EVOLUTION OF COASTAL SABKHAT

Coastal sabkha development results from the intersection of several critical components: a porous, clastic framework; a shallow water table; potential evaporation greater than precipitation; a source of eolian material and wind of significant velocity to transport it. The latter two requirements are critical to maintain a dynamic equilibrium between the elevation of the top of capillary zone and the elevation of the sabkha surface. Within the capillary zone, water and solutes rise in response to water evaporation at the surface. Retrograde minerals such as gypsum, anhydrite, calcite, and dolomite precipitate within this zone as thermalites rather than evaporites – there is no evaporation within this zone. Mineral precipitation in the capillary zone reduces the effective pore diameter, causing increased elevation of the capillary head and results in the capture of additional eolian material. This increasing surface elevation results in progradation of the sabkha seaward and creates additional pore space for mineral precipitation. Thus, it is possible to realize both an increase in sabkha area and significant thicknesses of retrograde minerals without tectonic changes in sea or land level or changes in climatic conditions. Chloride and nitrate minerals form as evaporites on the sabkha surface. However, overland runoff from infrequent storms removes much of this material, transporting it to the adjacent water body, thus preventing its accumulation with time.