RELATIONS BETWEEN DEEP AND SHALLOW STRATIGRAPHIC UNITS OF NORTHERN IRAQ DURING CRETACEOUS
Kamal H Karim1, Rabea K Al-Hamadani2 and Sirwan H. Ahmad3
Published in: Iranian Journal of Earth Sciences, Vol.4, No.2, p.75-103 (2012)
1Department of Geology, University of Sulaimani, Sulaimani City
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Abstract
The studied area includes Sulaimani, Erbil and Dohuk Governorates where Cretaceous rocks are well exposed in the High Folded Zone of North and Northeastern Iraq. This area is part of the northwestern boundary of Arabian platform near to Iranian border. In these governorates, the study tries to clarify the relations between deep and shallow stratigraphic units (formation or facies) during Cretaceous. The result of the present study is mainly depended on the field lateral tracing of the lithologic changes and paleontological evidences for indicating the new age termination of the units.
The result showed as a generalized and simple model of temporal and special relations between Cretaceous deep and shallow water formations. The Balambo (Kazhdumi in Iran), Kometan (Ilam), Shiranish-Tanjero (Gurpi) Formations (as deep facies) are indicated as lateral equivalent of shallow facies of Qamchuqa (Sarvak and Dariyian), Bekhme (has not equivalent in Iran yet) and Aqra Formation (Seymare or Tarbur) respectively. On the model, the transition zone is indicated where the deep facies, from the east, change to shallow ones at the west. The shallow facies consist of coarse grain limestone with fossils skeletons, bioclasts and intraclasts. The deep facieses are fine grain pelagic limestone with microfossils (panktonic forams and nanofossils) whiles the transitional facies (zones) generally consist mixture of the two as interfigering relation between the deep and shallow facies. As the changes of the lithology of transitional zones are relatively sudden, therefore, they are not designated as formation and not defined previously.
Introduction
The studied area is located in the Dohuk, Arbil and Sulaimaniya Governorates (Fig.1) in the High Folded Zone of Buday (1980). In this area extensive study is conducted from fifties of last century till now. These studies documented some relations (lateral lithologic change) between Early Cretaceous deep Balambo and shallow Qamchuqa Formations (Dunnington, 1958, Bellen et al, 1959). The former formation consists of about 600m of grey, massive detrital and biogenic dolomitic limestone of reefal origins while the latter one made up of yellowish white and well bedded pelagic limestone. The same relation is cited between Shiranish Formation (marlstone of Campanian-Maastrichtian) and Aqra Formations (Massive reefal limestone of Late Campanian-Maastrichtian) by Buday (1980) and Al-Ameri, and Lawa (1986). The lateral lithologic changes between other units are still obscure.
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1-Prof. University of Sulaimani, Department of Geology 2- Assistant lecturer, University of Mosul, Department of Geology 3- Assistant lecturer, University of Sulaimani, Department of Geology
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The present study tries to establish and define new relations between all Cretaceous deep and shallow water formations in the northeastern Iraq and modify the previously cited relations. The establishment includes combining new and old relations in simple and generalize model that change the previous idea about the geology of area in Dohuk, Arbil and Sulaimaniya Governorates.The model and included relations show temporal and special lateral distributions which are ascertained by finding transitional zones (common lithologies) between studied formations (Fig.2). The establishment of the discussed relations is mainly depending on fieldwork during which the lateral tracing of the relation is conducted and ascertained. The establishment is achieved with the aid of paleontologic evidences. All the previous studied are reviewed and checked in the field to indicate if there are any ideas that contradict the result of the present study. The study includes large areas of northern Iraq in the Kurdistan region about 250 and 50 kms long and wide respectively.
Discussion
The deep facies of the Balambo Formation (Early Cretaceous), in the east, changes laterally to shallow facies of the Qamchuqa Formation in the west. The transition zone between Balambo and Qamchuqa formation (Ameen, 2008; Karim and Ameen, 2009) can be seen in the area to the west and northwest of Sulaimaniyah City (Fig.1D). The facies of the transitional zone is generally intermediated between both formations, which are neither deep nor shallow. In some places can be seen as alternation of coarse and fine detrital limestone.
Any relations are not established previously between Kometan (Turonian–Early Campanian) and Bekhme Formations (Late Campanian-Maastrichtian) which are designated as formations of different ages. In the present study, a close relation is established between Kometan and Bekhme Formations and the age of both formations is changed to Turonian–Early Campanian Kometan Formation mainly in the east changes to Bekhme and Kometan Formation in the west. The transition zone can be seen in the area between Dokan and Smaqully Gorge, where the Kometan Formation gradually changes to massive Bekhme Formation. The transitional zone can be seen as alternation between white, fine grained beds and grey coarse grained ones (Fig.2). The best places to see the transition zone are eastern end of Makok Mountain about 3 km southwest of Ranyia town (Fig.3) and the area around the Hezob bridge at the latitude 36o 10– 18.48= N and longitude 44o 41– 12.88= E . Another place of the transition zone is the area around Sarochawa village about 20km northwest of Ranyia town where same features of the later mountain can be seen. It is important to note that, at few places to the west of the transition zone, Kometan Formation does not changes to Bekhme Formation totally but it remains in some places, but in reduced thickness. These places are such as the area around Shaqlawa (Taha, 2010) and about 4 km to the south of Khalyfan town. In the Bekhme Gorge, Ameen and Karim (2008) correlated lower part of the Bekhme Formation with Kometan Formation and changed the unconformable contact between Bekhme and Qamchuqa Formations to conformable.
The relations between Kometan and Bekhme Formations can be seen in the figure (1B) in which the facies map of Dunnington (1958) of Turonian–Early Campanian (Fig.1C) is modified to show lateral facies change of the former formation from east to the latter one in the west (see the green circle in the figure1B). Previously, Dunnington (1958) Buday (1980) and Jassim and Goff (2006) considered the Bekhme Formation as the age equivalent of Tanjero and Shiranish Formations.
Fig.(1) A, C, and D facies maps of Cretaceous (Dunnington, 1958) shows studied area which is located along and around the red line (AB). The line is the direction of geologic cross section in the figure 2. In this study, the map (C) is modified to the map B which shows lateral facies change of Kometan Formation to Bekhme Formation (green circle).
The same relation of the above formations can be established for Shiranish and lower part of the Aqra formations too. When one walks on the outcrops of the Shiranish Formation from Bekhme gorge (southwestern outlet) to Zanta gorge, can see gradual change of marly limestone to fossilliferous limestone of Aqra Formation. From the former Gorge, the upper part of Aqra Formation contains two interfingers (Fig.4). The two interfingers contain omphalocyclus large foram which found only in this formation (Fig.8) (see the section of paleontological evidence). Accordingly, there are two main futures of transition zones, the first one is gradual lithologic (or facies) change (Fig.5) and the second is interfingering between shallow and deep facies (Fig.4).
On Google image, the stratigraphic position of Shiranish Formation shows gradual thickness reduction on expense of Lower part of Aqra Formation. With this reduction, the white color succession (Shiranish Formation on Google earth) can be observed changes to grey succession of Aqra Formation. Near Bujeel town, the transition zone totally become resistive and covers (as Aqra Formation) the peak of the Perat Mountain (anticline). At the northeastern outlet of the Zanta gorge, the equivalent of Shiranish Formation can be distinguished, which consists of about 150 m of well bedded grey to bluish white limestone succession, which is occasionally fossiliferous (Fig.5).
Fig.(2) Geologic longitudinal section along the transect line AB (see facies maps A, B, C and D in the fig.1) shows relation between the shallow and deep formations. The diagram based on fieldworks between Sulaimaniya city and Aqra town. It shows spatial and temporal relations between Cretaceous formations. It appears that the geologic relations between Balambo and Qamchuqa formations are applicable for other units of Upper Cretaceous rocks. The dashed red lines are isochrones datums. 
Fig.(3) A) A lithology which is intermediate between Kometan and Bekhme Formation (transitional zone between the two formation as interfingers of shallow and deep facies) about 3 km to the west of Ranyia town (southeastern end of Makok Mountain), the rack is about 7m high. B) Enlarged interval between the two lines. C) Kometan formation in Dokan area.
Fig.(4) Southeastern outlet of Bekhme gore (directly to the west of Bekhme village) showing upper part of Shiranish Formation that contain two interfingers of Aqra Formation which contain Omphalocyclus. This interfingering is common feature of the transition zone.
Fig.(5) Bluish white to grey succession which represents the transitional zone between Shiranish and lower part of Aqra Formation at northeastern inlet of Zanta Gorge) about 7kms to the southeast of Aqra town. The beds of the succession are occasionally fossiliferous which is located under Tanjero Formation or its equivalent.
The same previous relations are true for Tanjero Formation (Maastrichtian Sandstone and calcareous shale), which changes to upper part of Aqra Formation. The change occurs in shorter distance than other formations. In Bekhme gorge Tanjero Formation exists mainly as soft green marl or calcareous shale (with thin interfigers of Aqra Formation) between Shiranish and Tanjero Formations, which is located in lowest elevation forming a small valley. Toward west, the soft part becomes resistive (changes to resistive limestone) and near Bujeel town it climbs the Perat Mountain up the peak.
In the southwest outlet of the Zanta Gorge it contain large foram of luftusia and finally in the Aqra town changes totally to typical lithology of Aqra Formation. The change of Tanjero to Aqra Formation is agree with conclusion of Walker et al (1983) whom clarified that the a basinal deep water shale (calcareous shale of Tanjero Formation in Bekhme Gorge) will always separate the terrigenous depocenter (Tanjero Formation in Dokan area) from the carbonate depo-center (Aqra formation near Aqra Town) . They added that the carbonate depo-center tends to remain nearly stationary for long periods. They further added that each basin will be filled slowly by shale deposition and will be flanked on both sides by thick, potentially more porous carbonates and coarser terrigenous clastics.
Paleontological study
Many detailed studies are conducted on the Aqra and Bekhme Formation such as (Al-Karadaghy, 1989; Ali, 2010 and Zebari, 2010 (they are detailed MSc theses), Al-Qayim and Shaibani, 1995; Jassim and Golf, 2006). These studies have not shown any direct evidence to indicate the age of the latter formation. Even Bellen et al (1959) have recorded many fossils in Bekhme Formation but today no one of them is used for age determination because they all have long rage of occurrence. In the present study, no fossils are found too in the Bekhme Formation for age determination due to intense dolomitizatiom and recrystallization. Therefore, effort is directed toward the base of Shiranish Formation which overlies latter formation. The samples of Shiranish Formation in Bekhme Gorge gave the age of early upper Campanian (Fig.6, 7 and 9). So effort of this study is successful for proving that the previous age determination of Bekhme Formation (Late Campanian-Maastrichtian) is not accurate. Therefore the age of the Bekhme Formation is older than Late Campanian (Middle Campanian and older). For this age termination, a section of boundary between Shiranish and Bekhme formations is take that overly the type section of this latter formation at Bekhme gorge. The section is located at the end of the tunnel that passes through northeastern limb of Perat anticline. Directly to the east of the northeast outlet of the tunnel, a well exposed section can be seen along the road cut (Fig.6). The latitudes and longitudes of this section is 36o 41– 45.20= N, 44o 17– 09.55=E.
The main sedimentological feature of this section is a glauconitic bed that is located between the two formations. It is about 10cm thick and contains grass green disseminated glauconite grains (Fig.2). R. Wetzel, 1950 (in Bellen et al.1959) cited abrupt contact between the two formations which is marked by condensation of planktonic foraminiferal fauna and by glauconite concentration, doubtfully conformable. The sample of the base of Shiranish Formation that are taken directly above the glauconitic bed, gave the Globotrancana calcarata which is index fossil of early upper Campanian (Fig.7 and 9). A another index fossil of the Late Campanian-Maastrichtian is omphalocyclus (Fig.8) which is not found in the Bekhme Formation by all the aforementioned studies but it found inside a fossilifetrous limestone in the upper part of Shiranish Formation at the Bekhme gorge. This latter limestone is interfinger of Aqra formation in the upper part of Shiranish Formation or in the lower part of Tanjero Formation.
Fig.(6) Contact between Bekhme and Shiranish Formation at northeastern outlet of theBekhme Gorge, shows inspected samples and location of Globotruncana (Radotruncana)calcarata (Early Late Campanian).
Fig.(7) Different section of Globotrancana calcarata which is index fossil of the base of Early Late Campanian in the base of Shiranish Formation in the Bekhme gorge, S.No. Sh2, 50X, normal light.
Fig.(8) Omphalocyclus in the Aqra Formation (that located at upper part of Shiranish Formation) in the Bekhme gorge which is not found in the Bekhme Formation neither in the present study nor in all previous ones . No. A3, 10 X, normal light.
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If the age of Bekhme Formation was late Campanian- Maastrictian as mentioned before, it must contained very clear and abundant fossils such rudists, loftusia, omphalocyclus . This is because the rock of the latter age contains these fossils in Iraq, Iran, Turkey Oman and Greece (see Sadiq, 2009, Abdelghany, 2003 and Ozcan, 2007) but unfortunately these fossils are not fond in Bekhme Formation. The occurrence of the Glauconitic beds below the Globotrancana calcarata Zone, convince one that the age of the top of the Bekhme Formation is for older than Middle Campanian. This is because always glauconitic bed is responsible for non-deposition and disappearance of sometime which may be coincides with Middle Campanian. The evidence for this assumption is the study of Al-Badrani et al (2012) whom concluded (by Nannofossils biozonation) that the lower part of Shiranish Formation has the age of Early Campanian in a section near Sulaimninyia city.
Fig.(9) Biozonation of the Late Cretaceous (Gradstein et al, 2004) in which the Early Upper Campanian is represented by Globotrucana calcarata (indicated by black arrow)
GENERALIZED MODEL
In the figure (9), a simplified graphical model is drawn to show generalized spatial and temporal lateral relations between deep and shallow Cretaceous formations. This model is based on fiedworks and lateral tracing in addition to palaeontologic evidences. The transition zone of each two related formations is indicated and located between the red dashed and bold green lines. All transitional zones of different ages and different formations are connected. The relations show that the facies changes (in the transitional zones) occur relatively suddenly which resemble step and ascend toward the west. It can be seen that there is no prominent unconformities between the different formations because the main tectonics was far from the studied area and located inside Iran to the northwest during Cretaceous. The continuous shallowing by reef growth and sediment fill is compensated by subsidence at the end of each reef building phase. In addition to reef growth, the area as Cretaceous forbulge is most possibly uplifted by tectonic load of Iranian plate.
Fig.(10) A simplified model shows generalized spatial and temporal relations between deep and shallow Cretaceous formation (based on fieldwork). The shallowness is compensated by subsidence therefore no unconformities are seen in the area.
CONCLUSIONS
· Generalized relations of deep to shallow facies changesare established across Sulaimaniyah, Erbil and Dohuk Governorates during Cretaceous for the first time.
· The relations are organized in simple graphical model of temporal and special relations between Cretaceous Balambo, Kometan, Shiranish (with Tanjero) Formations (as deep facies) are indicated as lateral equivalent facies of Qamchuqa, Bekhme and Aqra Formation respectively.
· On the model, the transition zone is indicated where the deep facies from the east, change relatively suddenly to shallow ones at the west.
· The paleontological evidences are used for the first time that shows that the previous age determination of Bekhme Formation was not accurate and it’s top is older than Late Campanian.
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