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A sabkha () is a predominately coastal, supratidal mudflat or sandflat in which evaporite-saline accumulate as the result of a semiarid to arid climate. Sabkhas are gradational between land and Intertidal zone zone within restricted coastal plains just above normal high-tide level. Within a sabkha, evaporite-saline mineral sediments typically accumulate below the surface of 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.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. 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.Warren, J.K., 2006. Evaporites: sediments, resources and hydrocarbons. Springer Science & Business Media.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.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. Evidence of Clastic rock sabkhas are found in the geological record of many areas, including the UK and Ireland. Sabkha is a phonetics transliteration of the Arabic language word sabaka used to describe any form of salt flat, including and . A sabkha is also known as a sabkhah, sebkha, or coastal sabkha.
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. 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, it has been proposed that the usage of this term be abandoned for playas and other intracontinental basins and flats.Briere, P.R., 2000. Playa, playa lake, sabkha: Proposed definitions for old terms. Journal of Arid Environments, 45(1), pp.1-7.
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 .
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.
These environments can be found laterally contemporaneous in parallel belts to the coast as well. , barrier islands, and oolite shoals form the barrier with the open shelf.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. These types of deposits are indicative of higher energy and protect the khor-lagoon environments, allowing for the growth of and algal and cyano-bacterial biofilm that prefer the more closed, lower energy environment. Inland of this are the supratidal sabkhas. The sabkhas can be as wide as when seaward of dune fields supplying large amounts of sediment. Sabkhas seaward of low outcrops of Miocene carbonate-evaporites or off the Oman fold and thrust belt can be as narrow as several hundred meters.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.
For example, large parts of the 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.
| Empty Quarter]]]] | Empty Quarter]]]] | 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 , with each continental sabkha about long and wide. White deposits of salt cover the surface of the continental sabkha. The Moreeb Dune, rising 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.
| Flooded dune field in the [[Lençóis Maranhenses National Park]] (Brazil).]] | Flooded dune field in the [[Badain Jaran Desert]] (China).]] |
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 subsurfaceButler, 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. via capillary action from brines brought up from the water table. In drier parts of the sabkha the gypsum can be altered to anhydrite and the aragonite can be dolomitized Diagenesis. 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. 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 that grade downward into more Bioturbation muds. The subtidal facies show carbonate 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 /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.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.
Some ancient analogs include immediate subsurface formations such as the Permian Khuff Formation, Jurassic Arab and Hith , and Tertiary sedimentary rocks. Similar deposits are also found in the Ordovician Williston Basin, the 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 in Australia.
Coastal sabkhas are composed predominantly of minerals such as calcite, 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.
Techniques used to improve sabkha soils for construction purposes include dynamic compaction.
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