Calcarenite is a specific type of limestone, a sedimentary rock defined primarily by the size of the grains that make up the rock mass, rather than its chemical composition alone. It is the carbonate equivalent of a sandstone, where the individual particles are composed of calcium carbonate instead of silica. The formation of calcarenite involves the mechanical transport and deposition of these carbonate grains in ancient sedimentary environments.
Defining Calcarenite: Classification and Grain Size
Calcarenite is formally classified as a clastic sedimentary rock, meaning it is formed from fragments that have been mechanically transported and deposited. As a limestone, it is composed predominantly (more than 50%) of carbonate material like calcite or aragonite. The term calcarenite is a textural designation combining “calcare-” (calcium carbonate) with “-arenite,” which denotes a rock made of sand-sized particles.
The definition is strictly tied to the Wentworth scale for grain size, specifying that the constituent grains must fall within the range of sand. This sand-sized interval spans from 2.0 millimeters at the coarse end down to 0.0625 millimeters at the fine end.
The particles within the rock are known as allochems, which are the transported and deposited fragments that give the rock its clastic texture. Because these particles are carbonate-based and have been moved, sorted, and redeposited, calcarenite is considered an allochemical limestone.
The presence of calcarenite often indicates a high-energy depositional environment, such as a shallow marine platform or a coastal setting. In these areas, water currents or wave action were strong enough to transport and sort the sand-sized grains, but not so strong that they would wash away all the finer silt and mud. The resulting rock fabric is typically grain-supported, meaning the sand-sized particles touch and support one another, with less fine-grained matrix material between them.
Primary Composition and Geological Formation
The material that forms calcarenite originates from the accumulation of various carbonate allochems. These grains can be biogenic, meaning they are fragments of marine organisms, or they can be formed through chemical precipitation. Common biogenic components include fragments of shells, corals, crinoids, and other skeletal debris from organisms that utilize calcium carbonate to build their structures. The specific mix of these allochems determines the rock’s sub-type, such as an oolitic calcarenite or a bioclastic calcarenite.
Chemical or biochemical processes also produce distinctive grains, such as ooids, which are spherical grains formed by the concentric precipitation of calcium carbonate around a nucleus. Other components include pellets, which are tiny, ovoid fecal remains of marine organisms, and intraclasts, which are fragments of an older, semi-lithified carbonate sediment that were eroded and redeposited.
The geological formation of calcarenite begins with the deposition of carbonate sand in a low-relief, typically warm-water marine environment. Once deposited, the loose carbonate sand undergoes a process called diagenesis, which transforms the sediment into a solid rock. This transformation involves burial, compaction, and the subsequent cementation of the grains. Cementation occurs as calcium carbonate-saturated water moves through the pore spaces between the grains. The dissolved calcium carbonate precipitates out of the water, forming a crystalline cement, often sparry calcite, that binds the individual grains together. The resulting rock is often still quite porous and permeable, which makes some calcarenites significant as hydrocarbon reservoirs or aquifers in certain geological settings.
Differentiating Calcarenite from Other Carbonate Rocks
Calcarenite sits within a broader granulometric classification system for limestones, which distinguishes carbonate rocks based purely on the size of their constituent grains. This system parallels the classification used for siliciclastic rocks like shale, sandstone, and conglomerate.
Carbonate rocks with grains coarser than the sand-sized range of calcarenite are termed calcirudite. Calcirudite is composed of fragments that are larger than 2.0 millimeters in diameter, similar to a conglomerate, and these larger clasts are sometimes referred to as lime-rubble.
On the finer end of the spectrum, carbonate rocks with grains smaller than the sand-sized range are classified as calcisiltite and calcilutite. Calcisiltite is composed of silt-sized grains, which are defined as being between 0.062 millimeters and 0.002 millimeters. The finest carbonate rock is calcilutite, which is made up of clay-sized particles smaller than 0.002 millimeters, essentially carbonate mud. The transition from calcarenite to calcisiltite and calcilutite reflects a change in the depositional setting from high-energy, current-agitated environments to much lower-energy, quieter water settings.