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 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 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 muds. The subtidal facies show carbonate grainstones 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 m/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. 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 total organic carbon up to 8.2% and hydrogen indices typical of marine type II kerogens. Some ancient analogs include immediate subsurface formations such as the Permian Khuff Formation, Jurassic Arab and Hith anhydrites, 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. See also References External links Sebkhat El Melah, Tunisia, from NASA's Earth Observatory Salt flats Geological processes Geography of the United Arab Emirates