{{Redirect|Sebkha|the city in Mauritania|Sebkha, Mauritania}} {{short description|Salt lake above the tide line, where evaporite deposits accumulate}} [[File:Sebkha El Melah - 2001.jpg|thumb|upright=1.5|Sebkhat (or Sebkha) El Melah, Tunisia in 2001, mostly dry. Note rectangular industrial evaporite pans, probably for sea-salt production, upper right. [[Landsat 7]] image.]] [[File:Sebkha El Melah - 1987.jpg|thumb|upright=1.5|Sebkhat El Melah in 1987, flooded. [[Landsat 5]] image.]]
A '''sabkha''' ({{langx|ar|سبخة}}) is a predominately coastal, [[Supralittoral zone|supratidal]] mudflat or sandflat in which [[evaporite]]-saline [[mineral]]s accumulate as the result of a semiarid to arid climate. Sabkhas are gradational between land and [[Intertidal zone|intertidal]] zone within restricted coastal plains just above normal high-tide level. Within a sabkha, evaporite-saline mineral sediments typically accumulate below the surface of [[mudflat]]s or sandflats. Evaporite-saline minerals, tidal-flood, and [[Aeolian processes|aeolian]] deposits characterize many sabkhas found along modern coastlines. The accepted [[type locality (geology)|type locality]] for a sabkha is at the southern coast of the [[Persian Gulf]], in the [[United Arab Emirates]].<ref name="NeuendorfOthers2005a">Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. {{ISBN|0-922152-76-4}}</ref><ref name="TuckerOthers2009a">Tucker, M.E. and Wright, V.P., 2009. ''Carbonate sedimentology.'' John Wiley & Sons. and Warren, J.K., 2006. Evaporites: sediments, resources and hydrocarbons. Springer Science & Business Media.</ref><ref name="Warren2006a">Warren, J.K., 2006. ''Evaporites: sediments, resources and hydrocarbons.'' Springer Science & Business Media.</ref><ref>J.K. Warren, 'Sulfate Dominated Sea-Marginal and Platform Evaporative Settings: Sabkhas and Salinas, Mudflats and Salterns', ''Developments in Sedimentology'' 50 (1991), pp. 69-187.</ref><ref name="Al-SayariOthers2012a">Al-Sayari, S.S. and Zötl, J.G. eds., 2012. ''Quaternary period in Saudi Arabia: 1: sedimentological, hydrogeological, hydrochemical, geomorphological, and climatological investigations in central and eastern Saudi Arabia.'' Springer Science & Business Media.</ref> Evidence of [[Clastic rock|clastic]] sabkhas are found in the geological record of many areas, including the UK and Ireland.<ref>{{Cite journal |last1=Thompson |first1=Jillian |last2=Meadows |first2=Neil S. |date=1997 |title=Clastic sabkhas and diachroneity at the top of the Sherwood Sandstone Group: East Irish Sea Basin |url=http://dx.doi.org/10.1144/gsl.sp.1997.124.01.15 |journal=Geological Society of London, Special Publications |volume=124 |issue=1 |pages=237–251 |doi=10.1144/gsl.sp.1997.124.01.15 |bibcode=1997GSLSP.124..237T |issn=0305-8719|url-access=subscription }}</ref> Sabkha is a [[phonetics|phonetic]] transliteration of the [[Arabic language|Arabic]] word ''sabaka'' used to describe any form of [[Salt pan (geology)|salt flat]], including [[salt marsh]]es and [[salt swamp]]s. A sabkha is also known as a ''sabkhah,'' ''sebkha'', or ''coastal sabkha''.<ref name="Al-SayariOthers2012a"/>
The term ''sabkha'' has been used as a general term for any flat area, coastal or interior, where, as the result of evaporation, salt and other evaporite minerals precipitate near or at the surface.<ref name="NeuendorfOthers2005a"/> The term '''continental sabkha''' is used for such environments found within deserts. Because of the confusion created by using ''sabkha'' for salt flats and [[Dry lake|playas]], it has been proposed that the usage of this term be abandoned for playas and other intracontinental basins and flats.<ref name="Briere2000a">Briere, P.R., 2000. ''Playa, playa lake, sabkha: Proposed definitions for old terms.'' ''Journal of Arid Environments,'' 45(1), pp.1-7.</ref>
==Origin and development==
=== Abu Dhabi sabkhas === The origin and progression of coastal sabkha development at the southern shore of the [[Persian Gulf]] was first discussed in detail in the seminal paper by Evans et al. 1969. The southern shoreline of the Persian Gulf is a shallow, low-angle carbonate ramp characterised by an evaporitic [[Supratidal zone|supratidal]] system passing offshore, via a broad carbonate–evaporite intertidal environment, into a carbonate-dominated [[Littoral zone|subtidal]] system. This is a low-energy setting with a small tidal range ({{Convert|1–2|m|ft|abbr=on}}) and low wave energy as a result of the limited fetch. High rates of evaporation result in salinities of 45–46 g l<sup>−1</sup> along the open-marine coast of [[Abu Dhabi]] and up to 89 g l<sup>−1</sup> in more-restricted lagoons.<ref>{{Cite journal|last1=Lokier|first1=Stephen|last2=Steuber|first2=Thomas|date=2009-04-01|title=Large-scale intertidal polygonal features of the Abu Dhabi coastline|journal=Sedimentology|language=en|volume=56|issue=3|pages=609–621|doi=10.1111/j.1365-3091.2008.00988.x|bibcode=2009Sedim..56..609L |s2cid=130763197 |issn=1365-3091|url=http://doc.rero.ch/record/16727/files/PAL_E3777.pdf }}</ref> The coast of Abu Dhabi is locally protected from open-marine conditions by a number of peninsulas and offshore shoals and islands associated with the east–west trending Great Pearl Bank.
[[Groundwater]] plays a key role in the formation of sabkhas. The phenomenon of groundwater discharging to the surface doesn't always result in visible open water. Instead, the water evaporates upon reaching the surface, leading to the formation of salt deposits. The salt flats of Abu Dhabi are a typical example of this, with the evaporation of water occurring from the capillary fringe – a subsurface layer where groundwater seeps up from a water table – which intersects the surface. This activity has contributed to the creation of an expansive salt flat, covering approximately {{Convert|36,000|km2|sqmi|abbr=on}}.
Much of the chemical content in these flats is attributed to groundwater that seeps to the surface. This seepage results in a concentration of these dissolved substances, which is estimated to be about ten times that found in seawater. The arid conditions of such regions are often characterized by sparse or even completely absent vegetation. This lack of plant cover allows [[aeolian processes]] to interact with the [[phreatic surface]] to form unique landforms, such as sabkhas.
Due to the minimal vegetation, aeolian activity has the capacity to cause deep erosion into the surface sediments. However, it's unable to displace material below the capillary zone due to the full saturation of this zone preventing its uplift by the wind. As a result, the Earth’s surface in such regions tends to mimic the shape and slope of the underlying water table. The resultant surfaces evolve into vast, flat, discharge areas where the process of [[evaporation]] leads to the accumulation of salts. Over time, these accumulations form crusts of a salty nature, characterizing the unique landscape of these regions.<ref>{{Cite journal |last1=Wood |first1=W.W. |last2=Sanford |first2=W.E. |last3=Al Habshi |first3=A.R.S. |date=2002 |title=Source of solutes to the coastal sabkha of Abu Dhabi |url=http://dx.doi.org/10.1130/0016-7606(2002)114<0259:sosttc>2.0.co;2 |journal=Geological Society of America Bulletin |volume=114 |issue=3 |pages=259–268 |doi=10.1130/0016-7606(2002)114<0259:sosttc>2.0.co;2 |bibcode=2002GSAB..114..259W |issn=0016-7606|url-access=subscription }}</ref>
===Khor lagoon=== In the ''khor-lagoon-sabkha'' model, an initial rise in sea-level floods coastal areas and creates shallow water features. If the features silt up, or the land rises, or the sea level falls, then the trapped water [[evaporite|evaporates]], leaving a flat salt pan, or ''sabkha''. If the coastal region has irregular [[topography]], then the flooding creates large independent creeks, or khors. A khor is a shallow, subtidal flat or tidal inlet. The inlet may host grey [[mangrove]]s, depending on whether less saline water is available from [[wadi]]s or [[groundwater]]. As sediment begins to accumulate, the khors become more shallow and form a [[lagoon]], or intertidal flat. The lagoons continue to fill until the lagoon floor is exposed at low tide, and the sabkha begins to form. A sabkha may be inundated during higher than normal spring tides, after rainstorms, or when driving winds push seawater onshore to a depth of a few centimeters. Mature sabkhas are only flooded after heavy rainstorms and may eventually coalesce to form a sabkha [[coastal plain]]. These coastal plains are very flat, with reliefs between {{Convert|10–50|cm|in|abbr=on}}, and their seaward slope can be as little as 1:1,000.
These environments can be found laterally contemporaneous in parallel belts to the coast as well. [[Coral reef]]s, [[barrier islands]], and [[oolite]] shoals form the barrier with the open shelf.<ref name="AlsharhanOthers2003a">Alsharhan, A.S. and Kendall, C.S.C., 2003. ''Holocene coastal carbonates and evaporites of the southern Persian Gulf and their ancient analogues''. ''Earth-Science Reviews'', 61(3-4), pp.191-243.</ref> These types of deposits are indicative of higher energy and protect the khor-lagoon environments, allowing for the growth of [[mangrove swamp]]s and algal and cyano-bacterial [[biofilm|mats]] that prefer the more closed, lower energy environment. Inland of this are the supratidal sabkhas. The sabkhas can be as wide as {{Convert|15|km|mi|abbr=on}} when seaward of dune fields supplying large amounts of sediment. Sabkhas seaward of low outcrops of [[Miocene]] carbonate-evaporites or [[alluvial fan]]s off the [[Oman]] [[fold and thrust belt]] can be as narrow as several hundred meters.<ref name="Al-Farraj2005a">Al-Farraj, A., 2005. ''An evolutionary model for sabkha development on the north coast of the UAE.'' ''Journal of Arid Environments'', 63(4), pp.740-755.</ref>
===Dune field=== If the coast has [[dune]] fields, then flooding creates many smaller pools between the crests of the dunes. In some parts of the world, these lakes can also form in inland deserts, filled by rain or a rising [[water table]] from underground [[aquifer]]s.
For example, large parts of the [[Rub' al Khali|Empty Quarter]] in [[Saudi Arabia]] and the southern [[UAE]] consist of patterns of high drifting [[barchan]] dunes alternating with continental sabkha filled with salt flats. In some places, the continental sabkha connect to form long accessible corridors into the desert.
{| align="center" | [[File:Rub' al Khali (Arabian Empty Quarter) sand dunes imaged by Terra (EOS AM-1).jpg|thumb | center| Dunes and salt flats in the [[Rub' al Khali|Empty Quarter]]]] | [[File:Rub-al-Khali 800.jpg|thumb | center| Dunes and salt flats in the [[Rub' al Khali|Empty Quarter]]]] | [[File:Liwa NASA.jpg|thumb|right|Dunes and salt flats south of the crescent-shaped [[Liwa Oasis]] (UAE).]] |}
The third picture shows the area south of the crescent-shaped [[Liwa Oasis]] in the southern [[UAE]]. The picture is about {{Convert|80|km|mi|abbr=on}}, with each continental sabkha about {{Convert|2–3|km|mi|abbr=on}} long and {{Convert|1|km|mi|abbr=on}} wide. White deposits of salt cover the surface of the continental sabkha. The [[Moreeb Dune]], rising {{Convert|120|m|ft|abbr=on}} above the continental sabkha, is located roughly in the middle of the picture. The border between [[Saudi Arabia]] and the [[UAE]] is shown in red.
The floor of a continental sabkha is usually a hard-packed combination of sand, mud and salt. It is easy to walk or drive 4x4 vehicles across the dry, continental sabkha. However, after rains and flash-floods, the continental sabkha fill with shallow layers of water, and cannot be crossed until they dry out to form a new crust. When the ground is partly dried, a salt crust forms over soft mud or hollow cavities, and vehicles become stuck after breaking through the crust.
{| align="center" | [[File:Lençóis Maranhenses 899a.jpg|thumb | center| Flooded dune field in the [[Lençóis Maranhenses National Park]] (Brazil).]] | [[File:Bilutu Peak.JPG|thumb | center| Flooded dune field in the [[Badain Jaran Desert]] (China).]] |}
==Climate effects== The climate is one of the main factors in sabkha development. Rainfall in this arid region usually occurs as thunderstorms and averages {{Convert|4|cm|in|abbr=on}}/year.<ref name="LokierOthers2008a">Lokier, S. and Steuber, T., 2008. ''Quantification of carbonate-ramp sedimentation and progradation rates for the late Holocene Abu Dhabi shoreline''. ''Journal of Sedimentary Research'', 78(7), pp.423-431.</ref> Temperatures can range in excess of {{Convert|50|C|F|abbr=on}} to as low as {{Convert|0|C|F|abbr=on}}. Humidity is linked to the wind direction, with humidity as low as 20% in the mornings from off the dry interior and building in the afternoon as a strong, onshore wind prevail. At night, relative humidity of 100% can lead to dense fogs.<ref name="PattersonOthers1981a">Patterson, R.J. and Kinsman, D.J.J., 1981. ''Hydrologic framework of a sabkha along Arabian Gulf.'' ''American Association of Petroleum Geologists bulletin'', 65(8), pp.1457-1475.</ref> Water temperatures vary by depth with shallow water being as much as {{Convert|10|C|F|abbr=on}} warmer. These high temperatures drive high rates of evaporation in the [[Persian Gulf]], as much as {{Convert|124|cm|in|abbr=on}}/year leading salinity to increase in the shallow lagoons to as much as 70 ppt.<ref name="AlsharhanOthers2003a"/> The net rate of evaporation from the sabkha can be as much as an order of magnitude less and has averaged {{Convert|6|cm|in|abbr=on}} for the last 4,000 to 5,000 years.<ref name="PattersonOthers1981a"/> The reasons for this are that the sabkha surface is not a free-water surface, the high humidity during the night, and vertical stratification of the air column. Despite the loss of water due to evaporation, the groundwater, never deeper than {{Convert|1.5|m|ft|abbr=on}}, flows seawards and is recharged by continental waters, rainstorms, and the northwest "shamal” gale-force winds that create waves of greater height than the intertidal height and drive water as much as {{Convert|5|km|mi|abbr=on}} inland over the sabkha to a depth of a few centimeters.<ref name="Al-Farraj2005a"/> The climate variations lead to the very dynamic nature of a sabkha. [[Halite]] is deposited on the surface of the sabkha and [[gypsum]] and [[aragonite]] precipitate in the subsurface<ref name="Butler1969a">Butler, G.P., 1969. ''Modern evaporite deposition and geochemistry of coexisting brines, the sabkha, Trucial Coast, Arabian Gulf.'' ''Journal of Sedimentary Research'', 39(1). pp 70-89.</ref> via [[capillary action]] from brines brought up from the water table.<ref name="AlsharhanOthers2003a"/> In drier parts of the sabkha the gypsum can be altered to [[anhydrite]] and the aragonite can be [[dolomitized]] [[Diagenesis|diagenetically]].<ref name="PattersonOthers1981a"/> Thermal contraction at night and expansion during the day leads to concave polygonal pans as the edges have been upturned, in part due to growth of evaporites wedging the crack apart.<ref name="AlsharhanOthers2003a"/> Below this is a gypsum mush where nodules of anhydrite and other sulfates may develop. These might also form a “chicken wire” crystalline structure. Below this are the intertidal deposits typified by laminated, organic-rich muds formed by the [[microbial mat]]s that grade downward into more [[Bioturbation|bioturbated]] muds. The subtidal facies show carbonate [[grainstone]]s and lagoonal muds.
These [[facies]] sequences, except for the [[halite]] that is frequently re-dissolved when wetted, can easily be preserved. Factors enabling preservation include the progradation of the sabkha with sedimentation rates of {{Convert|1|m|ft|abbr=on}}/1,000 years and the creation of Stokes surfaces. These surfaces are created by the deflation of the sabkha surface that is related to the level of the groundwater table acting as a local base level.<ref name="ShanleyOthers1994a">Shanley, K.W. and McCabe, P.J., 1994. ''Perspectives on the sequence stratigraphy of continental strata.'' ''American Association of Petroleum Geologists bulletin'', 78(4), pp.544-568.</ref>
==Hydrocarbon reservoirs== Sabkha deposits are believed to form some of the major subsurface hydrocarbon reservoirs in the Middle East (and elsewhere). The source of these [[hydrocarbons]] (both gas and oil) may be the microbial mats and mangrove [[paleosoils]], found in the sabkha sequence, that have a total organic carbon of up to 8.2% and hydrogen indices typical of marine type II [[kerogen]]s.<ref name="AlsharhanOthers2003a"/>
Some ancient analogs include immediate subsurface formations such as the [[Permian]] Khuff [[Geological formation|Formation]], [[Jurassic]] Arab and Hith [[anhydrite]]s, and Tertiary sedimentary rocks. Similar deposits are also found in the [[Ordovician]] [[Williston Basin]], the [[Permian Basin (North America)|Permian Basin]] in Texas, as well as the Jurassic [[Gulf of Mexico]]. Modern sabkhas are present in varying form along the coasts of [[North Africa]], [[Baja California]], and at [[Shark Bay, Western Australia|Shark Bay]] in [[Australia]].
==Geotechnical engineering== Execution of [[construction]] and [[civil engineering]] works in sabkhas must overcome a number of [[geotechnical engineering]] issues. Sabkha soils are often distinguished by their low strength, as the concentrated salt solutions found in sabkha brines can weaken the soil structure. In addition, the extreme climatic conditions under which sabkha deposits form – such as substantial temperature fluctuations and recurrent cycles of wetting and drying – can induce instability in these soils, and some minerals that act as binding agents, or 'cements', in these soils have a high solubility, which can potentially reduce the overall structural integrity.
Coastal sabkhas are composed predominantly of minerals such as [[calcite]], [[Dolomite (mineral)|dolomite]], and [[gypsum]]. These are accompanied by smaller quantities of [[anhydrite]], [[magnesite]], [[halite]], and [[carnalite]], as well as various other sulfates and chlorides. The [[groundwater]] in these areas is characterized by its high salinity, with sodium chloride levels potentially reaching up to 23%. This saline water often resides close to the ground level. Notably, the sodium chloride concentration can be substantial enough to pose a risk of [[corrosion]].
Conversely, the mineral composition within inland sabkhas tends to be more variable when compared to their coastal counterparts. The precipitated minerals within the soil of these inland sabkhas depend significantly on the specific composition of the local groundwater. Thus, the geological and chemical composition of these environments may differ widely based on regional groundwater characteristics.<ref>{{Cite book |last=Bell |first=Fred G. |title=Basic environmental and engineering geology |date=2008 |publisher=Whitless Publ |isbn=978-1-4200-4470-6 |edition=Reprinted |location=Dunbeath}}</ref>
Techniques used to improve sabkha soils for construction purposes include [[dynamic compaction]].<ref>{{Cite journal |last=Swann |first=L.A. |date=1984 |title=Improvement of Sabkha soils by preloading and dynamic compaction at a housing site in Saudi Arabia. |url=https://pascal-francis.inist.fr/vibad/index.php?action=getRecordDetail&idt=8967625 |journal=International Conference on Advances in Piling and Ground Treatment for Foundations |pages=101–118}}</ref> ==See also== *{{annotated link|Badain Jaran Desert}} *{{annotated link|Chott el Hodna}} *[[Garabogazköl]] *{{annotated link|Lençóis Maranhenses National Park}} *{{annotated link|Sabkhat Matti}} *[[salt pan (geology)|Salt pan]]
==References== {{Reflist}}
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==External links== *[http://earthobservatory.nasa.gov/IOTD/view.php?id=36093 Sebkhat El Melah, Tunisia], from NASA's [[Earth Observatory]]
[[Category:Salt flats]] [[Category:Geological processes]]