Tectonic Indications for Thickness Change of Geological Formations in South Eastern Part Area of Safen Anticline, North Eastern Iraq

Abstract


Introduction
Series folds of Iraqi foreland region is the outcome of the movement resulting from the convergence and subduction of the crust of the Neo-Tethys Ocean under the Eurasian plate, where the Alpine orogeny started in the Late Cretaceous and led to the subduction of the northeastern margin (Northern Iraq) of the Arabian plate under the Iranian-Turkish plates.As a result of this convergent movement, the normal displacements of the faults formed in the tension phase that preceded that stage turned into reverse displacements, which led to the growth of folds in northern Iraq as a deformation of the sedimentary cover that was deposited within the Neo-Tethys Basin, (Numan,1997 and2000).Listric faults responsible for the growth of folds were formed in northern Iraq during the Triassic age, where the Arab plate diverged from the Eurasian plate, (Numan and Al-Azzawi, 1993), and its reverse movement during the Alpine orogeny caused the emergence of different forms of folds in northern Iraq.Some folds (such as Safen Anticline) were affected by a complex faulting system that led to a more development of its northeastern limb compared to its southwestern limb, (Al-Azzawi, 2003).Geographically, the studied area is located in the southeastern part of the Safen Anticline in the northwestern part of Sulaymaniyah city in northeastern Iraq.The study area is located between the latitude: 36° 09' 35.39''-36° 12' 47.86''N and longitudes: 44° 38' 04 ''-44° 32' 47.72''E (Fig. 1).

Fig. 1. Location of the study area
Tectonically, the study area is located within the high folded zone of the Iraqi Western Zagros Fold-Thrust Belt (Fig. 2).

Fig. 2.
The tectonic divisions of the Iraqi Western Zagros Fold -Thrust Belt, shows the tectonic location of the studied area (Fouad, 2014) Stratigraphy, The exposed rocks in the studied area range in age from Early Cretaceous to the Early Eocene.They are represented by four Formations, which are from older to younger according to field observations, Qamchuqa, Bekhme, Shiranish, and Kolosh, (Fig. 3).Deposition of Qamchuqa formation in a shallow marine environment during the Alpian-Hetrovian.Both Bekhme and Shiranish were deposited in shallow reef and deep environments respectively , during the Campanian-Mastrachian age.The Kolosh formation was deposited in a deep marine environment during the Paleocene age, (Jassim and Goff, 2006).

The Relationship Between Faults and Folds
Folds are a type of plastic deformation, while faults are a type of brittle deformation, so having these two structures together in one place is an exciting thing that interests many researchers in this field.Some structural studies have found an explanation for the presence of these two structures together, which is that the first structure was formed at a different time from the formation of the second structure and under different pressure and temperature conditions (Van der Pluijm and Marshak, 1997).The growth of folds in north Iraq is related faults, (Al-Azzawi, 2003), where these faults may be formed as normal listric faults during previous tension time and showed reverse activation (thrusting) during last compression time, or may be formed as thrust fault during the fold development as result to increasing for the tectonic stress (Numan and Al-Azzawi, 1993).Generally, fault related fold mechanism of the fold thrust belts is divided in the types; detachment fold, fault bend fold and fault propagation fold, (Fig. 4).Fault propagation fold is the dominant mechanism in high and low folded zones of the Iraqi Western Zagros fold-Thrust Belt, (Fouad, 2014).Where the fault propagation fold shows the fore limp steeper than the back limp, and the dip of the back limp is parallel to fault surface that propagated the fold.Moreover, the back limp is high elevation area if compared with the fore limp area of the fold.According to this information's, can be understudied that the change of the elevation in the two limp areas causes different topography for the deposition basin at beginning start of fold growth and finally causes change in the thickness of the formations that deposit synchronously with the fold growth.But in sometimes, the change of thickness sediments may be present or absent because the normal slip that associated with strike slip faults, (Fig. 5).These uplifting was presented as result to normal slip associated with the strike movement and led to deposition of Bekhme formation in Safen Anticline area and absence of it in Khalakan Anticline area (Znad,2013;Ward, 2019).However, the strike slips movement was the dominant during the Late Cretaceous, some areas of anticlines within the High Folded Zone showed change in the thickness of the Late Cretaceous and Tertiary formations at the two limps as indication to the thrust movement of the listric (trust) fault that propagated fold, such as Bekher and Shaikhan anticlines (Al-Hubaty, 2008;Al-Khatony, 2009).Based on the surface measurements and the propriety seismic section transformed to depth, Safen anticline showed as fold related NE dipping thrust fault, (Fig. 7).

Calculation of the True Thickness
The Rowland method (Rowland, 1986) was adopted to find the real thickness of the geological formations from the drawn geological sections of the traverses that were identified within the study area.The width of the outcrop of the formations in the sections was calculated, which is represented by the confined distance between the contact surfaces of the formations, as well as the calculation of the dip angle for the layers of the formations, then the method of the trigonometric functions of the formations was applied when the angle of the slope is towards the dip of the layers because it corresponds to the position of the outcrop in the study area, equation and (Fig. 8).

Results
Three traverse's were chosen in the studied area, and used to drawing of three geological cross sections perpendicular to the fold axis, (Fig. 3).The three sections showed the flowing results: -

4.1.Thickness of the Geological Formations on the First Traverse
The Shiranish Formation varies in the thickness and dip on the two limbs of the fold in the study area, The dip of the formation in the northeastern limb is 44° while it 43° is in the southwestern limb, and the thickness of the formation in the northeastern limb is less than it is in the southwestern limb, with a thickness difference up to 7 meters, (Fig. 9) and (table 1).This discrepancy indicates the existence of a change in the topography of the surface of the depositional basin, which led to a difference in the depth of the basin between the two limbs of the fold due to the thrust movement for the Listric fault that propagated Safen Anticline.

Thickness of the Geological Formations on the Second Traverse
The measurement of the true thickness of the geological formations exposed in this traverse, represented by the Shiranish and Bekhme formations, showed that there is a discrepancy in the thickness and dip on the two limbs of the fold in the study area of the formations between the two limbs of the fold.The dip of the Shiranish Formation in the northeastern limb is 53° while it 46° is in the southwestern limb, and The dip of the Bekhme Formation in the northeastern limb is 51° while it 43° is in the southwestern limb, As the thickness of the Shiranish Formation in the northeastern limb is less than it is in the southwestern limb of the fold, with a thickness difference of up to 8 meters, (Fig. 10), and on the contrary in the Bekhme Formation, where the thickness of the formation increases in the northeastern limb more than it is in the southwestern limb with a difference Thickness up to 13 meters as shown in the table below, (Table 2).This variation in thickness indicates that there is a change in the depth of the basin and in turn affected the environment of deposition of the two formations within the study area, which led to the variation in thickness in the two limbs.

4.3.Thickness of the Geological Formations on the Third Travers
Measuring the true thickness of the geological formations in this traverse, represented by the Shiranish Formation and the Bekhme Formation, it was found that there are difference in the thickness and dip on the two limbs of the fold in the study area of the geological formations.The dip of the Shiranish Formation in the northeastern limb is 55° while it 40° is in the southwestern limb, and The dip of the Bekhme Formation in the northeastern limb is 55° while it 40° is in the southwestern, the thickness of the Bekhme Formation increases in the northeastern limb more than it is in the southwestern limb, with a thickness difference of up to 5 meters, (Fig. 11).This difference refers to the extension of the fault that affected the area in the second traverse and led to the occurrence of the same variation in thickness (Table 3).

Discussion
By comparison with the SW limb area, the large thickness of the Bekhme Formation in the NE limb area of the anticline may be interpreted as indication to continuance of the normal movement for the listric fault related Safen Anticline synchronously with Bekhme deposition time and led to downing the NE limb area for the fold as hanging wall if compared with the SW limb area, but the increasing of Shiranish thickness in the SW limp area (If compared with NE limb area) may be interpreted as indication to starting of the inverted movement (thrust movement) for the listric fault related Safen anticline synchronously with Shiranish deposition time (Fig, 12).

Fig. 9 .
Fig. 9. Geological cross section of the first traverse shows the change of Shiranish fn.thickness in both limp of Safen Anticline

Fig. 10 .
Fig. 10.geological cross section of the second traverse shows the change of Shiranish fn.And Bekhme fn.thickness in both limp of Safen Anticline

Fig. 11 .
Fig. 11.Geological cross section of the second traverse shows the change of Shiranish fn.And Bekhme fn.thickness in both limp of Safen Anticline

Fig. 12 .
Fig. 12.The proposed tectonic model of the sedimentation basin within the study area

Table 1 .
The true thickness of the Shiranish formation on the first traverse