Facies Association and Depositional Environment of the Sarmord Formation (Valanginian-Aptian), Kurdistan Region, Northeastern Iraq

Abstract


Introduction
The Sarmord Formation forms the lower and middle part of the AP8 tectonostratigraphic megasequence of Sharland et al. (2001).It was first mentioned by Wetzel (1950in Bellen et al., 1959) in the Surdash anticline-Sulaimanya area, of northern Iraq and consists of 455m of homogeneous brown and bluish marls alternating with marly limestones in a rhythmic manner.It is one of the widespread Cretaceous units distributed in the north and northeastern Iraq.Other than the type section, the Sarmord Formation crops out in the Shiranish Islam, Gara areas, Ser-Amadia, Rawanduz, Rania, Peramagroon, Surdash, and Balambo areas.In addition, it was penetrated in subsurface sections including Iraqi Petroleum Company (I.P.C) wells in the north and middle parts of Iraq (Kirkuk, Ain Zalah, Sasan, Ibrahim, Atshan, Najmah, and Makhul wells).The thickness of the studied formation in all areas is lesser than in the type locality and on average up to 100-150 m (Buday,1980).Bellen et al. (1959) dated the Sarmord Formation as Valanginian-Aptian.While Jassim and Goff (2006) suggested the Berriasian age in Kirkuk wells.After the brief description of the formation in Bellen et al. (1959), Chatton and Hart (1960) divided the formation into the Lower, Middle, and Upper Sarmord formations.Later, Ditmar and the Iraqi-Soviet Team (1971) dated the Lower Sarmord to a Tithonian -Berriasian age, the Middle Sarmord to a Valanginian-Aptian age, and the Upper Sarmord to an Albian age.While, Buday (1980) integrated the oldest two units into the Lower Sarmord and mentioned that the there is no Upper Sarmord in the High Folded Zone of NE Iraq, whereas the Sarmord type section represented the Lower Sarmord only.Since that time, there are several works were conducted on the Sarmord Formation.These are included the works of the Al-Abawi (1984), Al-Jaburi (1989), Al-Dujaili (1994), Al-Eisa and Al-Omeri (1996), Qaradaghi (2007), Basher and Samarraie (2012), Al-Dujile (2014), Karim et al. (2021), Saleh (2022) and Saleh et al. (2023).The main aspects of those works were stratigraphy, sedimentology and paleontology.The main goal of this study is to utilize a combination of field, petrographic observations, mineralogy and microfacies analysis to find out the depositional environment of the Sarmord Formation in two outcrop sections within the High Folded Zone and Imbrication Zone of northeastern Iraqi-Kurdistan region.

Geologic Setting
The Iraqi Mesozoic sequence was deposited in the northeastern passive margin of the Arabian craton (Numan, 1997).This margin progressed from the Permian and Triassic and was rifted in the Jurassic and Early Cretaceous, pursued by the Late Cretaceous and Early Tertiary sutures (Burchette, 1993).In the Early Cretaceous, the deposition occurred on an open platform and within the intrashelf basins of the Arabian Plate which be set to the north, east, and south by passive margins of Neo-Tethys (Ziegler, 2001).During the Berriasian to Barremian, the northeastern part of the Arabian plate that included northern Iraq (Kurdistan region) was covered by the Sarmord Formation which gradationally and laterally changes to the basinal facies of the Balambo Formation (Ahmad et al., 2016).After the Late Kimmerian uplift and during the Cretaceous time (Late Berriasian -Aptian) the continuous progressive Neotethys Ocean spreading happened and lead to produce Berriasian-Aptian sedimentary sub cycle, that involved Ratawi, Zubair, Garagu, Lower Qamchuqa, Lower Sarmord, and Lower Balambo formations (Buday, 1980), while most of the west and southwest of the country uplifted (Ditmar et al., 1971).During the Albian time, an extended transgression developed a shelf carbonate over most of the north and central parts of Iraq producing the Albian sedimentary sub-cycle, which included the Nahr Umr, Rim, Jawan, Upper Qamchuqa, and Upper Sarmord formations (Buday, 1980).
The Sarmord Formation cropped out in different locations from northern Iraq (Kurdistan Region), particularly in the High Folded and Imbrication Zones.For this reason, two surface sections (Barsarin in the Imbrication Zone) and (Hanjera in the High Folded Zone)) were chosen for the purpose of this study (Fig. 1).Barsarin section was selected near Barsarin village in the Balak valley, located nearly 12 km east of the Soran City and 16 km to the northeast of the Rawanduz Town in the Erbil Governorate with coordination (Lat. 36° 37' 16" N and Long. 40° 44' 20"E).While, the Hanjera section selected in the Hanjera village in front of the Kewa Rash Mountain about 2 km northwest of the Rania City in the Sulaymaniah Governorate with coordination (Lat. 36° 16' 55.38" N;Long. 44° 51' 45.24"E).The tectonic zones of the studied sections were located in the foreland basin of Iraq (Numan, 2000) (Fig. 2).The Imbrication Zone is characterized by the existence of various NW-SE trending thrusts and numerous normal and reverse faults.These faults led to the repetition of strata in the outcropped sequences and cause the de-ordering of the stratigraphic succession.

Materials and Methods
Fieldwork was done in the area around both the Barsarin and Hanjera areas include the general geology and structural relationships of the Jurassic-Cretaceous successions, to choose the suitable sections for the object of the study.In the studied sections, the outcrops were measured and described in detail, in terms of lithology, grain size and mineralogy.The main lithology of the studied formation is brown and bluish marls inter bedded with marly limestones in a rhythmic manner.
A total 40 carbonate samples were collected from both studied sections.Furthermore, several samples were taken from lower and upper contacts with the Chia Gara and Qamchuqa formations respectively in order to determine the position of the lower and upper contacts.The collection of samples was done usually at every change in lithology, and /or color (random sampling).The total of 40 thin sections was prepared in workshop of Scientific Research Center of Soran University.These thin sections were treated by the Alizarin Red Solution (ARS) according to the Friedman (1959) procedure for distinguishing between the calcite and dolomite minerals.Eight samples of marl from the Barsarin section were selected for XRD analysis, for identifying their clay components.Analysis was conducted in the Iraqi Geological Survey (GEOSURV) laboratories in Baghdad.

Stratigraphy
The stratigraphic succession of the study areas is complex due to the effect of the structural complications and tectonic regimes that affected the areas.The outcrops include; the Sarki and Sehkaniyan formations of the early Jurassic, the Sargelu Formation of the middle Jurassic, the Naokelekan and Barsarin formations of the late Jurassic and the Chia Gara Formation of the Late Jurassic-Early Cretaceous.The youngest formations in the area are the Sarmord Formation of the early Cretaceous and the Qamchuqa Formation of the middle Cretaceous.The Hanjera village is one of the most ideal areas for studying the Jurassic and Cretaceous formations in the Kurdistan region of Iraq.In this area, the Sarmord Formation is overlying the Balambo Formation and underlying the Qamchuqa Formation (Fig. 3a).In Hanjera area, dissimilar to the Barsarin area, the Sarmord Formation is dominantly composed of thick to massive-bedded, yellowish-grey marls interbedded with medium-bedded, yellowish-grey marly limestones containing iron oxide nodules and calcite veins (Fig. 3b and c).The nature of lower contact of the Sarmord Formation is gradational and conformable taken at the top of well-bedded, dark-grey, ammonite and belemnite-bearing limestones and shales of the Balambo Formation.Different kinds of bivalves and gastropods were also observed at the upper part of the Balambo Formation close to the contact with the Sarmord Formation.On the other hand, the upper contact of the Sarmord Formation with the Qamchuuqa Formation is sharp, taken at the base of the thick-massive bedded, greyish-brown, very hard, dolomitic limestone, dolostones and limestone of the Qamchuqa Formation.In the Barsarin section, the Sarmord Formation overlies the Chia Gara Formation and interfingered and underlies with the Balambo Formation (Fig. 4a).In the Barsarin section, thick intercalation between well-bedded, yellowish-grey, soft marls and marly limestones of the Sarmord Formation interfingered with the thin-medium bedded, dark grey, soft shales and limestones of the Balambo Formation containing ammonite and belemnite fossils (Fig. 4b).

Sedimentary Facies
The Sarmord Formation is a thick sequence exposed along wide areas within the Imbrication and High Folded zones of north and northeastern Iraq (Kurdistan region).The sequence is about 290 m interfingering with Balambo Formation (Aptian-Cenomanian) in Barsarin area and 320 m in Hanjera village.In Northern Iraq, the Berrisain -Vallanginan sequence shows a homogenous lithology with many aspects that are similar to the Aptian-Cenomanian Balambo Formation: a) the lithologies are dominated by marly limestone, yellowish grey-black shale and marl; b) the rhythmic alternation Between marly limestones and marls.c) the unit is also conformably and gradationally overlain by neritic carbonates of the Qamchuqa Formation, and underlain by bituminous limestone and shales of the Chia Gara Formation.Two litho facies characterize the Sarmord Formation in northern Iraq, which will be grouped into two lithological components for descriptive purposes; marly limestones and marl.

Marly limestone
Marly limestones from the Hanjera area are less abundant and less variable than those from Barsarin area, existed in a rhythmic interbedding with marls (Fig. 7) and only very rarely forming thin layers within the marl deposits.In both studied sections, Marly limestones occur as yellowish grey layers that are in general ranging between 20 and 45 cm thick, consisting of three main micro facies types (Table, 1): Lime Mudstone Microfacies, it is common in both studied sections in form of neomorphosed, radiolarian, calcispheres and calpionellids lime mudstone microfacies (Figs. 7 a, b, c and d).It comprises of micrite groundmass with rare fossil contents, commonly less than 10% (Dunham, 1962).There is a severe neomorphism of micrite to microspar observed within this facies particularly in the middle part of both studied sections.The significant diagenetic features occurred within this facies are neomorphism, silicification, and compaction.It coincides with SMF (3) of Flügel (1982) in facies zone FZ (1B) of Wilson (1975).
Lime Wackestone Microfacies, it is the most common facies in both studied sections particularly in the Barsarin section.It is observed all over parts of the Sarmord Formation in the form of radiolarian lime wackestone sub microfacies, sponge-calcispheres lime wackestone submicrofacies and globigerina lime wackestone submicrofacies (Fig. 7e, f, and g).Based on Dunham (1962), wackestone grains are ranging between 10 -50 % embedded in micritic groundmass.The main skeletal-grains of these facies include radiolaria, sponge spicules, calcispheres and planktonic foraminifera (Globigerina).Neomorphism and micritization are the main diagenetic processes that affected this facies.This facies corresponds to SMF (3) and FZ (1B) of Flügel (1982) and Wilson (1975) respectively.
Lime Packstone Microfacies, Packstone microfacies is composed of more than 50% of skeletal grains (Dunham, 1962).This microfacies is less common than the other facies types in both studied sections.The petrographic study showed that this microfacies is composed mainly of calcispheres within micritic matrix (Fig. 7h).The facies is equivalent to SMF (3) of Flügel (1982) and FZ (1B) of Wilson (1975).Neomorphism, micritization, and fracturing are the main diagenetic processes that affecting this facies.

Marl
In the Sarmord Formation, marl is the dominant facies all over the formation in both studied sections.It is occurred in a rhythmic manner interbedded with marly limestone beds and colored bluish grey.The ratio of marl units in Hanjera section is higher than those in Barsarin section (Figs. 8 and 9).XRD analysis, the XRD studies are carried out on the eight marl samples of the Sarmord Formation in Barsarin section for identifying their clay and non-clay contents.Analysis was conducted at the Iraqi Geological Survey (GEOSURV) laboratories in Baghdad, Iraq.Several XRD runs are applied (Fig. 10) for better identification of the clay and non-clay composition, followed the procedure of Brindly and Brown (1980); a-Acid-treatment (normal samples) b-Glycolated samples (using ethylene glycol) c-Heated samples (up to 550º for one hour).
The existing clay minerals in samples were discriminated according to first reflection (001) and other reflections.XRD analysis revealed the presence of illite, kaolinite, and chlorite clay minerals.Probable source of clay minerals, the formation of clay minerals is principally occurred during subaerial weathering and hydrolysis, although maybe form due to subaqueous weathering in the marine environment and during burial digenesis (Bogg,2006).The XRD results (Fig. 10) showed that the most common clay minerals in the The Sarmord Formation in both studied outcrops consist of illite followed by kaolinite and chlorite.The clay minerals in the Lower Cretaceous Sarmord Formation rocks indicate the prevailing climatic conditions and weathering processes at the source area.The dominant clay mineral is illite.Whereas, Kaolinite is the second most abundant clay in the studied sediments, and chlorite is relatively low abundant.In addition, mixed layer chlorite-illite clay minerals were found.Illite is probably of detrital origin and may be derived from the reworked sedimentary rocks and from muscovites that sustain many sedimentary cycles (Chamley, 1989).Illite could be derived from the diagenetic processes (Mesodiagenesis mainly).In some places, illite fibers and plates are occurred with kaolinite, referring their diagenetic origin (illitization of kaolinite) (Saleh, 2022).Illitization of kaolinite refers to an increase in burial depth and crystallinity as the paleotemperature increased.The transformation from kaolinite to illite might have been favored by potassium ions from the dissolution of K-feldspar and its overgrowths.When pore water transforms from acidic to alkaline condition, kaolinite can be altered to illite and/or chlorite (Weaver, 1989).The relatively high illite contents of the studied marl samples were produced by physical erosion of mica-bearing source rocks and indicate the hot arid climate during Lower Cretaceous in the northeastern Iraq.
Table1.Main microfacies types and their subdivisions with main diagnostic features of Sarmord Formation, Northeastern Iraq-Kurdistan region Calcisphere Lime Packstone -Calcisphere -Micratization +dolomtization Boggs (2006) mentioned that the proportion of illite and chlorite increases at the expense of kaolinite and smectite with time particularly older rocks.These trends are attributed to the diagenetic alteration of kaolinite and smectite to form illite and chlorite.Kaolinite is the second abundant clay minerals, resulted from the chemical weathering of acidic igneous and metamorphic rocks or their detrital weathering products under tropical to subtropical humid climatic conditions (Hendriks, 1985;Chamley, 1989).Kaolinite can crystallize during both shallow and deep burial (Weaver, 1989).The relative abundance of kaolinite as a detrital mineral may refer to nearness of the sediment source and deposition in relatively near-shore shallow marine environments, whereas smectite prefer the deeper deposition, more offshore environments (Flügel, 2004).Unfortunately, no smectites were recorded in the studied samples of Sarmord sediments.Therefore, it is suggested that all smectite of the studied samples were converted to illite during diagenesis.The identified chlorite in the studied samples, is associated with the weathering of rocks rich in ferromagnesian minerals typical of basic igneous and metamorphic rocks (Millot, 1970).It may also be formed by the diagenetic processes by alteration of smectite with burial depth and increasing temperature (Weaver, 1989;Tucker 2001).Additionally, chlorite may be transported as clastic particles to the depositional basin or may be produced by physical weathering under arid, hot climate conditions (Chamley, 1989).

Facies Association
Basinal facies association, this association is equivalent to SMF 3 of Flügel (1982) and FZ1B of Wilson (1975).It consists mainly of bluish marl and yellowish grey marly limestone within limemudstone, wackestone and packstone microfacies bearing deep water faunas.Rhythmic bedding structure consisting of marly limestones intercalated with marl is a general characteristic of the studied formation all over the northern Iraq (Jassim and Goff, 2006).In the marly limestones, pelagic faunas (macro and micro) are abundant.The pelagic macrofauna include ammonites.Whereas, micro-faunas include; abundant planktonic foraminifera's (Globigerina), radiolarians, calcispheres, sponge spicules, echinoderms, calpionellids and ostracods.Non skeletal grains include peloids.

Discussion
The microscopic inspection of the Sarmord limestone displayed different facies types of which similarity in litho and bio content was observed among the studied sections in northeastern Iraq during the Lower Cretaceous period (Saleh et al., 2023).This will refer homogeneity of both facies and environment that existed in both studied sections.Using the Dunham's (1962) terminology, it was showed that wackestone microfacies is the most abundant followed by mudstones and packstones dominated by micritic groundmass.The dominance of micrite within these facies was associated with the stagnant and calmness of sea bottom that led to lime mud accumulation (Dunham,1962).Some disparity in microfacies type takes place when the ratio of grain to matrix increases due to increase the rate of accumulation of grains to lime mud.The dominance of micritic groundmass led to reducing cementation, and indicating to missing of strong currents enable to washing out the micrite.
The recognized microfacies of the Sarmord Formation is correlated to standard microfacies (SMF) and facies zone (FZ1B) of Flügel (1982) and Wilson (1975), it is found that the Sarmord facies correspond to basinal associations of SMF3 which belongs to FZ1B.This means that application of such standard concepts would obscure most of the details and subdivisions of the Sarmord facies in northern Iraq.The bluish marl, encountered all over parts of the Sarmord Formation, would refer to periods of stagnation when in minimized sea current activities.
The ammonites are the less common and salient fauna in the Sarmord Formation, that represent marine animals had nektonic or nekto-planktonic mode of life and indicated pelagic (open sea), or relative deep-water deposits (Tucker, 2001).Additionally, Benton and Harper (1997) illustrated that those complex septa and sutures of ammonites may have increased the strength of ammonoid phragmocone, protecting the shell against possible implosion at deeper levels in the water column.Unfortunately, most of Sarmords ammonites are poorly preserved due to intensive neomorphism that made sutures and septa of shells hardly identifiable.
Radiolarians had been noticed mainly within the radiolarian lime mudstone and wackestone submicrofacies.Radiolarians are planktonic and believed to be exclusively marine organisms (Flügel, 1982).They (radiolarians) are occurred in open marine environments and in tropical regions (Friedman and Sanders, 1978).Bignot (1985) added that radiolarians are stenohaline faunas that were enduring only low saline sea waters.Radiolarians may tolerate deep marine conditions down to the CCD surface.It was noticed that the increase of radiolarians in the radiolarian lime wackestone submicrofacies was at the expense of decreasing calcispheres.This indicates the variation in depth in which each is seated.The Sarmord's radiolarians are characterized by bad preservation, most of them either dissolved leaving molds or replaced by calcite when conditions obtain variable pH.Although, in radiolarianbearing marly limestones, there are no clear -cut evidences for calcium carbonate solutions.Therefore, it is believed that radiolarian marly limestones are not comparable to recent deep-sea radiolarian oozes.Unwalled calcispheres are abundant in the Sarmord Formation, always associated with calcitized radiolarians.Such association causes difficulties in differentiating each of them.It is likely that most unwalled calcispheres are originated from spherical calcitized radiolarians.But generally, calcispheres have unknown systematic affinity and are regarded as remnants of pelagic organisms, due to their occurrence with planktonic foraminifera and radiolarians (Balaky et al., 2016).Calcispheres are more common and widely distributed in Tethyan realm than elsewhere, thus they were supposed to be formed by tropical-subtropical organisms.The pelagic origin of the Sarmord calcispheres is more acceptable due to their associations with pelagic organisms such as planktonic foraminifera, and radiolarians.
Based on all petrographic, facies, textural analyses, it is concluded that the Sarmord Formation in Barsarin and Hanjera sections was deposited at deep water basinal (outer ramp) environment.To imply all these interpretations a schematic 3 D-block diagram (Fig. 11) is designed.This model showed paleoenvironmental condition of Sarmord Formation in Northeastern Iraq (Kurdistan region).

Conclusions
The lithological composition of the Sarmord Formation in the northeastern Iraq, Kurdistan region comprises of rhythmic alternation between medium to thick bedded marly limestones with medium to thick bedded of marls.The petrographic components of Sarmord carbonates showed homogenous contents in all part s of the studied sections, which are: matrix mainly composed of micrite, non-skeletal grain involved peloids and skeletal grains showing a variety of pelagic (open sea) faunas which include ammonites, planktonic forams (Globigerina), radiolarians, calcispher, ostracods, bioclasts, echinoderm plates and rare calpionellids.X-ray diffraction analysis of selected marl samples of Barsarin section, revealed that the main clay minerals in Sarmord Formation are illite followed by kaolinite and chlorite.In addition, the mixed layers of illite-chlorite are also present.The high abundance of illite indicates that hot-arid climatic conditions prevailed during deposition of the Sarmord Formation in Imbrication Zone of Northern Iraq.According to Dunham's (1962) classification, and based on detailed microfacies analysis of limestones, three main microfacies types and 9 sub-microfacies are distinguished in the studied sections of the Sarmord Formation, these facies were subdivided according to their environmental interpretation, in to one basic type of facies association: Basinal Facies Association.The collection of petrographic, facies, and textural analyses, concluded that the Sarmord Formation in northeastern Iraq was deposited in a deep water, basinal (outer ramp) environment.

Fig. 3 .
Fig. 3. (a) The Out crop of the Sarmord Formation in Hanjera village, Lower contact of the Sarmord Formation with the Balambo Formation and the Upper contact with the Qamchuqa Formation.(b) thick to massive-bedded, yellowish-grey marls of the upper part of the Sarmord Formation.(c) mediumbedded, yellowish-grey marly limestones of the Sarmord Formation with common calcite veins

Fig. 4 .
Fig.4.(a) Out crop of the Sarmord Formation from Barsarin area, lower contact of the Sarmord Formation with underlying Chia Gara Formation and the Upper contact with overlying Balambo Formation.(b) Rhythmic manner of well-bedded, yellowish-grey, soft marls and marly limestones of the Sarmord Formation inter fingering with thin-medium bedded, dark grey, soft shales and limestones of the Balambo Formation

Fig. 11 .
Fig. 11.Depositional model of the lower Cretaceous Sarmord Formation in a ramp setting, NE.Iraq-Kurdistan region