Mineralogy and Geochemistry of Sandstones from the Tanjero Formation, Bekhme Gorge Northeastern Iraq: Implications for Paleoweathering and Provenance

Abstract


Introduction
The Late Campanian-Maastrichtian sequences are characterized by being complex and widespread in most regions of Iraq due to the tectonic factors related to the emergence and development of sedimentary basins, the sea level change factor also had an influential role in the variation in the thickness and diversity of the rocky formations of the sequences from one region to another (Jassim and Karim, 1984).The beginning of the closing of the Neo-Tethys marks the beginning of the Late Campanian-Maastrichtian cycle.This was followed by wide marine progress that covered all the lands of Iraq at the time, except for the Al-Rutba High (Jassim and Karim, 1984).A group of pre-formed faults resulting from the collision of the Arabian plate with the Turkish and Iranian plates dominated the formation of sedimentation basins whose axis extends Northwest -Southeast.This cycle also witnessed the peak of ophiolite eruptions, raising it high above sea level and subjecting its sediments to rapid erosion processes, as its products were deposited within a narrow basin along the northern and eastern edges of the Arabian Plate, which is currently knownforedeep basinas flysh sediments, the thickness of which exceeded 2000 m.These deposits extended from the south-eastern parts of Turkey through the north-eastern parts of Iraq to south-western Iran and reached as far as Oman.The (Late Campanian -Maastrichtian) cycle, includes eight formations: Hartha, Tayarat, Dekmah, Aqrah, Bakhma, Hadina, Shiranish and Tanjero (Jassim and Buday, 2006).The Tanjiro Formation consists of marl beds and carbonate sandstone with siltstone beds in its lower part.While its upper part is composed of silty marl, sandstones and limestone containing sand-silt size bioclastic.The upper part of the formation is interfingering with the carbonate rocks of the Aqrah Formation.The thickness of the formation varies from one region to another, and it is accompanied by a change in the size of the sediments as it becomes softer and the thickness of its beds decreases towards the southwest of the type section site.The presence of the Aqrah Formation was recorded in the upper part of it, in the form of a lens (Jassim and Buday, 2006).Sharbazheri (2008), explained that the contact surface between the Cretaceous and the Tertiary sediments is a harmonic surface between the Tanjiro and Kolosh Formations in the Sirwan Valley, during his study of the fossils of both ages, and indicated the presence of unconformable surface within the Tanjiro Formation in the Jawarta region, north-eastern Iraq.The age of the unconformable surface was estimated with a time period of 1.23 million years.Al-Rawi et.al.,(2002),conducted an extensive study of the formation in three sections: Sulaymani, Dokan and Rawanduz.They concluded that the lower part of the formation at Sulaymani section was deposited in a relatively shallow environment.Karim (2004), divided the Tanjiro Formation into three parts and considered the lower part regressive, the middle part progressive and the upper part regressive.Karim (2005), discussed the origin of the spherical structures in the Tanjiro Formation, in the Sulaymani region and concluded that these structures were formed during deep burial after deposition by the weight of the overlying rocks and tectonic stresses.Karim and Surdashy (2005a;2005b), determined the direction of the ancient currents using the sedimentary structures found within the Tanjiro Formation (south and southwest), and suggested merging the sedimentary basin of the Tanjiro Formation with the sedimentary basin of the Shiranish Formation and called the basin of the upper Cretaceous creeping continent of the Zagros belt and this name has been developed as an alternative to the Miogeosyncline.Karim (2007), observed the potential impact of storms on the sediments of the pre-continent basin in the upper Cretaceous, as a special case for identifying marine storm sediments in the Tanjero Formation Sulaymani and concluded that waves and currents led to re-deposition in the form of storm sediments.Malak (2015) mentioned that the lower contact of the formation is conformable with the underlain Shiranish Formation at Dokan area northern Iraq, which is composed of interbedded of green siltstone and sandstone layers.As a stratigraphic setting in Zagrose foreland basin system, Tanjero Formation deposited in foredeep depozone and its position occupied the upper clastic units at the trinity units that characterized the sediments of foredeep depozone ( Znad, 2013;Znad et al., 2020).Ali et al. (2022) studied the heavy minerals of the Tanjero Formation at the southwestern limb of the Surdash anticline in the northeast part of Sulaimanyia city.They were mentions that the most abundant heavy minerals are opaque (pyroxene, epidote, amphibole, chlorite, rutile, and garnet).The significant abundance of unstable and metastable minerals in the heavy mineral suites shows their direct derivation from the adjacent main basic igneous and metamorphic source rocks, which are situated in the Imbricate Zones in the northern and northeastern parts of Iraq.
The aim of the research is to determine the mineral components of sandstones in order to reach their name and to determine the source of these sandstones based on point counting results through which the percentage of mineral components was calculated.The geochemical analyzes were also relied upon to determine the maturity, amount of weathering,as well as to determine the origin of the sandstone based ongeochemistry.

Geological Setting
The study area is located northern Iraq, Erbil, within the High Folded Zone of the Unstable Shelf (Jassim and Buday, 2006).The Bakhme section is exposed on the northern flank of the Perat fold near the northern entrance to the Bakhme gorge through which the waters of the Upper Zab flow.The lithological section of the Tanjero formation exposed on the left shoulder of the river, at the intersection of longitude (36° 42' 27'' N) and latitude (44° 16' 26'' E), was studied (Fig. 1).

Materials and Methods
The research methods of this study included two stages: a field study and a laboratory study.The field study included several field trips to the study sections in the Bekhme Gorge (Fig. 2).During these trips, the best exposed paths were chosen, in terms of the clarity of the formation successions and the exposure of its lower and upper contact.The sampleswere collected according to the changes in lithology and many observations were recorded about the rock beds such as thickness, color and the relationship of the layers with each other, describing the nature of the stratification, as well as taking many photographs in the field.The sampling was done by taking 13 samples from the sandstone beds.
As for the laboratory study, it was represented in preparing the samples for the study by making thin sections of the 12 samples in the geological workshop of the Earth Sciences Department, College of Science, Mosul University.Then, all these slides were subjected to a precise petrographic examination using a polarizing microscope.

Fig. 2. Lithological column of the Tanjero Formation in Bekhme area
The petrography of the sandstone was carried out on the thin section of the sandstone beds.Modal analysis of the sandstone composition was determined by point counting of an average of 300 points per thin section of spacing of the largest grain size using the Gazzi- Dickinson method (Ingorsoll et al., 1984).The results are shown in Table 1.The major and trace element analysis of 10 samples were determined by X-ray fluorescence spectrometry at the University of Baghdad laboratories, Iraq.The best five samples were selected because of the similarity of the calculations and giving them the same results (Table 2).Moreover, some statistical processes were carried out using Excel software to determine the maturity, paleoweathering, and provenance of the Tanjero sandstones.

Results and Discussion
The Tanjero Formation appears in the Bakhme section with a thickness of 75 m.thick where it is bounded from the bottom by the Shiranish Formation.The lower contact surface of theTanjero Formation with the Shiranish Formation appears unconformable, indicative of the presence of a conglomeratic bed followed by the first bed of the Tanjero Formation, which consists of gray sandstone that contain at the bottom the gravel and pillow structures (Fig. 3 a).Followed by beds of brittle grey-brown marl, which contains remnants of iron oxides(Fig.3 b).It is followed by a succession consisting of alternating beds of hard sandstone (10-30 cm thick) containing pillow structures and small-scale cross-bedding and beds of black claystone and marl and a bed of coarse sandstone containing gravel in its lower part (2 cm) and traces fossils.Followed by thick beds of brittle grey marl and successive beds of brittle green sandstone, claystone and brittle marl.The remainder of the Tanjero Formation consists of thick beds of brown to green claystone alternating with brittle sandstone beds.It concludes with a bed of claystone beds representing the boundary between the Tanjero and Kolosh formations.

. Sandstone Petrography
The mineral components of the studied sandstones are illustrated in Table 1as mentioned above:

Quartz
Mono-and polycrystalline quartz constitute about 13% of the total components of the sandstone in the formation.Monocrystalline quartz forms about 10% of the total percentage of quartz (Fig. 4 a), while the percentage of polycrystalline was about 3% (Fig. 4 b) (Table 1).The monocrystalline quartz crystals are characterized by their small to medium sizes, regular shapes, semi-rounded edges and parallel extinction, with some inclusions recorded, such as iron oxides.however, indicates that quartz with a wavy extinction and the presence of impurities in it, as well as vacuoles, is usually sourced from granite igneous rocks (Folk, 1974).The polycrystalline quartz, the percentage is low (3%), and this is due to its weak resistance, which gives evidence of the conditions of its transfer from the source rocks to the deposition site.These crystals are characterized by their wavy to parallel extinction, irregular shapes, and angular to semi-rounded edges.In this regard, (Folk, 1974) mentionedthat this type of polycrystalline quartz is derived from metamorphic source rocks often .

Feldspar
The feldspar is present in small percentages, as its presence rate is about 8% of the percentage of the total basic components of sandstone in the formation (Fig. 5a).It is of two types, potassium feldspar, which reaches a rate of 6.3%, and the second type is plagioclase feldspar, which is less present, which reaches a rate of 2.1%.Most of the feldspar grains are represented by the orthoclase mineral, which is severely affected by the decomposition processes due to chemical weathering.The study also recorded the mineral plagioclase.Potassium feldspar is more stable during chemical weathering than plagioclase, so its presence is higher than the second, and the low ratio of both types of feldspar indicates that the source rocks from which sandstones are derived suffered from weathering processes and were transported a long distance away from their source.In this regard, (Boggs, 2006) points out that the plutonic and metamorphic igneous rocks are the good inheritance of the orthoclase and plagioclase minerals.

Rock fragments
The rock fragments constituted the highest percentage of the total components of the sandstones in the Tanjiro Formation (Fig. 5 b), with more than 50%.Where the pieces of its grains appear in varying sizes coarse to fine in size, irregular in shape and eroded edges.Carbonate rock fragments are the most common among the other types of rock fragments having about 15.5% while the percentage of flint fragments (chert) is about 13.9%.The current study also noted a high percentage of igneous rocks (13.7%) and metamorphic fragments (10.3%), which mostly consist of serpentinite.Boggs (2006) has mentioned that the ratio of rock fragments to the role of mechanical (physical) weathering was greater than the role of chemical weathering with the speed of burial and in general the short transportation distance, and this is evident from the high percentage of cutting flint.It is clear that the diversity of rock fragments indicates the diversity of the source rocks and the proximity to influential tectonic events and activities.

. Matrix and cement
The current study shows a high presence of matrix and cement within the ratios of the total components of sandstone in the Tanjiro Formation, where the ratio together (23%).The percentage of cement reaches 9% while the average percentage of the matrix reaches 14% which is mainly composed of clay and fine grains of silt.As for cement, it consists mostly of carbonate cement (4 a), and to a lesser extent of iron and silicate oxides.The high percentage of the matrix is due to the method of transporting and depositing the components of sandstone and their burial capacity, indicates that one of the most important features of turbidity deposits is the high content of the matrix, especially claystone and siltstone.

Classification of Sandstone
The classification of Williams et al. (1982) was based on the classification of Dott (1964), which depends on the percentage of matrix in sandstone is used in this study.If the matrix is less than 5%, the sandstone are called arenite, while if the matrix percentage is more than 5%, they are called wacke .This classification is more suitable for sandstone of the formation under study because it contains a high percentage of matrix of about 14.6 %.The point projections of the three sandstones components (Q, F, R) showed the accumulation of samples mainly within the (lithic wacke) (Fig. 6).

Geochemistry
The samples of the Tanjero sandstones show low silica, where the SiO2 contentranges from 28.85 to 35.05 wt% (Table 2).This is because the clastic sediments were partly derived from the nearby mountains causing recycled sedimentary sources or weathering of silicates that led to partial removal of feldsparsand ferromagnesian minerals.The weak negative correlation of SiO2 with K2O, MgO, and Na2O (r = -0.64,-0.62, and -0.57respectively) (Fig. 7) indicates the above information (recycling or weathering sediments).CaO and P2O5 show negativecorrelations with SiO2 (r = -0.83and -0.87 respectively) (Fig. 7) suggesting that they are associated with calcite and apatite.Al2O3,FeOt, and TiO2 have dispersed correlations with SiO2 (Fig. 7) that may derive from mixing minerals of mica, feldspare, hematite, and rutile.

Maturity and paleoweathering
The index of compositional variability (ICV) expresses the maturity of clastic sediments, where the ICV =(Fe2O3 + K2O + Na2O+ CaO + MgO + MnO)/Al2O3 (Cox et al., 1995).When the ICV is more than 1 refers to immature clastic sediments.On the other hand, when the ICV is less than 1, shows farther mature sediments (Cox et. al., 1995).The sandstones of the Tanjero Formation have ICV values of more than 1 (Table 2) indicating that they are immature clastic sediments.The high ICV values denote the occurrence of numerous non-silicate fragments especially carbonates, reflecting that the major elements of the Tanjero sandstones in the studied area may be influenced by diagenetic processes.The chemical index of alteration (CIA) refers to the degree of weathering (Nesbitt and Young, 1982).The equation that expresses to the CIA is [Al2O3/(Na2O + K2O + CaO* + Al2O3)] ×100.In this equation, the oxides indicate the molar content, but CaO* is the quantity of CaO integrated with the silicate portion of the samples, where CaO excludes that incorporated in the carbonate and phosphate concentrations (Nesbitt and Young 1982).Therefore, the CIA refers to the degree of weathering from feldspars to clays (Fedo et. al., 1995;Kautz and Martin, 2007).In the studied samples, the concentrations of CaO are greater than Na2O, where the amount of CaO is considered equivalent to Na2O (McLennan, 1993).the CIA values from 50 to 60 refer to the primary weathered sedimentary materials; 60 to 80 indicate moderate, and 80 to 100 show intensive weathering (Fedo et al., 1995).
In the studied samples, the CIA values range from 68.02 to 73.07 with an average of 69.83 (Table 2), this range of values refers to moderate chemical weathering of studied samples.Additionally, Fig. 8 of CIA vs. ICV confirms that the studied samples are geochemically immature and they were derived from moderate (weak to intensively) weathered source rocks.The A-CN-K diagram (Fig. 9) refers to increase weathering in the studied sandstone samples.The path of samples tends to the slight conversion from K-feldspar to plagioclase by Ca addition.

3. 2. Provenance
The sedimentary provenance of the Tanjero sandstones can be determined by the discrimination function diagram of the major oxides (Fig. 10) (Roser and Korsch, 1988).All the studied samples are plotted in the quartzose sedimentary provenance field.The TiO2 versus Zr diagram (Fig. 11) refers to that all provenance of studied sandstones is felsic igneous rocks.The studied samples were also projected on trigonometric diagrams (Fig. 12), which refers to the source rocks (Dickinson, 1985).The Qm-F-Lt diagram shows most of the studied samples are plotted in the lithic recycled field except two samples (T22 and T27) are located in the transitional arc field.This represents the part of the recycle orogeny field closest to the apex of the cut-off triangle (Lt).Igneous and metamorphic rocks are partially exposed in the regions of the orogenic belts where tectonic distortions prevail as a result of folding and thrust processes.The products of the weathering of the high orogenic belt are transported to the adjacent foreland basins (Dickinson and Suczek, 1979;Dickinson, 1985).The relatively high content of quartz and the commonality of rock cuts compared to feldspar reflect mainly mixed origin (sedimentary and metamorphic fragments) and secondarily igneous origin, indicating the derivation of the sandstones from the belt of recycle orogeny.It is possible to observe the types of rocks in the collision zone between the passive Arabian margin and the effective Iranian-Turkish margin that formed the orogeny belt along the collision zone to know the source rocks that formed parts of the Tanjero clastic rocks.The prevalence of clastic filling in the rocks is evidence of the rocky origin (metamorphic and igneous), and thus it is believed that the source rocks of the Tanjiro Formation are sedimentary, igneous, and metamorphic rocks.(Alavi, 2004;Jassim and Goff, 2006;Sissakian, 2013;Fouad, 2015) Correspondingly, The high ICV values (>1) indicated immature sandstone and confirm that the clastic sediments occurred in the tectonically-active-settings area (Cox et al., 1995).(Hayashi et al., 1977) Fig. 12. Tectonic provenance discrimination diagrams of the Tanjero sandstones.(Dickinson et al., 1985).Qm (monocrystalline quartz), F (total feldspar), and Lt (total aphanitic rock fragments) calculated from sandstone point counting(Table 1).

Conclusions
The Tanjero sandstones in the studied area have low silica contents; this is clearly shown by themoderate-negative correlation of silica with the most major oxides.The index of compositional variability (ICV) from the Tanjero sandstones is more than 1.The ICV firstly reflects immature clastic sediments, as well as, the high ICV values confirm that the clastic sediments occurred in the tectonically-active-settings area.The chemical index of alteration (CIA), the chemical index of weathering (CIW), and the plagioclase index of alteration (PIA) refer to that the source area encountered moderate chemical weathering.The tectonic discrimination diagrams of the Tanjero sandstones suggest the quartzose sedimentary provenance, felsic igneous rocks source, and transitional arc-lithic recycled settings.It is possible to observe the types of rocks in the collision zone between the passive Arabian margin and the effective Iranian-Turkish margin that formed the orogeny belt along the collision zone to know the source rocks that formed parts of the Tanjero clastic formation rocks.The Tanjero Formation was deposited in a foredeep depozone within the Zagros foreland basin system, its sediments derived from slice sheet that constitutes accretionary prism assemblage belong to a different origin.As the convergence pulses protracted between override Iranian plate and ridden plate.Arabian plate, this resulted in a movement of tectonic loads on the edge at Arabian plate.The consequence of this tectonic situation leads to migrate the depocenter of the foredeep depozone toward south-west.So the sediments of the Tanjero Formationhave been recycled in addition entrance of new sediments derived from constitutes of the accrete prisms (Tectonic load).

Fig. 1 .
Fig. 1.Geological and structural map of the study area showing the location of the studied section (After Zebari et al., 2019).

Fig. 3 .
Fig. 3. a: Pillow structures at the base of sandstones bed; b: marl contains iron oxides

Table 1 .
The major petrographic constituents of Tanjero sandstones in Bekhme section

Table . 2
. Major and trace elements concentration of the Tanjero sandstones