Published in modified form as "Tectonic and Geologic Evolution of Syria" by G. Brew, M. Barazangi, A.K. Al-Maleh and T.Sawaf, GeoArabia, 2001. All rights reserved.
For the first time, we document the tectonic evolution of all Syria throughout the Phanerozoic. These interpretations are based on a very extensive database, primarily seismic reflection data, well information, and surface geologic studies.
Syrian tectonic deformation is focused in four major zones that have been repeatedly reactivated in response to activity on nearby plate boundaries currently and throughout the Phanerozoic, especially Mesozoic – Cenozoic time. The most extensive zone is the Palmyrides, that includes the southwest Palmyride fold and thrust belt and the inverted sub-basins that are now the Bilas and Bishri blocks. The Euphrates Fault System and Abd el Aziz / Sinjar grabens in eastern Syria are large extensional features with a more recent history of compression. The final zone includes the Dead Sea transform plate boundary that cuts through western Syria.
Combining the interpreted history of these zones, together with analysis from the remainder of the country, we have constructed a model of tectonic evolution throughout Syria. Integration of lithostratigraphic information into our final model has refined the timing of specific events and provided a paleogeographic framework for the results. The model shows how specific deformation episodes within Syria have been penecontemporaneous with regional scale plate tectonic events. Following a relatively quiescent Early Paleozoic shelf environment, the northeast trending Palmyride / Sinjar trough formed across central Syria in response to regional Hercynian compression followed by Permo-Triassic opening of the NeoTethys Ocean and the eastern Mediterranean. The trough accumulated thousands of meters of clastic strata, and was the focus of Mesozoic carbonate deposition as subsidence continued. Late Cretaceous tectonics were dominated by extension in the Euphrates Fault System and Abd el Aziz / Sinjar graben in eastern Syria. Repeated collisions and continental margin shortening along the northern Arabian margin from Late Cretaceous to Late Miocene time caused platform-wide compression. This led to the structural inversion and shortening of the Palmyride trough and Abd el Aziz / Sinjar graben. This uplift and compression continues today under the influence of Arabia / Eurasia convergence.
The tectonic evolution of Syria has been critical to the hydrocarbon accumulations in the country. Hydrocarbons, with Miocene to Silurian age sources, are found predominantly in Mesozoic reservoirs with structural traps formed in response to Mesozoic extension and Cenozoic inversion tectonics. Some Paleozoic plays remain to be fully tested.
The 'Cornell Syria Project', active since the late 1980's, is an ongoing collaboration between Cornell University and the Syrian Petroleum Company (SPC), and recently with Damascus University. Our goal has been to analyze and map the tectonic history of structurally deformed areas of Syria, predominantly through geophysical analysis. Syria is part of the northern Arabian platform that has been proximal to active plate boundaries for most of the Phanerozoic, from Early Paleozoic ProtoTethys Ocean formation until today when plate boundaries still surround the country (Figure 5.1).
We show that regional plate tectonics strongly control the continental deformation in Syria (e.g. Brew et al., 1999). This deformation has been long-lived, episodic, and repeatedly focused in previously tectonized areas. Understanding this rich history can yield a fuller appreciation of plate tectonic processes. It can also provide a better understanding of the likely occurrence and distribution of natural resources. While not comparable with the vast reserves of the Arabian Gulf, the hydrocarbon resources of Syria are nonetheless economically important, and the potential for further significant discoveries remains.
Much of our previous work has concentrated on relatively distinct structural provinces within Syria. Our goal in this paper is to document the tectonic evolution of all Syria by integrating our previous interpretations with new regional structural maps and incorporating significant lithostratigraphic knowledge. After outlining the tectonic setting of the studied area, this paper continues with a very brief survey of previous work concerning Syria and a description of the newly expanded database used in the current work. We then describe the structure and interpreted evolution of specific, tectonically deformed zones within Syria. Our regional mapping is then discussed, encompassing a new lithostratigraphic chart, structural maps, and a new tectonic map for Syria. Our ultimate result is a regional evolutionary tectonic model for all Syria, set in a framework of plate tectonic events. We conclude by discussing the implications for hydrocarbon reserves in Syria.
Syria is close to the leading edge of a continent / continent collision where the Arabian Plate is converging on Eurasia at 18 ± 2 mm/year in a roughly north-northwesterly direction (McClusky et al., 2000). This collision is manifest in the active transform and convergent plate boundaries that currently surround Syria (Figure 5.1). The events on these boundaries, and their ancestors, have largely controlled Phanerozoic Syrian tectonics.
The most prominent current margin of the collision is the Zagros fold and thrust belt. These mountains, trending roughly northwest through western Iran and eastern Iraq, accommodate the convergence by widespread thrusting and folding with very significant shortening (e.g. Berberian, 1995). Along the northern Arabian margin the Zagros becomes the Eocene - Miocene age Bitlis suture of Eurasian and Arabian Plates (Hempton, 1985). To the northwest of the Arabian Plate the Mio-Pliocene age dextral North Anatolian Fault, and the sinistral East Anatolian Fault accommodate westward movement of the Anatolian subplate escaping under the influence of the convergence (McKenzie, 1970).
Coalescing with the East Anatolian Fault from the south is the Dead Sea Fault System. This system, that extends as far south as the Red Sea, separating Arabia from the African Plate (Levantine subplate). The Dead Sea Fault System is a sinistral transform fault accommodating the differential northward motion between the plates created by the opening of the Red Sea. Restraining bend geometry dominates the Lebanese portion of the sinistral Dead Sea Fault System (Walley, 1988). Total offset south of the bend is well established to be ~105 km (Quennell, 1984). Displacement north of the restraining bend has been documented at less than 25 km (Trifonov et al., 1991), but further work is needed to clarify and document this estimate. Several authors have suggested two phases of strike-slip motion on the fault, one pre-Miocene / Early Miocene and one post-Miocene (Freund et al., 1970; Quennell, 1984). This agrees with a widely accepted model in which Hempton (1987) documented a two-phase opening of the Red Sea. Hempton (1987) went on to correlate these phases of motion with the episodic deformation of many features in the northern Arabian platform, such as the Dead Sea Fault System. Bitlis suture, and Zagros fold and thrust belt. The findings of this paper largely support the model of Hempton (1987).
The work of the Syria Project has shown that, to a first-order, Syria can be divided spatially into four major 'tectonic zones' and intervening structural highs (see Figure 5.2 for locations). The first tectonic zone is the Palmyride area. Work there by Best et al. (1990; 1993), Chaimov et al. (1990; 1992; 1993), McBride et al. (1990), Al-Saad et al. (1991; 1992), Barazangi et al. (1993), Seber et al. (1993), and Alsdorf et al. (1995) documented a Late Paleozoic / Mesozoic depocenter trending northeast across central Syria (namely the 'Palmyride / Sinjar trough'). Compression in the Cenozoic has created the current topography above this trough (the Palmyride fold and thrust belt of the 'southwest Palmyrides' and the Bilas and Bishri blocks of the 'northeast Palmyrides'). This topography defines the areas that in totality we loosely call the 'Palmyrides'. Late Cretaceous rifting created the second tectonic zone, the 'Euphrates Graben' in the farthest southeast of Syria (Sawaf et al., 1993; Litak et al., 1998), and the associated 'Euphrates Fault System' tectonic zone that extends fully across Syria (Litak et al., 1997). Brew et al. (1999) mapped the evolution of the third tectonic zone, the Abd el Aziz / Sinjar area in northeast Syria that shows older association with the Palmyride trough, and more recent structural and stratigraphic similarities with the Euphrates Fault System. Most recent Cornell work has been focused on analyzing the Cenozoic evolution of the final zone, the Dead Sea Fault System in western Syria (Brew et al., 2000; Gomez et al., 2000).
These four tectonic zones have experienced the vast majority of tectonic deformation in Syria, while the stable zones remained structurally high and relatively undeformed. This follows the intuitively simple idea that a pre-existing weakness in the crust will be the focus of future strain accommodation. It has also been shown that the style of reactivation is dependent on the orientation of the tectonic zone to the prevailing stress direction. Detailed interpretations of the tectonics zones, and new ideas regarding their evolution, are further discussed in a later section, before being ultimately tied into our final regional tectonic evolution model for Syria.
Recent contributions to the understanding of Syrian stratigraphy and paleogeographic evolution are relatively numerous (e.g. Ponikarov, 1966; Al-Maleh, 1976, 1982; Al-Maleh and Mouty, 1983, 1988, 1992; Sawaf et al., 1993; Mouty, 1997a, 1997b, 1998). This work has concentrated on the extremely well exposed Mesozoic carbonate section in the Palmyride fold and thrust belt, the Syrian Coastal Ranges, and the Aafrin basin exposed in the Kurd Dagh mountains (Figure 5.2). In contrast, the predominantly clastic Paleozoic section and the Mesozoic of eastern Syria are known only from drilling data, and still present significant challenges to stratigraphic understanding. In addition to detailed mapping of the facies and biostratigraphic variations in the Mesozoic section, researchers have also made important progress in correlating formations regionally (e.g. Mouty, 2000). Currently this correlation, and many new regional maps, are being finalized by Al-Maleh et al. (2001).
The database available for this work is extremely extensive by academic research standards (see Figure 5.2 for data locations). It consists of roughly 18,000 km of migrated seismic reflection profiles , drilling records from over 400 different wells, 1,000 km of seismic refraction data, and numerous other datasets such as remote sensing imagery, topography, and geologic maps. We thank the Syrian Petroleum Company (SPC) for providing most of these data.
The seismic reflection data are mostly migrated hardcopies to 4 seconds two-way travel time. They are from a variety of vintages and show large variations in quality. The highest quality data are from the early to mid 1990's collected using Vibroseis sources with a very high fold of coverage. The poorest quality records were shot with dynamic sources and 6-fold coverage in the 1960's. In general the Cenozoic section is fairly unreflective, with the exception of some Miocene evaporite layers. The carbonate Mesozoic section forms very prominent seismic reflectors, and regional unconformities are easily distinguished. The clastic Paleozoic section is poorly reflective with the exception of several abrupt facies changes in Cambrian and Ordovician strata that form regionally observed reflectors. Data quality decreases markedly in areas of complex structure, most notably the deeper areas of the Euphrates Graben and most of the southwest Palmyrides. Recordings are also very poor in areas of Cretaceous limestone outcrop on the Bilas block, and basaltic outcrop in southwest Syria. Metamorphic basement does not form a clear event on reflection records, and so high-quality, multi-fold refraction data have been used to determine basement depth throughout Syria (Seber et al., 1993; Brew et al., 1997).
Formation top data are available for all of the more than 400 wells in the database. Various wire-line logs are available for around a quarter of the wells. These include sonic, density, gamma ray, and other assorted logs. Our stratigraphic data are based on these drilling records and fieldwork by the authors and others. Many of these data have been used in past interpretations of individual tectonic zones within Syria. For the first time, we consider all the data in totality for creating the present structural maps and tectonic model of all Syria.
The locations of our data, and all digitally held data, are stored within a GIS for easy retrieval and analysis. Many data interpretations have been conducted within the GIS, thus harnessing the power of multiple-dataset visualization, manipulation and combination. For more details see Brew et al. (2000).
The limitations of a printed journal do not allow a full appreciation of this digital approach, and space limitations allow only a fraction of our datasets to be shown here. Consequently, we are providing downloadable versions of many of our results and interpretations on the web (http://atlas.geo.cornell.edu/syria/welcome.html). The benefit of the digital distribution includes the facility for any reader to plot their own maps, displaying any of the available coverages, at any scale. The coverages can also be interrogated (for example, 'show only oil producing wells that penetrate deeper than 4000 m').
Previous work of the Cornell Syria Project has documented the structure and evolution of individual tectonic zones, based on subsets of the database discussed above. Here we discuss some past results and incorporate new and revised findings from the interpretations of these tectonic zones. These results will be integrated into our tectonic evolutionary model in a later section.
The Palmyride area is the most extensive and topographically prominent tectonic zone in Syria (Figure 5.2). Uplift in the Palmyrides is a relative recent phenomenon, however, and during most of the Phanerozoic the zone was a sedimentary depocenter (Palmyride / Sinjar trough), accumulating several kilometers of Paleozoic and Mesozoic strata through episodic rifting and broad subsidence.
Best (1991) was the first to identify Palmyride and describe in detail rift-bounding faults, and presented examples from around the Bishri block in the northeastern Palmyrides, many of which core previously interpreted structures of McBride et al. (1990). Chaimov et al. (1993) documented the southwestern Palmyrides to be controlled by Late Paleozoic and Mesozoic listric normal faults that were structurally inverted in the Neogene. Isolated seismic examples show faults penetrating deep into the Paleozoic section (Chaimov et al., 1992, 1993), and wells from the southwestern fold belt of the Palmyrides encounter repeated sections across reverse faults down to at least Lower Triassic levels. Stratigraphic relationships across these faults indicate movement at least as old as Middle Triassic. Unfortunately, poor seismic reflectivity of the older section and drilling strategy limited to Triassic objectives preclude documenting thickening of Paleozoic horizons that could be used to definitively date initial faulting.
A very significant portion of the Palmyride trough thickening can be related to broad subsidence rather than simple extensional faulting (Chaimov et al., 1992). In particular, the majority of the Triassic succession shows the typical form of a slow subsiding depocenter – perhaps the thermal subsidence phase above the Permo-Triassic rift. In the Jurassic, however, faulting dominated and many examples of this structural reactivation are found (e.g. Best, 1991; Chaimov et al., 1993; Litak et al., 1997). After gentle subsidence during the Early Cretaceous, in Cenomanian (and especially Maastrichtian) to Eocene time the Palmyride trough experienced accelerated subsidence (e.g. Mouty and Al-Maleh, 1983; Al-Maleh and Mouty, 1988; El-Azabi et al. 1998) with significant Late Cretaceous faulting in the northeast around the Bishri and Bilas blocks (Figure 5.2).
Since the Late Cretaceous the Palmyrides have been subjected to episodic compression leading to folding and the currently observed topography. About 400 km long and 100 km wide this topography can be divided into two distinct parts, the southwest Palmyrides, and the northeast Palmyrides which in turn consists of the Bilas and Bishri blocks. These areas have distinctly different Cenozoic histories as discussed below.
The southwest Palmyrides are dominated by a series of short, southeast verging reverse faults that core prominent surface folding. These short wavelength left-stepping anticlines have steeply-dipping (in some case overturned) forelimbs, and more shallowly dipping backlimbs. In general, the forelimbs become progressively steeper toward the southwest of the chain (Chaimov et al., 1993). The crests of the folds are generally 200 – 500 m above the surrounding topography.
Chaimov et al. (1993) argued for fault-propagation folding above inverted normal faults to form the southwest Palmyride folding and shortening. Many of these faults are linked by northwest striking sinistral transfer faults that are reactivated dextral transfers between the older normal faults. This model is supported by well data and outcrop evidence in the southwest Palmyrides that require significant reverse faulting. For example, at the Abou Zounar anticline ~70 km west-northwest of Damascus in the southwest Palmyrides, Triassic strata are thrust over the Santonian age Rmah formation (Mouty, 1997b). Coward (1996) also suggested that inversion of northwest-facing half graben could explain the Palmyride fault-propagation folds. He indicates decreasing fault dip in the shallow section to explain the tight folds. Chaimov et al. (1993) showed sub-parallel Upper Cretaceous and Lower Paleozoic horizons that argue against regional scale detachment development in the Palmyrides. However, Chaimov et al. (1992) did map a locally well developed Triassic detachment level that precipitated some fault-bend fold formation, especially in the northern area of the southwestern Palmyrides (Figure 5.3). As an extension of the detachment hypothesis, Salal and Seguret (1994) argued for three levels of detachment and very significant thrusting in the southwest Palmyrides.
To the contrary, Searle (1994) suggested there is only very minor reverse faulting in the Palmyrides. He mapped complex folding, often in the form of box folds, above a locally developed Upper Triassic detachment (the predominantly gypsum Hayyan formation). However, in reaching his conclusions Searle (1994) appears to have mapped only the central and northeast parts of the mountains.
Hence we interpret strong along-strike structural variations in the Palmyrides. Fault-propagation folding above reverse faults, occasionally above a locally well developed Upper Triassic detachment, appears to be the predominant shortening mode in the far southwestern Palmyrides. Folding, probably above the same detachment but with no appreciable thrusting, is predominant farther northeast. This would agree with the cross-sections of Chaimov et al. (1990) that show total shortening decreasing from ~20 km in the southwest Palmyrides to almost no shortening in the farthest northeast.
West of the tightly folded Palmyride anticlines, the Anti-Lebanon Mountains (Figure 5.2), form the highest topography in Syria. These mountains expose Jurassic and Triassic strata and most of the Cretaceous section has been eroded (Mouty, 1998). Walley (1998) suggests that the majority of Anti-Lebanon uplift was likely during the second-stage of "Syrian Arc" deformation in the Late Paleogene. Lebanese structures were later modified as part of the restraining bend architecture of the Dead Sea Fault System during the Neogene (Chaimov et al., 1990).
Northeast of the tightly folded Palmyrides the extensive, low-relief Al-Daww basin (90 x 25 km, Figure 5.2) lies between the southwest Palmyrides and the Bilas block. Seismic stratigraphic relationships in the Al-Daww basin date its formation to Miocene time onwards. This intermontane basin contains more than 2 km of Cenozoic strata.
To the north of the Al-Daww basin, the Jhar fault separates the southwest from the northeast Palmyrides (Plate 1). The Jhar fault has been traced nearly 200 km striking east-northeast and shows an average of 1 km of up-to-the north movement, and significant, but undetermined amounts of dextral strike-slip (Al-Saad et al., 1992). Surface mapping indicates Quaternary movement (Ponikarov, 1966). Well data indicate this was an active extensional fault at least as early as Jurassic time. Additional interpretations suggest this fault may be the surface manifestation of a Proterozoic suture zone, as discussed further in a later section. The structural inversion along the Jhar fault is controlling the southern edge of the Bilas block (Figure 5.2) in a style of thick-skinned deformation typical of the northeastern Palmyrides. Uplift within the Bilas block is dominated by strike-slip duplexing where relatively undeformed, large anticlines are bounded by steep faults that show very little shortening (Chaimov et al., 1990).
To the north and east of the Bilas block, the Bishri fault is a prominent right-lateral fault separating the Bilas from the Bishri block. Similar to the Jhar fault, the Bishri fault accommodates uplift of the Bilas block relative to the Bishri block. Folding directly adjacent to the fault again suggests a transpressional feature. Focal mechanisms also show these dextral and reverse components of slip (see later Tectonic Map, Plate 1, and Chaimov et al., 1990). Northeast striking Mesozoic normal faulting was more active in the Bishri block than in any other part of the Palmyrides. Total throw is often distributed amongst several closely-spaced, steep, deeply-penetrating faults (Figure 5.4, and see Best, 1991). Jurassic was a time of significant fault movement, a Jurassic thickness of up to ~900 m is reached in the Bishri block. The area is also the only part of the Palmyrides to show significant normal faulting in the Late Cretaceous (thickness up to ~1600 m, Figure 5.4). Cenozoic structural inversion of these faults is controlling the present northeast-plunging anticlinal morphology of the block, flanked by much smaller folds (McBride et al., 1990; Best, 1991).
In northeast Syria (and extending eastward into Iraq) there exist two prominent topographic highs, the Abd el Aziz and the Sinjar Uplifts (Figure 5.2) that suggest recent deformation. However, the origin of these features goes far back in geologic history.
From Late Paleozoic to Late Cretaceous time the Sinjar area and surroundings was the northeastern portion of the Palmyride / Sinjar trough. Strata are correlative throughout this trough, with some thinning of all formations above the Derro High (Plate 1). Accommodation space for sedimentation in northeast Syria was created largely through broad subsidence, although some Late Paleozoic and Mesozoic northeast striking faults have been identified (Brew et al., 1999). As in the Palmyrides, many thousands of meters of Late Paleozoic clastic strata and Mesozoic carbonates were deposited in this trough.
During Senonian time the formative Euphrates Fault System affected the southwestern portion of the Abd el Aziz / Sinjar area, forming faults that were to bound the western extent of the later deformation. But no significant extension occurred around the Abd el Aziz and Sinjar structures until the formation of a network of east - west striking faults in the latest Cretaceous that accommodated moderate extension. These normal faults (the largest of which were predominantly south-facing), and the resulting half graben, formed in the latest Campanian and Maastrichtian and accommodated up to 1600 m of syn-rift calcareous marly sedimentation (Figure 5.5). The Abd el Aziz and Sinjar graben were the most prominent of these features. Many of the extensional structures were linked by strike-slip faults that were also structurally reactivated Early Mesozoic northeast striking normal faults. The extensional event was contemporaneous with further extension in the Euphrates Graben. However, the cessation of the extension, indicated by termination of faulting, came abruptly at the end of the Cretaceous, a little later than the cessation observed in the Euphrates Graben.
The currently observed topographic highs (the Abd el Aziz and Sinjar Uplifts) are the result of structural inversion that has been ongoing throughout the Cenozoic, most particularly in the Late Pliocene – Recent (Kent and Hickman, 1997; Brew et al., 1999). Specifically, stress caused by collision along the northern Arabian margin is reactivating, in a reverse sense, the Late Cretaceous east - west striking normal faults causing fault-propagation folds above their tip lines (Figure 5.5). Some of the northeast striking faults were also reactivated, in a strike-slip and reverse sense, during Cenozoic compression. One such fault is bounding the present structural inversion of the Abd el Aziz Uplift. The three major systems of fault in the Abd el Aziz / Sinjar area (Late Paleozoic / Early Mesozoic northeast-southwest striking; Senonian northwest – southeast striking; and Maastrichtian east – west striking) are clearly illustrated in the structure maps we present below.
Based on limited data, similar deformation appears to have contemporaneously affected the Mesopotamian foredeep in the farthest corner of northeast Syria. There, reactivated Late Cretaceous faults are observed beneath tight, Late Cenozoic fault-propagation folds (Figure 5.2).
The Euphrates Graben is a fault-bounded rift studied extensively by Litak et al. (1998) and de Ruiter et al. (1994). Litak et al. (1997) further showed that the Euphrates Fault System, a related but less deformed zone of extension, extends from the Iraqi border in the southeast to the Turkish border in the northwest, including the Euphrates Graben. The Euphrates Fault System is relatively unexpressed topographically (Figure 5.2) because, unlike the other tectonic zones of Syria, it has experienced very little tectonic reactivation in the Cenozoic.
A Turonian age unconformity - probably marking pre-rift uplift - is extensively developed in the Euphrates Graben, and the underlying limestone are eroded and dolomitized. Extension then followed causing widespread redbed deposition (Litak et al., 1998) that graded into progressively deeper water carbonate facies. Senonian rifting, that resulted in around 6 km of extension and an undetermined amount of strike-slip movement, was accommodated on a distributed system of steep normal faults. This is unlike some more 'simple' grabens that are bounded by more clearly defined major faults (Litak et al., 1997). Transtensional deformation was increasingly dominant with time. The syn-rift carbonate deposition culminating in the Campanian – Early Maastrichtian with the deposition of up to 2300 m of deep water marly limestone within the graben (Figure 5.6). Extension stopped during the Maastrichtian.
Paleogene time was marked by widespread thermal subsidence above the aborted rift. This sag has been shown to fit theoretical models of thermal equilibrium after rifting (Litak et al., 1998) suggesting that likely the whole lithosphere was involved in rifting event. This is in contrast to the Abd el Aziz / Sinjar Graben that shows no clear port-rift subsidence. The relatively quiescent Paleogene tectonic regime is in contrast to the minor transpression and reactivation experienced by the Euphrates Fault System in the Neogene. Compressional features are very mild everywhere within the Fault System. They are most developed in the northwest where reverse and strike-slip movement, with some associated minor fault-propagation folding, is observed on reactivated Late Cretaceous normal faults.
The Dead Sea Fault System is a major transform plate boundary separating Africa (Levantine subplate) from Arabia, and accommodating the differential movement between them. Several authors have suggested two phases of strike-slip motion on the Dead Sea Fault System, a pre-Miocene / Early Miocene slip of 60 – 65 km, and post-Miocene slip of 40 – 45 km (Freund et al., 1970; Quennell, 1984). Along the northern segment of the fault the age and rates of faulting are unclear due to a lack of piercing points, although total post-Miocene offset has been reported as less than 25 km (Trifonov et al., 1991). These observations and work in the Palmyrides have been combined into a model in which the northern Dead Sea Fault has been active only during the second (post-Miocene) phase of Dead Sea Fault System motion. In this model 20 - 25 km of post-Miocene sinistral motion has been accommodated along the northern fault segment, and another 20 km in the shortening of the adjacent Palmyride fold and thrust belt (Chaimov et al., 1990). Ongoing work aims to clarify this issue.
The northern segment of the Dead Sea Fault System strikes parallel to the coast through western Syria. The fault is clearly defined topographically and structurally near the Lebanese border in western Syria (Walley, 1988), but becomes diffuse and distributed as it approaches and crosses the Turkish border (Figure 5.2). Along the fault in western Syria is the Ghab Basin (Figure 5.7), a deep Pliocene – Recent pull-apart structure (Brew et al., 2000). The Ghab Basin opened in response to a left-step in the fault, although sinistral motion fails to be fully transferred across the basin, resulting in the 'horse-tailing' of the fault system observed to the north. Extension in the basin is accommodated by a series of northwest striking normal faults and significant subsidence on the Dead Sea Fault that bounds the basin to the east. Late Quaternary volcanism is found at the north of the basin that indicates opening of the basin there has occurred since 2 Ma.
The Syrian Coastal Ranges, in places more than 1500 m high, occupy most of the Syrian onshore area west of the Dead Sea Fault and Ghab Basin. They extend from the Mediterranean coast to the Dead Sea Fault System and from Lebanon to Turkey. This extensive monocline exposes the Mesozoic section from Upper Triassic to Upper Cretaceous (e.g. Mouty, 1997). The area is characterized by extensive karst terrain, a gently dipping (~10°) western limb, and a chaotic, uplifted eastern limb where the oldest strata are exposed (Figure 5.7). Stratigraphic relationships indicate that the uplift of the Coastal Ranges has occurred since the Middle Eocene. They could be part of the extensive Syrian Arc deformation that has been documented in Lebanon and Israel (Walley, 2000). However, the Ranges are clearly bound to the east by the active Dead Sea Fault System (Figure 5.2) that, in our model, has only been active since the latest Miocene in its current position. This suggests that most of the uplift is post-Miocene. While some component of compression across the Dead Sea Fault may be causing uplift of the Coastal Ranges, isostatic and dynamic uplift are most probably the main driving force (Brew et al., 2000).
We now consider the structure and stratigraphy of Syria as a whole, rather than the physiographically distinct areas just discussed. The lithostratigraphic and structural mapping presented below is based on all available data from Syria, as well as work from previous Cornell researchers. In the section to follow 'Regional Tectonic Evolution', we will integrate these regional maps into our final model.
We have used extensive drilling records from Syria, together with surface observations, and preexisting studies (e.g. Ponikarov, 1966; Al-Maleh, 1976; Mouty and Al-Maleh, 1983; Al-Maleh and Mouty, 1988, 1992; Sawaf et al., 1993; Mouty, 1997a, 1997b; 1998) to construct the most accurate summary of Syrian lithostratigraphy. Figures 5.8, 5.9 and 5.10 compare and contrast lithostratigraphic evolution of all Syria. More detailed discussion of tectono-stratigraphy of individual tectonic zones is given chronologically in our final regional tectonic evolution model.
Figure 5.8 is a generalized lithostratigraphic chart showing the variations of Syrian strata in time and space. Note that we have used the time-scale of Harland et al. (1990) throughout this work. Most clearly illustrated is the shift from predominantly clastic Paleozoic deposition to Mesozoic and Cenozoic carbonates. Furthermore, numerous widespread unconformities, showing long-lived hiati and erosion, are observed throughout the section, most especially around Devonian and Late Jurassic times. The long-lived Rutbah / Rawda and Aleppo uplifts (Figure 5.2) show the least complete sections. Also of some note is the very prominent Palmyride / Bishri / Sinjar depocenter. For much of early Mesozoic time the Palmyride deposition was linked to the Sinjar area, whereas for the Upper Cretaceous Sinjar strata show much closer affinity to similar age rocks in the Euphrates Graben. This reflects the shift in tectonics from the Palmyride / Sinjar trough to the Late Cretaceous fault-bounded extension in eastern Syria. Figure 5.9 shows details of the shifting deposition throughout Syria. Note the limited Jurassic / Lower Cretaceous section caused by widespread erosion and non-deposition related to regional Late Jurassic / Early Cretaceous uplift. Preserved Cenozoic patterns are dominated by subsidence along the Euphrates Fault System.
The various formation names used in Syria are often site-specific (Figure 5.8), leading to a clutter and confusing nomenclature. Furthermore, different nomenclatures have historically been used by surface and subsurface geologists, compounding the already difficult task of correlating subsurface and surface formations. Paleozoic formations in particular are notoriously difficult to distinguish from scattered drilling penetrations, and are often poorly differentiated in drilling logs, rendering detailed chronology impossible (e.g. Ravn et al., 1994). Regarding Mesozoic nomenclature, several long-standing problems have hindered regional correlation. A well-known confusion involves the Kurrachine to Serjelu formations that, for many years, were regarded as Liassic in Iraq (as shown in Beydoun, 1991). More recent dating has established ages comparable with the similarly named formations in Syria (Middle – Upper Triassic) (Beydoun and Habib, 1995). In our discussion of Triassic and Jurassic strata we have used traditional formation names (as maintained by SPC) and their modern (Mulussa Group) equivalents because the older names are widespread in the literature. Al-Maleh et al. (2001) provide a more detailed description and discussion of Syrian Mesozoic stratigraphy and attempt a definitive regional correlation.
Figure 5.10 shows general thickness variations for all the major sediment packages. The main trends include a southward and eastward thickening of Early Paleozoic strata (Ratka well) caused by the Gondwana passive margin off the east of Syria at that time. In the Late Paleozoic and Mesozoic deposition shifted to the west (Abou Zounar section) as the Levantine passive margin developed (Best et al., 1993). From Late Paleozoic time onward the influence of the long-lived structural highs of the Rutbah / Rawda Uplift (Tanf well) and Aleppo Plateau (Khanasser well) are easily observed. Upper Paleozoic and Mesozoic strata are concentrated in the Palmyride / Sinjar trough, with significant along strike variation apparent (Bishri well and Derro well). Rapid thickness changes in eastern Syria are associated with Late Cretaceous basin formation (Ishara well in the Euphrates Graben and Tichreen well in the Sinjar graben), and the influence of Neogene Mesopotamian foredeep formation (Swedieh well). Finally, uplift and erosion of the Cenozoic section is observed in the Palmyrides (Balaas well and Abou Zounar section) and Sinjar Uplift (Tichreen well).
We present new subsurface structural maps of four horizons throughout Syria (Figure 5.11a-d). Each map shows the present depth to top of the subject horizon, along with current structure, and the sub-cropping formation on the top of each horizon. A fuller appreciation of how the depths of these horizons vary in respect to one another can be gained from a perspective view of the four surfaces, plus topography, shown together (Figure 5.12).
The non-uniform distribution in quality and quantity of geophysical data in Syria gives somewhat uneven coverage to any resulting map. Areas of highest data density are those of most hydrocarbon production. Hence, these structural maps are most accurate for the Euphrates Fault System, portions of the Palmyrides, and northeast Syria, and are least accurate for the Aleppo and Rutbah Highs. Furthermore, as data quality and density decreases with depth so does the accuracy of these maps. For example, 460 wells penetrate the top of Cretaceous horizon while only 190 reach as deep as the Paleozoic (Figure 5.2). The Lower Mesozoic and Paleozoic of the Palmyride region, where seismic data are generally not interpretable and deeper well penetrations few, has the least reliability of all the mapping for these strata.
The maps are presented at a small scale. In many cases, particularly in the east, the mapping was conducted at a much larger scale, typically 1:500,000. There are countless small structures beyond the mapping resolution, and in areas of very low data density some faults are undoubtedly not mapped even at the smaller scale. The chosen scale of presentation represents a compromise between these situations.
The maps are not structurally restored. They show present deformation rather than the structure and depths at the time of deposition of the target horizon. This is why, for example, the top Triassic horizon demonstrates reverse faulting in the southwest Palmyrides although at the time of deposition these were normal faults. The symbols on the faults are designed to show the approximate past history of fault movement. Also, present-day depths are shown, not those during deposition. For example, the top of Paleozoic in the Palmyrides is shown as predominantly uplifted (Figure 5.11d), whereas during deposition this area was a topographic trough. Full-scale restoration is a future planned project.
The top of Cretaceous horizon (Figure 5.11a) indicates the effects of Syrian Cenozoic compressional tectonics. Note the strongly inverted Palmyride trough, especially the Bilas block, and Abd el Aziz / Sinjar Uplifts. The large sag above the Euphrates Graben is a result of the Paleogene thermal subsidence. Recent basin formation in western Syria is also illustrated. In general, faulting in eastern Syria halted before the end of the Cretaceous. Hence, unless there has been Cenozoic reactivation and fault-propagation of these features, the faulting is not observed at the top Cretaceous level. The well-developed Al-Daww basin in the central Palmyrides is present at all stratigraphic levels.
The Lower Cretaceous sandstone, a good seismic reflector, forms many hydrocarbon reservoirs in the Syria, hence this horizon (Figure 5.11b) is of particular economic interest. As shown by the subcrop distribution, this sandstone was deposited across most of Syria except on the Rubah / Rawda Uplift that was exposed and from which these sands were largely derived.
This map shows the full extent of the Euphrates Fault System and Abd el Aziz / Sinjar deformation. Note the distributed nature of normal faulting in the Euphrates Graben with no major rift bounding faults. In northeast Syria the superposition of the three prominent fault directions is illustrated. This map, and the ones on horizons beneath it (Figure 5.11c,d), show generally very similar structures. This is because much of the structure in Syria is on deeply penetrating, high angle faults. The net sense of offset of any particular horizon changes down section; this is observed on many of the faults mapped here. However, the location of the faults remain essentially fixed at this scale of presentation. Although some faults only cut the lower portion of the sedimentary column, they are often either too small or too poorly imaged to be mapped. The biggest difference between these maps is the depth to top of the chosen horizon. Obviously, this is a function of the thickness of the strata above it. As we have seen (Figure 5.10), this can change considerable throughout Syria.
The Triassic subcrop distribution shows the extensive Mulussa F (Uppermost Triassic, Serjelu formation) deposition that covered much of the country. This formation marks the beginning of regional transgression that continued through the Early Jurassic. Note that some of the formation was removed by Late Jurassic / Early Cretaceous erosion; the original deposition was even more extensive. The underlying Mulussa group shows progressively limited extent up-section, showing the increasingly limited deposition as water depths decreased following rifting.
This map (Figure 5.11d) has the poorest accuracy of the four maps presented here due to severe decrease in the quality of seismic reflection data from Paleozoic depths, and lower density of well penetrations. As with the overlying horizons, the greatest depths are found in the Sinjar trough and the Euphrates Graben, and in isolated basins of western Syria. Note the broader downwarping at this level in the Sinjar area indicating the broad extent of the Triassic Sinjar trough.
The subcrop pattern is dominated by the Permian Amanous formation that was broadly deposited. This map also shows the continuation of the Permian Palmyride trough into the Sinjar area. Note that in the inverted areas of the Palmyrides and Sinjar uplifts, reverse faults are still shown at this level based on well and seismic data showing uplift across these structures. However, associated fold-propagation folds are greatly subdued or absent at this depth (Chaimov et al., 1993). Furthermore, in the southwest of the Palmyride fold and thrust belt, the top of Paleozoic is below the local Triassic age detachment, and therefore not significantly faulted or folded. However, in the Bilas and Bishri blocks, the thick-skinned deformation has affected all structural levels. Again, the quality of the mapping is relatively poor for these structures.
The new tectonic map of Syria (Plate 1) shows general tectonic elements, outcrop distribution, shaded relief imagery, and seismicity. The faults and folds shown in black were mapped on the surface by Ponikarov (1966) and Dubertret (1955), or from our surface observations and limited remote sensing imagery interpretation. The subsurface structure, in red, is modified from the top Lower Cretaceous structure map (Figure 5.11b). This level was chosen to represent the subsurface as most faulting cuts this horizon, yet it is still relatively close to the surface. As shown in Figure 5.11, the sense of motion on these faults may change according to the structural level considered.
This map, although relatively complete for this scale of presentation (1:1,000,000) is undoubtedly incomplete for some areas. The sense of motion on many of the mapped structures is also ambiguous. In particular, we have mapped many of the reverse faults that core the anticlines of the southwest Palmyrides as being reactivated normal faults. Although this is true for many of the faults, some may be thrust faults detached in the Triassic, not reactivated normal faults. Strike-slip activity is also extremely difficult to map accurately. On this map it is only noted where it is known with some certainty. Assuredly, many more faults have strike-slip components that are not identified by this map. The map shows again how most deformation in Syria is focused within the four major structural zones: the Palmyrides, the Abd el Aziz / Sinjar area (northeast Syria), the Euphrates Fault System, and the Dead Sea Fault System.
Earthquake locations are from the International Seismological Center (ISC) database (1964-1994), and locations from the local Syrian seismograph network (1995-1996). Some of the apparent clustering of locations is probably a consequence of the station distribution. For example, the apparent lack of events along the northern Dead Sea Fault system relative to the southern Dead Sea Fault system is a consequence of station distribution. Regardless, there is an obvious concentration of events along the Dead Sea Fault System, some events within the other Syrian tectonic zones, and very few events in the stable areas of Syria. The Harvard CMT focal mechanism (1977-1996), supplemented by work at Cornell, are only loosely constrained because of the relatively small size of the events involved.
Metamorphic basement in Syria is generally deep (> 6 km) and has not been penetrated by drilling. Furthermore, the basement does not form a good seismic reflector. Hence, we have mapped the basement using seismic refraction data (results shown in Figure 5.13, Seber et al., 1993; Brew et al., 1997). In addition, Moho depth beneath Syria has been estimated from receiver function analysis (E. Sandvol, personal communication, 2000). The limits of Moho depths shown on Figure 5.13 are calculated using a range of average crustal velocities (6.2 – 6.8 km/s).
The Bouguer gravity anomaly field for Syria (BEICIP, 1975) shows a clear difference between northern and southern Syria with the boundary roughly within the Palmyrides (Figure 5.13). Using the inputs for basement and Moho depths, we developed new gravity models along two profiles across the Palmyrides (see Figure 5.13 for locations).
The first profile (Figure 5.14a) crosses the Aleppo Plateau, southwest Palmyrides, and the Rutbah Uplift. The dichotomous 'observed' gravity anomaly (green circles) on either side of the Palmyrides is clear. External controls on Moho and basement depths, some projected tens of kilometers along strike into the section, are shown as white annotations in Figure 5.14. Using these constraints, we modified the density model until the 'observed' and 'calculated' anomalies were acceptably close (difference less than ~3 mGal). In the first instance, we investigated crustal-scale effects without concern for the second-order anomalies in the Palmyride area. The result (black line) shows that a large difference in crustal thickness and crustal density on either side of the Palmyrides is required to satisfy the gravity data. This change in crustal properties can be modeled to occur along the present position of the Jhar fault. Furthermore, a small 'crustal root', of the order 2 – 3 km, is required beneath the southwestern Palmyrides to satisfy the receiver function Moho depth. This is significantly different from Best et al. (1990) who, lacking information to the contrary, modeled the gravity response of Syria using a flat Moho.
Modeling the second-order anomalies along this transect (dashed pink model and anomaly in Figure 5.14a) shows that arbitrary, high-density intrusions beneath the Palmyrides can be used to match the observed anomalies very closely. These could perhaps be an extension of the Rmah trend of intrusions that is clearly imaged by the gravity data (Figure 5.13), and described by Best et al. (1990). However, lacking additional information, this second-order modeling should be considered ad hoc.
The second gravity profile also crosses the Aleppo and Rutbah highs, but traverses the Bilas block area of the Palmyrides (Figure 5.14b). Large density and thickness differences on either side of an interface at or near the Jhar fault are again required. There is no requirement for a well-developed crustal root, but a small flexing of the southern block on the southern margin of the Palmyrides improves the fit of the model.
When these results are taken in a regional context, they support the hypothesis that Syria, like the rest of the Arabian Plate formed through a Proterozoic amalgamation of microplates and island arcs, i.e., the Pan African system (Stoesser and Camp, 1985). This left a series of suture / shear zones underlying the continent that have acted as zones of weakness throughout the Phanerozoic. The difference in basement depth, and crustal thickness and density on either side of the Palmyrides could be indicating that northern and southern Syria are different crustal blocks, sutured along the Palmyride trend. Furthermore, the Jhar fault, one of the major structural features of the Palmyride area, could be marking the position of the suture (as first suggested by Best et al., 1990). Assuming this scenario, crust of 'Rutbah / Rawda Uplift' affinity would underlay the predominantly thin-skinned deformation of the southwest Palmyrides, while 'Aleppo Plateau' crust underlies the Bilas and Bishri blocks that exhibit predominantly thick-skinned tectonics. This might be demonstrating that the Proterozoic architecture of the Arabian Plate is controlling the style, as well as the location, of Phanerozoic deformation.
Walley (1998) went further to argue that the suture zone could be traced westwards through Lebanon. He correlated the deformation style of the north and south Lebanese Mountains with the northeast and southwest Palmyrides. However, Walley (1998) maps many tens of kilometers of north – south separation between the present locations of his 'Lebanese' and 'Syrian' sutures. Thus his model appears to require much more than the currently accepted ~25 km of translation on the northern Dead Sea Fault System.
The presence of a crustal root appears to follow the leading edge of the southern block. The root observed in Figure 5.14a and the flexure observed in Figure 5.14b can both be considered as bending at the leading edge of the southern block. This could be a loading effect created by the Palmyrides themselves preferentially affecting the Rutbah block, suggesting it may be the 'weaker' block. Alternatively, this could be explained by the proximity of profile 'a' to the Anti-Lebanon, a very significant load much larger than the Palmyrides. This is could be causing a crustal root beneath the southwestern Palmyrides, whereas in the northeast the loading is supported by the strength of the lithosphere.
Our regional tectonic evolutionary model (Plate 2) shows two different views of regional tectonics at twelve time-points throughout the Phanerozoic. On the left (parts 'a') are paleo-plate reconstructions around Arabia; on the right (parts 'b'), are shown schematic tectonic activity and the sedimentary environment within Syria at each time-point. Timelines of regional and local tectonic events are shown in the middle of Plate 2. The time-points on Plate 2 are also indicated on the stratigraphic record (Figure 5.8).
There are many paleo-plate reconstructions for the Tethys Ocean and the eastern Mediterranean evolution (e.g. Robertson and Dixon, 1984; Dercourt et al., 1986; Ricou, 1995; Stampfli et al., 2000). Shown here (Plate 2, parts 'a') are reconstructions mostly adapted from Stampfli et al. (2000) developed with other members of IGCP 369. They are shown necessarily approximate and generalized for this presentation. These reconstructions are still the focus of some debate, especially concerning the position of many microplates, and the exact timing of oceanization of the eastern Mediterranean. We show them as an aid to discussion, rather than an endorsement of validity. Nevertheless, the model of Stampfli et al. (2000) is broadly in agreement with our findings.
Some of the regional tectonic events depicted by timelines in Plate 2 are only approximately dated. The dashed bars illustrate some of this uncertainty and the approximate build-up and decay of the tectonism. Such details need not overly concern us since we are interested in the general scheme of plate divergence and collision; the reader is referred to the original sources for detailed discussions.
Note that in the discussions below we refer to present-day polarities. For example, what we refer to as an Early Paleozoic east-facing passive margin, was predominantly north-facing at that time (Plate 2, frame 1a), but was subsequently rotated approximately ninety degrees. All the frames in Plate 2 are oriented with north roughly toward the top of the page.
It has long been accepted that the southern Arabian Plate formed through Late Precambrian accretion of island arcs and microplates against northeast Africa, most probably between ~950 Ma and ~640 Ma (Beydoun, 1991) as part of the Pan African orogenic system. Suture zones relic from this accretion, and the Nadj faults that formed when these sutures reactivated, are well-exposed in the Arabian shield (Stoesser and Camp, 1985). Based on geophysical evidence (see discussion above and Best et al., 1990; Seber et al., 1993; Brew et al., 1997) we suggest that the northern Arabian Plate is a result of a similar concatenation. Specifically, we find that the current Palmyride fold and thrust belt may lie approximately above the location of a Proterozoic suture / shear zone. Reactivation of this crustal weakness appears to have profoundly affected the tectonics of Syria throughout the Phanerozoic with the formation of the Palmyride / Sinjar trough, and later the Palmyride fold and thrust belt.
From ~620 Ma to ~550 Ma continental failure and intracontinental extension followed the accretion. This included strike-slip movement on the Nadj fault system, and the deposition of thick Infracambrian and Early Cambrian syn-rift deposits (Husseini, 1989). Owing to their great depth, no direct dating of the oldest sediments in Syria is available. However, from refraction interpretation we infer Infra - Lower Cambrian strata between 1 and 2.5 km thickness across Syria (Seber et al., 1993; Brew et al., 1997). Significant thickness of Infracambrian age sandstone and conglomerates are also observed in southeast Turkey (the Derik and Camlipinar formations), and in Jordan (the Saramuj formation). Husseini (1989) suggested that these syn- and post-rift strata resulted from the 'Jordan Valley Rift' that formed between Sinai and Turkey during the Infracambrian.
The penetrated Cambrian section in Syria is composed of arkose sandstone with some siltstone and shale probably eroded from granitic basement in the south (Plate 2, frame 1a). The exception to the clastic Cambrian section is the Early - Middle Cambrian Burj limestone formation that is observed across all of Syria (Figure 5.8); contemporaneous carbonates are found in most parts of Arabia (Beydoun, 1991). The regional extent of this monotonous formation (both sides of the 'Palmyrides suture') is more evidence for the cessation of cratonization and regional intra-continental extension of northern Arabia before the Middle Cambrian (~525 Ma) as discussed above (Best et al., 1993).
An erosional unconformity at the top of the Cambrian sequence (Figure 5.8), is just one of many unconformities that punctuate the Paleozoic section. This was a time of relatively shallow water over much of Arabia, relatively minor eustatic variations easily caused hiati and erosion.
Ordovician strata are extensive across all Arabia, especially along the northern and eastern margins, deposited on the wide epicontinental shelf. The Syrian Ordovician section increases from 1.6 km beneath the Aleppo Plateau, to more than 3.5 km in the southeast beneath the Rutbah / Rawda Uplift (Figure 5.10). Wells in the west of Syria penetrate an almost wholly sandy Ordovician section, whereas those in the southeast encounter significant components of siltstone and shale (Figure 5.8 and Plate 2, frame 1b). These facies and thickness trends in Syrian Ordovician strata are indicative of the open marine conditions to the east. The source areas for the Ordovician, and other Paleozoic clastics, were the extensive Arabian and Indian shield areas exposed to the south (Plate 2, frame 1a), and an ever-increasing proportion of reworked sediments.
The top Ordovician unconformity is most probably related to falling sea levels during Late Ordovician Arabian glaciation following a drift to higher latitudes. Although not definitively identified in Syria, extensive glacial deposits are found farther south (Husseini, 1990). The far eastern areas of Syria, the Rawda High and western Iraq (Plate 2, frame 2b), were exposed during this Late Ordovician to Early Silurian regression. The Upper Ordovician Affendi formation is missing in the farthest southeast of Syria, and thinned dramatically above the Rawda High. Beydoun (1991) showed this exposed / structurally high area extending from Turkey to Saudi Arabia during the Late Ordovician and Early Silurian, and likely has some tectonic component of uplift.
Deglaciation in the Early Silurian, as Gondwana drifted towards the tropics, caused sea levels to rise sharply flooding much of Arabia and depositing what is now an extremely important regional hydrocarbon source rock (Husseini, 1991). In Syria, these Lower Silurian grapholitic shales (the Tanf formation, Figure 5.8), although now thickest beneath the Palmyride / Sinjar trough (Best et al., 1993), were probably deposited to ~500 to ~1000 m thickness across the entire region during this transgression (Plate 2, frame 2b).
The Syrian Lower Silurian section is directly overlain by Carboniferous clastics, marking an unconformity of major temporal and spatial extent. This hiatus, strong tectonism and volcanism are observed contemporaneously in many localities around northern Gondwana. Some authors cite two events, loosely referred to as 'Caledonian' and 'Hercynian', the first is of Late Silurian age and the other is of Middle to Late Devonian / Early Carboniferous age, (Husseini, 1992). The absence of preserved strata in Syria prevents such a distinction there. Suggestions of the cause of this tectonism include a regional compressive phase caused by the obduction of the ProtoTethys on what is now Iran (Husseini, 1992); uplift on the flanks of the PaleoTethys rifting (Stampfli et al., 2000); or a more localized thermal uplifting event (Kohn et al., 1992) (Plate 2, frame 2a).
In any event, Upper Silurian and Devonian strata are almost universally absent from Arabia and underlying Lower Silurian shales are substantially eroded. The current subcrop pattern of Silurian strata in Syria shows an elongate depocenter roughly along the trend of the current Palmyrides (Best et al., 1993), and thinned to absent Silurian to the north and south. This could be interpreted as evidence for an Early Silurian age initiation of the major Palmyride / Sinjar trough. However, based on slight angular unconformity observed at the top of Silurian (Best et al., 1993), we suggest that this subcrop pattern is a result of Late Silurian and Devonian preferential erosion on the Rutbah / Rawda and Aleppo structural highs southeast and northwest of the Palmyrides, respectively.
During both the Late Ordovician and Late Silurian / Devonian manifestations of the Rutbah and Rawda Uplifts the most prominent exposure appears to the east of the current structural and topographic high (compare Figure 5.2 with Plate 2, frame 2b), around the current location of the Euphrates Graben. Previous publications (e.g. Litak et al., 1997) have examined the possibility that the Euphrates Fault System may have formed above a Proterozoic suture / shear zone similar to that advocated beneath the Palmyrides. However, given little evidence of subsidence or faulting along the Euphrates trend before Late Cretaceous time, this is now discounted. The Rutbah and Rawda highs (Figure 5.2) were evidently connected through most of geologic time until Late Cretaceous dissection by the Euphrates Fault System. Other than a few episodes of minor subsidence, after emergence in the Devonian the basement-cored 'Rutbah / Rawda Uplift' remained structurally high for the rest of the Phanerozoic, strongly affecting Syrian tectono-stratigraphy. The difference in basement depth across the Euphrates Fault System (Brew et al., 1997) (Figure 5.13) could be explained by a continuation of the 'Palmyrides suture' to the east, combined with the deep-seated Euphrates faulting.
A very few wells in central and eastern Syria encounter what are thought to be latest Devonian age rocks (Ravn et al., 1994). No major hiatus between the Devonian and Carboniferous is observed (Ravn et al., 1994), and a possible Upper Devonian section is also found in western Iraq (Aqrawi, 1998). This could suggest that incipient subsidence along the Palmyride / Sinjar trough had begun in eastern Syria by latest Devonian time. However, several deep wells in the Palmyrides encounter Lower Silurian strata directly below the Carboniferous, indicating that the Palmyride / Sinjar trough was not undergoing large-scale subsidence before the Carboniferous.
In Carboniferous time the Palmyride / Sinjar depositional trough formed fully across central Syria, in strong contrast to the relatively uniform and parallel-bedded Early Paleozoic deposition. In various forms, this trough was the main depocenter in Syria from Carboniferous to Late Cretaceous time, continuously flanked to the northwest by the Aleppo Plateau and to the southeast by the Rutbah / Rawda Uplift. On many seismic lines the Carboniferous can be seen onlapping the Silurian (Brew et al., 1999) and over 1700 meters of Carboniferous sand, sandy shale, and some minor carbonates, were deposited in the Palmyride / Sinjar trough (Plate 2, frame 3b). We interpret this Carboniferous trough to be a broad crustal downwarping between anticlinoria identified to the north and south of Syria (Plate 2, frame 3b) (Gvirtzman and Weissbrod, 1984). This Devonian / Early Carboniferous age folding could also be responsible for the major Devonian hiatus observed in Arabia, as discussed above (Husseini, 1992). The cause of this folding could be the same as the Devonian / Carboniferous uplift, namely regional 'Hercynian' compression.
Alternatively, Stampfli et al. (2000) suggests that the Early Carboniferous was a time of continental rifting along the north African margin (and consequently in the Palmyride trough), possibly a precursor to the NeoTethys Ocean formation. The cause could be regional stress reorganization after the collision of the Hun superterrane and Laurussia (Plate 2, frame 3a). However, many previous models (e.g. Robertson and Dixon, 1984) envisage no such Carboniferous rifting along northern Gondwana. Hence, while poor seismic data beneath the Palmyrides prevent definitive detection of possible Paleozoic faults, we favor Carboniferous folding rather than initial rifting.
Interestingly, the Carboniferous (and Permian) trough are found along a trend parallel to, but a few tens of kilometers south of, the Mesozoic depocenter and present Palmyrides. This suggests that the locus of deposition shifted during the formation of the Palmyride trough, probably in response to differential uplift and subsidence of the bounding Aleppo Plateau and Rutbah / Rawda Uplift. Furthermore, the limit of the Paleozoic and Mesozoic Palmyride trough is fairly sharply defined to the northwest, while the trough shows a more gradual flank to the southeast showing a greater interaction of the Rutbah / Rawda Uplift with the Palmyride deformation (McBride et al., 1990).
Husseini (1992) identifies the Mid-Late Carboniferous and Early Permian as a time of regional glaciation in southern Arabia. Although glacial deposits have not been definitively observed in Syria, the thick siliciclastic Carboniferous strata are typical of northern Gondwana deposition of the time. The glaciation also contributed to the wide-spread Late Carboniferous / Early Permian hiatus observed throughout Syria (Figure 5.8).
Opening of the NeoTethys Ocean in the Permo-Triassic (Plate 2, frames 3a and 4a) instigated profound changes in regional tectonics that persisted until the final consumption of the NeoTethys in the Miocene. On the northern and eastern margin of Gondwana, oceanic spreading separated the Cimmerian superterrane (including present-day Iran) that proceeded to drift northeastwards (Stampfli et al., 2000). Along the northern African margin Permian and Early Mesozoic rifting is documented by Stampfli et al. (2000) as being the second phase of the extension that began in the Early Carboniferous. Other authors cite this event as the initiation of faulting (Robertson and Dixon, 1984).
Debate still surrounds the precise timing of tectonics in the eastern Mediterranean region. While consensus has generally been reached concerning the oceanic nature of the eastern Mediterranean crust (see Robertson et al., 1996), the exact initiation of spreading remains uncertain. Robertson et al. (1996) examined various NeoTethys models. They concluded the most promising reconstruction is similar to that of Robertson and Dixon (1984), who advocated Permo-Triassic rifting, under conditions of northward PaleoTethys subduction, leading to Triassic sea-floor spreading in the eastern Mediterranean.
The reconstructions that we show (Stampfli et al., 2000) are mostly similar to the model of Robertson and Dixon (1984). One of the main differences is the presence in the Stampfli model of oceanic, rather than continental, crust along north Gondwana at the end of the Permian. In any event, this set of models differs markedly from those advocating Early Cretaceous oceanization of the eastern Mediterranean (e.g. Dercourt et al., 1986). The models of Robertson and Dixon (1984), Stampfli et al. (2000) and others see the Early Cretaceous as a time of renewed activity in the eastern Mediterranean, rather than sea-floor spreading initiation.
Hence, we interpret the Late Permian development of the Palmyride trough to be a consequence of extension along the north African margin that lead to east Mediterranean sea-floor spreading. In this scenario, the Palmyride rift could be an aulacogen (e.g. Ponikarov, 1966; Best et al., 1993), and we note that in most respects the Palmyride rift fits the definition of an aulacogen as used by Sengor (1995). The plate reconstructions of Stampfli et al. (2000) favor this interpretation (Plate 2, frame 4a).
An enticing variation to this is the reconstruction of Walley (2000). He argues for two different Permo-Triassic extensional phases, one in the Late Permian – Early Triassic that opened the Palmyride rift, and another in the Mid-Late Triassic that led to the opening of the eastern Mediterranean in a slightly different direction. Thus, this model allows for Late Permian rifting of the Palmyrides while not requiring Permian sea-floor spreading of the eastern Mediterranean. Furthermore, in this scenario the Palmyride / Sinjar trough is not required to be an aulacogen. Additional work concerning the exact timing of faulting will help test this model further.
Syn-rift Permo-Triassic siliciclastic deposits are only preserved in the Palmyride / Sinjar trough where they reach more than 1000 m thickness. Our hypothesis of the Palmyride / Sinjar trough as a Late Permian aulacogen suggests that faulting may be responsible for most of the thickening in the trough. Rapid thickness changes observed in well data show that rift-bounding faults controlled at least some of the Permian deposition in the Palmyride trough, and deeply penetrating faults were imaged by Chaimov et al. (1992). Furthermore, stratigraphic relationships indicate that the Aleppo Plateau and Rutbah Uplift were emerged throughout the Permian, possibly uplifted flanks of the rift (Stampfli et al., 2000). Cohen et al. (1990) find Permian age normal faults in southwest Israel sub-parallel to the Palmyrides trend and increasing sediment thickness westward, consistent with the hypothesis of an aulacogen extending from the central Syria to the eastern Mediterranean. Beydoun (1981), based on limited data, also speculated on the occurrence of a Late Paleozoic / Mesozoic aulacogen extending through Lebanese territory. Unfortunately, poor seismic data limit our ability to better image structure at depth and hence obtain a complete picture of the style of deformation. We conclude that rifting – as opposed to downwarping and subsidence – controlled a significant proportion, if not the majority, of Permo-Triassic deposition.
The exception to the pattern of NE-SW rifting in Syria is the Derro high of central Syria (Figure 5.2). This area was a structural high in the Early Triassic and possibly the Carboniferous, and represents the 'Beida Arch' of Kent and Hickman (1997) that connects the adjacent Rawda and Mardin highs (Figure 5.2). The work of Brew et al. (1997) suggests that the Derro high is a basement uplift, partially bounded by faults.
Syn-rift deposition in the Palmyride trough appears to have continued into Early Triassic time. The 'Amanous Shale' formation (actually part of the Doubayat group according to Beydoun (1995), or the Mulussa group member A of most petroleum explorationists, Figure 5.8), is the uppermost syn-rift sequence consisting of sandstone and shale, with increasing dolomite and dolomitic limestone upwards in central Syria. The continuity from Amanous Sandstone (Permian) to Amanous Shale (Lower Triassic) sedimentation results in the lack of distinction between these two formations in many central Syrian wells, a common problem in northern Arabia (Gvirtzman and Weissbrod, 1984). See Al-Maleh et al. (2001) for a complete discussion of Syrian Mesozoic strata and depositional environments.
By the end of the Early Triassic, rifting in the Palmyrides had ceased, whereas spreading in the Eastern Mediterranean was still active. This is demonstrated by the 'Amanous Shale' formation that thickens westwards in Syria reaching more than 250 meters near the Levantine margin. Furthermore, stratigraphic thickening in Israel suggests that rifting may have been longer-lived there than in the Palmyrides (Flexer et al., 2000). The cessation of Palmyride rifting could be a consequence of the eastern Mediterranean spreading ridge moving away along a Levantine transform fault (Stampfli et al., 2000). With the removal of the spreading, rifting in the Palmyrides stopped. Alternatively, the hypothesis of Walley (2000) considers the extension in the Palmyrides and eastern Mediterranean as being two separate events that can be explained by a change in regional stress direction.
Timing of Palmyride rifting cessation is indicated by an extensive Early Triassic unconformity found in most parts of Syria (Figure 5.8) – most likely a post-rift unconformity, compounded by extremely low sea-levels (Haq et al., 1988). The only conformable Permian through Middle Triassic sequence is in central Syria were shaly dolomites of the 'Amanous Shale' formation (Mulussa A) grade into the overlaying Kurrachine Dolomite (Mulussa B). This area, with the deepest depocenter, remained submerged as all other areas were exposed and eroded.
While the syn-rift Permian and earliest Triassic clastics are confined to the Palmyride / Sinjar trough, the first post-rift formation, the Middle Triassic Kurrachine Dolomite (Mulussa B) is spatially extensive, covering most of Syria (Figure 5.11c). This formation shows facies variations between dolomite and limestone, with greater carbonate content in paleogeographically deeper waters. Thus, Middle Triassic formations overlay Permian, Carboniferous, and sometimes even Silurian strata (Figure 5.11d). This extensive Early / Middle Triassic strata across almost all Syria indicates that the Paleozoic stratigraphic arrangements we observe today are not a consequence of Late Mesozoic or Cenozoic erosion. These post-rift strata are predominantly restricted-water carbonates and evaporites, as opposed to the overwhelmingly clastic syn-rift fill (Figure 5.8). This is a consequence of a drift to lower latitudes (Plate 2, frame 5a), lack of sediment source areas after plate reorganization, and shallower, more restricted waters of the post-rift environment. The evaporitic content generally increases up-section in the Triassic, indicating progressive shallowing.
The extents of the various Triassic formations are progressively more limited to the internal Palmyride / Sinjar trough through time (Figures 5.8 and 5.11c). However, some transgression of younger formations beyond the limits of older formations (especially on the Aleppo Plateau, Figure 5.11c) suggests the influence of minor sea-level variations on a relatively flat platform (Sawaf et al., 2000). This pattern is partially influenced by extensive Late Jurassic and Early Cretaceous non-deposition and erosion on the Aleppo and Rutbah / Rawda highs that removed much of the Lower Mesozoic section from those areas. Furthermore, this erosion led to anomalously thick preserved Triassic formations in the Palmyrides that were previously interpreted as evidence of Palmyride Triassic rifting (McBride et al., 1990).
The exception to progressively restricted Triassic formations is in southeast Syria where Triassic onlap developed along what is now roughly the axis of the Euphrates Fault System. The members of the Mulussa group progressively onlap the Rutbah / Rawda Uplift to the southeast, with a full Triassic sequence present near the Bishri block, but only the Mulussa F found in the southeast (Guyot and Zeinab, 2000). The Triassic onlaps Carboniferous and, in the extreme southeast, Silurian strata (Figure 5.11d) on the persistent Rutbah / Rawda high.
Subsidence curves from within the Palmyride trough shows a decreasing subsidence rate typical of post-rift subsidence (Sawaf et al., 2000; Stampfli et al., 2000) and indicate that this thermal relaxation probably continued until Early Cretaceous time. Well and seismic data show no widespread Triassic faulting around the Palmyrides, although some faults are locally observed (Best, 1991). Broad subsidence was the dominant control of the Triassic depocenter.
Triassic sedimentation halted before the deposition of the youngest Triassic Mulussa F formation (Serjelu). This interruption is marked by emergence and erosion, especially of the Aleppo and Rutbah / Rawda highs (Figure 5.8 and 11c). The subsequent Mulussa F deposition shows a distinct facies change, being largely clay and siltstone, as opposed to the underlying carbonates and evaporites. These clastics were sourced from the Rutbah Uplift in the south and southwest that remained exposed during the Late Triassic, with increasing calcareous content northward. The Mulussa F formation marks the beginning of regional transgression that continued through the Early Jurassic as described by Mouty (2000).
From a regional perspective Syria changed during the Permo-Triassic from being an east-facing to a westward-facing passive margin (Best et al., 1993). This occurred as the Levantine passive margin formed in western Syria to a backdrop of the continued formation of the eastern Mediterranean. This margin development, linked to the continued post-rift subsidence in the Palmyrides, is shown by preservation of more than 1.6 km of Triassic-Jurassic sedimentation along the present coastline. Triassic strata in Lebanon are very similar to those in Syria. In fact, a Triassic evaporite layer is found off-shore Lebanon coring compressional features (Beydoun and Habib, 1995) in a very similar way to the salt in Syria (Chaimov et al., 1990; Searle, 1994). In northeast Syria thickening of the Triassic eastwards indicates that the Sinjar region was linked to another major basin that was developing along the northern passive margin of Gondwana (Lovelock, 1984), as well as being influenced by subsidence along the Palmyride / Sinjar trough (Brew et al., 1999).
There is some overlap and confusion in the literature concerning the nomenclature of the Rutbah Uplift compared to the 'Hamad Uplift'. The term Hamad Uplift was first introduced by Mouty and Al-Maleh (1983). They used it to describe the northeast – southwest trending topographic high they documented in the paleogeographic environment of the Mesozoic Palmyrides. This usage distinguished the Hamad from the 'Rutbah Uplift' that is often used to describe uplift in western Iraq. Later authors largely failed to follow the nomenclature of Mouty and Al-Maleh (1983). Some oil company workers (e.g. de Ruiter et al., 1994) referred to the 'South Syrian Platform', thus distinguishing this from the Rutbah Uplift, but it is unclear if the Hamad and the South Syrian Platform are anything more than superficially synonymous.
The past work of the Cornell Syria Project has defined the Rutbah Uplift as a large, basement cored uplift dating since at least the Paleozoic. It covers western Iraq, parts of Jordan, and southern Syria (Figure 5.2). In this paper we acknowledge that the Hamad Uplift is a second-order feature on the north edge of the Rutbah Uplift that influenced Mesozoic paleogeography of the Palmyrides. However, we will maintain consistency with past work by not using the name 'Hamad Uplift', but rather using 'Rutbah Uplift' to include all elevated areas in southern Syria.
The transgression begun in the latest Triassic continued through the Early Jurassic. Characterized by limestone, dolomite, and occasional marl (Mouty, 2000), it progressively replaced the Triassic lagoonal evaporitic deposition with characteristically deeper water facies (Figure 5.8). The transgression covered all Syria except the Rutbah / Rawda (including the present Euphrates Graben area) and Aleppo / Mardin highs that remained emerged throughout the Jurassic (Mouty and Al-Maleh, 1983; Mouty, 2000). During the Jurassic, the Palmyride / Sinjar trough extended through southwest Syria (up to 2100 m of Jurassic section) and Lebanon (up to 2250 m) toward the still developing eastern Mediterranean (Walley, 2000).
Liassic tholeiitic basalts found in the Anti-Lebanon (Mouty, 1998, 2000) and Israel (Wilson et al., 1998), illustrate the continued rifting activity along the eastern Mediterranean margin. As a possible consequence, the Liassic was a time of renewed regional faulting activity in the northern Arabian platform (Wilson et al., 1998). Seismic profiles and wells throughout the Palmyrides, especially around the Bishri and Bilas blocks (Figure 5.4), demonstrate Jurassic age faults (Best, 1991; Chaimov et al., 1992; Chaimov et al., 1993; Litak et al., 1997), possibly a reactivation of Permian rift-bounding faults. Paleozoic faults reactivated in the Jurassic have been identified in Israel (Flexer et al., 2000).
Minor Lower Jurassic thickness changes (few tens of meters) within southwestern Palmyride anticlines (Mouty, 1997) are only a hint of the larger architecture of the time. Stratigraphic relationships preclude these thickness changes being due to later erosion. Two Jurassic depocenters are evident along strike in the Palmyrides, one centered around the current Bilas block, and one around the Bishri block (Sawaf et al., 2000). Widespread Jurassic faulting clearly focused deposition in these areas, with less significant accumulation in the southwest Palmyrides and Sinjar area. This further indicates that the Jhar and Bishri faults are old structural features.
Regression, beginning at the base of Bathonian (Plate 2, frame 6b), is evidenced by thinning of the Middle Jurassic formations eastward, the full sequence of Middle Jurassic formations showing this is not an erosional artifact (Mouty, 2000). However, a more pronounced regression, that was accompanied by widespread erosion, is recorded beginning in Kimmeridgian strata, and most of Syria was uplifted by the end of the Kimmeridgian (Mouty, 2000). Consequently, Jurassic strata are only preserved in the deepest areas of the Palmyride / Sinjar trough. The Upper Jurassic and Lower Cretaceous was a time of major sedimentary hiatus. Tithonian through Barremian strata are almost entirely absent from Syria (Figure 5.8), and much of the rest of northern Arabia (see summary in Guiraud, 1998), in concert with globally low sea levels. Heavily karstified surfaces further attest to long-lived exposure of the Jurassic limestone, except in the eastern Mediterranean basin where subsidence continued. Oxfordian – Kimmeridgian alkaline volcanics, with continuing volcanism through to Aptian time, were identified in the Anti-Lebanon, the Syrian Coastal Ranges, the Palmyrides, and other parts of the eastern Mediterranean (Mouty et al., 1992). Laws and Wilson (1997) combined observations of volcanism, regional titling and uplift to suggest mantle plume activity centered around Syria in the Late Jurassic and Early Cretaceous (also see Wilson et al., 1998). Garfunkel (1992) goes on to suggest that the Darfur volcanism in North Africa is the present expression of this still-existent hot spot.
The Late Jurassic non-depositional hiatus and erosion continued well into the Cretaceous. This extensive unconformity together with widespread Early Cretaceous volcanics (as far afield as the Euphrates and Sinjar areas) has led to suggestions of continued mantle plume activity (Laws and Wilson, 1997; Wilson et al., 1998). The somewhat accelerated deposition and fault reactivation found in the Sinjar area (Brew et al., 1999) and the Palmyrides (Chaimov et al., 1992) at this time could also be a result of this regional volcanic event. In a possibly connected event, accelerated spreading in the eastern Mediterranean may have also contributed to the pronounced Late Jurassic / Early Cretaceous faulting (Robertson and Dixon, 1984).
The regional Early Cretaceous transgression that followed uplift covered most areas of the northern Arabian platform with hundreds of meters of fluvial-deltaic to shallow marine sands (maximum >550 m in Bishri block). This Cenomanian and Early Cretaceous Rutbah sandstone in eastern Syria has largely equivalent Aptian and pre-Aptian members in the Palmyrides (Palmyra sandstone, Mouty and Al-Maleh, 1983), Lebanon (Gres de Base, Dubertret, 1955; Tixier, 1972) and Aafrin Basin (Al-Maleh, 1976). The only region of Syria not covered by the Rutbah sandstone or equivalent was the Rutbah / Rawda Uplift (Figure 5.11b and Plate 2, frame 7b). These areas were still elevated, as they had been for most of the Phanerozoic, and eroding Carboniferous and Permian sandstone as the source for the sandstone (Figure 5.11c,d) (Guyot and Zeinab, 2000).
Several interesting facies variations within the Lower Cretaceous sandstones reveal ambient paleogeographic conditions. Palmyra sandstone mapped in the Coastal Ranges is generally much more marly than in the Palmyrides, indicating deeper water to the west. The Gres de Base sand shows significant thickening toward Lebanon, with observations of limited chalk showing occasional shallowing (Tixier, 1972). This fits within a scenario of a continually subsiding eastern Mediterranean passive margin. The Rutbah and Palmyra sandstones become progressively more shaly and carbonaceous to the north, reflecting increasing distance from sediment source (the Rutbah Uplift).
In central and western Syria slow subsidence continued after the sandstone deposition. In general this broad Albian – End Cenomanian carbonate platform deposition (sometime referred to as the 'Middle Cretaceous', Mouty and Al-Maleh, 1983) is distinctly different from the underlying sandstones and overlying Senonian transgressive strata. The Zbeideh and Abou Zounar formations (Figure 5.8) mark two cycles of a shallowing depositional environment superposed on a general trend of decreasing water depth, suggesting a decreasing rate of subsidence. As with the majority of the Cretaceous and Jurassic strata, these formations show clear trends indicating deeper water depth, less restricted circulation, and a smaller proportion of clastics in the west and southwest. For example, in the Euphrates Graben in eastern Syria, the Cenomanian – Turonian Judea limestone (Figure 5.8) suggests marginal to shallow water depths and calm conditions. The equivalent Palmyride strata (Abou Zounar and Abtar formations) show medium depth to shallow marine conditions. The Cenomanian in the Coastal Ranges and Anti-Lebanon shows increasing marl with occasional planktonic foraminifera and pelagic, open marine facies (Slenfeh and Bab Abdallah formations). The northwestern Kurd Dagh region records hemipelagic strata. Maximum 'Middle Cretaceous' transgression is recorded around the Early Cenomanian to Early Turonian, coincident with all-time global maximum sea levels (Haq et al. 1988).
The first hint of Euphrates rifting activity comes in Turonian / Coniacian time. The initiation is marked by a widespread unconformity and associated volcanics and anhydrite. The underlying Judea formation is eroded and dolomitized. This could mark the pre-rift unconformity created by initial heating and uplift of the lithosphere under conditions of incipient rifting. Subsequent redbed deposition that was restricted to eastern Syria (Derro redbeds, Figure 5.8) marks the next stage in this evolution.
The exact cause of the Euphrates rifting is still unclear. Alsdorf et al. (1995) suggested that Latest Cretaceous continental collision along the northern margin of the Arabian Plate caused tensional forces orthogonal to the collision, thus creating the Euphrates Fault System and Abd el Aziz / Sinjar faulting. However, the much earlier initiation of faulting in the Euphrates Graben and the increasing tectonism away from the collision tend to invalidate this suggestion. Lovelock (1984) was the first to suggest trench-pull as a possible passive rifting mechanism. By Senonian time subduction in the NeoTethys was approaching the Arabian margin, and continued to approach until latest Cretaceous collision (Plate 2, frames 8-10a). This could explain the increasing extension with time, and the cessation of rifting with the collision of the trench and the northern Arabian margin during the Maastrichtian. However, the stresses created by such a distant trench may not be sufficient to cause the observed extension. Furthermore, the presence of pre-rift unconformity and volcanics might favor an active rifting scenario. This could be associated with the Early Cretaceous phase of plume activity observed in western Syria. Geochemical findings of deep mantle material in limited Late Cretaceous volcanism, if made, might suggest the plume over the trench-pull hypothesis.
The Senonian was a time of global high sea levels, and also a time of subsidence throughout the northern Arabian platform. In the Palmyrides facies suggest a clear increase in water depth after Turonian time. The Soukhne group (Rmah and Sawwaneh formations) exhibit increased marl and decreasing calcareous content. The top of the Soukhne group (Upper Campanian) is marked by a locally phosphatic limestone bed (Al-Maleh and Mouty, 1988). Studies of the Soukhne group (Mouty and Al-Maleh, 1983) show differentiation between pelagic and hemipelagic strata recorded in the Bilas area, and shallower conditions on the southern margin of the Palmyrides that was not completely submerged until the Late Senonian. This caused the preferential development of phosphatic deposits along the southern margin (Al-Maleh and Mouty, 1992).
Significant Late Cretaceous faulting in the Palmyrides is only observed in the Bishri area. Even so, central Syria at this time was undergoing accelerated regional subsidence that covered all areas. This was possibly due to the influence of northeast - southwest directed stress that we have invoked as the cause of formation of the Euphrates Fault System formation as discussed above.
On a regional scale Bartov et al. (1980) reported significant Santonian structural inversion in northern Sinai. However, Guiraud and Bosworth (1997) note that this was an isolated case, and was generally minor compared to later events. They claim no widespread compression of the "Syrian Arc" (inverted structures sub-parallel to the eastern Mediterranean coast from Sinai to Syria, see Walley, 2000) is observed before Maastrichtian time.
Although the Senonian was the time of significant rifting in the Euphrates Fault System (discussed below), similar large scale faulting is not observed in the Abd el Aziz / Sinjar area until Maastrichtian time. Deposition in the northeast of Syria was limited during the Late Cretaceous (excluding Maastrichtian), often not more than a few hundred meters of strata are encountered. The depositional environment was calm, with massive limestone prograding from Turkey in the north and mudstone deposition farther south (Kent and Hickman, 1997). Towards the southwest of this area, the northwest striking faults of the Euphrates Fault System controlled deposition (Brew et al., 1999).
The Euphrates Fault System rifted across oblique-normal faults from Santonian time onwards, although was most active during the Campanian and Early Maastrichtian. The first graben-fill were the Rmah chert in the west (directly equivalent to the Palmyride Rmah chert), and the Derro redbeds in the east (Figure 5.8) deposited during southeastward transgression. Progressively deeper water carbonate facies of the syn-rift sequence then filled the graben. This culminated in the accumulation of up to 2300 m of pelagic marly limestone of the Shiranish formation. At this time the Euphrates Fault System and Bishri depocenters were linked by a fault-controlled topographic low (Figure 5.9).
We suggest, as originally proposed by Lovelock (Lovelock, 1984), that Euphrates rifting was driven by trench pull of the approaching subduction zone in the NeoTethys (Plate 2, frames 9a,b). The Wadi Sirhan graben in Jordan (Figure 5.1) shows a very similar orientation, and timing of evolution, to the Euphrates Fault system (Litak et al., 1997). This suggests that the tensional forces responsible for transtension in the Euphrates were transmitted across the Arabian Plate and were causing similarly oriented extension in Jordan.
During the Late Cretaceous the Aafrin Basin formed around the northwestern corner of the Arabian platform, roughly along the line of the current Syrian / Turkish border. The basin has subsequently been inverted in the Kurd Dagh mountains (Figure 5.2). As in other areas of Syria, subsidence and deposition in the Aafrin basin was increased throughout the Senonian. The basin fill contains progressively deeper water facies from this period (Al-Maleh, 1976; 1982). Hemipelagic open marine strata of Santonian age lay beneath pelagic Campanian strata. The beginnings of recognizable Aafrin Basin geometry developed in the Campanian. Again, this may be related to the increased stress within the platform as a consequence of subduction approaching from north and northeast. It may also be related to the loading of ophiolites that were being progressively obducted onto the northern Arabian margin a short distance north of the basin. Surface mapping shows a typical preserved Santonian – Campanian section of more than 200 m (Al-Maleh, 1976). During this time, pelagic open marine strata were deposited in the Coastal Range area. However, this area was not a significant depocenter in comparison to the Aafrin Basin.
The Early Maastrichtian was marked by accelerated deposition throughout the Palmyrides. This was the initiation of a major phase of Palmyride trough development recorded by the deposition of the carbonate pelagic Maastrichtian to Lower Eocene age Bardeh formation (Mouty and Al-Maleh, 1983). The Bardeh formation (its lower part equivalent to the Shiranish of Euphrates and northeast Syria, Figure 5.8) has been studied extensively in outcrop (e.g. Al-Maleh and Mouty, 1988; El-Azabi et al., 1998). It shows great contrast to the depositional environment of the majority of Senonian Palmyride strata. The Bardeh formation was monotonously deposited marl contained very few quartz grains, with some planktonic and occasional benthic foraminifera, indicating great water depths in a low-energy open marine setting (El-Azabi et al., 1998). Thickness changes within the Bardeh formation emphasize the continuous development of the Palmyrides with the thickest strata recorded in the central areas.
Minor compression and uplift are well documented in the Palmyrides and the foothills of Turkey (Chaimov et al., 1992) in the latest Cretaceous. A coincident minor sedimentary hiatus at the Cretaceous / Tertiary boundary is observed in the Bardeh formation (El-Azabi et al., 1998). This is also regarded as one of two prominent phases in the development of the Syrian Arc that caused inversion of Permo-Triassic normal faults along the Levant margin (Guiraud and Bosworth, 1997). This transition from an extensional to a compressional regime was due to collision of the Arabian platform with the intra-oceanic subduction trench in the north and east (Plate 2, frame 10b), as first suggested by Lovelock (1984). This event was related to widespread Maastrichtian southward obduction of ophiolites along the northern and northeastern margin of Arabia (Hempton, 1985). This was not the final Eurasian - Arabia collision, however, and the NeoTethys Ocean, with associated subduction, persisted to the north and east (Plate 2, frame 10a).
The significant period of Late Cretaceous deformation in northeast Syria began in the latest Campanian or earliest Maastrichtian (Brew et al., 1999). The boundary between the Soukhne (Massive Limestone) formation, and the syn-extensional Shiranish is unconformable, (Kent and Hickman, 1997) suggesting this is the major pre-extensional unconformity. The Shiranish is predominantly a marly limestone with occasional sands eroded from exposed areas to the north (Kent and Hickman, 1997). It correlates with the Shiranish in the Euphrates Fault System. Extension took place on east-west striking faults that are limited to the west by the Euphrates faulting, and coalesce with Zagros deformation to the east in Iraq (Figure 5.11b). This extension created the Abd el Aziz and Sinjar half grabens (Figure 5.5). This faulting and half graben formation ultimately led to the deposition of up to 1600 m of Shiranish strata (Figure 5.9).
We suggest that these east - west oriented faults formed as a consequence of tension created by subduction located to the north and northeast margins of the Arabian peninsula (Plate 2, frame 10a). Perhaps the strain was accommodated in the Abd el Aziz / Sinjar area because it represented a structurally weak zone of thick sedimentation on the northern edge of the Palmyride / Sinjar trough. A gradual shift in the orientation in this subduction zone might explain the transition from general northwest - southeast extensions in the early Senonian (manifest by the Euphrates and Wadi Sirhan grabens) to more north - south extension in the Maastrichtian (Adb el Aziz and Sinjar half grabens). This was also the time of maximum extension in the east-west trending Anah graben (Plate 2, frame 9b) (Ibrahim, 1979). The relative southerly advance of ophiolitic nappes that were to obduct onto the northern margin could have contributed to normal faulting in northeast Syria through loading effects (Yilmaz, 1993).
Facies changes from marly limestone to lime grainstone (Kent and Hickman, 1997), and the abrupt termination of faulting at the top of Cretaceous level, together with a post-extension unconformity, signal the end of Late Cretaceous extension in northeast Syria. This was caused when Arabia collided with a NeoTethys subduction zone, as discussed above.
While a vast thickness of the Shiranish formation continued to be deposited in the Euphrates Fault System during the Maastrichtian, subtle indications suggest a reorienting stress direction, and a slowing of extension before final abortion of the rifting. Litak et al. (1997) reported that strike-slip features that are more common amongst the northwest - southeast striking faults of the Euphrates deformation, than amongst the west-northwest - east-southeast striking features. Furthermore, faults within the Shiranish formation were less active during the Maastrichtian, faulting ceased before the end of the Cretaceous (Guyot and Zeinab, 2000), and an unconformity is observed within the Shiranish formation (Litak et al., 1998). These observations could be explained by reorientation of extension from southwest - northeast to north - south) in agreement with that suggested for the Abd el Aziz / Sinjar area above, related to reorienting NeoTethys subduction (Plate 2, frame 10a).
Early Maastrichtian time saw continued subsidence and pelagic deposition in the northeast – southwest trending depocenter of the Aafrin basin. More than 600 m of Maastrichtian strata are found in measured sections exposed by Cenozoic basin inversion (Al-Maleh, 1976). However, during Maastrichtian time ophiolitic nappes encroached on the northwest margin of the basin, hence these areas experienced considerable shallowing. To the southeast and especially the southwest, the basin remained and Maastrichtian turbidities deposited there contain considerable ophiolitic ditrital content (Al-Maleh, 1976). Stratigraphically above the ophiolite, clastic lenses within the Uppermost Cretaceous strata indicate transgression after ophiolite emplacement.
In the Early Maastrichtian, the Coastal Ranges show a continuation of Campanian depositional trends with marly strata and only limited subsidence. The Late Maastrichtian is marked by the initial uplift of the Coastal Ranges (Brew et al., 2000). This is recorded stratigraphically by an angular unconformity between Paleocene and Maastrichtian strata (Ponikarov, 1966). This uplift occurred as part of the development of the 'Syrian Arc', resulting from collision along the northern Arabian margin as discussed above.
The Paleogene was largely a time of quiescence in the northern Arabian platform. All areas remained under marine conditions with extensive pelagic deposition. In the Euphrates and Wadi Sirhan Graben areas, widespread thermal subsidence following Late Cretaceous rifting (Plate 2, frame 11b). The Paleocene Kermev formation in the Euphrates Graben contains more chert than underlying Shiranish, and indicates shallowing water. This progressive shallowing is indicated throughout the Paleogene section here, and in the Abd el Aziz / Sinjar area.
During the Paleogene the Palmyrides area continued the prominent subsidence begun in the Maastrichtian, and deposition of the Bardeh formation continued. The Paleocene portion of this again shows monotonous pelagic marly limestone deposition with high levels of nanoplankton. The Lower Eocene Arak Flint marks the relatively shallower conditions that interrupted this period. Upper Eocene and Oligocene strata (the Abiad group) show continued subsidence. Facies are sandy limestones of shallow water origin related to the oncoming regression in the Palmyrides, Anti-Lebanon and Aafrin basin.
Chaimov et al. (1992) documented minor tectonism in the southwest Palmyride fold and thrust belt in Middle Eocene time, Late Eocene is clearly documented as the main stage of Syrian Arc deformation (Guiraud and Bosworth, 1997). This included uplift of the Syrian Coastal Ranges (Brew et al., 2000) that is recorded by a stratigraphic gap during the Late Eocene and Oligocene in the Coastal Range area. This 'Syrian Arc' development also included formation of the major topographic elements in Lebanon (Walley, 1998). Minor shortening in southern Turkey (Hempton, 1985), very minor transpression in the Euphrates Fault System (Guyot and Zeinab, 2000), and minor compression in the Abd el Aziz uplift (Kent and Hickman, 1997) are all reported for this time.
Hempton (1985) documented the Middle to Late Eocene as the initial period of final collision on the northern Arabian margin. This final obliteration of oceanic lithosphere thus formed the Bitlis suture in the western part of the northern Arabian margin (Plate 2, frame 11a). The plate-wide compression caused by this suturing can explain the numerous instances of Mid-Late Eocene compressional tectonics mentioned above.
Miocene time witnessed the final transition to continental conditions in Syria. One exception to this was the Lataqia / Aafrin basin along the northwestern margin of Arabia that includes Miocene marine strata. Marine incursions of the eastern Mediterranean Sea into western Syria continued well into the Pliocene (Ponikarov, 1966).
After the Middle to Late Eocene suturing of Africa / Arabia to Eurasia convergence between the plates was partially accommodated by the shortening and thickening of the Arabian continental margin (Hempton, 1985). The stress created by this ongoing convergence continued the formation of the compressional features that began forming in the Mid-Late Eocene, but at a slower rate. This stress regime was changed again by the Late Oligocene / Early Miocene initiation of continental stretching and rifting in the Red Sea. Rifting in the Red Sea lead to first phase of motion along the southern Dead Sea Fault System (Hempton, 1987). This, in turn, precipitated accelerated and still ongoing uplift of the Palmyrides (Chaimov et al., 1992).
By the Mid-Late Miocene the colliding edge of the northern Arabian continental margin had reached full thickness. This occurrence is thus regarded as the terminal suturing of Arabia to Eurasia. In the model of Hempton (1987) this collision can be correlated in time with the cessation of the first phase of Red Sea rifting and Dead Sea Fault System movement.
Hempton (1987) suggests that around the end of the Miocene the North and East Anatolian Faults formed to accommodate the continued convergence of Arabia and Eurasia. This coincided with a resumption of extension in the Red Sea, leading to full-scale sea-floor spreading. This also lead to the second phase of motion along the Dead Sea Fault System. This episode of movement caused a shift in the path of the fault north of Israel, and hence formed the Syrian portion of the Dead Sea Fault System (Chaimov et al., 1990).
Late Miocene onwards is marked as a time of increased compression in Syria, presumably caused by the cessation of shortening along the northern margin. Evidence for increased compression includes accelerated basin inversion of the Palmyride fold and thrust belt (Chaimov et al., 1992), minor shortening in the northwest portion of the Euphrates fault system, the Turkish foot hills, and the Zagros (Litak et al., 1997), and minor shortening in the Abd el Aziz uplift (Kent and Hickman, 1997). Furthermore, Feraud et al. (1985), using dykes and volcanic alignments as paleostress indicators, documented a shift in maximum stress direction from roughly northwest – southeast to north – south around the end of the Miocene.
Full-scale inversion of the Abd el Aziz and Sinjar uplifts did not take place until the Late Pliocene (Brew et al., 1999). Fault-propagation folds forming above reactivated Late Cretaceous east – west striking normal faults have created the current east – west trending topography. While small outcrops of Senonian strata are found on the Abd el Aziz structure, Cretaceous levels are more extensively exposed on the Sinjar Uplift in Iraq owing to increasing fault inversion to the east. Inversion in the Euphrates Fault System, however, is very minor and transpression is largely limited to the northwest segment of the system. This could be a consequence of the Abd el Aziz / Sinjar structures accommodating most of the Late Cenozoic strain. Also the oblique angle that the Euphrates Fault System forms in relation to the Alpine collision favors strike-slip reactivation (which is harder to detect in subsurface data). Seismicity, Quaternary volcanism (Plate 1) and very minor Quaternary faulting suggests the aborted graben in eastern Syria are still actively inverting (Ponikarov, 1966).
To the northeast of the Sinjar area, sediment thickness increases rapidly into the Mesopotamian Foredeep (Figure 5.11a). This depression formed due to the loading of the Zagros Mountains located to the northeast. In Syria some small Zagros-related folding is observed, with deeper structure reminiscent of the Sinjar graben. Well data indicate more than 1.3 km of Neogene clastic fill (Figure 5.9), shed from the uplifting Zagros since the Mid-Late Miocene terminal continental collision along this margin.
A series of eruptive volcanics from 24 – 16 Ma is found throughout western Syria, with the exception of the Coastal Ranges. As noted by Mouty et al. (1992), this period roughly coincides with the final stages of Arabian – Eurasian convergence. Interestingly, a gap in volcanism between about 16 and 8 Ma roughly corresponds to the episode of inactive Red Sea spreading, and no movement on the Dead Sea Fault (Hempton, 1987). Penecontemporaneous with renewed movement on the transform, the volcanism shifted from the Aleppo Plateau to locations along the Dead Sea Fault in Syria. In particular, formation of the northern Ghab Basin appears to have focused the most recent volcanism there from 1 – 2 Ma (Devyatkin et al., 1997). Holocene volcanic centers south of Damascus show strong alignments trending about north-northwest (Plate 1). This could be reflecting a modern stress direction trending north - south (Feraud et al., 1985), or evidence for reactivation of the underlying Wadi Sirhan structures that strike in a very similar direction (Figure 5.1).
Currently the Palmyride region is deforming by dextral transpression (Chaimov et al., 1990; Searle, 1994), under the influence of compression from the north and northwest (Plate 2, frame 12b). Evidence for active deformation on the Jhar fault includes small Quaternary offsets (Ponikarov, 1966) and seismicity. Additional, possible dextral strike-slip faults on the Aleppo Plateau (Plate 1) have also been identified (McBride et al., 1990). Our analysis suggests that the northeast trending faults mapped from the Bishri block towards the Abd el Aziz (Alsdorf et al., 1995) (Figures 5.2 and 5.11b) could be acting to translate right lateral shear away from the Palmyride region. The exact interaction between the Palmyrides, Euphrates, and Sinjar tectonic zones is still unclear. Forthcoming GPS surveys should help to resolve many of these uncertainties in the current tectonics of Syria.
Estimated recoverables from Syria are about 2.5 Bbbl of oil and 8.5 TCF of gas (Oil & Gas Journal, December 1999). The vast majority of hydrocarbon discoveries have been made in three of the four major Syrian tectonic zones (Figure 5.2) and hence understanding the tectonic evolution of these structures is critical to hydrocarbon exploration. The Dead Sea Fault System is host to some hydrocarbons in Israel, but none have been found so far in this zone in Syria. The three hydrocarbon-bearing zones are all abandoned rifts, with varying degrees of subsequent structural inversion. As a gross generalization, the source and reservoir rocks of Syria were deposited under the extensional conditions in the Late Paleozoic and Mesozoic, and traps were formed by Mesozoic extension and Late Cenozoic compression (Figure 5.15).
The discoveries in the Palmyrides are generally gas because of the greater paleo-burial depths of source rocks relative to elsewhere in Syria (Figure 5.11c). Most of the gas is found in the Triassic carbonate section, especially the Middle Triassic Kurrachine Dolomite formation; fracturing largely controls porosity as primary porosity is poor (3 to 10%; Al-Otri and Ayed, 1999). This reservoir is sealed by the Kurrachine Anhydrite formation, and was charged by Permo-Triassic and Carboniferous shale (0.8% - 5% TOC; Al-Otri and Ayed, 1999). Traps have been created in Late Paleozoic / Mesozoic fault blocks and the folds created during structural inversion and shortening (Figure 5.15).
A combination of oil and gas are produced from the Bishri block (Figure 5.2) in the transition between the Euphrates and Palmyride petroleum systems. Lower Cretaceous sandstone is the most common reservoir and fault blocks the usual trap. Potential Upper Cretaceous source rocks have not been sufficiently buried to reach full maturity in the Bishri block, and are positioned structurally higher than the reservoirs (Figures 5.11a and b). Hence, charging of the Bishri system may have resulted from westward migration of Upper Cretaceous oil originating in the adjacent Euphrates Graben (Illiffe et al., 1998).
As clearly demonstrated in Figure 5.2, the hydrocarbon discovery wells in the Abd el Aziz / Sinjar area are most directly correlated with current topography. The main trapping mechanism in the northeast is Late Pliocene fault-propagation folding, therefore indicating very recent oil migration (Figure 5.15). The degree of structural inversion is critical to successful trapping. In Turkey, greater shortening has breached many of the fault-propagation fold reservoirs. Some deeper traps are fault blocks. Source rocks in northeast Syria are commonly Cretaceous and Triassic (Ala and Moss, 1979). Reservoirs are predominately Mesozoic and Cenozoic fractured carbonates and many fields have multiple objectives in the Miocene, Cretaceous and Triassic (Figure 5.15). Sealing is accomplished by shale and evaporites that are distributed throughout the Mesozoic and Cenozoic sections.
The Mesopotamian foredeep, in far northeastern Syria is the longest established production in the country, within many different fields (see well distribution in Figure 5.2). Trapping is in the simply folded strata of Late Cretaceous and Cenozoic strata, charged by Late Cretaceous and Triassic sources. Late Cretaceous fault blocks may also be trapping deeper reserves.
Although mostly unknown before the 1980's, the Euphrates Graben harbors the most important hydrocarbon plays in Syria. More than 400,000 barrels of light, sweet crude are estimated to be produced daily from the graben, out of a national average of 540,000 barrels (Oil & Gas Journal, December, 1999). The bulk is from the Lower Cretaceous Rutbah sandstone (Figure 5.8), a high porosity (up to 20%) fluvio-deltaic sandstone with well maintained permeability, that was deposited during the Neocomian transgression in eastern Syria (Plate 2, frame 7b). Minor production comes from other levels (Figure 5.15) and trapping is most commonly in fault blocks (Figure 5.6), similar in many respects to the North Sea. Alternating carbonates and evaporites of the transgressing Triassic (Figure 5.8) have created a series of potential reservoir / seal pairs, and the widespread Serjelu (Mulussa F) could be a reservoir quality sandstone. The Thayyem field is typical of the southern Euphrates Graben, it was the first discovery in the Euphrates Graben in 1984, and is still the most productive. Rutbah sandstone forms the reservoir that is both charged and sealed by Upper Cretaceous marly limestone of the Shiranish formation. The Shiranish, deposited under widespread extension in eastern Syria (Plate 2, frame 9b), has been juxtaposed against the Rutbah by the Latest Cretaceous normal faulting that created the rotated fault block trap (Litak et al., 1998). While appreciable structural inversion in the northwest of the system may have breached some reservoirs, further southeast trapping has been enhanced by the very mild folding resulting from the Cenozoic compression.
In all of Syria, declining yields have pushed the search for hydrocarbons deeper, and exploration now focuses on Paleozoic plays. Grapholitic shale source rocks of the Silurian Tanf formation and Lower Ordovician Swab formation (Figure 5.15), as well as Late Paleozoic shales, are found through most of the Middle East (Husseini, 1990). Tests show 2 - 5 % TOC in the Tanf formation increasing southwards with perhaps up to 16 % TOC in Iraq (Aqrawi, 1998). Drilling from the Rutbah uplift, however, shows an over-mature Tanf formation, reinforcing that a viable source is the most elusive component part of a Paleozoic play.
Paleozoic reservoir rocks in Syria could include Permo-Carboniferous and Ordovician sandstones (both up to 25 % porosity) that are present at various depths over most of the region (Figure 5.11d). The Akkas oil shows (oil from a Lower Silurian sandstone and gas from the Upper Ordovician, sourced and sealed by Lower Silurian shales) in Iraq (Aqrawi, 1998), and Paleozoic discoveries in Euphrates Graben confirm viable Paleozoic plays in southeast Syria. The presence of suitably-aged structural traps could be the main control on this play (Aqrawi, 1998). The Maghlouja well on the Abd el Aziz structure had shows of gas in the Silurian section, and limited shows of relatively light oil (39 API gravity) in the Upper Ordovician Affendi formation (Kent and Hickman, 1997). Perhaps this oil was sourced in the Silurian and migrated after fault inversion juxtaposed that unit with the Ordovician in the Neogene, thus providing insufficient time for economically adequate charge.
Integration of a vast amount of detailed geophysical and geological data, together with experience based on many previous investigations, has allowed us to compose a new regional tectonic evolutionary model for Syria. We find that tectonic deformation within specific Syrian tectonic zones was often contemporaneous with deformation in other adjacent zones. Moreover, in almost all cases these episodes of tectonism can be related to activity on nearby Arabian Plate margins. In particular, the opening and closing of the various elements of the Tethys Ocean throughout the Phanerozoic profoundly affected Syrian tectonic evolution.
After Proterozoic cratonic accretion, for the vast majority of the Phanerozoic time Syria was part of the northern passive margin of Gondwana bordering the Tethys Ocean. Gentle Early Paleozoic subsidence of this east-facing margin led to the regional accumulation of thick clastic deposits eroded from nearby shield areas. For most of this time Arabia experienced either glacial-fluvial or marginal marine conditions, that changed to a shelf environment during frequent transgressions. Towards the end of the Paleozoic Hercynian compression, followed by extension related to opening of the NeoTethys Ocean led to the formation of the Palmyride / Sinjar trough that accumulated over two thousand meters of clastic Carboniferous and Permo-Triassic strata.
After drifting to lower latitudes in the Mesozoic, huge carbonate platforms developed on the exceptionally wide northern Arabian epicontinental shelf. Thermal subsidence above the Permo-Triassic Palmyride rift created a thick Triassic and Jurassic section in the Palmyrides, enhanced by periods of reactivated faulting. Development of the east Mediterranean, west-facing, passive margin also concentrated deposition in that area and was another dominant control on Mesozoic sedimentation. Observations of extensive Late Jurassic / Early Cretaceous uplift, widespread volcanism and renewed fault activity have led to suggestions of contemporaneous mantle plume activity.
Barremian - Aptian transgression deposited thick fluvio-deltaic sands across much of Syria. In the Late Cretaceous a northeast – southwest directed regional extension dominated. This led to the formation of the Euphrates Fault System, and accelerated subsidence elsewhere. Shifting to a more north - south extension direction in the Maastrichtian caused the opening of the Abd el Aziz, Sinjar and Anah grabens. Collision along the northern margin in the latest Cretaceous with associate ophiolite emplacement terminated extension and caused slight uplift in the Syrian Arc, including the southwest Palmyrides.
The thick carbonate sequences continued to form in the Paleogene, with some uplift and compression in Mid-Late Eocene time related to initiation of final collision along the northern Arabian margin. Neogene clastics indicate the shift to the continental conditions that prevail today. This occurred in tandem with renewed compressional tectonics related to terminal suturing on the northern margin causing the majority of the Palmyride uplift and inversion of the Abd el Aziz / Sinjar structures. Pliocene development of the northern Dead Sea Fault System led to the creation of the Ghab Basin, and added to the compressional tectonics within the platform.
The combined effect of this complex tectonic evolution has been to form conditions highly suitable for the preservation of hydrocarbon resources. Reservoirs are formed in the extensive clastic and carbonate deposits, most particularly in the Mesozoic, with shaly sources throughout the section. The traps are most often structural in fault blocks or fault-propagation folds.
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Figure 5.1: Regional tectonic map of the northern Arabian Plate and surrounding regions showing the proximity of Syria to many active plate boundaries. Leb. = Lebanon, NAF = North Anatolian Fault.
Figure 5.2: Map showing topography of Syria, seismic reflection and well data locations, and locations of other figures in this paper. Wells colors indicate depth of penetration, symbols show best available knowledge regarding hydrocarbon status of the wells as summarized from various literature sources. The map projection listed is used for this and all subsequent maps.
Figure 5.3: Block model of Abou Rabah anticlinal structure in the northern part of the southwestern Palmyrides. Surface is Thematic Mapper (TM) imagery draped over topography. Seismic lines CH-36 (dip line) and CH-45 are shown. See Figure 5.2 for location. View is looking towards the northeast. See annotation for horizontal and vertical scales.
Figure 5.4: Interpretation of migrated seismic profile from the southwestern edge of the Bishri block in the northeastern Palmyrides (seismic profile ALAN-90-10). Surface is Thematic Mapper (TM) imagery draped over topography. See Figure 5.2 for location. See annotation for horizontal and vertical scales, depths are below sea level.
Figure 5.5: Block model of the Abd el Aziz uplift in northeast Syria. Surface is Thematic Mapper (TM) imagery draped over topography. Seismic lines UN-350 (dip line) and SY-48N are shown. See Figure 5.2 for location. View looking towards the southwest. See annotation for horizontal and vertical scales, depths are below sea level.
Figure 5.6: Block model for Euphrates Graben, location shown in Figure 5.2. Surface is Thematic Mapper (TM) imagery draped over topography. Seismic profiles are PS-11 (dip line) and PS-11. View looking towards the southwest. See annotation for horizontal and vertical scales, depths are below sea level.
Figure 5.7: Block model for Coastal Ranges / Ghab Basin along the Dead Sea Fault System in western Syria. Along-basin profile is GA-6 and cross-basin profile is GA-3. Surface is Thematic Mapper (TM) imagery draped over topography. See Figure 5.2 for location. View looking towards the southwest. See annotation for horizontal and vertical scales.
Figure 5.8: Generalized lithostratigraphic chart for all Syria based on extensive surface observations and drilling records. Time intervals are not drawn to scale. Red dots and numbers correspond to time points on Plate 2. Note the alternative formation names for the Lower Mesozoic section in the Euphrates Graben System (Mulussa A, B, C etc.). See text for full discussion.
Figure 5.9: Isopach maps of Syria showing the present thickness of the four major Mesozoic and Cenozoic sedimentary packages, as derived from well and seismic data. Contour interval is 250 m in each frame. See text for discussion.
Figure 5.10: 3-D fence diagram generalizing the current sedimentary thickness variations in Syria. The view is from the northwest with illumination from the north. The name of the well used in the correlation and its total depth are shown at the top of each data point. Vertical and horizontal scales change with perspective.
Figure 5.11: Maps of Syria showing depth, structure, and stratigraphy of various subsurface geologic horizons derived from seismic and well data. Colors in each map represent best estimates of depths to chosen horizon, black contours indicate extents of uppermost subcropping formation of the chosen horizon, and faults and folds are marked in red. Surface geology modified from Ponikarov (1966). Surfaces shown are (a) top Cretaceous, (b) top Lower Cretaceous, (c) top Triassic, (d) top Paleozoic. In (a) only two different formations subcrop, except in exposed areas. Therefore, a stippled pattern is used to show where the Soukhne formation subcrops, and the Shiranish formation subcrops in all other areas. There is only one Lower Cretaceous formation, therefore in (b) the stippled pattern indicates an absence of the Lower Cretaceous. The Lower Cretaceous is present in all other areas.
Figure 5.12: Perspective views of the four structural surfaces shown in Figure 5.11. (a) View from the southeast with ten times vertical exaggeration to illustrate some of the through-going structural relationships. (b) View from the north.
Figure 5.13: Map of Bouguer gravity field of Syria (BEICIP, 1975) shaded with topography imagery. Black numbers indicate depth to top of metamorphic basement determined from seismic refraction profile (black lines) interpretations. White numbers indicate approximate depth to Moho near seismograph stations (white triangles). Red lines are gravity profiles shown in Figure 5.14. All depths are in kilometers below sea level. Note the abrupt along strike variation in gravity anomalies in the Palmyrides coincident with topographic change. Note also the contrast between Bouguer anomalies north and south of the Palmyrides. See text for full discussion.
Figure 5.14: Gravity models through central Syria, see Figure 5.13 for profile locations. Densities in g/cm3 are given parenthetically. Constraints on the model - other than through gravity modeling - are shown in white. (a) Profile across Aleppo Plateau, southwest Palmyrides, and Rutbah uplift. The modeled anomaly is shown both with and without two otherwise unconstrained intrusive bodies in the Palmyrides that can be used to map the second-order gravity anomalies. (b) Profile sub-parallel to profile (a), but across the Bilas block, a significant crustal root is not indicated by gravity modeling.
Figure 5.15: Chronological chart showing development of most significant stratigraphic and structural elements in selected hydrocarbon provinces. Proven elements are shown as solid colored lines, uncertain elements as dashed lines. A generalized distribution of the relevant stratigraphy is also shown. Tectonic events generalized from Plate 2. Formations names vary in Turkey. Patterns indicate lithologies, same legend as Figure 5.8. Red dots refer to time-points detailed in Plate 2.
Plate 1: Tectonic map of Syria representing the current significant structural elements in the country. Surface geology is modified from Ponikarov (1966), modified using the volcanic aging results of Devyatkin et al. (1997), and Lebanese geology from Dubertret (1955), and is shown shaded with topographic imagery. Surface mapped tectonic elements modified from Ponikarov (1966) and Dubertret (1955), in addition to our own mapping, are shown in black. Tectonic elements that are only identified in the subsurface are shown in red. See legend for additional information and see Chapter 5 for complete discussion.
Plate 2: Syrian tectonic evolution model showing regional plate reconstructions (left), timelines of significant regional and local tectonic events (center), and Syrian tectonic evolution (right). Note that the plate reconstructions (after Stampfli, 2000) are simplified and are shown for orientation only. In each plate reconstruction frame, north is approximately upward, and present Arabia is highlighted, however each frame is not to scale relative to the others. For the Syrian tectonic frames, no palinspastic reconstruction is attempted; the tectonics are shown in the correct position for the time of emplacement. Modern-day geography fixed on central and eastern Syria is shown for reference. Facies distributions, water depths, and tectonic elements in Syrian frames are generalized. See Chapter 5 for full discussion.
