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Geologie Spojených arabských emirátů (Geology of UAE)

26.06.2011  |  61× přečteno      vytisknout článek 

Geologie Spojených arabských emirátů (Geology of UAE)

The Emirates landscape splits naturally into two distinct regions - the vast areas of recent sediments (Gulf shoreline and desert) and the narrower, but magnificent, mountain chain.

 The Gulf shoreline is a classic carbonate barrier coast and for this reason it is visited regularly by eminent geologists who come to study the modern-day equivalents of many oil-bearing reservoir rocks.

The Gulf waters contain many coral banks and reefs although the number of species of coral and other shell-life that can tolerate the high salinity is few. These coral patches give way to a series of fringing islands which stretch south westwards from Gharab, Sadiyat and Abu Dhabi to Halat al Bahrani, Abu al Abyad, and to Bazm al Gharbi. Rounded grains of lime (called ooliths) are washed by currents and waves driven by the dominant northwest trade winds into shoals and islands that protect the lagoons behind.

The shallow waters of the lagoons cover vast areas that fill and drain with each tide. As this great volume of water drains through only a few channels, so the currents within these channels become very strong (often as much as five knots). Small pockets of mangrove grow along the edges of the lagoons and a black, mat-like area of a primitive species of algae, often mistaken for oil pollution, forms along the tidal limits of the lagoon.

Behind this lie the extensive salt flats known as 'sabkha' that are only covered by the sea during freak storm conditions. This coastal complex is backed by recently abandoned salt flats and a series of low escarpments of Tertiary rocks (called the Lower Fars limestones) that can be seen along the Abu Dhabi-Tarif road.

Behind these low escarpments stretch the vast trains of sand dunes that slowly move south eastwards blown by the ever-present northwest winds. The dunes vary in size and color. Near the coast they are cream to buff but towards the interior they become a deep red-brown color, as at Al Ain, along the Al Ain-Dubai road and towards Dhaid. In the southern part of the Emirates, the dunes become giant-sized as near Al Liwa, Qusahwira and Al Hamrah. This is the northeastern tip of the Empty Quarter (Rub al Khali). The red-brown color of the sands is due to oxidizing conditions of the arid climate. The sand grains often have a thin gray coating of clay that is oxidized to a rusty ferric iron material. These sand dunes are separated by areas of gravel - the coarse grains and pebbles that the wind cannot move, or by areas of inland 'sabkha' within which the lovely 'desert roses' form. Desert rose is the name given to a form of interlocking crystals of gypsum (CaSO42H2O - the same mineral as plaster-of-Paris). The crystals grow easily but slowly in the sub-surface ground waters where evaporation has caused the salinity to be sufficiently high for the mineral to precipitate.

Backing this desert scene is a long range of stark, jagged mountains from which huge fans of gravel and boulders spread out, as near Shwaib and Al Madam. The present-day scenery is a relic of post-Pleistocene times (i.e. since the Ice Age) when the climate was far wetter than today. Large rivers raced off the mountains distributing vast amounts of pebbles and boulders as huge fans abutting the mountains and filling up the valleys to form almost flat-topped boulder beds. As the climate became warmer and drier again, the rivers became smaller. At this time also, the land generally uplifted a little and the rivers cut down through these ancient fans and boulder beds to form modern small but impressive wadis, as along the Masafi-Fujairah road and those wadis southwest of Dibba.

The surface boulders and pebbles have lain baking in the sun for several thousand years. They have been polished smooth by the desert winds and have 'sweated out' iron, magnesium and manganese salts to produce a distinctive black mineral surface and a dark appearance to the plains, as inland of the Fujairah-Dibba road.

The hard rocks of the sub-surface are several thousand meters thick. They are mainly limestone (often oil or gas-bearing), evaporates, shale and sandstones, similar to those that are accumulating at the surface today (Gulf coastline deposits backed by desert sands). The ancient seas in which these sediments accumulated have fluctuated during the last 500 million years. Sometimes they extended as far as the Saudi Arabian massif, at other times they shrank to less than their present extent.

The mountains of the Saudi Arabian desert separate the desert from the Red Sea and African continent nowadays as they have done for the past hundreds of millions of years. However, the UAE-Oman mountain chain has only been a barrier for the last 15-20 million years.

The Arabian Peninsula has moved gently around the Earth's surface, but has always remained or near the tropical zones. The climate and seas have therefore usually been warm tropical or subtropical often with desert lands bordering them.

These deeply buried sediments that depict this history of ancient Gulf desert regimes lie beneath the blanket of recent deposits. Individual beds of rock spread for miles like a layer of a cake, extending from Saudi and Qatar through Abu Dhabi and across, beneath the Gulf waters, towards Iran.

During the mid-Tertiary times earth movements folded and tilted these rocks and sometimes they break through the recent cover to form scenic mountains or 'jebels' as at Jebel Hafit, where Oligocene and Eocene rocks outcrop; and at Jebel Huwayyah, where Upper Cretaceous rocks are present. More on: http://www.enhg.org/bulletin/b26/26_02.htm

Geologická mapa Spojených arabských emirátů je k vidění na adrese (Geological map of UAE) : http://www.bgs.ac.uk/research/international/uae.html


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