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Reservoir Characterization, Research & Consulting (UK) Ltd, Ty New
dd, 5 Greys Hollow, Rickling Green, Saffron, Walden, Essex CB11 3YB, UK
During Quaternary time, the southern flank of the Arabian Gulf can be considered as a classic, foreland basin ramp setting in which the sedimentological processes operating changed drastically along strike. The combined effects of sea-level fluctuations and wind, which have dominated the Quaternary coastal morphology and coastal evolution, are observed at a resolution rarely achieved in the stratigraphic record of palaeoramp sequences. The facies distributions and diagenetic characteristics of the relatively thin Pleistocene-Holocene strata provide valuable insights into the complex stratigraphy that can develop within an inner ramp and its continental fringe.
In the UAE, Pleistocene sediments comprising mainly aeolian dunes overlie a deeply eroded topography of Upper Miocene strata. These sediments become increasingly carbonate rich and calcite cemented northwards towards the coast. Here they experienced severe aeolian deflation due to the later marine flooding of the aeolian sources on the formerly exposed areas lower down the ramp profile, and wind erosion along the coastal area because of a lack of sediment supply. The antecedent topography of Miocene and Pleistocene escarpments, mesas, islands, cemented seif dunes and coastal deflationary ‘Stokes surfaces’ provided a highly varied Holocene ramp margin undergoing coastal progradation (with offlap) during the 1–2 m relative sea-level fall of late Holocene time. Since then, marine carbonate deposition and evaporite diagenesis have dominated the Holocene sequence, but it is aeolian processes that largely control their ultimate distribution within the constraints of the contemporaneous sea-levels. Longshore currents, lateral and leeward accretion, infilling of stranded lagoons and deflation on a massive scale testify to the crucial role of wind in the development of this proximal ramp. The Holocene aeolian sediments, though extensive, are very different in character from the Pleistocene sediments.
Two generations of Holocene supratidal anhydrites are recognized. The earlier one was partly transgressive, but the later one is regressive. In addition to the present-day oolith factories on the ebb tidal deltas at the seaward ends of tidal channels between the barrier islands of central Abu Dhabi, oolith factories also exist on exposed beaches and tidal embayments of western Abu Dhabi. They also occurred within Pleistocene and Holocene embayments. Flat-topped and sometimes shingled hardgrounds representing isochronous events extend over very large areas. So also do commonly observed iron oxide bands formed beneath the sabkha surfaces.
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