Hydrogeology of the Damascus Basin (Southwest-Syria)
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HYDROGEOLOGY OF THE DAMASCUS BASIN (SOUTHWEST-SYRIA) Reinhard WOLFART Bundesanstalt für Bodenforschung, Hannover ABSTRACT The description of the geology and hydrogeology of the Damascus Basin is based on research carried out in Syria in 1961/62. Relations between tectonics and groundwater collective systems are demonstrated. The investigation of thegroundwater chemistry showed that there are four chemical types of groundwater restricted to parts of the Damascus Basin. Favourable conditions for extraction of groundwater only exist in the range of the collective systems. The Damascus Basin covers 8450 km2. Framed by mountaineous ranges the basin opens in the Damascene plain iowards east to the Syrian desert. 1. GliOLOGY The Damascus Basin is situated at the northern slope of the Arabian shield where sedimentation since Cambrian time partly was marine-epicontinental, partly terrestrial. From the Damascus Basin itself prejurassic deposits are not known. In the area of the Syrian-Lebanese mountains in the Jurassic sedimentation of limestones and dolomites (more than 1.400 m thick) prevailed. After sedimentation of marls and biohermal limestones (100-150 m) the Jurassic terminated with a regression which lasted till the Aptian. The formation of the basis sandstone during the continental Neocomian — in the Lebanon up to 200 m thick — in the area of the Syrian-Lebanese mountains indicates the beginning of the great marine transgression of the Cenomanian. During Aptian and Albian marine limestone and terrestrial sandstone were sedimented which grade into lagoonal faciès with gypsum towards east. In the Cenomanian and Turonian marine limestone (up to 600-700 m) were deposited all over Syria. Preponderately marly-chalky sediments were formed from the beginning of the Coniac till the end of Eocene, only for short periods interrupted by sedimentation of chert and banked lime- stone. The thickness of the sedimentary beds of the Upper Cretaceous and of the Lower Tertiary amounts to 1,500 m in the Damascus Basin. The regression of the sea during Oligocène was followed by differentiation of tectonical movements during Miocene/Pleistocene causing the formation of depres- sions and raising areas. The tectonical events were accompanied by intensive volcanic activity which culminated in the eruption of "plateau-basalts (up to 1,500-1,600 m thick) in the southern part of the Damascus Basin (Jebel Druse). The foreland depres- sions were filled with the debris of the raising mountains; the zones of most intensive subsidence shifted repeatedly. During Miocene and Pliocene the synclines of Barada and Emjar el Aassal were filled with fluviatile conglomerates and lacustrine marls or limestones of a thickness of up to 700 m. At the end of Pliocene/beginning of Pleisto- cene the decisive step in the structural formation of the Damascus Basin happened by subsidence of the Damascene depression. In a first period the Damascene depression was filled up with fluviatile sediments probably at a climate which was relatively poor in rainfall. In a second period in the centre of the Damascene besides fluviatile sedi- ments lacustrine limestones and marls were precipitated at a more humid climate. Total thickness of the sedimentary filling of the Damascene depression amounts to about 450 m. Structure of the Damascus Basin is marked by the concordant NE and NW lines of the lincamcnttectonical joint system. Where the most important rhegmatic zones — "Sirhan-Zonale" (SE-NW) and "Palmyra-Zonale" (SW-NE) — cross one another 402 there is situated the deepest part of the Damascene depression. While the Sirhan-Zonale mainly consists of a group of more or less parallel faults and joints, in the area of the Palmyra-Zonale besides this a system of NE striking anticlines and synclines is deve- loped. Frequently the synclines run into faults. The anticlines show diapiritic features. According to this the germanotype orogenesis is modified by the passive participation of evaporites in the region of the Palmyra-Zonale. The western frame of the Damascus Basin shows germanotype tectonics. It is part of the graben-horst-complex of Lebanon and Syria which is principally formed by tectonical movements within the meridional rift zone; as a whole, however, it follows the NE direction of the rhegmatic system. According to this, the deflexion of the Syrian-Lebanese graben-horst-complex from the SN to the SW-NE direction should essentially to be traced back to the pre-cxistencc of the SW-NE running Palmyra- and Bckaa-Zonale. 2. OROGRAPHY The Damascene plain ("Damascene") — in the centre situated at an elevation of 590-600 m above sea level — is surrounded by a mountaineous frame locally incised by valleys which partly radially run into the Damascene. Thus, the Damascene Basin is a closed basin. Some wide shallow valleys furnished with low water divides crossing those valleys are cut through the mountaineous frame connecting the southern and eastern part of the Damascus Basin with the neighbouring closed basins. The highest elevations of the mountaineous frame are to be found in the Mount Hermon (2,814 m above sea level), Antilebanon (2,420 m), Jebel Druse (1,799 m) and in the region of the Palmyrean Chaines (up to 1,500 m). 3. CLIMATE In the different parts of the Damascus Basin climatic conditions are strongly modified by the Orography. In Hermon and Antilebanon region there prevails moun- tain climate with frost and precipitation up to 1,500 mm in wintertime. The other parts of the mountaineous frame in the south and north are belonging to a subtropical area with winterrains (about 200 to 350 mm); periods of frost happen rarely. East of Damascus the Damascene and the eastern part of the mountaineous frame arc to be classified as a subtropical zone of arid steppes with precipitation less than 200 mm/ annum. Annually the Damascus Basin receives about 1,600 millions m3 of rainfall. The precipitation has an average content of dissolved solids of about 6 ppm. Mean annual temperature of Damascus amounts to 17.7°C. Around Damascus the annual potential evaporation is estimated to be about 1,500-1,800 mm. 4. HYDROGRAPHY Closed basins are situated nearly all around the Damascus Basin. There are regions draining to the Mediterranean Sea only west and northwest of the Damascus Basin. Amongst the closed basins of southern Syria only in the Damascus and in the Dead Sea Basin perennial drainage of surface water takes place. The two main rivers of the Damascus Basin — Nahr elAouaj and Nahr Barada — reach the centre of the basin (about 590 m above sea level) only at rise. They are fed by karst springs in the western 403 mountaineous frame. The biggest one of those springs (Ain Fijé) has a mean annual discharge of 6.4 nvVsec. at a quotient of fluctuation of 2.5. Maximum discharge is to be observed 2-3 months later than maximum of rainfall; accordingly maximum dis- charge falls into the arid time. 5. HYDROGEOI-OCY OF THE DAMASCUS BASIN According to the existing knowledge of the conditions in the mountaineous frame of the Damascus Basin — Hermon, Antilebanon, Palmyrean Chaînes and basaltic area in the southern part — it can be considered as a fact that there are developed several groundwater storeys which are relatively weakly permeable outside of the tectonical zones. Within taphrogenetic and rhegmatic zones the existing groundwater storeys are locally broken through by joint systems. Consequently, within those zones there is developed only one single body of groundwater prcponderately of vertical extent which runs into the different storeys. In the Damascene, there are developed groundwater storeys of relatively great horizontal extent corresponding to the alter- nation of gravel, lacustrine limestone and marl. The different groundwater storeys hydraulically are but weakly effective in the Damascus Basin, because (here are no beds between them really acting as aquicludes and because there is a very effective communication between the different storeys in the range of the tectonical zones. Therefore, the hydrogeological circumstances can be considered as if a single ground water body would be present in a first storey. In the Damascene the substratum con- sisting of marine deposits in general acts as a base for this first storey. In the Damascus Basin the groundwater table is adapted to the base level, i.e. to the groundwater table in the area of the former lakes in the eastern Damascene. According to the geological conditions in great regions of the western and northern mountaineous frame the groundwater table is situated at a relatively high level within those regions compared with the base level in the Damascene. Consequently, the groundwater table is rather steeply sloped between the Damascene and the north- western frame. Thus, Zcbedani and Jairoud Graben, for example, are hydrologically rather independent from the Damascene. Different from the sedimentary part of the mountaineous frame the groundwater table in the basaltic areas (Jebcl Druse etc.) essentially is adapted to the base level in the Damascene showing normal sloping towards the base level. Consequently to relatively low rainfall and to recently raised exploitation of groundwater there is to be observed a permanently progressive falling of the groundwater table in the area of the Damascene. In the area of the former lakes in the eastern part of the Damascene the groundwater tabel was 3-5 m below surface in 1962. In the Damascus Basin the drainage system can be outlined as follows. Within the mountaineous frame there is developed a network of groundwater collective systems within the tectonical zones. Functionally this network of groundwater collec- tive systems is comparable with a surface drainage system. Just as a river a groundwater collective system acts as a local drainage base level. After collection in the tributary collective systems of the mountaineous frame which run parallel to the border of the Damascene frequently, the groundwater pours into the main collective systems radially directed towards the collective basin of the Damascene.