摘要: Description　　Site # 1: The Qattara DepressionThe Qattara Deperssion in the noerthern part of the Western Desert is the largest natural closed depression of the Eastern Sahara. It is a region where sa

Description

　　Site # 1: The Qattara DepressionThe Qattara Deperssion in the noerthern part of the Western Desert is the largest natural closed depression of the Eastern Sahara. It is a region where salt weathering appears to be particularly effective. In spite of earlier research in the 1940’s and 1950’s, the origin of this Depression is still a geological puzzle. A common origin by wind deflation to a base level controlled by the ground water table, is the generally accepted explanation. Other explanations include solution, mass-wasting followed by wind deflation, or that the depression was originally excavated as a stream valley, to be subsequently modified by karstic activity, and was further deepened and extended by mass-wasting, deflation, and fluviatile processes. It has also been suggested that the Depression is of structural control origin.The Qattara Depression forms one of the most significant geomorphological features of the NW desert of Egypt. It is a closed inland basin bounded from the north and werst by steep escarpments, with an average elevation of 200 masl. Towards the south and east the floor of the Depression rises gradually from 60 mbsl to the general desert level of 200 masl again. The Depression has an area of some 18,130 to 19,500 sq. km and an average depth of 60 m below sea level (bsl); the lowest point, lat the SW part, being 134 m bsl. The Depression is estimated to have an excavated volume of 3200 cu. Km. Within the Depression, cones, towers, mushroms, and plateau-like hils, ranging in height from 5 to 30 m, are common, especially near the western scarp. Sinkholes and caves are also common in the northern Diffa Plateau, separating the Depression from the Mediterranean frontal plain. The northern and western escarpments are dominated by large mass-wasted blocks.The Depression is excavated into northerly dipping Miocene and Eocene rocks. Sandy and clayey layers of the Lower Miocene Moghra Formation form its bottom and surroundings, where the elevation ranges from 50 to 80 m bsl. In some areas, the Moghra sediments occur as small plateaus and dissected hills within the sabkhas. Middle Eocene calcareous sediments of the Mokattam Formation form the southern scarp of the Depression. The Upper Eocene-Oligocene Dabaa Formation underlies the SW part of the Depression, including all areas below 100 m bsl. It consists of black shales and contains abundant gypsum veins and shark teeth and remains. The northern steep escarpment is associated with the Middle Miocene calcareous sediments of the Marmarica Formation, with a thickness of a few meters at the rim of the Depression, increasing to several hundred meters at the coast, where Pliocene carbonate rocks are exposed.Over large areas of the floor of the Depression, the bedrock is covered by younger deposits including wind-blown sand, sabkhas, and Quaternary evaporite sediments. The sands that cover most of the Depression are associated with moist sand sheets with adhesion ripples at the surface in the NE part, and large parallel longitudinal lunette, seif, and complex dune belts in the southern part. The dune axes trend nort-north-west-south-southeast, parallel with the prevailing wind direction. The dunes are composed mostly of quartz sand mixed with minor carbonate, mud, shale, and gypsum fragments. Near the SW part of the Depression, the dunes are black, named El-Ghorood El-Sood (= black dunes), due to their high content of black shale fragments, derived from the Dabaa Formation.Since the Qattara Depression forms the deepest point in the Western Desert, groundwater flow in all aquifers bordering the area is consequently directed to this final base level. Most of the groundwater comes from the Moghra Aquifer system, recharged from four sources: the Nubia Sandstone aquifer in the south, Nile water in the east, saline water from the Mediterranean Sea to the north, and rain water. In contrast, in the western part of the Depression, groundwater seepage is recharged from Nubian and Upper Cretaceous-Eocene aquifer systems. The near-surface groundwater ranges in salinity (mostly sodium chloride) from 3.3 g/l around the Moghra Lake, to 38.4 g/l at the center, to about 300 g/l in the sabkha area to the west. An exception to this east-west increase in salinity is found around fresh water springs such as Bir (= well) Qifar area. This increase in salinity from 43.6 to 421.0 g/l, has been attributed to the leaching of salts by surface water and groundwater, and to excessive evaporation of groundwater at or slightly below the surface in the lowest part of the depression.Site # 2: The Great Sand SeaThe Great Sand Sea is one of the largest unbroken mass of sand dune areas of the world, and home to not one living soul. It was first discovered by the German explorer Gerhard Rohlfs in 1875. It is in fact the third largest. It is a desert in its own right, spanning 650 km between Siwa Oasis in the north and the Gilf Kebir Plateau in the south. Its average width is about 300 km from the Libyan border to the west and Farafra Oasis Depression to the east. Parallel dune ridges run north-south for hundreds of kilometers. There is not a single water point in the whole area. Totalling the size of England, the sand accumulation varies in shape, colour, and geologic origin from one place to another. In the northern region of the dunes, south of Siwa, there is minor greenery, with calcareous outcroppings that reveal the marine origin of the strata. Further south and towards the central parts of the dune field, is a land of pristine sands with undulating whaleback dunes and minor crest dunes. Through this central region patrols of the British Long Range Desert Group made their way to attack German posts in Libya, during the early years of WWII. Their more than 60-year old tire tracks are still visible on certain surfaces. On the eastern edge lie the Nummulite scarp and the only water source, Ain Dalla. The southern region of the Great Sand Sea is a mysterious place dotted with legends of lost oases, disappearing armies, and secret caravan routes.In the west of this region lies the enigmatic silica glass – ultra pure glass, 98% silica - discovered by P. Clayton in 1932 and believed by some scientists to be the result of a meteoritic impact. Another theory has it that these pieces are remains of a dried up lake. The silica field is located in the Great Sand Sea around 25° 17' N and 25° 33' E, and about 50 km from the Libyan border, between two dune ridges 200 m high. It is the world’s only known field of silica glass. Some chunks weigh 5 kgm, half-buried like icebergs in the reddish sand. Some lying windblown on the desert floor, scoured by millennia of sand storms, into lustrous prisms of glass. Prehistoric Man had discovered this Libyan Desert Glass (LDG) and carved knives and other sharp-edged tools from it. The Ancient Egyptians carveda scarab from it, deposited in Tutankhamon’s tomb. They are the purest glass ever found. Over a thousand tons of it are strewn across hundreds of kilometers of bleak desert. Some of the chunks weigh 26 kilograms, but most exist in in smaller, angular pieces looking like sherds left when a giant green bottle is smashed by colossal forces.Pure as it is, LDG does contain tiny bubbles, white wisps, and inky black swirls. The whitish inclusions consist of refractory minerals, such as cristtobalite. The ink-like swirls, are rich in iridium, diagnostic of an extra-terrestrial impact – meteorite or comet. The iridium leads to the heart of the LDG problem. At least three “minor” problems bedevil the generally acceptable impact theory: (1) The surface of the Great Sand Sea shows no sign of a giant crater. Neither do microwave probes deep into the dsand by satellite radar. (2) LDG seems too pure to be derived from a messy cosmic colloision. Known impact craters, such as that at Wabar, Saudi Arabia, are littered with bits of iron and other meteorite debris. Not so at the LDG site. (3) LDG is concentrated in two areas. One is oval-shaped, the other circular, 6 km wide and 21 km in diameter. The ring’s wide center is devoid of LDG.Could there have been a “soft” projectile impact; that is the detonation of a meteorite, perhaps 30 meters in diameter, 1o km or so above the Great Sand Sea? The blast of hot air might have melted the sand beneath. Such a crater-less impact is thought to have occurred in the 1908 Tunguska Event in Siberia. Another theory has a meteorite glancing off the desert surface, leaving a glassy crust and a shallow crater that was soon filled in. But if there are two known fields of LDG, where there two meteorites in tandem. How much more LDG lurks beneath those sands (G. Wright 1999).Through this region the Senussi used to travel between Kufra Oasis in southern Libya to Dakhla Oasis in southern Egypt. The area hides underground a huge fossil water reservoir joining Egypt, Libya, the Sudan, and Chad.Site #3; Wadi Sannur CaveWadi Sannur Cave was discovered during blasting for alabaster in the famous alabaster quarries, 70 kms south- east of the city of Beni Suef, in the Eastern Desert of Egypt. These quarries produce some of the finest alabaster material the world has ever known, and it is from these quarries that the Ancient Egyptians fabricated the wonderful alabaster ware, like those found in King Tutankhamon’s tomb. The Wadi Sannur Cave dates back to the Middle Eocene, 40-45 million years ago. It contains structures known as statalactites and stalagmitic formations, in a fantastic presentation. It is the largest natural subterranean chamber known in Egypt, with a characteristic crescent shape that may be unique in the world. Such caves are extremely rare in arid environments and are therefore useful for understanding the geological history of these regions and the evolution of their palaeoclimatology, their geohydrology, their geomorphology, and their biota.