The northwestern segment of the Zagros Orogenic Belt of the Kurdistan Region of Iraq includes the Zagros Suture Zone which is consisting of allochthonous Tethyan Triassic–Eocene thrust sheets. The zone is bounded by the Zagros Main Reverse Fault in the northeast, and the Zagros Thrust Front in the southwest. Parts of this zone’s rocks are represented by stacks of thrust megasheets obducted over the autochthonous Cretaceous–Cenozoic sequence of the Arabian Plate margin. Each sheet represents a specific Tethyan tectonostratigraphic facies, and includes (from older to younger): isolated Triassic carbonate platforms (Avroman Limestone), Jurassic carbonate imbricates (Chia Gara, Sargelu and other formations), radiolarites (Qulqula Group), sedimentary mélange (sedimentary-volcanic units of the Qulqula Group), ophiolites (Mawat and Penjwin ultramafics complexes), and Cenozoic fore-arc volcano-sedimentary sequences (Walash Group). Petrography, facies interpretation and lithostratigraphic correlation of these allochthons along four traverses across the Zagros Suture Zone of the examined area indicate that they evolved during the closure of the Neo-Tethys Ocean. Their stacking pattern and tectonic association resulted from two important events: the Late Cretaceous obduction processes, and the Late Miocene–Pliocene collision, uplift, folding and suturing between the Arabian Plate and the Sanandaj-Sirjan Block of Iran. Based on these field observations and by using the model of the Iranian Zagros evolution, a tectonic scenario is proposed to explain the history and evolution of the Zagros Suture Zone in this area.
The NW-trending Zagros Orogenic Belt extends about 2,000 km from the Anatolian Fault of southeastern Turkey to the Makran Zone in southern Iran, and is part of the Alpine-Himalayan Mountain Chain (Figure 1). The belt resulted from the Late Cretaceous and Cenozoic convergence of the Iranian terranes and the Arabian Plate, when the intervening Neo-Tethys Ocean went through a succession of subduction, obduction and collision stages (Alavi, 1994; Sharland et al., 2001; Agard et al., 2005). An integral part of the belt is preserved in the mountainous region of northeast Iraq (Figures 2 and 3). However, this part of the belt remains less studied and underexplored as that of the southeastern Iranian and Omani parts. The present study offers a general review of the tectono-stratigraphic architecture of the area as an integral contribution to the understanding of the geological history of the Zagros Orogenic Belt. We present the results of detailed field examinations, petrographic studies, and litho-stratigraphic correlation of the exposed tectono-stratigraphic units of this zone. Future studies aimed at obtaining biostratigraphic control and petroleum geological and geophysical data are planned.
The study area is located in the Sulaimani Governorate of the Kurdistan Region of northern Iraq. It extends from the Mawat area to the east of Sulaimani City down to the surroundings of Halabja (Figures 4 and 5). Four traverses were selected perpendicular to the tectonic strike of the Zagros Orogenic Belt to cover the regional tectono-stratigraphic domain of the area. Each traverse represents an elongated block, a few kilometers wide and tens of kilometers long and covers a particular portion of the suture zone.
(1) Traverse 1 is oriented N20°E and located northeast of Sulaimani City. It extends from the axis of Azmur Mountain to the Iraqi-Iran border (Figure 6).
(2) Traverse 2 is oriented N35°E and covers the Penjwin-Said Sadiq area (Figure 7).
(3) EW-oriented Traverse 3 extends from the Iraqi-Iran border to the town of Khurmal (Figure 8).
(4) EW-oriented Traverse 4 is located to the east of Halabja City, and connects Byara to Tawela (Figure 9).
Within each of these traverses several side traverses and sections were measured to document lithological character and associations. Major structural features such as shear zones, boudinage, major and minor folds and faults are reported. Lithostratigraphic correlation was conducted to construct the local and regional stratigraphic architecture within the postulated tectonic framework (Figure 10). Samples were collected from each unit to investigate various lithologic features. Each sample was thin-sectioned for petrographic studies to define lithology, sedimentary components, and to differentiate between mineralogical and textural criteria of each lithologic association.
Bolton’s contribution to the geology of the Zagros Suture Zone remains the foundation of all succeeding studies and geological mapping campaigns of the area (Bolton, 1955, 1956, 1958a, b, c). His 1:100,000-scale geological maps of the Kurdistan region in Iraq were the first, and are still considered the basic background for all recently published geologic maps of the area. Bolton’s stratigraphic nomenclature of the area remains unchanged, though revision is needed in the light of later geological concepts such as plate tectonics and sequence stratigraphy, and newly accomplished work on the area.
The most recent geologic map of the area was compiled by Ma’ala (2008) at a scale of 1:250,000 for the Geological Survey and Mineral Investigation State Organization of Iraq (GEOSURV). It is considered the most reliable mapping resource of the Zagros Suture Zone (Figure 5). The ages of the stratigraphic units of the zone that are discussed in our study are mainly taken from the chart that accompanies the geologic map (Figure 10).
The tectonic review by Jassim and Buday (2006) applied new vision to the area’s evolution. Ibrahim (2009) presented a regional review of the tectonic style and evolution of the Zagros Orogenic Belt of the Kurdistan Region of Iraq using a scaled analogue sand model. Other works include local studies or mapping of specific areas and/or general reviews of the tectonic setting of northeastern Iraq (Al-Mehaidi, 1975; Al-Fadhli et al., 1979, 1980; Buday, 1975, 1980; Buday and Jassim, 1987; Ameen, 1979, 1991; Al-Saedi, 1992; Marouf, 1999; Numan, 1997, 2001; Stevanovic and Markovic, 2003; Al-Qayim, 2004; Al-Eli, 2004; Al-Kherasan, 2007; Karim, 2007; Karim et al., 2009; Al-Hassan, 2009; Banks, 2011).
REVIEW OF THE REGIONAL GEOTECTONIC SETTING
The northeastern margin of the Arabian Plate initially formed by the Mid-Permian rifting and the Triassic break-up of Pangaea and the opening of the Neo-Tethys Ocean. As a result an extraordinary wide and shallow marine shelf developed over the northeastern Mesozoic Arabian passive margin (Beydoun, 1991). Stable tectonic conditions typify the Jurassic and most of the Cretaceous with shelf carbonates and evaporites, and deep-marine marlstone basinwards (Figure 10). Tectonic conditions were subdued until the opening and extension of the South Atlantic, which was transmitted onto the northeast margin of the Arabian Plate by accentuating subduction of the Neo-Tethys beneath Eurasia. This led to the tilting and rapid drifting of the Arabian Plate margin causing compression, and eventually, the obduction of young Neo-Tethys ophiolitic oceanic crust onto the northeast margin of the Arabian Plate (Sharland et al., 2001).
The emplacement of the ophiolites is considered to have taken place during the Late Cretaceous and includes: ophiolitic mélange, metamorphosed massive ophiolite of supra-subduction zone setting, sheared and massive carbonates and radiolarites (Jassim and Goff, 2006). These units extend in age from Triassic to the end of the Eocene (Figure 10), and are stacked over the Arabian Plate margin forming the components of the Zagros Suture Zone. This emplacement event is associated with a foreland basin formation in response to the successive loading and flexuring in front of the advancing emplaced blocks and along the northeastern margin of the Arabian Plate (Al-Qayim, 1993, 1994, 2004, 2011; Jassim and Goff, 2006). The foreland basin is filled with a thick flysch sequence of siliciclastics derived from the uplifted thrust sheets. Foreland flysch sedimentation progressively shallowed until termination by the early Mid-Eocene time where red, sub-continental siliciclasts prograde from the northeast and cover most of precursor foreland area. During Late Eocene the whole area was denudated and a clastic-free period of sag-interior basin sedimentation dominated the southwestern part of the foreland area and is characterized by cyclic shallow-marine ramp carbonates and basinal marlstone (Al-Qayim, 2006).
The present-day tectonic configuration of the northeast part of the Arabian margin resulted from the final closure of the Neo-Tethys Ocean and continental collision between the Arabian Plate margin and Eurasia (Sanandaj-Sirjan Block) during Early to Mid-Miocene (Jassim and Buday, 2006). The excessive compression resulted from the opening of the Red Sea speedup deformation and resulted in inversion, uplift, shortening of the margin sequence, whereby the Zagros Orogeny was evolved. The replacement of the marine conditions by clastic sedimentation of molasse type over the newly developed Mesopotamian basin, indicate the overwhelming uplift and active erosion of the hinterland (Jassim et al., 2006). Progressive shortening led to the folding, faulting and imbrication of the of the Arabian Plate margin sequences accompanied by the overthrusting of the Tethyan accretionary prism components, which led to the final development of the Zagros Suture Zone.
This tectonic history is demonstrated by the development of several major tectonic boundaries and zones aligned along the Zagros tectonic strike (NW-SE), from NE to SW: (1) “Shalair Zone” of the Sanandaj-Sirjan Block; (2) “Zagros Main Reverse Fault”; (3) “Zagros Suture Zone”; (4) “Zagros Thrust Front”; (5) “Zagros Imbricate Zone”; (6) “High Zagros Reverse Fault”; (7) “Zagros Fold Zone” divided by the “Zagros Mountain Front Fault” into the “High Folds Zone” and “Low Folds Zone”; (8) “Zagros Foredeep Fault”; and (9) “Mesopotamian Zone” (Figures 2 and 3).
TECTONIC BOUNDARIES AND ZONES
The major boundaries and encompassed tectonic zones of northeast Iraq are reviewed below with proposed nomenclature and definitions. We follow the Zagros terminology used in the international literature and our newly proposed tectonic model (Al-Qayim, 2004; Figures 2 and 3).
Most of the boundaries remain the same in their trends and delineation as proposed by previous authors (Bolton, 1955, 1956, 1958a, b, c; Buday, 1980; Buday and Jassim, 1987; Al-Kadhimi et al., 1996; Jassim and Goff, 2006; Al-Qayim and Lawa, 2006; Ibrahim, 2009). The description of these features in the study area is supported by our field observations.
Shalair Zone of the Sanandaj-Sirjan Block
The “Shalair Zone” comprises metamorphosed Triassic–Cretaceous units of the Zagros Belt in Iraq (Buday and Jassim, 1987). It is bound to the southeast by the Zagros Main Reverse Fault of Iran (Berberian and King, 1981). The zone forms an integral part of the Sanandaj-Sirjan Block of Iran (Stöcklin, 1968; Al-Qayim, 2004). It is located at the extreme northeastern part of the studied area, with limited exposure in the Shalair Valley area (Figure 2). Exposures of the Shalair Zone are governed by a major EW-trending mega-anticlinorium with the Shalair Valley being axial to it (Buday and Jassim, 1987). The major units include the old crystalline rocks of the Mishav Complex, which consists mainly of granite intrusions. The Upper Triassic recrystallized limestone of the Darokhan Unit is exposed in the core of the Shalair Anticline, and often imbricated with the Shalair phyllites (Jassim and Goff, 2006). The third unit is the Kata Rash Group, of the Cenomanian arc volcano-sedimentary complex of andesite and rhyolite, and the fourth unit consists of Maastrichtian–Paleocene carbonates (Piran Limestone), which occur as isolated thrust sheets over older units.
Oweiss (1984) considered this zone to represent an intra-continental shear zone of regionally metamorphosed units. Jassim and Goff (2006) reviewed the tectonic classification of Iraq, hence they put forward the denomination “Zagros Suture Zone” when classifying the allochthonous thrust blocks of the Zagros Orogenic Belt in northern Iraq. They included the Shalair Zone in the Zagros Suture Zone. The present authors believe that the Shalair Zone is part of the Sanandaj-Sirjan Block and thus should be excluded from the Zagros Suture Zone of the Neo-Tethys domain. The description given by Jassim and Goff (2006) for the basic characteristics of this zone support this conclusion: (1) The age of its units extends from pre-Permian to Cenozoic; (2) it includes the innermost metamorphosed rocks of the area; and (3) it structurally represents the highest thrust sheets with EW-trending lineaments and major structures.
Zagros Main Reverse Fault
The Zagros Main Reverse Fault represents the suture line between the Zagros Suture Zone in the Arabian Plate and the Sanandaj-Sirjan Block (Figure 2). It is a NW-trending fault system that extends from the north and northeast of Iraq to the north of Bandar Abbas in Iran (Berberian, 1995). In the study area this fault is recognized in the Penjwin-Halabja area only, several kilometers to the northeast end of the Traverse 2 area. The fault appears mainly as a reverse 40°NE fault, separating the metamorphic rocks of the Shalair Zone from the ultra-basics rocks of the Penjwin Ophiolite Complex (Ibrahim, 2009). On the surface it is covered in large areas by Quaternary sediments of the Shalair Valley. In other areas, the fault line is marked by an elongated valley segment covered by recent sediments and running in front of a major thrust sheet.
Zagros Suture Zone
In this paper we use the term “Zagros Suture Zone” for the region bounded by the Zagros Main Reverse Fault to the northeast, and the Zagros Thrust Front to the southwest (Figure 4). It includes intensively tectonized allochthonous thrusted sheets of a Tethyan ophiolite, radiolarite, platform carbonates and sedimentary-volcanic suites (Figure 2). The sheets were thrusted west-southwestwards over the Arabian Plate’s margin in different order and succession and were caught between the Arabian Plate’s margin and the Sanandaj-Sirjan Block (Buday, 1980). Its components belong to a possibly subduction-related complex (Al-Qayim, 1993, 1994; Numan, 1997, 2001). These sheets were grouped into the following units from east to west and from topographically higher to lower units: Triassic–Jurassic carbonate platform (Avroman Limestone); Mawat and Penjwin Ophiolite complexes of basic and ultra-basic units, radiolarian chert and limestone, mudstones, and conglomerates (Qulqula Group); the Cenozoic sedimentary-volcanic unit (Walash Group); the metamorphosed sedimentary unit (Naupordan Series); and the Jurassic Limestone Imbricates. The age of these units is given in Figure 10.
Rock units of this zone exhibit displacement and crustal thickening (Figure 11b). The amount of displacement decreases towards the folded zone. The NE-dipping thrust faults, the SW-verging overthrusts, and the tight anticlines are the most prominent structures, forming “piggy back” imbricate stacks. Geomorphologically, the Zagros Suture Zone exhibits continuous high mountain ranges with deeply incised valleys, having a very rough morphology due to the imbrication of the thrust sheets. The deformation of this zone is the product of long and complicated geodynamic evolution resulting from compression and shortening due to the Late Cretaceous obduction of the ophiolite-radiolarite suites, which culminated during the late Cenozoic continent-continent collision of Arabia and Eurasia (Alavi, 2004; Jassim and Goff, 2006; Agard et al., 2011).
Synonyms: The Zagros Suture Zone has been variably named: “Thrust Zone” (Bolton, 1958; Buday, 1980; Buday and Jassim, 1987; Marouf, 1999), “Nappe Zone” (Dunnington, 1958), “Zagros Thrust Zone” (Buday and Jassim, 1987), “Zagros Crush Zone” (Al-Qayim, 1993, 2004), “geosyncline thrust region” (Al-Kadhimi et al., 1996), “Subductional Tectonic Facies” (Numan, 1997, 2001), “Zagros Suture Zone” excluding the Shalair Zone (Jassim et al., 2006), and “Zagros Imbricate Zone” (Ibrahim, 2009).
Correlative Zone in Iran: The Zagros Suture Zone of Iraq continues into Iran where it is known as the “Zagros Crush Zone” (Wells, 1969; Haynes and McQuillan, 1974; Setudehnia, 1978; Stoneley, 1981; Agard et al., 2005), “High Zagros Thrust Belt” (Berberian and King, 1981; Berberian, 1995; Blanc et al., 2003; Alavi, 1994, 2007; Navabpour et al., 2007; Ghasemi and Talbot, 2006; Robin et al., 2010), and “Zagros Imbricate Zone” (Hessami et al., 2001; Sepehr and Cosgrove, 2004; Mouthereau et al., 2006).
Zagros Thrust Front
The “Zagros Thrust Front” is a prominent tectonic boundary, which is characterized by a NW-trending fault front (in most cases a low-angle thrust fault), which separates the Zagros Suture Zone from the Zagros Imbricate Zone. The boundary is equivalent to the “Zagros Exposed Thrust Front” in Iran (Alavi, 1994, p. 235). In northeast Iraq it consists of connected segments of thrust faults with different orientations depending on the geometry of the fore-thrusted blocks of the Zagros Suture Zone (Figures 2 and 3). This boundary runs within the studied area, by swinging around the forerunning thrust sheets of the Zagros Suture Zone.
This fault line represents an important morpho-tectonic feature separating the different thrust sheets of the suture zone to the northeast from the sedimentary sequence of the Zagros Imbricate Zone to the southwest. At Mawat area, it is associated with a linear valley due to low lithologic contrast on both sides (i.e. the Walash Group in the northeast and the siliciclastic sediments of the Red Beds Group from the southwest). In other areas it can be recognized by an up to 500 m high fault scarp especially between the thrust front of the Qulqula Group and the Cretaceous sequences of the imbricate zone as in the area to the northeast of Said Sadiq (Figure 11a).
Zagros Imbricate Zone
This “Zagros Imbricate Zone” is bounded to the northeast by the Zagros Thrust Front and the southwest by the High Zagros Reverse Fault (Figure 2). The sedimentary sequence of this zone includes parautochthonous plate margin sediments of the Cretaceous marine carbonates (Balambo Formation), Upper Cretaceous foreland flysch (Tanjero Formation), and molasse sequence of the Paleogene foreland, i.e. the Suwais Red Beds (Figure 10). The zone is intensively deformed and greatly shortened as compared to the rest of the Arabian margin strata. Deformation resulted from shortening due to the displacement of the suture zone masses and the succeeding collision event. Imbrications are evident from the structural style, which is characterized by tight, overturned, high-amplitude, dense folds intersected by intense faulting (Figure 11c).
This zone is equivalent to the “Balambo-Tanjero subzone of the Imbricate Zone” of Jassim and Goff (2006). It is differentiated from the thrusted Tethyan sheets of the Zagros Suture Zone by its parautochthonous sedimentary components of the Arabian Plate margin. It differs from the Zagros Folded Zone by its intensive degree of deformation and imbrications. Imbricate structures indicate multiple phases of deformation. Folding and re-folding includes mesoscopic chevron and box folds, in addition to multiple faulting. These were associated by thrusting, mostly semi-parallel to their fold axis (Al-Fadhli et al., 1980). Folds are intricate isoclinal anticlines lacking expressive synforms. The zone is disrupted by a dense net of reverse faults; each often dislocates the axis of anticlines southwestwards, manifesting typical structural imbricate morphology (Buday and Jassim, 1987).
High Zagros Reverse Fault
The NW-trending “High Zagros Reverse Fault” separates the Zagros Imbricate Zone from the Zagros Folded Zone (Figure 2). It is interpreted as a deep basement fault in Iran because it is associated with outcrops of Hormuz salt intrusions (Mobasher and Babaie, 2007), and there is some seismic activity with strike-slip focal mechanism solutions (Aziz Zadeh, 1997). The fault has been active since Permian time (Bahroudi and Talbot, 2003). In northeast Iraq, it dips 65°NE and is clearly expressed on the surface as an axial fault along the Balambo and Azmur anticlines (Ibrahim, 2009). The more competent Balambo and Kometan formations were thrusted along the fault over the low-elevated incompetent rocks of the Shiranish and Tanjero formations. The fault has been displaced dextrally as in the Halabja-Said Sadiq area due to the effect of the dextral strike-slip NS-trending longitudinal faults (Ibrahim, 2009).
Zagros Folded Zone
The “Zagros Folded Zone” lies between the High Zagros Reverse Fault to the northeast and the Zagros Foredeep Fault to the southwest (Berberian, 1995; Ibrahim, 2009). It is the most extensive region of the Zagros Orogenic Belt (Figure 2) and named here after Agard et al. (2005) to emphasize the role of the folding process in the development of its structures. It continues into Iran as the “Zagros Simple Folded Zone” (Stöcklin, 1968; Alavi, 1994; Berberian, 1995; Hessami et al., 2001; Bahroudi and Talbot, 2003; Homke et al., 2009).
The fold structures in this zone are linear, curvilinear, asymmetrical, doubly-plunging, high-amplitude, convergent and divergent and are arranged in an en-echelon pattern on the surface (Buday and Jassim, 1987). They are foreland-and hinterland-verging folds. The zone consists dominantly of Jurassic–Cretaceous passive margin sequences and the overlying Upper Cretaceous–Paleogene foreland sequence (Figure 10). The anisotropy of the mechanical properties of the stratigraphic units was one of the main reasons for the geometrical variations of these structures, especially the northeastern part. Most of the folds were propagated by major axial, NW-trending faulting (Ibrahim, 2009). The physiographical features of this zone are inherited from the folded nature of the mountain range, which is composed of parallel sets of NW-trending anticlines and synclines. The whole zone is characterized by intermountain plains, which represent broad synclines filled with Quaternary sediments (Figures 2 and 4).
Zagros Mountain Front Fault
The “Zagros Mountain Front Fault” is a NW-trending deep reverse blind fault, which shows a pure dip-slip movement (Figure 3). The hanging-wall is represented by the High Folds Zone of ca. 1,200 m elevation. The footwall is represented by the Low Folds Zone of ca. 700 m elevation (Buday and Jassim, 1987). The fault is displaced right-laterally by many transversal faults, among them the longitudinal Khanaqin Fault, which displaces it by about 140 km on the eastern part of the study area (Ibrahim, 2009). The strike of the Zagros Mountain Front Fault swings from NW in the study area, to west to the north of Erbil City (Figure 2). The vertical displacement across the fault is often marked by a series of prominent scarps, ridges or fault-bend anticlines. The Sagirma and Darbandi-Bazian anticlines are surface expressions of the fault in the study area.
Zagros Foredeep Fault
The NW-trending “Zagros Foredeep Fault” separates the Zagros Folded Belt from the Mesopotamian Zone (Figure 2). It is a reverse fault, and its surface expression is recognized by the chain of narrow anticlines that developed as a result of SW-directed deep fault-propagation structures marking the delineation of this fault boundary. This chain includes from southeast to northwest: Mansuriah, Hemrin South, Hemrin North and Makhul anticlines.
ROCK UNITS OF THE ZAGROS SUTURE ZONE
The following review is a general compilation of previous works on the area as well as the new data of stratigraphic and tectonic field observations collected from four selected traverses across the suture zone (Figure 4). Unit names used here are based on current stratigraphic status and description proposed by Bolton’s studies (1955-1958), and the review of Jassim and Goff (2006). New interpretations are given in later sections incorporating previous discussion with new observations. Since the components of the suture zone include thrusted sheets of different lithologies, ages, and tectonic positions, the description focuses on the most significant regional tectonic blocks. The order bears no chronostratigraphic significance but follows tectonic sequence from upper to lower sheets.
Triassic Avroman Limestone
The Avroman Limestone is a massive limestone body with exposure limited to the area along the Iraq-Iran border to the east of the line connecting Kaolos, Khurmal, and Byara (Figure 5). This body represents a small portion (ca. 27 km x 2 km) of a large limestone body extending into Iran and widely known as the Bisotun Limestone unit or microcontinent (Bordenave and Hegre, 2005). The Avroman Limestone in the study area was first introduced by Bolton (1958) to designate the massive carbonate sequence forming the high grounds of Jabal Avroman (Hawraman) during his mapping of the Halabja area.
The Avroman Limestone comprises 800 m of light gray-brownish, milky-white, thick-bedded-to-massive, hard, and fossiliferous limestone, with a wide range of shallow-water fossils including laminated algae, oolites, oncolites, shells of bivalvia and brachiopods. Bivalvia and oncolites are the most recognizable fossil contents along the Zalam Valley to the east of Khurmal (Figures 12a and 12b). However, examination of samples from the Avroman to the northeast of Said Sadiq shows that the Avroman Limestone consists predominantly of gray oncolitic limestone with subordinate occurrence of oolites, pellets and bioclasts. All these components indicate a shallow isolated platform (Karim, 2007). The high purity of the limestone indicates shallow carbonate platform far from continental clastic influence (Jassim et al., 2006).
The Avroman Limestone is well exposed in the area of Traverse 3 (Figure 8). It clearly displays a low-angle thrust boundary with the underlying cherty conglomerate unit of the Qulqula Group. The boundary zone is about 1.0 meter thick and highly sheared. The Avroman front has a cataclastic appearance especially in the lower part due to its fragmentation and crushing during thrusting and transportation over the Qulqula Group rocks (Figure 12c). The fragmented rocks are arranged in planar fabrics producing anastomosing cleavage perpendicular to the direction of shortening indicating considerable distance of transportation (Figure 12d). The upper boundary of the unit is not clear because most of its extension lies in Iran.
The dating of the Avroman Limestone using microfossil assemblages from the Traverse 3 area indicate Late Triassic (Norian–Rhaetian) (Jassim and Goff, 2006), which is equivalent to the lower part of the Bisotun Limestone unit of Iran of Triassic–Cenomanian age (Bordenave and Hegre, 2005). The lower boundary of the Avroman Limestone is characterized by concordant volcanic bodies (Figure 8). These extrusive igneous rocks are composed of alkaline olivine basalt and trachyte, and their ages were determined with K/Ar dating of the diabase flow at 220 ± 20 Ma, Norian (Jassim and Goff, 2006). The alkaline basalt indicates active continental rifting during the Late Triassic. Three volcanic bodies were identified. The small one is of trachyte type and emplaced at the boundary zone. The second one occurs in the lower part of the Avroman Limestone and the third in the upper part of the cherty conglomeritic unit of the Qulqula Formation (Figure 12e). The volcanic bodies were emplaced tectonically as cold magmatic bodies within the examined area. This is evidenced by the lack of any thermal metamorphic effects on the host Avroman or other rocks, together with slickensides seen in the field on the extrusive igneous rock surfaces at the contact with the host Avroman rocks. The slickensides also indicate the direction of last movement of the extrusive igneous bodies towards the southwest.
To the east of Tawela (Traverse 4 area), the Avroman Limestone can be seen as a high mountain in the far site within Iran, overthrusting the Qulqula Group in Iraq. The contact seems to run along a valley passing parallel to the border (Figure 12f).
The Qulqula Group represents the deep-marine sequence of radiolarian chert, mudstones, and limestone with basic volcanic intrusions. The upper part is a thick conglomerate unit (Buday and Jassim, 1987). Due to repetition, the group constitutes the thickest and most extensive thrust sheet of the Zagros Suture Zone. The Qulqula Group in the studied area is distributed in almost two separate blocks. The larger northern block occurs at Mawat-Penjwin areas (Traverses 1 and 2, Figures 6 and 7). A smaller block was recognized at the Khurmal-Tawela areas (Traverses 3 and 4) (Figure 5). Its rock units appear on the eastern side of the Traverse 1 area, and in the middle of Traverse 2 area, and the eastern part of Traverses 3 and 4 (Figures 7 to 9). The radiolarite-proper units, in general, have uniform, thin-to medium-bedded siliceous limestone, with chert beds of different colors, thickness and forms. This lithological feature is the most diagnostic for identification of the Qulqula Group.
The group was first introduced by Bolton (1955, Figure 13) as the Aptian–Albian Qulqula Radiolarian and Qulqula Conglomerate formations. Buday (1980) interpreted the group as Late Jurassic (Tithonian) to Santonian sequences deposited in the Neo-Tethys Ocean. Both formations are closely associated with each other, and seemingly occurred as a single unit or thrust sheet. However, it always includes exotics units, and inter-sequence bodies from lower or adjacent units, which were incorporated during thrusting and movement of the major radiolarite block. The boundary with the underlying units is always tectonic and represented by a low-angle and eastward-dipping thrust zone. Since Bolton’s work in 1958 several attempts were made to classify the group into further smaller units as summarized (Czech Team, 1976; Jassim and Goff, 2006; Figure 13):
The Czech Team (1976) divided the Qulqula Group into four main units:
(a) Tithonian–Cenomanian Qulqula Unit 1 (QU.1) consisting of chert with siliceous mudstone (QU.1a), chert with rare mudstone (QU.1b) and limestone, and undifferentiated limestone (QU.1c).
(b) Valanginian–Early Aptian Qulqula Unit 2 (QU.2), limestones with cherty layers and nodules.
(c) Late Aptian–Cenomanian Qulqula Unit 3 (QU.3) consisting of limestones with chert.
(d) Albian–Cenomanian and Turonian Qulqula Unit 4 (QU.4) comprised of limestone conglomerates and breccias.
The Qulqula Group is intensively deformed with multiple faulting and imbrications. This increases the complexity of its stratigraphy by repetition and overturning. The deformational structures include fracturing of all types, joints, reverse and thrust faults and folding of chevrons, overturned, and asymmetrical types. In some parts especially, at the front of the thrust sheet, intense deformation becomes usual.
In the area studied, the Qulqula Group generally consists of the following units, reading from west to east, and from top to the bottom. The numbering of these units follows the general classification of the group in the area.
Unit Q1: Dark Grayish-Green Shale
A dark grayish-green shale unit generally consists of a crushed and occasionally sheared sequence of fissile shale, or marly shale with exotic bodies of rocks (Figure 14a). Exotics include smashed volcanic bodies of up to 5 m in size that are recognized as dykes or sills within the highly deformed shale (Figure 14b). This unit is recognized from the studied area of Traverses 2 and 4 (Figures 7 and 9). The unit termed informally as the “Hawar shale unit” from a typical and thick section exposed near Hawar Village, 8 km to the northeast of Halabja of Traverse 4 area (Figure 5). At this location the unit is about 70 m thick and consists of fissile-to-foliated dark-gray to green shale (Figure 14c). Volcanics are either concordant or discordant with the sequence.
The boundary with the overlying reddish siliceous mudstone unit is tectonic and characterized by shear zone of a thrust plane. Limestone horizons of possible Jurassic age overlie parts of this unit in some places (Figure 14d), protecting them from erosion as in the outcrop near Kolitan Village, along the road to Penjwin. Exotic carbonates or olistostromes are frequent in this unit. It is recognized with different sizes and types, and usually embedded within the sheared part of this unit (Figure 14e). In other case the unit rests directly on the passive margin pelagic, and Cretaceous marly limestone of the Balambo Formation. The boundary is represented by a shear zone in which blocks of the Balambo limestone were embedded into the gray shale of the lower part of the locally termed “Hawar unit” (Figure 14f). This thrust boundary locally shows reverse segments with angles reaching 70°. It dips eastwards and forms a local segment of the Zagros Thrust Front.
Unit Q2: Reddish Siliceous Mudstone
The reddish siliceous mudstone unit is highly sheared and crushed due to its ductile nature and is characterized by thin-to-medium-bedded siliceous mudstone, silty shale, chert, and limestone alternating with the dominating red to brown shale (Figures 15a, b and c). It often occurs as less than 100-meters-thick patches or slivers within the Qulqula Group sequence. Small-to-medium-sized olistostromes of volcanic bodies and limestone units are smashed and incorporated into the sequence (Figure 15d). Other occasional components of this unit include tuffaceous/pyroclastic material or horizons, and a black irregular pillow lava horizon which can be recognized along the road to Tawela (Figures 9, 15e and 15f). Along the Zalam Valley and by the Ahmad Awa Gorge, reddish, thin cherty beds or siliceous limestone can be distinguished, alternating with the siliceous mudstone beds. Intensive shearing of the unit due to its ductile nature produces varieties of deformational structures including; folding, refolding, faulting, and imbrications. These deformational features are manifested clearly when the beds become thinner and more siliceous (Figure 15f). Thickness is variable due to erosion and in extensive exposures usually forms outstanding features or ridges as seen near Kanarue (Figure 15d).
Unit Q3: Thin-Bedded Radiolarian Chert and Limestone
The thin-bedded radiolarian unit is locally termed as the “Qulqula-proper” or the “Radiolarites Facies”. It is the most dominant and characteristic lithologic unit of the Qulqula Group. It can be identified from many areas, and on all major Qulqula thrust sheets. It is recognized from the eastern margin of Traverse 1 area, and as intermediate sheets within the Qulqula Group outcrop of Traverses 3 and 4 (Figures 8 and 9). The unit is well exposed along a road cut near the village of Nal Pareze along the road to Penjwin of the Traverse area 2 (Figure 7), where a thick sequence of deformed, thin-to-medium (occasionally thick) uniformly bedded, silicified limestone and chert is seen (Figure 16a). Dark gray to buff marlstone or silicified shale often occur as thin inter-layers. In some places only silicified limestone occurs, with almost no marlstone interlayers (Figure 16b). This unit occurs as a major sliver or thrusted sheet over other units of the Qulqula Group. Deformation is displayed in regional folding and local refolding due to imbrications and faulting which repeat the sequence in multiple segments (Figure 16c). The dip of the strata is locally variable due to the intense deformation, but commonly indicates regional southwestward movement.
Chert beds are either primary of radiolarian siliceous wackestone to packstone (Figure 16d), or secondary irregular inter-beds with no fossils (Figure 16e), which resulted from diagenetic replacement. Chert beds occur in gray, whitish-gray, green (Figure 16f), dark gray, and reddish colors with red shale interlayers (Figure 17a). The limestones are usually medium to thick beds of bioclastic wackestone to packstone. Slices of this unit are frequently embedded in the reddish siliceous mudstones indicating complex and severe mixing of the different units during thrusting and transportation (Figure 15d). Bed thickness is often uniform especially when it lacks shale interlayers (Figure 17b). Concordant volcanics of green or reddish bodies intruded the sequence in different sizes and types and are recognized in Traverses 2 and 4 areas (Figure 17d). The boundary with the underlying unit is sharp and characteristically represented by a shear zone (Figure 17c).
Another outcrop of this unit is recognized along the road from Chwarta to Kanarue of Traverse 1, where medium-to-thickly bedded limestone alternates with greenish and brownish marlstone (Figure 17e). Chert nodules and chert interlayers become common upwards with thickness reaching 20 cm. Marlstone interlayers on the other hand decrease upwards. The beds are highly squeezed and crushed and display intermixing with other units of the group (Figures 17e and 17f).
Unit Q4: Thick-bedded Shallow-marine Carbonate
This unit consists of dark gray, weathered to milky white to buff color, thick-bedded, marine limestone. It exhibits massive carbonate features often separated by a thin marlstone interlayer (Figure 18a). The limestone is of grainstone to packstone types. The grains are dominated by pellets, ooids, lithoclasts and bioclasts of shallow marine conditions (Figures 18b and 18c). Karim et al. (2009) examined similar limestones from Kaolos of Traverse 2 area (Figure 7), and inferred that it was deposited in a shallow-marine environment with active detrital conditions. The limestone beds often are associated with thin dark chert layers of seemingly diagenetic origin.
In many cases beds of this unit are thrusted over or squeezed into the underlying units as a result of intense deformational features of the Qulqula units as the case of a fresh outcrop exposed along the road from Chwarta to Penjwin (Figure 17c).
Near the border town of Tawela the limestone beds become thicker and fossiliferous (Figure 9). Petrographic examination of these rocks shows domination of oolitic-pelloidal packstone with an indication of winnowing suggesting shallow-marine environment of active shoals and energetic ridges (Figure 18d). These shallow-marine limestones grade upwards into deep-marine limestones and chert reflecting cyclicity and fluctuation of depositional water depth (Figure 16b). The age of this unit using micropaleontological analysis is believed to range between Tithonian to Berriasian (Jassim and Goff, 2006). However, Karim et al. (2009) from their study on these limestones from Said-Sadiq area and based on stratigraphic position suggest an Early Cretaceous age.
Cherty Conglomeratic Limestone Unit Q5 (Qulqula Conglomerate Formation)
The cherty conglomerate limestone unit always occurs on top of the Qulqula Group, and in many cases occurs at the boundary zone between the Avroman Limestone unit and the under-thrusted Qulqula Group units. This unit appears in the Traverse 2 area, along the road to Penjwin around Kani Manga Village (Figure 7). However, the best exposure of this unit is located along Permaron Valley sides of Traverse 3 area between the villages of Ta Ta and Sargat (Figures 8 and 18e), and along the border zone with Iran, in a mountain opposite to Tawela of Traverse 4 area (Figure 2).
Bolton (1955; Figure 13) named this unit as “Qulqula Conglomerate Formation” and described it as consisting of thick lenticular beds of conglomerates, composed of pebbles, and small boulders of limestone, and to a lesser extent chert. Both the limestone and the chert boulders and pebbles were derived from the underlying Qulqula Radiolarian Formation. This unit is considered to be of Albian–Cenomanian age (Jassim and Goff, 2006). Unit Q5 in this study is referred to the cherty brecciated conglomerate part of the Qulqula Conglomerate Formation of Bolton (1955) and Jassim and Goff (2006).
A detailed section of this unit has been measured and documented within the Traverse 3 area, along the Permaron Valley from the Ta Ta Village down to Sargat Village, about 5 km to the east of Khurmal (Figure 8). The outcrop here is the thickest and the best-exposed and in the area which is considered as the type section for this unit. The section is about 500 m thick and consists of repeated sequences of cherty detrital limestone or calcirudite horizons. Four major horizons were recognized in this area, which implies repetition of the basic unit several times either by multiple thrusting, or folding to form a series of topographic ridges throughout the section (Figure 18e). Each horizon is 10–30 m thick and consists of gray, hard, detrital or fragmented limestone with angular dark gray to brown chert pebbles. These conglomerate horizons alternate with reddish, silty shale, siliceous mudstone and thin chert interlayers (Figure 18f). These reddish siliceous horizons range between 10–40 m in thickness.
The grains of the cherty conglomeritic horizons range in size from boulders to sand, and are usually embedded in calcarenite matrix. The limestone fragments are of dark gray fossiliferous grains of different sizes, which range from granules to boulders, and are often rounded-to-subrounded (Figures 19a and 19b). The large pebbles are often angular to subangular and mainly consist of monotonous dark gray fine-grained limestone (Figure 19c), densely packed in lighter color matrix. In other cases grains are polygenic and poorly sorted, especially when their size is relatively smaller (Figure 19d). In this case the variety of grains includes limestone, chert, red mudstone, and siltstone (Figure 19e). The chert fragments are the characteristic grain type. They often have a dark color and angular form, and usually protrude out upon weathering due to their relative hardness (Figure 19f).
The cherty detrital limestone of unit Q5 is usually bottomed by layers (up to 10 m in thickness) of detrital limestone or conglomerate lacking fragmented chert. In places, discontinuous, irregular thin beds of chert often ranging from 5-40 cm in thickness may occur (Figure 20a). These chert beds are dark in color and commonly are intersected by joints or micro-faults of different attitudes (Figures 20b and 20c) reflecting shearing and intensive internal deformation. Recementation and reorientation of these jointed chert beds are characteristic at Traverse 3 near Hani Dang Village (Figure 20d), and Traverse 4 area near Tawela town (Figure 20e). It indicates active late diagenetic alterations accompanying the tectonic deformation.
Another important and extensive outcrop of this unit is recognized around Kani Manga Village of Traverse 2, about 3 km southwest of Penjwin (Figure 7). The fragments here are either homogenous (Figure 19c), or heterogeneous (Figure 19d), and the outcrop section shows less duplication or repetition and more massive body as compared to its occurrence at Traverse 3 area. The occurrence of this unit here without the anticipated overlying Avroman Limestone, and the occurrence of the Suwais Red Beds next to it instead, possibly implies that erosion had removed the Avroman Limestone from the area. The sole occurrence of this conglomerate represents a relict of the cherty conglomeritic limestone (unit Q5), which exposes the younger and underlying Red Beds as a tectonic window. This area represents the northern narrow tip of the Avroman Limestone block in the area and thus can be easily removed.
The boundary of this unit with the overlying Avroman Limestone is well exposed near Ta Ta of Traverse 3 area. It is characterized by a shear zone, and marked by a thick horizon of trachyte volcanic sill (Figure 12e). The lower boundary with the underlying red mudstone units is also tectonic and commonly shows sharp faulted transition.
Due to the high lithologic contrast between the cherty conglomerate horizons and the shale interlayers, the unit better demonstrates the style and degree of deformation of the Qulqula Group in general. This is well displayed in an outcrop located above the village of Sargat of Traverse 3 (Figure 20f). At this outcrop multiple chevron folds are common and resulted from combined faulting and folding effect.
The overall highly tectonized nature, the fragmented fabric of its rocks, the association with volcanics, and the dominating mixture of limestone and chert fragments suggest that severe crushing resulted from the thrusting of the Avroman block over the Qulqula Radiolarite unit is responsible for the formation of unit Q5.
Ultrabasic rocks and associated igneous bodies are the most interesting features of the Zagros Suture Zone. These bodies were recognized in five different areas by Bolton (1955, 1956, 1958) and documented by Jassim et al. (1982) to include basic, ultrabasic, and metamorphic thrusted blocks. Al-Mehaidi (1975) and Buda and Al-Hashimi (1977) introduced the term “ophiolites” for the ultrabasics of the Mawat area. Buday (1980) gave a comprehensive review of the geology and origin of these ophiolite complexes including those of the Zagros Suture Zone in Iraq. Both the Mawat and Penjwin complexes are exposed in the study area and represent the largest and most complete sequences. They are considered to belong to a single emplacement episode, which took place during the Albian–Cenomanian between 110–80 Ma (Aziz et al., 2011). The Mawat Ophiolite Complex, however, is better differentiated and more complete compared to the Penjwin Complex (Jassim and Goff, 2006). The Mawat Ophiolite Complex is fully exposed in the Traverse 1 area (Figure 6), and part of the Penjwin Ophiolite Complex appears at the Traverse 2 area (Figure 7). The review below is compiled from previous works in addition to the authors’ field and petrographic observations.
Mawat Ophiolite Complex
Mawat is a high mountainous area to the northeast of Sulaimani City, and generally includes stacks of sheets thrust over the imbricate units of the Arabian margin (Figure 4). The highest sheet represents an elongated igneous body extending north-south, about 25 km long and 7–12 km wide, and occupying an area of about 250 sq km. The dominating hard crystalline mafic and ultramafic rocks of the Mawat Ophiolite Complex, control the topographic pattern and geological fabric of the area (Figure 6), and consists of 600–1,000 m thick volcanic, plutonic, meta-volcanics and meta-sedimentary rocks of Cretaceous age (Jassim and Goff, 2006). These magmatic rocks are highly sheared and bear characters of ophiolite suites (Al-Mehaidi, 1975; Buda and Al-Hashimi, 1977; Buday, 1980) or dismembered ophiolites (Aswad, 1999).
The Mawat Ophiolite Complex is generally dominated by a plutonic body of ultrabasic rocks including: pyroxenite (Figure 21a), layered and coarse crystalline gabbros (Figure 21b), diorites, and dolerite dykes. Gabbro is the main component of the complex followed by ultramafic rocks of dunite, harzburgite, lherzolites and pyroxenite, with minor diorite and dolerite intrusions. The harzburgite rocks consist of Mg-rich olivine, which is variably serpentinized with subordinate orthopyroxinite and chromite. Serpentinization of the olivine occurs on grain margins mostly, or along cracks. It most commonly occurs in shear zones and at tectonic contacts (Jassim and Goff, 2006). This massif is overlain by a “roof unit” of 600 m of interbedded marble, basalt and calc-schist unit, which is known locally as the Gimo Group (Al-Mehaidi, 1975). The third important unit of the Mawat Group which comprises amygdaloidal and occasionally pillowed and spilitic flows, 3-10 m thick (Figures 21c and 21d). The largest and most characteristic body is the banded gabbro, which occurs either as layered or laminated fabrics. It is intruded by a coarse crystalline gabbro body in the east, which crops out near the eastern thrust contact. The western contact of the banded gabbro body with the meta-volcanics is a NS-trending shear zone (20-200 m wide) along which the gabbro body margin is highly deformed and crushed, with small basic and acidic intrusions (Jassim and Goff, 2006).
The Mawat Ophiolite Complex overlies the Walash Group all around with a thrust boundary. The contact is marked by a 25-50-m-thick serpentinite horizon. This serpentinite is highly sheared, and according to Aziz (2008), is of two types:
Ophiolite-serpentinite associates are localized at the base of the Mawat thrust sheet. They occur as massive or sheared pale-green serpentinites, 5-10 m thick.
Ophiolitic mélange serpentinites occur as intra-formational bodies or within the lower part of Walash volcano-sedimentary sequence. These serpentinites occur as olive-black colored massifs. The best outcrops of these serpentinite bodies occur and are quarried near Betwat and Kunjirin villages (Figures 21e and 21f), with a total thickness of 40-50 m.
Petrologic studies of these serpentinite bodies show that they represent isolated bodies of lizardite and chrysolite. They are believed to have developed through intra-oceanic subduction (Mohammad, 2004).
Penjwin Ophiolite Complex
The Penjwin igneous complex is dominated by a NW-trending ultramafic elongated body surrounding Penjwin City (Figure 7). It constitutes part of an incomplete ophiolite sequence (Aswad, 1999; Jassim and Goff, 2006). It comprises dunite, pyroxenite, layered gabbro (Figure 22a), and diorite in contact with a volcano-sedimentary sequence referred to locally as the “Gimo Group” (Jassim and Goff, 2006). It shows a continuous sequence from peridotite (dunite, harzburgite, websterite and bronzitite) at the bottom through amphibolitized gabbro to diorite (Figure 22b) at the top (Mohammad, 2008). Pyroxenite is present in very small bodies. The gabbro body directly overlies the ultramafic body. Irregular pegmatite gabbro and pyroxinite dykes occur in the dunite member of the complex. These bodies are interpreted as segregation from, and trapped bodies of, partial melt in the dunite residue (Al-Hassan and Hubbard, 1985). To the west of the ophiolite complex the “Gimo Group “ crops out and comprises schist, phylitic schist, and phyllite, commonly associated with lenticular and recrystallized limestone, calc-schist, meta-tuffaceous and amphibolites (Jassim and Goff, 2006). Highly sheared serpentinite offshoots penetrate the gabbro in this area (Figure 22c), and are believed to have formed by subsequent mobilization along the shear zone (Jassim and Goff, 2006).
The Penjwin igneous complex was thrust over the clastic rocks of the Walash and Suwais Red Beds, along a low-angle reverse fault (Figure 7). The contact is tectonic and often associated with shear serpentinites, which acted as a lubricant layer between the igneous body and the ductile underlying sedimentary sequence. The northeastern contacts of the complex are a major thrust zone with the Shalair metamorphic group. It corresponds to the Zagros Main Reverse Fault of southwest Iran.
This sedimentary-volcanic unit often occurs below both ophiolite sheets of Traverses 1 and 2 (Figures 6 and 7). The Walash Group, which is named by Bolton (1958) after the Walash Village in the Rowanduz area, is of Paleogene age. The type locality description of the group shows local differences, prompting Jassim and Goff (2006) to compile a composite section of the group. This section includes the following units from base upwards: (1) basal red beds, (2) lower basaltic lava, (3) middle red beds, (4) upper basaltic-andesitic flows and pyroclastics, and (5) upper red beds.
A limited outcrop of the Walash Group is exposed on the right-hand side of the road to Penjwin of Traverse 2 area, about 1.5 km to the north of Kani Manga Village. The group here has been confused with red beds sequence. Both are deeply eroded forming major valleys in the area with thick cover of recent sediments, which has lead to its overlooking in the geological map of the area (Figure 7). The exposed section represents the upper part of the Walash sequence. It is about 10 m thick (the rest is covered by recent sediments). Here it is characterized by buff-to-olive-green silty calcareous shale alternating with thin-to-medium-bedded sandstone units. Sandstone beds are graded with a sharp lower boundary and transitional upper boundary (Figure 22d). The upper boundary of the Walash Group, with the overlying serpentinite body, is seemingly transitional reflecting contact alteration (Figure 22e).
The best-exposed and most complete outcrop of the Walash Group is located in the Traverse 1 area, around Chwarta, which is slightly different from Rowanduz type-section area. It is dominated by a buff-to olive-green flysch-type sequence, which is characteristically intruded by different types of volcanic bodies. The group surrounds the ophiolite body as a narrow outcrop belt of less than one kilometer to the west and more than 10 km to the southeast side of the ophiolite body (Figure 6).
The group varies greatly in thickness due to intensive faulting of the area. However the maximum thickness does exceed 200 m. The upper part of the series shows foliation and shearing with frequent occurrences of volcanics. The lower part, on the other hand, shows less shearing, limited volcanic influences and a proper flysch-type sequence (Figures 23a and 23b).
The sedimentary sequence displays cyclic sedimentation of occasionally sheared, fissile, calcareous, silty shale (Figure 23c), alternating with thin to thick beds of medium to coarsegrained, graded bedded sandstone (Figure 23d). Petrographic examination of these sandstones shows it is dominated by calcarenite, rich in benthic foraminifera (Figure 23e). Occasional thin marly limestone to limestone horizons were recognized. Shale intervals range between one and ten meters in thickness, and are commonly dark gray and yield Paleogene planktonic foraminifera. The group’s upper boundary with the overlying ophiolite sheet is highly tectonized and locally associated with a thick serpentinized horizon.
The volcanics vary in size and composition, which display a variable weathered appearance. In most cases they are concordant within the sedimentary sequences (Figures 23a and 23b). They include tuffs, lapillis, pyroclasts and thin sills of light-gray to reddish-gray color. These volcanics are classified into basic dykes, lava flows of spilitic diabase, pyroxene-bearing spilitic basalt (Figure 23f), spilite, and intermediate volcanic (pyroxene andesite, pyroxene-amphibole andesite, and altered andesite (Aziz, 1986; Jassim and Goff, 2006). Locally, as in the area to the north of Chwarta, the group is capped by isolated limestone bodies of nummulitic shoals (Al-Hashimi, 1975). These bodies represent a shallowing-upward sequence of slope to shallow platform facies. These limestones display intensive shearing in the form of stylolite networks, imbricate microfaults and fractures, which imply transportation during successive deformational phases (Surdashy, 1997).
The lower boundary of the Walash Group is a tectonic boundary and represented by a segment of the Zagros Thrust Front. This boundary in the Traverse 1 area is less expressive and can be followed along valleys separating the color-contrasted sediments of the buff-green Walash Group from the underlying Suwais Red Bed Group. The lower boundary in Traverse 2 area with the Suwais Red Bed Group is concealed under recent sediments.
Isolated Jurassic limestone hills crop out as out-of-place ridges within younger strata. On the geologic map of the area they appear as small, elongated, patches of limestone bodies distributed in front of or within the Qulqula Group outcrop belt along three NW-trending faults. It appears in Traverses 2 and 3 areas (Figures 7 and 8). These exotic Jurassic limestone bodies have been defined as carbonates of the Sehkaniyan, Sargelu, Naokelekan and possibly the Barsarin formations and named as “Jurassic Imbricates” by Jassim and Goff (2006). However, the Czech Team (1976) characterized them as carbonates of the Sargelu and Sehkaniyan formations. The Jurassic Imbricates, disregarding their origin, were exhumed to the surface along NE-dipping, deep-seated ramp thrust faults during the Late Miocene–Pliocene tectonic event when the whole Cretaceous–Pliocene sedimentary cover was up-thrust over the Upper Jurassic decollement unit (Ibrahim, 2009).
Exposures of these imbricates were examined around Kolitan and Kaolos villages and along the road from Said-Sadiq to Penjwin of Traverse 2 area (Figure 7). The outcrop near Kolitan generally shows the occurrence of massive-to-thick-bedded gray, hard limestones with microfacies similar to the Upper Jurassic Chia Gara Formation (van Bellen et al., 1959-2005) (Figure 24a). Another outcrop near Kaolos Village shows dark gray stromatolitic fabric similar to the Sargelu Formation. Fracturing is common, indicating shearing. Near Ahmad Awa (Traverse 3), these limestone bodies steeply protrude as a fault scarp of about 25 m above the surroundings, facing southwest with the scarp face striated by slickensides (Figure 24b). The regional dip of these limestone bodies is to the northeast. Their patchy distribution within the clastic parts of the Qulqula Group protects the underlying gray or red mudstone, and forms isolated hills especially along the Said Sadiq-Byara road.
ROCK UNITS OF THE ZAGROS IMBRICATE ZONE
The stratigraphic units of the Zagros Imbricate Zone are represented by Cretaceous–Cenozoic sediments of the Arabian margin. Because of their marginal location they underwent intensive deformation but without actual displacement. The thrusted sheets and blocks of the Zagros Suture Zone lie directly on these units with the Zagros Thrust Front separating the two zones. An important part of the deformational features and stacking pattern of these thrust sheets depends on the type and nature of the sediments of the imbricate zone. The review of these units here is to help understand that inter-relationship. The review covers the units starting from the closest one to the suture zone.
Suwais Red Beds Group
The Suwais Red Bed Group outcrop surrounds the thrust sheets of the Zagros Suture Zone in a narrow belt swinging about the forethrust blocks as in the Traverse 1 of Mawat area, or behind the thrust block due to the erosion of a “tectonic window” as in Traverse 2 of Penjwin area (Figure 7). The outcrop of the Suwais red beds around Chwarta City of Traverse 1 area represents the most extensive in the studied area. It exhibits a thick siliciclastic sequence with characteristic overall red color. It typifies the Zagros Imbricate Zone by reflecting different types of deformational imbricate structures of multiple faulting and refolding structures. The intensive deformation and the ductile nature of its clastic sediments amplify the imbrication of its rocks to form, in some cases, misleading angular unconformities with over-thrusted strata. Imbrication of this unit is intensified because it is sandwiched between the hard rocks of the thrust sheets of the suture zone to the northeast and the carbonate-clastic sequence of the imbricate zone to the southwest.
Al-Mehaidi (1975) subdivided the group in the Chwarta area into four units (from bottom to top): (1) fine clastic of ferruginous red shale, blue siltstone, and sandstone with some conglomerate and limestone beds, (2) gray and thickly-bedded sandstone and thin red claystone, and (3) thickly-bedded to massive conglomerate with pebbly sandstone interlayers, and (4) a unit of limited thickness, consisting of alternating sandstone, pebbly sandstone, conglomerate and red shale. Al-Barazanji (2005) revised the stratigraphic and sedimentological status of the Red Bed Group around Sulaimaniyah and proposed a new subdivision scheme of six units: (1) Lower Fine Red Clastics Unit; (2) Lower Conglomerate-Clastic Unit; (3) Sandstone Unit; (4) Mixed Fine and Coarse Clastic Unit; (5) Upper Conglomerate Unit; and (6) Upper Fine Clastic Unit. The age of the series is believed to range from Paleocene to probably Mid-Miocene (Al-Mehaidi, 1975). The depositional setting of the red beds is discussed by Al-Qayim (2000) and Karim et al. (2007), who suggested that it represents the coastal equivalent of the Paleogene Kolosh Formation flysch sediments of the Zagros Foreland Basin.
About 3 km to the southeast of Penjwin Town a narrow outcrop of the unit is recognized between the Walash-Penjwin Ophiolite Complex sheets, on one side, and unit Q5 sediments of the Qulqula outcrop on the other (Figure 7). The tectonic and geological setting of this outcrop indicates its possible occurrence as a tectonic window breached behind the Qulqula Radiolarite Sheet, and then overthrusted by the Walash Group and the Penjwin Ophiolite body from the northeast. The thrust line is covered by recent sediments with a low lithologic contrast. The sequence is partly covered by the valley-fill recent sediments. The thickness of the exposed section exceeds 300 m. The general lithologic features are alternating medium-to-thick bedded sandstone units with red silty mudstone (Figures 24c and 24d), or shale. Sandstone horizons are sometimes thick, coarsegrained and occasionally bottomed by a layer of coarse conglomerate (Figure 24e). The upper parts of these sandstones are often laminated. Shale interlayers are thicker and often covered by recent sediments. These sediments seems to be equivalent to unit four (Upper Sandstone and Shale Unit) of Al-Mehaidi (1975) subdivisions which can be seen near Maokaba to the east of Sulaimani City (Figure 24f).
The Balambo Formation represents the Early Cretaceous, pelagic argillaceous-carbonate unit of the pre-foreland sequence of the Arabian margin (Buday, 1980). The formation crops out extensively in the study area, especially at the southwestern part of Traverses 1 and 2 areas (Figures 6 and 7). The exposed section of the formation in this area is about 225 m and generally consists of well-bedded argillaceous gray limestone, shale with occasional black chert horizon. The sequence shows alternation of thinly-bedded globigerinal limestone and marlstone. Limestone beds are occasionally silicified and marly. Its age extends from Late Aptian to Turonian (Ghafor et al., 2004; Abawi and Hammoudi, 2008). The lower contact of the formation is not exposed and therefore only the upper part is represented in this area. The formation is exposed at the crestal and upper parts of both flanks of Azmur Mountain to the east of Sulaimaniyah (Figure 6). The southwestern flank of the anticline is steeper and intersected by an axial reverse fault. This is considered to be an important segment of the High Zagros Reverse Fault and represents the southwestern limit of the imbricate zone in this area (Figure 25a). The northeastern flank shows intensive imbrication where imbricate structures are manifested as tight and chevron folds, mesofaults assemblages, multiple reverse faulting, shear and orthogonal joints and in-sequence repetition. Mesoscopic analysis of these structures reveals a maximum principle stress axis trending in NE-SW and ENE-WSW direction (Al-Jumaili and Adeeb, 2010).
An extensive exposure of the formation is reported from the Halabja area. Only a small part of this exposure appears at the lower left corner of Traverse 4 area (Figure 9). The Balambo Formation in this area shows alternating thick beds of dark gray limestone with dark gray marlstone. In the Wazgil Valley, near Hawar Village to the east of Halabja, the formation is over-thrusted by unit Q1 of the Qulqula Group (Dark Gray Shale Unit). The boundary is tectonic and highly sheared whereby extracted blocks and pieces from the Balambo Formation, incorporated into the Qulqula shale. Other imbrication structures are also clearly recognized in the middle part of the Balambo Formation. At this part the sequence becomes medium to thin bedded limestone with no or little marl interlayers. The Balambo Formation sequence in many places was located close to the Zagros Thrust Front Fault where active compression was absorbed by the folding and faulting of the imbricate zone (Figure 25b).
The Kometan Formation represents the pelagic facies of the Lower Cenomanian–Turonian sequence of north Iraq (van Bellen et al., 1959-2005). It is exposed in the Traverses 1 and 2 areas (Figures 6 and 7). The best outcrop in the study area, however, is exposed around Azmur Mountain of Traverse 1 area, which unconformably overlies the Dokan Formation. Biostratigraphic analysis of this boundary indicates missing strata of the Gulneri Shale Formation and imply a hiatus of Late Albian to Mid-Cenomanian (Abawi and Hammoudi, 2010). It consists of white-to-light-gray, hard, uniform and medium-bedded limestone up to 50 m thick. It is characteristically stylolitic and jointed and occasionally cherty. The uniform stratification of the formation and its fresh exposures along the road across Mount Azmur display some of the imbricate structures such as second-phase of folding and faulting. The upper contact of the formation with the overlying Shiranish Formation is unconformable, and marked by a thin horizon of greenish-gray pebbly and glauconitic limestone (van Bellen, et al. 1959-2005).
The Shiranish Formation of the early Zagros Foreland Basin is of Late Campanian–Maastrichtian age (van Bellen et al., 1959-2005). It represents part of the pre-flysch sequence of the foreland basin. It is followed by the flysch sediment of the Tanjero Formation. The boundary between the two units is transitional. It is well exposed around Azmur Mountain of Traverse 1 area, with relatively thin occurrences on the mountain’s southwestern limb due to the replacement by its stratigraphic equivalent the Tanjero Formation. Its lithology shows bipartite subdivision, the lower part formed alternating gray, thick-bedded marly limestone with bluish-gray marlstone, the upper part completely consists of greenish gray calcareous marlstone which becomes silty near the top (Al-Qayim et al., 1986). The thickness of the formation in the Azmur Anticline is about 50 m. However, it shows variability due to its ductile nature and tectonic flowage upon folding.
The upper contact with the Tanjero Formation appears gradational, with lithologic differences, and can be placed at the first appearance of a thin sandstone bed (van Bellen et al., 1959-2005). This indicates the progradation of the clastic wedge of the Tanjero Formation from northeast onto the foreland basin. Deformational structures are less expressive due to the generally homogenous lithologies and flowage nature of its argillaceous-dominated parts.
The Tanjero Formation represents the thick Upper Cretaceous–Paleogene flysch sequence of the early Zagros Foreland Basin, which developed during the early stage of collision between the Arabian margin and the Tethyan subduction complex to the northeast (Al-Qayim, 1993). The formation is exposed over most of the study area. However, it is best exposed and documented in the Traverse 1 area (Figure 6). The sequence reaches 1,000 m in thickness, and characteristically consists of turbidite facies representing a submarine fan complex (Jaza, 1992; Al-Qayim, 1994; Karim, 2004). The rocks are generally characterized by olive to buff green rhythmic alternation of shale, siltstone, and sandstone, with less frequent conglomerate.
The formation is widely exposed, over both limbs of the Azmur Anticline. However, on the northeastern limb of the anticline the formation is characterized by the occurrence of the so-called Aqra tongue, which represents the fossiliferous limestone equivalent of the reefal Aqra Formation of Erbil-Dohuk area. This lenticular body extends as a prominent ridge for about 18 km along the anticlinal limb (Lawa et al., 1998). It consists of 50-150-m-thick sequence of alternation of thick-bedded marly fossiliferous limestone and silty and marly shale interlayers. Limestone beds are rich in reefal fauna especially rudists. It becomes thicker and more massive towards the top. According to van Bellen, et al. (1959-2005) the age of the formation is considered to be Late Campanian to Maastrichtian throughout. Abdel-Kireem (1986) however, in his study of the formation in Sulaimaniyah area, interpreted its age between Mid-to Late Maastrichtian.
The upper contact of the Tanjero Formation with the overlying Suwais Red Beds Group is seemingly conformable in some places, and unconformable in others. In areas of possible unconformable relations thrust faulting is postulated and might lead to the misinterpretation of that relationship (Al-Qayim and Lawa, 2006). The Tanjero Formation sediments better exhibit imbricate structures due to their inhomogeneous lithologic character.
In this section we use our detailed fieldwork and observations in the Sulaimaniyah area to correlate Iraq’s rock units with those in the Iranian Zagros Suture Zone. Recent studies in the Kermanshah and neighboring areas by Mohajjel et al. (2003), Agard et al. (2005) and Robin et al. (2010) have been referred to and are used to support our interpretations of these units in the study area.
Bisotun Isolated Platform
The Avroman Limestone Unit of Eastern Halabja area has a limited distribution within Iraq. It is an elongated massive limestone body, which continues across the Iranian border to form a huge block of similar lithologies and sedimentary facies. Fossil assemblages of the Avroman Limestone in the study area indicate (Late Triassic) Norian–Rhaetian age (Jassim and Goff, 2006). The major Iranian integral part is termed Bisotun Limestone. It is a 3,000-m-thick block of shelf carbonate deposits ranging in age from Late Triassic to Cenomanian. It characterizes the Zagros Suture Zone of Iran, especially to the north of Kermanshah (Ricou et al., 1977; Mohajjel et al., 2003; Bordenave and Hegre, 2005; Agard et al., 2005). Their different sedimentary facies and thicknesses from the Arabian margin, and their age, suggests deposition in a distinct paleogeographic domain, separated from the Arabian Platform by a radiolarite trough (Ricou et al., 1977), during Mid-Triassic rifting (Mohajjel et al., 2003; Bordenave and Hegre, 2005; Agard et al., 2005). We have no reason to differentiate the Avroman Limestone of Iraq from the Bisotun Limestone of Iran. The similarity in age and its tectonic position and the thrusting over the radiolarite of the Qulqula Group indicate a similar origin. Thus we interpret the Avroman Limestone of the Zagros Suture Zone of Iraq as an integral part of the Bisotun paleogeographic-paleotectonic setting.
The radiolarian chert and limestone of units Q3 and Q4 of the Qulqula Group in all examined areas seem to bear typical characteristic features of the radiolarite facies. Their uniform bedding, the deep water and pelagic facies of the bedded radiolarian chert and its variable colors (dark gray, light-gray-reddish, and green), and the associated siliceous shallow-marine limestone, are characteristic features of radiolarite facies. The intensive structural deformation as compared to the deformation of the other neighboring units, and association with other suture zone components, all reflects their origin and tectonic history. The age of these radiolarite facies at the studied area ranges from Valanginian to Albian (Jassim and Goff, 2006). Stratigraphic evidence for the commencement of the obduction process of the Tethyan ophiolite-radiolarite and other units over the Arabian margin goes back to the Late Turonian (Sharland et al., 2001). The clear tectonic discontinuity between the Qulqula sheets and the underlying Balambo Formation limestone of the Arabian margin witness their displacement. Therefore, these tectonic facies were conceivably obducted as slivers from a deep oceanic regime onto the Arabian margin during Late Cretaceous convergence (Al-Qayim, 1994; Numan, 1997; Al-Qayim, 2004; Jassim and Goff, 2006; Ibrahim, 2009).
Similar facies have been recognized from different areas in the Iranian Zagros Suture Zone (Pamic et al., 1979); from the Chega Gorge area around Kermanshah (Alavi, 1994); to the east and southeast of Kermanshah (Mohajjel et al., 2003); from Kermanshah to Dorud (Agard et al., 2005); Pichakun Mountains, Neyriz area, and Pichakun Nappes (Robin et al., 2010). These radiolarites correspond to units QU-2 and QU-3 of Jassim and Goff (2006).
The fine-grained clastic parts of the Qulqula Group, which includes units Q1 and Q2 in this study, were excluded from the radiolarite facies due to their different lithologic association and the high degree of deformation and mixing with exotic bodies and other tectonic facies. It is considered a part of the “mélange facies” which are closely intermixed with the Qulqula Group during thrusting and transportation over the Arabian margin.
A mélange according to Flower and Dilek (2003) is a tectono-stratigraphic facies composed of blocks of oceanic rocks, platform carbonates, and metamorphic rocks occurring beneath ophiolite complexes and their metamorphic soles. It is commonly made of imbricate thrust sheets whose direction of tectonic transport is in accordance with the ophiolite emplacement direction. Detrital materials are of platform carbonates ranging from 1 cm to km-sized blocks of deep-water limestone, locally intercalated with sheared radiolarian chert, black chert, and litharenite, which occur in a clay-rich argillaceous matrix. Locally, this mixture shows a well-developed scaly fabric, and often represents distal parts of passive-margin sequences on which Neo-Tethyan ophiolite were emplaced. The sedimentary mélange unit shows locally chaotic, olistostromal characters with occasionally preserved layering and stratigraphy.
Similar facies from the Zagros Suture Zone at the Neyriz Ophiolite Complex in western Iran, known as the “Color mélange”, represents a subduction-accretion complex developed during the collision of rollback continental margin (Sarkarinejad, 2003).
The dark-grayish-greenish gray shale unit Q1 and the reddish siliceous mudstone unit Q2 of the Qulqula Group are believed to represent a sedimentary mélange, incorporated into the Qulqula Radiolarite blocks during emplacement and transportation over the Arabian margin. Both are argillaceous-carbonates and represent a deep-marine sequence with preserved original stratigraphic features. Crushed volcanics are intermixed with these sedimentary sequences and may be responsible for their variegated colors and chaotic fabric. Olistostromes of limestone, bedded chert, of different sizes characterize these units in different localities, especially where intensive folding and faulting has affected these sheets. Similar features were recognized by Stoneley (1981) along the Zagros Suture Zone in Iran, to the southeast of Saadatabad. He considered these units as a part of the mélange zone which represents shelf to-deep-marine facies of the Neo-Tethys Ocean and part of the Arabian-Zagros platform. The estimated age of these units ranges between Maastrichtian and Early Miocene.
The cherty carbonate conglomerate unit Q5 seems to show a completely different lithologic character as compared to the other units of the Qulqula Group. Fragmented and bedded chert is characteristic in a conglomeritic and brecciated limestone matrix. The carbonate grains are similar to the Avroman Limestone facies. Some grains are dark gray, fossiliferous limestone, sometimes rounded, while others are angular, ranging from sand to boulders size. Chert fragments are dark brown to gray, always angular with variable grain size and sometimes taking specific euhedral forms. Bedded chert can exceed 50 cm in thickness with intensive folding and fracturing and often occurs at the bottom the fragmented cherty limestone units. The close association of this unit to Avroman Limestone in all the examined localities, and the similarity of its carbonate detritus to the carbonate facies of the Avroman Limestone, the dominating carbonate cement, the occurrence of bedded chert support its formation in a deep-marine environment developed next to the Avroman Block during pre-displacement time.
The abundance of chert fragments of similar types to the radiolarite chert, the mixture of both units’ material, the shearing and intensive deformation suggest that this unit was crushed by the thrusting processes of the Avroman Block over the Qulqula radiolarian rocks, and finally, sandwiched between the two units as thrusting progressed. The wide range of ages (Cenomanian–Turonian) given to the unit by the Czech Team imply both matrix and grain-derived age. Robin et al. (2010), from their examination of the Pichakun Nappes to the north of the Neyriz area of the Zagros Suture Zone of Iran, recognized similar conglomerate facies and suggested that its deposition might have been in a narrow deep trough located next to the seaward edge of the isolated carbonate platform of the Avroman Limestone. Their thick section along Permaron Valley of Traverse 3 area may be related to the repetition of their facies (Figure 8), and may have resulted from internal multiple imbrications and faulting, which support its crushed-in-between origin.
Exhumed Jurassic Imbricates
The Qulqula sheets in the studied areas include exotic marine Jurassic limestone bodies cropping out in frontal parts of these sheets. These limestones can be kilometers in dimension, and have been examined from different localities in the studied area. Fossil contents and micro-facies type indicate their belonging to the Sehkaniyan, Sargelu and Naokelekan formations (Jassim and Goff, 2006). Field and petrographic examination of some of these bodies in the study area show similarities to the Chia Gara Formation as in the outcrop near Kolitan Village. Others show dark gray to black stromatolitic limestone of the Sargelu Formation as in the outcrop to the north of Kaolos Village. Association of some of these limestone blocks with fault scarps as is the case around Ahmad Awa of Traverse 3 area (Figure 8), the-out-of-place positions within the Qulqula Group, and their similarities to the platform carbonate of the Arabian margins suggest that it represents an exhumed limestone block emplaced by fault imbrication in front or within the obducted radiolarite slivers during its final stage of emplacement.
There is little doubt about the ophiolitic origin of the ultramafic rocks of the Mawat and Penjwin-Halabja areas. Comparative petrologic study between the two complexes shows similarities in most respects, and that they have undergone similar magmatic and post-magmatic histories. Even the metamorphism of parts of these bodies is similar and is believed to have developed upon marginal shearing during upwards movement and emplacement of these ophiolite masses (Jassim and Al-Hassan, 1977). Geochemical analysis of Mawat ophiolites indicates its ophiolitic origin and relation to the Eastern Mediterranean ophiolite suites (Al-Hashimi and Al-Mehaidi, 1975; Buda and Al-Hashimi, 1977). Comprehensive geochemical investigation of differentiation trends of these ultramafic bodies shows that they belong to a young Mesozoic orogenic phase (Jassim et al., 1982). Comparative petrochemistry of both complexes shows that both belong to Albian–Cenomanian ophiolite suites (Jassim and Al-Hassan, 1977; Aswad, 1999). In addition, Aswad (1999) indicated that these units represent dismembered ophiolite sequences.
Jassim et al. (2006) using TAS plots of geochemical data from ophiolite complexes believe that the Mawat Complex is a more complete sequence as compared to the Penjwin Massif. These units were obducted on the Arabian margin as a result of the collision with the Iranian Plate during Late Miocene time (Buda and Al-Hashimi, 1977). Aswad (1999) believed that the origin of these ophiolites could be related to a mid-oceanic ridge domain. The mode of emplacement of these units over the Arabian Plate margin is believed to have taken place by thrusting over the Qulqula during the early stage of collision (Early–Mid-Miocene). As collision progressed the ophiolite bodies slid down by gravity over the Cenozoic cover (Aswad, 1999, and Farjo, 2008) (Figure 6).
Emplacement of the Mawat Complex was believed to have occurred together with the Walash Group over the sediments of the Suwais Red Beds Group during the collision process (Buda and Al-Hashimi, 1977). A similar mechanism is postulated for the Harsin-Sahneh (Kermanshah area) in the Iranian Zagros (Agard et al., 2005). The geochemical constraints of the Mawat ophiolites pointed to an intraplate-island-arc geotectonic setting. The mid-Cretaceous age of the subvolcanic member of the oceanic crust of the ophiolitic massifs of Mawat is confirmed by K-Ar dating of 97-118 Ma (Aswad and Elias, 1988). This is a time preceding their emplacement over the Arabian margin and is correlative with the Kermanshah area ophiolites. The origin of the serpentinite imbricates, which are associated with the ophiolite complexes, is related to two different settings: (1) highly sheared serpentinites at the lower contact of the ophiolitic massifs (upper allochthonous), and (2) an ophiolitic mélange on the base of lower allochthonous nappe showing a block-in-matrix aspect, with cm-to-km fragments (Aziz, 2008).
Paleogene Fore-arc Basin
The sediments of the Walash Group, its stratigraphic position, microfossil assemblages, and its association with arc volcanics in Traverses 1 and 2 areas show evidences of a Tethyan flysch basin. The cyclic alternation of sandstone and shale, the graded bedding, groove-casted, sharp bottom sandstone horizons in a silty calcareous shale of deep-marine environment, all indicates flysch-type sequence (Al-Qayim et al., 2012). The associated volcanics, which become common near the top of the sequence are of two types: (1) basic dykes, lava flow of spilitic diabase, pyroxene-containing spilitic basalt and spilite, and (2) an intermediate volcanic (mainly andesites) sequence. Both types are believed to represent arc volcanic suites (Jassim et al., 1982; Aziz, 1986).
The Paleogene age of the Walash volcanites is confirmed by 40Ar/39Ar ages of 32-43 Ma (Koyi, 2006). Their occurrence within a flysch-type sedimentary sequence suggests that they were deposited in a fore-arc basin developed in front of the Sanandaj-Sirjan Block (Figure 26d). Similar tectonic facies have been recognized from the Kermanshah area of the Zagros Suture Zone of Iran (Mohajjel et al., 2003; Agard et al., 2005). The combination of the volcanic rocks within a flysch basin suggests a fore-arc setting (McCarron and Smellie, 1998). Such a basin formed in front of an arc system behind the Late Cretaceous obducted sheets of ophiolite-radiolarite suites (Al-Qayim, 2004). Tectonic juxtaposition of the Walash and ophiolite nappes occurred during the collapse and subduction of a Paleogene arc basin at the end of the Paleogene (Aziz, 2008).
TECTONIC EVOLUTION: DISCUSSION
The stacking pattern of the major sheets or blocks of the Zagros Suture Zone from bottom to top seems to be persistent in most of northeastern Iraq and always shows two different stacking sets. The first is (from bottom to top) the sedimentary mélange sequence followed by the radiolarites, and the above-all Bisotun Limestone Block. Boundaries between these blocks or units are always tectonic and marked by major thrust faults with displacement towards the west-southwest over the Arabian margin.
The second stacking sequence includes (from the bottom) the fore-arc basin sequence of the Walash Group and the ophiolite massifs. The Walash Group has an irregular belt of outcrop, and always occurs underneath the ophiolite bodies. Its lower boundary is presumably a major thrust plane, the Zagros Thrust Front. However, it is often hidden beneath valley-fill recent sediment or missed due to the weak lithologic contrast between the underlying siliciclastic sediments of the Suwais Red Bed Group and the overlying shale of the Walash Group (Figure 7). The upper boundary with the ophiolite displays a low-angle thrust fault and is commonly marked by a zone of serpentinization. Emplacement of this highly-sheared serpentinite is believed to have developed as a meta-ultramafic mélange, which acts as a lubricant for the ophiolitic Alpine-type harzburgite and dunite during the Late Cretaceous obduction (Mohammad, 2008; Aziz, 2008).
Intersection between these two seemingly different stacking sheets can be seen to the east of Mawat area, and the area south of Penjwin Town, which indicates separate tectonic packages different in time, movement direction, original domain, and deformation style. The estimated ages of these units, their tectono-stratigraphic characters, manner of stacking and imbrications, and the analogy with other models suggested for the same zone southeastward into Iran (i.e. Mohajjel et al., 2003; Alavi, 2004; Agard et al., 2005), are used here to construct the following scenario of the tectonic evolution of the Zagros Suture Zone of northeastern Iraq (Figure 26). Previous local studies and scenarios of this tectonic evolution (e.g. Numan, 1997; Al-Qayim, 2004; Aziz, 2008; Ibrahim, 2009; Aqrawi et al., 2010) are well considered in this scenario. The scenario assumes that the two stacking patterns are linked to two separate and most influential tectonic events.
The first is the obduction of radiolarite-mélange, and associated magmatic bodies (Aziz, 2008) over the Arabian margin during the period from the Coniacian to the Campanian (Sharland et al., 2001; Agard et al., 2005; Jassim and Goff, 2006; Al-Qayim, 2010; Figure 26c). Accompanying to that was the evolution of a foreland basin in front of the obducted blocks. The types and composition of the detrital grains of the foreland basin’s flysch sediments (Mid-Maastrichtian-Eocene) support this inference (Al-Rawi, 1980; Al-Qayim, 1993, 1994, 2004; Karim, 2004).
The second major event is related to the post-Mid-Miocene continental collision of the Arabian Plate margin with the attached formerly obducted slivers on one side and the Sanandaj-Sirjan Block on the other (Figure 26f). The intra-orogenic quiescent interval from the end of the filling of the foreland basin (late Mid-Eocene) to the Mid-Miocene was characterized by a clastic-free sedimentation in a sag-interior basin over the Arabian Plate (Al-Qayim, 2006). The clastic sedimentation resumed in the area by the introduction of the shallow-marine mixed siliciclastic-carbonate sediments of the Mid-Miocene Fatha (Lower Fars) Formation, which signaled the commencement of the early stage of the continent-continent collision (Figure 26e). At the same time the stacked ophiolite and fore-arc sequence of the accretionary prism, which had developed in front of the Sanandaj-Sirjan Block was also emplaced over the Arabian Plate margin. As the collision progressed compression and the intensive degree of shortening led to the “over-passing” of the newly emplaced blocks of the accretionary prism over the old stacking blocks of the Cretaceous obduction. Uplift and shortening due to this collision was responsible for the major deformation observed in the Zagros Suture Zone (Oweiss, 1984). The Zagros Orogenic Belt thus evolved progressively, with continuing shortening with the degree of deformation increasing towards the Zagros Suture Zone.
The points below summarize the main tectonic events in a chronological order, and shown schematically by cross-sectional presentation (Figure 26) the evolution of the Zagros Suture Zone.
Mid-Triassic rifting, separation, and development of the Bisotun carbonate block (Avroman Limestone) on a horst block off the Arabian margin forming an isolated carbonate platform of Tethyan regime (Mohajjel et al., 2003; Bordenave and Hegre, 2005; Agard et al., 2005).
This resulted in the development of a long-lasting radiolarite graben trough in between the Arabian passive margin and the Bisotun Block (Mohajjel, et al., 2003, and Bordenave and Hegre, 2005). The deep sub-oceanic trough was filled with radiolarian limestone, chert and shale of the Qulqula Group which lasted from Mid-Jurassic through the Cretaceous (Buday and Suk, 1978; Jassim and Goff, 2006).
Late Cretaceous obduction of radiolarite, associated sedimentary mélange, Bisotun Block and the ophiolite mélange over the Arabian margin was a result of continuous convergence and subduction of Arabia (Mohajjel et al., 2003; Alavi, 2004; Agard et al., 2005). As a result, an early deformation and metamorphism phase is evident from intra-basinal unconformities (van Bellen et al., 1959-2005; Dunnington, 1958), and the emplacement of serpentinite bodies (Jassim and Goff, 2006; Mohammad, 2008; Aziz, 2008). Consequently, the Arabian margin became the site of a flysch-dominated foreland basin with flexural subsidence resulting from the loading of obducted sheets, which contributed to the huge amount of the detrital sediments of the basin (Al-Qayim, 1993, 1994, 2004; Alavi, 2004).
A second and major thrusting event post-dated the deposition of the Suwais Red Beds (Paleocene–Miocene), as a result from a continental-continental collision between the Arabian margin and proto Zagros, and the Sanandaj-Sirjan Block (Alavi, 2004; Agard, et al., 2005; Jassim and Goff, 2006).
As a result accretionary prism stacking of ophiolite complexes (Mawat-Penjwin complexes) and the underlying fore-arc sheet (Walash Group) were deeply emplaced, together and simultaneously over the previously obducted units, and in some places, over-passing them onto the Arabian margin sequence.
Final suturing further complicated the area by generating a progressive shortening and intensify deformation of the Arabian margin and the successor foreland sequence with ultimate growth of the Zagros Suture Zone, the Zagros Imbricate Zone and the Zagros Folded Zone.
A representative segment of the Zagros Suture Zone, located in Kurdistan Region, northeastern Iraq, preserves an important episode of the convergence history between the Arabian Plate and the Sanandaj-Sirjan Block. Major components of this zone include: an early stacking related to the Coniacian–Campanian ophiolite-radiolarite obduction of over-thrusted sheets, which includes: the sedimentary mélange (volcanics and siliceous siliciclastic units of the Qulqula Group (units Q1 and Q2), radiolarites (bedded chert and siliceous limestone of the Qulqula Group, units Q3 and Q4), and the Triassic platform carbonate (Avroman Limestone Formation). The second and succeeding stacking sheets are related to the accretionary prism developed on the northeastern margin of a remnant Neo-Tethys Ocean. It includes the fore-arc flysch sequence (Walash Group), and the overlying ophiolite sheets (Mawat and Penjwin complexes). The emplacement of this package is associated with the Mid-Miocene continental collision of the Arabian Plate with the Sanandaj-Sirjan Block. Tectono-stratigraphic analysis of these units contributes to the constraints of the convergence history of Zagros in this area, which shows little differences from that of other parts along the Zagros Orogenic Belt.
The authors would like to thank the Swedish Research Council, VR and SIDA for their financial support to this joint research project between Uppsala University, Sweden, and Sulaimani University, Iraq. The critical review and valuable suggestions of Roger Davies and Moujahed Al-Husseini of Geo-Arabia are sincerely acknowledged. The authors thank GeoArabia’s Assistant Editor Kathy Breining for proofreading the manuscript, and GeoArabia’s Designer Arnold Egdane for designing the manuscript.
ABOUT THE AUTHOR
Basim Al-Qayim earned his BSc and MSc degrees in Geology from the University of Baghdad, Iraq, and his PhD in Stratigraphy from the University of Pittsburgh, USA. Since then he has taught in several universities including Salahaddin University (Iraq), University of Baghdad (Iraq), Sana’a University (Yemen), and University of Sulaimani (Iraq). Since 2004 he has been working as Professor of Geology at Sulaimani University, Iraq. Basim has published more than 60 scientific papers on the geology of Iraq. He is a member of the editorial board of two national geological journals. His research interests include sequence stratigraphy, tectonostratigraphy, sedimentology, and petroleum geology of the Zagros Fold-and-Thrust Belt of Iraq.
Azad Omer earned his BSc at Sulaimani University, Iraq, and his MSc in Structural Geology from Mosul University, Iraq. During the years 1987-1993 he worked as a supervisor in the Geotechnical Laboratory of Sulaimani, Iraq. After that he joined the Department of Geology, Sulaimani University as an Assistant Lecturer. In 2009 he successfully defended his PhD thesis on the structural style of the Zagros segment of Iraq, and earned his degree from the Department of Geology, Sulaimani University. He is now working as a Lecturer at the same department. His research interests include the structural and tectonic evolution of the Zagros Fold-and-Thrust Belt of the Kurdistan region, Iraq.
Hemin Koyi earned his BSc in 1982 from Sulaimani University, Iraq, and his MSc in 1987 and PhD in 1989from Uppsala University, Sweden. He worked as a Research Fellow at the Bureau of Economic Geology at the University of Austin in Texas (USA) between 1991 and 1993. In 1994, he was awarded his Habilitation and in 2003 became a Professor in Tectonics and Geodynamics at Uppsala University. Hemin is the Head of Solid-Earth Geology at Uppsala University and the Director of the Hans Ramberg Tectonic Laboratory, Chairman of the Evaluation Committee for Geology and Geophysics at the Swedish Research Council (VR), and a member of the Research Council for Natural Sciences at the Danish Research Council (FNU). He has also served on the editorial board of the Journal of Petroleum Geology since 1994. Since 2010, Hemin has been a Guest Professor at the China University of Petroleum in Beijing, and since 2012, an Elected TecTask Officer of the International Union of Geological Sciences (IUGS) Commission. Hemin’s main research interest is deformation geology, structural geology and tectonics, in particular modelling of rock-deforming processes in the crust and mantle lithosphere, including hydrocarbon-related and exploration structural geology, structural control of groundwater accumulation and aquifer deformation, and rock deformation.