2. DESCRIPTION OF THE STUDY AREA
2.2 G EOLOGICAL F RAMEWORK
2.2.3 Regional Geology of the wider area of interest
Along the lower Jordan/ NE Dead Sea foothills area three consolidated sedimentary sequences can be identified, which are separated from each other and from the underlying crystalline basement by major unconformities. The Cambrian to Early Ordovician Ram Group represents a fluvial sandstone- transgressive siltstone/limestone-regressive mature sandstone cycle draping all over the crystalline basement with southward increasing thickness from an estimated 700 m around the Wadi Zarqa area to about 1 km near Madaba. It comprises the Disi Aquifer, which was encountered in well JICA No. 5 S of Kafrein.
The overlying Permian to Jurassic Hudayb, Ramtha, Azab Groups (formerly agglomerated as Zarqa Group) consists of unconformity-bounded marginal marine sandstone-carbonate-shale associations most of which are classified as aquitards. To the SE all of them wedge out so that a total thickness of an estimated 1 km around Wadi Zarqa is reduced to about 250 to 300 m south of Kafrein where the Jurassic Azab has tapered out completely and only the lower part (eg. Scythian to Anisian) of the Triassic Ramtha Group intercalated between the Ram Group and the following Cretaceous strata. SE of Wadi Naur-Wadi Kafrein the Ladinian and Carnian gypsiferous formations (Umm Tina, Abu Ruweis) were completely removed by pre-Cretaceous erosion (Bandel and Khoury 1981).
Above the early Cretaceous transgressive unconformity follows the sandy Kurnub Group (Neocomian), the limestone-marl sequences of the Ajlun Group (late-Albian to end of Turonian) and the predominantly chalky Belqa Group (Coniacian to Eocene). The fluvial to marginal marine sandstones of the Kurnub Group increase in thickness northwestward attaining 180 to 200 m at the NE end of the Dead Sea and 250 to 300 m in the Wadi Zarqa area. They form the Kurnub aquifer (Fig.
2.2-6 and Fig. 2.2-8) characterized by low salinities.
The lower Ajlun Group (Naur, Fuheis, Hummar, Shueib Formations; A1 to A6) with 300 m of subtidal to peritidal marls, nodular limestones, massive limestones reflect several transgressive to highstand cycles of a permanently submerged carbonate platform which however ends with a late middle Turonian lowstand generating upper Shueib F. shallow water dolostones and marls grading S-ward into supratidal gypsiferous claystones (Schulze et al 2003). Thus, mainly due to the Fuheis and the Shueib marls and claystones the lower Ajlun (A1 to A6) forms a multilayer aquitard, in some areas even an aquiclude with embedded aquifers e.g. the massive limestones of the upper Naur and of the Hummar Formations. In contrast, the overlying ca. 100 m of Wadi as Sir Formation (A7) of the area consists mainly of well bedded and massive limestones of a prograding and agrading carbonate platform and together with the basal Belqa Group forms the important Upper Aquifer (A7-B2 aquifer).
After an episode of non-deposition the pelagic chalks of the Wadi Umm Gudran and Amman cherty limestone Formations of the Belqa Group were deposited, which on the western-facing slopes are preserved in synclines. Only on the crest of the Transjordanian Mts. and beyond they form a gently
eastward sloping continuous layer where they are overlain by bituminous marls and chalks of the B3 aquitard (Muwaqqar Form.).
Fig. 2.2-6: Stratigraphic table of Rock Units in the wider area of interest (modified after Shawabkeh 2001).
2.2.3.2 Unconsolidated rocks - Jordan Valley Group
Three different continental depositional environments have played a major role since the development of the Jordan Valley basin. These environments are terrestrial/fluvial, deltaic/limnic, and limnic/brackish environments.
Upper Pliocene to lower Pleistocene Unit
As stated above, the early history of the Jordan Valley Group is not fully known. In the study area the above described Shagur Formation is believed to be the oldest formation deposited in the Jordan Valley (Upper Pliocene – Lower Pleistocene). The Shagur Formation consists of massive, crudely bedded fluvio- limnic conglomerates that alternate with crudely bedded travertine and marl or claystone. This unit overlies unconformable older consolidated rocks. Clasts are angular to subangular and siliceous cemented. In the type localities in the east and south the terrestrial to fluvio- limnic Shagur Formation has a thickness of approximately 75 m. However, thickness changes significantly within the different facies types (Bender 1965). While the Shagur Formation crops out in the east and southeast of the study area, it was not reported, neither in the deep oil wells Jordan Valley 1 and 2, nor in the wells drilled by the Ministry of Water and Irrigation. No outcrop of the Ghor al Qatar Formation exists in the study area.
Pleistocene Unit
During Pleistocene times, three members can be distinguished. The three members (the coarse clastic, the silt and the lacustrine) are a vertical and lateral facies succession from terrestrial/fluvial, to deltaic/limnic and limnic/brackish lake environments. As stated above, Bender (1965) divided these different depositional environments into two members: Samra and Lisan. But in order to avoid confusion with the word Samra, the term Pleistocene Aquifer will be used for the coarse clastic and
2. Description of the study area
the silt member henceforward. The “Pleistocene aquifer” consists of the coarse clastic and the silt member. However, exploitable water resources are principally restricted to the coarse clastics member.The lacustrine member (Lisan Formation) consists of marl, gypsum and silt, and is generally considered an aquiclude, void of exploitable water. It crops out mainly in the west of the study area.
Since the Lisan Lake reached an elevation of –180 m above mean sea level its sediments can be found in incised channels of the major wadis up to the margin of the Jordan Valley (Fig. 2.2-8). Its total thickness is around 40 m. The coarse clastic member consists of gravel, interbedded with clay, sand and marl horizons. Both, the coarse clastic and the silt member underlie, overlie, or interfinger with the Lisan Formation .
Holocene or sub-recent Unit
This unit is built up of sub-recent terrigenous sediments deposited along the outlets of major wadis.
These alluvial fans are still accumulating as a result of large floods. They consist of debris from all neighbouring lithologies and are deposited according to their transport energy. The biggest components are found close to the apex and the smallest close to the fan margin. The transport normally takes place along alternating channels or after very heavy rain storms as sheet, or debris flow. Thus permeable horizons alternate with less permeable lithologies within these deposits. It is believed that the thickness maximum is near the valley margins, thinning out towards the centre of the basin. Well depth is rarely beyond several tens of metres. Often the alluvial aquifer directly overlies the Pleistocene gravel aquifer and because of that is hydraulically interconnected with this aquifer.
Since the study area is made up of this unconsolidated strata a special chapter (chapter 4.2) is devoted to the sedimentology of the unconsolidated strata.
2.2.3.3 Geologic Structure
Pre-Cretaceous structural imprint on the subsurface rocks of the area is virtually unknown. Triassic and/or Jurassic phases of extension are indicated by a number of basaltoid dykes and sills in the Ramtha and Azab Groups which did not intrude the Cretaceous cover (e.g. Bandel and Khouri 1981).
The structures most pertinent to groundwater flow is manifested in the unconformity between the Turonian limestones/ dolomites and the Senonian chalks. Thus the variable stratigraphic gap is associated with the embryonic ‘alpidic’ phase of folding in the Syrian Arc and is followed by the Coniacian-Santonian world-wide transgression (Braun et al. 1987; Haq et al. 1987). The Syrian Arc System that formed in early Senonian (Bentor and Vroman 1951, 1954, 1960; Bartov 1974) evolved under a regional WNW-ESE compressional stress field that dominated the region until Miocene (Eyal and Reches 1983; Eyal 1996). These structures were modified during the following Miocene to present-day deformation associated with shear on the N-S-trending Dead Sea Transform Fault (DSTF) which in the area runs close to the River Jordan.
The Syrian Arc deformation produced the following fold and fault structures exposed in the eastern slopes of the southern Jordan Valley between Wadi Zarqa and the eastern shore of the Dead Sea.
Salameh (1980) and Mikbel and Zacher (1981, 1986) on the base of structural field measurements, aerial photograph interpretation gave a time frame between Maastrichtian and Oligocene/Miocene for the evolvement of the Wadi Shueib Anticline and Amman-Halabat structure or Amman Flexure. The major fold and fault structures are listed from N to S (Fig. 2.2-7):
a dome-like structure around Salt where the base of the Cretaceous (bCr) rises to +400m aSL;
the Wadi Shueib Syncline, where bCr drops to -500 m bSL near Shuna;
the Wadi Shueib NW -facing monoclinal flexure plus associated normal faults;
the Wadi Shueib composite anticline where bCr between Shuna and Kafrein rises to -250m;
the NNE-trending Kafrein normal fault and associated faults bounding the Kafrein syncline to the W;
the Kafrein asymmetric syncline (bCr ca -400 to -800m):
the NE- to ENE- trending Amman flexure and associated faults facing toward NW;
the gently warped dip slope SE of the Amman flexure where bCr rises again to about +300m aSL.
Fig. 2.2-7: Main structural features in the wider area of interest.
These structures are modified or enhanced by sets of normal faults trending NNW and NW which may be regarded as syn- and antithetic to the subsiding Jordan Valley depression.
The network of Wadis flowing down the western slopes of the Transjordanian Mountains toward the Jordan Valley depression seem fairly well controlled by the above structures of the Cretaceous rocks. For example wadis follow synclines (Wadi Shueib, Wadi Kafrein etc). They also follow a combination of structural dip with NW-trending antithetic or ac-faults on the dip slope SE of the Amman Flexure (Wadi Hisban/ ar Rama, Wadi al Muhtariqa, Wadi al Hiri E of Suwayma). This may reflect some of the controls geologic structure exerts on subsurface flow pathes of groundwater in the consolidated rock sequences.
2.2.3.4 Subsurface contacts between consolidated rocks and unconsolidated Jordan Valley Group sediments
Due to the spatial variability of the hydraulic potential and the multilayer nature of aquifers and aquitards in the consolidated rock sequence hydraulic contacts with the unconsolidated valley sediments play a major role in the hydraulic budget.
At the foothills of the highlands near Al Karameh, South Shuneh, Kafrein, Sweimeh the base of the Jordan Valley Group sediments (bJVG) crops out at an altitude of ca. –200 m b.s.l. A preliminary analysis of reflection seismics in combination with the deep wells JV1 and JV2 shows that the late Cretaceous structures continue beneath the younger Jordan Valley Group sediments (Heinrichs et. al 2004; AlZoubi et al. 2006). The same data reveal a drop of the bJVG along the Wadi Shueib composite anticline from –200 m at the surface to about –500 m b.s.l. at 4 to 5 km distance WSW of the outcropping contact. This dip corresponds roughly to the dip of the Cretaceous strata E of the contact (Fig. 2.2-9). It can therefore be concluded that up to this location on this anticline Naur and Kurnub Formations are directly overlain by unconsolidated sediments. Further W and toward SW seismic data suggest that higher stratigraphic levels of the Cretaceous are preserved and in contact with the unconsolidated sediments in spite of the bJVG dipping steeply toward W and toward the Dead Sea in the south. A similar inference can be made for the Wadi Shueib Syncline where the Wadi as Sir Formation should be continuous for some distance under the base of the unconsolidated sediments. At the moment low density of seismic information does prevent tracing the major faults into the subsurface of the Jordan Valley.
2. Description of the study area
Fig. 2.2-8: Geological Map of the wider area of interest (after McDonald and Partners 1963; Shawabkeh 2001;
Diabat and Abdelghafoor 2004).
Fig. 2.2-9: Geological cross section in the northern part of the wider area of interest (slightly modified after Shawabkeh 2001). Location of the cross section is indicated in fig. 2.2-8.