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Location map. The Bekaa Valley, shaded. Inset box: Lebanon position within the eastern Mediterranean region.
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... Detailed mapping of the Bekaa basin shows that the Bekaa megabasin comprises three secondary basins enhanced by transverse uplifts (Lateef, 2004b). There are also minor folds and oblique faults in north Bekaa (Al Hermil locality) and middle Bekaa (Maqna locality) that produce additional local basins. ...
... These secondary structures bring notable variation in the scale (thickness) of the sedimentary fill (see e.g. Lateef, 2004b;2006a). The site of the current study exemplifies one such secondary structural setting where a faulted minor anticline divides the depression into two daughter basins ( Figure 2). ...
Initial Ar40/Ar39 chronology of two basalt bodies from northeast Bekaa Valley-Lebanon provided 10.4 ± 0.37 Ma and 10.87 ± 0.31 Ma eruption ages. These ages indicate basal Upper Miocene magmatism, which introduce new volcanic event into the geological history of Lebanon. The obtained chronology suggests volcanic eruption coeval with Middle-Upper Miocene tectonic stage and the major folding/ compression of Late Miocene in Lebanon that could suggest this volcanic activity as a precursor to the strike-slip tectonics along the Levant fault system and major folding and faulting of Lebanon at later times.
By mid-Cenozoic, earth movements have established, underwater, the three sub-parallel morpho-structural units in Lebanon of which the Bekaa syncline is the middle fold. From this point on, the following events are recognized: 1) Late Eocene-Oligocene. Intensified inversion. Sea retreat and establishment of continental conditions in the Bekaa basin. 2) Middle Miocene-Late Miocene. The first orogenic phase. Tectonism evidenced in north Bekaa by basalts dated 10.4-10.87 Ma (early Late Miocene). Important uplift of bounding heights and down-warping of the Bekaa basin. The Birth of some strands of the Lebanese segment of the Dead Sea Fault System (DSFS). Wide-spread lacustrine conditions. Alluvio-lacustrine sedimentation. Paleoenvironmental homogeneity prevailed. 3) Pliocene-Middle Pleistocene. The second orogenic phase commenced in Late Pliocene and culminated in Middle Pleistocene. Formation of Al Yemmouna Fault and associated small pull-apart basins. Intensified erosion of high lands and gravel deposition in the Bekaa basin. Within the basin proper, small-scale transversal uplifted zones developed (Tamnine and Kneissa-Laboua highs). Segmentation of the Bekaa Valley into three sub-basins. Alluvial fans merged across the basin. Lacustrine conditions of middle-south Bekaa contracted to the south. Small-scale folding, flexuring and faulting produced other daughter basins in middle Bekaa (Maqna) and north Bekaa (Al Hermil). In the southern basin, continuation of subsidence, deposition of fine sediments and commencement of lithotectonical zonation. In the middle Bekaa sub-basin, peripheral and quasi-basinal gravel aggradations continued on the eastern side. On the western side fine grained sediments were dominant. In the north sub-basin, the created daughter basin hosted a shallow-lake marsh with luxury vegetation (Al Asi Formation). By late Middle Pleistocene, Mount Lebanon probably achieved its highest orography. At this time, basin inversion occurred in north Bekaa. The highest altitude of Mount Lebanon enhanced inception of semi-arid conditions in north Bekaa. Commencement of chemogenic deposition (Ain al Zarqa Caliche Formation). 4) Late Pleistocene. Orogeny declined. Structures of the previous inherited phases were sculptured and denuded. The south Bekaa turned to lake-marsh conditions and fine clastic sedimentation. In the middle Bekaa, alternation of wet, dry and similar to-day climate conditions with characteristic sedimentation in western sector. Gravely deposition by spasmodic flash floods processes started to accumulate. In north Bekaa basin, caliche deposition developed thick calcrete armour. Environmental inhomogeneity was established in the basin. 5) Holocene. Early Holocene witnessed wet phase of unknown spatial extension. In late Holocene or historical period, climate shifted toward dryer conditions establishing present day climate. Man-effects exacerbated the natural factor and reduced vegetation cover. In the south basin, fine clastics sedimentation and water-logged conditions continued throughout the Holocene to present day. In the middle Bekaa, proluvial gravel aggradations on the eastern side penetrated deep into the basin during the early wet interval. In late Holocene, the middle Bekaa basin was stabilized and sedimentation was replaced by pedogenesis and calcrete soil formation. In the north basin aridity and chemogenic deposition continued. The thick calcrete armour preserved undulatory relic landscape. Spasmodic events (flash floods) proceed as important depositional environments in both middle and north basins. Environmental inhomogeneity developed steadily in the Bekaa Valley paralleling the increase of the role of orography as climate modulator. INTRODUCTION The Bekaa valley, Lebanon, is a structural basin sandwiched between the western Lebanese and the eastern Lebanese anticlines (Fig. 1). The basin and the overshadowing mountain belts comprise the principal morphostructural zones of Lebanon. The study of the targeted basin and the involved geological structures as well as the geological history of the Bekaa Valley (~900 sq. km) can't be discussed satisfactorily, without incorporating all Lebanon (10,300 sq. km) and also to a less extent the surrounding regions. Our knowledge of the geology of this region is based on works that started to accumulate since the early years of the 20th Century (e.g. Kober, 1915). The painstaking field works of L. Dubertret and collaborators (Dubertret, 1929-1963) were pivotal in establishing the geological picture of Lebanon and the Bekaa basin. Early geological works from adjacent areas, particularly in relation with the Red Sea rift and the Levant Fracture/Dead Sea Fault System (DSFS) improved our understanding of the geology of Lebanon and the Bekaa Valley (e.g. Quennell, 1958; 1959; 1984, Freund, 1965; Bender, 1983). In the last three decades a number of other valuable studies became available providing further insights on the basic geological issues of the region (see text for references). One notes that a great part of the available literature is dedicated to mountain building processes. Paleoenvironmental information from Lebanon, and the Bekaa Valley, are alarmingly scarce and are even lacking for the older part of the record. This imposes serious constraints on deciphering the geological history the basin. Recently, the present author obtained few paleoenvironmental indications that are effectively incorporated in this paper. Because of lack of data, paleo-orientations of the Lebanese structures are not incorporated in the drawings of the present paper; hence all figures depict present day orientation throughout
