The Paleogene Carbonate Banks of the Zagros Foreland Basin’s Forebulge, Northern Iraq: Stratigraphic Architectures and Facies Associations

Abstract


Introduction
The prolific Zagros foreland basin holds about (98%) of Arabian plate region hydrocarbons, which make about (66%) oil and (31.5%) gas of the world oil and gas reserve (Beydoun, 1991).The Zagros foreland basin is referred here to represent the Early Zagros basin, which extends in age from the Middle Turonian to the Eocene (Lutetian) stage.This basin isdifferentiated from the rest of the Zagros Basin sequencebased on the sequence stratigraphic framework of the originally known "Zagros Foreland Basin" of NE Arabia (Al-Qayim, 2019;Al-Qayim, 2021).Basin dynamics is related to African -Eurasian plate convergence and flexural subsidence, which is commenced as early as the late Turonian (Stoneley, 1990;Alavi, 2004;Robertson, 2006;Homke et al., 2009;Leturmy and Robin, 2010;Saura et al., 2011;Al-Qayim, 2019;Al-Qayim, 2021, Al-Qayim, 2022).The depositional history of this basin passes during the Paleogene period through a complete cycle of a foreland basin evolution, includinga final filling andshoaling stage (Al-Qayim et al., 2012, Al-Qayim, 2019).This stage had displayed the occurrence of full set of the foreland basin depozones suggested by DeCelles and Giles (1996) including the foredeep zone, which is represented by the flysch sediments of the Kolosh Formation.The backbulge zone is represented by the basinal marlstone of the Aaliji Formation (Al-Qayim, 2021).To the northeast of the foredeep zone of the basin, an intermontane basin is developed with a thick sequence of subcontinental red siliciclasts of the Suwaiss Red Beds Series (Al Barzinji, 2005;Karim et al., 2008).This localized basin represents the wedge-top depozone of the basin (Al-Qayim, 2021).The forebulge of the basin had being marked by a NW-SE trending chain of intra-basinal highs, and represented by a series of carbonate banks known as the Sinjar Formation.These banks are developed as coralgal reefal limestone and associated shoals and lagoonal facies.Similar banks with similar facies type also developed over the Sinjar Mountain area from which, these bank limestone were originally named.
The origin, sedimentary facies associations and stratigraphic architecture of these bank carbonates are considered as a key subject to the basin depositional history and evolution.
Facies and depositional environment of the Sinjar carbonate banks had being investigated by many authors (Al-Siddiqi, 1968;Shathaya 1980;Mallick and Al-Qayim, 1985;Al-Qayim, et al. 1988;Al-Qayim, 1995;Jassim and Goff, 2006;Daod, 2009;Amin et al., 2005;Tamar-Agha et al., 2015;Alsultan, 2016;Karim, 2016;Al-Dulaimi and Al-Dulaimi, 2017;Karim et al., 2018;Fattah et al., 2020;Al-Qayim and Barzani, 2021).The interesting association of these banks with the ramp facies on both sides, was not well explained.The purpose of this study is to further investigate the nature, origin as well as the sedimentary facies associations of these banks in details and in a basin-wide sense, to evaluate their mutual relationships to basin evolution.The focus of the paper is northeast Iraq, but references were made to the equivalent units and basin parts over the Iranian side.This regional correlation was attempted to support the regional understanding of the basin evolution.

Geological Setting
The Zagros foreland basin evolved during the Late Cretaceous time of the tectonic convergence between the Afro-Arabian continental margin and Eurasia.The sequence of the tectonic events involved and includes 1) subduction of the Afro-Arabianmargin, 2) obduction of the Tethyan masses (radiolarites, ophiolites, Triassic carbonate platform and the exhumed Early Mesozoic slices of the Afro-Arabian margin) over the Arabian margin.These processes generate flexuring and active subsidence which leads to the formation of the Zagros foreland basin (Stoneley, 1990;Beydoun et al., 1991;Sharland et al., 2001;Alavi, 2004;Robertson, 2006;Agard et al., 2011;Homke et al., 2009;Leturmy and Robin, 2010;Saura et al., 2011;Al-Qayim et al., 2012, Al-Qayim, 2019, Al-Qayim, 2021).The basin is progressively developed in front of the advancing Zagros orogeny, which is culminated bythe subsequent continentalcontinental collision of Arabia with Eurasia in post Oligocene -Miocene transition (Alavi, 2004;Agard et al., 2005;Jassim and Goff, 2006;Homke et al., 2009;Moutheraue, 2011;Al-Qayim et al., 2012;Al-Qayim, 2021).The Zagros Fold-Thrust belt represents the external and spectacular part of the Zagros Orogeny.In northern Iraq, this belt is, tectonically divided into the Imbricate, High Folded, Low Folded, and the Mesopotamian Zone (Fig. 1).The Zagros foreland basin extends mainly over most of northern and northeastern Iraq and over southwestern Iran with regional NW-SE trend.The sediments of the foreland basin considered to range in age from the Turonian up to the recent time including the full record of AP9, AP10, and AP11 megasequences of the Arabian plate (Sharland et al., 2001).However, Al-Qayim (2019) had recognized that each of these megasequences represents a different sedimentary basin setting.Each megasequence has different tectonic environment, subsidence history, stratigraphic architecture and sedimentary fill sequence (Al-Qayim, 2022).He suggested that the megasequence (AP9) of Sharland et al. (2001) represents the complete sequence of the Early Zagros Foreland Basin.The stratigraphic boundaries of this megasequence were revised accordingly, based on; correlation of cross-basin unconformities, stratigraphic relationships of the sedimentary units and nature of tectonostratigraphic facies (Al-Qayim, 2019).This sequence includes three important super sequences: the Middle Turronian-Middle Campanian (AP9a), the Middle Campanian-Maastrichtian (AP9b), and the Paleocene-Early Eocene supersequence (AP9c) (Fig. 2).
The supersequence (AP9c)includes the carbonate banks of the Sinjar Formation and the associated units.These units include the olive green flysch sequence of the Kolosh Formation, which fill out the foredeep area.The Sinjar limestone Formation of the forebulge banks, and the pelagic light gray marlstone of the Aaliji Formation occupied the back-bulge area, (Fig. 3).The carbonate banks were developed over elevated highs representing the evolving forebulge of the basin (Al-Qayim, 2019).Upon basin shoaling a lagoonal dolostone and dolomitic limestone of the Khurmala Formation was developed behind the carbonate banks and over the shoaling foredeep area, marking the final phase of marine sedimentation of the basin (Al-Qayim, 2019) (Fig. 3) as well.Following this carbonate deposits a wedge of subcontinental red siliciclastic sediments of the Gercus Formation prograde from the northeastern hinterland and cover the foredeep and forebulge areas.These red sediments mark the molasses phase of the Early Zagros forland basin.Outcrops of these Paleogene stratigraphic units were cropped out at flanks of the high folded zone and cores of some anticlines of the low folded zone.The line connecting the carbonate banks runs NW-SE and passes nearby the line separating the high folded zone from the low folded zone of the generally trending NW-SE Zagros Thrust -Fold Belt.The low folded zone consists of a series of widely spaced, low amplitude gentle folds.Middle Miocene to Pleistocene sediments dominate such topography.The high folded zone, on the other hand, is characterized by high topographic and structural relief.It contains a number of high amplitude, narrowly spaced tight anticlines separated by narrow deep synclinal valleys with overturned folds of different geometries and sizes.The landforms, which is dominated by erosionresistant Cretaceous rock units, are highly structured and reflected on the topography as high rugged anticlinal mountains (Fouad, 2015).

Material and Methods
The focus of this paper is to examine the microfacies and the facies associations of the carbonate banks, which are developed over the forebulge depozone of the Zagros foreland basin.
Samples and stratigraphic measurements of the bank facies successions are taken from (15) localities, which covers most parts of the forebulge area of the Zagros foreland basin in northern Iraq.These localities are: Shiranish, Dohuk (Perfat and Berbuhar sections), Spindar, Aqra, Bekhme, Shaqlawa, Haibat Sultan, Dokan, Daratw, Branan Mt., Sagirma Mt., Derbandikhan, Sirwan Valley, and Sartaki Bamo.Stratigraphic measurements includes lithostratigraphic logging, sedimentological characters and stratigraphic relations.Nearly (200) samples were made as thin-sectiones for microfacies analysis and examined under advance polarizer microscope (Lietz type) to support facies interpretation following the standard procedures set by (Walker and James, 1992;Flugel, 2010;James and Dalrymple, 2010).Other data sources are based on facies reinterpretation and stratigraphic revaluation of nearly (40) surface and subsurface sections over most part of the basin, which are previously presented by other workers.The subsurface measurements were taken from specific final well reports and from the formations top correlation tables for formations of northern Iraq oil fields (Information Department, INOC, 1986).These additional sets of data are used to support regional basin stratigraphic architecture.

Lithostratigraphy
The Paleocene-Lower Eocene supersequence (AP9c) is previously recognized by Buday (1980) as an individual basin-wide sequence.It represents the third and the upper second-order sequence of the Zagros foreland basin, and named as (AP9c) by Al-Qayim (2019).This supercycle represents the filling stage of the basin, and includes the stratigraphic units of the carbonate banks and the associated units (Fig. 3) above.
Rock units of the Paleocene-Eocene sequence is exposed on flanks of the high folded zone and cores of the low folded zone of northern Iraq.These rock units are represented (from northeast to the southwest direction) by the Kolosh and Gercus Formations (over high folded zone) Sinjar/ Khurmala Formations are distributed at both high and low folded zone and the Aaliji Formation over the low folded zone and the Mesopotamian tectonic zones.A brief stratigraphic review of each of these units is given below:

Kolosh Formation
It is the flysch type sediments, which fills the foredeep zone of the Early Zagros foreland basin.It's type section area is at Kolosh Village, near Koysanjaq.The formation is (777m) thick.It is characterized by dark gray shale intercalated with fine sandstone, in addition to limestone fingers of the Sinjar Formation at higher part (Bellen et al., 1959).
The shale is often dark gray, silty, and rich in organic matters, which alternate with medium to thin beds of sandstone forming a typical cycles of a distal turbidite sequence (Al-Qayim and Salman, 1986).The formation is commonly and unconformably overlay the Maastrichtian sediments of the basin including Tanjero, Shiranish, and/or Aqra Formation (Bellen et al., 1959).The red beds of the Gercus Formation overlay the Kolosh Formation, with probable unconformable contact (Bellen et al., 1959).The formation crops out mainly at the area between the high folded zone and the low folded zone in localities such as Banik, Shiranish, Germawa, Bekhair Mt., Sundur, Jebel Maqlub, Shaqlawah, Smaquli, Dokan, Surdash, Bazian, Ghilizarda, Kashti, Nador, Derbandikhan, and Sartaki Bamo (Bellen, et al., 1959).Measured thicknesses of the formation from outcrops at 15 localities show high variability, which range from 40 meters at Aqra locality to (550m) at Daratw locality of Sulaimaniyah area (Table 1).The variation in the formation thickness probably due to tectonic deformation and/or basin configuration (Znad and Aljumaily, 2019).Petrofacies analysis of its detrital grains indicates deposition in a peripheral foreland basin of recycled orogeny (Al-Qayim, 1994).The occurrences of the limestone and the dolomitic limestone of the Sinjar and/or Khurmala Formation at upper part of the formation is characteristic and reported by many workers such as (Bellen et al. 1959;Al-Qayim and Salman, 1986;Al-Qayim et al., 1988;Lawa, 2006).

Khurmala Formation
The formation is identified by Bellen et al. (1959) at type-section from the subsurface well (K-114) of Kirkuk oil field.The thickness of the formation reaches (185m) (drilling thickness), and composed of porous dolomite with shale interlayers especially at the upper part.The dolostone contains ghosts of restricted fauna such as miliolids valvulinids, clavulinids, small gastropods and algal fragments.The upper part composed of dolomitic marly limestone, which is intercalated with gray to buff calcareous shale (Bellen et al., 1959;Barzani and Al-Qayim, 2019).The measured thicknesses from few outcrops of the formation show that it did not exceeds (100 m) including the shale interlayers (Table 1).However in wells sections, it shows higher thickness, probably because it represents a drilling thickness, (Table 2).The Khurmala Formation is considered as the lagoonal equivalent of the Paleocene-Lower Eocene reefal limestone of the Sinjar Formation (Bellen et al., 1959).However, Al-Qayim (2023), using lithostratigraphic principles in identification and recognition of the formation, emphasize the domination of the dolostone lithology as an important factor in recognizing the occurrence of the formation.At Aqra (Al-Qayim, 1995) and Geli Zenta (Asaad and Balaky, 2018), and Gara Mountain (Barzani and Al-Qayim, 2019), the Khurmala Formation occurs with typical sequence dominated by variable crystalline fabrics of a dolostone.It is sitting directly and unconformably over the Cretaceous carbonate unit of the Aqra-Bekhme Formation.In other areas, it is interdigitates with siliciclastic sediments of the Kolosh Formation (Bellen et al., 1959).The red siliciclastic sediments of the Gercus Formation or the fossiliferous limestone of the Avanah Formation is usually overly the Khurmala Formation unconformably.Similar to the Sinjar Formation, the Khurmala Formation was reported from different parts of the foredeep area to be conventionally interfingering with the upper part of the Kolosh Formation, which witness the gradation relationship.

Aaliji Formation
Aaliji Formation is the pelagic sediments that had developed in an intra-shelf basin over the backbulge area.It is defined, for the first time by Bellen et al., (1959) from an outcrop section in Syria.A supplementary section is selected from Iraq at well K-109 of Kirkuk oil field.The formation here is (167m) thick, and consist of grey and light-brown argillaceous marl, marly limestone and shalewith occasional microscopic fragments of chert and rarely scattered glauconite.It is unconformably overlained by the basinal Jaddala Formation, or conformably by the Kolosh Formation and unconformaly underlained by the Upper Cretaceous basinal sediments of the Shiranish Formation (Bellen et al. 1959).The formation is generally occupied most of the Mesopotamian tectonic zone, to the west-southwest of the line connecting the carbonate banks of the forebulge area (Fig. 3) above.Outcrop of the formation are few and always represented by thin tongues of marlstone at the bottom of the Kolosh Formation siliciclastic sediments (Table 1.).However, most of the studied sections of the formation are in subsurface wells, and often show complete and thicker occurrences (Table -2).Sediments of the formation is reported to include fingers of nummulitic and bioclastic limestone at different levels and from different localities such as Sinjar area (Bellen et al., 1959), well Mushorah-1, well Ain Zala-26, well Damir Dagh-1, well Kirkuk-116, andwell Chemchamal-2 (Al-Siddiqi, 1968, Al-Banna et al. 2014), well Ain Zala-16 (Kassab, 1972), and well Alan-1, Butmah -5 (Starkie, 1994) and other wells(Table 2.).The age of the formation based on planktic foraminifera or calcareous nannofossils is estimated as Middlle Paleocene to Early Eocene (Bellen et al., 1959;Shihab, 2008;AL-Hyaly and Al-Badrani, 2019).

Stratigraphic Architecture
The Paleogene bank carbonate strata of the Sinjar Formation is originally, examined from the Sinjar area of northwestern Iraq.The Sinjar Trough is a major east-west trending extensional half-graben basin in northwestern Iraq and northeastern Syria.It began to develop in the Late Cretaceous (Maastrichtian) due to a transtensional tectonics, then passing through several inversion stages since the Paleocene as a results of the Zagros Orogeny (Brew et al. 2001;Grabowisky, 2014).The carbonate bank of the Sinjar area is developed when the Sinjar basin began to inverted, where part of the Upper Cretaceous -Paleocene strata were uplifted and eroded.This bring the area into a setting with a proper condition for the Paleogene carbonate bank (Sinjar Formation) to develop unconformably over the eroded Maastrichtian Shiranish Formation (Bellen et al., 1959).The Sinjar area was first to be examined for the Sinjar Formation and thus taken as the type area.Most of the formation lithologic characters were first documented from this area.Then after, intensive stratigraphic studies on the Sinjar Formation were conducted, elsewhere, such as (Al-Saddiqi, 1968;Shathaya, 1980;Mallick and Al-Qayim, 1985;Khadouri, 1989;Amin et al., 2005;Daod, 2009;Tamar-Agha et al., 2015;Alsultan, 2016;Karim, 2016;Karim et al., 2018;Fattah et al., 2020;Al-Qayim and Barzani, 2021;Al-Qayim, 2022).These studies had shown that the Sinjar limestone Formation represents the bank limestone of an inner ramp, which includes the following basic facies: Coralgal Bank, Back Bank Lagoonal facies, Nummulitic Fore-bank Shoal Facies, and Fore bank Slope Facies (Amin et al., 2005).The intercalations of the Nummuliticbioclastic limestone of the Sinjar Formation with the pelagic marlstone of the Aaliji Formation southwestwards are considered to represent the middle-outer ramp association (Alsultan, 2016).On the other hand, the carbonate intercalations within the Kolosh Formation, which extends in northeastwards direction into the adjacent foredeep area are considered as the middle-outer ramp facies.These ramp facies associations were seemingly, developed on both sides of the Sinjar banks of the forebulge area forming an intra-basin double-sided ramp setting, (Fig. 3) above and (Fig. 4).
The intensive stratigraphic works on the Sinjar bank of the Sinjar area are applied as a model to further understand facies association and stratigraphic architecture of the stratigraphically equivalent forebulge carbonate banks of the same basin.
The lithostratigraphic measurements of the studied outcrop localities (surface sections) are compiled, correlated and integrated with the previously studied sections and subsurface well data reveal an interesting stratigraphic architecture of a bank-ramp facies associations.
(8) lithostratigraphic cross-sections (a-h) were constructed almost perpendicular to the forebulge axis to show lithostratigraphic build up and facies distribution (Fig. 5).The cross-section orientation is controlled by the distribution of the examined localities.For subsurface well data, the final well reports of the examined wells are used to reinterpret the stratigraphic data within the proposed regional facies association scheme.Regional correlation of the facies and the stratigraphic associations of the surface sections with the subsurface sections were conducted to integrate the regional facies framework.The lithostratigraphic correlation at each cross section and among adjacent cross-sections of the examined localities shows that a chain of algal-foraminiferal carbonate banks were developed over a line runs parallel to the forebulge depozone, which is developed within the Paleogene Zagros foreland basin.Although the lithostratigraphic characters of the Sinjar area bank and the forebulge banks are closely similar, however, the origin of these two carbonate bank settings are different.The Sinjar bank is developed in response to basin inversion of the Sinjar Trough (Brew et al., 2001) within the back-bulge area of the Zagros foreland basin.The forebulge banks on the other hand were developed over an intra-basinal elongated high.This high grew gradually when basin flexuring wave is developed, forming a submerged incipient shallow forebulge.The emergence of this high within the basin, thus developed a chain of isolated carbonate banks over the forebulge area (Al-Qayim, 2019;Al-Qayim, 2021).Comparison between facies association of the forebulge banks with the Sinjar bank shows that all of them have similar facies and microfacies proxies.
The correlation among the examined localities across the forebulge zone are displayed in Figure ( 5), which shows eight cross sections taken in general NE -SW direction.These correlation sections indicated that these banks growth is concentrated in a three isolated areas.These areas include: Dohuk, Kirkuk, and Sulaimaniyah areas, and informally called here as Dohuk, Kirkuk, and Sulaimaniyah banks.Evidences for the disconnection of the carbonate bodies over the forebulge are shown by the intermixture of the foredeep flysch sediments of the Kolosh Formation with the back-bulge marlstone of the Aaliji Formation at many areas (Bellen et al., 1959).The organic coralgal reef buildups are recognized in some cases, as a small and localized bodies of patch reef type.These reefs, however, control deposition and facies distribution of the bank and the associated ramps.Ramps on both sides of the forebulge were developed not until banks sediment were well established, and after adjacent slopes became gentle enough for ramp setting.This is more true for the foredeep area where slope is originally and relatively steep.The bank-side facing the offshore (backbulge zone) develops a " windward ramp" with gentler slope which seems to have developed earlier than the other side.The facies of this ramp is reported from wells recognized at cross-sections of figure (5 a, b, c, d, e, and f).The bank side which faces the foredeep area, develops a "leeward ramp", which is evolved in a later time during the shallowing of the foredeep area.This ramp is recognized from sections shown in figure (5 b, d, e, f, g, and h).The ramp terminology and classification used here followed Burchette and Wright (1992) and Bosence (2005).
Stratigraphic correlation shows that the bank of Sulaimaniyah area represents the most extensive, well developed, and better established ramp setting on both side of the bank (Fig. 6).Thus, it is considered here as a model for the rest of the forebulge carbonate banks.The lateral extension of this bank is quite noticeable and extends from Daratw to Chemchamal area.The leeward ramp sediments are reported from areas like Daratw, Sagirma Mt. through Branan Mt.Its thickness is measured from Branan Mt. locality reach (200m) of carbonate intercalations of the ramp sediments within the foredeep siliciclatics of the Kolosh Formation.The windward ramp sediments of this bank is reported from well Ch-2, which is represented by the intercalations of limestone rich in Nummulite, Discocyclina and Assilina, with the globigerinal marlstone sediments of the Aaliji Formation (Al-Siddiki, 1968).

Bank-Ramp Microfacies and Facies Associations
Field stratigraphic description, lithologic characters, and the microfacies data of the studied surface localities leads to the recognition of the basic sedimentary facies of the bank-ramp system.The bank facies represent a shallow marine foraminiferal-coralgal reef and associated shoals, lagoons and in some cases tidal flat facies.The bank and the associated facies forms the main parts of the inner ramp.The middle-outer ramp facies on the other hand were developed on both sides of the bank and each has its own stratigraphic relations.Paleoecological considerations and paleoenvironmental interpretation of the different microfacies and the construction of the sedimentary facies scheme is assisted by standard schemes presented by Hottinger (1983), Burchette andWright (1992), andFlugel (2010).Previous studies on facies, paleontology and paleoecology of the Sinjar Formation such as (Al-Siddiqi, 1968;Amin, 2005;Daod, 2009;Al-Dulaimi and Al-Dulaimi, 2017), were reviewed to assist in constructing the paleoenvironmental model.In addition, reference to the Iranian equivalent banks (i.e.Taleh Zang Formation of Southwest Iran) is attempted, to cover regional aspects of the forebulge bank facies associations using studies such as (Adabi and Zohdi, 2008;Bagherpour and Vaziri, 2011;Zohdi et al., 2013).

Carbonate Bank ( Inner Ramp)
The basic components of the inner ramp facies is the coralgal carbonate bank and its associated facies.The coralgal carbonate bank, which is evolved in Late Paleocene-Early Eocene time is represented by a massive, thick-bedded, and fossiliferous limestone body.These limestones often form a steep cliff over the soft sediments of the underlying Kolosh Formation (Fig. 7a, and b), or protrude as a weathering resistant limestone ridge (Fig. 7c and d)).The bank carbonates is well developed at Sulaimaniyah area, and recognized from many localities such as Dokan, Daratw, Branan, Sagirma, Darbandikhan, Sartaki Bamo and Serwan Valley localities.In most cases the bank limestone forms a single and massive limestone horizon (Fig. 7c), or in some cases three units can be differentiated reflecting a cyclic development (Fig. 7d, and e).Thickness of the bank limestone is variable and generally reach (70 m) as in Branan, Sagirma, and Derbandikhan.Thickness variation witness the patchy distribution and the discontinuity of this reef belt.Bank carbonates are, also well developed over the inverted Sinjar basin forming the proper type of the Sinjar bank.
In other cases Nummulite shoalfacies dominate the bank margins, mainly with robust type of nummulitids forming limestone with Nummulitic grainstone to packstone microfacies (Fig. 9a and b).This Nummulitic shoals/fore-bank shoals are developed during periods of inactive reef growth.Other shoals-associated with larger benthic foraminifera including lokharitia, rotalina, and lepidocyclina (Fig. 9c).Bioclasts are common components and often includes; red algal fragments (Fig. 9d), bryozoa and shell fragments (Fig. 9e).Locally, an Intra-reef pond faciesrich in soritids is developed within the reef interior area as a local lagoonal-type facies (Schlager, 2005).These facies are developed near reef surface and are represented by pelloidal-miollids packstone with common sortids and algal bioclasts.In some localities, such as Derbandikhan the algal reef limestone is reported to be mixed with conglomerate, especially, at lower part (Fig. 9f).

4.3.1.1, Lagoonal Facies (Inner Ramp)
Behind the bank and over the foredeep area, a body of medium to thick-bedded lagoonal limestone, of up to (70 m) thick, thinning upwards sequence is developed representing the Back-bank Lagoon(Fig.10a).Microfacies of this semi-restricted lagoon includes, miliolid grainstone to packstone, rotalid packstone to wackestone, algal bioclastic packstone of dacycladacean algae, and localized alveolinids grainstone to packstone (Figs.10b, c, d, and e).Each of these microfacies occupy certain part of the back-bank lagoonal environment.Similar microfacies are reported from the Talih-Zang Formation of Iran Zagros and interpreted as lagoonal facies (Adabi et al., 2008;Bagherpour and Vaziri, 2011).At certain areas such as Aqra, Gara Mountian, Gully Zana and certain wells of Kirkuk area, this lagoonal limestone was replaced by dolostone and intensively dolomitized limestone forming the sequence of the Khurmala Formation (Figs. 11a, b, c, and d).The back-reef lagoon at these areas seems to have progressively become isolated with increasing salinity, which promote intensive dolomitization (Lith et al., 2002).

Tidal flat dolostone (Inner Ramp)
Shoreward of the lagoonal area, as a tidal flat is developed whereby horizons of stromatolitic limestone or dolostone is often recognized, sometimes associated with karstified upper surface (Figs.11a, b, and c ).The cyclic and shallowing upwards sequence of the this lagoon is consist of 30-200 cm thick cycles of carbonate horizons (limestone or dolostone), which is followed by thin mudstone or shale (Fig. 11d).The carbonate units are often topped by a thin, laminated stromatolitic horizon (Fig. 11a).When the lagoonal sequence is consist mainly of limestone, it is considered as a part of the Sinjar Formation.However, when lithology is dominated by dolomitic limestone or dolostone, then it is considered as part of the Khurmala Formation (See Al-Qayim, 2023).Each of these dolomite horizons is characterized by fine to coarse crystalline dolomite with ghosts of the remaining original lagoonal fossils (Figs.11e and f).The typical dolomitc sequence of the Khurmala Formation is recognized from areas such as Aqra (Al-Qayim, 1995), Zenta Gorge (Asaad and Balaky, 2018), Gara Mountain (Barzani and Al-Qayim, 2019), and Dohuk area (Al-Qayim and Barzani, 2021).

Ramp Associations
Ramp facies are developed on both side of the forebulg banks, in most areas, forming leeward and windward ramp settings.Leeward ramp however, is better developed at Suliamaniyah area, and thus receive more attention here.The middle and outer ramp facies was clearly differentiated in many localities of the Sulaimaniyiah bank such as Branan Mt. (Figs. 7a, and Fig. 12a) and Sagirma Mt. (Figs. 13a, b, and c).However, in most cases both facies are intermixed, and hard to differentiate.Below is review of the facies association of each ramp type.The windward ramp is mainly recognized from the subsurface sections, and show less differentiated parts.

4.3.2.1.Leeward Ramp
This ramp is developed on the leeward side of the bank, which is facing the foredeep zone of the basin.It is developed in later stage of the basin filling when the foredeep trough reaches the shoaling stage and thus a distally steepened ramp settingis established.Sediments over this ramp is represented by a mixed siliciclastic-carbonate sequence characterizing the upper part of the Kolosh Formation.The middle ramp facies is differentiated from the outer ramp in few localities amongst the Branan Mt. and Sagirma Mt.

Middle Ramp
It is a carbonate -dominated facies, which consists of cyclic sequence of thinning upwards cycles of gray to buff argillaceous limestone (Fig. 7a).These limestones are interlayered with grayish green silty calcareous shale and, sometimes, thin sandstone of the Kolosh Formation.Ramp sequence thickness is variable and range from (50 m) at Baranan or thin and undifferentiated sequence in Serwan Valley and Sartaki Bamo sections (Figs.12b and c).Microfacies of these carbonates ranges between bioclastic benthic foraminiferal grainstone to packstone.Foraminifera of these microfacies is dominated by LBF of flat and discoidal phragminids such as Nummulites, Lepidocyclina, Discocyclina and Assilina (Fig. 12d).The shale interlayers thins upwards the sequence and represent part of the Kolosh Formation, which are rich in planktonic foraminifera.

Outer Ramp
This sequence is characterized by mixed sequence of Kolosh siliciclastic sediments with less frequent, carbonate horizons (Figs.12a, 13b).The carbonates are represented by layers of thin to medium bedded, buff, gray, bioturbated, and fossil-rich limestones (Figs.13a and c).Fossils are dominated by large and flattened benthic foraminifera of Assilina, Discocyclina, Lepidocyclina and Nummulites in addition to bank-derived bioclasts (Figs.13c and d).In addition to occasional autochthonous fauna of open marine biota such as echinoids and bryozoa.Rip off clasts of shale and siltstone are commonly intermixed with the sediments of these carbonate beds (Fig. 13c).The siliciclastic interlayer are characterized by relatively thick (5-20 meters) dark gray to black, calcareous silty shale, and occasionally sandy shale of the Kolosh Formation (Fig. 13b).Outer ramp sequence has shown variable thickness due to intermixture with the Kolosh sediments, however, it often display thicker section relative to the middle ramp section (Fig. 12a).The outer ramp facies is often mixed with the middle ramp facies in a way makes it hard to differentiate between the two facies as in Serwan Valley locality (Fig. 12b), Sartaki Bamo locality (Fig. 12c) , Derbandikhan locality (Fig. 12e), and KalkSmaq locality (Fig. 13e).

Windward Ramp
At the southwestern side of the bank another ramp setting is developed down into the back-bulge basin of the Aaliji Formation.Unfortunately, no outcrop reported for this ramp system, however, subsurface sections into the basinal sediments of the Aaliji Formation at wells in many oil fields reveal the occurrence of such a ramp sediments.The Ramp on this side is rather gentler but less differentiated into mid-outer ramp facies.In general, the ramp facies here is recognized as the fingers of the Sinjar Formation bioclastic limestone within the globigerinal marlstone and marly limestone of the Aaliji Formation.These fingers are extended towards the west -southwest direction, and developed in bundles of beds of variable thicknesses.This facies association is reported from wells widely distributed in the back-bulge area including: Ain Zala-26, Butmah-2, Guware-2, Qara Chuq-2, Bai Hassn-54, Kirkuk-116, Chemchamal-2, and Kur Mor-3.Three wells were chosen to demonstrate the proper relations of the windward ramp facies with the Aaliji Formation sediments.These wells are Ain Zala-26, Kirkuk-116, and well Chamchamal-2 (Fig. 14).The stratigraphic column of well Ain Zala -26 shows two intercalation zones between Aaliji and Sinjar Formations reflecting the influence of the windward ramp facies.The upper zone is about 35 m. thick, and shows fingers of Sinjar Formation bioclastic limestone intermixed with the basinal Aaliji globigerinal marlstone and shale.(Fig. 14).The limestone fingers are thinning upwards and are rich in Nummulites, Discocyclina, Rotalina as well as Globigerina.Other components includes Assilina, Alveolina in addition to fragments of algea, miliolids and Orbitolites (Al-Saddiqi, 1968).The lower zone consists of two limestome fingers, both up to (25 m) in thickness, and consist of bioclastic limestone rich in red algae of Lithophyllum Sp., Lithothaminium Sp. in addition to Globigerina Spp. (Al-Saddiqi, 1968).
Similarly, well Kirkuk-116 shows the same ramp influence, which is represented by up to (100 m) thick carbonate dominated -sequence in the middle of the Aaliji Formation.It is characterized by alternation of several thin limestone horizons of the Sinjar Formation intermixed with the globigerinal marlstone of the Aaliji Formation.These limestones are partly dolomitized and are rich in benthonic foraminifera mainly of Assilina, Discocyclina, and Nummulites in addition to the occurrence of Operculina SP., Orbitolites, Rotalia, and Kathina (Al-Siddiqi, 1968).Infrequent bioclasts of red algae and miliolids, are also At Sulaimaniyah area, the windward ramp sequence is documented in the Paleogene section of well Chamchamal-2 (Fig. 14).The bank carbonate section, is represented by the Sinjar Formation, which is overlained by 78 meters thick section of the lagoonal marly limestone and dolostone of the Khurmala Formation (Al-Siddiqi, 1968;Starkie, 1994).The carbonate bank is represented by a fairly, thin section of 30 m of the Sinjar Limestone Formation, which is consist of clayey algal limestone with alveolina (Al-Siddiki, 1968).

Paleoenvironmental Model
The paleoenvironmental implication of the existed fauna especially the larger benthic foraminifera, the interpretation of the sedimentary facies, and their distribution, and association with the studied Paleogene carbonate bank are used in establishing a simple paleoenvironmental model for the studied sequence.This model is intended to show relations between facies and subfacies and their components.The using data are collected from the examined surface sections in addition to the compiled and reinterpreted stratigraphic data from previously studied localities.For general environmental interpretation of the paleontological contents, standard paleoenvironmental and paleoecological models were consulted such as; Hallock and Glenn (1986), Buxton and Pedley (1989), Racey (2001), Jorry (2004), andRasser et al. (2005).The carbonate facies the Sinjar Formation is dominated by Larger Benthic Foraminifera (LBF).These bioclasts represents the most important carbonate grains of a larger benthic foraminifera-dominated ramp environment, which is typical of most Paleogene carbonate sequences (Banerjee et al., 2018).Nummulitic accumulations, however, are of special interest due to its dynamic biofabrics, and sensitivity to water depth as well as, its symbiotic relationship to the photosynthetic algae (Aigner 1985, Beavington-Penny andRacey, 2004).In addition, the standard global microfacies description of Flugel (2010) are referred to in microfacies classification.Previous facies interpretation of the carbonate rocks of the Sinjar Formation such as: (Amin et al., 2005;Al-Banna et al., 2012;Tamar-Agha et al., 2015;Al-Sultan, 2016;Ameen and Mardan, 2019) were also considered.Studies on the equivalent units in Iran (Taleh Zang Formation) such as Adabi and Zohdi (2008), Bagherpour andVaziri (2011), andZohdi et al. (2013) were used for analogy and assistance in constructing the paleoenvironmental model.
The massive and thick-bedded limestone unit of the Sinjar Formation represents the bank carbonates of the inner ramp facies.It includes varieties of subfacies reflecting different environments such as coralgal reef, intra-reef miliolid ponds, nummulitic fore-bank shoal, fore-bank bioclasts, alveolinids shoals, lagoonal foramol, and stromatolitic tidal limestone.The intercalation of the Sinjar limestone fingers with the upper part of the Kolosh Formation represents the "leeward ramp" facies (Fig. 15).These fingers are rich in diagnostic Paleogene larger benthic foraminifera assemblages of the middle-outer ramp such as Assilina, Discocyclina, Nummulites, Operculina, Kathina and Lokharitia, and their bioclasts.These fauna are typically intermixed with pelagic marlstone of the Kolosh Formation, which is rich in Globegerina.The Sinjar Formation's limestone fingers within the Aaliji basinal marlstone represent the "windward ramp" facies.

Discussion
During the Paleogene, the Zagros foreland basin was tectonically mobile and its depocenter continuously and gradually shifted southwestwards (Ibrahim, 1979;Lawa, 2013).Basin depozones were inherited from the Cretaceous foreland basin system, however it was developed into a new system.The forebulge was not established yet, until Late Paleocene, whereby it became a prominent intra-basinal high with regional NW-SE trend, parallel to the main Zagros tectonic strike.At this time, two types of intra-basinal carbonate banks were initiated similarly, but due to different tectonic mechanisms.
The Sinjar bank (type area of Sinjar Formation) had developed over the inverted Sinjar trough of the foreland basin during the successive stages of the convergence history of the basin (Brew et al. 2001, Grabowisky et al., 2014).Originally, this bank is intensively studied and is considered as a model of typical microfacies and facies type and relations for the other group of the carbonate banks.The second group of the Paleogene carbonate banks of the Sinjar Formation, is distributed over the forebulge depozone.These banks form an elongated and discontinuous belt of three localized and informally named banks as; Dohuk, Kirkuk, and Sulaimaniyah bank.The change in the position of the forebulge of the basin towards the southwest from the Upper Cretaceous to the Paleogene reflects new basin evolution stage.This causes the shift of the basin depocenter southwestwards to reach maximum rate of (22 km) during early Ypresian time, with infill sediments rate in the foredeep slows down to a level of 0.14 mm/ a (Saura et al., 2011).The forebulge carbonate banks were initiated when the forebulge sea bottom reaches the proper water depth that suitable for organic buildups.Indications of possible exposures of certain parts of the forebulge before the growth of these carbonates are reported from two areas.These indications are represented by a mixing of an extra-basinal conglomerate with the reef limestone in the form of pebbly limestone at the bottom of bank carbonate of Derbandikhan locality at Sulaimaniyah, and Perfat locality of Dohuk area (Figs.16a and 16b).As basin shoaling developed, specially over the forebulge area, the slopes on both sides of the forebulge become gentle enough to establish the ramp setting.A windward ramp into the Aaliji Formation sediments of the back-bulge zone was developed, and a leeward ramp setting is established into the foredeep zone of the Kolosh Formation sediment northeastwards.It seems however, that the windward ramp had developed earlier than the leeward ramp, where ramp sediments reported from the lower part of the Aaliji Formation at Ain Zala well -26 with Early Ypresian age (Al-Siddiqi, 1968).The leeward ramp sediments often associated with the upper part of the Kolosh Formation of the foredeep area indicating younger association.
As shoaling continued with drop of basin subsidence rate (Leturmy and Robin, 2010), sediment fill rate decrease (Saura et al., 2011), and bank carbonate spread regressively, over the foredeep area which become a semi-isolated and extensive carbonate lagoon.The lagoonal carbonates often overlay the leeward ramp deposits as in Haibat Sultan, Kalksmaq, Branan Mt., and well Kd-1, or sits directly over the flysch deposits of the foredeep area as in Aqra and Bekhme areas.In other cases the lagoonal carbonate are reported to overlay the bank carbonates as in localitiesof wells: Tq-1, K-116, Kc-1, and Dokan area.The lagoonal carbonate also reported to cover the windward ramp deposits (as in wells .This indicate a regional regression over most parts of the basin including the backbulge area.Further isolation generate a localized area of intensive dolomitization within the lagoon, which develops the sequence of the Khurmala Formation.

Conclusions
Integrated stratigraphic data, facies analysis and microfacies investigations from surface and subsurface sections of the Paleogene Zagros foreland basin sequence of northeast Iraq represents the research approach of this work.In addition, re-interpretation of related previous stratigraphic studies are assisted in revealing the interesting outcomes on the origin of the stratigraphic architecture and facies relations of the Paleogene forebulge carbonate banks.The main conclusions can be summarized as follow: 1.The Zagros foreland basin depo-zonation in NE Iraq is well manifested during the Paleogene time.The forebulge depozone of the basin become prominent, shallow, and suitable enough for carbonate bank initiation not until Late Paleocene.The other depozones such as the wedge top, the foredeep, and the back-bulge were continued developing from ancestral counter parts in earlier Paleocene time.2. The Paleogene carbonate banks of the basin grew on two different type of intra-basinal highs.The first is associated with basin inversion high of the Sinjar trough forming the Sinjar bank.The second banks group is related to the development of the basin forebulge depozone.The latter forms a chain of elongated carbonate banks, which are segmented basically into three informally recognized banks from NW to SE: Dohuk, Kirkuk, and Sulaimaniyah banks.3.Both types of the carbonate banks were represented by the Sinjar Formation and have similar microfacies and depositional facies types.4. Middle-outer ramp facies were developed on both sides of the forebulge banks when water depth and slop become favorable for ramp setting.The carbonate bioclastic intercalations within the upper part of the Kolosh Formation of the foredeep area formed the leeward ramp facies.On the opposite side, the intercalation of the Sinjar Formation with the basinal marlstone of the Aaliji Formation at the backbulge area made the sediments of the windward ramp facies.5. Facies components of both ramp types are represented by bioclastic limestone or marly limestone rich in diagnostic LBF including; Nummulites, Orthophragminids, Assilina, Lepidocyclina, Rotalina as well as planktonic foraminifera.6. Microfacies analysis and paleoenvironmental interpretation indicate that the carbonate banks represent the inner ramp facies, which are dominated by Tidal flat, coralgal reef, miliolid lagoonal limestone, back-bank alveolinids shoals, and forebank nummulitic shoals as well.

Fig. 1 .
Fig.1.(A) Location map (Google-Earth satellite image) of the study area within the Zagros foreland basin, and (B) tectonic map of northern Iraq showing the studied localities with reference to the tectonic divisions (Division nomenclature after Fouad, 2015).

Fig. 3 .
Fig.3.A paleogeographic map of the Paleogene -Early Eocene Zagros foreland basin of northern Iraq showing the regional facies belts and basin depozones.

Fig. 4 .
Fig.4.Schematic 3D facies model of the Paleogene Carbonate bank and the associated stratigraphic units, Zagros foreland basin, NE Iraq.

Fig. 5 .
Fig.5.Lithostratigraphic correlation ofcross-sections showing facies interpretation and related depozones of the Paleogene carbonate bank-ramp system across the forebulge area of the Zagros foreland basin.

Fig. 7 .
Fig.7.(a) Massive Sinjar limestone body of the Sulaimaniyah Bank at Sagirma Mt. overlaying mixed Sinjar limestone-Kolosh siliciclasts of the inner leeward ramp; (b) Steep cliff of the Sinjar limestone overlaying leeward ramp sediments at KalkSmaq locality; (c) Protruding ridge of Sinjar limestone at Bekhme locality; (e) Three distinctive and thick limestone horizons of the Sinjar Limestone Formation at Serwan Valley locality; (e) Cyclic Sinjar limestone horizons at Branan Mt, Sulaimaniyah Bank.

Fig. 8 .
Fig.8.(a) Coralline limestone of the bank lower part of the Perfat section, Dohuk area; (b) Algal bioclastic limestone of the Sulaimaniyah Bank, Sagerma section; (c) Nummulitic shoal limestone of the lower part of the Sagirma section, Sulaimaniyah Bank; (d) Solitary Sacleractinian coral limestone, middle part of the bank facies, Serwan Valley section; (e) Coral boundstone of the carbonate bank at Bekhme section; (f) Encrusted coralline algal boundstone from the Bekhme section.Bar when unspecified is equal to 1 mm.

Fig. 13 .
Fig. 13.(a) Lapidocyclina-Assilina packstone of the outer ramp facies, Sagirma Locality; (b) Thin bedded horizons of the outer ramp facies within the dark gray siliciclastic sediments of the Kolosh Formation at Sagirma Mt; (c) Bioclastic limestone rich in Assilina and Lepidocyclina with imbricated rip-off mud clasts of the outer ramp facies Sagirma Mt.Locality; (d) Discoidal and flat Nummulites concentrated in argillaceous limestone bed of the outer ramp facies, Branan Mt.Locality; (e) Mixed middle and outer ramp facied at Kalksmaq Locality; (f) Dark gray Kolosh Fm. underlay the carbonate bank of the Sinjar Fm. with hard to recognize ramp sediments, Bekhme gorge Locality.(Kh-Khurmala Fm., K-Kolosh Fm., B-Bank, R-Ramp).

Fig. 14 .
Fig.14.Stratigraphic columns of oil wells from different part of the back-bulge basin, showing the interrelation of the windward ramp facies with the sequence of the Aaliji Formation.(BBL-Back Bank Lagoon, B-Bank, WRF-Windward Ramp Facies, BB-Back bulge Basin).
The basic faunal components (especially LBF) of these fingers are typical of ramp facies.The proposed paleoenvironmental model of the studied sequence is shown in Figure (15).

Fig. 15 .
Fig.15.Schematic facies spectrum and paleoenvironmental model of the forebulge Paleogene carbonate banks -ramp systems, showing distribution of major faunal and bioclastic groups.Zagros foreland basin, NE Iraq.

Fig. 16 .
Fig.16.(a) Extra-basinal conglomerate intermixed with the bottom sediments of an algal limestone bed of the carbonate bank at Derbandikhan Locality; (b) Pebbly reefal bank limestone at the bottom of Perfat section, Dohuk area.

Table 1 .
Measured thicknesses of the outcrop sections of the Paleogene banks and associated stratigraphic units.

Table 2 .
Thicknesses of the Paleogene banks (Sinjar Fm.) and associated stratigraphic units at subsurface well sections.(Based on Final Well Report estimations and tops correlation chart of INOC).