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OOID SHOALS |
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LINKS |
Three major west to east geomorphic provinces form the general coastal complex of the Emirate of Abu Dhabi. Each of these may contain shoals on which ooids accumulate. For instance ooid sands accumulate and cover extensive areas in the waters of less than ten meters of eastern Abu Dhabi along the northern flanks and crests of the ebb tidal deltas sited flanking the barrier islands between Ghunat and Ras al Hanjure to the Dhubaiya peninsula (Kinsman, 1964b; Purser and Evans, 1973). Others areas of ooid accumulation are found in the beaches flanking Jebel Dhanna, on the sands accumulating on the shoals just south east of Sir Beni Yas (Purser and Evans, 1973), and also seaward of the al Qanatir Bu Sharah Shelf and channel region. The ooid particles are sheathed in aragonite needles which are nucleated about carbonate pellets and bioclastic grains. quartz and other siliciclastic grains are uncommon as the nucleus of the ooids and in most cases are non existent as the cores of these carbonate grains.. |
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EBB DELTA OOID SHOALS OF EASTERN ABU DHABI
East of the peninsula of Dhabiaya are the Abu Dhabi Barrier Islands:
- Halat al Baharani
- Abu Dhabi
- Sadiayat
The have accumulated on a narrow shelf open to heavy seas, have steep beach faces and landward form large subaerial dunes that prior the building boom of the United Arab Emirates often had a height greater than 5 m. Lieing between and just north of these islands are series of ebb tidal deltas and their axial tidal channnels. The axial tidal channels are up to sixty feet deep and are entrenched in the Pleistocene and maybe the Tertiary too. The ebb deltas are the site of shoals on which ooids accumulate. The ooids are being reworked by the tides and onshore nothern winds (Shamals) to form a series of megaripples (Kinsman, 1964b; Purser and Evans, 1973). Locally the shoals are accreting on the aeolian and marine Pleistocene. Also these islands and peninsula of Al Dhabaiya are nucleated around Pleistocene aeolian calcareous sands and marine sediment, the miliolite that is locally elevated some 2 to 4 m above sea level (Kirkham, 1998b). The Holocene sediments accumulating on the seaward side of these low islands are largely mix of ooids, bioclastic debries and aeolian carbonates which in turn reach only a few meters above sea level. |
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COASTAL BARCHAN DUNES OF OOIDS
These images record how onshore winds often carry the ooids from the tidal deltas to form the coastal dunes discussed briefly above, some of which are barchanoid. This can be seen on this aerial photograph from the 1950s before the building boom that has overtaken Abu Dhabi city.At 24 °N, the U.A.E. has an arid, subtropical climate, with an annual temperature range 50 to 0 °C. This explains the restricted character of the vegetation, though diurnal dews associated with moisture brought inland from the humid Arabian Gulf by the late afternoon and evening onshore winds (Kendall and Skipwith, 1969; Ministry of Water, Oman 1995). During cloudless nights radiation of heat into the atmosphere causes a drop in temperature over the land areas and initiate the precipitation of dew along the near shore areas and sometimes the development of thick morning fog (dhabab). These dews support a local quite dense cover of halophytes on the dunes that line the immediate coast just landward of the berms formed on most of the beaches. This vegetation is also found on some of the hills that occur in the nearshore. The dews lose their intensity and eventually disappear landward across the dune belt, and this decrease in precipitation is matched by a loss of the vegetation cover found close to the coast. The result is that the vegetation dies as the coast progrades seaward. As indicated elsewhere it also explains the widespread occurrence of carbonates and evaporites of this area. |
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CORAL REEFS LINE SOME TIDAL CREEKS
As occurs in similar settings in the Bahamas small areas of reef are often found lining the margins and floors of the tidal channels and seaward or north of central portions of the barrier islands between these tidal deltas as laterally discontinuous reefs (Evans et al., 1964). These latter channels are the site of the accumulation of well abraded mollusks, echinoids, foraminifera, and corals. Kinsman (1964b) was the first to record coral reefs developing along the seaward flanks of the inter island tidal channels and in the seaward portions of the lagoons where interestingly tidal channel waters provide enough circulation to enable coral growth. |
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"DRUMB STICK" SHAPED BARRIER ISLANDS PROTECT LAGOONS TO THEIR SOUTH
The"drumb stick" or ‘T' shaped barrier islands are oriented with the top of the ‘T' lying seaward and parallel to the trend of the coast at right angles to the dominant wind direction (the Shamal). Their long leaward tails or stems extend south in response to the aeolian transport of carbonate sand and the lateral accretion of sand spits by longshore currents. They subdivide and restrict the circulation in the series of protected macrotidal carbonate mud prone coastal lagoons. Tidal movement in these lagoons tends to be at right angles to the coast and landward of the barrier islands, the lagoonal sediments are commonly dominated by lime mudstones with scattered skeletal fragments derived from in situ growth of organisms. Where sand size carbonates occur in the seaward portions of the lagoons these are often composed of fecal pellets. Close to the edges of the oolite deltas, where they are subject to flood tides, grapestones accumulate.
At low tide extensive parts of the lagoon floor are exposed and evaporation of the intertidal waters results in increased salinities. This is matched by a decrease in echinoids, corals, and algae and an increase in gastropods and imperforate foraminifera. Here, as in other north facing protected coastal areas of the United Arab Emirates, the accumulation of lagoonal sediments is being encroached on and terminated by sediments from tidal flat settings (often lined by mangroves and cyano bacterial mats) that in turn are flanked and being encroached on by supratidal salt flat (sabkha) evaporite rich sediments. This coastal complex lies just north of a series of an earlier high energy beach system, demarked by stranded beach ridges dominated by the shells of cerithid gasteropods. |