GEOLOGY AND FOSSILS
Most of the surface of the present day UAE is a sand desert, stretching from the Arabian Gulf coast south to the unbroken and uninhabited sands of the Empty Quarter, and east to the gravel plains bordering the Hajar Mountains. The desert is a geologically recent feature, the result of prolonged subaerial erosion and deposition in an arid environment. The sands overlie the thick, oil-rich sedimentary strata of the Arabian Platform which constitutes the bedrock of most of the UAE, but the oil producing rocks are nowhere exposed at the surface, and are known only from drilling.
In many areas near the coast, the sand is stabilized by vegetation, although the natural flora has been altered in recent times by extensive grazing of domesticated animals. Further inland the sands may be quite barren, as few plants can successfully colonize the mobile dunes.
Sand dune formation is controlled by a combination of wind strength and direction, and sediment supply. In detail, however, the formation of dune patterns is complex and remains poorly understood. Within a given area the dune pattern may be quite regular, but also very intricate. Physical features are typically created on several different scales: giant sand ridges on a scale of hundreds of metres to a few kilometres, sand dunes measured in metres to tens of metres, and ripples on a scale of centimetres to a metre or more. This hierarchy can be readily observed in the deserts of the UAE.
Since dune patterns vary with wind direction, seasonal or occasional variations in wind direction introduce new elements into the overall pattern. These elements may reinforce or cancel each other, in the same manner as ocean waves. In addition, because sand dunes cannot move or change as quickly as ocean waves, past history may play a significant part in what we see today. Despite relatively consistent prevailing wind directions in the present day UAE, dune patterns and alignment vary considerably from area to area .
The Effects of Climatic Change
The largest dune features of the present day UAE, including the major transverse dunes of Liwa and Manadir, the smaller eroded dune ridges of the Northern Emirates, and the longitudinal dunes of the south-west, are believed to date from the most recent glacial period, more than 10,000 years ago. A glacial origin for these major features is consistent with the fact that they do not seem to be aligned with today's prevailing winds.
The present day wind regime appears to be transporting material from the coast inland and reworking the surface of the major earlier structures without, so far, removing or reorienting them. For example, one may observe between Abu Dhabi and the Liwa oasis that extensive tongues of pale sand resembling a choppy sea of dunes (aligned NE-SW) are filling in broad troughs between higher, flatter ridges of red sand (aligned WNW-ESE). The latter are known as suruq , or easy travel zones, and are interpreted as the eroded cores of older, larger ridges. Further inland, however, the major transverse dune ridges of Liwa and Manadir are neither in motion nor are they being eroded at the present time, although the smaller dunes on their surfaces conform to present day winds.
In addition to changes in wind regime, the UAE deserts have experienced changes in rainfall at various times in the past. This is indicated by the widespread occurrence of outcrops of lightly cemented, cross-bedded dune sands. These were cemented by the precipitation of calcium carbonate and other salts from ground water at a time when the water table was higher than it is today. Other evidence of higher rainfall in the past includes playa lake sediments, horizons containing abundant fossil roots and burrows, fossil reeds, crocodile bone, freshwater mollusc shells and trails, and fragments of ostrich eggshell. Occasional gravel deposits, often preserved as low, flat-topped hills or mesas , testify to the presence of rivers.
Some of these features may be attributable to the alternation of so-called pluvial (wet) and inter-pluvial (dry) periods recognized elsewhere and believed to correlate with the stages of Pleistocene glaciation. Arid conditions in the UAE predated the Pleistocene, however. The widespread Miocene deposits of the Baynunah Formation (c.6-7 million years old) in the west of Abu Dhabi are interpreted as a major river system that watered a semi-arid, subtropical savannah. The Baynunah formation contains the fossilized remains of early relatives of elephants, hippopotamus, horses, cows, crocodiles, turtles and other animals. The intervening Pliocene is not known from the UAE, but was a period of drought in both East Africa and the Mediterranean.
Sabkha is the Arabic term for low-lying saline flats subject to periodic inundation. Three types are recognized, based on their environment of formation. All are found in the UAE. Coastal sabkha, as the name implies, forms at or near the marine shoreline. Fluvio-lacustrine (ie river-lake) sabkha is formed in association with riverine drainage patterns in arid areas. Inland or interdune sabkha is found in low-lying basins within the sand desert.
All sabkhas share certain characteristics. Although they are restricted to hot, arid regions, the sabkha surface is always very close to the local water table, usually within about a metre. Groundwater is drawn towards the surface by capillary action and evaporates in the upper subsurface in response to the high temperatures. There it deposits dissolved salts, including calcium carbonate, gypsum (CaSO4-2H2O), anhydrite (CaSO4) and sodium chloride or halite (NaCl), which precipitate in that order. These salts create a hard, impermeable crust in a zone about half a metre below the surface. This crust, along with high salinity, discourages all plant growth. The crust also impedes the drainage of surface water, so that after rains the sabkhas flood. The surface water then evaporates over time, often leaving behind a dazzling white crust of salt.
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