Coastal and continental sabkhas of Barr Al Hikman, Sultanate of Oman

The Barr Al Hikman sabkhas, on the east coast of the Sultanate of Oman, cover about1400km2 of low-lying topography (the peninsula is about 30km wide). They formwithin a thin irregular soft-sediment veneer over a complex geological substrate whererecent sea-level fall, together with structural movement, has provided emergent, marineto lagoonal carbonate host sediments of varying thickness, as well as ponded depressions,suited to the development of evaporite deposits. The sabkhas develop under anextremely arid climate with high evaporation in spite of frequent early-morning highrelative humidity. Evaporite and thermalite minerals form either as halite-dominateddeposits in coastal low-lying to ponded environments, or as gypsum layers in soft grainyhost sediments. Geomorphological analysis has been made on satellite images and hasbeen tested by ground truthing. Sediment sampling and sedimentological analyses havealso been carried out. The sabkhas may be grouped into two types: coastal sabkhas andcontinental sabkhas, based on their geomorphology and likely hydrogeology. Lowerlying coastal sabkhas (1-5m altitude) are occasionally flooded, sometimes by runoffafter short heavy rainfall and sometimes by unusually high-water marine influx, to feedextensive perennial halite deposits. The slightly higher continental sabkhas (lyingbetween 5 and 15m altitude), may be fed exclusively by continental groundwaters,and are depleted in halite by surface runoff after occasional rains. A dual permeabilitysystem is proposed that might govern flow in the continental aquifer (moderate to lowpermeability in the non-lithified Quaternary to older lithified deposits; conjectural highpermeability along fault and fracture conduits). Dolomite is forming in high-salinitycoastal lagoons; a microbially mediated origin is preferred, as opposed to hypersaline ormixing-zone chemical precipitation. An older generation of sabkha deposits, preservedas terraces and patches of exposed evaporite, may possibly be linked to higher sea-levelsix thousand years ago. A summary comparison with the Abu Dhabi sabkhas revealssome commonalities, but many contrasting features.