Carbonates (in brief)

Carbonates
Carbonate rocks form from ions in seawater. Thus, their deposition and accumulation is somewhat different than it is for siliciclastic sediments. They do not require that sediment is transported into the environment. Rather, they require specific chemical, temperature, and biological conditions in the environment where they form.

Most carbonates (during Phanerozoic time) are created by living organisms as shells and skeletons. (During Precambrian time, microbial communities strongly influenced carbonate mineral precipitation.) Corals, snails, clams, etc. are good examples. The reaction to form the carbonate minerals calcite or aragonite (which have the same mineral formula) is:

Ca²⁺ + CO₃^2- = CaCO₃

Because HCO₃^- is more abundant in seawater than CO₃^2-, the actual reaction that takes place is:

Ca²⁺ + 2HCO₃^- = CaCO₃ + H₂O + CO₂

The = means the reaction can go either way. If it goes from left to right, calcium carbonate minerals - calcite or aragonite - form. If it goes to the left, calcium carbonate minerals dissolve.) The chemical reaction to form the minerals is most likely in warm water, particularly in warm agitated waters. Breaking waves help get rid of the CO₂ produced by the reaction, which makes the reaction proceed even faster. Also, some corals contain photosynthetic organisms within their tissues, and those organisms consume CO₂, which also helps with the mineralization process. Thus, carbonate minerals form in warm, shallow seawater. The accumulation of carbonates creates “carbonate platforms” around many tropical islands (e.g. Caribbean islands, Bahamas, Hawaii, etc.) and along tropical shorelines (Florida, Great Barrier Reef of Australia, etc.).

Channels south of Highbourne Cay, Bahamas:

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This atol NW of Hawaii consists entirely of a reef that built up to sea level and the extinct volcano that underlies it subsided:

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One major source of carbonate sediments is from calcifying algae. These organisms are very abundant. They produce the minerals as sand or mud grains (depending on the species) within their tissues. When the algae die, the carbonate grains are released into the environment to be transported by waves and currents. At this point, the grains behave more or less the same as siliciclastic grains, with coarser sediment requiring high flow speeds to be transported, and mud-sized grains requiring very low flow speeds to settle from suspension. In shallow environments, muds accumulate in deeper areas of lagoons or get transported off shore into deeper waters. The grains get concentrated into shoals where water speeds slow down, for example, where water is channeled through a reef into a lagoon. These grains can also grow through carbonate mineral precipitation forming coated grains, or ooids. Thus, they get coarser with time.

A second distinctive feature of carbonates is the growth of reefs. Corals and other skeletal organisms grow well in high energy zones with breaking waves. Their skeletons make them resistant to erosion, and the breaking waves enhance carbonate mineral formation. Also, the precipitation of more carbonate as cements makes the structures hard and very resistant to erosion even though they are in high energy zones with breaking waves. These reef ecosystems can grow very quickly, creating a topographic high located off shore. This high induces more breaking waves, changing the energy distribution across the carbonate platform. The distribution of grain sizes around a reef depend on the flow speeds, similar to the dependence for siliciclastic grains, but the reef itself is cemented in place and provides a unique environment. Grains that are broken off tend to be transported to the inside or outside of the reef where water depths increase and flow speeds slow.

As reefs grow upward, they create very steep slopes, sometimes almost vertical slopes. These slopes can be unstable long-term, and they can fail, creating breccia in deep water and inducing turbidites as in siliciclastic sediments.

See figure 15.12 in Nichols, Edition 2 for the distribution of environments across a reef.