LAUSR

Microbial mats and gypsum exist in concentration ponds due to the first stage of evaporation, whereas halite is present in the crystallizers with continuous seawater evaporation, in the saltwork of Dhahban area, north of Jeddah city, Saudi Arabia. Several gypsum and halite facies types are distinguished in response to differences in microbial activity, salinity, topography of the ponds, water depth and wave energy. Two gypsum facies are recognized; planar–wavy gypsum microbialite and domal gypsum stromatolite at the relatively low salinity, inlet margin, and the relatively high salinity, outlet margin of the concentration ponds, respectively. They consist of fine and coarse, fibrous and swallowtail twin gypsum crystals that nucleate on the surface of microbial mats. Halite crystallizes at the brine surface as floating cumulates and rafts, and at the bottom of the ponds as vertically elongated chevrons and cornets. Lateral chevrons grow at the bottom of the ponds and at the brine surface in association with vertical chevrons and halite spheroids, respectively. Fluid-inclusion bands are regular and symmetrical in halite crystals, or show asymmetrical and indistinct layers in chevrons, and as a continuous layer over top of cumulates and rafts. This study suggests that saltworks represent a valuable ‘laboratory’ for the study of the primary depositional features of gypsum and halite crystals that are related to microbial activity, salinity, depth and/or energy conditions. The results can be used to infer physical energy conditions for deposition of gypsum and halite in ancient evaporitic basins.