阿布扎比地质概况.pdf

Geologically, the UAE occupies a corner of the Arabian Plat, a body of continental rock that has remained relatively stable since the Cambrian Period more than 500 million years ago Fig. 1. The Arabian Plat encompasses not only present-day Arabia but also the shallow Arabian Gulf which is not a true ocean basin and the rocks of the Zagros Mountains of Iran. In addition, for most of its history the Arabian Plat has been part of the larger Afro-Arabian continent, and the two have behaved as a unit in response to plate tectonic movements Fig. 2. Only 25–30 million years ago, with the initial opening of the Red Sea, did Arabia begin to separate from the African plate. The Precambrian history of the UAE is somewhat speculative. Precambrian rocks do not outcrop in the UAE, nor are they known from drilling ination, but exposures of Precambrian sedimentary rocks in neighbouring Saudi Arabia and the Sultanate of Oman indicate that this region participated in the late Precambrian glaciations that are known from geologic evidence in many disparate parts of the present-day globe. Since the middle Cambrian Period, not long after the first appearance of abundant fossilisable life s, the area that now constitutes the UAE has been generally at or near the edge of the Afro-Arabian continent, often covered by a shallow sea. Early Cambrian sediments on both sides of the Arabian Gulf, including thick accumulations of salt, suggest that the region may have been at that time the site of early-stage rifting of a larger continent to a new ocean basin. This interpretation would account for the UAE’s subsequent position at the continental margin. Movements of the Afro-Arabian plate during the Palaeozoic caused Arabia to pass near the South Pole in the Ordovician Period, and the UAE may have become glaciated along with most of North Africa. By the mid-Palaeozoic the Afro-Arabian continent was itself part of the larger southern supercontinent of Gondwana, which began to break up in the Permian and Triassic. Since the end of the Palaeozoic, the UAE has remained in tropical or subtropical latitudes. Despite its travels, the UAE appears to have remained tectonically relatively stable. The exception is the creation of the Hajar Mountains along its eastern margin, discussed separately below. The geological history of the country as a whole has therefore been primarily a history of the advance and retreat of the sea in response to global, rather than local, tectonic and climatic fluctuations. THE OIL AND GAS RESERVOIRS Over time, sediments accumulated on the continental shelf that was to become the UAE. Isolated pre- Permian exposures in the UAE reveal shallow-water terrigenous sediments sandstones and shales. These were probably relatively thin overall and may have been largely removed by intermittent emergence and erosion. Later, in the tropical Mesozoic seas, thick sequences of carbonate rocks GEOLOGICAL OVERVIEW IN ADDITION TO ITS WELL-KNOWN OIL ANDgas reserves, the UAE is endowed with a number of other significant and unusual geologic features and environments that are both well exposed and readily accessible. This chapter reviews the overall geological history of the UAE and then examines in more detail the geology of the present-day sand deserts, the sabkhas and the Hajar Mountains. A GEOLOGICAL HISTORY FIGURE1 The geological timescaleFIGURE2 The Afro-Arabian plate, highlighting the UAE G E O L O G I C A L O V E R V I E W 4 14 0 T H E E M I R AT E S – A N AT U R A L H I S T O R Y 0 – 50 – 100 – 150 – 200 – 250 – 300 – 350 – 400 – 450 – 500 – 550 – 600 – 650 – 700 – 750 – 4600 – CRETACEOUS JURASSIC TRIASSIC PERMIAN CARBONIFEROUS DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN PRE-CAMBRIAN PRE-CAMBRIAN PALAEOZOICMESOZOIC CENO- ZOIC MILLION YEARS QUARTERNARY TERTIARY HOLOCENE PLEISTOCENE PLIOCENE MIOCENE OLIGOCENE EOCENE PALAEOCENE into the Empty Quarter. Subsurface structures may be very gentle, but gradients of as little as 2 per cent can be sufficient to permit migration and entrapment of crude oil. Shallow-water sedimentation continued through the early Tertiary over most of the UAE, but regional uplift began in the late Oligocene ca. 30 million years ago. Subsequent early Miocene sediments consist of salt and gypsum, and the area has been above sea level since the end of the Miocene ca. 5 million years ago. In the west of Abu Dhabi, exposures of terrestrial deposits and vertebrate fossils of late Miocene age ca. 6–8 million years ago indicate an environment of riverine savannah at that time as discussed by Whybrow et al. in the chapter The Fossil Record. The sea also retreated briefly at various times throughout the earlier Mesozoic and Tertiary depositional history, intermittently exposing areas of low relief as at present. This is shown by occasional gaps in the sedimentary record and by sedimentary features indicating surface erosion, such as the development of palaeo-soils. One such example is a regional hiatus in marine sedimentation which occurs at the Cretaceous–Tertiary boundary. Since the mid-Cretaceous ca. 100 million years ago, local topographic highs and major structural traps for petroleum have been created by salt domes rising from the thick Cambrian salt deposits that underlie many areas of the southern Arabian Gulf at depths of some 10,000 metres. Today these salt domes several of the UAE’s offshore islands, including Sir Bani Yas, Das, Arzanah, Zirku and Sir Abu Nu’air, and the coastal hills of Jebel Dhanna. Jebel Ali, on the coast of Dubai Emirate, may be related to the mobilisation of later, Miocene, salt deposits. The salt domes pierce and disrupt the successive overlying strata, bringing up fragments of rock units not otherwise exposed to view, although the salt itself is normally dissolved before reaching the surface. On Sir Bani Yas Island, the most well-developed salt dome in the UAE, the rising salt plug has pushed up hills to almost 150 metres high. These are littered with a medley of colourful shales, lavas and, most strikingly, specular haematite, a variety of iron oxide Fe2O3, whose fine, platy crystals a glittering carpet along narrow gulleys. Sulphur is often concentrated above salt domes by chemical processes, probably from associated gypsum CaSO4-2H2O, and sulphur was mined at Jebel Dhanna, probably ca. the seventeenth to nineteenth centuries. MOUNTAIN BUILDING The exception to this generally placid but productive history lies in the Hajar Mountains in the north-east of the country and along the border with the Sultanate of Oman. There, earth movements driven by plate tectonics – and therefore ultimately by convection within the earth’s mantle – caused the crust and upper mantle of the LEFT The author on a trail in the northernmost Hajar Mountains, called the Ru’us al-Jibal, where the thick carbonate sediments of the Tethys Ocean are well-exposed. RIGHT The mountains of Ru’us al-Jibal drop steeply to the sea. G E O L O G I C A L O V E R V I E W 4 34 2 T H E E M I R AT E S – A N AT U R A L H I S T O R Y – limestones CaCO3 and dolomites CaMgCO32 were deposited. The late Permian and Mesozoic seas of the UAE were part of a major ocean that existed to the north of Arabia during that time, separating the Afro-Arabian continent from the Eurasian continent. This palaeo-ocean, known to geologists as Tethys, at one time extended westwards through the present-day Mediterranean countries and eastwards to the Himalayas Fig. 3. Fossiliferous limestones and dolomites of late Permian to late Cretaceous age ca. 260 to ca. 65 million years ago are the rocks in which the UAE’s abundant hydrocarbon reserves are typically found. Some of these rocks represent depositional environments much like today’s Arabian Gulf shores, but they are now buried at depths of 2,500 metres to 8,000 metres. The lower part of the Mesozoic sequence, in particular, includes many sabkha deposits indicative of a restricted shallow-water environment. The source of the oil and gas itself was probably abundant organic material deposited in these same sediments – the remains of algae and other micro-organisms that flourished in the warm tropical seas. As they were buried deeper and deeper, insoluble organic residues were broken down by heat to crude oil and natural gas. The fluid and relatively buoyant oil and gas then migrated upwards as the sediments were compacted, using porous rock units and fractures as pathways, until they were trapped by an impermeable layer or structural barrier. It is ironic that these enormously important hydrocarbon reservoir rocks are almost nowhere exposed at the surface within the UAE. Apart from the rocks of the Hajar Mountains, however, there is little surface outcrop of any kind throughout eastern Arabia, and most of what is known in detail of the geological history of the Arabian Plat in this area comes from drilling and seismic ination. Two persistent regional structural features recognised by petroleum geologists are a major ridge running north-east to south-west through the Qatar Peninsula and a parallel and adjacent major trough running through western Abu Dhabi andFIGURE3 The Tethys Ocean in the late Permian Burrow casts and the enigmatic fossil Cruziana at lower left, believed to be trilobite tracks, from the scarce Palaeozoic rocks of the UAE. GONDWANA TETHYS OCEAN deep ocean, then lying to the north-east, to be forced over the edge of the Arabian Plat and its cover of shallow-water sediments. This process created a structure of massive superimposed sheets nappes of diverse rock types that now appear to have been shuffled, like cards, on a grand scale Fig. 4. The result was fortuitous for geologists, since the Hajar Mountains today constitute the world’s finest and most extensive surface exposure of rocks of the oceanic crust and upper mantle. These nappes were emplaced during the late Cretaceous, from about 90–75 million years ago. In the process, the area was raised above the sea, creating a chain of rugged islands in the area of the present-day Hajar Mountains. That original relief was rapidly eroded, however, and much of the area again became a site of shallow marine deposition by the end of the Cretaceous. The height and rugged topography of today’s Hajar Mountains is the product of renewed uplift and erosion due to regional forces commencing in the late Oligocene ca. 30 million years ago and continuing to the present. This regional uplift is believed to be related to the gradual opening of the Red Sea and to the ongoing convergence of the Afro-Arabian and Eurasian plates, which is responsible for the Zagros Mountains of Iran and other mountain chains from the Alps to the Himalayas. Geologically recent events, such as Pleistocene glaciation and its associated effects on climate and sea level, have put the finishing touches to the present-day geology of the UAE as a whole. These events are discussed in more detail by Evans and Kirkham in the following chapter, The Quaternary Deposits. TOP The wadis at the tip of the Musandam Peninsula have been flooded through a combination of tectonic subsidence and post-glacial rise in global sea level. BELOW The salt dome at Sir Bani Yas has carried up fragments of a medley of rock units not otherwise seen at the surface. G E O L O G I C A L O V E R V I E W 4 54 4 T H E E M I R AT E S – A N AT U R A L H I S T O R Y FIGURE4 Geological cross-section of the UAE BELOW LEFT The sediments found in the Ru’us al-Jibal correlate with the oil-bearing strata buried thousands of metres beneath the surface in the western UAE. BELOW RIGHT The purple-brown patina known as ‘rock varnish’ covers ancient gravel plains. It is now known to be caused by manganese-fixing bacteria. Salt diapirs Oil Fields Dune Sands Continental Shelf Sediments Cambrian to Recent Shallow water Ophiolite Cretaceous oceanic crust and upper mantle Metamorphic sole Metamorphosed sediments Hawasina Mesozoic deep ocean sediments Exotics Isolated carbonate massifs Scale vertically exaggerated ca. 10 x Nappes Thrust Sheets WEST Offshore island ARABIAN GULF Hajar Mountains GULF OF OMAN Abu Dhabi areaAl Ain area EAST PRE-CAMBRIAN PRE-CAMBRIAN G E O L O G I C A L O V E R V I E W 4 7 of which, when approached from the west, showed up in turn as a wavy silver-blue wall, three to four feet high, running out of sight to north and south along the top of an orange-red slope a mile wide’. The limited observations permitted in that area are consistent with the ation of such dunes by the asymmetric development of small barchans ed atop an older ridge, perhaps under the influence of a consistent, oblique, subsidiary wind direction. However, linear dunes having the same alignment continue across the UAE border for hundreds of kilometres south- south-west into the Empty Quarter, where they are much larger and more extensively developed as they are elsewhere in the world, with well-developed slip faces on both sides. For this a more comprehensive explanation is obviously required. Whatever their mode of origin, linear dunes are thought to have been a more prominent feature of the UAE landscape in the past. The sand deserts of the central and eastern UAE are characterised by a pattern of large relict dune ridges aligned in a more or less west–east orientation from WNW to ESE in the south to WSW to ESE in the north-east. These were once considered the remains of transverse dunes, but are now interpreted to have ed as linear dunes Glennie 1991; 1996; 1997, probably aligned with stronger and more consistent winds prevailing during LEFT ‘Fossil’ dune sands were cemented at a time when groundwater levels were higher than today. BELOW An inland sabkha lake after rains at Qaraytisah, in the Eastern Desert. 4 6 T H E E M I R AT E S – A N AT U R A L H I S T O R Y Most of the land surface of the present-day UAE is a sand desert, stretching from the Arabian Gulf coast south to the uninhabited sands of the Empty Quarter, the Rub’ al-Khali, and east to the gravel outwash plains bordering the Hajar Mountains. The desert is a geologically recent feature, the result of prolonged subaerial erosion and re-deposition in an arid environment. The sands overlie the thick, oil-rich sedimentary strata of the Arabian Plat which constitute the bedrock of most of the UAE, but most of the oil- and gas-producing rocks are not exposed at the surface, and are known only from drilling. The desert sands vary in both composition and . Near the coast they consist mostly of calcium carbonate CaCO3 material derived from the carbonate sediments, seashells and coral reefs of the coast. Further inland, the sand consists predominantly of quartz grains. Quartz SiO2 is the most common rock-ing mineral and is a stable end-product of the chemical weathering of most rock types other than carbonates. However, even the well-weathere