Carbonates are a group of sedimentary rocks that are fundamentally different from the siliciclastic rocks on which we have been focussing.
The materials that are deposited as carbonate sediments have their ultimate source in the dissolved constituents of surface waters, derived by weathering of source rocks. These materials can be deposited either by direct inorganic precipitation, or by biological intervention: organisms extract minerals from water to build skeletal structures, and those are deposited as discrete particles when the organisms die.
They represent a different fraction of the products of weathering, and therefore have a different chemical composition. Whereas sandstones concentrate SiO2, and Al2O3 is fractionated into mudrocks, the carbonates concentrate CaO and MgO. (the CO2 is atmospheric carbon)
Where do carbonate rocks form?
The shallow waters of the oceans and most lakes are saturated with CaCO3, and can precipitate it readily. Most CaCO3 precipitation in the ocean is mediated by organisms in shallow-water environments. Life-forms (plants as well as animals) discovered calcite as a useful and readily availble structural material in the latest Precambrian, and nearly all carbonate sediment since then has formed largely through the agency of organisms.
The average limestone contains less than 5% silicate minerals. Generally, significant carbonate accumulations form only in areas where there is little terrigenous sediment input. Fine-grained sediment in suspension in the water makes it cloudy, decreases light penetration, and reduces organic activity.
Carbonates do not form in the deep oceans because the solubility of CaCO3 increases with depth, and so deep ocean water is undersaturated with respect to calcite. The Lysocline is the depth at which carbonate dissolution begins, and the Carbonate Compensation Depth is the depth below which no carbonate is found. This depth varies from 3-6 km, depending on bottom water temperature and carbonate supply.
Controls on carbonate deposition:
Temperature Calcite solubility is less in warm waters, so it is easier for organisms to extract it. Therefore most carb. production occurs on tropical shelves.
Light Carbonate ecosystems are plant-based and so dependant on photosynthesis. In addition, many corals live in close symbiosis with algae, so they are restricted to the photic zone. In the pelagic oceans, carb. plankton thrive in the upper few tens of m. of the water column.
Oxygen Wave activity mixes air with seawater and oxygenates it, so that more organisms can grow and thrive.
Productivity Abundant nutrients are necessary to promote organic growth. (Too many nutrients however can limit carbonate production because corals etc. are outcompeted by algae).
Turbidity Low sediment input is required. Turbid water is dim...less light; plus the fine suspended sediment can clog the feeding and breathing apparati of invertebrates. Too many nutrients can also cause increased turbidity.
Water depth Water depth controls carb. accumulation, not just because of light penetration, but because of pressure and cold. The dissolution of carbonate is favoured by increased P and decreased T. The Lysocline is the depth at which carbonate dissolution begins, and the Carbonate Compensation Depth is the depth below which no carbonate is found. This depth varies from 3-6 km, depending on bottom water temperature and carbonate supply. It is shallower in general in polar regions and deeper in the tropics.
The role of organisms
It is the chemical characteristics of seawater that have led to the significance of carbonate rocks. There are many ions in solution in seawater, but most are not easily available to organisms because seawater is not saturated with them. Organisms use CaCO3 rather than eg. NaCl because of this saturation. Other minerals such as halite or gypsum are not used because seawater at its normal concentration is undersaturated with respect to these minerals. They can form only from much more concentrated brines, which is why they occur as evaporite deposits.
It had been recognised for a long time that the skeletal remains of organisms were a significant component of limestones, but it was only in the late 50s and early 60s that far more than shelly material was contributed by organisms. Invertebrate fecal material is a significant volumentric component of carbonate sediment, as is the fine-grained cominuted material produced by rasping and burrowing organisms, and the fine-grained skeletal material that is contributed by cyanobacteria and many of the algae.
Of what do carbonate rocks consist?
There are three important carbonate minerals in sediments:
Aragonite (CaCO3) This is one of the dominant minerals in modern carbonate sediments. It is a metastable polymorph of calcite. Many modern carbonate-secreting organisms secrete aragonite rather than calcite, depending on the specific amino-acid composition of the protein matrix in which the mineral is forming. It is not generally stable under diagenetic conditions, and is converted quickly to calcite after the death of the organism. Aragonite in rocks older than the Cretaceous is very rare.
ORGANISMS: some sponges, corals, Halimeda, some molluscs, some bryozoans, some arthropods,
High-mag. calcite (CaCO3 containing more than 4-5 mole% MgCO3) This form of calcite is more stable in seawater than low-magnesium calcite, but in many other environments (eg. freshwater, many diagenetic environments) it is not stable, because the substitution of Mg into the calcite crystal lattice causes positional disorders and frequently it is converted to low-magnesium calcite after the organism's death.
ORGANISMS: many crustaceans, all echinoderms, all brachiopods, some calcareous sponges
The above two are the most common minerals in modern carbonate accumulations. The following two are the most common in ancient rocks.
Low-mag calcite (CaCO3 containing less than 4% MgCO3). Stable, resistant to diagenetic recrystallisation. This is secreted by some modern organisms, but more commonly it forms by recrystallisation of aragonite (and low-mag calcite) diagenetically.
ORGANISMS: articulate brachs., forams, molluscs, barnacles
Dolomite (CaMg(CO3)2) This mineral is not secreted by any organisms known. It is very rare in modern deposits, and where found is generally secondary. It becomes more common in older limestones, where its increasing prominence is ascribed to progressive diagenetic reaction of calcite with Mg-rich brines.
Carbonate mud, or micrite (term introduced by Folk, 1959) largely composed of microcrystalline needles of aragonite (most modern muds) or calcite (most limestone muds), is abundant in both modern and ancient carbonates. By some estimates it is the volumetrically most abundant constituent of limestones.
The origin of micrite has been controversial for a couple of decades; but it is now generally conceded that there are three main sources:
(a) Inorganic precipitation
Although shallow marine waters are oversaturated with respect to both calcite and aragonite, in fact neither precipitates very readily under normal marine conditions. For many years the existence of "whitings" (concentrations of suspended carbonate mud) in shallow carbonate environments was cited as evidence that, under some conditions, direct inorganic precipitation from seawater does take place.
Most evidence now indicates that in the open marine environment this is very rare. Generally, whitings consist of suspended bottom sediment and not newly-nucleated inorganic precipitates. This has been shown by compositional and isotopic comparasons between the whiting muds, the seafloor muds, and the ocean water.
Atomic testing produced excess C14 (radiogenic atoms) in seawater. If mud in whitings had precipitated from this water, its isotopic composition should record the radiogenic signature; but when tested this was found not to be the case. The whitings isotopic composition was in fact identical with the bottom sediments.
Direct inorganic precipitation does seem to cause whitings in areas like the Dead Sea, where the water chemistry differs considerably from the open ocean, but it is now agreed that inorganic precipitates are a negligible contributor to micritic sediment.
(2) Mechanical or biological abrasion
Larger carbonate particles can be abraded to mud size just as can siliciclastic particles. However, even in very low-energy settings carbonate mud is produced in abundance by biological abrasion. Some organisms ingest carbonate and abrade it internally, then excrete it. Some abrade it externally: snails have a "rasping tongue" called a radula that they use to scrape at corals to get at the polyp inside. Just like sandpaper, this produces micritic dust. Many organisms, both macroscopic and microscopic, bore into carbonate material either for food or shelter. Bivalves do it, and so do endolithic algae.
(3) Production of aragonite by algae
Many algae produce microcrystalline needles of aragonite within their tissues. When the algae die and decay, the micrite is released as sediment. Studies have shown that this is an enormous source of micrite, and probably the single most important source through the Phanerozoic.
Neuman and Lands study (1975, J.Sed. Pet. v. 45. p. 763-786-see handout) of the production of calcareous mud in the Bight of Abaco showed that calcareous algae alone could not only account for all mud deposited there since the post-glacial sea-level rise, but also supplied twice as much sediment to the surrounding deep-water basins.
Micrite may also form diagenetically: neomorphic degradation. often biologically mediated (mirco-organisms)
(3) Grain types
One of the fundamental realisations of modern times that led to a better understanding of carbonate environments was that in contrast to siliciclastic sediments, which most commonly have a unimodal size distribution, carbonate sediments are generally bimodal, and can be considered as a mixture of sand-sized (or larger) grains, and carbonate mud. This derives largely from the classic work of Folk.
The sand-size material is formed by many processes, generally at or near the site of deposition, and is broken down into a number of grain types or allochems.
Bioclasts Skeletal material in carbonates may consist of whole fossils, (micro or macro), or broken fossil fragments. They are the most common grain type in carbonates. The kind of skeletal material present is a function of the age of the rock and also the depositional environment. Certain organisms or groups of organisms are often diagnostic of specific water depths or energy levels or salinities.
Ooliths or ooids "coated carbonate grains that contain a nucleus". They represent inorganic precipitation from seawater of aragonite (modern) on a nucleus. They have a concentric internal structure, indicating sequential deposition of layers. The breaks have been described as "microunconformities" representing changes in conditions and cessation of precipitation. They form only in areas with strong bottom currents, so that the grains are rolling continuously. Ancient oolites may have a radial structure, indicating recrystallisation of aragonite to calcite.
Oncoloites Similar to ooliths, but form by bacterial or algal coating of agitated grains. They also have a concentric internal structure but it is less regular.
Peloids Grains that show no internal texture and are composed of crypto-crystalline calcite or carbonate mud. Generally of fecal origin, and in that case the are generally in the silt to fine sand size range, oval, uniform in size and shape and rounded. They may be dark in colour because of organic material. Peloids also form by the activites of micro organisms, especially endolithic algae. These organisms bore into carbonate material, especially skeletal material but also ooliths, and extract organic substances. In the process they convert the aragonite or calcite into very fine-grained micrite.
Lithoclasts Lithoclasts are rock clasts incorporated in carbonate sediment. The most common type are intraclasts, which are ripped-up pieces of penecontemporaneous sediment, commonly generated during storms. They are more common in carbonates than in siliciclastic sediments because lithification is much more rapid in carbonates. Carbonate sediment is commonly partially lithified while still at or near the sediment-water interface. Clasts of lithified material that have no evident genetic relationship to the sediment, eg. clastic rock fragments, are interpreted to have come from outside the depositional basin and are therefore called extraclasts.
Grapestones Aggregate grains, bound together. Studies of modern grapestones have shown that they are bound by micritic material that contains tubes and organic structures, and seems to result from the activites of tiny worm-like creatures and micro-organisms, including cyanobacteria. They seem to form during times of non-deposition when wave and current activity are also low. They are rarely described from the rock record because they are commonly distorted or destroyed during diagenesis.
Sparry calcite refers to large (greater than 0.02mm or medium silt and above) crystals of calcite that generally grow during diagenesis. They may form in two ways:
Micritic grains, and non-calcite allochems, may recrystallise to coarse calcite. Important to distinguish from cement, but often difficult. This is called NEOMORPHIC spar
Calcite may precipitate in void spaces and pores. This suggests that the pores were open, indiciating an energetic environment
Grain size, sorting and rounding
In siliciclastic rocks, the size, shape and sorting of grain populations are determined by the transport history and the mechanism of transport and depotision. In carbonates however interpretation of these features is less straightforward because of the biological origin of many of the grains.
Example: the predominant grains in a limestone might be a particular species of ostracod (bivalved crustacean) that were buried in the mud in which they lived. These grains would be "very well sorted" and "very well rounded", but this is not directly related to the current strengths in their depositional environment.
Similarly, fecal pellets show a biologically determined high degree of "sorting" and "rounding".
The kinetic energy in a carbonate depositional environment is generally determined by:
(a) the presence or absence of micrite in the rock
Generally, allochems and mud will be produced togehter at or near the site of deposition. If the deposit contains no mud particles, that indicates removal of the fine material by active currents ("winnowing out").
(b) the degree of abrasion and breakage of skeletal allochems
If recognisable allochems show no damage, this indicates a lack of transport, and therefore an energy level below that necessary to transport the fragments. They give an upper limit to the energy level. The smaller the particle, the lower the upper limit. Unbroken ostracods indicate a lower maximum energy level than do unbroken oyster shells. The absolute energy level in both cases may however have been the same.
The more fragmented and broken the particles, the greater the energy level. But again, it takes less effort to fragment bryozoans than large bivavles.
(c) the presence of oolites always indicates active bottom currents
(d) the presence of traction structures (cross bedding for example) can be used to infer energy level in the same way as for siliciclastic rocks.
Classification of carbonates
The classification of siliciclastics is based on their mineralogical composition; but because limestones are almost mono-minerallic, and because the mineralogy of carbonates is more a function of time than any depositional or provenance process (will cover this more when we discuss carbonate diagenesis) we take a different approach to their classification.
The main parameters used in the classification of carbonates are
(1) the grain (allochem) types present
(2) the micrite to grain ratio
The most widely used classifications are those of Folk (1962) and Dunham (1962; usually presented in modified form). They are shown on pages 209 to 211 of Boggs, and will be compiled and given to you as a working handout in lab.
Folks classification is allochem- and cement-based (compositional)
Dunhams classification is grainsize- and sorting-based (textural)
They are descriptive classifications, but the descriptions convey depositional information. Sparites (Folk) or Grainstones (Dunham) lack micrite, and imply an energetic environment. The classifications do not consider the degree of breakage of grains.