Carbonate platform

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The Bahama Banks are an example of a carbonate platform Bahamabank.jpg
The Bahama Banks are an example of a carbonate platform

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. [1] Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually microbes) 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. [2] 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, [3] the platform on which the Great Barrier Reef is growing, and the Maldive atolls. [4] All these carbonate platforms and their associated reefs are confined to tropical latitudes. [5] Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.

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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. [1] [6] This implies for example that carbonate platforms may grow far from the coastlines of continents, as for the Pacific atolls.

The mineralogic composition of carbonate platforms may be either calcitic or aragonitic. Seawater is oversaturated in carbonate, so under certain conditions CaCO3 precipitation is possible. Carbonate precipitation is thermodynamically favoured at high temperature and low pressure. 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. [6]

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. [6]

Platforms produced by the "tropical factory"

In these carbonate factories, precipitation is biotically controlled, mostly by autotrophic organisms. Organisms that build this kind of platforms are today mostly corals and green algae, that need sunlight for photosynthesis and thus live in the euphotic zone (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. [6] The depositional profile of a Tropical factory is called "rimmed" and includes three main parts: a lagoon, a reef 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. [7] 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. [6] [7] This process accumulates coral debris in clinoforms. The maximum angle that a slope can achieve is the settlement angle of gravel (30–34°). [8]

Platforms produced by the "cool-water factory"

In these carbonate factories, precipitation is biotically controlled by heterotrophic organisms, sometimes in association with photo-autotrophic organisms such as red algae. The typical skeletal association includes foraminifers, red algae and molluscs. 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. [9] 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 [9]

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. These conditions often prevail in the thermocline, for example at intermediate water depths below the ocean's mixed layer. [6] The most important component of these platforms is fine-grained carbonate that precipitates in situ (automicrite) by a complex interplay of biotic and abiotic reactions with microbes and decaying organic tissue. [6] Mud-mound factories do not produce a skeletal association, but they have specific facies and microfacies, for example stromatolites, that are laminated microbialites, and thrombolites, that are microbialites 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. [6]

Geometry of carbonate platforms

Several factors influence the geometry of a carbonate platform, including inherited topography, synsedimentary tectonics, exposure to currents and trade winds. Two main types of carbonate platforms are distinguished on the base of their geographic setting: isolated (as Maldives atolls) or epicontinental (as the Belize 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. [6]

Generalized cross-section of a typical carbonate platform. CARBONATIC PLATFORM EN.PNG
Generalized cross-section of a typical carbonate platform.

T-type carbonate platforms

The depositional profile of T-type carbonate platforms can be subdivided into several sedimentary environments. [1]

The carbonate hinterland is the most landward environment, composed by weathered carbonate rocks. The evaporitic tidal flat is a typical low-energy environment.

An example of carbonate mud sedimentation in the internal part of the Florida Bay lagoon. The presence of young mangroves is important to entrap the carbonate mud. FangoCarb1.jpg
An example of carbonate mud sedimentation in the internal part of the Florida Bay lagoon. The presence of young mangroves is important to entrap the carbonate mud.

The internal lagoon, 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), green algae produce great volumes of carbonate mud. Rocks here are mudstones to grainstones, depending on the energy of the environment.

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

The slope 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. [8] The slope contains coarser sediments than the reef and lagoon. These rocks are generally rudstones or grainstones.

The periplatform basin is the outermost part of the t-type carbonate platform, and carbonate sedimentation is there dominated by density-cascating processes. [10]

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 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. [6]

C-type carbonate platforms

C-type carbonate platforms are characterized by the absence of early cementation and lithification, and so the sediment distribution is only driven by waves 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., rhodoliths) 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. [7] [6]

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. 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. [7] [11]

The Cimon del Latemar (Trento province, Dolomites, northern Italy) represents the internal lagoon of a fossil carbonate platform. Continuous sedimentation took place in an environment as the one described in the image of the Florida Bay and, given a strong subsidence, led to the formation of a sedimentary series that therefore acquired considerable thickness. Bigcimon.jpg
The Cimon del Latemar (Trento province, Dolomites, northern Italy) represents the internal lagoon of a fossil carbonate platform. Continuous sedimentation took place in an environment as the one described in the image of the Florida Bay and, given a strong subsidence, led to the formation of a sedimentary series that therefore acquired considerable thickness.

Carbonate platforms in the geological record

The Calcaires du Bou Dahar record a well preserved carbonate platform, including it's sedimentary cycles and facies. Jbel Bou Dahar early Mesozoic map.jpg
The Calcaires du Bou Dahar record a well preserved carbonate platform, including it´s sedimentary cycles and facies.

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 archaeocyatha. During Paleozoic brachiopod (richtofenida) and stromatoporoidea reefs were erected. At the middle of the Paleozoic era corals became important platforms builders, first with tabulata (from the Silurian) and then with rugosa (from the Devonian). Scleractinia become important reef builders beginning only in the Carnian (upper Triassic). Some of the best examples of carbonate platforms are in the Dolomites, deposited during the Triassic. This region of the Southern Alps contains many well preserved isolated carbonate platforms, including the Sella, Gardenaccia, Sassolungo and Latemar. The middle Liassic "bahamian type" carbonate platform was notoriously widespread in the western Tethys Ocean, including the Rotzo Formation on Italy, the Aganane Formation & the Calcaires du Bou Dahar of Morocco (Septfontaine, 1985) is characterised by the accumulation of autocyclic regressive cycles, spectacular supratidal deposits and vadose diagenetic features with dinosaur tracks. [12] In the same area the overlying Tafraout Group (Toarcian-Aalenian) records a massive Siliclastic-Carbonate Platform. [13] Other coeval records of platforms are seen in the Adriatic Platform, in areas such as the Dinaric Alps (The Budoš Limestone of Montenegro), Rotzo or the Podpeč Limestone of Slovenia. [14] Others regional records include examples like the Coimbra Formation (Sinemurian) in the Lusitanian Basin. [15] The Aalenian Iberian Platform includes Volcanic intrusions that led to ephemeral islands, like in the El Pedregal Formation. [16] The Cretaceous (Campanian) Calcare di Aurisina in Italy, records a series of islands adjacent to a shallow carbonate sea in a platform built by bivalvia (rudists). [17] 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 Arabic platform, Oman and Abu Dhabi (Septfontaine & De Matos, 1998) with the same microfauna of foraminifera in an almost identical biostratigraphic succession.

Aganane Formation platform of Morocco with first order autocyclic regressive cycles Bloc diagramme Lias Haut Atlas.jpg
Aganane Formation platform of Morocco with first order autocyclic regressive cycles
Aganane Formation metre-scale peritidal sedimentary cycles in two outcrops. The two outcrops are 230 km apart. Storm beds and possibly tsunamites include abundant reworked foraminifera. This image is an example of the continuity of peritidal cycles in a carbonate platform environment. Cycles regressifs de comblement.jpg
Aganane Formation metre-scale peritidal sedimentary cycles in two outcrops. The two outcrops are 230 km apart. Storm beds and possibly tsunamites include abundant reworked foraminifera. This image is an example of the continuity of peritidal cycles in a carbonate platform environment.
Virtual metric "shallowing upward sequence" observed all along (more than 10,000 km) the south Tethyan margin during middle Liassic times. The (micro)fossils are identical till Oman and beyond. Cycle virtuel emersif.jpg
Virtual metric "shallowing upward sequence" observed all along (more than 10,000 km) the south Tethyan margin during middle Liassic times. The (micro)fossils are identical till Oman and beyond.

Sequence stratigraphy of carbonate platforms

With respect to the sequence stratigraphy 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. [1] Among these peculiarities, carbonate platforms may be subject to drowning, and may be the source of sediment via highstand shedding or slope shedding. [6]

Drowning

Drowning of a carbonate platform is an event where the relative sea level rise is faster than the accumulation rate on a carbonate platform, which eventually leads to the platform to submerge below the euphotic zone. [18] In the geologic record of a drowned carbonate platform, neritic deposits change rapidly into deep-marine sediments. Typically hardgrounds with ferromanganese oxides, phosphate or glauconite crusts lie in between of neritic and deep-marine sediments. [18]

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 any relative sea level rise that is caused by long-term subsidence, or changes in eustatic sea level. 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". [18]

Since drowning of carbonate platforms requires exceptional rise in the relative sea level, only limited number of processes can cause it. According to Schlager, [18] only anomalously quick rise of relative sea level or benthic growth reduction caused by deteriorating changes in the environment could explain the drowning of platforms. For instance, regional downfaulting, submarine volcanism or glacioeustasy could be the reason for rapid rise in relative sea level, whereas for example changes in oceanic salinity might cause the environment to become deteriorative for the carbonate producers. [18]

One example of a drowned carbonate platform is located in Huon Gulf, Papua New Guinea. 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 to cover the coral reefs. [19]

Plate movements carrying carbonate platforms to latitudes unfavourable for carbonate production are also suggested to be one of the possible reasons for drowning[ further explanation needed ]. [18] [7] For example, guyots located in the Pacific Basin between Hawaiian and Mariana Islands are believed to be transported to low southern latitudes (0-10°S) where equatorial upwelling occurred. [7] 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[ further explanation needed ]. [7] [20]

Highstand shedding

Highstand shedding and slope shedding SlopeShedding.png
Highstand shedding and slope shedding

Highstand 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. Flat topped, rimmed platforms with steep slopes show more pronounced highstand shedding than platforms with gentle slopes and cool water carbonate systems. [21]

Highstand shedding is pronounced on tropical carbonate platforms because of the combined effect of sediment production and diagenesis. [6] Sediment production of a platform increases with its size, and during highstand 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. [6] The effect of increased highstand production is enhanced by the rapid lithification of carbonate during lowstands, because the exposed platform top is karstified rather than eroded, and does not export sediment. [6]

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 microbialites, 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. [11]

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:

See also

Footnotes

  1. 1 2 3 4 Wilson, James Lee (1975). Carbonate facies in geologic history. Berlin: Springer-Verlag. ISBN   978-0387072364. OCLC   1366180.
  2. Carannante, G.; Esteban, M.; Milliman, J. D.; Simone, L. (1988-11-01). "Carbonate lithofacies as paleolatitude indicators: problems and limitations". Sedimentary Geology. Non-tropical shelf carbonates-modern and ancient. 60 (1): 333–346. Bibcode:1988SedG...60..333C. doi:10.1016/0037-0738(88)90128-5. ISSN   0037-0738.
  3. Geologic Map of Florida [ permanent dead link ]
  4. "Bahamas Introduction". www.tamug.edu. Archived from the original on 2009-11-22. Retrieved 2006-03-09.
  5. "ReefGIS - Location of Coral Reefs - Reef Basemap". Archived from the original on 2008-05-16. Retrieved 2007-03-12.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Schlager, Wolfgang (2005). Carbonate sedimentology and sequence stratigraphy. SEPM Concepts in Sedimentology and Paleontology. ISBN   978-1565761162.
  7. 1 2 3 4 5 6 7 Pomar, L. (September 2001). "Types of carbonate platforms: a genetic approach". Basin Research. 13 (3): 313–334. Bibcode:2001BasR...13..313P. doi:10.1046/j.0950-091x.2001.00152.x. S2CID   129859376.
  8. 1 2 Kenter, Jeroen A. M. (1990). "Carbonate platform flanks: slope angle and sediment fabric". Sedimentology. 37 (5): 777–794. Bibcode:1990Sedim..37..777K. doi:10.1111/j.1365-3091.1990.tb01825.x. ISSN   1365-3091.
  9. 1 2 Pomar, L.; Hallock, P. (2008-03-01). "Carbonate factories: A conundrum in sedimentary geology". Earth-Science Reviews. 87 (3–4): 134–169. Bibcode:2008ESRv...87..134P. doi:10.1016/j.earscirev.2007.12.002. ISSN   0012-8252.
  10. Roberts, Harry H.; Wilson, Paul A. (1992-08-01). "Carbonate-periplatform sedimentation by density flows: A mechanism for rapid off-bank and vertical transport of shallow-water fines". Geology. 20 (8): 713–716. Bibcode:1992Geo....20..713W. doi:10.1130/0091-7613(1992)020<0713:CPSBDF>2.3.CO;2. ISSN   0091-7613.
  11. 1 2 3 Kenter, Jeroen A.M.; Harris, Paul M. (Mitch); Della Porta, Giovanna (2005-07-01). "Steep microbial boundstone-dominated platform margins – examples and implications". Sedimentary Geology. 178 (1–2): 5–30. Bibcode:2005SedG..178....5K. doi:10.1016/j.sedgeo.2004.12.033. ISSN   0037-0738.
  12. Burgess, C. J.; Lee, C. W. (1978). "The development of a Lower Jurassic carbonate tidal flat, central High Atlas, Morocco; 1, Sedimentary history". Journal of Sedimentary Research. 48 (3): 777–793. Retrieved 20 April 2023.
  13. Krencker, F.-N.; Fantasia, A.; El Ouali, M.; Kabiri, L.; Bodin, S. (2022). "The effects of strong sediment-supply variability on the sequence stratigraphic architecture: Insights from early Toarcian carbonate factory collapses". Marine and Petroleum Geology. 136: 105469. doi:10.1016/j.marpetgeo.2021.105469. ISSN   0264-8172.
  14. Pantić, N. K. (1952). "Liassic flora from Budos mountain - Montenegro". Glasnik Prir. Muzeja SRP. Zem. 5 (1): 293–308.
  15. Duarte, Luís Vítor; Silva, Ricardo Louro; Azerêdo, Ana Cristina; Comas-Rengifo, María José; Mendonça Filho, João Graciano (2023-01-13). "Shallow-water carbonates of the Coimbra Formation, Lusitanian Basin (Portugal): contributions to the integrated stratigraphic analysis of the Sinemurian sedimentary successions in the western Iberian Margin". Comptes Rendus. Géoscience. 354 (S3): 89–106. doi:10.5802/crgeos.144. ISSN   1778-7025.
  16. Cortés, J. E.; Gómez, J. J. (2018-04-16). "The epiclastic barrier-island system of the Early‒Middle Jurassic in eastern Spain". Journal of Iberian Geology. 44 (2): 257–271. doi:10.1007/s41513-018-0061-7. ISSN   1698-6180.
  17. Jurkovšek, Bogdan; Biolchi, Sara; Furlani, Stefano; Kolar-Jurkovšek, Tea; Zini, Luca; Jež, Jernej; Tunis, Giorgio; Bavec, Miloš; Cucchi, Franco (2016-08-12). "Geology of the Classical Karst Region (SW Slovenia–NE Italy)". Journal of Maps. 12 (sup1): 352–362. Bibcode:2016JMaps..12S.352J. doi:10.1080/17445647.2016.1215941. hdl: 11368/2888161 . ISSN   1744-5647.
  18. 1 2 3 4 5 6 Schlager, Wolgang (1981). "The paradox of drowned reefs and carbonate platforms". Geological Society of America Bulletin. 92 (4): 197. Bibcode:1981GSAB...92..197S. doi:10.1130/0016-7606(1981)92<197:tpodra>2.0.co;2. ISSN   0016-7606.
  19. Webster, Jody M; Wallace, Laura; Silver, Eli; Potts, Donald; Braga, Juan Carlos; Renema, Willem; Riker-Coleman, Kristin; Gallup, Christina (2004-02-28). "Coralgal composition of drowned carbonate platforms in the Huon Gulf, Papua New Guinea; implications for lowstand reef development and drowning". Marine Geology. 204 (1): 59–89. Bibcode:2004MGeol.204...59W. doi:10.1016/S0025-3227(03)00356-6. ISSN   0025-3227.
  20. Hallock, Pamela; Schlager, Wolfgang (August 1986). "Nutrient Excess and the Demise of Coral Reefs and Carbonate Platforms". PALAIOS. 1 (4): 389. Bibcode:1986Palai...1..389H. doi:10.2307/3514476. ISSN   0883-1351. JSTOR   3514476.
  21. Schlager, Wolfgang; Reijmer, John J. G.; Droxler, Andre (1994). "Highstand Shedding of Carbonate Platforms". SEPM Journal of Sedimentary Research. 64B. doi:10.1306/D4267FAA-2B26-11D7-8648000102C1865D.

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A sabkha is a coastal, supratidal mudflat or sandflat in which evaporite-saline minerals accumulate as the result of semiarid to arid climate. Sabkhas are gradational between land and intertidal zone within restricted coastal plains just above normal high-tide level. Within a sabkha, evaporite-saline minerals sediments typically accumulate below the surface of mudflats or sandflats. Evaporite-saline minerals, tidal-flood, and aeolian deposits characterize many sabkhas found along modern coastlines. The accepted type locality for a sabkha is at the southern coast of the Persian Gulf, in the United Arab Emirates. Evidence of clastic sabkhas are found in the geological record of many areas, including the UK and Ireland. Sabkha is a phonetic transliteration of the Arabic word used to describe any form of salt flat. A sabkha is also known as a sabkhah,sebkha, or coastal sabkha.

<span class="mw-page-title-main">Cyclic sediments</span>

Cyclic sediments are sequences of sedimentary rocks that are characterised by repetitive patterns of different rock types (strata) or facies within the sequence. Processes that generate sedimentary cyclicity can be either autocyclic or allocyclic, and can result in piles of sedimentary cycles hundreds or even thousands of metres thick. The study of sequence stratigraphy was developed from controversies over the causes of cyclic sedimentation.

<span class="mw-page-title-main">Depositional environment</span> Processes associated with the deposition of a particular type of sediment

In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases, the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available.

<span class="mw-page-title-main">Aganane Formation</span> Geologic formation in Azilal Province, central Morocco

The Aganane Formation is a Pliensbachian geologic formation in the Azilal, Béni-Mellal, Ouarzazate, Tinerhir and Errachidia provinces, central Morocco, being the remnant of a local massive Carbonate platform, and known mostly for its rich tracksites including footprints of thyreophoran, sauropod and theropod dinosaurs. It may also include the fossiliferous levels of the Calcaires du Bou Dahar, if true, it would be one of the richest Early Jurassic formations in the entire tethys area.

<span class="mw-page-title-main">Shallow water marine environment</span>

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.

<span class="mw-page-title-main">San Cassiano Formation</span>

San Cassiano Formation (Anisian-Carnian) is a geologic formation located on the Southern Alps in the Dolomites. These Triassic dolomites are considered to be a classic example of ancient carbonate platforms. As the allochthonous elements in the Shale strata show a good preservation, fossils and microbialites contained in these elements are useful in detailed geochemical analyses.

Automicrite is autochthonous micrite, that is, a carbonate mud precipitated in situ 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.

The Budoš Limestone is a geological formation in Montenegro and maybe Albania, dating to 192-182 million years ago, and covering the Pliensbachian-Toarcian stage of the Jurassic Period. It is located within the High karst zone, and represents a unique terrestrial setting with abundant plant material, one of the few know from the Toarcian of Europe. It is the regional equivalent to the Toarcian-Aalenian units of Spain such as the Turmiel Formation and the El Pedregal Formation, the Sinemurian Coimbra Formation in Portugal, units like the Aganane Formation or the Tafraout Group of Morocco and others from the Mediterranean such as the Posidonia Beds of Greece and the Marne di Monte Serrone of Italy. In the Adriatic section, this unit is an equivalent of the Calcare di Sogno of north Italy, as well represents almost the same type of ecosystem recovered in the older (Pliensbachian) Rotzo Formation of the Venetian region and the Podpeč Limestone of Slovenia, know also for its rich floral record.

<span class="mw-page-title-main">Tafraout Group</span> Geological formations in Morocco

The Tafraout Group is a geological group of formations of Toarcian-Aalenian age in the Azilal, Béni-Mellal, Imilchil, Zaouiat Ahansal, Ouarzazate, Tinerhir and Errachidia areas of the High Atlas of Morocco. The Group represents the remnants of a local massive Siliciclastic-Carbonate platform, best assigned to succession W-E of alluvial environment occasionally interrupted by shallow marine incursions and inner platform to open marine settings, and marks a dramatic decrease of the carbonate productivity under increasing terrigenous sedimentation. Fossils include large reef biotas with richness in "lithiotid" bivalves and coral mounts, but also by remains of vertebrates such as the sauropod Tazoudasaurus and the basal ceratosaur Berberosaurus, along with several undescribed genera. While there have been attributions of its lowermost layers to the Latest Pliensbachian, the current oldest properly measured are part of the Earliest Toarcian regression ("MRST10"), part of the Lower-Middle Palymorphum biozone. This group is composed of the following units, which extend from west to east: the Azilal Formation ; the Amezraï Formation ; the Aguerd-nˈTazoult Formation ; the Tafraout Formation & the Tagoudite Formation. They are connected with the offshore Ait Athmane Formation and the deeper shelf deposits of the Agoudim 1 Formation. Overall, this group represents a mixed carbonate-siliciclastic system of several hundred meters thick, dominated by deposits of shallow marine platforms linked to a nearby hinterland dominated by conglomerates. The strata of the group extend towards the central High Atlas, covering different anticlines and topographic features along the mountain range.

<span class="mw-page-title-main">El Pedregal Formation</span>

The El Pedregal Formation is a geological formation of Early Aalenian-Early Bajocian age in the Iberian Basin of W Iberian Peninsula. This is allocated in the East-Iberian area, that during the Middle Jurassic was part of a Carbonate platform system, influenced by tectonic activity and fault lines, along the Iberian and Catalan Coastal mountain ranges of Spain, with an exposure of up to 500 km. This carbonates are allocated on the Chelva Group, that was network of carbonate platforms, with shallow areas forming around elevated blocks created by tectonic forces. Deeper marine environments developed between these blocks, which were likely connected to the open ocean. The Internal Castilian Platform was linked to the Iberian Massif, while the El Maestrazgo High separated two marine platforms: the External Castilian and Aragonese. Further to the northeast, the Tortosa Platform was bordered by the Tarragona High and Catalan Massif to the north and the El Maestrazgo High to the south. The Beceite Strait acted as a transition zone between the Aragonese and Tortosa platforms.

The Podpeč Limestone is a geological formation of Pliensbachian-Earliest Toarcian age in southern and southwestern Slovenia, including South-West of Ljubljana or nearby Mount Krim, with other isolated locations such as in the Julian Alps. This unit represents the major depositional record of the Adriatic Carbonate platform, being known for its shallow marine-lagoon deposits and its bivalve biota, that are abundant enough to give the vulgar name to this unit sometimes in literature as the "Lithiotis Horizon". Is a regional ecological equivalent to the Veneto Rotzo Formation, the Montenegro Budoš Limestone or the Moroccan Aganane Formation. Its regional equivalents include the hemipelagic Krikov Formation at the Tolmin basin.

<span class="mw-page-title-main">Calcaires du Bou Dahar</span>

The Calcaires du Bou Dahar is a geological formation or a sequence of formations of Late Sinemurian to Pliensbachian-Toarcian boundary age in Bni Tadjite, the Central High Atlas, Morocco. This unit represents an excepcional record of an evolving reef complex, platform slopes and a emerged shoal developed inside a carbonate platform, recording the evolutionary cycles of this environment with notorious precision, also yielding what is considered one of the greatest/most diverse marine biotas of the entire Jurassic Tethys Ocean. The Bou Dahar carbonate platform shoal stands prominently and structurally above surrounding plains, spanning 35-40 km in length and 4-15 km in width, with a relief of 100-450 m. This carbonate formation originated on metamorphosed Silurian to Ordovician siliciclastic rocks and tholeiitic volcanic layers tied to Central Atlantic Magmatic Province basalts, forming a corridor oriented WSW to ENE. Surrounding alluvial plains expose green marls, shales, and dark lime-mudstones representing basinal deposits contemporaneous or subsequent to the platform. It has been considered to be a sequence of different coeval inner geological formations, including the Foum Zidet Formation, the Aganane Formation and Ouchbis Formation, but is usually interpreted as a single major unit due to it´s unique preservation.

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