Bulletin 37 - March 1989: The Coastal Sabkha of Abu Dhabi

The Coastal Sabkha of Abu Dhabi

The coastal area of Abu Dhabi includes several small shoals, islands, protected lagoons, channels and deltas, an inner zone of intertidal flats (where algal mats are well developed) and broad areas of supratidal salt flat designated as 'sabkha'. Sabkha is the local Gulf Arabic word for flat, salt-crusted desert.

There are two types of sabkha, coastal and continental. They have developed where hard rock lies below the groundwater table, allowing water to remain close to the surface and susceptible to evaporation.

Radiocarbon dating has indicated that the coastal sabkha of Abu Dhabi was formed within the past 7000 years (Evans 1969). The same author states that the "transgression began in this area about 7000 years ago and reached an apparent high about one meter above its present level somewhat prior to 4000 years B.P. Since then, progration of intertidal and supratidal sediments has taken place; this began 3750 years ago." This means that the land area is presently expanding seaward. We can expect the lagoonal areas to eventually dry out and the islands to coalesce. Since 1960 the Abu Dhabi region has become the model area for sabkha evaporites.

The sabkha area runs approximately parallel to the coast and in some places is up to 16km wide. It has a slope that averages 1:3000 and lies above the level of present-day high tide. Extensive flooding occurs when offshore winds (the 'shamal') combines with high spring tides and during sporadic torrential rainfall. After the floods, the pools on the sabkha evaporate and leave salt-crusts 7 to 8cm thick. These crusts are later dissolved when the surface is flooded or washed by rain.

The Abu Dhabi monthly average air temperature ranges from 47' to 12' but the temperature of the sabkha surface can reach 60' or more in summer. As a consequence, evaporation is intensive and the salinity of the open Gulf waters ranges from 40% to 45% while the lagoons can reach a high of 70% in their nearshore areas.

In a depositional environment like the sabkha, with strong evaporation, the pore waters become highly concentrated and are drawn towards the surface, causing the precipitation of evaporites - halite, gypsum and anhydrite, together with some authigenic minerals: aragonite, calcite, dolomite, celestite and magnetite.

The distribution or occurrence of the three more typical minerals of the sabkha - gypsum, anhydrite and dolomite - can be summarized as follows:

a. Gypsum

Gypsum (CaSO4.2H2O) is commonly associated with the algal mats. The algal mats are well developed in the intertidal flats and are composed of blue-green algae. The mats stabilize the muddy sediment and in depth produce a characteristic laminated structure. In some places, the gypsum can make up more than 50% of the top meter of the sediment. Gypsum crystals are mostly discoidal and flattened and oriented with their plane in a vertical position. Crystal sizes up to 1cm are frequent but they can reach 10 to 25cm. Some crystals are clear while others contain calcium carbonate inclusions, even whole gastropods that usually turn out to be Cerithium.

 

b. Anhydrite

Anhydrite is an orthorhombic sulfate, CaSO4, characteristic of the evaporites. There are two kinds of anhydrite. One, with a cottage cheese texture, is now considered to originate by secondary replacement of gypsum. The other occurs more inland and appears as nodules and contorted layers, and is of primary origin.

c. Dolomite

Dolomite (CaMg(CO3)2) first appears at the junction between the algal mats and the sabkha and increases in amount inland. By about 10 km inland the whole of the top meter of sediment may be replaced by fine-grained (2 to 3mm) calcium-rich, disordered dolomite.

Access to the sabkha is much easier today than in the past. Today it can be reached with a normal saloon car. The best way to explore the sabkha is now to the west of Abu Dhabi town, between Al Dhabayah (Zubbaya) and Abu Al Abyadh islands.

At Al Dhabayah, 45 km to the west of Abu Dhabi [Fig. 3], algal mats, mangrove, channels, beach ridges and sabkha are easily visible on the western side of the island. However, the best place to study the sabkha is on the road to Al Rafiq Island that is 13km further to the west of Al Dhabayah [Fig. 3].

There are several aspects to bear in mind when planning a field trip to the sabkha:

1. The best time is between November and April.
2. Visits should be made at low tide.
3. Old shoes and clothes should be worn.
4. A shovel is needed in order to dig sample pits to see the vertical sequence and also (if necessary) to dig out your vehicle.
5. Sample bags are needed.

G. Evans et al (1969) - Stratigraphy and Geologic History of the Sabkha, Abu Dhabi, Persian Gulf in Sedimentology 12, 145-159.

Definitions:

Evaporites: A sediment resulting from the evaporation of saline water. The minerals are formed in the reverse order of their solubilities, i.e. the least soluble first: calcite and dolomite, gypsum and anhydrite, halite (rock salt), and potash and magnesium salts.