A supratidal sabkha plain at Rabigh area, Reel Sea coast, Saudi Arabia occupies an extensive flat depression between highly elevated Quaternary coral reef terraces, aeolian and fluvial sandy hills. Based on the local topography, mineralogy, sedimentology, hydrology, textures and structures of the evaporite sediments, the sabkha plain is subdivided into three basins, which are: (1) Three saline pans occupy the lowest topographic depressions. The first is the coastal saline pan located near the Red Sea shoreline, which receives its water from the Red Sea directly through an inlet or by seepage influx through a barrier. The second is represented by two inland saline pans, which are located to the east and receive their water from the Red Sea seepage, as well as from sporadic and torrential rains kept in the surrounding dunes and fluvial sands. (2) Saline mudflats that surround the saline pans, and are moistened most of the year by groundwater seepage from the Red Sea and from sporadic rains. (3) Dry sandflats that are located between the saline mudflats zone and the surrounding high lands of the Quaternary coral reef terraces, dunes and fluvial sandy hills.
Deposition of the evaporite sediments in the sabkha plains took place through the three stages; flooding, evaporative concentration and desiccation. During flooding stage in winter time, inflow of water led to: (a) inundation of the lowest depression with brackish water, (b) partial dissolution of the last dry-season halite crust, (c) moistening of the surrounding slightly elevated lands, and (d) deposition of muddy layers from suspension in the saline pans. During summer time, the evaporative concentration stage is characterized by: (a) deposition of halite layers in the form of rafts, cumulates, skeletal crystals, chevrons and cornets in the saline pans, (b) displacive growth of lenticular, prismatic and nodular gypsum in the saline rnudflats, and (c) displacive growth of prismatic gypsum in the sandflat zone. The desiccation stage is characterized by: (a) further growth of clear halite cement in the dissolution cavities of the surface halite crust, (b) continuous growth of displacive clear halite and gypsum in the dry saline pans and mudflats, respectively, which rupture the sediment surface into tepee polygonal structure, (c) efflorescent precipitation of halite and polyhalite on the surface of the mudflats and sandflats, (d) desiccation of the mudflat surface into mudcracks, and (e) formation of adhesion ripples and sand sheets over the sandflat sediment surface.
Mineralogical composition of the evaporite sediments in the sabkha plain indicates that: (a) halite is the dominant mineral in the saline pans, (b) gypsum dominates the saline mudflats in a relatively large amount than in the sandflats, (c) polyhalite is recorded, in a decreasing order of abundance from the sandflats, the mudflats, and the saline pans.
Chemical compositions of the brine indicate an origin from recent marine water, with small contribution from deep meteoric water in the sandflat zone.
The present model of sabkha formation in Rabigh area is useful in interpretation of similar recent and ancient evaporites on the Saudi and Egyptian sides of the Red Sea with respect to their mineralogy, texture, structure, paleoclimate and paleohydrology.
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