Aspects of the Abu Dhabi Sabkha

Abstract

The Flandrian transgression inundated an embayed Abu Dhabi coastline defined by headlands dominated by Miocene and Quaternary continental strata. Subsequent coastal development has produced the sabkha where Holocene marine carbonate dominates the outer reaches of the coastal plain whilst the inner parts are dominated by Pleistocene aeolian sediment which also underlie the Holocene sequence. The distribution of the Holocene carbonates, mainly formed under a forced regression, has been controlled by the formation of two barrier lagoon systems, the first of which is represented by a severely deflated storm beach system and the second by the present-day barrier island complex. The early storm beaches are still active along the seaward margin of Sabkha Matti, beyond the western end of the protective barrier island complex. The small shallow lagoons behind the deflated storm beaches have been infilled with marine and aeolian carbonates and early Holocene evaporites including transgressive and regressive anhydrite. The currently active lagoons behind the frontal barrier island complex have been progressively infilled by both mainland coastal accretion and barrier island enlargement through leeward accretion. Such leeward accretion has sometimes dominated the two processes. The modern intertidal microbial mat is separated from, and partly overlaps, the buried regressional microbial mat and seems to represent a renewed eustatic transgression in response to global warming. Whilst the mainland coastal sabkhas are expanding seawards, they are also extending landwards as a result of aeolian deflation. Marine and aeolian processes are therefore important in the bidirectional enlargement of the coastal sabkhas. Late Holocene peritidal and sabkha evaporites are forming along the mainland coastline seawards of the deflated storm beaches, but they are very rare on the barrier islands. Some banded anhydrites with planar bases and containing bouquets of anhydrite which replaced vertical gypsum crystals, plus interbedded microbial mat, indicate former salinas on the sabkhas. Holocene dolomite is difficult to detect in the field. Much of the dolomite is likely to have formed inorganically, perhaps encouraged by the presence of Mg-Si clay such as palygorskite, but there is increasing evidence of microbially induced dolomitization within the microbial mats. There is also evidence, from sulphide concentrations, of dolomite resulting from the activity of sulphate-reducing bacteria, possibly within former salinas. Whilst several Holocene hardgrounds are known to exist, they tend to display remarkably flat tops and irregular bases and may extend over large areas. They may act as chronological markers although it is possible that they may be diachronous. The once pristine sabkhas are now so spoiled by petroleum and civil engineering activities that Abu Dhabi and the global community are in grave danger of completely losing the best natural example of coastal sabkhas in the world although as yet there are regrettably only very limited efforts to try and preserve a small area as a site of special scientific interest.