in beachrocks or reefs. It may also be deposited as micrite, as in grapestones. High-magnesian calcite forms a micrite cement in the deep-sea environment or in beachrocks or a fibrous cement in reefs. Low-magnesian calcite forms a drusy calcite mosaic under fresh water conditions or occurs in the shape of rods or wedge-shaped crystals in freshwater limestones, such as in tufa or cave pearls. It occurs as a cement in marine rocks in which it has been emplaced post-depositionally by fresh water. Low-magnesian calcite micrite is formed in fresh water, such as in lakes, where it makes up oncolites or massive limestones. Texturally fresh water lowmagnesian calcite micrite is indistinguishable from aragonite micrite or from marine highmagnesian calcite micrite or its low-magnesian diagenetic equivalent.