UAE Jurassic


The Holocene sediments of the United Arab Emirates have been used to interpret portions of the sediments of the underlying upper portions of the Upper Jurassic Arab Formation (Fig. 7). Comparisons of present day sedimentary sequences with this part of the formation suggests that some the sequences of litho facies of the Arab Formation, particularly in central Abu Dhabi where the Hith Anhydrite pinches out, belong to an inner shelf lagoon tidal flat sabkha sequence (Alsharhan and Kendall, 1994; Alsharhan and Whittle, 1995; Silwadi et al., 1996). However it should be recognized that the Tithonian aged Hith Anhydrite, a major hydrocarbon seal in the Arabian Gulf region, most frequently contains less than 20% carbonate over much of the area it occupies, often in the form of thin laminations. This lack of carbonate, local layers of salt, and the predominance of anhydrite argue a playa setting (Warren and Kendall, 1985) for this extensive deposit, which covers a much greater geographical area than the current coastal area of the Southern Arabian Gulf and has a much thicker sequence of sediments dominated by evaporites, with much less carbonate than in the Holocene sabkha sediments of Abu Dhabi. In contrast to central Arabia, the Hith beneath the southern Arabian Gulf appears to have accumulated in a sabkha setting matching the present day supratidal evaporite depositional settings along the Arabian Gulf coastal area. Like these settings these deposits have a relatively limited areal extent. This interpretation is supported by the interbedded relationship of the Hith anhydrites with tidal flat carbonates and the local predominance of horizontally flattened nodules and enterolithic layers of anhydrite. These latter features match some of the characteristic fabrics found in the Holocene coastal sabkhas of the United Arab Emirates. Here the setting of the Hith Anhydrite was apparently dominated by salt flat encrusted plains and playas formed in an essentially arid climate. Along its seaward margins we believe storm activity accompanied by exceptionally high tides resulted in flooding of the sabkha surface and deposited "storm sediments". Since the sabkha had a very flat surface, such sediments would have been deposited over extensive areas. The formation mudflats characterized this intertidal setting and cyano bacterial mats, while the periodic flooding produced influxes of shallow subtidal inner lagoon sediments. Beach bars probably developed close to the seaward margins of the local Arab Formation shelf, and shoals were formed at the landward limits of this formation by high storm tides. The shallow subtidal (lagoonal) setting of the Arab Formation in the UAE is characterized by marine biota and sediments that became progressively coarser grained landwards. Coarser grained sediments may have formed proximal to offshore bars in the outer lagoon. Channels running into the lagoon from the intertidal zone may also have produced an influx of channel outwash bioclastic coarse grained sands. Shoals are interpreted to have divided the lagoons from the intertidal zones. Channel sands dependent on tidal energy tended to be restricted to lagoons and intertidal settings. The shelf setting extended seawards from the outer reaches of the lagoon, with the inner boundary generally delineated by offshore bars.

The Asab Oolite (the lateral equivalent of the Hith Formation) in eastern Abu Dhabi is composed mainly of well sorted oolitic grainstone. The ooids have cortices composed of thin laminae, and the nuclei composed of bioclastics and occasional foraminifera (Alsharhan, 1989; De Matos, 1994). There is local evidence of vadose meniscus cements and absence of marine cements, suggesting accumulation on high energy wave dominated tidal barrier island complex. This is analogous to the present day barrier island complex located along eastern coast of Abu Dhabi. The Manifa reservoir in central Abu Dhabi represents the back barrier facies to the Asab oolite. It consists of dolomitic lime mudstone and oolitic limestone and has a restricted fauna. This is broadly analogous to the present day Abu Dhabi lagoon at Al Dhabaiya and Al Qanatir Islands where there is currently a low diversity hypersaline fauna.


Monday, May 23, 2016
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