Recent evaporite deposition associated with microbial mats, Al-Kharrar supratidal–intertidal sabkha, Rabigh area, Red Sea coastal plain of Saudi Arabia

The supratidal–intertidal sabkha of the Al-Kharrar area, Red Sea coast, Saudi Arabia, contains the evaporite minerals gypsum, anhydrite, and halite. Microbial mats flourish adjacent to the sabkha evaporites in tidal flats and pools of the Al Kharrar lagoon. Desiccation and decay of some microbial mats in tidal flat areas have led to precipitation of gypsum and halite there. The evaporite minerals have been precipitated through displacive, inclusive, and replacive growth within mud, sand, gravelly sand, and bioclastic sediment of the sabkha. Gypsum occurs as lenticular and tabular crystals whereas anhydrite occurs as nodular (individual, mosaic, and enterolithic) and pseudomorphs of lenticular gypsum crystals that grew displacively and replacively near the surface of the sabkha. Halite exists as a diagenetic cement within the sabkha sediment, or as primary rafts and skeletal crystals in desiccated tidal pools with salinity over 220‰. Microbial mats are growing on the surface of the upper tidal flat areas and in pools at a salinity range of 80–110‰, and they lead to biostabilization of the sediment. They have induced a range of sedimentary surface structures (MISS) including gas domes, reticulate patterns, tufts, pinnacles, wrinkles, and microbial shrinkage cracks. The occurrence, abundance, and association of evaporite minerals and MISS are controlled by local environmental factors such topography of the sabkha, emergence or submergence of tidal areas, surface area of the evaporite basin, contribution of meteoric water from floods from the adjoining Red Sea Mountains, and water salinity. These factors promote the growth of the microbial mats in the winter months, and deposition of evaporite minerals during summer months. Field and petrographic data indicate that the main recharge to the sabkha area is from tidal flow and water seepage from the Al-Kharrar lagoon. The results of this study indicate that within a small sabkha area of Al-Kharrar (3 × 17 km), a large variation in evaporite mineral types and morphologies grade into and are associated with MISS due to local environmental parameters. The interpretation of this association of evaporite minerals and MISS provides useful data for understanding the mechanisms responsible for precipitation of evaporite minerals and formation of MISS.