middle east the geological background p. e. kent · central to the area is the arabian peninsula,...

Q. Jl Engng Geol. 1978 vol 11 pp. 1-7, 5 figs. Printed in Northern Ireland.

Middle East The Geological Background

P. E. Kent Natural Environment Research Council, Alhambra House, 27-33 Charing Cross Road, London WC2H OAX

Introduction The Middle East is an area of widely varying geog- raphy and topography, mostly characterised either by low rainfall or full desert conditions. These present an abundance of engineering and environmental problems. It has diverse mineral resources of which only one---oi l~has been developed on a large scale in modern times, Fig. 1.

Central to the area is the Arabian peninsula, which is geologically a prolongation of the African Conti-

nent. The various sedimentary and structural belts further east and north-east are peripheral to the edge of this old nucleus. It is, consequently, convenient to describe the region in terms of four parallel zones; the Arabian shield, shelf and the submerged sedimentary terrain of the Gulf; the Iranian coast and foothills; the mountain ranges of the Zagros; and finally the high Iranian Plateau. Geological columns through Saudi Arabia, Kuwait, Iran and Iraq are indicated on Fig. 3, and a generalised cross section shown in Fig. 4.

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Arabia

The Arabian peninsula has three major elements, see Fig. 2.

The Arabian Shield

The westernmost element is the outcrop of the Crys- talline Shield, which flanks the Red Sea from Jordan to the Yemen, widening to 400 km in a broad fiat

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dome in west-central Saudi Arabia. The crystalline rocks have a peneplaned surface which dips gently eastwards beneath the strata of the Interior Homo- cline (see Fig. 4).

The interior Homocl ine and Cont iguous basins

The sediments directly overlying the Crystalline Shield are the Palaeozoic elastics, which are succeeded

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ARABIAN SHELF Interior Homocline

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FIG. 2.

Structural divisions of the Arabian Peninsula.

Basins



M I D D L E E A S T - - T H E G E O L O G I C A L B A C K G R O U N D 3

by Permian and Mesozoic rocks, dipping gently north- eastwards. The older Palaeozoics are undisturbed at outcrop, but appear to have been dislocated--at depth further east--perhaps by faulting before being bevelled by the Permian seas. The outcropping sediments form parallel escarpments, some of which are continuous for 500-1000 km. This outcrop trends south-eastwards from southern Iraq across Saudi Arabia, hooking round north-eastwards in Dhofar and Southern Oman parallel to the Arabian Sea coast. North-east of the Homocline, broad contiguous basins, largely of Ter- tiary rocks blanketed by sand and gravel, extend to the Gulf. Only in this third area is gentle folding developed.

The lowest elements in the Arabian sedimentary column, outcropping in the west of the Homocline, are some 2000 m of Palaeozoic rocks--Cambrian, Ordovi- cian and Devonian, largely of sandstone facies. Above these are Permo-Triassic rocks (outcropping in a wide continuous belt in the west) which consist of about

1000 m of marine beds, mainly limestone and dolo- mite, alternating with continental clastics. The suc- ceeding Jurassic and early Cretaceous rocks, some 1700 m thick, are characteristically limestones with interbedded anhydrites, including the late Jurassic Arab Formation. The evaporite elements are better developed down dip and such anhydrites as the Titho- nian Hith are important cap rocks in the Arabian oilfields. The Cretaceous and Tertiary, 700-1100m thick at outcrop, include both sands and limestones; indeed in this part of the sequence thick deltaic sands provide the main oil-bearing reservoirs of Kuwait (Fig. 3). In the southwest, in the Yemen and Aden, extensive floods of lavas were poured out on to the Jurassic rocks and the crystalline Basement during Cretaceous times.

No orogeny affected Arabia in Phanerozoic time, and block fault movement occurred, the Red Sea graben developed from the Miocene onwards, and late movements produced the Yemen and Asir blocks. The

FIG. 3. Geological columns in the Middle East.

EPOCH SAUOI ARABIA KUWAIT I R A N I R A 11 . . . . . . . o B A K H T Y A R I ° o o o o o o o o o [

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4 P.E. KENT

gentle folds of the oilfield belt, initiated during the Lower Cretaceous, are believed to be due to flowage of deep-seated Cambrian/Infra-Cambrian salt.

Surface movement in the desert suffers little impedi- ment from topographical features and is exceptionally easy; drilling rigs are commonly towed from location

to location without dismantling. Surface water is usually non-existent: oases are infrequent or absent over much of the area. The main exception to the sparsity of surface water is the basaltic area of the south-east, where complex tanks have been built to collect the meagre run-off. In many places underground water is

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saline or (in parts of Kuwait) heavily contaminated with hydrocarbons, and consequently the areas where sweet water is available at depth are correspondingly valuable. In a region with minimal rainfall, however, conservation of any underground fresh water must be an absolute priority. The geology of the surface rocks and the problems of construction in Saudi Arabia are described by Fookes (this vol.).

The Gulf Islands

The Gulf Islands present a series of engineering problems associated with the development of offshore oil fields within the submerged section of the sedimentary region, see Dennis (this vol.). In the Gulf area folding is very gentle and the full succession is much the same as in Arabia. Pliocene and Miocene rocks rest on a long succession of limestones which are separated by cap rocks of shale or (more frequently) of anhydrite, so that they form reservoirs with independent pressure regimes. The late Jurassic reservoirs, in particular, contain important oilfields, and attention is now being given to the potential of the underlying Permian for the production of gas.

Deep-seated salt has erupted through the Mesozoic and Tertiary cover to produce surface highs which form most of the Gulf Islands. At the surface they are composed of salt dome detritus, are totally waterless and have cavernous surfaces composed of a melange of ancient rocks from depth, together with varying quantities of gypsum and salt. Since the oilfields with their production units are out at sea, piled into the sea floor, it is the usual practice to use the small islands as collecting, separating, and shipping points. Because of the incoherence of the surface rocks, however, location of tank farms or large structures on the islands

has to be carried out with care. Good harbours are absent, and the gently shelving island shores have therefore involved the building of artificial sea-walls for construction of sheltered anchorages.

The provision of fresh water is a permanent prob- lem on the islands, for not only are the surfaces quite barren but drilling has shown that underground waters are saturated with chloride or sulphate salts, and sea- water distillation is necessary for fresh water supplies.

I ranian coast and foothi l ls

Areas of plain extend north and northeast of the head of the Gulf, built up by the alluvium of the Tigris, Euphrates and other rivers. These are well watered and fertile, the location of civilisations for past millennia. Discovery of oilfields in the deep- seated rocks has led to extensive development, and the broad level spreads of alluvium present ideal conditions for the construction of pipelines~the gradients being so even that the earlier lines in Iran relied on gravity flow for tens of miles from the foothills oilfields to the refinery at Abadan.

Behind the Coastal Plains the ground rises into the foothills ( 150 -300mOD) consisting largely of the broken and dissected ground of the Fars Group (Miocene), made up of relatively soft sediments including shales, marls, sandstones and gypsum beds. The lowest formation (Gachsaran), is comprised of interbedded gypsum and red marls; at depth it in- cludes a large proportion of incompetent sodium chloride (halite) which has flowed under the super- incumbent load, resulting in complex dislocation of the cover rocks, independent of the relatively simple mas- sive oil-bearing rocks at greater depth. Above this, the middle of the Group is a marine interval of grey beds

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oc : : u c : : : Upper Cret" eous ........................................... ~E....:.S. t oe M ~ MiddJe Cret eous ..........................................

Lower Creloceous ............................................ J . . . . . i ............................................................. I l lJ l l i l l Triossic ............................................................. J^^ T.-. ̂ ̂ ^1

FIG. 5.

Cross-sections of the Zagros Foothills and Outer Ranges.



6 P . E . KENT

(Mishan Formation), and this in turn is followed by a thick series of red marls with interbedded sandstones (the Agha Jari Formation) which is the tectonic equiv- alent (in a different facies) of the Alpine flysch (Fig. 3).

It is in this belt that many of the well-known oilfields of Iran have been developed, commonly tap- ping reservoirs whose structures bear no relation to the convoluted strata nearer the surface (Fig. 5). The badland topography of this belt adds to the complica- tions of road-building, but apart from the roads tend- ing to be rather circuitous, does not produce special problems. Water supply is still a concern but within the oilfield area is pumped from the few rivers which cross the foothills belt.

The disturbances of the surface rocks of the Fars Group were largely produced by simple flowage, with a relatively small part played by fracture. It is likely that movement is still taking place over much of the belt and in fact a case has been documented near the head of the Gulf where a canal some 2000 years old has had its bed arched and dried out by a slowly developing anticlinal fold (Lees & Falcon 1952). Sig- nificantly, earthquakes are not especially associated with this belt despite its complexity, and the rocks presumably come within the category described by Ambraseys (this vol.) as being aseismic because they have yielded without fracture.

Towards the mountains, the surface geology of the foothills belt is often formed by dissected gravel fans, which range in date from the original Mio-Pliocene development of the orogenic belt to modern times. The older rocks are the Bakhtiari Conglomerates, analogous to the Swiss molasse, which are coarse torrential gravels deposited in depressions and down- warps at the time when the mountains were rising. Later conglomerates are derived from the continuing erosion of the exposed limestone anticlines. The re- cent conglomerates and gravel fans are an important source of underground water which have been tapped by the Iranian water engineers over many thousands of years; the most common method being the use of underground conduits ('ganats') which head into the water table and are designed with a gentle gradient to intersect the more steeply sloping ground surface; sometimes emerging miles from their source lower down the fan surfaces. The more sophisticated exam- ples include inverted siphons to transmit the water beyond ravines.

The Zagros Mountains

The Zagros Mountains provide a wealth of geological interest as well as numerous engineering problems. They are characterised by very long continuous anticlinal belts produced by major upfolding of the deep- seated limestone floor; individual wall-like ranges may extend for 100 or 200 kms without any break.

The succession in the mountains is shown in Fig. 3. The soft rocks of the Fars Group were eroded from the anticlines as the mountains grew, and are now limited to synlical belts. The hard outer carapace of the mountains was originally the immediately underlying Asmari Limestone, historically the most important reservoir where it is effectively sealed by a cap rock in the foothills, but this has frequently been breached to expose limestones lower in the succession--notably the middle Cretaceous Bangestan limestone, the various Lower Cretaceous and Jurassic limestones and the Permian (Figs. 3, 5). Most of these units are in the range of 500-1000 m thick, separated by argillaceous groups which weather to produce lower ground between the limestone eminences. Lower Palaeozoic rocks are occasionally exposed in the cores of folds of abnormally high amplitude.

The folding of the limestone sequence shows inter- esting features in relation to the strength of materials. The limestone units, hundreds of metres thick, are made up of individual beds commonly 0.5-15 m in thickness. The radius of curvature in the tighter folds may be in the same order as the thickness of the major units; and for this to take place without extensive faulting implies bed-over-bed sliding of most of the individual beds. Collapse features have developed on the flanks of the steeper folds, the outer skins of limestone folds (unsupported after erosion of the soft beds in the synclines) having kinked, subsided or progressively turned outwards as flaps independent of the anticlinal cores, to rest upside down on the Fars valley-fill (Harrison & Falcon 1936). Both the main folding and the collapse structures take place with little significant faulting at the surface; that which is visible relates to tension in outer limestones of the packet, or to flanking thrusts of synclines due to space problems. The major folds, conversely, have a much steeper buried southwestern flank (approaching verti- cal in some cases) and it is believed that many of these are over-thrust at depth.

On many of the ranges, the mountain flanks are so steep and high that even the mule tracks are difficult and hazardous. At the present time no major road crosses any of them. The rivers, however, provide gorge-like cuts through the wall-like, steeply dipping limestones which may be as much as 1000-2000 m deep, locally widened into steep valleys in the softer rock intervals. The belts of lower country between the outer ranges include areas of badland cut in the softer late Tertiary strata, which provide no particularly easy routes for road-building. Between the inner ranges, however, most of the synclines are marked by broad fertile alluvial plains which are usually well-watered from the melting snows on the adjoining high mountains.

Overall, there is a north-easterly rise in the levels of the mountain crests; the plains between them are also stepped up to higher and higher levels towards the



MIDDLE EAST--THE GEOLOGICAL BACKGROUND 7

inner Zagros-- the 'Iranian Staircase' as it has been called. Routes across the mountains pick their way from plain to plain by way of passes which in former days were extremely difficult to cross, but which are now being replaced by major roads cut through the narrow limestone gorges, at considerable cost in ex- plosives.

The alternation of the major ranges with their limestone walls and the widened valleys and plains provide a major potential for dam location; with such projects as the Reza Shah dam on the Karun River designed to provide water supply for the developing towns of the coastal plain. Dam location has to be somewhat selective in that rivers which flow past surface salt intrusions are too highly saline for agricultural use or human consumption, and the emphasis has to be placed on those rivers which are fresh. Avoid- ance of leakage through the surrounding limestone terrain is a serious concern.

Small streams feeding the rivers are common in the high plains, but the lower country is mainly desert. There, water supply is being developed currently by a programme of developing underground aquifiers in the gravel-fill of the plains between the mountains. An extensive new agricultural industry is, for example, being developed at Bandar Abbas based on supplies of this kind. As on the other side of the Gulf, it is essential that the off-take is balanced against the meagre rainfall to ensure that these projects, like some of those of ancient times, do not come to grief through exhaustion of the stored water resources.

Earthquakes are a geological hazard in some parts of the Zagros, for example, in Southern Fats Province, where the city of Lar in particular has been destroyed repeatedly. Freshly collapsed rock scarps due to local quakes are not uncommon in this area, but shocks of extreme severity seem to be rather localised in their effects.

From an engineering point of view, the exposed mountains have the great advantage that the high rate of erosion eliminates the occurrence of any significant amount of weathered or oxidised rocks, and stability of slopes due to these factors is hence not one of the problems. The danger of rock falls from earthquakes nevertheless has to be taken seriously in engineering construction.

The Iranian Plateau

Northeast of the Zagros, and extending to the Elburz mountains, interior Iran presents a series of wide

plains locally interrupted by small ranges of isolated hills, sometimes of limestone, sometimes of bedded igneous rocks. A large part of the surface rocks are Eocene and Tertiary marls; the scattered hills are either of Miocene limestone (which is locally oil- bearing when it occurs at depth, as at Qum) or of Eocene tufts and lavas. Pre-Cambrian crystalline rocks form ranges to the east of the plateau, and more varied hill masses occur in the south. Mesozoic coal is an economic mineral mined in the Elburz and also in the Merman area. Where fresh water is abundant (particularly near the major ranges), the plains are well-watered and are now under regular cultivation. For large cities such as Isfahan the local water supply is being augmented by driving tunnels through the limestone ridges of the high Zagros, taking water from the headwaters of rivers which drain to the Gulf.

The central plains are, however, part of an internal drainage basin and the more easterly parts constitute a salt desert, in which the soil is saturated with sodium chloride and gypsum, often churned up by its repeated re-crystallisation. In the Dasht-e Kavir some of the mountains are themselves emergent salt plugs. The salt desert is uninhabited and likely to remain unin- habitable, but elsewhere development of agriculture and industry is proceeding on a generous scale, both in the plains adjoining the Zagros on the west and the Elburz Mountains in the north. Communications are easy, but central Iran is an area particularly prone to earthquakes, in which the consequent constructional problems will have to be faced.

ACKNOWLEDGEMENTS. Thanks are due to the British Petro- leum Company for providing the figures for this paper.

References

AMBRASEYS, N. N. (this vol.). Middle East--A reappraisal of the seismicity.

DENNIS, J. A. N. (this vol.). Middle East--Offshore Struc- tures.

FOOKES, P. G. (this vol.). Middle East--Inherent Ground Problems.

HARRISON, J. V. ~¢ FALCON, N. L. 1936. Gravity Collapse Structures and Mountain Ranges, as exemplified in South-Western Iran. Q. Jl geol. Soc. Lond. XCll (1), 91-102.

LEES, G. M. & FALCON, N. L. 1952. The Geographical History of the Mesopotamian Plains. Geogrl J. CXIil (1), 24-39.



middle east the geological background p. e. kent · central to the area is the arabian peninsula,...

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