The Abu Dhabi Sabkha Environment (United Arab Emirates): Facies Stacking Patterns, Evaporite Overprint, and Microbial-Mediated Dolomite
Vertical and lateral sabkha sequences studied along the Abu Dhabi coastline at Mussafah Industrial Channel and in the vicinity of Al-Qanatir Island are unique modern day geologic examples that add greatly to the interpretation of features observed in core and outcrops deposited many millions of years ago.
The vertical sabkha sequence formed during the post-glacial Flandrian transgression, resulting in the reworking of Pleistocene aeolian dunes (radiocarbon age approx. 24,000 to 23,000 years before present) and the deposition of intertidal to shallow subtidal Holocene microbial mat, rooted and microbial-laminated lagoon, tidal-channel, tidal-delta, and longshore beach bar and beach spit deposits (approx. 6,600 to 5,000 years before present). Recent find of whale bones within tidal-channel deposits overlying the microbial mat further document the initial Holocene transgression. During a subsequent slight sea-level fall (regression), these carbonates were partly to completely overprinted (replaced) by gypsum and anhydrite.
The lateral sabkha sequence displays supratidal (upper, middle, and lower sabkha), intertidal (microbial mat) and lowermost intertidal to shallow subtidal (lagoon and tidal-channel) facies belts. Radiocarbon age-dating of ten hardground and three microbial mat samples show an age range from approx. 3,500 years before present (supratidal environment: subsurface hardground) to approx. 900 years before present (intertidal environment: subsurface microbial mat); thereby supporting the seaward progradation of the facies belts since the last Flandrian sea-level highstand.
Significant amounts of dolomite were found within the rooted and microbial-laminated lagoonal carbonates, the subsurface and surface crinkly-laminated microbial mats, some of the tidal-channel deposits, and within some of the Pleistocene carbonate-rich sands. The dolomite is very fine-crystalline and displays spherical morphologies as well as subhedral to euhedral dolomite rhombs. The formation of dolomite is interpreted to be related to dolomite-mediating microbes, which form the widespread microbial mat along the Abu Dhabi coastline. Biopolymers of microbial origin, referred to as exopolymeric substances (EPS), are interpreted to play a key role in the formation of low-temperature dolomite. Mineralization through excretion of EPS could be a widespread, but yet underestimated, process occurring today and during the geological past.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009