Calcarenite

Calcarenite is a clastic carbonate rock composed predominantly of sand-sized calcium carbonate grains (about 0.062 to 2 millimeters in diameter). It is often described as the carbonate equivalent of a sandstone because the grains are in the sand size range and the rock is cemented by carbonate minerals, usually calcite.

The grains in calcarenite are typically bioclastic fragments (such as shells or foraminifera), ooids, peloids, or intraclasts. The spaces between grains are commonly filled with calcite cement (spar) or micritic calcite, giving the rock cohesion.

In classification schemes, calcarenite is used to denote a sand-sized carbonate rock. In Folk nomenclature it emphasizes grain size and carbonate composition, while some authors use calcarenite to distinguish carbonate sandstones from siliciclastic arenites.

Depositional environments are usually shallow, tropical to subtropical marine settings with sufficient energy to remove fines, such as beaches, dunes, lagoons with open circulation, oolitic banks, or reefal fore-reef sands.

Porosity in calcarenites depends on the balance between cementation and dissolution; they may have good intergranular porosity in less cemented examples but can lose porosity to calcite cement. Diagenetic processes such as dolomitization or dissolution can alter porosity and permeability.

Because of their porosity and permeability, calcarenites can serve as reservoirs in petroleum geology and groundwater systems. Their study requires petrographic analysis to identify grain types and cement and to interpret depositional history.