Editing Phosphorite (section)

==Depositional environments==

Phosphates are known to be deposited in a wide range of [[depositional environment]]s. Normally phosphates are deposited in very shallow, near-shore marine or low energy environments. This includes environments such as supratidal zones, littoral or intertidal zones, and most importantly estuarine.<ref name="Baturin"/> Currently, areas of oceanic upwelling cause the formation of phosphates. This is because of the constant stream of phosphate brought from the large, deep ocean reservoir (see below). This cycle allows continuous growth of organisms.<ref name="Middleton"/>

''Supratidal zones:'' Supratidal environments are part of the tidal flat system where the presence of strong wave activity is non-existent. Tidal flat systems are created along open coasts and relatively low wave energy environments. They can also develop on high energy coasts behind barrier islands where they are sheltered from the high energy wave action. Within the tidal flat system, the supratidal zone lies in a very high tide level. However, it can be flooded by extreme tides and cut across by tidal channels. This is also subaerially exposed, but is flooded twice a month by spring tides.<ref name="Boggs">Boggs, Sam, Jr. (2006). ''Principles of Sedimentology and Stratigraphy'' (4th ed.), Pearson Education Inc., Upper Saddle River, NJ, pp. 217–223 {{ISBN|0321643186}}</ref>

''Littoral environments/intertidal zones:'' Intertidal zones are also part of the tidal flat system. The intertidal zone is located within the mean high and low tide levels. It is subject to tidal shifts, which means that it is subaerially exposed once or twice a day. It is not exposed long enough to support the growth of most vegetation. The zone contains both suspension sedimentation and bed load.<ref name="Boggs"/>

''Estuarine environments'': Estuarine environments, or estuaries, are located at the lower parts of rivers that flow into the open sea. Since they are in the seaward section of the drowned valley system they receive sediment from both marine and fluvial sources. These contain facies that are affected by tide and wave fluvial processes. An estuary is considered to stretch from the landward limit of tidal facies to the seaward limit of coastal facies. Phosphorites are often deposited in fjords within estuarine environments. These are estuaries with shallow sill constrictions. During Holocene sea-level rise, fjord estuaries formed by drowning of glacially-eroded U-shaped valleys.<ref name="Boggs"/>

The most common occurrence of phosphorites is related to strong marine upwelling of sediments. Upwelling is caused by deep water currents that are brought up to coastal surfaces where a large deposition of phosphorites may occur. This type of environment is the main reason why phosphorites are commonly associated with silica and chert. Estuaries are also known as a phosphorus “trap”. This is because coastal estuaries contain a high productivity of phosphorus from marsh grass and benthic algae which allow an equilibrium exchange between living and dead organisms.<ref name="D.R Pevear">{{cite journal|title=The estuarine formation of United States Atlantic Coastal Plain phosphorite |url=http://econgeol.geoscienceworld.org/content/61/2/251.abstract<!--broken doi-->|author=Pevear, D. R.|journal=Economic Geology|year= 1966|volume= 61|issue=2|pages=251–256|doi=10.2113/gsecongeo.61.2.251|bibcode=1966EcGeo..61..251P |url-access=subscription}}</ref>

===Types of phosphorite deposition===

*Phosphate nodules: These are spherical concentrations that are randomly distributed along the floor of continental shelves. Most phosphorite grains are sand size although particles greater than 2&nbsp;mm may be present. These larger grains, referred to as [[Nodule (geology)|nodules]], can range up to several tens of centimeters in size. Phosphate nodules are known to occur in significant quantities offshore northern [[Chile]].<ref>{{Cite journal|title=Potential mineral resources of the Chilean offshore: an overview|journal=[[Andean Geology]]|url=http://www.andeangeology.cl/index.php/revista1/article/view/V47n1-3260/html|last1=García|first1=Marcelo|volume=47|pages=1–13|last2=Correa|first2=Jorge|issue=1|year=2020|last3=Maksaev|first3=Víctor|last4=Townley|first4=Brian|doi=10.5027/andgeoV47n1-3260|doi-access=free}}</ref> 
[[File:Estonian shelly phosphorites.jpg|thumb|right|Shelly phosphorite from Estonia]]
*Bioclastic phosphates or bone beds: Bone beds are bedded phosphate deposits that contain concentrations of small skeletal particles and coprolites.<ref name="Prothero"/> Some also contain invertebrate fossils like brachiopods and become more enriched in P<sub>2</sub>O<sub>5</sub> after diagenetic processes have occurred. Bioclastic phosphates can also be cemented by phosphate minerals.<ref name="Baturin"/> 
*Phosphatization: Phosphatization is a type of rare diagenetic processes. It occurs when fluids that are rich in phosphate are leached from guano.<ref name="Prothero"/> These are then concentrated and reprecipitated in limestone. Phosphatized fossils or fragments of original phosphatic shells are important components within some these deposits.

====Tectonic and oceanographic settings of marine phosphorites====

*Epeiric sea phosphorites: [[Epeiric sea]] phosphorites are within marine shelf environments. These are in a broad and shallow cratonic setting. This is where granular phosphorites, phosphorite hardgrounds, and nodules occur.<ref name="Middleton">Middleton V. Gerald, 2003 Encyclopedia of Earth Sciences series. Encyclopedia of Sediment and Sedimentary Rocks. Kluwer Academic Publishers. Dordrect, Boston, London. pp 131, 727, 519–524.</ref>
*Continental margin phosphorites: Convergent, passive, upwelling, non-upwelling. This environment accumulates phosphorites in the form of hardgrounds, nodules and granular beds.<ref name="D.R Pevear"/> These accumulate by carbonate fluorapatite precipitation during early diagenesis in the upper few tens of centimeters of sediment. There are two different environmental conditions in which phosphorites are produced within continental margins. Continental margins can consist of organic rich sedimentation, strong coastal upwelling, and pronounced low oxygen zones. They can also form in conditions such as oxygen rich bottom waters and organic poor sediments.<ref name="Middleton"/>
*Seamount phosphorites: These are phosphorites that occur in seamounts, guyots, or flat topped seamounts, seamount ridges. These phosphorites are produced in association with iron and magnesium bearing crusts. In this setting the productivity of phosphorus is recycled within an iron oxidation reduction phosphorus cycle. This cycle can also form glauconite which is normally associated with modern and ancient phosphorites.<ref name="Middleton"/>
*Insular phosphorites: Insular phosphorites are located in carbonate islands, plateaus, coral island consisting of a reef surrounding a lagoon or, atoll lagoon, marine lakes. The phosphorite here originates from guano. Replacement of deep sea sediments precipitates, that has been formed in place on the ocean floor.<ref name="Middleton"/>