Geologie Libanonu

THE GEOLOGY OF LEBANON
A SUMMARY

C. D. Walley

1.1 THE MAIN FEATURES OF LEBANON

Figure 1 gives a basic geological map of Lebanon. Figure 2 shows the main structural features and Figure 3 gives a sketch cross section across the region.

The three fundamental divisions of Lebanon; a western Mount Lebanon rising to 3083m, a central Bekaa Valley and an eastern Jebel Lubnan al Sharqi or Anti-Lebanon Range¹ with Mount Hermon at 2814 m should be known by every student. It is worth remembering that the Bekaa is almost everywhere above 850m in altitude and that this is as high as some of the highest mountains of many countries. One other feature should mentioned at this point because it is frequently overlooked. This is that the continental shelf of Lebanon is very narrow indeed (it has a maximum width of ten kilometres, see Fig 3) and drops down abruptly to water depths of 1500m. We know very little about what lies off the coast of Lebanon and the area has not been mapped in detail². Beyond this shelf break, which is cut by deep canyons, lies what must be ancient ocean crust.

The rivers of Lebanon can be divided into two groups. The first group is made up of the east-west rivers, mostly cut into steep gorges, which drain Mount Lebanon. The second group is that of the two large rivers of the Bekaa; the Litani, which flows south and eventually cuts through to the Mediterranean and the Nahr al Assi which flows northwards into Syria.

There is a hierarchy of folds in Lebanon. The major geological structures of the area, Mount Lebanon, the Bekaa ³ and the Anti-Lebanon (see Figs. 1 and 2) are basically two very large NNE-SSW trending anticlines separated by a large syncline. They have however been broken up and disrupted later by a series of major and minor faults. These form what we can call the first order structures or megastructures.

Smaller folds occur locally but, in general, the brittle limestone rocks of the region have deformed more by faulting than folding. Perhaps the most spectacular folds are the overturned beds at Nabi Ayoub along the southwestern part of the Baruk-Niha ridge. Immediately east of the Yammouneh Fault a number of small NE-SW trending anticlines occur. Other good folds occur in the Tripoli area (i.e. at Jebel Terbol).

A major fold that is widely seen is the NNE-SSW trending Western Lebanon Flexure which runs from the western edge of the Chouf up to the latitude of Tripoli inland of the coast. This feature is technically a monocline and in places gives steep and even vertical dipping rocks.

Lebanon is cut by faults of every scale. Figure 2 merely shows some of the main ones. The longest fault in Lebanon is the Yammouneh Fault that runs along the western margin of the Bekaa and links the major fault of the Jordan Valley to the Ghab Valley Fault of Northern Syria. This is a lateral, or strike slip fault and is the Lebanese segment of the Dead Sea Transform Fault (see Section 2.5). It originated around 12 to 10 million years ago as the boundary between the Arabia Plate and the Levantine part of the African Plate and has ben moving since. The result of this is that the Bekaa has moved some 50 km northwards with respect to Mount Lebanon. The evidence suggests that the Yammouneh Fault has not moved for many thousands of years; and whether it is dead or dormant is not clear. We would dearly love to know will move again. Like many large faults the Yammouneh Fault is not actually very impressive on the ground and is often only marked by a wide breccia zone.

The Roum Fault, which runs from near Marjayoun towards Beirut is probably where most of the plate tectonic motion is going on now and may be the present plate boundary between the Arabian and the African Plate. One model is that the plate motion has fairly recently (in geological terms) switched from the Yammouneh to the Roum Fault. The last recent earthquakes in Lebanon have been along this fault including the Chhim earthquake of 1956 that caused many deaths and much damage. One slightly worrying point is that the Roum Fault seems to be on line for Beirut. If it does have an active fault segment near (or even under) the capital then that must raise the earthquake risk. Earthquakes are discussed below in Section 2.7.

There are other major faults particularly in the Anti-Lebanon. The main highway to Damascus shows a good deal of faulting in the road cuts as it passes through this area. The Serghaya Fault in particular is apparently another major strike slip fault.

There many other faults in Lebanon with displacements ranging from a few centimetres to several kilometres. Working out which are major faults, and which are minor, is not easy.

There is no current volcanic activity within Lebanon. However within the last ten million years there was large-scale basaltic volcanism both in the extreme north of Lebanon with the Homs Basalts, which extend into the Akkar and in the extreme southeast where the Golan and Jebel Druze volcanics occur round Mount Hermon. The Golan volcanism in particular seems to have died out very recently, probably within the last 10,000 years. The very much older Jurassic and Cretaceous volcanics are discussed in connection with the geological history in Section Two.

Older and subtler volcanic features can be seen exist in a number of Late Jurassic volcanic vents. There is a good one at Aintoura on the Dour Choueir-Zahle road. But even here you need a lot of imagination to see it as smoking volcano emitting lava.

Almost all the rocks in Lebanon are sedimentary rocks and most of these are pale limestones. These and/or the snow cover may be the origin of the name as L-B-N is 'white' in the Semitic languages. Despite the vast thicknesses of limestone I have to say that the variation in limestone types is rather limited; much of it is so fine grained that it needs a microscope to show any interesting features. The most varied sequence of sediments is that which extends from Late Jurassic to the Middle Cretaceous (see Fig. 5) and shows a considerable variety of limestones, sandstones, clays and volcanic ashes. The ashes tend to weather to a bright red or purple colour and to give fertile soils.

The only igneous rocks are basaltic flows and intrusions of a variety of ages. The only metamorphic rocks are confined to narrow bands around the edges of the intrusions.

Given that Lebanon is largely Jurassic and Cretaceous limestones and rocks of this period are generally very fossiliferous the sequence here is frankly disappointing. In the Jurassic there are, especially towards the top, beds with corals and sponges (often turned into brown weathering silica), enormous echinoid spines and the odd bivalve. The Lower Cretaceous sandstones have plant debris and occasional amber lumps. These are rather dull and cracked and certainly not of jewellery quality. These contain good insects but the reports of preserved DNA in them (and in any other rocks) have now been more or less discounted; Jurassic Park is pure fiction. The rest of the Cretaceous has a number of bivalves and gastropods in it, particularly at specific levels. There are some ammonites in the higher beds. The redeeming feature of the Lebanese fossil record is, of course, the famous Late Cretaceous 'Fish Beds'. These are world class features and probably rank in the top twenty or thirty localities in the world. They are also very important historically; some of the earliest reports of fossils refer to 'fish in the rocks of the mountains of Lebanon'. The four outcrops known present a remarkable picture of life in the sea a hundred million years ago complete with worms, squids, the occasional octopus, prawns and of course fish. Although we might expect to have dinosaur fragments in our thick Jurassic and Cretaceous rocks and as far as I know no dinosaurs have been found in Lebanon, probably because the rocks were mainly marine during these times.⁴

The Cenozoic rocks have some fossils. The Eocene limestones is often rich in the giant coin shaped foraminifera Nummulites. There are some vertebrate bones in the Late Cenozoic gravels. Some of the cave systems have good mammal fossils, including those of the bear. One feels that there should be fossils of Neanderthal man but I think there are no reports from Lebanon

A useful little guide book to local fossils is 'Les Fossiles du Liban: Guide Practique' by Arslan, S.; Gèze, R. and Abdul-Nour, H. published in 1995.

There are few 'exciting' minerals in Lebanon. Good crystals of calcite are common and sometimes calcite or quartz filled geodes occur in rocks. Metallic minerals appear to be confined to the iron ores of haematite and limonite.

I find it useful to explain to students what is happening in the modern Middle East in plate tectonic terms. Firstly, it helps them understand Lebanon Secondly the Middle East area provides excellent examples of plate tectonic processes.

Figure 4 shows the region diagrammatically. The overall theme for the last sixty million years has been that the great oceanic seaway, the Tethys, which once lay to the north of the Arabia, has almost been entirely consumed as Africa and Arabia have collided with Eurasia.

The first main collision between what we can call the Africa-Arabia Plate and Eurasia took place around 40 million years ago. As it continued it gave rise to the Taurus and Zagros Mountain Ranges. For reasons not yet fully explained, but possibly due to the nature of the collision the Arabia part of the African Plate kept on moving north and rifted away from the larger African body. The spreading ocean that resulted is the Red Sea and Gulf of Aden. The movement of this new plate past the Levantine protrusion of the African Plate has given rise to the Dead Sea Transform Fault zone.

In summary we have

1. A divergent or spreading boundary is well shown by the Red Sea which now forms what can be called an embryonic ocean.
2. The continent-continent type of destructive or convergent boundary is well seen in the Taurus and Zagros ranges which represents where the old Tethys ocean has been completely destroyed. An ocean-continent collision can be seen along the northern part of the Gulf of Oman along the edge of the Makran and along the arc which extends from Cyprus to the northeast. In both cases fragments of the Tethys are still being consumed.
3. The Dead Sea Fault System forms an excellent transform margin between the Arabian Plate and the Levantine part of the African Plate. Both are moving northwards but the Arabian plate is moving faster.

The Middle East has therefore all three types of plate boundary and if textbooks were written here rather than in the States we would doubtless be in it as a wonderful example.

Lebanon has only limited geological resources. Iron oxides occur locally in the mountains and were some of the first iron ores to be exploited; by the 7th century BC Lebanese iron was being exported to Babylon. Unfortunately iron smelting requires high temperatures and as wood was the only fuel this was a major factor in the catastrophic deforestation. The limestone has also been used as a building stone and for fertiliser and cement. The last two processes also use wood and have contributed to the deforestation.

There is no oil known and the general opinion is rather negative about prospects. The main problem seems to be that the heavy faulting has fractured any possible oil reservoirs and allowed water to get in. However drilling programs have been minimal and oil may exist at depth. The narrow continental shelf means that any offshore oil fields, even if they exist are not likely to be extensive. Some bitumen has been recorded along the southern flanks of the Bekaa but this may only be a leakage of immature oil and may be no indicator of major oil fields at depth.

The chief natural resource is water. The mountains give a high rainfall (widely over a meter a year in Mount Lebanon), and the porous fractured limestone makes an excellent aquifer which are refilled over spring and early summer by the slow melting of snow. The resulting abundant springs and rivers, unique to the Arab world, gave the country its once abundant forests and legendary fertility. However due to the steep slopes and the stony, shallow soils this fertility has proved hard to harness for agriculture and the removal of the forests has tended to produce only short lived farming land.

Stone for building and cement can also be accounted a resource and the frequently enormous, visually and environmentally appalling quarries bear witness to this. I am not against all quarries but there are ways of designing and planning them so that they have minimal impact on the environment. This is - to say the least -not the practice here.

A main concern in Lebanon is that of earthquakes as the area is in an active region. Beirut has been destroyed many times by earthquakes and tsunami ('tidal waves') most notably in 551 AD. Lesser quakes have occurred since. Unfortunately prediction of such events is still in its infancy and we have no way of knowing when the next major quake may occur. Even small earthquakes may trigger landslides.

A subtler hazard is that of soil erosion. The steep slopes of Lebanon and the high rainfall means that the soils, in many cases the product of thousands of years of formation, are easily eroded. Deforestation and the reckless building of the last ten years, has made this problem even worse. These soils are not now being replaced. Related to this are widespread landslides on various scales due to the steep slopes and wet winters. The loss of trees, and extensive urbanisation has again only made this worse.

A final geological concern is of the pollution and contamination of the underground water supplies due to poor waste disposal practices. The complex network of underground fissures that makes up the main aquifers means that pollutants can circulate rapidly and unpredictably. The chief dangers here come from the 'ordinary' unspectacular pollution of aquifers by sewage and agricultural chemicals. The widespread use of large quantities of pesticides are a major concern. The seems little doubt that the uncontrolled shooting of the birds has caused such an explosion in insect numbers that people are forced to use pesticides. A far better practice would be to leave the birds to naturally control the insects and so keep pesticides out of our drinking water.

It can be said that it is the geology that has largely controlled the history of Lebanon.

1. It has given Lebanon its high mountains and inaccessible valleys. Positively, this makes the area an excellent refuge for minorities. Negatively, this leads to isolationism, a clan system and makes a centralised state difficult.
2. It has given the country good ports. This is in marked contrast to the coast of Palestine. The Lebanon region is also protected by Cyprus from worst winter storm waves.
3. It has given the region its fertility with the high rainfall and excellent springs. However this is localised, demands hard work to farm due to the steep slopes and is easily destroyed. As a result wealth based on agriculture has not proved easy and many Lebanese have traditionally migrated or gone into commerce. This has been encouraged by the lack of mineral wealth.

PART 2: AN OUTLINE OF THE GEOLOGICAL HISTORY OF LEBANON

This is a summary of the geological history of Lebanon modified extensively from a longer (and more technical) guide that I produced for my students. I have included with it the latest summary stratigraphic table for Lebanon (Fig. 5).

Rather than go through the sequence unit by unit I have presented the geological history of Lebanon here in terms of five broad summary episodes each distinguished by varying plate tectonic and depositional style. Each episode more or less corresponds to a 50 million year interval starting from around 250 ma (million years ago).

The oldest rocks seen at the surface in Lebanon are Early Jurassic, perhaps 200 million years old. This is a very recent age when set against the 4.6 billion year age of the earth. The handful of wells drilled for oil prior to 1975 never penetrated any older rocks. This level of ignorance about the subsurface, coupled with the only rudimentary geophysical data, means that Lebanon is very poorly studied indeed. There are few, if any, other countries in the world whose geological history goes no further back than the surface rocks and there are certainly very few whose pre-Jurassic history is so sketchy.

On the basis of the adjacent countries we can speculate at what must have happened in Lebanon before the Triassic. There was probably a major mountain building episode around 800-600 million years ago, deposition of an interrupted sequence of sandstone and clays during the Early Paleozoic, some sort of uplift during the Devonian and Carboniferous and a flooding by shallow limestone seas during the Permian. In terms of plate tectonics the Lebanon area was part of the supercontinent Gondwana during Paleozoic to Triassic time.

Although there are no rocks of this time period known in Lebanon we can make a fairly good guess as to what events took place in this area from data from surrounding countries.

After the relatively high sealevels in the Permian the seas began to fall during the Triassic so that by the end of the Triassic evaporites and shallow water limestones were probably being deposited.

During the Late Permian to Triassic the supercontinent Gondwana began to break up with the formation of a series of rifts and opening oceanic seaways. This trend towards breakup was to continue until the middle of the Cretaceous. During the Triassic the Tethyan Ocean progressively opened westwards round the northeastern to northwestern margins of what is now Arabia. It is probable that by the Late Triassic sea floor spreading had opened a narrow NE-SW aligned ocean whose eastern margin lay just to the west of the present day continental slope, some 10-20 km west of the modern shoreline (Fig 6a).

The oldest rocks seen in Lebanon are Lower Jurassic in age. The main part of the Jurassic sequence in Lebanon is extremely thick (possibly greater than two km) but poorly known largely due to the cliff forming and monotonous character. This massive Jurassic sequence (the Kesrouane Formation) occurs essentially in three main areas. These are a) Mount Lebanon north of the Damascus Road (the Metn and Kesrouane), b) the Chouf and Jebel Barouk and c) the central and southern Anti-Lebanon.

For most of the Jurassic time (210-144 million years ago) the Lebanon region appears to have been a stable area upon which marine limestones were deposited. Over this area sea levels gradually rose during the Early and Middle Jurassic so that shoreline and tidal flat limestones and evaporites were gradually replaced by shallow marine limestone muds and sands with local patches of corals and sponge reefs.

Whether any seafloor spreading occurred offshore Lebanon at this time is unclear, but until the Late Jurassic the region appears to have been tectonically quiet.

At the start of the Late Jurassic further tectonism began to occur in the region. This probably mainly involved the break up of the area into a distinct series of blocks, some of which rose above the sea and became covered by soils. There was the widespread eruption of basalt lava and ashes from a number of vents. It is probable that this tectonism is related to a renewed phase of breakup of Gondwana; similar Late Jurassic rifting is known across Africa and into southern Arabia. This volcanic and tectonic phase was relatively temporary and there was renewed flooding of the Lebanon area during the last part of the Jurassic to give further limestone deposition. However sometime, either at the very end of the Jurassic or during earliest Cretaceous time, the area underwent more block faulting giving emergence and erosion that possibly lasted for 10 or so million years. The result of this is that the sandy Lower Cretaceous sandstones of the Chouf Formation rest unconformably upon the Jurassic limestones. Associated with this pre-Chouf Sandstone uplift was another phase of basaltic volcanism which continued in places into the middle part of the Cretaceous. In addition to this it seems tectonism it seems as if seafloor spreading continued in the offshore area until mid Cretaceous time.

During the Early Cretaceous Lebanon was covered by a series of swamps, rivers and deltas which has given a widespread sequence of sands and shales up to 500m thick. These Early Cretaceous strata are widely worked for building sand. They also contain good fossil amber with well preserved insects.

During the later part of the Early Cretaceous sea levels began to rise and marine incursions became increasingly prominent. The supply of sands into the Lebanon area began to wane switching off almost totally during the middle Cretaceous when a sea level rise brought in a widespread pure limestone deposition, locally with reefs, across the area. After a brief return to sandstone and clay deposition sea levels rose further to give a return to widespread limestones.

After the seafloor spreading which marked the first three episodes the Late Cretaceous saw a major switch in the tectonic pattern as Eurasia and the Africa-Arabia Plate began to come closer together causing the start of the closure of the Tethyan Ocean. Although any collision zone was well offshore and far to the north and northwest of the Lebanon area the first compressional effects seem to have been felt across the area during the Late Cretaceous. This gave rise to the first gentle uplifting of the Mount Lebanon and Anti-Lebanon area so that the main features of Lebanon started to form at this time.

A more obvious feature was that of the very high sea levels which dominated most of Late Cretaceous and Early Tertiary time. These contributed to thick sequences of pale fine limestones and chalks. It is during this time the 'fish beds' formed in local areas of oxygen shortage close to the edge of the carbonate platform. The fine grained limestones seem to cross the Cretaceous-Tertiary boundary with no major change. Whatever did kill off the dinosaurs and ammonites left no obvious signs of its action here.

The last fifty million years has seen an enormous change in the area from the Mid Eocene time when the area was covered by shallow seas in which limestones were being deposited to its present state of being an emergent and eroding land mass.

At the start of this episode the Africa-Arabian Plate was just beginning to collide with Eurasia and there was still a substantial Tethyan Ocean present (Fig 6b). As the plates collided the geology changed. Substantial uplift occurred in the Late Eocene and Oligocene giving a major emergence and the marking out of the threefold NNE-SSW trending pattern of modern Lebanon. During this time the sea was progressively pushed out of the Bekaa depression and restricted to shallow marine incursions along the line of the present day coast. The erosion of some of the main river valleys of Mount Lebanon may have started at this time.

Perhaps ten million years ago the area began to be dissected by the first motions along the faults of the Dead Sea Transform Fault System. These caused new tilting and uplift and caused major disruption of drainage patterns. At the end of the Miocene the Mediterranean dried up and during this time the river valleys may have cut down across the continental shelf.

There has been continued uplift and local tilting over the last five million years and some major disruption of river courses as various blocks slide against one another due to the strike slip faulting. Uplift and local tilting is evidently continuing; evidences for this are the numerous raised beach levels and the continuing seismicity. A classic case of this is the Litani River which, at one point, probably originally flowed due south into the Hula and Jordan valley areas but which had its path blocked by uplift and basaltic volcanism, redirecting it eastwards to the Mediterranean.

Superimposed on the effects of these tectonic events have been the major climatic and sea level changes of the last part of the Cenozoic. The lowering of temperatures over the last two million years gave rise to frequently wetter and colder conditions during the Pleistocene . Some of the best evidence for this can be seen in the way that the southern part of the Bekaa (from Rayak southwards) appears to have been sporadically covered by a large lake with a well marked shoreline at around 970m. The last remains of this lake system can be seen in the Aammiq Wetlands area. The extent to which glaciers were present on the tops of the highest peaks during the Pleistocene glacial periods is uncertain. The evidence suggests that limited glacier systems existed at altitudes above 2500m at the coldest times giving glacial moraines at such places as the Cedars at Bcharré.

From 10,000 years ago the area warmed up and reforestation occurred. Human activity however started to negatively affect the environment on a large scale from around 4000 BC onwards; a process that has increased alarmingly in the 20th Century

Mapa Libanonu: http://almashriq.hiof.no/ddc/projects/mot/maps/lebanon/arabicmap528-lines.html

Obr.1: http://almashriq.hiof.no/ddc/projects/geology/geology-of-lebanon/fig1.html

Obr.2: http://almashriq.hiof.no/ddc/projects/geology/geology-of-lebanon/fig2.html

Obr.3: http://almashriq.hiof.no/ddc/projects/geology/geology-of-lebanon/fig3.html

ZDROJ: http://almashriq.hiof.no/ddc/projects/geology/geology-of-lebanon/