Evaporites space rocks precipitated from saturated surface or near-surface brines by hydrologies pushed by solar evaporation (e.g., Warren, 1999, 2006).

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From: regional Geology and Tectonics: Phanerozoic Passive Margins, Cratonic containers and global Tectonic Maps, 2012

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Peter Sonnenfeld, in Encyclopedia of Physical scientific research and modern technology (Third Edition), 2003

II.B Lacustrine Evaporites

Lacustrine evaporites accumulate only in basins with insufficient drainage, specifically closed or endorheic basins, frequently fed through a groundwater influx. Few of these have small or no relief and feeble prices of subsidence (e.g., the Chad container in Africa), others space tectonic basins with rapid subsidence and an excellent relief. Due to the fact that of the great diversity of solutes carried in waters of continent origin, a an excellent number of mineral can form that room then classified by the leading anion.

The succession of precipitates somewhat differs in polar lakes from the in the subtropical zone since of the better hydration potential in cold temperatures. Because that instance, gypsum does not precipitate in the visibility of salt bicarbonate brought by groundwater; calcium carbonate and sodium sulfate kind instead. Mirabilite and thenardite are uncovered wherever the brine approaches freezing temperature in the winter, in ~ which time the bicarbonate solubility rises drastically. Seawater concentrated by evaporation precipitates gypsum at 4.5-fold concentration; focused by freeze-drying it returns mirabilite, the hydrated sodium sulfate, currently at 4-fold concentration. Mirabilite is additionally common in supraglacial and periglacial meltwater lakes, wherein it is acquired from redissolved thenardite, the anhydrous salt sulfate. Bloedite, the hydrated sodium-magnesium sulfate, crystallizes in cold lakes even before halite saturation is reached. Seawater focused by evaporation precipitates halite at 10-fold concentration, concentrated by freeze-drying; below −23 °C, the precipitates hydrohalite, the hydrated form of sodium chloride, in ~ 8-fold concentration.

Lakes external the polar program concentrate your brines through evaporation in the summer. Just in rare cases is there precipitation through freeze-drying in the winter, since the latter produces a less concentrated brine that have the right to redissolve the precipitate once the ground warms up. A vertical and horizontal zonation that precipitates develops in that the many soluble compounds take trip the farthest.

If the resource waters are highly carbonated, one of two people by redissolution the limestone, or indigenous rotting plant material, castle precipitate very first calcite and even monohydrocalcite. Amongst the three anhydrous calcium lead carbonate minerals, vaterite is built into some gastropod shells yet eventually converts come aragonite. Aragonite is the desired precipitate of numerous marine organisms, calcite of lacustrine ones. In time, however, all aragonite, whether inorganically precipitation or constructed into shells and also skeletons, converts to calcite. Aragonite of Mesozoic period is really rare, pre-Permian aragonite is unknown. Calcium carbonates as rocks are referred to as limestones.


Because the the lower calcium/sodium ratio in terrestrial runoff contrasted to seawater, the progressive concentration of brines in alkaline lakes leads climate to a preferential precipitation that sodium and compound sodium–calcium carbonates. All the hydrated varieties of mineral in this team are unstable and also either convert in time come anhydrous varieties or redissolve (Table I). Castle are discovered exclusively in lacustrine environments. Soda lakes type mainly top top bedrock composed of crystalline rocks or polymictic, feldspathic or argillaceous sandstones; magnesium-rich brines type where carbonated groundwater comes in contact with an easy igneous rocks.


KaliciniteKHCO3
FairchilditeK2CO3·CaCO3
BuetschliiteK2CO3·CaCO3·2 H2O
ThermonatriteNa2CO3
Soda (natron)Na2CO3·10 H2O
NahcoliteNaHCO3
WegscheideriteNa2CO3·3NaHCO3
TronaNa2CO3·NaHCO3·2 H2O
NyerereiteNa2CO3·CaCO3
ShortiteNa2CO3·2CaCO3
PirssoniteNa2CO3·CaCO3·H2O
GaylussiteNa2CO3·CaCO3·5 H2O
BurbankiteNa2CO3·(Ca, Ba, Sr, La, Ce)4(CO3)4
EiteliteNa2CO3·MgCO3
NorthupiteNa2CO3·MgCO3·NaCl
TychiteNa4Mg2(CO3)2(SO4)3
BradleyiteNa3PO4·MgCO3
DawsoniteNaAl(OH)2CO3
BurkeiteNa2CO3·2Na2SO4
HanksiteNa22K(CO3)2(SO4)9Cl
SideriteFeCO3 through Mg: Fe under.05
Sideroplesite(Mg, Fe)CO3 with Mg: Fe = 0.5–.30
Pistomesite(Mg, Fe)CO3 with Mg: Fe = .30–1.0
Mesitite(Mg, Fe)CO3 v Mg: Fe = 1–3
Breunerite(Mg, Fe)CO3 with Mg: Fe = 3–20
MagnesiteMgCO3 through Mg: Fe over 20
BarringtoniteMgCO3·2 H2O
NesquehoniteMgCO3·3 H2O
LansforditeMgCO3·5 H2O
Artinite2MgCO3·Mg(OH)2·3 H2O
Dypingite4MgCO3·Mg(OH)2·3 H2O
Hydromagnesite3MgCO3·Mg(OH)2·3 H2O or 4MgCO3·Mg(OH)2·4 H2O
Huntite3MgCO3·CaCO3
DolomiteMgCO3·CaCO3
Calcite, aragoniteCaCO3
MonohydrocalciteCaCO3·H2O
TrihydrocalciteCaCO3·3 H2O
PentahydrocalciteCaCO3·5 H2O
IkaiteCaCO3·6 H2O

Magnesite is the only kind of pure magnesium carbonate and is discovered in lacustrine environments either precipitated straight or together a bacterial modification product of calcite. It can also be uncovered along the rims of naval lagoons and sebkhas with a considerable influx of continental waters. Dolomite, the calcium-magnesium twin carbonate, is found in a few instances close to the interface in between groundwater discharge and seawater influx together cryptocrystalline soft paste that at some point hardens right into a really fine-grained rock, quite different in appearance indigenous sucrosic dolomites that space evidently second after limestone. Whether this dolomite is truly primary or calcite converted within a season come dolomite is not always clear.

Many together hypersaline lakes build a high alkalinity and a high pH sufficient to dissolve any kind of swept-in silicates. These might be precipitated as framework silicates in the zeolite team (e.g., sodium–aluminum silicate mineral hydrated to a differing degree) uncovered in number of lakes in north central Africa. Hydrated calcium–aluminum silicates or calcium–sodium aluminum silicates are likewise known however are lot rarer. However, the mineral stability is ephemeral: in time the minerals are significantly dehydrated and also turn right into authigenic feldspar crystals, or the sodium and alumina space leached out and they convert into members the the opal group and hence into a selection of quartz or chert. If the resource waters are rich in liquified silica, a collection of hydrous silica compounds (related come opal) are precipitated. They room all unstable and convert in time to anhydrous silica cement or to some range of chert. The finish conversion come chert may take number of million years, however none of the complex hydrous silica compounds common in dried-out Neogene lake bed are recognized from more ancient sediments.


Further concentration the a lacustrine brine stripped the carbonate ion leads come the precipitation the sulfates or of equivalent magnesium compounds. The sulfate ions can type sodium, magnesium, and also calcium sulfates but only after ~ the progressive concentration has actually stripped the brine of practically all carbonate and also bicarbonate contents (Table II). Gypsum does no precipitate in the presence of sodium bicarbonate; instead, calcite and also mirabilite, the hydrous salt sulfate, or thenardite, the dehydrated salt sulfate, space formed.


MineralFormulaSp. Gr.MineralFormulaSp. Gr.
Lacustrine sulfates:
CelestiteSrSO43.971Hanksite2Na2CO3·9Na2SO4·KCl2.570
PolyhaliteK2SO4·2CaSO4·MgSO4·2 H2O3.000BurkeiteNa2CO3·2Na2SO42.570
AnhydriteCaSO42.980UklonskoviteNaOH·MgSO4·2 H2O2.450
GlauberiteNa2SO4·CaSO42.850GypsumCaSO4·2 H2O2.330
BassaniteCaSO4·H2O2.761BloediteNa2SO4·MgSO4·5 H2O2.280
ThenarditeNa2SO42.698Hydroglauberite5Na2SO4·3CaSP4·6 H2O1.510
AphthitaliteNa2SO4·3 K2SO42.656MirabiliteNa2SO4·10 H2O1.490
Bruckerite2Na2SO4·CaSO4·2 H2O
Marine sulfates:
Ca-sulfates:(decreasing calcium content)Ammonium sulfates:
AnhydriteCaSO42.980Boussingaultite(NH4)2SO4·MgSO4·6 H2O1.723
BassaniteCaSO4·H2O2.761Mg-sulfates:(decreasing magnesium-content)
GypsumCaSO4·2 H2O2.330KieseriteMgSO4·H2O2.570
GoergeyiteK2SO4·5CaSO4·H2O2.770SanderiteMgSO4·2 H2O1.757
GlauberiteNa2SO4·CaSO42.850LeonhardtiteMgSO4·4 H2O
PolyhaliteK2SO4·2CaSO4·MgSO4·2 H2O3.000AlleniteMgSO4·5 H2O
SyngeniteK2SO4·CaSO4·H2O2.597SakiiteMgSO4·6 H2O
Sr-sulfates:(decreasing strontium content)EpsomiteMgSO4·7 H2O1.677
CelestiteSrSO43.971Na-sulfates:(increasing salt content)
KalistrontiteK2SO4·SrSO43.320AphthitaliteNa2SO4·3 K2SO42.656
K-Mg sulfates:(decreasing potassium content)BloediteNa2SO4·MgSO4·5 H2O2.280
Kainite4KCl·4MgSO4·11 H2O2.190Loeweite6Na2SO4·7MgSO4·15 H2O2.423
LangbeiniteK2SO4·2MgSO42.829Vant'hoffite3Na2SO4·MgSO42.694
PicromeriteK2SO4·MgSO4·6 H2O2.150D'ansite9Na2SO4·MgSO4·3NaCl2.650
LeoniteK2SO4·MgSO4·4 H2O2.201ThenarditeNa2SO42.698

Potassium is screened out of the groundwater by sorption and also ion exchange, leading to a substantial enrichment in sodium cations. In a very few instances, potassium sulfate minerals are uncovered in lacustrine environments, wherever the groundwater is particularly enriched in potassium and also has no possibility to rotate some clay right into illite. Stole sulfates perform not form either crusts or bedded deposits; they occur almost exclusively together halos roughly weathering sulfides. Top top dissolution in one aqueous environment, they become bivalent iron-organic complexes.


Chlorine contents of igneous and metamorphic rocks is minimal. Wherever groundwater contains significant quantities of chloride anions, these are usually due to contamination by discharging calcium chloride enriched formation waters or come dissolution of ancient marine or lacustrine chloride deposits (Table III). Circulating groundwater can easily dissolve precipitation chlorides and also deliver them to the nearest surface ar flow; the occurrence of salt creeks or salt rivers is, therefore, quite usual even exterior the arid belt.


Chlorides in maritime environments
HaliteNaClSylviteKCl
CarnalliteKCl·MgCl2·6 H2OBischofiteMgCl2·6 H2O
TachyhydriteCaCl2·2MgCl2·12 H2ORinneiteFeCl2·3KCl·NaCl
Chlorides in polar lacustrine environments
HaliteNaClSylviteKCl
HydrohaliteNaCl·2 H2OCarnalliteKCl·MgCl2·6 H2O
AntarcticiteCaCl2·6 H2O
Rare accessory minerals
Sal AmmoniacNH4ClBaeumleriteCaCl2·KCl
ChloraluminiteAlCl3·3 H2OErythrosideriteFeCl3·2KCl·H2O
RokuehniteFeCl2·2 H2OMolysiteFeCl3
DouglasiteFeCl2·KCl·2 H2O or FeCl2·2KCl·2 H2O

Reprecipitation of this solutes leads to secondary, nonmarine chloride store that display all the aspects of maritime evaporite deposits. However, together evaporite sequences present no evidence of any type of connection come a coeval ocean and also most the the time carry out not exhibition the complete marine sequence of evaporites, either gift devoid of calcium sulfates or that potash deposits. Gift redeposited and also recrystallized, the bromine/chlorine proportion in recycled evaporite shop is but a fraction of that discovered in comparable marine deposits. Such internal salt containers are uncovered in Europe, Asia, Australia, and North America. Mesozoic and also Cenozoic salt accumulations in both phibìc Dakota and also Arizona show up to it is in in this category. For example, the great Salt Lake appears to derive its salt contents from a leaching the Jurassic salts from the Wasatch Range.

Borates kind a an excellent variety of character language minerals that are typical as nests in both lacustrine and marine evaporites, wherever there is either a syngenetic or an epigenetic volcanic resource of boron nearby. Boron is also liberated out of the decomposing lattice that swept-in muscovite flakes. On occasion it leads to the development of a few slivers the authigenic tourmaline.


The evaporite chemical sedimentary rocks space rare, however extremely essential commercially as the raw products for the chemistry industry. As the name suggests, the evaporites consists a suite of minerals formed from the evaporation of sea water. They have tendency to take place in limited sedimentary basins in cyclic assignment that begin with carbonates (limestone and/ or dolomite), overlain through sulphates (gypsum and/or anhydrite), halite (sodium chloride), and then a variety of potassium salts, including carnallite and polyhalite (Figure 8).



As the surname suggests, that was as soon as thought the evaporites formed exclusively from the dry out the enclosed marine basins. This forced improbably big volumes of sea water to provide the resultant evaporites. The is currently realized that plenty of evaporites actually form in sabkhas (Arabic because that salt marsh) indigenous the instead of of pre-existing rocks, principally carbonates, by circulating brines. Evaporites should thus more correctly be termed ‘replacementites’. Evaporites are explained in much more detail in Sedimentary Rocks: Evaporites.


Scott A. Elias, in Encyclopedia that Geology (Second Edition), 2021

Evaporites

Evaporite salts may form anywhere on earth where evaporation over precipitation. Their preservation is managed by the nature that the post-depositional climate, the ingress the groundwaters, and also subsequent interment history. Modern seaside evaporites accumulate in between 15° and also 35° latitude, yet intracontinental evaporites might occur beyond 50°, sometimes being frozen for number of months of the year. Therefore, salts execute not necessarily imply permanent warmth. The formation and also preservation the the much more soluble salts of potassium need low atmospheric humidity (less 보다 50%). Salts may form very rapidly under ideal conditions (100 m in 1 Ka), so short-lived episodes of evaporation may cause a good thickness of salts.


Frederick L. Schwab, in Encyclopedia the Physical scientific research and modern technology (Third Edition), 2003

V.A Evaporite

Evaporite shop (largely bedded halite, sylvite, gypsum, anhydrite, and various potash salts) form from the precipitation of solid mineral crystals indigenous a concentrated solution the salt- or freshwater, in various other words, indigenous brine. Evaporite deposits may be either marine or nonmarine (lacustrine) in origin.

Modern evaporites space relatively restricted in occurrence and also are developed in areas where the quantity of water shed by evaporation over rainfall or flow via rivers and streams. Usual examples of together localities include the an excellent Salt Lake of Utah, the semidry it s not for long time playa lakes that the Nevada–California great Basin, the marginal coastal salt flats (sabkhas) around the Persian Gulf, and also the various salt pans, marine salinas, and also lagoons the India, Cyprus, and the Gulf the Suez. Ancient evaporites space much more extensive regionally than these modern analogs and suggest special situations in the geology past vital for the deposition that basinwide or also continent-wide evaporite deposits.

The mineralogy the evaporite shop is complex, with practically 100 minerals possibly present. However, only about a dozen mineral species are major constituents. These incorporate (1) carbonate mineralscalcite, aragonite, magnesite, and dolomite; (2) sulfate mineralsanhydrite, gypsum, kieserite, langeinite, kainite, and polyhalite; and (3) chloride mineralshalite, sylvite, and carnallite. These minerals room not just soft, but really soluble, and thus are rarely visible in outcrop, gypsum and also anhydrite gift the key exceptions. The structure of evaporites is invariably crystalline, v grain size ranging from coarse to fine crystalline.

All evaporite shop ultimately form from the evaporation that a fresh- or saltwater brine. It is possible to kind brines straight from seawater simply by isolating a volume of seawater indigenous the open up ocean, for example, in ~ a lagoon separated indigenous the open s by some sort of topographic barrier (a sandbar or organic barrier like a reef). Seawater can likewise be secluded in shallow pools left in the intertidal zone the arid regions. Freshwater brines form in arid regions identified by inner drainage (i.e., no drainage operation to the ocean).

Laboratory experiment have exactly traced the evolution of brine formation and also precipitation. The salinity (amount of liquified constituents) of typical seawater is 3.5% or 35,000 ppm, many of which is sodium and chlorine. If the volume of typical seawater is decreased by evaporation to one-fifth the original, precipitation of evaporites starts in an orderly fashion. The most insoluble constituents prefer gypsum solidify first. Ongoing evaporation to one-tenth the original volume generates salt chloride (rock salt). Other evaporite minerals monitor in order of raising solubility, back the methodical ordered appearance visible in laboratory experiments is rarely perfectly visible in the vertical sequence of actual evaporite deposits.

The amount of evaporated water (1000 m) represented by a 13-m-thick bed of halite is startling! In yes, really fact, few if any type of sedimentary petrologists think that immense volumes of evaporates were generated during a single, an easy episode the evaporation of a body of seawater. Instead, castle invoke long-term intervals the earth history characterized by elevated temperature and decreased precipitation. Together climatic problems are coupled v the visibility of broad, regionally considerable marine shelves that can be consistently transgressed and regressed through epeiric seas and also that were bordered by sabkhas, broad, low-lying salt flats inundated through the sea in arid regions. Repetitious, even diurnal illustration of tidal advance and retreat across such vast areas might sequentially ridge infinitely thin evaporite beds on peak of one an additional to create thick composite part (for example, the Silurian evaporites of the Michigan Basin; Permian rocks that Texas and new Mexico).

Evaporites are significant sedimentary rocks. They suggest peculiar climatic and also tectonic conditions. Likewise evaporite beds, an especially salt, frequently coincide with lubricating horizons along which an excellent fault slices move. Finally, evaporites are vital strictly from an economic viewpoint, both in their very own right, and also due to your plasticity, which enables them to rise up v overlying crust together (salt) domal structures that commonly create traps because that petroleum.


M. Bąbel, B.C. Schreiber, in writing on Geochemistry (Second Edition), 2014

9.17.24.1 Paleogeographic Indicators

Evaporite development requires a relatively minimal climate, supporting a an adverse water balance. Ancient saline giants additionally needed details paleogeography related to tectonic pulses that Earth and also accommodation space for deposition. The evaporite deposits type and are preserved largely in locations of minimal rainfall and also elevated evaporation. Today, they room largely concentrated within 2 ‘dry’ subtropical high-pressure belts in between 15ᵒ and 35ᵒ latitude indigenous the equator (Borchert and also Muir, 1964; Lotze, 1938). Compilation by Ziegler et al. (2003), Zharkov (1998, 2001), and Chumakov and also Zharkov (2003) proved that indigenous the Permian till now, the evaporite deposition prevailed within the 2 belts 10–40ᵒ N and 10–40ᵒ S, apparently concerned hot and also dry climate problems created through the to decrease branches of the Hadley cells. This shows up to show that global atmospheric circulation was roughly the very same as now at least due to the fact that Permian. In particular, Ziegler et al. (2003) noted that the distribution of evaporites over time was connected to ease of access of epeiric and also shelf sea containers within lower latitudes rather than to global events of dry climate.

Some old evaporite basins, favor the gigantic Permian containers of north hemisphere, prolonged up come ~ 50ᵒ latitude (Trappe, 2000; Ziegler et al., 2003). The evaporites are an excellent paleogeographic indicators suggesting the position of the Phanerozoic basins did no extend past 35–50ᵒ latitude (Briden and Irving, 1964; Parrish et al., 1982; Rees et al., 2004; Zharkov, 1998). They have the right to be provided for the mediate of paleogeographic reconstructions based upon paleomagnetic data (e.g., Drewry et al., 1974; Evans, 2006).


Grant M. Young, George E. Williams, in Encyclopedia that Geology (Second Edition), 2021

Evaporites

Evaporite deposits are rocks composed largely of minerals produced by evaporation of saline solutions. They selection from carbonates come sulfates, chlorides and potassium-rich salts. These precipitates indicate that the climate to be warm and also arid. Nodules the the sulfate mineral gypsum and anhydrite are existing in the Paleoproterozoic Gordon Lake formation (Fig. 4) in Canada, and also similar-aged deposits happen elsewhere. For example Melezhik et al. (2005) reported widespread event of syn-depositional marine anhydrite with Ca-sulfate pseudomorphs in the c. 2100 Ma Tulomozero development in Fennoscandia. They thought about these rocks to indicate that surface seawaters to be oxidized and seaside playa and sabkha settings were sulfate-rich. Prior to ~ 1.7 exchange rate years ago (Ga), however, over there is one apparent lack of enormous sulfate evaporites, possibly due to a dearth of sulfate in human being oceans, so the evaporation might have created halite (NaCl) before gypsum. The soluble and also mobile nature of evaporite deposits might have decreased the likelihood of long-term preservation.

Basin-scale Proterozoic evaporites incorporate the ~ 850 Ma bedded gypsum- and anhydrite-rich rocks the the Shaler Supergroup ~ above Victoria Island in the Canadian Arctic and also in the northern Canadian Cordillera. Comparable deposits are maintained in Australia, Namibia and main Africa. Most of this rocks were deposited in comprehensive intracratonic or marginal maritime basins and also would have actually removed substantial quantities of sulfate from civilization oceans and oxygen native the atmosphere. Inferred paleolatitudes the such old evaporite store (Evans, 2006) said that they created at similar paleolatitudes come their modern-day counterparts. The was, however, concluded by Li (2013) the all the ~ 825 Ma evaporite containers formed in ~ high latitudes. They said that these inconsistent findings may have resulted indigenous erroneous correlations with various other deposits for which paleomagnetic data room available, or the they may be concerned “true polar wander” events throughout which Earth's whole outer covering is assumed to have undergone quick displacements.


Marine evaporite sulfate incorporates the sulfur isotopic signature from the seawater in i m sorry the mineral precipitated. Due to the fact that marine evaporite sulfate minerals have actually high concentration of sulfate, it is straightforward to use them for isotope analyses. Indeed, fluctuations that the seawater δ34S through time have been originally revealed by evaporite-derived sulfate δ34S data (Claypool et al., 1980; Strauss, 1997; Strauss et al., 2001). Claypool et al. (1980) very first presented a compilation of marine evaporite sulfate δ34S data because that the terminal Precambrian and also the Phanerozoic. The period curve that the evaporite-derived δ34S is defined by a discernible rise from about 18‰ to an typical value as much as 35‰ during the late Neoproterozoic, followed by a pronounce 25‰ decline throughout the Paleozoic to a Permian minimum at ~ 10‰, and also a steady rise towards the modern oceanic worth of 21‰ (Fig. 1; Claypool et al., 1980). Although big temporal gaps exist in the evaporite record, their job-related demonstrated that the sulfur isotope ratio of seawater sulfate had undergone an extensive changes end geological time and laid the groundwork because that our current understanding the the Phanerozoic sulfur cycle and also S-isotope stratigraphy. Further studies presented much more comprehensive evaporite sulfate δ34S records with a thoroughly compilation the the evaporite data covering the Proterozoic and the Phanerozoic (Crockford et al., 2019; Strauss, 1997, 1999; Strauss et al., 2001), which substantially extend the applications because that stratigraphic correlations.



However, there room some fundamental limitations when considering the use of the evaporite records for stratigraphy. First, the evaporite-derived δ34S records are not continuous for some geological intervals. Because of the sporadic event of evaporite store in sedimentary records, the temporal resolution that this record is relatively low (but additionally see Bernasconi et al., 2017). This disadvantage to be partially get rid of by extrapolating between the discrete data to adjust to almost right a best-estimated period curve (Claypool et al., 1980). However, this smoothing results in masking that finer alters in the δ34S signatures during times of fast perturbations. Second, the age control on this document is imperfect. As result of the scarcity that fossils in evaporite settings, the is difficult to correlate to biostratigraphy which constrains the stratigraphic position of the boundaries, and also thus evaporite records are typically accompanied with large age uncertainties. Depending upon the certain time intervals, the age uncertainties gift in the evaporite record are top top the stimulate of tens of million year or higher (Strauss, 1997). Third, the spread in the δ34S data at each time suggest interval is an extremely large, making it daunting for stratigraphic correlations. The selection of the δ34S worths is approximately 5‰ for most of the time intervals and can be as high together 25‰ for part older sections (e.g., the Devonian; Claypool et al., 1980; Strauss, 1997). This is regardless of minor sulfur fractionation (0–2.5‰) throughout evaporite crystallization (Raab and also Spiro, 1991) indicating other controls on the measure up δ34S.


M.R. Talbot, in Encyclopedia the Geology, 2005

Evaporites

Evaporite minerals space deposited from saline lakes, wherein they kind as a result of the evaporative concentration of lake water (see SEDIMENTARY ROCKS | Evaporites). Meanings of what constitutes a saline lake vary, depending on the criteria used. The most practical an interpretation is a organic one, based on the monitoring that usual freshwater organisms room generally absent from waters who salinity over 5‰. A bewildering number of different salts have been tape-recorded from saline lake basins, including some, such together borates, nitrates, and sodium carbonate minerals, that are of significant economic importance. The commonest lacustrine evaporites room gypsum (CaSO4 · 2H2O), halite (NaCl), and also a range of sulphates, carbonates, and also bicarbonates the sodium. I m sorry minerals room precipitated is established by the chemistry that the inflowing water. This is in significant contrast come the formation of naval evaporites, where the uniform composition of seawater ensures that only a minimal suite of minerals have the right to occur.


Because the precipitation that salts requires the evaporation of large volumes of lake water, their development and build-up tend to take place in lakes that room hydrologically closeup of the door (i.e. They have actually no outflow) and in areas with a dried or arid climate. In this settings, water loss as result of evaporation have the right to exceed inflow. Together lakes have tendency to alternating between brief periods that flooding, adhering to rainfall, and also longer periods of saline or even totally desiccated conditions. Because of their large variations in volume, plenty of saline lakes space surrounded by extensive salt flats, whereby salt precipitation may proceed owing to groundwater seepage and loss through capillary effects (Figure 10).


*

Platform evaporites are comprised of stratiform beds, typically  50 m thick and composed of stack Table 7 and Fig. 15A). Usually, communication salts were deposited in laterally substantial (> 50–100 km wide), hydrographically-isolated, sub-sealevel, marine-seepage lagoons (salterns) or evaporitic mudflats (sabkhas and also salinas) (Fig. 16). These areas are hundreds to more than a thousands kilometers in width and also have no same-scale contemporary counterparts (Fig. 17). Systems of platform evaporite prolonged as prevalent depositional sheets across big portions that hydrographically-isolated maritime platform areas that pass seaward across a subaerial seepage barrier into open maritime sediments.




Fig. 17. Epeiric (epicontinental) seaways the covered large areas that the continental interior with shallow marine waters are recognized as intracratonic basins, if epeiric seaways that formed as very wide shelf edges follow me the continent margin specify pericontinental seaways. Waters were really shallow over areas that were hundreds to thousands of kilometers across. Neither setting has a contemporary counterpart; pericontinental seaways that evolve into salterns and or evaporitic mudflats are an ext typical the marine-margin eustatic layouts on the planet in greenhouse climate mode, once the absence of permanent polar icecaps, particularly coupled with higher rates of seafloor spreading, expected the continental freeboard was much larger than it is today. Periplatform evaporites favored by greenhouse eustacy (relatively steady at fourth order 100,000 year sealevel cycle) with extr isolation (tectonic or 5th order sealevel-fall induced) required to deposit communication evaporites. Icehouse eustacy way high amplitude and high frequency oscillations, bring about hydrologic instability that the shelf and also so no platform evaporites in icehouse times uneven hydrographical isolation is collection up ~ above a tectonically-isolated intracratonic sag container (aulacogen).


In naval margin epeiric settings, such together the Jurassic Arab/Hith and Permian Khuff cycles of the Middle east or the Cretaceous Ferry Lake Anhydrite in the Gulf the Mexico, this platform evaporites are intercalated v shoalwater marine-influenced carbonate shelf/ramp sediments, which consequently pass basinward throughout a subaerial sill into open maritime carbonates (Fig. 18A and B). Landward castle pass right into arid zone continental siliciclastics or carbonate mudflats. Communication evaporite deposition developed in both pericontinental and also epicontinental settings, at time of low-amplitude 4th and fifth order sealevel changes, i m sorry typify greenhouse eustasy (Figs. 13 and also 16). Communication evaporites additionally typify the saline step of some components of intracratonic and other basinwide settings (Fig. 16). Platform evaporites cannot form on carbonate shelves in the high-amplitude, high-frequency sealevel changes of icehouse eustasy. The 100 m + amplitude oscillations the Icehouse times mean sealevel drops off the shelf edge roughly every 100,000 years, so that any kind of evaporite that had formed on the communication is subaerially exposed, leached and removed. The only known marine-edge evaporites tied to the penultimate interglacial highstand room small-scale highly-eroded salina remnants that locally chop out on the Sharma shore of the Red Sea, Egypt.


Fourth order high-amplitude icehouse eustatic cycles have tendency to avoid laterally-continuous lead carbonate sediment obstacles forming in ~ the top of the shelf to slope break, and also so icehouse evaporite systems room usually not hydrographically secluded (drawdown) in ~ the communication scale. Rather, icehouse eustasy favors nonmarine as the leading evaporite style, follow me with tiny ephemeral marine-margin salt bodies, together seen today in the Holocene halite and gypsum bed of Lake Macleod in coastal West Australia.

Ancient communication evaporite successions may contain halite beds, particularly in intracratonic basinwide settings, but periplatform environments, exterior of intracratonic basins, are typically dominated through 5–40 m special Ca-sulphate bed intercalated through normal-marine communication carbonates (Table 7). The lateral level of this epeiric platform sulphate bodies, favor the middle Anhydrite Member that the Permian Khuff development of Saudi Arabia, with a current area of more than 1,206,700 km2, and the Hith Anhydrite the the UAE and Saudi Arabia (1,068,540 km2 in size), constitute several of the most aerially-extensive evaporite beds ever deposited (Fig. 18A).


Evaporites precipitate with unique textures in a variety of hydrologically contrasted setups that room dependent ~ above brine stability and related to prices of temperature and salinity change in the mom liquor. Sometimes crystals remain where they precipitate, other times they space mechanically or geochemically reworked, or undergo partial levels of dissolution and also fractional recrystallization (Figure 1). Crystals may an initial precipitate at the air–brine user interface in rafted crystal clusters that then sink to form cumulate beds, or can be blown come the strandzone (e.g., halite rafts crystallizing in ~ the air–brine user interface in Lake Guilietti in Ethiopia; Figure 1). Climate again, instantly after lock crystallize, precipitates deserve to sink indigenous the uppermost water fixed to ultimately collect as pelagic accumulations top top the shallow or deep brine floor (‘rain native heaven’ deposits). Seasonal or longer term changes in the chemistry and also salinity in the upper water column way many such pelagic deposits space millimeter-scale laminates consisted of of mineral double or triplet layers. In a holomictic (unstratified) water mass, rough circuit centimeter–decimeter scale crystals can type as bottom-nucleated inclusion-entraining precipitates, commonly at the base of a water shaft that is tens of centimeters to meter deep. Together crystals, it is in they gypsum swallowtails or halite chevrons, tend to be written of alternate inclusion-rich and also inclusion-poor laminae and also micro-laminae, reflecting rapid transforms in chemistry or temperature the the shallow water mass. Table 1 list the mineralogy that the an ext common evaporite salts.


If a brine shaft remains both supersaturated and holomictic to greater depths, climate evaporites can accumulate in ~ the deepwater basic of a brine column that is hundreds of meters deep. This is the case today in the North container of the Dead Sea in the middle East, wherein a salt bed, comprised of a meshwork the inclusion-free, randomly aligned centimeter-scale halite crystals, is accumulating and also has been doing so since the North basin brine column came to be holomictic in February 1978. Before 1978 and also for at the very least the preceding 400 years, the brine column of North basin (>370 m water depth) was a meromictic (density-stratified) hypersaline system. Climate pelagic millimeter-scale laminites accumulated on the exact same deep bottom and also were created of alternating calcite and also aragonite lamina, along with minor centimeter-scale gypsum crystal clusters or rosettes. The absence of inclusions in the halite mesh top top the present-day deep bottom reflects the greater stability of chemical problems on the deep bottom, through the corollary that growth-aligned evaporite crystals, well-off in entrained inclusions of brine (e.g., chevron halite in death Valley, California, or carbonate pellets in swallow-tail gypsum in Marion Lake, Australia) show precipitation in lot shallower water depths (decimeter to meter depths). That is, inclusion-rich set bottom expansion textures in both halite and also gypsum kind in well-mixed brines that room shallow sufficient to endure short-term changes in saturation and growth rate, coupled to short-term changes in water temperature or salinity.

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If currents and also waves rework the bottom crystals, climate ripples and dunes are the dominant texture in the accumulating evaporite bed. The presence of evaporite equivalents of lead carbonate ooids, specific gypsolites and also halolites, suggest oscillating bottom currents at the moment the crystals grew. Brine floor instability, pertained to seismic events, halokinesis, or tectonism, can also lead come the formation of slumps and debris flow textures in a salt bed deposited at any kind of water depth. The lowering of suprasalt brine obelisk salinity or the lowering the the regional water table can create karst cones and also breccias in an evaporite bed. For more details on the significance of the assorted evaporite textures, the leader is referred to Lowenstein and Hardie (1985), Warren (2006), and Orti et al. (2010).