Limestone inside Upper Bakhtiari Formation



Abstract: Thick beds of detrital and stromatolitic limestones are found for the first time inside Upper Bakhtiari Formation in two different localities in the Sulaimani Governorate, NE-Iraq. The first locality is Dokan area at the northwestern limb of Kosrat anticline while the second one is located in the Garmian area between Chamchamal and Qadir Karam towns. The limestones change laterally and vertically to conglomerate of the latter formation.The beds are located on or inside the conglomerate of latter formation and only in one place it changes laterally to green marl. Petrographically the limestones consist of alternation of limestone layers of intraclastic, oolitic and pisoidal and oncoidal lithology. The lithology indicates relatively sudden environment changes and unstable energy regime which most possibly indicated deposition in freshwater lakes. The lakes are formed due fluvial activities such as river cutoff (oxbow lake) and river damming by rock slides or river plugging. The paleogeography of the Upper Miocene and Pliocene had assisted the deposition of the limestone in the lakes due to compartment of source areas. In many areas, the source area was consisted of limestone terrains during latter two ages and from these terrains the carbonate rich solution and clastics are supplied to the rivers and the lakes in which carbonates had   deposited.

Keywords: Bai Hassan Formation, freshwater limestone, limestone beds, Upper Bakhtiari Formation, stromatolitic limestone


  1. Introduction

The Upper Bakhtiari is well known for its polymictic conglomerate in Iran and Iraq and Turkey which consist of mixture of gravels and boulders of cherts, limestones and Igneous clasts. While the record of occurrence of bedded limestone in the Upper Bakhtiari Formation is the first one in the present study. But it does mean that it has not some indirect background in the literatures as Karim and Taha (2012)[1] has studied in detail a succession of limestone conglomerate that belong to Upper Bakhtiari (Bai Hassan) Formation. they cited that  the succession is about 300m thick and located around Dokan dam; therefore they call it Dokan Conglomerate. They attributed the deposition of limestone conglomerate to nature of the source area which was consisted of exposure of limestone such as Qamchuqa Formation. In Iran, Sahraeyan (2013)[2]  has recorded clast-supported limestone conglomerate under Bakhtiari Formation. He assigned it as a non-marine depositional system of braided rivers.

According to Jassim and Goff (2006)[3],  the Bakhtiari Formation (including Upper and Lower Bakhtiary) was first described from Iran by Busk and Mayo in 1918 and the formation was recognized by Bellen et al. (1959)[4]  with using same term in Iraq too. In Iraq the name of the Lower and Upper Bakhtiari formations are changed to Mukdadiya and Bai Hassan formations   respectively by Al-Rawiet al. (1993)[5]. Buday (1980)[6]  cited that the formation in  is largely made up of coarse and thick fluviatile and estuarine conglomerates, products of erosion of the rising mountain ranges, deposited in one or more sinking troughs in front of those ranges. They added that in the folded zones residual and terrace gravels of Recent and Pleistocene age rest with high angular discordance on deeply eroded Upper Bakhtiari and older formations. The formation is composed almost purely of terrigenous clastics from silt size to boulder conglomerates. In general, the grain size of the clastics increases upward. The prevalence of the conglomerates in the upper part of the formation had been the main’ reason for considering that part as an independent formation (Bellenet al.1959)[4].

The Upper Miocene- Pliocene conglomerate consists of very thick pile of pebbles, pebbly sandstone and interbedded with red claystone and erosional surfaces. This pile includes Mukdadyia and Bai Hassan (Lower and Upper Bakhtiari) Formations. The thickness of the latter Formation (mainly as conglomerate) is more than 1500m and is located mainly in the Low folded Zones. The paleocurrent of this conglomerate is towards southwest (Karim et al., 2009)[7]. In addition to conglomerate, Bai Hassan Formation contains thick beds of claystone and sandstone in fining upwards cycles. The lower contact with Mukdadiyah Formation is conformable and marked at the base of the first conglomerate bed (Al-Musawi, 2008)[8].


  1. Location and Geomorphology

During fieldwork, on the outcrops of the Upper Bakhtiari Formation, limestone beds are found in two different areas inside the formation. The first locality is located about 10 kms to the northwest of Dokan Town at 500 m to the southeast of Mewzha village on the northeastern side of Kosrat Mountain(or anticline) inside the High Folded Zone (Fig.1, 2 and 3).The latitude and longitude of thelocation are 36o0628.52= N  44o4804.46= E .This location is looking (from the south) over the Dokan lake and Marga plain.These beds are referred to in the ext as “northern limestone”

      The second one is located between the three villages of Hanara (at southwest), Mordana (at the west) and Hama Sherif (at northeast). These villages are located at 18 and 3kms to the south of and northwest of Chamchamal and Takyiay Jabary towns in the side Low Folded Zone (Fig.4 and 5).  At this location, two outcrops are found which are separated by 700mand the latitude and longitude of the outcrops are 35o2238.3= N  44o4547.5= E.and 35o220.3= N  44o4619.5= E. These beds (two outcrops) are referred to in the text as “southern limestone”.


  1. Materials and Method of The Study

The study depends both on the field work and microscopic study of the samples by which the lithology and fossils of the samples are studied.  Fifteen thin sections are prepared for petrographic study and three samples are prepared and sent to England for nannofossils analysis   by Nannofossils Consultancy Ltd, London. The location of the limestone beds are plotted on topographic maps which later changed to geological map. The previous studies reviewed and used to find the previous study about Bakhtiari Formation. The main material of this study is beds of limestone that are apparently associated with the conglomerate of Upper Bakhtiari Formation.

Bakhtiari ls f1Fig.(1)Tectonic map of the  Northern Iraq (Jassim and Goff, 2006)[3] on which the location of  the limestone outcrops are shown


Bakhtiari ls f2Fig. 2: Geological map of the studied area (modified from Karim and Taha, 2012)[1]


Bakhtiari ls f3Fig.(3)Bedded (freshwater) limestone inside Dokan Conglomerate south and southeast of Mewzha village.  

Bakhtiari ls f4Fig.(4)Topographic map of the area between Chamchamal and Qadir Karam towns shows the location of the two outcrop of limestone inside Upper Bakhtiari Formation

Bakhtiari ls f5Fig.(5)Outcrop of the limestone bed  of the  southern limestone near Takyai Jabari town. The gradation

of the limestone to conglomerate can be seen in the photo C.


  1. Petrography of The Limestone

Both the northern and southern limestones are nearly similar in lithology as they   consist of massive or well bedded milky to grey limestones and they are hard and coherent.  In both locality, they laterally and vertically change to conglomerate and the transitional zone consist of pebbly limestone. In one locality, the southern limestone changes to green marl from which samples are taken for age determination by nannofossils.  

4-1-Detrital limestone

In the published work of Karim and Taha (2012)[1] a 300m of limestone conglomerate succession (Dokan conglomerate) is found in the same area of the northern limestone. The latter two authors proved that Dokan conglomerate is proximal sediment of Upper Bakhtiari Formation.  This succession can be considered as the detrital limestone of very coarse grain (in the size of gravels).  Since the northern limestone of the present study is associated with the Dokan conglomerate, therefore the detrital limestone of sand size is not strange to the studied area as both derived from limestone terrain source area.

The detrital limestone beds are clearer in the northern limestone than the southern one and they consist of crudely laminated beds about 30-100 cm thick. Under the 10X hand lens the detrital grain can be seen as white and rounded grains more or less similar to cluster of rice grains. Perographically, they consist, under polarized microscope, of fine grain of dark color which are sorted and rounded with the sand size. The grains have more or less diffused boundaries and milky color and calcitic matrix (Fig.6). The beds of the detrital limestone change laterally and vertically to limestone conglomerate. No dolomite lithoclasts and fossil bioclasts are observed among the grain. This may due to weakness (against reworking) of these two grain types while the pure limestone grains are most resistive to breakdown especially in the fresh water. The detrital limestone contains of petrified plant roots fragments and calcified plant debris. The fragment are dark brown in color and has the thickness of 5- 20 mm, the fibers and cortex cell of the petrified root remains can be seen on the surface (Fig.7). The debris exists as elongate and randomly arranged fragments which appear as scratches of knife on rocks (Fig.7).


Bakhtiari ls f6Fig.(6) A) elongate superficial ooids and B) detrital limestone grains in the  northern limestone.

Bakhtiari ls f7Fig.(7)A) plant debris that preserved the fibers and cortical cells of root. B) The limestone contains plant debris that is replaced (by calcite) elongate plant debris, vugs and fenestrae.  Both samples are from Mewzha village


4-2-Stromatolitic limestone

Stromatolitic limestone is nearly similar to detrital limestone in color and laminations but the lamination are more crumpled and with lighter and darker color tones between the alternating lamina. Under polarized microscope the limestone appear as alternation of dark and light color fine grain limestones which are crinkled and may be associated with detrital limestone. These grains, as a lamina alternate with stromatolitic limestone lamina. Both types of limestones have the milky color and  have characteristics of  lacustrine limestone  that are given by Freytet and Verrecchia (2002)[9] whom stated that they  are varved, laminated, homogeneous, peloidal, brecciated, gravelly, bioturbated (burrows), bioclastic, algal, and stromatolitic. They added that detrital beds are sometimes present and are interpreted as bottom set deposits.

The stromatolitic limestone (Fig.9 B) contains fenestral texture which determined by Flugel (2010)[10], that is results from grain bridging and gas bubble Formation associated with decomposition or organic material.  After that, the open spaces of the fenestral fabrics are filled with sparite.  Friedman (1995) observed same texture (or facies) in the upper parts of tidal flatsof Arabian Gulf.

The Evidence that Upper Bakhtiari Conglomerate contain of fresh water limestone is the content of petrified plant roots from which the fibers and cortex sell can be seen on the surface of petrified root remain (Fig. 7). These limestones are deposited in the lakes that were formed due to streams channel abandonment or due to growth of anticlines in front of streams or in front of alluvial fan.


4-3-Oolitic limestone

Ooids are one of the constituents of recent and ancient carbonate successions in the studied area since Khanaqa et al (2013)[11], Ameen (2008)[12] and Karim et al. (2009)[7] found ooids in the recent, Cretaceous and Jurassic rocks respectively.  According to Davies et al., (1978),three conditions are usually  considered necessary for ooids formation which are  (1) water supersaturated with aragonite or high-Mg calcite, (2) availability of   a of nuclei, and (3) presence of the above two  condition in an agitation milieu. Pacton et al (2012)[13] discussed the role of the microbs in formation of ooids.

In the southern and northern limestones, coating grains are abundant in the form of pisolite and irregular oncoids. But well developed ooids are not found which notifies important signature which will be discussed in the other sections. The observed ooids are radial ones which are consisting of the radial needles and have fibrous form. The needles are needles are usually truncated by the next layer and arranged as concentric dark and pale grey laminae within the cortex (Fig.8A).When the grain has only one dark micritic thin layer it called cortoid. According to Flugel (2004)[10] the radial ooids are formed in quiet water of marine and nonmarine environment

4-4-Oncoid and pisoid

Oncoids are generally similar to tangential ooids which have size of larger than 2mm in diameter. Ameen (2008)[12], Daod and Karim (2010)[14] and Karim et al (2011)[15] had found oncoids in Cretaceous, Jurassic and in recent sediments in the studied area. According to Boggs (2009)[16], pisoids are coated grains that resemble ooids but they differ in theirinternal structures and they are generally less uniform in shape and larger (several millimeter to centimeters). Most of them are non-marine in origin.

The thin sections study shows that both southern and northern  limestones contain  large (more than 2mm in diameter) irregular coated grainsthat satisfy the above characteristics of both oncoids and pisoids but they are  more close to  the latter one (Fig.9). Under polarized microscope both limestone are similar to calcrete as they show signs of both leaching, deposition and encrustation. In rare case the dripstones can be seen

Bakhtiari ls f8Fig.(8A)Radial ooids and,  B)  with internal sedimentation (dark areas)  limestone  in the northern limestone, S6, 20X.

Bakhtiari ls f9Fig.(9)Highly  amalgamated  limestone with irregular pisoid in the southern limestone in Qadir Karam area. B) Stromatolitic limestone with birds eye


  1. Origin of The Limestone Bed

The record of the limestone beds inside the Upper Bakhtiari Formation is   extraordinary event since this formation is known for its prevalence of conglomerate and other clastics. However, Karim and Taha (2012)[1] found 300 meters of limestone conglomerate in Dokan area and they proved that it belongs to Upper Bakhtiari Formation. The proving was based correlation of the limestone conglomerate with its equivalent inside proper Upper Bakhtiari Formation near Taqtaq town. In Iran, Sahraeyan et al. (2013) [2] have found limestone in the southwest Iran in Upper Fars (Aghajari) Formation which is equivalent to Lower Bakhtiari Formation of Iraq. They cited that the limestone exists in a section which is about 2221 m thick and consisted of sandstone, claystone and conglomerate.

 The deposition of limestone beds in side Upper Bakhtiari Formation is due to the fact that in many places the source area was compartmentalized during Upper Eocene – Pliocene to different drainage basins with different rocks outcrops. Some basins were consisted of limestone source area while others were consisted of mixed rocks (sedimentary and ophiolite) source area. According to Karim and Taha (2012)[1], from the limestone outcrops  limestone conglomerates were deposited in the proximal areas (High and Imbricated Zones) while the cherts and ophiolites were derived  from the Thrust Zone and deposited as mixed sediments in the distal areas. The sediment area transported from source areas to the side of deposition by river.

It was possible that these rivers are suffered from one of two processes, the first is natural damming by mass wasting (large slides and debris flows) the second is oxbow cutoff of meandering river. The developed fresh water lake was site of deposition of detrital and stromatolitic limestone on the previous coarse clastics. The lateral gradation to pebbly limestone and then to ortho- conglomerate is the evidence to its deposition in fluviatile environment (Fig.5C). In the slides, desiccation cracks (Fig.11 left), bioturbated and highly amalgamation limestone (Fig.12), can be seen which denote deposition in   variable flow regime and terrestrial environments. 

The accurate age determination of the limestone inside Upper Bakhtiari Formation is difficult.  As mentioned before, marlstone occurs with the southern limestone from which three samples are taken and send to England for age determination. Only one sample was contained index fossils and it gave the age of Middle Miocene to Late Miocene (Fig.10).

Bakhtiari ls f10Fig.(10)The nannofossils in the green marl  that is associated with limestone beds in the southern area (near Tayi Jabari Village)


Bakhtiari ls f11Fig.(11) Left)  cracks in detrital limestone  which may be formed by desiccation, Right)  vugy and pisoidal limestone of southern limestone


Bakhtiari ls f12Fig.(12) A)Vugs and bioturbation clasts (dark pieces near the finger) in malgamated  limestone beds in the southern limestone.  B) Iron rich laminae


  1. Conclusion

1-The limestone beds are found in two different localities inside Upper Bakhtiari for the first times.

2-The limestone bed are consist of massive to crudely laminated layers that are composed of detrital or stromatolitic or oolitic beds.

3-They derived from limestone source areas during Upper Miocene and Pliocene

4-They are deposited in fresh water (Fluviatile) environment of unstable and high energy condition.

5- The paleogeography of the Upper Miocene–Pliocene was of different contrast topography of high mountain,   low land with dominate of fluvial and fresh water lakes deposits.



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Post Author: Professor Kamal Haji Karim

Professor at Department of Geology, University of Sulaimani, Kurdistan Region, Iraq