Sabkha is a transliteration

Transliteration

Transliteration is a subset of the science of hermeneutics. It is a form of translation, and is the practice of converting a text from one script into another...

 of the Arabic

Arabic language

Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...

 word for a salt flat. Sabkhas are supratidal, forming along arid coastlines and are characterized by evaporite-carbonate deposits with some siliciclastic

Siliciclastic

Siliciclastic rocks are clastic noncarbonate sedimentary rocks that are almost exclusively silica-bearing, either as forms of quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or deposited through some mechanical process, such as stream deposits that are...

s. Sabkhas form subaerial

Subaerial

The term subaerial is mainly used in geology to describe events or structures that are located at the Earth's surface...

, prograding and shoaling-upward sequences that have an average thickness of a meter or less. The accepted type locality is along the coast of the Persian Gulf

Persian Gulf

The Persian Gulf, in Southwest Asia, is an extension of the Indian Ocean located between Iran and the Arabian Peninsula.The Persian Gulf was the focus of the 1980–1988 Iran-Iraq War, in which each side attacked the other's oil tankers...

 in the United Arab Emirates

United Arab Emirates

The United Arab Emirates, abbreviated as the UAE, or shortened to "the Emirates", is a state situated in the southeast of the Arabian Peninsula in Western Asia on the Persian Gulf, bordering Oman, and Saudi Arabia, and sharing sea borders with Iraq, Kuwait, Bahrain, Qatar, and Iran.The UAE is a...

. This article outlines research into the evolution, hydrocarbon potential, climate, and dynamic nature of these deposits.

The origin and progression of sabkha development in the Persian Gulf

Persian Gulf

The Persian Gulf, in Southwest Asia, is an extension of the Indian Ocean located between Iran and the Arabian Peninsula.The Persian Gulf was the focus of the 1980–1988 Iran-Iraq War, in which each side attacked the other's oil tankers...

 is discussed by Al-Farraj (2005). In the khor-lagoon-sabkha model presented, an initial rise in sea-level floods coastal dune fields forming embayments between the dune crests. As the sediment is re-worked by the dominant winds and currents, spits, that parallel the shore, create a khor

Khor

Khor is an ancient region located in the south of ancient Syria, probably around modern Lebanon. It has long been an outpost of ancient Egypt....

. A khor is a shallow, subtidal flat or tidal inlet in which grey mangrove

Grey Mangrove

Grey mangrove may refer to either of two species of mangrove:* Avicennia marina - occurring around the Indian Ocean and into the western Pacific Ocean as far as New Zealand...

s may or may not grow depending on whether less saline water is available from wadi

Wadi

Wadi is the Arabic term traditionally referring to a valley. In some cases, it may refer to a dry riverbed that contains water only during times of heavy rain or simply an intermittent stream.-Variant names:...

s or groundwater

Groundwater

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

. As sediment now begins to accumulate, these khors shallow and begin to form a lagoon

Lagoon

A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

, an intertidal flat. The lagoons continue to shallow until the lagoon floor is exposed at low tide. This is the beginning of sabkha development. An immature sabkha will 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 eventually coalesce to form a sabkha coastal plain

Coastal plain

A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...

. These coastal plains are very flat, with reliefs between 10-50cm, and their seaward slope can be as little as 1:1,000. The flatness of a sabkha is enhanced by aeolian siliciclastic

Siliciclastic

Siliciclastic rocks are clastic noncarbonate sedimentary rocks that are almost exclusively silica-bearing, either as forms of quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or deposited through some mechanical process, such as stream deposits that are...

 dust being deposited in the topographic lows with most of the relief caused by evaporites.

These environments can be found laterally contemporaneous in parallel belts to the coast as well. Coral reef

Coral reef

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s, barrier islands, and oolite

Oolite

Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...

 shoals form the barrier with the open shelf. These types of deposits are indicative of higher energy and protect the khor-lagoon environments, allowing for the growth of mangrove swamps and algal and cyano-bacterial mats

Biofilm

A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...

 that prefer the more closed, lower energy environment. Inland of this are the supratidal sabkhas. The sabkhas can be as wide as 15 km when seaward of dune fields supplying large amounts of sediment. Sabkhas seaward of low outcrops of Miocene

Miocene

The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 carbonate-evaporites or alluvial fans off the Oman fold and thrust belt can be as narrow as several hundred meters.

The climate is one of the main factors in sabkha development. Rainfall in this arid region usually occurs as thunderstorms and averages 4cm/year. Temperatures can range in excess of 50°C to as low as 0°C. 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. Water temperatures vary by depth with shallow water as much as 10+°C warmer. These high temperatures drive high rates of evaporation in the Persian Gulf

Persian Gulf

The Persian Gulf, in Southwest Asia, is an extension of the Indian Ocean located between Iran and the Arabian Peninsula.The Persian Gulf was the focus of the 1980–1988 Iran-Iraq War, in which each side attacked the other's oil tankers...

, as much as 124 cm/year leading salinity to increase in the shallow lagoons to as much as 70ppt. The net rate of evaporation from the sabkha can be as much as an order of magnitude less and has averaged 6cm for the last 4,000 to 5,000 years. 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 1.5m, flows seawards and is recharged by continental waters, rainstorms, and the NW “shamal” gale-force winds that create waves of greater height than the intertidal height and drive water as much as 5km inland over the sabkha to a depth of a few centimeters.

The climate variations lead to the very dynamic nature of a sabkha. Halite is deposited on the surface of the sabkha and gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

 and aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

 precipitate in the subsurface via capillary action out of brines brought up from the water table. In drier parts of the sabkha the gypsum can be altered to anhydrite

Anhydrite

Anhydrite is a mineral – anhydrous calcium sulfate, CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium and strontium sulfates, as might be expected from the...

 and the aragonite can be dolomitized diagenetically. 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 microbial mats that grade downward into more bioturbated

Bioturbation

In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

 muds. The subtidal facies show carbonate grainstones and lagoonal muds.

These facies

Facies

In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....

 sequences, except for the halite

Halite

Halite , commonly known as rock salt, is the mineral form of sodium chloride . Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities...

 that is frequently re-dissolved when wetted, can easily be preserved. Factors enabling preservation include the progradation of the sabkha with sedimentation rates of 1m/1000 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.

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 total organic carbon up to 8.2% and hydrogen indices typical of marine type II kerogen

Kerogen

Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...

s.

Some ancient analogs include immediate subsurface formations such as the Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 Khuff, Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 Arab and Hith Anhydrite

Anhydrite

Anhydrite is a mineral – anhydrous calcium sulfate, CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium and strontium sulfates, as might be expected from the...

, and Tertiary sedimentary rocks. Similar deposits are also found in the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 Williston Basin

Williston Basin

The Williston Basin is a large intracratonic sedimentary basin in eastern Montana, western North and South Dakota, and southern Saskatchewan known for its rich deposits of petroleum and potash. The basin is a geologic structural basin but not a topographic depression; it is transected by the...

, the Permian Basin in Texas, as well as the Jurassic Gulf of Mexico

Gulf of Mexico

The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

. Modern sabkhas are present in varying form along the coasts of North Africa

North Africa

North Africa or Northern Africa is the northernmost region of the African continent, linked by the Sahara to Sub-Saharan Africa. Geopolitically, the United Nations definition of Northern Africa includes eight countries or territories; Algeria, Egypt, Libya, Morocco, South Sudan, Sudan, Tunisia, and...

, Baja California

Baja California

Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...

, and at Shark Bay

Shark Bay, Western Australia

Shark Bay is a World Heritage Site in the Gascoyne region of Western Australia. It is an area centred approximately on , 800 kilometres north of Perth, on the westernmost point of Australia. An expedition led by Dirk Hartog happened upon the area in 1616, becoming the second group of Europeans...

 in Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

.

Sources

• Al-Farraj, A., 2005. An evolutionary model for sabkha development on the north coast of the UAE, Journal of Arid Environments, v. 63, p. 740
• Alsharhan, A.S., and Kendall, C.G.St.C., 2003. Holocene coastal carbonates and evaporites of the southern Persian Gulf and their ancient analogues, Earth-Science Reviews, v. 61, p. 191
• Butler, G.P., 1969. Modern evaporite eposition and geochemistry of coexisting brines, the sabkha, Trucial Coast, Arabian Gulf, Journal of Sedimentary Petrology, v. 39, no. 1, p. 70
• Lokier, S.W., and Steuber, T., 2007. Seasonal dynamics of a modern sabkha surface, Geophysical Research Abstracts, v. 9
• Patterson, R.J., and Kinsman, D.J.J., 1981. Hydrologic Framework of A Sabkha Along Arabian Gulf, AAPG Bulletin
  AAPG Bulletin
  The AAPG Bulletin is a monthly peer-reviewed scientific journal covering geosciences and associated technologies relating to the energy industry. It is an official journal of the American Association of Petroleum Geologists.-Abstracting and indexing:...
  
  , v. 65, p. 1457
• Patterson, R.J., and Kinsman, D.J.J., 1982. Formation of Diagenetic Dolomite in Coastal Sabkha Along Arabian (Persian) Gulf, AAPG Bulletin, v. 66, no 1, p. 28
• Shanley, K.W., and McCabe, P.J., 1994. Perspectives on the Sequence Stratigraphy of Continental Strata, AAPG Bulletin, v. 78, no. 4, p. 544
• Warren, J.K, and Kendall, C.G. St. C., 1985. Comparison of Sequences Formed in Marine Sabkha (Subareal) and Salina (Subaqueous) Settings—Modern and Ancient, AAPG Bulletin, v. 69, no, 6, p. 1013

External links

• Sebkhat El Melah, Tunisia, from NASA's Earth Observatory