UAE Reefs

Some Holocene Reefs of the UAE
Coral Reefs

Corals grow profusely on the offshore shoals, along the northern or windward edge of the offshore bank seaward of the Khor al Bazam and along the seaward edge of the coastal terrace (Kendall and Skipwith, 1969b). Small reefs also line tidal channels and parts of the lagoon edge. Acropora sp., Porites sp. and several species of brain coral in conjunction with calcareous algae have been found to be the main reef builders. Kinsman (1964a) found similar corals form in the Abu Dhabi reefs. As in the Bahamas (Newell and Rigby, 1957), much of the coral examined in the Khor al Bazam was being colonized and bored by other organisms. Focusing on the coral reefs of the Khor al Bazam these can be subdivided into a coral reef front and reef platform.

The coral reef front forms the seaward edge of a coral embankment in which the coral growth is most prolific. The slope of the reef front is dependent on rate of coral growth, on the initial bottom slope, and the intensity of waves and currents. The reef front may take several forms:

(a) straight and lobate vertical fronts
(b) spurred and pinnacled front and
(c) terraced front (Kendall and Skipwith, 1969b).

None of these reef forms exhibits a Lithothamnion ridge or is marked the rubble zone common in the Pacific reefs.

a) Straight and lobate vertical fronts
These are more common and form a vertical wall of corals and calcareous algae. They occur around the northeast edge and northern tip of the Hail bank and north of Abu al Abayad (Landsat image). Kendall and Skipwith (1969b) have shown that poorly developed examples of this front partially rim the coastal terrace that lines the southern side of the Khor al Bazam.
The straight and lobate fronts are probably the products of the relative intensities of the cross currents and the oncoming waves. The straight fronts form in strong cross currents; spurred fronts where wave currents develop perpendicular to the front: and lobate fronts are formed in intermediate conditions. All forms can be seen in the reefs lying seaward of Abu al Abayad and Marawah. The slope of the face of the reef front is probably matches that of the surface on which the reef was initiated. This is believed to be a cliff surface of miliolite.
Vertical fronts undoubtedly once extended along the length of the northern edge of the inner coastal terrace that lines the souther margin of the Khor al Bazam. Old coral reefs can locally be seen to underlie the seaward portions of this terrace. Coral growth here is inhibited, as the offshore bank has become a more effective barrier to water circulation. The current lagoonal waters have elevated salinities and temperatures that are probably too high while the turbulence of breaking waves is too low for the development of a flourishing reef along this coastal terrace margin. As the Khor al Bazam fills and these conditions become more extreme, coral will eventually cease to grow. The terrace is thus a dying fringing reef. Kinsman (1964a) recognized the effect of these high salinities in the enclosed lagoons south of Abu Dhabi


b) Spurred and Pinnacled Fronts
Coral pinnacles are sharply projecting vertical colonies of corals (Shepard 1948). Similar structures have been referred to as coral heads (single corals) and coral stacks, which are upright structures of many corals (Storr, 1964).
Coral pinnacles occur north of the offshore bank near Salaha and opposite Marawah and northwest of Abu al Abayad (Landsat image 9). Pinnacles are also present in the shelter of the back reef lagoons, as just north of the peninsula of Al Dhabaiyah where they are associated with micro atolls. These pinnacles are usually about 10 to 20 m in diameter and may coalesce to form a coral ridge or small coral embankment. Micro atolls (Krempf, 1927) are a halfway stage in the outward growth of coral pinnacles and the generation of coral ridges and small coral banks. The atolls are needle shaped pillars, rimmed entirely by living coral, with the exception of the top where the central corals are deprived of nutrients, smothered by sediment, and exposed at low tide so that they die leaving a bald patch. The atolls are best developed in the back reef off Al Dhabaiyah Peninsula (Landsat image 10).
Spurred and grooved fronts are associated with pinnacles. Shinn (1963) reviewed the development of spurs and grooves, using evidence collected on the Florida reef tract. He suggested that spurs are growing seaward but are periodically trimmed back by severe storms. Detritus is removed from between the spurs by storms, which prevents the infilling of the grooves. The spurs grow seaward since only those coral heads that point directly into the heavy seas are resistant to destruction. Newell et al. (1951) discussed similar features in the Bahamas and Cloud (1959) studied them in Saipan. In the Khor al Bazam, three forms of spurs and grooves occur, which are distinguished by size and position on the ree
  • Seaward of the reef front, spurs can be as much as 2 km long, 400 m wide and between 5 to 10 m high. Grooves may be 800 m wide. The most striking examples of these spurs and grooves occur just north northwest of Abu al Abayad and north of Marawah (Landsat image 7, 10). These spurs and the long axes of most of the pinnacles are aligned along the dominant wind direction. Once pinnacles begin to grow on the sea floor, they trap sediment on their leeward side. While the pinnacles expand radially to form micro atolls, coral and calcareous algae fix their sediment tails. These tails gradually extend toward the reef front, which they join to form a spur. An oriented colony protected in the lee of another colony grows seaward to connect to it; this increases the linearity of the growth centers (Shinn, 1963). Once initiated, spurs grow slowly seaward and grooves between spurs are maintained by wave and tidal scour. Spurs could not develop on the reef front where strong cross currents occur. These large spurs and grooves also could be solution features formed along joint planes during a low Pleistocene sea level, since joint directions and present dominant wind direction coincide.



  • Spurs and grooves on the reef front range between 3 and 5 m high, are sometimes over 16 m wide, and can be between 400 m and 800 m long. Grooves are between 3 and 6 m wide. These features resemble the spurs and grooves of Florida and Spain. They are interpreted to have been initiated as patch reefs that developed tails that became fixed by coral. Any gaps between adjacent patch reefs are kept clear by heavy storms (Shinn, 1963) and are maintained as the spurs grow forward. The best example of these features can be seen northeast of Marawah (Landsat image 7).


  • On the reef flat behind the reef front, spurs can be as much as 1 km long, 0.5 to 2 m wide, and 0.5 to 1 m high. The grooves are some 2 to 6 m wide. These lines of coral occur to the northeast of Marawah (Landsat image 7) and northwest of Salaha (Kendall and Skipwith, 1969b) (Landsat image 11). Like pinnacles and patch reefs, individual coral colonies act as sediment traps. Tails of sediment develop behind the coral and are fixed by coral growth. Once the coral line is initiated, it grows shoreward. Cloud (1959) suggested that similar features in the back reef lagoon of Saipan Mariana Islands of Mexico are produced by movement of sediment trains that scour deeply into the floor of the lagoon and reef. Potholes within grooves observed in Saipan and on the Alacran Reef Complex, Campech Bank, Mexico (Kornicker and Boyd, 1962) were not seen in the Khor al Bazam region of the Arabian Gulf.

c) Terraced fronts
These consist of two or more levels of coral growth and occur along the southeastern edge of the Hail bank and in front of the Janana Salaha bank. Each terrace is believed to have similar origin and to have formed on offshore bars parallel to the offshore bank. The different levels indicate different periods of coral growth. Eventually the lowest tidal level will limit their upward growth. The lower level terraces and the seaward spurs and pinnacles occur at the same depth. Thus, they probably began growth at the same time.
Coral fronts may begin growth on bars, since these bars have some permanence. Aerial photographs show little alteration and Cloud (1959) has shown that although the sand may be in constant motion, corals may colonize it. In the Khor al Bazam, acropora was observed growing on shells on banks of moving sand.

The reef platform is the top of any reef structure that is fairly flat (Storr, 1964). Most of the offshore bank north of Khor al Bazam and the shelves north of Abu al Abayad and north of Al Dhabaiyah may therefore be considered reef platform (Kendall and Skipwith, 1969b). The platforms are largely covered by coral/coralline algae sand and underlain by miliolite (Block diagram). The sand accumulates as megaripples that are just awash at low tide, as is the Florida reef platform (Ginsburg, 1956). Parallel ribbons of coral and seaweed occur to the south of Hail, where they trend north northwest and south south east. Between Jananah and Salaha they trend east west. The ribbons are perpendicular to both the expected direction of waves refracted against the reef front and to ebb and flow tidal currents. They extend almost to the southern edge of the Jananah Salaha section of the offshore bank. Here tidal channels cutting across the bank have been unable to erode the coral but have deepened the areas between the ribbons.
Hollows or depressions in the reef platform and reef fronts range from 2 to 10 m deep and can be from 0.40 to 2 km wide. They occur southeast of Hail or on the seaward side of the Janana Salaha bank, and to the north of Al Dhabaiyah Peninsula and Abu al Abayad (Landsat image 10). They have vertical walls that are rimmed by actively growing coral and are being filled by reef debris washed in by tidal currents and waves. It is unlikely that all the hollows on the reef have the same origin. Some are depressions left as the reef front advances. Large hollows southeast of Hail are of this type and represent old fluvial channels cut into the Miliolite at low Pleistocene sea level, now partially overgrown by coral. Still other hollows in reefs to the north of Salaha are remnant features left by random growth of coral spurs and ridges. Other hollows are erosional features such as abandoned tidal channels. Two hollows just north of Salaha may well be this type of feature. The last possibility is that the hollows represent solution at low Pleistocene sea levels similar to the "blue holes" of the Bahamas (Newell and Rigby, 1957).


Tuesday, March 26, 2013
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