Modern Evaporite Deposition and Geochemistry of Coexisting Brines, the Sabkha, Trucial Coast, Arabian Gulf

ABSTRACT Near Abu Dhabi, Trucial Coast, diagenetic evaporite minerals are developed within an accumulation of detrital Pleistocene sands and Recent carbonate sediments which form a supratidal flat (sabkha) 8 miles wide. Field studies were carried out by the author and his co-workers of Imperial College in 1964. A further visit to the Trucial Coast was made by the author in 1967 under the auspices of the Dry Lands Research Institute, Riverside. A study of 260 interstitial brine samples showed that the distribution of the evaporite minerals, which include gypsum, anhydrite, dolomite, and balite, is largely controlled by the hydrology of the area. Interstitial brines within the sabkha sediments are replenished with sea water by abnormally high tides and storm surges. These surface waters, which soak down into the sediments, are concentrated both by solution of soluble salts and by evaporation. The subsequent equilibration between brines and host sediments leads to precipitation of aragonite, gypsum, and halite; alteration of gypsum to anhydrite, and dolomitization of carbonate sediment. A direct correlation exists between the frequency and extent of flooding, the mineral phases developed, and the chemical composition of interstitial brines. Flooding of a 2- to 3-mile wide belt roughly concentric to lagoon margins, occurs at monthly or longer intervals. In this belt, a surface gypsum crystal mush up to 12 inches thick is present which, landwards, is overlain and progressively replaced by anhydrite. Dolomitization is extensive toward the landward margin of this zone. From the lagoon laterally across the 2- to 3-mile wide belt to its landward margin, chlorinity increases from 44 to 155; sulfate concentration rises from 6 to 17 gm/kg and then falls to less than 3 gm/kg; the Mga2+/Ca2+ mole ratio increases from 5.3 to 35 and then decreases to about 4. Concentration of these components decreases with in reasing depth below the sediment surface. Gypsum is the stable calcium sulfate mineral in contact with brines of chlorinities less than 145 and anhydrite at chlorinities greater than 145. An additional 5-mile wide zone is flooded only at 4- to 5-year intervals. The original gypsum crystal mush in this zone is completely replaced by up to a 12-inch thick layer of chicken-wire mesh structure anhydrite, and the carbonates are highly dolomitized. Interstitial brines are Saturated with NaCl at chlorinity 160 and specific gravity 1.20. These brines contain less than 1 gm/kg sulfate and have Mg2+/Ca2+ mole ratios of about 4. In this zone, constant Mg2+/Ca2+ values with increasing magnesium concentration suggest that there has been an approach to mutual equilibrium between brines and evaporite-carbonate minerals. The remaining 1 to 2 miles at the landward margin of the sabkha are underlain by Pleistocene sands and are not flooded by marine waters. Ground waters in this zone are in part terrestrial in origin. Inland across this zone, chlorinity decreases from 160 to 107; the sulfate concentration is 2 gm/kg and the Mg2+/Ca2+ mole ratio averages 3.5. Locally, anhydrite is hydrated to gypsum where brine chlorinity is less than 145. Processes of gypsum precipitation and dolomitization are essentially restricted to the 2- to 3-mile wide belt close to the lagoon margin. In the mid-sabkha, equilibrium conditions probably prevail between brines, diagenetic minerals, and host sediments. Diagenesis at the landward margin of the sabkha is in part retrogressive. Both a progressive and a retrogressive diagenetic facies can be visualized as moving out with the progradation of the shore line. The similarity between the stratigraphy and mineralogy of some Ordovician, Silurian, Pennsylvanian, and Jurassic evaporite sequences and the sabkha on the Trucial Coast suggest that evaporites of supratidal origin may be more common in older evaporite rocks than previously realized.