Automicrite

Last updated April 28, 2019

Automicrite is autochthonous micrite, that is, a carbonate mud precipitated in situ (no transporting) and made up of fine-grained calcite or aragonite micron-sized crystals. It precipitates on the sea floor or within the sediment as an authigenic mud thanks to physicochemical, microbial, photosynthetic and biochemical processes. It has peculiar fabrics and uniform mineralogical and chemical composition.^[1]

Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm formed by the recrystallization of lime mud.

Carbonate minerals Nickel–Strunz 9 ed mineral class number 5

Carbonate minerals are those minerals containing the carbonate ion, CO₃²⁻.

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO₃). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".

Environments where automicrite is formed

Automicrite deposits are found in different environments, as lakes, tidal flats, or the aphotic zone of marine slopes and basins. However, the environment where automicrite is most common is in carbonate mud mounds, a type of carbonate platform. In this kind of carbonate platform, common in the geological past but nearly absent today, automicrite deposits made carbonate structures of many tens to hundreds of metres of relief called "mud mounds". Automicritic mud mounds seemed to form more easily in areas of low sedimentation rate of siliciclastic extrabasinal sediment.^[2]

The aphotic zone is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone. Most food in this zone comes from dead organisms sinking to the bottom of the lake or ocean from overlying waters.

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today’s reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha or extinct cnidaria were important reef builders.

Usually, the presence of mud in sedimentary rocks is an indicator of low-energy conditions in a depositional system, so in high-energy conditions it is nearly impossible to find mud. Instead, automicrite can precipitate internally in cavities and sediment pores below the sediment–water interface, also in sediments that are formed in high-energy condition. Automicrite can also crystallize in suspension within cavities and then deposit on the cavity bottom. Usually, this process forms peloids, a kind of carbonate grains make by automicrite.^[3]

Peloids are allochems that are composed of micrite, irrespective of size, shape, or origin. The two primary types of peloids are pellets and intraclasts. Another type of peloid is pseudo-oolith.

Features

In present carbonate platforms, automicrite is composed by aragonite needle-shaped crystals which have dimension of 1- 5 μm. Instead, in old limestones micrite is composed by μm-size calcite crystals.^[3]

Formation

Knowledge of the automicrites generation processes allow to make paleo-environmental interpretations, so it can become good instrument for basin analysis. Carbonate mud or micrite may originate through several processes, including the abiotic precipitation from highly supersaturated seawater or precipitation induced by microbial activity.^[4]

Sedimentary basin analysis is a geologic method by which the history of a sedimentary basin is revealed, by analyzing the sediment fill itself. Aspects of the sediment, namely its composition, primary structures, and internal architecture, can be synthesized into a history of the basin fill. Such a synthesis can reveal how the basin formed, how the sediment fill was transported or precipitated, and reveal sources of the sediment fill. From such syntheses models can be developed to explain broad basin formation mechanisms. Examples of such basinal environments include backarc, forearc, passive margin, epicontinental, and extensional basins.

1. Abiotic micrites precipitation

There are two kinds of “inorganic” micrite:^[3]

- Internal micrite which precipitates inside cavities and inter-granular pores of sediments. It is a pore-filling micrite which has peloidal or clotted textures, and when it is found in limestones it may prove that marine lithification occurred.

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO₃). A closely related rock is dolomite, which contains a high percentage of the mineral dolomite, CaMg(CO₃)₂. In fact, in old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.

- Seafloor micrite which precipitates at the sediment–water interface.

The equilibrium relationship for ${\ce {CO2, CaCO3}}$ in water is:

${\ce {H2O + CO2 + CaCO3 (solid) <-> Ca2+ (aq) + 2HCO3- (aq)}}$

Through temperature increase, pressure decrease, or a decrease in pH, the loss of ${\ce {CO2}}$ makes the reaction shift to the left. While ${{\ce {CaCO3}}}$ should precipitate spontaneously from seawater, which is supersaturated with respect to calcium carbonate, calcite precipitation is inhibited by the presence of Mg in seawater and aragonite crystallization is inhibited by organophosphatic molecules. In modern environments, carbonate mud seems to form spontaneously in seawater in whitings.^[3]

Whitings are clouds of suspended carbonate crystals (aragonite and Mg-calcite) that make the sea white. This phenomenon is common in tropically environments such as the Great Bahama Bank. Some attribute the formation of whitings to resuspension of carbonate sediment from sea bottom by waves.^[4]

2. Precipitation induced by microbial activity

Precipitation of carbonate mud from seawater may be triggered by biological activity as photosynthesis. In fact, the metabolism of some organisms remove dissolved CO₂ from seawater and thus promotes the precipitation of carbonate. In present sedimentary environments, the production of carbonate mud by cyanobacteria is known to occur in peritidal environments (Bahamas, Florida Bay, Persian Gulf) and in highly saline lakes (Coorong, Australia; Lake Tanganyika, eastern Africa). In this process is relevant the rule of filamentous cyanobacteria, in fact the calcite crystals formed within the organic filaments of cyanobacteria are retained after the death of the organism and consequently end up in the formation of layered carbonate mud rocks, the stromatolites. Micritic rocks of the geological past, e.g., the stromatolites of the Late Triassic Hauptdolomit of the Alps, may have had a similar origin. In many Jurassic micritic and peloidal limestones, remains of benthic coccoid cyanobacterial mats have been found.^[1]

Also bacterial sulfate reduction may promote carbonate mud precipitation from seawater. Bacterial sulfate reduction may be represented by the simplified reaction:

2CH + SO₄²⁻ → HCO₃⁻ + HS⁻ + H₂O

This process has the potential to promote precipitation by producing hydrogen carbonate ions, which are one of the reactants in the precipitation of carbonate from seawater.

Related Research Articles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Calcium carbonate is a chemical compound with the formula CaCO₃. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of pearls and the shells of marine organisms, snails, and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous.

The lysocline is the depth in the ocean below which the rate of dissolution of calcite increases dramatically.

Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, Ca_{C_{O₃_{_{_{_{_{_{_{_{_{_{_{_{_{_{_{_{_{(the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.}}}}}}}}}}}}}}}}}}}

Evaporite A water-soluble mineral sediment formed by evaporation from an aqueous solution

Evaporite is the term for a water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes. Evaporites are considered sedimentary rocks and are formed by chemical sediments.

Ooids are small, spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas. After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. Oolites usually consist of calcium carbonate; these belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad in the Czech Republic.

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO₃) and dolomite, also known as dolostone, which is composed of the mineral dolomite (CaMg(CO₃)₂).

Calcite compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite lags behind the rate of solvation, such that no calcite is preserved. Aragonite compensation depth describes the same behaviour in reference to aragonitic carbonates. Aragonite is more soluble than calcite, so the aragonite compensation depth is generally shallower than the calcite compensation depth.

The Folk classification is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas.

Bristol Lake is a dry lake in the Mojave Desert of San Bernardino County, California, 42 km (26 mi) northeast of Twentynine Palms.

Sedimentary exhalative deposits are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir, resulting in the precipitation of stratiform ore.

Beachrock is a friable to well-cemented sedimentary rock that consists of a variable mixture of gravel-, sand-, and silt-sized sediment that is cemented with carbonate minerals and has formed along a shoreline. Depending on location, the sediment that is cemented to form beachrock can consist of a variable mixture of shells, coral fragments, rock fragments of different types, and other materials. It can contain scattered artifacts, pieces of wood, and coconuts. Beachrock typically forms within the intertidal zone within tropical or semitropical regions. However, Quaternary beachrock is also found as far north and south as 60° latitude.

Calcite sea Sea chemistry favouring low-magnesium calcite as the inorganic calcium carbonate precipitate

A calcite sea is one in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic are characterized by aragonite seas.

Aragonite sea Chemical conditions of the sea favouring aragonite deposition

An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The chemical conditions of the seawater must be notably high in magnesium content relative to calcium for an aragonite sea to form. This is in contrast to a calcite sea in which seawater low in magnesium content relative to calcium favors the formation of low-magnesium calcite as the primary inorganic marine calcium carbonate precipitate.

Cementation (geology) Process of chemical precipitation bonding sedimentary grains

Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together. In this way sand becomes "sandstone", and gravel becomes "conglomerate" or "breccia". Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz or silica phases like cristobalite, iron oxides, and clay minerals, but other mineral cements also occur.

Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils.

Shell growth in estuaries is an aspect of marine biology that has attracted a number of scientific research studies. Many groups of marine organisms produce calcified exoskeletons, commonly known as shells, hard calcium carbonate structures which the organisms rely on for various specialized structural and defensive purposes. The rate at which these shells form is greatly influenced by physical and chemical characteristics of the water in which these organisms live. Estuaries are dynamic habitats which expose their inhabitants to a wide array of rapidly changing physical conditions, exaggerating the differences in physical and chemical properties of the water.

Microbialite is a rock or benthic sedimentary deposit made of carbonate mud that is formed with the mediation of microbes. The constituent carbonate mud is a type of automicrite, or authigenic carbonate mud, and therefore it precipitates in situ instead of being transported and deposited. Been formed in situ, a microbialite can be seen as a type of boundstone where reef builders are microbes, and precipitation of carbonate is biotically induced instead of forming tests, shells or skeletons. Bacteria can precipitate carbonate both in shallow and in deep water and so microbialites can form regardless of the sun light.

References

1 2 Erik., Flügel (2010). Microfacies of carbonate rocks : analysis, interpretation and application. Munnecke, Axel. (2nd ed.). Heidelberg: Springer. ISBN 9783642037962. OCLC 663093942.
↑ Keim, Lorenz; Schlager, Wolfgang (1999). "Automicrite facies on steep slopes (Triassic, Dolomites, Italy)". Facies. 41 (1): 15–25. doi:10.1007/bf02537457. ISSN 0172-9179.
1 2 3 4 Sam., Boggs (2009). Petrology of sedimentary rocks (2nd ed.). Cambridge: Cambridge University Press. ISBN 9780511516429. OCLC 500960599.
1 2 Wolfgang, Schlager (2005). Carbonate Sedimentology and Sequence Stratigraphy. SEPM (Society for Sedimentary Geology). Tulsa, OK. ISBN 978-1565761162. OCLC 61364867.

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[:0-1] 1 2 Erik., Flügel (2010). Microfacies of carbonate rocks : analysis, interpretation and application. Munnecke, Axel. (2nd ed.). Heidelberg: Springer. ISBN 9783642037962. OCLC 663093942.

[2] Keim, Lorenz; Schlager, Wolfgang (1999). "Automicrite facies on steep slopes (Triassic, Dolomites, Italy)". Facies. 41 (1): 15–25. doi:10.1007/bf02537457. ISSN 0172-9179.

[:1-3] 1 2 3 4 Sam., Boggs (2009). Petrology of sedimentary rocks (2nd ed.). Cambridge: Cambridge University Press. ISBN 9780511516429. OCLC 500960599.

[:2-4] 1 2 Wolfgang, Schlager (2005). Carbonate Sedimentology and Sequence Stratigraphy. SEPM (Society for Sedimentary Geology). Tulsa, OK. ISBN 978-1565761162. OCLC 61364867.