Several anticlines in northern Iraq and Syria have been studied through the construction of balanced and restored cross sections. Based upon structural analysis, each of the studied anticlines is a fault-propagation fold that developed due to Zagros-related, recent inversion of much older normal faults. Studies on the Iranian part of the Zagros Fold Belt have suggested that the regional variation in the character of the fold belt is related to weak detachment surfaces in the stratigraphic section, primarily the decollement developed near the top of the Hormuz Salt where the salt is present. No evidence for Hormuz Salt has been found within the Kirkuk Embayment, and although detachment surfaces contribute the area’s structural character, the prominent folds seem to originate mainly from basement involved faults.
Two distinct inversion structural trends exist: E-W system and a NW system of inverted grabens. In Syria, several of the faults associated with the EW-trending system cut the basement on seismic data and have stratigraphic relationships indicating that their displacement originated in the Neoproterozoic. In Iraq, the thicker sedimentary section did not allow the deep parts of the fault systems to be imaged on the available seismic. While the NW fault system of inverted normal faults could be linked to the Zagros Orogen by a decollement surface in the sedimentary section, regional relationships and potential-field data suggest that this trend also is basement involved and has a Neoproterozoic origin.
The Zagros Fold Belt extends more than 1,800 km from southern Anatolia through northern Iraq and Iran to the Strait of Hormuz (Figure 1). The fold belt has been studied extensively since 1855 when Loftus presented one of the earliest technical papers on the region to the Geological Society (London). The majority of published articles regarding the Zagros are in international journals and focus on the Iranian part of the orogenic belt, especially those published within the last few years (Bahroudi and Talbot, 2003; Blanc et al., 2003; Bosold et al., 2005; Sherkati et al., 2005; Authemayou et al., 2006; Hessami et al., 2006; Alavi, 2007; Stephenson et al., 2007).
A review of previous works indicated that the genesis of the anticlinal structures of northern Iraq has long been a subject of debate. Conclusions of early investigators were strongly influenced by the part of the orogen they studied as indicated by the discussion following Henson’s 1951 paper presented at the Third World Petroleum Congress. He recognized four structural trends based on topography and tectonic maps: (1) N-S East African, (2) E-W Tethyan, (3) NW Erythrean and (4) NE Aualitic. Henson found no direct evidence that the features along these structural trends had any common genetic relationships, but noted several episodes of faulting along each of the four structural trends. From this observation, he suggested that there was recurrent movement on basement fracture systems aligned along these trends.
Lees (1952) viewed: “the Iraq-Persian mountain belt as the product of extreme compression with the development of thrust sheets.” He dismissed the emphasis Henson placed on block faulting, and his opinion was that Henson’s 1951 interpretation was “quite misleading”.
Accumulation of geological data since the early 1950s has only enhanced the importance of these four structural trends (Figures 1a, b). Structures of Henson’s Tethyan Trend and the Aualitic Trend are now commonly referred to the Taurus and Palmyra trends, respectively. Subsequent authors have provided evidence for both lines of argument, again depending upon the part of the orogen investigated. Those workers describing structures in the northern and northeastern part of the mountain belt have seismic and geologic data to illustrate thrust faults with flat and ramp geometry (Blanc et al., 2003; McQuarrie, 2004; Bosold et al., 2005; etc.).
Studies of the fault trends in the southern part of the Arabian Plate (e.g. Moore, 1979; Berberian and King, 1981; Stoeser and Camp, 1985; Alsharhan and Nairn, 1997; Al-Husseini, 2000; Hessami et al., 2001) have demonstrated that these trends are repeatedly reactivated basement structures related to the Proterozoic accretion, which assembled the Arabian Plate. In northern Iraq, investigations based primarily on satellite imagery and potential field data (e.g. Buday, 1980; Ameen, 1991; Jassim and Goff, 2006) have been used to propose similar tectonic histories and origins of structural styles generally involving basement blocks bounded by strike-slip faults resulting from transpression. Where individual structures in northern Syria and northern Iraq have been studied in detail (Kent and Hickman, 1997; Brew et al., 1999; Marouf and Al-Kubaisi, 2005), the structures have been interpreted as fault propagation folds originating from reactivated normal faults. Kent and Hickman (1997) extrapolated their detailed study of Jabal Abd Al Aziz using surface geology obtained from field mapping and published geologic maps to propose that the surface anticlines of southern Anatolia, northern Syria and the adjacent region in northern Iraq belonged to an inversion terrain. However, we did not attempt to distinguish where the surface structures related to inversion of deep-seated normal faults transitioned into structures related to flat-and-ramps in sub-horizontal faults might occur.
If, as McQuarrie (2004) proposes, the large-scale structure of the Zagros Fold Belt is controlled by the presence or absence of the Hormuz Salt and a weak decollement zone, then knowledge of the depth and character of the detachment zones for the region’s fault systems is key to understanding the regional geology. The work by Carter and Gillcrist (1994) is particularly noteworthy in the literature of the northern Arabian Plate because they demonstrate that the well-documented Proterozoic – Paleozoic outcrops in the Derik-Mardin area are related to Late Cetaceous and Miocene inversion of a deep-seated normal fault with an E-W orientation.
The present study provides subsurface data and interpretations of several anticlines in northern Iraq and suggests that the hydrocarbon-producing anticlinal structures are recent secondary features related to faults systems of greater antiquity and geologic significance.
DATA AND METHODOLOGY
The author was in Iraq with the U.S. Department of State, Iraq Reconstruction Management Office from mid-2006 until mid-2008. During the spring and summer of 2007, he presented a seminar on techniques for interpreting geologic structures to employees of the Ministry of Oil, Iraq. A goal of the seminar was to include a workshop in which techniques learned in the seminar would be applied to structures within the country. The Bashiqa and Mansuriya anticlines were selected as candidates for workshop projects. Initial examination of the data and models derived with pencil and paper demonstrated that the initial assumptions of fault-bend and fault-propagation fold geometries (e.g. Suppe, 1985) of typical foreland basins are not applicable to most of the anticlinal structures. An extensive literature study of the Zagros Fold Belt was undertaken to obtain the area’s structural context and to develop a more suitable method of analysis.
The work presented here is based on limited well and 2-D seismic data for only a few of the structures of northern Syria and Iraq and minor amounts of unpublished Iraqi Petroleum Company (IPC) data. These are the data available to the author for public display and represent only a part of a larger dataset available for this study. The dataset that can be made public is insufficient for a comprehensive investigation of the structures of Iraq or the northern Arabian Plate and limits the conclusions that can be made from the evidence provided. Thus, this work is intended to be only a general overview of the large-scale fabric of the region. However, in an attempt to address the shortcomings of the data that can be displayed, the author has relied on data from the numerous published reports on the area, including the recent publication of studies of the Zagros Fold Belt in Iran (Sherkati et al., 2005; Alavi, 2007, etc.).
To insure the proper geographic relationship of features discussed herein, figures from the references cited were georeferenced in WGS 1984 geographic coordinate system and included in a digital mapping project. The project’s dataset includes well and seismic locations, topography, geology, and geographic and cultural features in addition to the imported figure elements. The maps in figures within this paper were derived from the resulting integrated dataset that allows stratigraphic data and structural elements from a variety of sources to be discussed and compared within a single spatial reference.
REGIONAL STRUCTURAL SETTING
Most of the recent descriptions of the Zagros Orogeny use some modification of the structural zones proposed by Falcon (1969) which include: (1) the Arabian Platform, (2) a broad simply folded belt, (3) a narrow imbricated belt, and (4) a thrust belt. These zones are identified on the basis of abrupt geomorphic changes often corresponding to major surface faults (Figure 1b).
The Zagros Fold Belt is separated from the Iranian Plateau by a structural zone composed of the Main Recent Fault system and the Main Zagros Reverse Fault system. Partitioned strain along these fault systems accommodates the oblique plate convergence at this suture zone (Hessamie et al., 2001; Bahroudi and Talbot, 2003; Authemayou et al., 2006; Sarkarinejad and Azizi, 2008). Lateral movement on the Main Recent Fault is dissipated within the Zagros Fold Belt along N-S dextral faults such as the Khanaqin, Hendijan, and Kazerun faults. These faults segment both the Imbricate Belt and the Simply Folded Belt (Figure 1b).
Detailed studies of the Imbricate Zone in Iran indicate that it is a complex system of NW-striking oblique-slip thrusts with typical ramp-flat geometry (Bosold et al., 2005; Sarkarinejad and Azizi, 2008). Seismic data show that the High Zagros Fault, which separates this zone from the Simply Folded Zone, is a low-angle thrust fault (Boslod et al., 2005).
Simply Folded Zone
The Simply Folded Zone lies between the Zagros Deformational Front and the High Zagros Fault. The Simply Folded Zone generally increases in width along strike from the northwest toward the southeast. The zone is segmented into regions of differing topographic expression. The trends of regions of elevation change correlate, in most cases, with N-S dextral faults. Ethno-geographic names proposed by Oberlander (1965) are commonly used to designate these subdivisions. From northwest to the southwest, the subzones names are the Taurus Province, Kirkuk (Kurdistan) Embayment, Lurestan (Pusht-e-Kut) Province, Dezful Embayment, and Fars Province. Variation in elevation between provinces has been attributed to the relative weakness of the basal decollement and the presence or absence of the Hormuz Salt (McQuarrie, 2004).
The N-S fault trend, to which the faults that segment the Simply Folded Zone belong, has a well-documented history of recurrent movement. As discussed below, deposition of the Hormuz Salt occurred in N-S trending grabens (Stnöcklin, 1968; Talbot and Alavi, 1996), which formed following the N-S fabric established by the Amar Collision (Al-Husseini, 2000). Faults bounding the Hormuz grabens were reactivated in the mid-Carboniferous transpressional event, the Permian – Triassic and Late Cretaceous (Wender et al. 1998; Al-Husseini, 2000, 2004). Growth interpreted from surface geomorphology indicates that renewed uplift on the structures related to this fault trend is either ongoing or recently ceased (Adasani, 1967).
Within the Simply Folded Zone, authors have described nearly all fold types common in sedimentary rocks. The discussion follows the early debate between basement-involved folds (Henson, 1951) and detachment folds (Lees 1952) originating from faults within the sedimentary section. In the Kirkuk Embayment, Ameen (1992) interpreted folds as basement-involved, forced-folds and inversion structures. Colman-Sadd (1976) interpreted the folds of the Simply Folded Zone to be parallel folds formed by buckling of competent units between an upper and lower detachment surface. In the Lurestan Salient, Alavi (2007) interprets the folds to be dominantly fault-propagation folds or fault-bend folds. Sherkati et al. (2005) document multiple decollement zones and the detachment folding. Recent papers on the structure of the central Zagros (Blanc et al., 2003; Shertaki and Letouzey, 2004) illustrate cross sections in both the Lurestan Salient and the Dezful Embayment with both decollement horizons and basement involvement. These authors suggest that the basement faults of the Proterozoic Najd system may have been reactivated later (Proterozoic to Permian – Triassic) as extensional faults and further reactivated as thrusts during late Cenozoic compression.
The thickness of the stratigraphic column in the Simply Folded Zone ranges from a minimum of 1–2 km in the northwest to estimates of more than 12 km (Dunnington, 1958; James and Wynd, 1965; Falcon, 1974; Koop and Stoneley, 1982; Jassim and Goff, 2006). The sedimentary sequence includes rocks from Neoproterozoic to Quaternary age. For convenience and simplicity in relating the regional stratigraphic relationships to the structural geology, the stratigraphic column is generally devided into five mechanical units as proposed by O’Brien (1950; Colman-Sadd, 1978; Sattarzadeh et al., 2000; Sherkati and Letouzey, 2004; see Figure 2): (1) Basement Group, (2) Lower Mobile Group (Ediacaran – Cambrian Hormuz Salt or its equivalent), (3) Competent Group (composed of two sub-groups, Paleozoic clastic rocks and Mesozoic – Paleogene carbonate and evaporite rocks), (4) Upper Mobile Group (the Miocene Gachsaran [Lower Fars] Formation) and (5) Incompetent Group (Miocene and younger rocks). Two regional detachments occur in the upper and lower mobile groups defining these units (Figure 2). Other bedding-plane detachments may occur locally within the Competent Group, primarily in areas where thick evaporite sequences are developed.
While these divisions are sufficient for discussion of regional tectonic relationships, they obscure the complex stratigraphic details observed in northern Iraq. For detailed understanding of individual structures, a significantly refined stratigraphic understanding is necessary. The distinctive character of each provinces of the Simply Folded Zone derives from the variation in the character of these structural units, particularly the Lower Mobile Group and the Competent Group. While detachment surfaces define the Upper and Lower Mobil groups, decollements also occur in various parts of the Competent Group (Blanc et al., 2003; Sherkati and Letouzey, 2004; Bosold et al., 2005).
The structures described below are generally within the region of relatively simpler stratigraphy of the Arabian shelf. The larger part of the stratigraphic column is composed of sedimentary wedges that predates the Tertiary – Quaternary Zagros Orogeny and contains several passive-margin, rift and drift sequences (Koop and Stoneley, 1982).
Lower Mobile Group
The Hormuz deposition occurred in N-S trending grabens in the eastern Arabian Plate. The grabens have abrupt NE and NW boundaries. A halite facies in the south transitions to the dolomite Soltanieh facies northward (Stnöcklin, 1968; Talbot and Alavi, 1996). Combining elements of previous interpretations; (1) distribution of Hormuz evaporite facies (Talbot and Alavi, 1996), (2) Late Precambrian – Early Cambrian paleogeography of Iraq (Jassim and Goff, 2006), (3) faults at the Upper Carboniferous subcrop (Al-Husseini, 2004), and (4) the Amar Arc (Al-Husseini, 2000) yields a possible shape of the Ediacaran – Cambrian Hormuz Salt basins (Figure 3). In this view, the Hormuz Salt basins are a complex rift system (Husseini and Husseini, 1990) open to the northeast in which salt deposition is segmented into three sub-basins by the Qatar Arch and the Amar Arc.
Although not well constrained on the west and southern boundaries, the map shows the generalized Early Cambrian basin geometry. This basin geometry was inherited by the Paleozoic and Mesozoic sedimentary units in both the Mesopotamian and the Widyan basins. The Ediacaran and Cambrian sediment distribution in the Hormuz Salt basin was influenced by three of the regional trends identified by Henson (1951), the N-S, NW and the NE trends. Extension on the E-W South Mardin Fault System occurred during deposition of the Ediacaran – Cambrian sediments on the flank of the Mardin-Urfa High (Rigo de Righi and Cortesini, 1964; Carter and Tumbridge, 1992), but this area was almost certainly outside of the region of salt deposition.
Uncertainty exists in the lithology, thickness and distribution of Ediacaran – Cambrian stratigraphic units. What is known is derived from widely distributed, limited outcrops in structurally complex regions, from deciphering relationships in extruded salt masses and from geophysical data with unequal and incomplete distribution. The original thickness and distribution of the Hormuz Salt may be indeterminate because of salt mobilization and dissolution even if the structural control on deposition is firmly established.
Despite this uncertainty, a general regional facies distribution for the Neoproterozoic – Cambrian system has been developed. In the Proterozoic – Cambrian Lower Mobile Group, salt occurs in sufficient thickness to form salt diapirs south of the Kaserun Fault System (Talbot and Alavi, 1996). North of this boundary fault, between the Fars Province and the Dezful Embayment, salt crops out only in fault zones indicating a major thinning across the Kaserun lineament (Sattarazdeh et al., 2002).
Toward the northeast, the Hormuz Formation changes to more normal-marine facies consisting of the dolomites and shales (Stnöcklin, 1968; Talbot and Alavi, 1996). The northwestward facies change continues to southern Turkey where at the Zap Anticline (Figure 4) 3,000 m of fluvial-deltaic to fluvial sandstones and shales crop out. The base of the unit is not exposed and the only fossil described to date is a single fragment of Archaecyathus. An Ediacaran to Early Cambrian age is assigned to the unit based on its position below the overlying Lower – Middle Cambrian Koruk Dolomite (Dean, 1980; Janvier et al., 1984). These rocks are assigned to the Zabuk Formation by Dean (2006), who suggested a more limited age of Early Cambrian.
Farther west, on the Mardin-Urfa High near the town of Mardin, ca. 2,000 m of felsic porphyry and clastic rocks are overlain by a thinner sequence (300–500 m) of fluvial conglomerates, sandstones and red beds. These units are overlain in turn by more than 1,950 m of sandstone, siltstone and shale (Rigo de Righi and Cortesini, 1964; Ketin, 1966). Positive ages for these units are lacking, but the lower unit of volcanic rocks and clastic beds, the Derik Formation, has been tentatively assigned to the Neoproterozoic (Dean, 2006). The second unit of fluvial quartzite, the Sadan Formation, may be Neoproterozoic to Cambrian age, while the third unit, the Zabuk Formation, is generally considered Early Cambrian (Rigo de Righi and Cortesini, 1964; Carter and Tunbridge, 1992; Dean, 2006). Following previous workers, these units are approximately time equivalent to the Ediacaran to Early Cambrian Hormuz Formation (Talbot and Alavi, 1996). These data suggest that the Hormuz Salt basin was a silled basin open to normal marine waters or an intra-cratonic basin as proposed by Al-Husseini (2000).
McQuarrie (2004) suggests that the large-scale features of the Zagros Fold Belt are controlled by lateral ramps influenced by the presence or absence of Hormuz Salt along the strike of the fold-thrust belt. This interpretation suggests that a weak decollement provided by Hormuz Salt exists in the Fars and Lurestan regions, but not in the Dezful and Kirkuk embayments (Figure 1b). The presence of a weak Lower Cambrian salt detachment under the Fars and Lurestan provinces allowed deformation to propagate farther in these regions. However, the western and northern extent of the Hormuz Salt is not sufficiently constrained to test this hypothesis. Furthermore, the Jabal Sanam Salt Dome (Figure 3) in southern Iraq (Al-Naqib, 1970) indicates the presence of Hormuz Salt in southern Iraq and possibly into the Lurestan region. The Ediacaran – Early Cambrian correlation noted above provides no evidence for Ediacaran – Cambrian salt units in the Kirkuk Embayment.
The Paleozoic to Mesozoic age Competent Group does not behave as a single structural unit along strike of the orogen. Decollement zones in the Competent Group exist in rocks of Triassic, Jurassic, mid and late Cretaceous, and Paleocene age (Blanc et al., 2003; Sherkati and Letouzey, 2004; Bosold et al., 2005; Sherkati et al., 2005; Alavi, 2007). The existence of several possible decollement zones within the Competent Group and their effect on the mechanical behavior of that group provides for many of the local variations in structural style.
Regionally the Competent Group can be divided into two sub-units. The lower unit includes Cambrian to Carboniferous sediments composed mostly of quartzite, sandstone, and shale with rare carbonate units. The upper subunit includes Permian to Miocene age rocks that are mostly carbonates, evaporites and shale.
In northern Iraq, the lower competent unit can be subdivided into the Proterozoic rift unit and the Lower Paleozoic shelf deposits. The character and distribution of the Proterozoic rocks in not known outside of where they crop out in southern Anatolia. The Cambrian shelf units thin and gradually become finer grained toward the north and northeast, whereas the Ordovician and Silurian rocks thin dramatically toward the northeast from the Khleisia area to the outcrops near of the northern Iraqi border.
The upper competent unit also has two parts, but tectonic changes in the depositional basin that occur in the Cretaceous produces a less defined boundary between the two parts. The lower part consists of shallow-marine units that thicken in the increased accommodation space provided at extensional faults surrounding regional structural highs (Figure 5). This pattern of cyclic sedimentation persisted in the south and west from the Permian to the Cretaceous. The group of carbonate, evaporite and shale formations contains more clastic units at its base and in its southern periphery. However, all the lower and middle Cretaceous of northeastern Iraq is represented by a single unit, the Balambo Formation. The formation is a thick sequence of deep-water sediments containing globigerinal or radiolarian microfossils (Dunnington et al., 1959).
The uppermost part of the upper Competent Group marks the beginning of the Zagros Orogeny. The character of the group changes rapidly in both the temporal and spatial dimensions. In the Campanian, rudist reef limestones surrounded low regional highs with intervening oligosteginal (calcisphere-bearing) marls. Extensional faulting during the Late Campanian and Early Maastrichtian opened grabens into which the sequence of olistostromes to marls of the Shiranish Formation was deposited (Weber, 1964; Hart and Hay, 1974; Kent and Hickman, 1997; Marouf and Al-Kubaisi, 2005). In the west, subsidence in the major rift basins continued into the Miocene. The developing Zagros Foreland Basin was filled from the northeast starting in the Miocene with the Tanjero Clastic Formation. The formation grades vertically from globigerinal marls at its base to silty marls, siltstones, sandstones and conglomerates at its top (Dunnington et al., 1959). A similar pattern of basin fill continued through the Early Miocene with clastic units deposited in the northeast, limestones, and marls in the southwest (van Bellen et al., 1959-2005).
Upper Mobile Group
The Upper Mobile Group consists of the Dhiban, Jeribe, and Lower Fars (Gachsaran) formations that contain variable thickness of anhydrite, salt, limestone, shale and sandstone. The mobility of the unit is a function of the amount of salt and anhydrite in a particular area. The unit is responsible for the disharmony between surface structures and subsurface structures in the Simply Folded Belt, and often discussed “tectonic problems” of the Zagros Fold Belt.
The Incompetent Group includes the Upper Miocene to Recent Upper Fars, Lower and Upper Bakhtiari formations and alluvial units. These sediments are marls, shales, sandstones and conglomerates derived from the Zagros Orogeny.
The Kirkuk Embayment includes the surface structures of the Simply Folded Zone in northern Iraq and is commonly included as a subdivision of the Zagros Orogenic Belt (Beydoun et al., 1992; Sattarzadeh et al., 2002; Bahroudi and Talbot, 2003; Authemayou et al., 2006; Alavi, 2007). However, the large-scale surface structures, which include the hydrocarbon-producing structures of the Kirkuk Embayment are not typical foothill structures developed from decollements within the foreland basin sediments. Although the folds modify and deform sediments derived from the Zagros highlands and deposited into the developing foreland basin, the faults from which they originate are wholly developed within the pre-Zagros stratigraphy. An isopach (Figure 5) of the Upper Miocene to Pliocene Upper Fars and Lower Bakhtiari formations by Dunnington (1958) shows a synorogenic depocenter near the Iraq-Iran border. This map reportedly omits local thinning over crests of rising folds. Notable is the isopach thinning over the Aziz-Bashiqa Trend and the thin veneer of these orogenic units over most of northern Iraq.
The limits of the Kirkuk Embayment are defined by changes in topography and the correlated structural changes. The northeastern boundary is the Mountain Front Flexure and associated imbricated thrusts. The higher elevation of the folds toward the southeast into the Lurestan Salient may be the result of a lateral ramp at the Khanaqin lineament (McQuarrie, 2004). The topographic and structural changes to the northwest along strike near the borders with Syria and Turkey will be shown to reflect the interaction between the Mardin-Urfa basement high and the Mountain Front faults. The division between the Arabian Platform and the Simply Folded Belt is usually placed at the topographic break south of the Makhul-Hemrin structural trend (Figures 1 and 4).
While most workers (Figure 1) label the Makhul-Hemrin structural trend as the “Zagros Deformation Front”, seismic data indicate several similar structural trends further to the southwest in the Mesopotamian Basin subsurface. Notable is the Tikrit-Amara Trend, which includes a number of anticlines accompanied by normal faults (Buday and Jassim, 1980). Addition of the buried structures to the map would suggest that the “Zagros Deformation Front” is a more diffuse than abrupt boundary.
The structures on the Tikrit-Amara Trend are traps for several oil fields including Tikrit, Balad, East Baghdad, Ahdab and Dujaila fields. Because of the economic importance of the hydrocarbon traps and their more recent discovery than the oil fields of the surfaces anticlines to the northeast, the structural trend has been studied with more advanced subsurface techniques. These studies have concluded that the trend is composed of horsts and grabens related to basement uplifts that are dissected by strike-slip faults (Buday and Jassim, 1980; Aljawadi, 1990), positive flower-structures developed along strike-slip faults (Ibrahim, 1998), or a more complex “super-position” of structures (Schafer and George, 2008). While the importance of strike-slip faulting may vary from one study of this trend to the next, there is agreement that faults are the primary structures and that they have had a complex displacement history that may involve crystalline basement.
The western boundary of the Kirkuk Embayment is complicated by the intersection of the Zagros Fold Belt and the Mardin-Urfa High (Figure 1). Maps made by Temple and Perry (1962) show all Permian and most Triassic units absent over the high as well as Neoproterozoic units subcropping at the base of Cretaceous unconformity at its western end. Well log data in northern Syria indicates that the Mardin-Urfa High uplifted episodically, as shown in a cross section from the Markada-101 Well in northern Syria to the Ackakale-1 Well in Turkey near the Syrian-Turkish border (Figure 6, see Enclosure). Seismic data indicates thinning of upper Paleozoic and Mesozoic strata onto the feature (Figure 7). Although much of the stratigraphy is missing at erosional unconformities, depositional thinning is apparent in units with dramatic changes in thickness occurring in the upper Paleozoic and Mesozoic stratigraphic units stepwise at NE-trending faults. The seismic data also shows Upper Cambrian and Ordovician units thickening on the northern side of faults that terminate at the base of Silurian marker. Faults with thicker Lower Paleozoic strata on the down-thrown northern side is similar to the faulting described by Rigo de Righi and Cortesini (1964) and Carter and Tumbridge (1992) as occurring during deposition of the Neoproterozoic and Cambrian on the flank of the Mardin-Urfa High.
A structural cross section drawn from the Khleisia-1 Well in Iraq to outcrops in the Zap Valley of Southern Anatolia illustrates a similar increase in the number and thickness of Mesozoic and Paleozoic units in a stepwise manner from the Khleisia High toward the northeast across NW-trending faults (Figure 8, see Enclosure). In this figure, the down lap and erosional thinning of the Silurian and Ordovician stratigraphy reduces the relief on the projected top of Proterozoic basement for the Khleisia High when compared to the Mardin High.
At the junction of the Iraq-Syria-Turkey border, definition of the Simply Folded Zone becomes difficult. Faults on the north and northeastern flanks of the basement high bring rocks as old as Neoproterozoic to outcrop (Figure 1). The Mardin-Urfa and Khleisia highs are separated by the Palmyra-Sinjar Trough, which has been active since Late Devonian (Figure 6). Either the zone is constricted between the Mardin-Urfa High and the Zagros Mountains front fault, or the definition must include the inverted structural zones of central Syria (Figure 1c).
STRUCTURES OF THE SIMPLY FOLDED ZONE
Structures of the Simply Folded Zone in northern Iraq fall into two distinguishable groups, those that trend NW and those that trend E-W. NW-trending structures occur generally in the south while E-W-trending structures occur in the north. A study of six structures within the Simply Folded Zone was undertaken to determine their origin and their regional context.
In one of the most recent detailed studies of seismic and well data in northern Iraq, Marouf and Al-Kubaisi (2005) presented interpreted seismic over the Kirkuk and Mansuriya structures, two of the structures discussed below. Their important study draws similar general conclusions to the work described below that the major anticlines are fault-propagation folds formed in response to faults that had normal displacement during deposition of the Maastrichtian and Middle Miocene and reverse displacement in the Pliocene. Differences in details of the Marouf and Al-Kubaisi (2005) study and the interpretations illustrated below occur because their seismic interpretations are done only in the time domain, without an attempt to verify that the interpretations are viable by restoring their interpretations.
In this study, balanced and restored cross sections based on interpretations of similar structures within the region are the basis for deriving the conclusions. Because the interpretations are based on degraded paper seismic profiles displaying data that may not have had optimum processing for structural interpretation, the interpreted structures are not unique. Methods for balancing cross sections and restoring cross sections do not “prove” an interpretation. The methods do indicate that the interpretation is physically possible. Choosing structural styles that have been demonstrated within the region to guide an interpretation, likewise does not insure a “correct” interpretation, but provides the interpretation with plausibility.
The Makhul-Hemrin structural trend is distinctive among the anticlinal trends of northern Iraq in that it is also a linear positive feature in the Bouguer gravity (Jassim and Goff, 2006). Jassim and Goff (2006, p. 51) describe the Makhul-Hemrin Trend as “the longest anticlinal chains in the Middle East”. They speculate that the trend marks a fault zone, which is the boundary between the East Arabian and Zagros Proterozoic terranes and associate its origin with development of the Neoproterozoic Najd Fault System. The Makhul-Hemrin Trend is generally designated the “Zagros Deformational Front”. Lovelock (1984) interpreted the Makhul-Hemrin as an important transcurrent fault zone. Two structures on this trend, the Makhul and Mansuriya anticlines, were studied.
The only subsurface data available to the author for the Makhul Structure were the two runs of a gamma ray-neutron log for the Makhul-2 Well drilled in 1955. This log is in two segments that cover only part of the drilled section (Figure 9a). Although a schematic cross section by R.M. Ramsden (Dunnington, 1960) shows the well penetrated overturned lower Jurassic section (Figure 9b), the official formation tops recorded for the well indicate only a repeated Lower Jurassic section (IOEC, unpublished data).
Interpretation of this log reveals a set of marker beds A, B, and C appear to be the same unit, i.e. unit C, inverted and repeated (Figure 10). By reversing and inverting a copy of the log and correlating the copy to the original, identification of a section of the lower Alan, Mus and upper Adaiyah formations which occurs, from bottom to top, in normal, inverted and then repeated sequence can be made. The interpretation also identifies two fault surfaces and a stratigraphic marker in the Alan. The separation of a ductile upper unit from a basal competent zone occurs at a point marked (“Top”) a few meters above the Alan stratigraphic marker, “M”. The base of the repeated section identifies stratigraphic location of the interpreted footwall flat.
The thicknesses of the hanging-wall, overturned section and the footwall section are 166, 168 and 170 m respectively. The variability of measurement occurs because, although there is good correlation between units, there is not a precise peak-to-peak correspondence. The relatively uniform thickness between the units implies that the difference in the angle of dip for the units should not exceed ca. 20°. If the footwall beds are essentially horizontal, then the limbs of the fold must also be nearly horizontal.
This information was used to construct a diagrammatic cross section illustrating the recumbent fold within the core of the Makhul Anticline (Figure 11). The interpreted cross section presents an extreme fold over a rather apparently simple fault that originates in a detachment in the Jurassic Adaiyah Formation. The fault ramp angle is undefined by the log data, and although the diagrammatic cross section honors the well data, the unit areas do not balance. To test conceptual viability, a balanced cross section was constructed (Figure 12). The balanced cross section requires ductile deformation in the upper Alan and Sargelu formations. This interpretation also requires an upper detachment near the Gotnia-Sargelu contact.
The absence of data in the Sargelu-upper Alan section of the well supports the interpretation of a ductile zone. The limited drilling history derived from the log headers indicates drilling and logging problems were encountered through the Sargelu and upper Alan formations. Drilling in the only
other deep well on this structure, Makhul-1, reached total depth near the top of the Upper Jurassic Gotina. Reported and apparent drilling, logging, and casing difficulties in the Gotnia-upper Alan interval on this structure suggest that the Makhul-2 Well encountered mechanically-unstable and deformed rocks, as this part of the stratigraphic column is represented in logs of wells on nearby structures. The proposed cross section satisfies all available data and is remarkably similar to Ramsden’s interpretation made relatively soon after the well was drilled (Dunnington, 1960) despite the author’s lack of dip data which was available to Ramsden.
Gravity and magnetic data suggest that the Makhul-Hemrin Trend is related to basement-related structure and/or composition (Sayyab and Valek, 1968; Jassim and Goff, 2006). However, the simple structure modeled derived for Jabal Makhul is not compatible with a through-going, large-displacement strike-slip fault; although, the general character of the Makhul-Mansuriya Trend does fit the description of the Najd Fault System at outcrop in Saudi Arabia (Moore, 1979). Both trends are composed of individual faults segments of approximately 50 km or less and not a through-going master structure. The topographic expression of the Makhul-Mansuriya Trend is compatible with reverse displacement and indicates little or no evidence of recent lateral motion. Vertical displacement, either positive or negative, occurs at steps, branches or bends along strike-slip faults because of concentration of strain at these locations (e.g. Davis and Reynolds, 1996). The Makhul, Hemrin and Mansuriya anticlines occur along the straight segments of the fault system, and no structures occur at the fault bends, branches or intersections. This is particularly well illustrated when topographic data and the geologic map of the southern area of Kirkuk Liwa (Al-Naqib, 1960) are combined (Figure 13). On this map, the Makhul-Hemrin-Mansuriya Trend is segmented into two systems that intersect at N50°W and N35°W. The fault trends are consistent with faults developed by N47°E oriented extension that predates the compression resulting in folding.
Changes in trend of the anticlinal axes occur at the gaps between many of the area anticlines (e.g. location 1 in Figure 9a). Jassim and Goff (2006) interpret this deflection of the anticlinal axes as the intersection of the anticlinal trends with NE-oriented transverse faults. This offset and narrowing of the fold axis is more likely the result of thrusting along a shallow detachment in the Lower Fars. The phenomenon termed the “frontal pucker” described by Ion et al. (1951) at Agha Jari oil field in Iran is common to most of the folds in the Simply Folded Zone (Figure 14a). As illustrated in Figure 14a, fault-tip puckers occur at the tip of a detachment faults. The geologic map of Qsar-e-Shirin (British Petroleum Company, 1963) (Figure 14b) provides a map view of the “frontal pucker” and an opportunity to compare the “down-plunge” map view with the cross section view of similar structures. This map also shows that the termination of slip at the sides of the decollement creates a “lateral pucker”.
A similar change in orientation of the fold axis occurs along strike north of the Makhul-1 Well (Figure 9a). This is interpreted to be a “lateral pucker” caused by dissipation of the amount of ductile, upper Alan Formation accumulated on the upper flat of the detachment surface.
The Mansuriya Structure is the trap for an undeveloped oil and gas field in shallow reservoirs below the Lower Fars regional seal (Al-Ameri et al., 2008). Although no wells on the structure penetrate formations below the Upper Cretaceous Shiranish Formation, a 24 fold, 2-D seismic dataset exists over the structure. Although the complete dataset was available for inspection and interpretation, only seismic line PIK-6 was provided for publication (Figure 15a). The seismic data were interpreted using paper sections (Figure 15b), converted from time to depth using Geo-Logic Systems’ LithoTect on scanned data (Figure 15c), and restored to a pre-deformation state (Figure 15d).
Unfortunately, the lack of deep well control in either the northeast or southwest of the Mansuriya Structure prohibits correlation of the seismic reflection horizons to formation tops. Thus, only the seismic stacking velocities are available for converting the seismic data from time to depth. The oldest formation top identified by well control is the Shiranish. Uncertainty in stratigraphic correlations is amplified by facies changes that are known to occur in the Mesozoic and Lower Cretaceous units from the Khleisia High toward the northeast. Additionally, the fault extends deeper than the limit of the seismic data.
If the structural complexities of the Makhul Structure occur within the Mansuriya Structure, they would not be resolvable in the seismic data. However, there is relatively good seismic character correlation of units on both flanks of the structure, which indicates a general thickening of most stratigraphic units toward the northeast. In addition, the system of bounding faults of the inverted graben occurs on several of the seismic lines over the structure. The increased thickness of the Shiranish Formation within the bounding faults defines an inverted graben. The deep structure to the northeast (b-B, Figure 15b) and the balancing difficulty of adding area from both additional Shiranish Formation and ductile Jurassic units over a recumbent fold makes the simpler inverted normal fault solution preferable. This interpretation also shows the weakness of focusing only on the single anticline and its main associated fault and of single structure interpretations and restorations in regions of co-evolving structures, especially in areas with relatively low-friction detachment surfaces.
The greater Mansuriya Structure (Figure 15b A-B) together with the Makhul Structure provides several clues to the structure of northern Iraq. The deep structure on PIK-6 offers the possibility that the deep structure north of Mansuriya accommodates the shortening of the Jurassic section at Makhul. This suggestion may indicate that the deep, south-dipping fault (B) and the Mansuriya Fault (A) form a large-scale graben (A-B) that is geometrically self-similar to the smaller-scale grabens (A-a and b-B) of the Mansuriya Structure (Figure 15b). The distance (ca. 30 km) between faults A and B (Figure 15b) is nearly the same distance as the distance between the Syrom Fault and the J-1 Fault that bound the Jabal Abd Al Aziz inversion structure in Syria (Kent and Hickman, 1997).
Existence of large-scale, deep-seated inverted grabens could explain the anticline trends that appear to occur in pairs of high and low amplitude folds (e.g. Kirkuk-Bia Hassan/Jambar, Makhul-Khanuqin or Najma/Jawan-Qasab (Figure 4). An additional possibility is that the structures in northern Iraq are disharmonic, in the truest sense, with the folds arising from multiple detachments that are not physically linked. The implication is that detachments at different stratigraphic levels accommodate shortening of different parts of the stratigraphic section, but do not originate from a master detachment.
The restored depth interpretation (Figures 15c and d) suggests the following event sequence. The Mansuriya Fault has been active at least since the Jurassic or Triassic. An episode of normal displacement occurred during deposition of Lower Cretaceous (?) and/or Jurassic – Triassic (?) units. During deposition of the Upper Cretaceous Shiranish, additional normal movement on the fault developed a graben into which that unit thickened. The fault was reactivated by compression during the Pliocene and reverse displacement on the fault has occurred sporadically to the present-day. Early growth of the Mansuriya Anticline is reflected in the facies and thinning of the overlying Jeribe and Dhiban formations (Al-Hadidy, 2007). Later more rapid growth created an impediment to Bakhtiari sediment transport from the south toward the north, the opposite of the current situation. After deposition of the northward prograding, lower Bakhitiari Group, movement along the Lower Fars detachment formed thrust imbricates over the crest of the structure.
The above interpretation differs from that of Marouf and Al-Kubaisi (2005) in several attributes. The most important difference is that they depict the main fault and its principle antithetic fault as consisting of several secondary faults that form a “reverse fault fan”. The “reverse fault fan” of the main fault originates from a single bend in the main fault. This interpretation may seem reasonable given that reactivation a normal fault usually results in a reverse fault that does not actually follow the same fault trace over a large distance, but follows a lower-angle trajectory. However, work with higher quality seismic data on similar structures in the region, where well control exists that penetrates the entire faulted section (e.g. Kent and Hickman, 1997), indicate the original normal fault and the subsequent reverse fault are nearly coincident and often not resolvable in poorer quality seismic data. In the Mansuriya seismic data, the author could identify only one main fault that could be correlated throughout the dataset.
An additional problem of the interpreted “reverse-fault fan” is that the main bend in the interpreted fault, or the accumulated displacement of the bend, does not create a fault-bend fold/folds as would be expected.
The Kirkuk Anticlinal Trend is another long sinuous structural trend with structural apexes at the straight segments. No seismic data were available for the Kirkuk Trend. However, Marouf and Al-Kubaisi (2005) provide an interpreted seismic line in the Kirkuk area. This line shows an anticline with a fault on each flank dipping toward the center of the structure. The two faults do not intersect within the imaged region. Their interpretation of the seismic data is collaborated by the work done using well control (Gaddo and Hussein, 1967).
An unpublished report on the Cretaceous reservoirs of Baba Dome, Kirkuk Field (Gaddo and Hussein, 1967) provided by the Ministry of Oil supplies the data for the following insights into the Kirkuk Structure. A line cross section in the report is reproduced digitally along with the log data (Gaddo and Hussein, 1967) (Figure 16a). The original line drawing and the computer generated cross section are essentially identical excepting the addition of the well log curves and the lithological marker and formation horizons. The resulting cross section is restored to the uppermost Cretaceous with the top of the Shiranish as horizontal (Figure 16b).
According to Gaddo and Hussein (1967), at Baba Dome, Kirkuk Field, the Upper Maastrichtian and the uppermost part of the Lower Maastrichtian Shiranish is absent below Paleocene deposits. Despite the loss of the formation by soft sediment deformation or slumping (Hart and Hay, 1974), the Shiranish thickens with nearly twice the section below the lithologic marker “M” (Figure 16a) over the structure crest when compared with to the thickness on the flank. Of the nine cross sections along the length of Baba Dome made by Gaddo and Hussein only the three at the northwest end include the graben-bounding faults, and these faults are indicated as conjectural (Figure 16c). However, in their discussion of the structure, they state that “Baba Dome is the result of several orogenic and epeirogenic movements” (Gaddo and Hussein 1967, p. 8), with earlier Cretaceous faulting followed by the main deformation during the Miocene – Pliocene.
In Late Cretaceous, faulting occurred during deposition of the Shiranish and was associated with two sets of perpendicular faults. Additionally, faults are encountered in the Cretaceous interval in several of the wells, and a fault cut exists in Well K-175 that cuts out 82 m (270 ft) of the lower part of the Upper Qamchuqa Formation. Based on the isopach thickness changes of the Shiranish around Well K-175, Gaddo and Hussein (1967) interpret the well to be located in the ‘central’ graben of Baba Dome. A Shiranish-filled graben at the crest of the Baba Dome begs comparison of the structure to the Mansuriya Anticline.
The map (Figure 16c) of Kirkuk Structure by Gaddo and Hussein (1967) shows the secondary fault trend active during deposition of the Shiranish Formation was oriented N-S. Neither the log nor survey data for the section below the top of the Qamchuqa in the K-175 Well are available to confirm their interpreted N-S fault, but clearly, the north graben fault in the original section would not encounter the lower part of the Upper Qamchuqa indicating the necessity of an inferred N-S fault. Their study is one of the few places where N-S faults are demonstrated or inferred in northern Iraq. The occurrence of a few faults on a single structure is not sufficient to define a regional fault system. However, the suggestion that all four of the regional fault trends were active as normal faults during the Maastrichtian implies regional extension rather than regional transtension.
The Aziz-Bashiqa Trend of inverted grabens extends from the Mountain Front Fault on the east to the Abba Fault in the west (Figures 1, 4 and 5). The southern boundary is related to the Syrom-Sinjar Fault System. The northern boundary is somewhat arbitrary because of the intersection of the trend with the Palmyra Graben System. However, seismic data in the region where these two structural trends intersect indicate northward thinning of Lower Paleozoic strata at E-W and NW-trending faults. A seismic line across the Souedie and Karatchok fields suggests that both the Souedie and Karatchok fault systems at the northern boundary of the Aziz-Bashiqa Trend originate in rocks older than the Middle Cambrian Burj Formation (Figure 17a and b). Stratigraphic relationships interpreted from the seismic data and supported by well log correlations indicate that these two fault systems have had episodic displacement from at least the Late Ordovician. The development of the carbonate ramp developed in Massive Limestone and thickness changes in the Kermav Formation on the up-thrown side of the Souedie Fault System (Figure 17b) are the more apparent stratigraphic changes related to these fault systems.
Structures of the Aziz Bashiqa Trend are classic inverted grabens as described in detail by Kent and Hickman (1997). However, Weber (1964) was the first to describe the Sinjar trough and surrounding area as an area of structural inversion based on seismic interpretation. Given the quality of the data availably to Deutsch Erdol and that the work was carried out before concepts of basin inversion were popularized, Weber’s paper and its conclusions are remarkable. Hart and Hay (1974) similarly recognized Ain Zalah as an inverted graben based on well log correlations and identified a “fairly well defined belt of horst and graben structures developed on an east-west trend which can be traced through northeast Syria and north Iraq” (Hart and Hay, 1974, p. 981).
Lovelock (1984) and Kent and Hickman (1997), respectively, document that Jabal Sinjar and Jabal Abd Al Aziz anticlines originated by inversion of Late Cretaceous grabens. This system of generally EW-trending grabens developed after deposition of the “Massive Limestone” (Pilsener-Bekhme-Hartha Limestone) in Campanian time and filled with Upper Campanian – Maastrichtian Shiranish Formation. NE and NW faults segmented the E-W graben trend into highs and lows during the deposition of the Shiranish. The Shiranish Formation can be divided into at least three subunits with variation in thickness, which can be related to displacement on these faults (Hart and Hay, 1974; Kent and Hickman, 1997).
Ameen (1992) interprets all of the anticlines of northern Iraq as either inverted grabens or half grabens. While this interpretation is consistent with this author’s observations, a necessary distinction between the E-W Aziz-Bashiqa Trend and the NW Zagros structures is proposed. While the Aziz-Bashiqa structures may be genetically related to the Taurus Thrust Belt as the NW-trending folds are to the Zagros Thrust Belt, most of the Aziz–Bashiqa structures are separated from the orogenic belt by the Proterozoic Mardin-Urfa High. The presence of a basement high that has only a thin sediment veneer and a long geologic history makes a direct linkage of these structures to the thrust belt by a detachment surface within the stratigraphic section unlikely. The thickness of the Shiranish Formation in the grabens of the Aziz-Bashiqa Trend is dramatically greater than that found in the grabens developed along normal faults of the Zagros Trend.
Linkage of the Zagros inversion structures to the Zagros Thrust Belt trend through a master decollement in the Neoproterozoic – Lower Cambrian or lower Paleozoic stratigraphy is more feasible, but unnecessary. This suggests that both the Aziz–Bashiqa Trend and the Zagros Trend structures in the Kirkuk Embayment are related to extension parallel to the passive north and northeastern edges of the Arabian Plate, which were reactivated by collisions of the Arabian Plate with the Anatolian and Iranian plates. Although the main Taurus and Zagros collisions appear to have occurred nearly simultaneously, absence of upper Shiranish and Lower Tertiary sediments in the Zagros structures compared with thick sediments of these ages in the Aziz-Bashiqa structures suggests a slightly younger age for the initial Taurus compression.
Jabals Ibrahim and Alan are located west of Mosul within the area of intersection. Jabal Ibrahim’s long axis is oriented NW on the Zagros Trend whereas Jabal Alan’s long axis is E-W on the Aziz-Bashiqa Trend (Figure 18a). A cross section transecting from Ibrahim-1 to Alan-2 was created to illustrate the style of structures using a single seismic line. The cross section encounters faults of various orientations which yield uncertainty in correlations which cannot be resolved with a single seismic line. Given the uncertainty in correlations and unit seismic velocities, the cross section and restoration are only approximations, rather than rigorous or precise exercises. However, they do give a good first assessment of the structure (Figures 18b to d).
Topographic trends were used as proxy for fault locations and projected into the cross section. Intersections of the topographic trends correlate with the zones of relatively poor continuity in the seismic reflectors interpreted as fault zones (Figure 18a). The low angle of several of the faults in the cross section is the result of apparent dip generated by the oblique intersection of the faults with the line of section.
The cross section is located at the southern flank of the Aziz-Bashiqa inversion trend and crosses only the southern edge. The profile shows a region of general uplift and inversion rather than simply smaller inverted graben/anticlines. Jurassic stratigraphic units thicken up-dip toward the north from the Ibrahim Well to the Alan-2 Well. This section shows thinning of the Cretaceous interval into the center of the graben trend with the abrupt thinning of units underlying the Shiranish on the southern edge of the trend. This stratigraphic relationship is a variation of a similar situation on the northern edge of the Aziz-Bashiqa Trend seen in seismic line SY-34 (Figure 7). The jabals mark the areas of greatest extension and Shiranish depocenters.
Jabal Bashiqa / Maqlub / Kand
Investigation of the Bashiqa Structure initiated this study. The anticline was selected as a problem for a structural workshop at the Ministry of Oil offices in Baghdad. The topographic contours on the nearly unbroken Pila Spi surface essentially provide structure contours on that surface (Al-Naqib, 1959). Because the Pila Spi Formation underlies the Lower Fars decollement, the Pila Spi surface could provide an opportunity to use surface geology to resolve subsurface structure. Although high-altitude photographs show a lateral pucker typical of deformation above the Lower Fars/Dhiban upper detachment at the structure’s flanks, assuming the absence of these units over the structural crest would eliminate complication rising from using surface geology proved incorrect. Detailed study of the surface structure and geomorphology of the Bashiqa Structure indicate detachment surfaces in the Gercus Formation that underlie the Pila Spi Formation (Salih and Al-Daghastani, 1993). Imbricate thrusts originating from detachments in the Gercus Formation occur of both the southwest flank and the northwest flank of the structure.
The Bashiqa Structure has a roughly rhombic perimeter elongate to the northwest (Figure 19a). While the geometry might suggest a feature more readily associated with strike-slip faulting, the same geometry would result from the intersection of the Taurus and the Zagros structural trends producing a structure that is a composite of features common in both trends. Interpretation of seismic lines over the Bashiqa Structure require assumptions and projections based on structural models as used previously in Mansuriya interpretation. However, the Ministry of Oil released only the KA-32 line from the Bashiqa dataset for publication. This seismic line was selected by the author because examination of seismic lines adjacent to but not on complex structures often yields greater insight than do data directly across these features (Kent and Dasgupta, 2003).
Seismic line KA-32 crosses the Bashiqa Structure at the western end. The line extends to the north across the fault system associated with the Kand structural trend. The seismic line is composed of three segments of 12 to 24 fold data (Figure 19b). Only the north end of the section was imported into Geo-Logic Systems’ LithoTect program for structural interpretation. Stacking velocity data was used for depth conversion. Stratigraphic control was provided by data from the Kand-1 Well. Stratigraphic data for the Kand-1 Well were derived from formation tops in Al-Hadidy (2007) and isopach values given in Jassim and Goff (2006).
The interpretation (Figure 19c) suggests that large-scale normal faults displace the entire section through the Paleozoic. Antithetic faults to the major faults have developed with roll-over into the master faults. Normal displacement occurred through deposition of the Shiranish Formation. Near the end of the Cretaceous, compression reactivated the master faults and their antithetic faults. This deformation creates an oddly asymmetric geometry of folds with the steep dip toward the direction of compression due to the thickening of the Shiranish Formation in that direction.
The cross section was restored to verify the interpretation it displays is viable (Figure 19d). The paper seismic section provided for study shows a history of several alternate interpretations. The most recent of the previous analyses interpreted the Kand Fault as a reverse fault that dipped toward the south. Areas where there is limited poor quality data invariably produce multiple possible interpretations of that data. Defense of the interpretations presented herein is that they
honor all available well and seismic data;
are rational within the regional context; and
are restorable to the resolution of the data.
Additionally, the interpretation of the Kand Structure matches both the expected geometry of a listric normal fault and an inverted normal fault.
All of the structures examined with the possible exception of the Makhul can be associated with preexisting normal faults in the available or published seismic data. Although there is no seismic available for the Makhul Structure, potential field data suggests that it is related to a prominent discontinuity in the basement. Although an interpretation of the existing dataset cannot confirm a deep detachment in the Lower Paleozoic section as the origin for some of the region’s structures, the Makhul Structure is also the only structure where a detachment in the stratigraphic section can be demonstrated.
Of the four trends recognized by early workers and described by Henson (1951), all but the N-S regional trend are easily identified in northern Iraq as related to fault systems that originated in the Paleozoic or earlier and have experienced multiple episodes of displacement. The three identified regional fault trends may have originated independently, but all were active, including the N-S trend, in the Late Cretaceous during deposition of the Shiranish and subsequent inversion of the Shiranish grabens. This recent history suggests the possibility of earlier episodes of concurrent displacement and implies that the Neoproterozoic – Cambrian Hormuz Basin and its related structures have influenced sedimentation throughout geologic history of the Arabian Peninsula.
Discounting shallow structures that originate from decollements in the Upper Mobile Group (dominantly the Lower Fars or Fatha evaporites), the foreland basin sediments are inconsequential to the development of the area’s oil producing structures. The structures in the foreland basin sediments obscure the deeper structures, and therefore from a trap-mechanism point-of-view contribute little to the region’s economic geology. The significance to the petroleum system of the foreland basin sediments to the region’s petroleum system(s) is further diminished by detrital hydrocarbon shows in Cretaceous units that indicate mature and migrating hydrocarbons before any of the units of the Upper Mobil Group were deposited (Dunnington, 1958).
Henson’s (1951) E-W Tethyan Trend is now more commonly described as the Taurus Trend (e.g. Ameen, 1991) and originated in the Neoproterozoic as normal faults with syn-tectonic accumulation of Neoproterozoic and Cambrian sediments (Carter and Tumbridge, 1992). This trend in Iraq and northern Syria is represented by the Aziz-Bashiqa inversion trend. Extension on this fault system and contemporaneous normal displacement on the NW and NE fault systems created several depocenters where thick sections of Maastrichtian to Miocene sediment accumulated. Tertiary sedimentation was interrupted several times by mild episodes of inversion with the main inversion event occurring in the Pliocene – Pleistocene (Gaddo and Hussein, 1967; Kent and Hickman, 1997).
The NE Aualitic Trend correlates with the Palmyra inversion structural trend. Faults in this system in northern Iraq and Syria have displacement, early as Devonian as illustrated by thickening in the Palmyride-Sinjar graben trend (Figure 5). The tectonic history of this fault system is similar to the Taurus Trend described above.
The NW-trending Zagros Fault System (NW, Erythrean) is a composite fault set oriented between N35°W and N30°W. The earliest movement on this system in northern Iraq, documented with the available seismic, is normal displacement inferred from thickening of Jurassic and Triassic units across the Mansuriya and Kand faults (Figures 14a and 19c). However, potential field maps of northern Iraq and the cross section from the Khleisia High to the Zap Valley suggests that like the previously described trends, the NW fault trend has a history beginning in the Neoproterozoic – Cambrian. This set of faults was reactivated with normal displacement in the Maastrichtian to form half grabens in which the Shiranish Formation was deposited. As described for the previous two trends, inversion on the Zagros Trend began in the Paleocene, but the structures persisted. The result is anticlines in which the inverted Shiranish section is eroded. and Miocene units are thin or absent over the structural highs.
The question of whether to regard the structures of the Kirkuk Embayment as “structures of the foreland” and foreland basin structures may seem trivial since the anticlines clearly result from compression associated with the Zagros Orogeny. However, the structures of the Kirkuk Embayment discussed herein are not structures developed over ramps on thrust faults in sediments derived from an evolving thrust stack. The anticlines are fault-propagation folds developed by the most recent tectonic event in a long history of the underlying faults that existed well before the Zagros Orogeny. Describing these structures simply as Zagros foreland basin structures ignores the faults as the primary structures and their greater significance to the geologic history and petroleum geology.
At the local scale understanding the anticlinal structures may be paramount for the hydrocarbon trap, the understanding the faults provide understanding internal geometry, stratigraphy and migration corridors. However, on a regional scale, the fault systems and their development provide the framework for understanding the region’s stratigraphy and structural development. This is demonstrated by the influence of the presence or absence of the Hormuz Salt and the elements that controlled its deposition and preservation on the regional character of the Zagros Orogenic Belt.
This study is the out-growth of work began as a technical seminar provided to the Iraq Oil Exploration Company (OEC) in Iraq. The author is indebted to the employees of OEC for their contributions and their friendship. Discussions with Dr. Elbir, Dr. Ghazi, Dr. Falal, Dr. Khalaf, and many others provided the author with a greater understanding of Iraqi geology and great respect for Iraqi geologists. The Iraqi Ministry of Oil provided well and seismic data for this study, and it is used herein with their generous permission. The librarian is the explorationist’s best friend is an undeniable truth. The contribution this work may make is in large part the result of aid that I have received from the Ministry’s librarians. I am indebted to the library at the Ministry of Oil and to my dear friends in the Ministry library.
Although research for this paper began while the author was in Iraq, the work as presented herein was not funded, supervised or otherwise managed by the governments of the United States or Iraq. Several individuals have reviewed this work. The author is particularly indebted to Dr. R.G. Hickman, Dr. A.H. Horbury and the anonymous reviewers provided by GeoArabia for their comments and suggestions. The interpretations, errors and omissions are the sole responsibility of the author. The opinions and characterizations in this article are those of the author, and do not necessarily represent official positions of the United States Government. GeoArabia’s Arnold Egdane is thanked for designing the paper and Joerg Mattner for supplying and enhancing the map material.
ABOUT THE AUTHOR
W. Norman Kent has a BSc in Geology from the University of Arizona and an MSc in Geology from Northern Arizona University. He has more than thirty years experience in exploration petroleum geology with projects in the United States, Canada, Newfoundland, Morocco, Syria, Jordan, Iraq, Turkey, India and China. He has contributed to significant discoveries in Alaska and India. His publications include articles on structural geology and hydrocarbon exploration in the American Association of Petroleum Geologists Bulletin, GeoArabia, the Journal of Marine and Petroleum Geology and the Oil and Gas Journal. He is the owner and principle interpreter for Kent GeoScience Associates, a geological consultancy that provides exploration assistance to petroleum companies working in areas with complex geology and difficult data acquisition.