Stratigraphy of Sartaq-Bamo Area, Northeastern Iraq

Stratigraphy of Sartaq-Bamo Area, Northeastern Iraq

Kamal Haji Karim
Published In: Journal of Iraqi Geological Society, 1997

Mapping, lithologic description, correlation and mutual relations of stratigraphic units (formations) of the area are discussed. As compared to other localities of northern Iraq of the same tectonic position boundary between High Folded and Low Folded Zone, four stratigraphic differences are found and discussed in detail, they are:
1. Sinjar Formation (Teleh Zang Formation in Iran) is represented only by single shoal nummulitic facies.
2. Gercus Formation(Kashkan Formation in Iran) is composed of sandstone and conglomerate, without red
claystone and grades laterally to Avanah vertically to Pila Spi Formation.
3. Recognition of Avanah Formation for the first time in the area, which intertongues, with Pila Spi Formation.
4. Relatively rapid thickness variation of Sinjar Formation and Gercus Formation are observed in the area, which is attributed to irregularity of basin configuration.

The studied area is located about 35km southeast of Darbandikhan town at the intersection of latitude 34 58 and longitude 45 52 from northeastern Iraq. The area tectonically represents the boundary between High Folded and Low Folded Zones (Buday and Jassim, 1984). The area includes two mountains, Khushk Mountain which lies to the north of the area and Bamo Mountain occupies the eastern part of the area (Fig. l and 2), and between the two mountains there is a deep flat bottomed Sartaq valley, the place where the present study is achieved. The valley, being surrounded by vertical cliffs and ridges, it can be reached only through Sartaq Gorge, about 20m wide and 600m long, here a paved new road excavated recently which cuts the vertical beds perpendicularly to the strikes,(fig.3A) consequently new fresh outcrops are exposed along both sides of the road.
In contrast to vertical beds (Fig.3A), there are horizontal ones near Kani-Zhalla village (Fig.2) between the two extremes there are some intermediate dip angles of about 30 degrees to the west near Sartaq village (Fig.3B). Along the southeastern side of a back thrust (opposite to the main Zagros thrust) can be seen in the layers of Sinjar Formation (Fig.4) while the northwestern side show no such thrust(Fig.5), . The area is mainly occupied by a part of a large elongated asymmetrical anticline, runs from Darbandikhan area through the studied area to Iranian border at the south -east, with western limb being steeper than the eastern.

The availability of good outcrops and an excavated section in Sartaq valley led to detail fieldwork.
The fieldwork included, boundary indication, lithologic description and sampling of three selected sections, they are:
1.Sartaq Gorge section
2.Kani Zhalla section
3.Bamo Mountain section
The Bamo Mountain section is similar to the Gorge section so it is not mentioned in the description. The remained efforts are directed toward interpretation of mutual relations between different rock units, their chronologic succession and environmental discussion.
The stratigraphic sections in the area have four differences as compared to sections of the same tectonic position, boundary between High Folded and Low Folded Zones (Fig. l), in other localities of northern Iraq as following:

Fig. (1) Location and geological map of the studied area

Fig.(2) Cross section of the left side of Sartak Bamo Gorge and Khushk mountain (30km east of Darbandikhan town.

Fig.(3) A) Sartak Gorge about 20m wide and 200m high with vertical beds cut through Sinjar, Pila Spi and Avanah formations. B) Northeastern side of the Sartag valley showing deformation of the units (60degrees at background and about 25 degrees at forground).

Fig.(4) Back-thrust in the southeastern side of Sartak Valley. This thrust is opposite (toward northeast) to the main Zagros Thrust (fore-thrust) in which the direction of the thrust is toward southwest. The thickness of the rocks is about 30m.

Fig.(5) northern side of the Sartak valley showing the high thickness (massive ) of the Pila Spi Formation.

1. Sinjar Formation (Upper Paleocene-Lower Eocene): Boundary and facies
This Formation is represented exclusively by single facies of shoal Nummulitic limestone (Nummulitic packstone to wackstone, fig. 7) which is underlined conformably by Globigerinal green marl of Kolosh Formation (Middle Paleocene), (Fig.6 and 8-1, 8-2 and 9-1). The gradational nature of boundary between Kolosh and Sinjar Formations are exposed in the form of alternating beds of green marl and Nummulitic sandy limestone (calcarenite), the later changes toward Sinjar Formation to Nummulitic limestone or (Nummulitic packstone to wackstone), in addition to this the cooked samples of Kolosh Formation contain brachiopod shells and show remarkable increase of iron oxides and coarse clastics toward the boundary at the expense of Globigerina foraminiferas.
In other locality of Northern Iraq, Sinjar Formation is well known for reefal association facies (reef; back reef and fore reef), which has been studied by (Bellen et. al, 1959; Al-Saddiki, 1968; Shathaya, 1980; Al-Surdashy, 1988). Outside the studied area, the type locality from Sinjar area is the only place has been reported to have Nummulitic limestone. The thickness of this Formation in the Gorge section reaches 60m but at Kani Zhalla is exactly 20m, the reduced thickness in the later section is probably returned to nearness of source area and topographic higher land consequently more clastic sediments influx than the Gorge section as reflected by deposition 60m of conglomerate and sand stone in Kani Zalla section (see point no.4 for details).

2- Gercus Formation (Lower to Middle Eocene):
Lithology change and lateral gradation to Avanah Formation
The stratigraphic position of Gercus Formation between Sinjar Formation and Pila Spi Formation has been filled by different sediments as following:
A: Kani Zhalla section (Fig.5) contains thick sequence of 40% sandstone and 60% conglomerate; the total thickness of this sequence is about 70m with brown to dark brown color and with some red colors as coating of weathered surfaces. The conglomerate is composed exclusively of moderately sorted and subrounded chert gravels but the sandstone beds are composed of lithicarenite, according to lithology of conglomerate the possible source area of these sediments is Qulqula Group.
B: The Gorge section Fig.2 and 3A) contains well-bedded sequence of yellowish marly limestone with sparse large nummulites. This sequence is located between Sinjar and Pila Spi Formations, about 40m thick and the frequency of nummulites decreases upward. In this study it is assigned as Khurmala Formation due to stratigraphic position and lithological similarities with the formation in the type section. Close inspection of this interval shows a several thin beds of red claystone (10-20) cm thick, The thin beds are surrounded from the top and bottom by wavy, distorted and oxidized surfaces with iron oxides as one laminae of 1cm thick, occasionally the overlaying surface shows nodules of red claystone surrounded by chalky limestone, softer and more porous than the host rock. A close relation can be established between clastic sequence of Kani- Zhalla and marly limestone of the Gorge section, if the former section is considered to be closer to shore line than the later one, therefore the conglomerate beds are correlated with red claystone, on the other hand, sandstones are correlated with marly limestone of the Gorge section, this based on the fact that the high energy during conglomerate deposition enable the fine clastics (clay) to reach the basin (Gorge – section) and to deposit as red clay or to mix with carbonates and precipitate as marl but during sand deposition only the dissolved carbonate sediments are reached the basin with very little clay forming marly limestone after deposition and lithification. In these connections, (Blatt et. al, 1980) state that sediments become finer toward the basin therefore can be used as paleocurrent direction finder.
C:- The upper part of nummulitic marly limestone contains gypsum nodules Fig.9-3) which spreaded through the thickness of 2m ; they are spherical and with some angularity, about 10 nodules were found in 1 sq.m., they range in diameter from 2cm to 5cm and no relations are observed between bedding planes and distribution of the nodules. The occurrence of gypsum nodules are well studied in recent studies of Arabian Gulf by (Shearman, 1963; Butler, 1969), there the nodules are formed by early diagenetic processes in supratidal environment known as Sabkha. Nodular gypsum or anhydrite are also recorded from several ancient Formations which are reviewed by (Dean and Schrieber, 1978) , in addition to this gypsum lenticules are mentioned by (Bellen ,et. al., 1959) at the type locality of Gercus Formation, however the coexistence of gypsum nodules and nummulites in the marly limestone shows some ambiguity, their interpretation needs the following explanations:-
The formation of gypsum nodules and precipitation of nummulitic marly limestone cannot occur together because of environmental differences of the two processes, the first happens in hypersaline lagoon or in hot and saline supratidal environment (Dean and Schreiber, op. cit), on the contrary, the second is found in open marine environment of normal to warm temperature (Henson, 1950; Al-Hashimi, 1972). Therefore, their present coexistence is returned to later growth of gypsum nodules after a fluctuation of sea level (minor regression) by which the shoal or subtidal environment of nummulite are changed to supratidal environment of gypsum nodules analogous to supratidal of Arabian Gulf at present time .The thin beds of red claystone and iron oxide surfaces are evidences of subaerial exposure of the marly.

Fig. (6) Stratigraphic column of the both sides Sartak Bamo Valley

3. Avanah Formation (Middle to upper Eocene).
The upper parts of the sections are composed of white fossiliferous limestone occasionally changes to porcelaneous limestone with thickness no less than 170m, in addition to large Alveolina foraminiferas (Fig.5 and 9-2) contains some small nummulites. The field and thin section studies are proved that this type of the limestone in the area belongs to Avanah Formation, which is as far as the author aware, this study is the first one to mention the existence of this Formation in the Northeastern Iraq. The middle part of this Formation contains 20m of recrystallized dolomitic limestone; this type of occurrence is treated by (Bellen et al, op. cit) as Pila Spi tongues at the type locality section from Kirkuk well no.116 on the Avanah dome.

Relation between Gercus Formation and Marly Limestone (Khurmala Formation)
As compared to original description by (Bellen, et al. l959) there is two lacks in the lithology of Gercus Formation in the studied area. The first is the absence of red claystone, the main constituent of Gercus Formation. The second one is that the red color cannot be recognized except for some red spots on weathering surfaces which can be observed only by the close inspection, yet it has been designated as Gercus Formation because of analogy in lithology and stratigraphic position to Gercus Formation. If we compare between Gercus Formation of Sartaq-Bamo area and that of the type locality the former is similar to the lower part of the type locality section which is composed dominantly of sandstone and conglomerate (Bellen, et. al. 1959). On the other hand the nummulitic marly limestone of Gorge section, besides of equivalency in time, stratigraphic position and source area to clastic sequence (Gercus Formation) of Kani-Zhalla section, no longer can be recognized as Gercus Formation because of lithologic differences, so it is more convenient to be named as Khurmala Formation (Fig.6).

4. Tectonism and thickness variation in the area:
Another important stratigraphic feature in the area is equality of dip angles of the Pre-┬ČOligocene Formations and the Middle-Miocene Formation (Fatha Formation), which can be observed clearly near Horeen village (Fig.2). This bedding relation shows that the structural features now observed in the area, such as folds, and faults are younger than Middle Miocene except for some irregularities (topography) in the basin configuration during Sinjar and Gercus deposition. These irregularities are caused remarkable thickness variation of these two Formations in the area. In the Darbandikhan area according to (Jassim et al., 1974) these irregularities are returned to Middle Eocene faulting which runs parallel and adjacent to Darbandikhan anticline. But in the present study these irregularities in the studied area are suggested to be formed from factors other than faulting as following
A. the Kolosh basin represents turbidite sediments (Al-Hashimi, 1985), this characteristic of the basin created several relief features with relatively high topography such as submarine fans, canyon and submarine channels, therefore, deposition of Sinjar and Gercus over these irregularities cause variance of their thickness. B. Differential compaction of soft sediments of the Formations such as Shiranish, Tanjero and Kolosh .The inequality in compaction may be between marl and sand or between thick sediment and thin one. C.The Kolosh basin is narrow rapidly subsiding trough (geosyncline) (Buday, 1980.) so it is familiar to have some degrees of slope and cause thickness variation.

Fig.(7) the sculpture of the nummulite of the Sinjar Formation as exposed by differential weathering

Fig.(8) different part of the Sartak Bamo Valley showing exposed formations

Fig.(9) different constituents of the exposed formations in the Sartak Bamo Valley

The stratigraphic study of Sartaq-Bamo area led to the following conclusions
1. Unlike other localities of Northern Iraq (boundary between High Folded and Low Folded Zones), Sinjar Formation is composed of shoal nummulitic Limestone (Nummulitic packstone) only. 2. Recognition of Avanah Formation in the area for the first time. 3. Both Sinjar and Gercus Formations show notable lateral thickness variation at the expense of each other, the rapid variation is attributed to shallow environment of deposition and irregularity in the basin. 4. Gercus Formation is composed of sandstone and conglomerate only without red claystone and grade laterally to Avanah Formation and vertically to Pila-Spi Formation
5.Pila-Spi and Avanah Formations in contrast to Gercus and Sinjar Formations show homogeneous and continuous lateral extension because of the period of tectonic quiescence. 6. Observation of interlingering of Pila-Spi and Avanah in the area, which is reflected by alternating of dolomite and fossiliferous limestone.
7. All exposed Formations in the area have intergradational contacts except for Avanah and Fatha Formation. 8.The depositional environment of the formations range from deep marine for Kolosh Formation, shoal for Sinjar Formation, fluviatile or deltaic environment for Gercus Formation, restricted lagoon for Pila-Spi and normal lagoon to marine environment for Avanah Formation in addition to possibility of Sabkha as the environment of lower part (marly limestone) of Avanah Formation.

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