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

Abstract

The Barr Al Hikman sabkhas, on the east coast of the Sultanate of Oman, cover about 1400km 2 of low-lying topography (the peninsula is about 30km wide). They form within a thin irregular soft-sediment veneer over a complex geological substrate where recentsea-level fall,together with structural movement,hasprovided emergent,marine to lagoonal carbonate host sediments of varying thickness, as well as ponded depressions, suited to the development of evaporite deposits. The sabkhas develop under an extremely arid climate with high evaporation in spite of frequent early-morning high relative humidity. Evaporite and thermalite minerals form either as halite-dominated depositsincoastallow-lyingtopondedenvironments,orasgypsumlayersinsoftgrainy host sediments. Geomorphological analysis has been made on satellite images and has beentestedbygroundtruthing.Sedimentsamplingandsedimentologicalanalyseshave also been carried out. The sabkhas may be grouped into two types: coastal sabkhas and continental sabkhas, based on their geomorphology and likely hydrogeology. Lower lying coastal sabkhas (1–5m altitude) are occasionally flooded, sometimes by runoff after short heavy rainfall and sometimes by unusually high-water marine influx, to feed extensive perennial halite deposits. The slightly higher continental sabkhas (lying between 5 and 15m altitude), may be fed exclusively by continental groundwaters, and are depleted in halite by surface runoff after occasional rains. A dual permeability system is proposed that might govern flow in the continental aquifer (moderate to low permeability in the non-lithified Quaternary toolder lithifieddeposits; conjectural high permeability along fault and fracture conduits). Dolomite is forming in high-salinity coastal lagoons; a microbially mediated origin ispreferred, as opposed to hypersaline or mixing-zone chemical precipitation. An older generation of sabkha deposits, preserved as terraces and patches of exposed evaporite, may possibly be linked to higher sea-level six thousand years ago. A summary comparison with the Abu Dhabi sabkhas reveals some commonalities, but many contrasting features.