Étude géomorphologique de la basse-vallée du Nahr el Kébir

Résumé En

The geomorphology of the area of the Nahr el Kebir is of considerable interest since it is very rich in both Quaternary shorelines and fluviatile terraces, in which there are numerous flint artifacts. 1. General View The Nahr el Kebir lies between two different geological areas : one is the Baer and Bassit mountainous area to the North, with greenstone rocks (ophiolites) which were formed here in the Maestrichtian ; the other is the Alaouite mountain range to the South, an anticline of Jurassic and Cretaceous carbonaceous rocks. Between these two mountain areas there was once an important Neogene gulf, in which sedimentation was mainly marly. Deposits of Pliocene age are very thick in the lower valley of the Nahr el Kebir (460 m of marls). According to Soviet geologists, the Nahr el Kebir lies exactly along an important accident, a geosuture, separating the Arabian Platform from the Tauric structure. This accident does not seem to have moved to any great extent during the Quaternary. The lower part of the Nahr el Kebir valley has cut widely into Pliocene marls, and is sometimes 6 km wide. It is incised in a plateau, smoothly sloping towards the West, and forming a glacis ; in the South this is exclusively cut into the Pliocene, but also into rocks of different age and hardness in the North. The fluviatile terraces of the lower Nahr el Kebir valley occur down its length as buttes and hills, whereas on the plateau it is marine deposits which outcrop frequently, with the main shoreline at about 80 - 85 m above sea-level. 2. The marine deposits of the plateau The topography of the plateau is very regular, except for some small breaks of slope. Fluvial deposits are the exception, while in contrast, marine deposits frequently occur, easily recognisable by their darker soils. They are formed, sometimes of sandstone or of conglomerates, up to 15 m thick, and sometimes of very rolled flint gravels or pebbles. The fauna, firmly cemented, is not rare, but it is banal. The most important marine formation is connected by the 80 - 85 m shoreline, and can be seen between Fidio and Hennadi, as well as, in the North, between Qannjra and Kirsane. Marine deposits are very abundant down to 30 m along the Nahr Snoubar ;they rise gently and continuously from the present shoreline to the foot of the Cheikh Mohammed butte ; the same transgressive formation sometimes fossilised an erosional topography and we know of several shorelines with two unconformable marine series (el Haqime and Boustane). Along the Nahr el Kebir valley the faciès is fluvio-marine, with alternating marine (with fauna) and fluviatile series. This transgressive formation, which is continuous for about 60 km, from the Nahr el Arab in the North to Banyas in the South, certainly belongs to the penultimate interpluvial ; the Tyrrhenian with Strombus bubonius does not outcrop at the mouth of the Nahr el Kebir or in the Jebleh area, but it exists at Banyas, and near the Nahr el Arab it overlaps, with an unconformity, the Khellale- Hennadi Formation, which corresponds to the Jbailian of Lebanon. The Baksa Formation is relatively well preserved on both sides of the Nahr el Kebir between 90 and 135 m. At Jabal Idriss, it has a fluvio-marine faciès. Probably it belongs to the antepenultimate interpluvial and corresponds to the Zaqrounian of Lebanon. The Mchairfet Formation rises up to 190 m and is of great age (pre-Günz). Its altitude is approximately the same on both sides of the river, so that the Nahr el Kebir geosuture has not operated very much since the lower Quaternary. 3. Fluviatile terraces Like the Orontes river, the Nahr el Kebir has a complete series of fluviatile terraces, the best in the entire Levantine coastal area. These terraces are generally stepped, owing to the wide incisions into the valley cut during the Quaternary. The fluviatile accumulations must be correlated with the glacial periods of temperate zones (regression periods) with a climatic control. The material is generally coarse, but it also includes thick, silty series. The buttes, benches and hills are generally structural surfaces. However, the two first Wurmian terraces were cut by two subsequent erosional surfaces ; the younger the terraces, the smoother the terrace profile becomes. The glacis slope is 1.3 % ; the Mindel terrace slope is 1 % ; the Riss, 0.55 % ; the Würmian, 0.33 %, probably because of an epeirogenic rise of this area during the Quaternary. Finally, these terraces decrease in age towards the East ; the axis of the valley has moved progressively eastward, because erosion is easier in Pliocene marls ; also there may have been a slight tilting of the basin of the river. The altimetric situation, stratigraphy, faciès, surface alterations and artifacts allow us to propose a relative chronology of these terraces mainly due to the fact that South of Snoubar, the Jinndiriye Formation lies unconformably over the Hennadi-Kellale Formation : The lower terrace (Qo) is quite important (3 or 4 m) and contains potsherds. Three formations are Wurmian (QI). Two Ech-Chir Formations (QIa and QIb) contain Middle Paleolithic, but not Upper Paleolithic, artifacts, and are dated from the first half of the Würm. The first one is mainly silty ; both are 30 m thick. In the Ech-Chir-Khoura area the second one lies unconformably over the first. Near Jinndiriye, and Haqime, a colluvial-alluvial deposit with red, clayey matrix (Jraïmaqiye Formation) contains some Upper Paleolithic flints (QIc). The Jinndinye Formation (QII, Rissian) has been very well preserved on the right bank, in the high cliff at Jinndiriye, Jabal Aamrouniye, Jabal Souayate, where the deposits are sometimes 30 m thick, coarse in the lower part, then silty (but not loessic, as stated by Wendt, 1971), then with a rarely-preserved pebble series at the top. The apparent base of this formation is at about 65 m above the talweg of the Nahr el Kebir. The older formations are much rarer and only exceptionally form terraces. The Berzine Formation (QIII , Mindelian) is preserved only in three or four outcrops, but was clearly identified in Berzine, where it is at + 100 m, i. e. 90 m above the alluvial plain, and where we found many artifacts. The Sitt Markho Formation (QIV, Günz) is know from only one outcrop, at 110 m above the river, and about 10 m below the plateau. The numerous decalcified flints, the altitude and the geoche- mical alteration of pebbles, show that this formation is very old.