A carbonate platform is a

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by

sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...

organisms whose skeletons build up the

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...

or by organisms (usually

microbes A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

) which induce

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

precipitation through their

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...

. 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 Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

, transparency and pH-Value. For example, carbonate sedimentation along the

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...

n coasts takes place everywhere but at the mouth of the Amazon River, because of the intense

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids ...

of the water there. Spectacular examples of present-day carbonate platforms are the

Bahama Banks The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island ...

under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

platform, the platform on which the

Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...

is growing, and the

Maldive Maldives (, ; dv, ދިވެހިރާއްޖެ, translit=Dhivehi Raajje, ), officially the Republic of Maldives ( dv, ދިވެހިރާއްޖޭގެ ޖުމްހޫރިއްޔާ, translit=Dhivehi Raajjeyge Jumhooriyyaa, label=none, ), is an archipelag ...

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gr ...

s. All these carbonate platforms and their associated

s are confined to tropical latitudes. Today's reefs are built mainly by

scleractinian corals Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mo ...

, but in the distant past other organisms, like

archaeocyatha Archaeocyatha (or archaeocyathids 'ancient cups') is a taxon of extinct, sessile, reef-building marine sponges that lived in warm tropical and subtropical waters during the Cambrian Period. It is believed that the centre of the Archaeocyatha or ...

(during the Cambrian) or extinct

cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

(

tabulata Tabulata, commonly known as tabulate corals, are an order of extinct forms of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to ...

and

rugosa The rugosa, also called the tetracorallia or horn coral, are an extinct order of solitary and colonial corals that were abundant in Middle Ordovician to Late Permian seas. Solitary rugosans (e.g., '' Caninia'', '' Lophophyllidium'', '' Neoza ...

) were important reef builders.

Carbonate precipitation from seawater

What makes carbonate platform environments different from other depositional environments is that carbonate is a product of precipitation, rather than being a sediment transported from elsewhere, as for sand or gravel. This implies for example that carbonate platforms may grow far from the coastlines of continents, as for the Pacific atolls. The

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

ogic composition of carbonate platforms may be either calcitic or aragonitic.

Seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...

is oversaturated in carbonate, so under certain conditions CaCO₃ precipitation is possible. Carbonate precipitation is thermodynamically favoured at high temperature and low

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

. Three types of carbonate precipitation are possible: ''biotically controlled'', ''biotically induced'' and ''abiotic''. Carbonate precipitation is biotically controlled when organisms (such as corals) are present that exploit carbonate dissolved in seawater to build their calcitic or aragonitic skeletons. Thus they may develop hard reef structures. Biotically induced precipitation takes place outside the cell of the organism, thus carbonate is not directly produced by organisms, but precipitates because of their metabolism. Abiotic precipitation, by definition, involves little or no biological influence.

Classification

The three types of precipitation (abiotic, biotically induced and biotically controlled) cluster into three "carbonate factories". A carbonate factory is the ensemble of the sedimentary environment, the intervening organisms and the precipitation processes that lead to the formation of a carbonate platform. The differences between three factories is the dominant precipitation pathway and skeletal associations. In contrast, a carbonate platform is a geological structure of parautochotonous carbonate sediments and carbonate rocks, having a morphological relief.

Platforms produced by the "tropical factory"

In these carbonate factories, precipitation is biotically controlled, mostly by

autotrophic An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

organisms. Organisms that build this kind of platforms are today mostly

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...

s and green algae, that need sunlight for photosynthesis and thus live in the

euphotic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...

(i.e., shallow water environments in which sunlight penetrates easily). Tropical carbonate factories are only present today in warm and sunlit waters of the tropical-subtropical belt, and they have high carbonate production rates but only in a narrow depth window. The depositional profile of a Tropical factory is called "rimmed" and includes three main parts: a

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') ...

, a

and a slope. In the reef, the framework produced by large-sized skeletons, as those of corals, and by encrusting organisms resists wave action and forms a rigid build up that may develop up to sea-level. The presence of a rim produces restricted circulation in the back reef area and a lagoon may develop in which carbonate mud is often produced. When reef accretion reaches the point that the foot of the reef is below wave base, a slope develops: the sediments of the slope derive from the erosion of the margin by waves, storms and gravitational collapses. This process accumulates coral debris in clinoforms. The maximum angle that a slope can achieve is the settlement angle of gravel (30–34°).

Platforms produced by the "cool-water factory"

In these carbonate factories, precipitation is biotically controlled by

heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

ic organisms, sometimes in association with photo-autotrophic organisms such as red algae. The typical skeletal association includes foraminifers, red algae and

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

. Despite being autotrophic, red algae are mostly associated to heterotrophic carbonate producers, and need less light than green algae. The range of occurrence of cool-water factories extends from the limit of the tropical factory (at about 30◦) up to polar latitudes, but they could also occur at low latitudes in the thermocline below the warm surface waters or in upwelling areas. This type of factories has a low potential of carbonate production, is largely independent from sunlight availability, and can sustain a higher amount of nutrients than tropical factories. Carbonate platforms built by the "cool-water factory" show two types of geometry or depositional profile, i.e., the homoclinal ramp or the distally-steepened ramp. In both geometries there are three parts: the inner ramp above the fair weather wave base, the middle ramp, above the storm wave base, the outer ramp, below the storm wave base. In distally steepened ramps, a distal step is formed between the middle and outer ramp, by the in situ accumulation of gravel-sized carbonate grains

Platforms produced by the "mud-mound factory"

These factories are characterised by abiotic precipitation and biotically induced precipitation. The typical environmental settings where "mud-mound factories" are found in the Phanerozoic are dysphotic or aphotic, nutrient-rich waters that are low in oxygen but not

anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...

. These conditions often prevail in the thermocline, for example at intermediate water depths below the ocean's

mixed layer The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...

. The most important component of these platforms is fine-grained carbonate that precipitates in situ (

automicrite ''Automicrite'' is autochthonous micrite, that is, a Carbonate minerals, 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 sedi ...

) by a complex interplay of biotic and abiotic reactions with microbes and decaying organic tissue. Mud-mound factories do not produce a skeletal association, but they have specific facies and microfacies, for example

stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). T ...

s, that are laminated

microbialite Microbialite is a benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbes. The constituent carbonate mud is a type of

s, and

thrombolite Thrombolites (from Ancient Greek θρόμβος ''thrómbos'' meaning " clot" and λῐ́θος ''líthos'' meaning "stone") are clotted accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary g ...

s, that are

microbialite Microbialite is a benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbes. The constituent carbonate mud is a type of

s characterized by clotted peloidal fabric at the microscopic scale and by dendroid fabric at the hand-sample scale. The geometry of these platforms is mound-shaped, where all the mound is productive, including the slopes.

Geometry of carbonate platforms

Several factors influence the geometry of a carbonate platform, including inherited topography, synsedimentary

tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

, exposure to currents and

trade winds The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisp ...

. Two main types of carbonate platforms are distinguished on the base of their geographic setting: isolated (as Maldives atolls) or epicontinental (as the

Belize Belize (; bzj, Bileez) is a Caribbean and Central American country on the northeastern coast of Central America. It is bordered by Mexico to the north, the Caribbean Sea to the east, and Guatemala to the west and south. It also shares a wate ...

reefs or the Florida Keys). However, the one most important factor influencing geometries is perhaps the type of carbonate factory. Depending on the dominant carbonate factory, we can distinguish three types of carbonate platforms: T-type carbonate platforms (produced by "tropical factories"), C-type carbonate platforms (produced by "cool-water factories"), M-type carbonate platforms ("produced by mud-mound factories"). Each of them has its own typical geometry.

T-type carbonate platforms

The depositional profile of T-type carbonate platforms can be subdivided into several sedimentary environments. The carbonate hinterland is the most landward environment, composed by weathered

carbonate rocks 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 rock (also known as doloston ...

. The evaporitic

tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal fl ...

is a typical low-energy environment. The internal

, as the name suggests, is the part of platform behind the reef. It is characterised by shallow and calm waters, and so it is a low-energy sedimentary environment. Sediments are composed by reef fragments, hard parts of organisms and, if the platform is epicontinental, also by a terrigenous contribution. In some lagoons (e.g., the

Florida Bay Florida Bay is the bay located between the southern end of the Florida mainland (the Florida Everglades) and the Florida Keys in the United States. It is a large, shallow estuary that while connected to the Gulf of Mexico, has limited exchange o ...

), green algae produce great volumes of carbonate mud. Rocks here are mudstones to

grainstone Under the Dunham classification (Dunham, 1962) system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as ''a carbonate-dominated ...

s, depending on the energy of the environment. The

is the rigid structure of carbonate platforms and is located between the internal lagoon and the slope, in the platform margin, in which the framework produced by large-sized skeletons, as those of corals, and by encrusting organisms will resist wave action and form a rigid build up that may develop up to sea-level. Survival of the platform depends on the existence of the reef, because only this part of the platform can build a rigid, wave-resistant structure. The reef is created by essentially in-place,

organisms. Today's reefs are mostly built by hermatypic corals. Geologically speaking, reef rocks can be classified as massive boundstones. The

slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...

is the outer part of the platform, connecting the reef with the basin. This depositional environment acts as sink for excess carbonate sediment: most of the sediment produced in the lagoon and reef is transported by various processes and accumulates in the slope, with an inclination depending on the grain size of sediments, and that could attain the settlement angle of gravel (30-34°) at most. The slope contains coarser sediments than the reef and lagoon. These rocks are generally

rudstone Rudstone is a type of carbonate rock. The Dunham classification (Dunham, 1962) did not consider grain size as a criterion for the description of carbonate lithologies. In an attempt to rectify this perceived deficiency, Embry & Klovan (1971) intr ...

s or

s. The periplatform basin is the outermost part of the t-type carbonate platform, and carbonate sedimentation is there dominated by density-cascating processes. The presence of a rim damps the action of waves in the back reef area and a lagoon may develop in which carbonate mud is often produced. When reef accretion reaches the point that the foot of the reef is below wave base, a slope develops: the sediments of the slope derive from the erosion of the margin by waves, storms and gravitational collapses. This process accumulates coral debris in clinoforms. Clinoforms are

beds A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many be ...

that have a sigmoidal or tabular shape, but are always deposited with a primary inclination. The size of a T-type carbonate platform, from the hinterland to the foot of the slope, can be of tens of kilometers.

C-type carbonate platforms

C-type carbonate platforms are characterized by the absence of early cementation and

lithification Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificatio ...

, and so the sediment distribution is only driven by

waves Waves most often refers to: *Waves, oscillations accompanied by a transfer of energy that travel through space or mass. *Wind waves, surface waves that occur on the free surface of bodies of water. Waves may also refer to: Music *Waves (band) ...

and, in particular, it occurs above the wave base. They show two types of geometry or depositional profile, i.e., the homoclinal ramp or the distally-steepened ramp. In both geometries there are three parts. In the inner ramp, above the fair weather wave base, the carbonate production is slow enough that all sediments may be transported offshore by waves, currents and storms. As a consequence, the shoreline may be retreating, and so in the inner ramp there may be a cliff caused by erosional processes. In the middle ramp, between the fair weather wave base and the storm wave base, carbonate sediment remains in place and can be reworking only by the storm waves. In the outer ramp, below the storm wave base, fine sediments may accumulate. In distally steepened ramps, a distal step is formed between the middle and outer ramp, by the in situ accumulation of gravel-sized carbonate grains (e.g.,

rhodolith Rhodoliths (from Greek for ''red rocks'') are colorful, unattached calcareous nodules, composed of crustose, benthic marine red algae that resemble coral. Rhodolith beds create biogenic habitat for diverse benthic communities. The rhodolithic gr ...

s) only episodically moved by currents. Carbonate production occurs along the full depositional profile in this type of carbonate platforms, with an extra production in the outer part of the middle ramp, but carbonate production rates are always less than in the T-type carbonate platforms.

M-type carbonate platforms

M-type carbonate platforms are characterized by an inner platform, an outer platform, an upper slope made by microbial boundstone, and a lower slope often made by

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

. The slope may be steeper than the angle of repose of gravels, with an inclination that may attain 50°. In the M-type carbonate platforms the carbonate production mostly occurs on the upper slope and in the outer part of the inner platform.

Carbonate platforms in the geological record

Sedimentary sequences show carbonate platforms as old as the Precambrian, when they were formed by stromatolitic sequences. In the Cambrian carbonate platforms were built by

. During

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

(richtofenida) and

stromatoporoidea Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Ordovician through the Devonian. They were especially abundant and important reef-formers in the Silurian and most of the Devonian.Stock, C.W. 2001, Stroma ...

reefs were erected. At the middle of the Paleozoic era corals became important platforms builders, first with

(from the Silurian) and then with

(from the Devonian).

Scleractinia Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a ...

become important reef builders beginning only in the

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227 m ...

(upper

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

). Some of the best examples of carbonate platforms are in the

Dolomites The Dolomites ( it, Dolomiti ; Ladin: ''Dolomites''; german: Dolomiten ; vec, Dołomiti : fur, Dolomitis), also known as the Dolomite Mountains, Dolomite Alps or Dolomitic Alps, are a mountain range located in northeastern Italy. They form pa ...

, deposited during the Triassic. This region of the Southern

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Swi ...

contains many well preserved isolated carbonate platforms, including the Sella, Gardenaccia, Sassolungo and

Latemar The Latemar is a mountain in the Dolomites on the border between South Tyrol and Trentino, Italy. It is a popular hiking destination. It is best known for its view from the Karersee Lake Carezza ( it, Lago di Carezza; german: Karersee) is a s ...

. The middle Liassic "bahamian type" carbonate platform of Morocco (Septfontaine, 1985) is characterised by the accumulation of autocyclic regressive cycles, spectacular supratidal deposits and

vadose The vadose zone, also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone, the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure ("vadose" is fr ...

diagenetic Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

features with

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

tracks. The Tunisian coastal "chotts" and their cyclic muddy deposits represent a good recent equivalent (Davaud & Septfontaine, 1995). Such cycles were also observed on the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

Arabic platform, Oman and Abu Dhabi (Septfontaine & De Matos, 1998) with the same microfauna of foraminifera in an almost identical biostratigraphic succession. Bloc diagramme Lias Haut Atlas

In the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

period there were platforms built by

bivalvia Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biva ...

(

rudists Rudists are a group of extinct box-, tube- or ring-shaped marine heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organis ...

Sequence stratigraphy of carbonate platforms

With respect to the

sequence stratigraphy Sequence stratigraphy is a branch of geology, specifically a branch of stratigraphy, that attempts to discern and understand historic geology through time by subdividing and linking sedimentary deposits into unconformity bounded units on a variety ...

of siliciclastic systems, carbonate platforms present some peculiarities, which are related to the fact that carbonate sediment is precipitated directly on the platform, mostly with the intervention of living organisms, instead of being only transported and deposited. Among these peculiarities, carbonate platforms may be subject to drowning, and may be the source of sediment via highstand shedding or slope shedding.

Drowning

Drowning of a carbonate platform is an event where the

relative sea level Relative sea level (abbreviated as RSL) is defined as the sea level that is observed with respect to a land-based reference frame. It is often contrasted with eustatic sea level, which is a measure of the total mass or volume of the oceans. Relative ...

rise is faster than the accumulation rate on a carbonate platform, which eventually leads to the platform to submerge below the

. In the

geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sand ...

of a drowned carbonate platform,

neritic The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminate ...

deposits change rapidly into deep-marine sediments. Typically

hardground Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor (Wilson and Palmer, 1992). A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone ...

s with

ferromanganese Ferromanganese is a ferroalloy with high manganese content (high-carbon ferromanganese can contain as much as 80% Mn by weight). It is made by heating a mixture of the oxides MnO2 and Fe2O3, with carbon (usually as coal and coke) in either a bla ...

oxides,

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

glauconite Glauconite is an iron potassium phyllosilicate ( mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Gre ...

crusts lie in between of

and deep-marine sediments. Several drowned carbonate platforms have been found in the geologic record. However, it has not been very clear how the drowning of carbonate platforms exactly happen. Modern carbonate platforms and reefs are estimated to grow approximately 1,000 μm/yr, possibly several times faster in the past. 1,000 μm/yr growth rate of carbonates exceeds by orders of

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

any

rise that is caused by long-term subsidence, or changes in

eustatic sea level The eustatic sea level is the distance from the center of the earth to the sea surface. An increase of the eustatic sea level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or more mid-oceanic ridges. ...

. Based on the rates of these processes, drowning of the carbonate platforms should not be possible, which causes "the paradox of drowned carbonate platforms and reefs". Since drowning of carbonate platforms requires exceptional rise in the

, only limited number of processes can cause it. According to Schlager, only anomalously quick rise of

or benthic growth reduction caused by deteriorating changes in the environment could explain the drowning of platforms. For instance, regional downfaulting, submarine

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

or glacioeustacy could be the reason for rapid rise in

, whereas for example changes in oceanic salinity might cause the environment to become deteriorative for the carbonate producers. One example of a drowned carbonate platform is located in

Huon Gulf Huon Gulf is a large gulf in eastern Papua New Guinea. It is bordered by Huon Peninsula in the north. Both are named after French explorer Jean-Michel Huon de Kermadec. Huon Gulf is a part of the Solomon Sea. Its northern boundary is marked by ...

Papua New Guinea Papua New Guinea (abbreviated PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini), officially the Independent State of Papua New Guinea ( tpi, Independen Stet bilong Papua Niugini; ho, Independen Stet bilong Papua Niu Gini), is a country i ...

. It is believed to be drowned by rapid sea level rise caused by deglaciation and subsidence of the platform, which enabled coralline algal- foraminiferal nodules and halimeda

limestones Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms when t ...

to cover the

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. C ...

s. Plate movements carrying carbonate platforms to

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

s unfavourable for carbonate production are also suggested to be one of the possible reasons for drowning. For example,

guyot In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed .Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...

an and Mariana Islands are believed to be transported to low southern

s (0-10°S) where equatorial

upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...

occurred. High amounts of nutrients and higher productivity caused decrease in water transparency and increase in bio-eroders populations, which reduced carbonate accumulation and eventually led to drowning.

Highstand shedding

Highstand A raised shoreline is an ancient shoreline exposed above current water level. These landforms are formed by a relative change in sea level due to global sea level rise, isostatic rebound, and/or tectonic uplift. These surfaces are usually exp ...

shedding is a process in which a carbonate platform produces and sheds most of the sediments into the adjacent basin during highstands of sea level. This process has been observed on all rimmed carbonate platforms in the Quaternary, such as the

Great Bahama Bank The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island ...

. Flat topped, rimmed platforms with steep slopes show more pronounced highstand shedding than platforms with gentle slopes and cool water carbonate systems. Highstand shedding is pronounced on tropical carbonate platforms because of the combined effect of sediment production and

diagenesis Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

. Sediment production of a platform increases with its size, and during

highstand A raised shoreline is an ancient shoreline exposed above current water level. These landforms are formed by a relative change in sea level due to global sea level rise, isostatic rebound, and/or tectonic uplift. These surfaces are usually exp ...

the top of the platform is flooded and the productive area is bigger compared to the lowstand conditions, when only a minimal part of the platform is available for production. The effect of increased highstand production is enhanced by the rapid

of carbonate during lowstands, because the exposed platform top is

karstified Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

rather than eroded, and does not export sediment.

Slope shedding

Slope shedding is a process typical of microbial platforms, in which the carbonate production is nearly independent from sea level oscillations. The carbonate factory, composed of microbial communities precipitating

microbialite Microbialite is a benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbes. The constituent carbonate mud is a type of

s, is insensitive to light and can extend from the platform break down the slope to hundreds of meters in depth. Sea level drops of any reasonable amplitude would not significantly affect the slope production areas. Microbial boundstone slope systems are remarkably different from tropical platforms in sediment productions profiles, slope readjustment processes and sediment sourcing. Their progradation is independent from platform sediment shedding and largely driven by slope shedding. Examples of margins that may be affected of slope shedding that are characterized by various contributions of microbial carbonate growth to the upper slope and margin, are: * the Canning Basin in Australia * the Guilin platform in the southern China * the Permian of the US Permian Basin * the middle Triassic carbonate platforms of the

Gallery

File:Cycle émersif Maroc.jpg, "Shallowing upward" cycle in the middle Lias of the high Atlas (Morocco). Algal dolomitized laminations on top. File:Cycle émersif Oman.jpg, "Shallowing upward" cycles in the lagoonal Lias of the Musandam Peninsula. (N-Oman). File:Cycle émersif Musandam.jpg, "Shallowing upward" liassic cycles arranged in decametric sequences, Musandam Peninsula, (N-Oman). File:Cycle émersif Laghdar.jpg, "Shallowing upward" cycle in the Middle Jurassic (Saghtan form.) of the jbel Laghdar Range (Oman). File:Desiccation Rnim.jpg, Desiccation figures on top of a regressive sequence; Middle liassic, High Atlas, Morocco. File:Calcretes et ammonites.jpg, Ammonites and belemnites washed over a supratidal surface (calcretes and "

teepees A tipi , often called a lodge in English, is a conical tent, historically made of animal hides or pelts, and in more recent generations of canvas, stretched on a framework of wooden poles. The word is Siouan, and in use in Dakhótiyapi, Lakȟó ...

"); Middle Liassic of the High Atlas, Morocco. File:Brèche de tempête.jpg, Hurricane breccia cemented (early diagenesis) at the surface of a bed, top of a regressive, metric, sequence. Middle Lias, High Atlas. File:Pisolithes vadoses.jpg, Vadose ferrugenous pisolites (soil) and coastal (tempestite) sediment with birdseyes in an outer platform environment. Aerial diagenesis. Middle liassic, High Atlas, Morocco. File:Keystone vugs meniscus.jpg, Meniscus and point contact cement in a marine grainstone with displaced foraminifera (by tide and hurricanes) on the supratidal flat of the middle liassic platform of Morocco. Top of emersive cycle. Middle Atlas. File:Calcretes remaniés dolomie.jpg, Reworked calcretes concretions from the supratidal environment in a marine (dolomitised) sediment displaced by hurricanes on the inner platform flat. Top of emersive sequence. High Atlas, Morocco. File:Ciment stalactite vadose.jpg, Stalactitic cement in sediment from the supratidal zone, vadose environment, top of "shallowing upward" sequence. Middle Liassic, High Atlas. Thin section. L = 0.3 mm. File:Traces dino géant.jpg, Giant dinosaur tracks (sauropod) on top of a regressive sequence, Middle Liassic, High Atlas, Morocco. File:Calcretes et birdseyes.jpg, Vadose stalactitic cement filling an horizontal cavity in a marine coastal sediment, outer platform. Birdseyes in the allodapic (tidal or tempestite) grainstone point to an aerial diagenesis. High Atlas, Morocco. File:Séquences Lias Todhra.jpg, Autocyclic filling (metric to hectometric) sequences in the Middle Liassic lagoon, South (Todhra) of the High Atlas, Morocco. File:Teepee supratidal flat.jpg, "Teepee" structure, due to increasing sediment volume by dolomitisation on the inner platform supratidal flat. Top of emersive cycle. Middle Lias, High Atlas. File:Carottes chott tunisien.jpg, Quaternary to recent equivalent of a "shallowing upward sequence", cores in a Tunisian "chott", intertidal laminations in yellow. File:Tunisie teepee.jpg, Recent "teepee" structures in a Tunisian salt lagoon, "chott". File:Tunisie carottes Zarzis.jpg, Recent equivalents of "shallowing upward sequences", cores in a Tunisian salt lagoon, "chott". File:Tunisie gypse.jpg, Top of a regressive sequence with algal laminations (yellow) and crystallised gypsum, salt lagoon "chott", Tunisia. File:Tunisie tempête.jpg, Eolian bioclastic (calcareous algae and porcellaneous foraminifera) sand dune on Tunisian shore.

Footnotes

References

* Davaud E. & Septfontaine M. (1995): "Post-mortem onshore transportation of epiphityc foraminifera: recent example from the Tunisian coast line". ''Jour. Sediment. Research'', 65/1A, 136–142. * Bosellini A., 1984, "Progradation geometries of carbonate platforms: examples from the Triassic of the Dolomites, northern Italy". ''Sedimentology'', Vol. 31, pp. 1–24
"Bahamas Introduction" (Accessed 3/8/06)

* Pinet P. R., 1996, ''Invitation to Oceanography''. St. Paul: West Publishing Company, * Septfontaine M. 1985, "Depositional environments and associated foraminifera (lituolids) in the middle liassic carbonate platform of Morocco." ''Rev. de Micropal.'' 28/4 265–289. See also www.palgeo.ch/publications. {{Physical oceanography, state=collapsed Carbonates Sedimentology Marine geology . Submarine topography Sedimentary rocks