Bulletin 1 - March 1977: Sand, Sabkha and Shells



Sand, Sabkha and Shells

by John Scott

A summary of the talk presented by John Scott to the ENHG on 31st January 1977.

The main sedimentary environments of the coast around Abu Dhabi, and the associated marine invertebrate faunas were described; these may be summarized as follows:

The sabkha between the dunes and the sea is a faunal and floral desert. It is the site of precipitation of various minerals from saline water (e.g. anhydrite and gypsum). The sabkha is at present growing to fill the lagoons.

The algal mats are areas of dense filamentous algae that grow in sheltered locations between normal high water mark and low water. Apart from algae and bacteria, other forms of life are rare.

The lagoons are found between the islands and consist of a complex series of shallow shoals and banks with relatively deep channels in between. To the northwest they pass into the open sea. At the landward side of the lagoons, salinity is high and the annual and daily fluctuations in temperature are large. Some mangrove is found at the head of the lagoons. Towards the open sea the salinity decreases and the temperature range becomes less. The fauna becomes more diverse as the lagoons become less restricted to the open sea. In the inter-tidal areas crabs are very common. Large numbers of small gastropods (marine snails) are also found in parts of the lagoon. In the inner areas the sediment is often made up of fecal pellets, probably produced by these snails. Towards the mouth of the lagoons the sediment is coarser, being mainly lime sand derived from animal skeletons. Corals with an associated rich fauna of bivalves, gastropods, sea urchins and starfish are found in patches in the outer lagoon. This fauna is a depauparate Indo-Pacific fauna. The number of coral species, for instance, is less than in the Indian Ocean, although all the species found around Abu Dhabi occur in the Indo-Pacific. Similarly the bivalve fauna is less diverse; the giant clam, for example, is missing from the coral reefs around Abu Dhabi and indeed throughout the Gulf.

The islands are accumulations of lime sand and are elongate parallel to the direction of the prevailing wind between north and northwest. The islands form from sands generated in the surrounding shallow water around a core of solid rock (either cemented desert sand or shallow water limestone).

The oolite deltas form at the junction of the lagoons and the open sea in areas of high tidal currents. The oolites are composed of chemically precipitated lime in the form of small sand-sized spheres.

To the northwest of the lagoon, island and oolite delta complex lies the open water of the Gulf.


 


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