Modes of Formation, Sedimentary Associations, and Diagnostic Features of Shallow-Water and Supratidal Evaporites

Production of brine depends critically on favorable physiographic setting and climate. The physiographic requirements are not common on a large scale, but are common on a small scale in the form of lagoons, pans, and exposed supratidal surfaces. Such environments may be present on the peripheries of large brine basins. The most common evaporite environment is the exposed salt flat or sabkha,(FOOTNOTE 3) a fairly level, salt-encrusted surface that only occasionally is inundated. Coastal sabkhas are supratidal surfaces formed by depositional offlap of marine sediments, and the associated evaporites are precipitated from seawater-derived basins. Continental sabkhas comprise continental or earlier cycle marine sediments, and the associated evaporites are precipitated from eva orated continental waters. Sabkhas are equilibrium geomorphic surfaces, their levels being dictated by the local level of the groundwater. Water-table depths in very few places exceed 1-2 m. Sabkhas are formed where the eolian sand supply is limited and where hard rock lies below the level of the groundwater table. Framework sediments of coastal sabkhas may range from almost pure carbonate to noncarbonate, depending on local supply of terrigenous detritus. Sedimentary evidences of emergence and desiccation are abundant. Evaporite pans may be associated with sabkha environments, but they have distinctive characteristics. Illustrative examples from the Persian Gulf and Baja California are given. Particularly significant are the high rates of shoreline regression (1-2 m/yr), which would all w a sabkha 100-200 km wide to be formed in only 105 years. The concept of a mega-sabkha, as being an evaporite environment rather different from a sabkha, probably is erroneous. Sabkha diagenesis involves interstitial emplacement of evaporite minerals within the host sediments and changes in the host sediments themselves, such as dolomitization. Evaporites are restricted largely to the upper levels, but dolomitization may extend in depth as brines move downward and seaward through the sediment wedge. Sabkha gypsum crystals typically are lensoid and commonly contain inclusions, in contrast to brine-pan gypsum crystals, which are elongate and free from inclusions. Sabkha anhydrite is typically nodular. Gypsum and anhydrite emplaced diagenetically within sabkha sediments in places are crudely layered but not laminated, whereas sedimentary lamination of detrital sediment and evaporite minerals commonly is well developed in brine-pan environments. Brine pans usual y accumulate more soluble evaporites than sabkha areas because of the greater evaporation losses from exposed surfaces. In addition to diachronism of sedimentation, coastal sabkhas show diachronism of diagenesis, the evaporite-mineral belts moving progressively seaward as depositional offlap proceeds. Early stages of diagenesis are masked by later stages and, during this process, the area occupied by the younger, more soluble evaporite minerals increases. Data from the Persian Gulf indicate that with time the coastal sabkha zone dominated by gypsum is shifted progressively seaward but its area remains roughly constant, whereas the area dominated by anhydrite progressively increases. Thus a fossilized sabkha will not necessarily contain all the minerals, or even traces of the minerals, that were developed within it during early diagenesis, even if late diagenetic changes are minor. Many early diagenetic minerals are ephemeral phases only.