Carbonate rocks

Carbonate rocks are made of particles (composed >50% carbonate minerals) embedded in a cement. Most carbonate rocks result from the accumulation of bioclasts created by calcareous organisms. Therefore carbonate rocks originate in area favoring biological activity i.e. in shallow and warm seas in areas with little to no siliciclastic input. In present day Earth these areas are limited to ± 40 latitude in region away or protected from erosion-prone elevated continental areas.

Two classification schemes are in common use by those who work on carbonate rocks. Although you will use only the Folk classification in lab, you should also become familiar with the Dunham classification since it is widely used as well.

The Folk classification, divides carbonates into two groups. Allochemical rocks are those that contain grains brought in from elsewhere (i.e. similar to detrital grains in clastic rocks). Orthochemical rocks are those in which the carbonate crystallized in place. Allochemical rocks have grains that may consist of fossiliferous material, ooids, peloids, or intraclasts. These are embedded in a matrix consisting of microcrystalline carbonate (calcite or dolomite), called micrite, or larger visible crystals of carbonate, called sparite. Sparite is clear granular carbonate that has formed through recrystallization of micrite, or by crystallization within previously existing void spaces during diagenesis.

Dunham Classification is based on the concept of grain support. The classification divides carbonate rocks into two broad groups, those whose original components were not bound together during deposition and those whose original components formed in place and consist of intergrowths of skeletal material. The latter group are called boundstones (similar to biolithite of the Folk classification). The former group is further subdivided as to whether or not the grains are mud-supported or grain supported. If the rock consists of less than 10% grains it is called a mudstone. If it is mud supported with greater than 10% grains it is called a wackstone. If the rock is grain supported, it is called a packstone, if the grains have shapes that allow for small amounts of mud to occur in the interstices, and a grainstone if there is no mud between the grains.


Dunham Classification


Folk classification

Most modern, and probably most ancient, carbonates are predominantly shallow water (depths <10-20 m) deposits. This is because the organisms that produce carbonate are either photosynthetic or require the presence of photosynthetic organisms. Since photosynthesis requires light from the Sun, and such light cannot penetrate to great depths in the oceans, the organisms thrive only at shallow depths. Furthermore, carbonate deposition in general only occurs in environments where there is a lack of siliciclastic input into the water. Siliclastic input increases the turbidity of the water and prevents light from penetrating, and silicate minerals have a hardness much greater than carbonate minerals, and would tend to mechanically abrade the carbonates. Most carbonate deposition also requires relatively warm waters which also enhance the abundance of carbonate secreting organisms and decrease the solubility of calcium carbonate in seawater. Nevertheless, carbonate rocks form in the deep ocean basins and in colder environments if other conditions are right.

The principal carbonate depositional environments are as follows:

• Carbonate Platforms and Shelves. Warm shallow seas attached the continents, or in the case of epiric seas, partially covering the continents, are ideal places for carbonate deposition. Other shelves occur surrounding oceanic islands after volcanism has ceased and the island has been eroded (these are called atolls). Carbonate platforms are buildups of carbonate rocks in the deeper parts of the oceans on top of continental blocks left behind during continent - continent separation.

• Tidal Flats. Tidal flats are areas that flood during high tides and are exposed during low tides. Carbonate sands carried in by the tides are cemented together by carbonate secreting organisms, forming algal mats and stromatolites.

• Deep Ocean. Carbonate deposition can only occur in the shallower parts of the deep ocean unless organic productivity is so high that the remains of organisms are quickly buried. This is because at depths between 3,000 and 5,000 m (largely dependent on latitude - deeper near the equator and shallower nearer the poles) in the deep oceans the rate of dissolution of carbonate is so high and the water so undersaturated with respect to calcium carbonate, that carbonates cannot accumulate. This depth is called the carbonate compensation depth (CCD). The main type of carbonate deposition in the deep oceans consists of the accumulation of the remains of planktonic foraminifera to form a carbonate ooze.

Upon burial, this ooze undergoes diagenetic recrystallization to form micritic limestones. Since most oceanic ridges are at a depth shallower than the CCD, carbonate oozes can accumulate on the flanks of the ridges and can be buried as the oceanic crust moves away from the ridge to deeper levels in the ocean. Since most oceanic crust and overlying sediment are eventually subducted, the preservation of such deep sea carbonates in the geologic record is rare, although some have been identified in areas where sediment has been scraped off the top of the subducting oceanic crust and added to the continents, such as in the Franciscan Formation of Jurassic age in California.

• Non-marine Lakes. Carbonate deposition can occur in non-marine lakes as a result of evaporation, in which case the carbonates are associated with other evaporite deposits, and as a result of organisms that remove CO2 from the water causing it to become oversaturated with respect to calcite.

• Hot Springs. When hot water saturated with calcium carbonate reaches the surface of the Earth at hot springs, the water evaporates and cools resulting in the precipitation of calcite to form a type of limestone called travertine.

Embry & Klovan (1971) classification modified the Dunham scheme by further subdividing coarse-grained skeletal deposits and organically formed or organically bound carbonate rocks. The five new terms add to the descriptive capability of the Dunham classification in the area of biogenic deposits, especially reefs and bioherms.


Embry & Klovan (1971) classification