Calcilutites

Calcilutite is a fine-grained limestone rock defined by a predominance of micrite, a microcrystalline calcite mud. In hand specimen it appears uniform and lacks visible grains, with any fossils typically present only as microfossils or submillimeter inclusions. The defining characteristic is grain size; calcilutite contains carbonate particles smaller than about 0.062 mm (63 microns).

In lithologic terminology, calcilutite is the limestone counterpart to calcarenite. Calcarenite is dominated by sand-sized carbonate grains, whereas calcilutite is composed mainly of mud-sized material. The term is often used within Folk’s carbonate classification to denote a lime mudstone that is micritic in texture.

Depositional environments for calcilutite are typically low-energy marine settings where fine carbonate mud can settle and accumulate. Such environments include protected lagoons, sheltered shelves, outer carbonate ramps, and deeper-water carbonate basins. Diagenesis can lead to compaction and cementation, often producing a denser, more cohesive rock; sparry calcite cement may fill pore spaces.

Thin-section or micrographic examination is usually required to confirm its micritic texture and to identify any microfossils or pellet-like grains. Calcilutites can occur as extensive beds within carbonate sequences and may be interbedded with coarser carbonates such as calcarenites or calcirudites, depending on changes in energy and sediment supply.

Overall, calcilutite serves as a descriptive term for a micrite-dominated, fine-grained limestone, reflecting a low-energy carbonate mud environment and distinguishing it from coarser, grain-supported carbonate rocks.