The Role of Heavy Minerals in Understanding the Provenance of Sandstone: An Example from the Upper Cretaceous Tanjero Formation, Surdash Region, Northeastern Iraq

Abstract


Introduction
Heavy minerals (specific gravity > 2.89) are useful indicators for provenance and source rocks of sediment and sedimentary rocks for determination and understanding of diagenetic processes, since the various types of source rocks yield varied heavy mineral suites (Morton, 1985).So, the provenance of sandstone (Hubert, 1971;Schafer and Dorr, 1997;Uddin and Lundberg, 1998) and interpreting source rock geology including mineralogical composition and transport processes (Pettijohn et al., 1987;Da Silva and Vital, 2000) are widely used in sedimentological studies according to the nature and distribution of heavy minerals.The Tanjero Formation is an important formation in stratigraphic column in Iraq, which is an Upper Cretaceous (Campanian-Maastrichtian) lithological unit that was cropped out at the High Folded and Imbricate Zones in Northeastern Iraq (Buday, 1980) and extends as a range towards northwest-southeast along the Iranian border.Many researchers were studied Tanjero Formation in all aspect among those studied we mention (Al-Rawi, 1981;Abdul-Kareem, 1986;Jazza, 1992;Lawa et al., 1998;Al-Rawi and Al-Rawi, 2002;Karim, 2004;Karim and Surdashy, 2005;Al-Zubaidi et al., 2016;Ali and Mohmmad, 2018;Jassim and Al-Hazza, 2020).The formation is essentially composed of a thick sequence of clastic sandstone rocks, claystone, calcareous shale and marl with thick beds of limestone and conglomerate (Buday, and Jassim, 1987;Karim, 2004).Sissakian et al. (2016) delineated that this formation in Surdash anticline, is separated into two members: Lower Member by Approximate thickness of about 300 meter which comprises of alternation of sandstone, mudstone, shale, conglomerate, claystone and sandy limestone and Upper Member is composed of 659 meters of alternation of sandstone, mudstone and silty claystone, with silty claystone and mudstone dominating.This research aims to conduct studies on the heavy mineral assemblage and it discusses the distribution patterns and their corresponding provenance and source rocks of these minerals in the study area.

Location and Geological Background
The study area is located in Sulaimaniya near Surdash village about 40km to the northwest of Sulaimaniya city.The sampled representative section falls within the High Folded Zone (Buday and Jassim, 1987), between longitudes 45° 04' 45.12" -45° 03' 53.28" E and latitudes 35° 51' 8.64" -35° 50' 42.72" N (Fig. 1).The Tanjero Formation was deposited in Mesopotamian Foreland Basin in the western Zagros Belt, northeast Iraq, (Buday, and Jassim, 1987).Where Hassan et al. (2014Hassan et al. ( , 2015) ) explained that the formation was derived from the Thrust Belt and uplifted Zagros Fold as revealed by their petrology and geochemical evidence.The Tanjero Formation was formed when the Neo Tethys Ocean closed as the Arabian and Iranian plates collided through the Late Cretaceous and Miocene, which led to the deposition of a thick sedimentary succession in the Mesopotamian foreland basin south of the Zagros Fold-thrust Belt (Jones et al., 2020).It stretches as narrow northwest -southeast belt near and parallel to the Iranian border (Karim and Surdashy, 2005).Folding and thrusting dominate the Zagros Orogen is characterized by rough mountains with an irregular steep dendritic drainage pattern overlaid on structurally complicated terrain (Buday, 1980).The Ueumich-Dokhtar Magmatic Arc, the Sanandaj-Sirjan Zone, and the Zagros Orogen can be separated into four subparallel tectonic zones.The Mesopotamian Foreland Basin and the Zagros Fold and Thrust Belt (Berberian and King, 1981;Alavi, 1994;Mohammed et al., 2014, Yahya, andAl-Shammary, 1993).The study area represents as a part of the Mesopotamian Foreland Basin formed as a result of the continuing collision between the Arabian and Iranian plates, which resulted in the consumption of Neo Tethys (Ali et al., 2013).

Materials and Methods
Twenty samples of sandstone were collected from field to analyzing heavy minerals, from the Tanjero Formation cropped out at southwestern limb of Surdash anticline near Surdash village of northeast part of the Sulimaniya city of Iraq (Fig. 1).Each sample represents a geological cycle of sandstone unit; all samples were dried by air before being separated into sand fractions (250-63 μm) from other sizes and components.The heavy fraction was separated from the light fraction using bromoform (specific gravity = 2.89; Carver, 1971;Mange and Maurer,1992).Each heavy fraction sample was washed with acetone on filter paper, dried, and mounted with Canadian balsam (RI = 1.52) on slides for mineral identification using a polarized binocular microscope.Heavy mineral preparation and extraction were carried out at Kirkuk University, Department of Applied Geology, Faculty of Science.The Ribbon technique, which is the utmost prevalent technique for heavy minerals, was used to perform the quantitative analyses (point counting) (Mange and Maurer, 1992).According to Cascalho and Fradique (2007) heavy mineral suites and their source rocks are shown in Table1.For heavy mineral fractions, the counting data were converted to numeric percentages as shown in Table 2. Heavy mineral assemblages have been identified and classified as opaque and transparent heavy minerals (Deer et al. 1992;Mange and Maurer 1992), as shown in Tables 2 and 3.  Jassim et al.,1987).

Heavy Mineral Petrography
The analyzed samples of sandstone consist of diverse heavy mineral assemblages.The quantitative point counting results for the heavy minerals composition of the Tanjero Formation are distinguished by an abundance of opaque minerals, which account for 34% of the overall heavy mineral contents.Unstable (amphibole and pyroxene) and meta-stable minerals (epidote, garnet, staurolite and kyanite) are representing more than 29%, 19% respectively of the transparent heavy minerals.Whereas, flaky minerals (chlorite and biotite) are present in most of the samples, while Some samples contain small or traces amount into low fraction of ultra-stable (zircon, tourmaline and rutile) minerals.Heavy mineral suites and their source rocks are shown in Tables 1 (Cascalho and Fradique 2007), whereas shows the semi-quantitative abundances of heavy (opaque and non-opaque) minerals in the Tanjero sandstone samples.

Opaque minerals
The opaque minerals represent the largest percentage of heavy minerals, this is due to the minerals crystallization in all types of igneous, sedimentary and metamorphic rocks (Neese, 2000), which are identified in all of the analyzed sandstone samples, with ranges of 19.0 -42% and an average of 33.85% (Table 2).The identified opaque grains mostly have a variety of shapes, but most are cubic (pyrite grains Plate 1) and rhombohedra (Plate 2).The presence of opaque minerals in the analyzed samples implies igneous and metamorphic source rocks.Odumoso et al. (2013) illustrates that the abundance of heavy opaque minerals show that deposition took place in oxic environments (oxygen-rich) conditions.

• Unstable minerals
Pyroxene grains are the most common type of transparent unstable heavy mineral investigated.Pyroxene content varies between 7.81 and 20.97%, with an average of 15.06 percent (Table 3).Pyroxene can be found in igneous (basic and ultra-basic) source rocks (Tucker, 1991).It is the most common ferromagnesian rock-forming mineral group.They may be found in almost all types of igneous and metamorphic rocks and crystallize under a variety of circumstances (Mange and Morton, 2007).Pyroxene has a color ranges from colorless to pale yellowish green, with weak or no pleochroism, and is partially to weakly corrode with parallel extinction.Because of the solutions, pyroxene grains have a saw-tooth digenetic characteristic (Plate 1), in addition to displaying pitting and weathering characteristics.Under weathering conditions, amphibole and pyroxene grains are prone to instability (Tucker, 1985).According to Yue et al. (2019) the varies shape of unstable mineral in the studied sediment indicates the multiple sources of these sediments, the difference in the transportation distance and the sedimentation speed, and that chemical weathering was not severe, as evidenced by the prevalence of unstable minerals.Amphiboles represent the second abundant constituent of the transparence unstable heavy minerals in the studied section of the Tanjero Formation.Hibbard (2002) indicated that the amphibole is one of the primary minerals that forms the basic igneous rocks.Ehrmann and Polozek (1999) explained that the presence of amphibole an indication of igneous source rock.They are considerably complex mineral groups that can be found in a diversity of igneous and metamorphic rocks (Pettijohn et al. 1973;Mange and Morton, 2007).The amphibole group mineral percentage varies between 4.81-23.41%,and average of 14.17% (Table 3).They are mainly composed of hornblende, with subordinate actinolite.The hornblendes are distinguished by their green to brownish green color and high relief pleochroism (Plate 1).The actinolite is colorless with a few grains of distinctive light grey, with no pleochroism and a low extinction angle, it crystallizes in metamorphic rocks with a regional metamorphism and under low intensity metamorphic conditions within the facies of green schist (Rymond, 2010).

• Metastable minerals
Epidote grains are the most prevalent metastable minerals in the samples of the Tanjero sandstone.The epidote percentage ranges from 0.77 to 28.49%, with an average of 14.57% (Table 3).Epidote granules are brown to yellowish green in color and are sub-rounded to angular in shape (Plate 1).This mineral is commonly found in igneous rocks and is most numerous in green schist and epidoteamphibolite facies rocks (Asiedu et al., 2000); where it predominates in low-grade metamorphic rock (Pettijohn et al., 1973;Mange and Maurer, 1992).Kerr (1959) illustrated that the epidote is one of the metastable minerals are crystallizes in metamorphic rocks under medium to low intensity metamorphism conditions.
The average garnet content in the Tanjero sandstone samples is 3.44%, with a range of 0 to 12.70% (Table 3).It is isotropic, with a high relief shape that's typically colorless but occasionally brownish.The majority of the crystals are hexagonal and angular (Plate 1).Under both weathering and burial diagenetic conditions, garnet is a highly stable mineral (Garzanti et al., 2013).It is particularly common in metamorphic rocks (Mason and Berry, 1968).The presence of garnet in the studied samples indicates that the source rocks are metamorphic or sedimentary rocks.Whereas, the percentage of kyanite ranges from 0.78 to 2.77%, with an average of 1.48% (Table 3).Kyanite occurs in a metamorphic rock.It has colorless and elongated with a high relief subhedral shape (Plate 1).Staurolite percentage range from 0.00 to 1.72 % with an average of 0.38 % (Table 3).Staurolite grains were recognized by their distinctive shape and light yellowish color (Plate 1).Other important physical and optical properties identified in the grains include strong yellow to yellowish gold pleochroism, straight extinction, fairly high relief, low birefringence color, and hackly fracture nature.It is commonly found in moderate to high grade regional metamorphic phase (Elsner, 2010).On the other hand, Mason and Berry (1968) demonstrated that this mineral is used as an indicator mineral to evaluate the temperature, depth, and pressure that the rock succumbs to metamorphism.

• Ultrastable Minerals
Rutile grains are quite common in virtually all studied sandstone samples, ranging from 2.47 to 14.62 %, with an average ratio of 5.65% (Table 3), and their colors are distinguished by dark red and dark brownish yellow (Plate 2).Some of the distinguishing optical properties of the studied samples include a high refractive index, high in relief, weak pleochrosim, and the absence of cleavage.The majority of the rutiles in this research include prismatic or elogated that are irregular and angular to sub angular grains.Rutile is a prevailing secondary mineral in felsic igneous and metamorphic rocks (Boggs, 1995;Meinhold, 2010).The tourmaline content ranges from 0 to 6.20%, with an average of 1.13% (Table 3).Tourmaline grains have a strong pleochroism that ranges from light brown green to yellowish brown tone.Some of the common characteristics include moderately relief, parallel extinction, and high order birefringence (Plate 2).Tourmaline is a mechanically and chemically ultra-stable detrital heavy mineral that is abundant in detrital sediments and sedimentary rocks.Due to its abundance in a variety of rock types, tourmaline has proven particularly helpful as a provenance mineral (Ayofe and Anthony, 2020).The existence of tourmaline in various colors and forms is connected with the presence of transparent heavy minerals, particularly rutile; it is derived from felsic igneous and metamorphic rocks (Tucker, 1985;Elawi, 2005).Zircons is colorless mineral with extremely high relief that are characterized by rounded to sub-rounded elongated grains (Plate 2).The majority of the Tanjero sandstone samples contain comparatively low contents of zircon, ranging from 0.0 to 3.51%, with an average of 1.54% (Table 3).This mineral is regarded one of the ultra-stable minerals that can resist weathering and erosion as well as numerous deposition cycles.Zircon possesses tetragonal symmetry, but its internal crystal structure is frequently disrupted by bombardment from radioactive element decay, which is found in minor levels in many zircons (Klein, 2002;Al-Malabeh et al., 2017).Its presence suggests the existence of acidic igneous source rocks (Sangeeta and Pandey, 2017;Jassim and Al Haza, 2020;Ali, 2021).Whereas, Speer (1982) explained that zircon is a useful indicator of the provenance of rock; consequently, the euhedral shape of zircon indicates acidic igneous rock, while the rounded shape indicates high-grade metamorphic rocks.

• Flaky minerals
In the present study, quantitative average values of chlorite represented 6.47% of the total component of non-opaque heavy minerals in the sandstone samples (Table 3).Chlorite grains typically green to pale green in color and tend to vary in shape from rounded to sub-round (Plate 2).Some grains have fractures and include opaque minerals as inclusions.Chlorite is usually secondary in origin, derived from metamorphic rocks and formed by the alteration of ferromagnesian silicate minerals (Hibbard, 2002).While, the biotite grains have high relief, brown to reddish brown and light red flakes with a prominent cleavage and are extremely pleochroism.The observed mineral grains are mainly angular to sub-angular and were detected with a small relative percentage (0-9.70%)and an average ratio (2.31%) in some of the sandstone samples in the studied area (Table 3; Plate 2).

Evidence for Source Rocks and Tectonic Provenance
The ultra-stable heavy mineral assemblages in the studied Tanjero sandstone samples can be used as an index for the maturity of the sediment.The quantitative maturity of the studied Tanjero sandstone samples has been assessed by the zircon, tourmaline, and rutile (ZTR) index (Hubert, 1962).It is derived from acidic igneous rocks and also indicates the intensity of weathering processes (humid climate).The ZTR index is high in precipitation environments because it can withstand long transportation distances and resist weathering processes.Prothero and Schwab (2014) illustrated that the ZTR index is a useful method of determining how weathered (chemical and mechanical) sediment might be; it focuses on heavy minerals like zircon, tourmaline, and rutile.The ZTR index is used to ensure mineralogical maturity by identifying how mature, sub-mature, and immature minerals are in sediment samples.This index is usually high in the littoral or beach depositional environment because of the high energy of the environment and long transport distances from the source area (Adedoyin et al., 2021).
(1) Mineralogically mature sediments have a ZTR value greater than 75, whereas immature to submature sediments have a ZTR index of less than 75 (Hubert, 1962;Suleiman et al., 2015;Oni and Olatunji 2017).The rising value of the ZTR index corresponds to an increase in zircon, tourmaline, and rutile contents and a decrease in the amount of transparent heavy minerals.The ZTR index of the Tanjero sandstone samples was obtained to be 12.46% (Table 4; Fig. 2).The heavy minerals in the studied area were characterized by a low proportion of zircon, tourmaline, and rutile (ZTR index), which is utilized as a hint for identifying the source and a measure of the sediment maturity.According to Hubert (1962), Tanjero sandstone samples are mineralogically immature sediments.Heavy minerals represented important evidence in determining the type of source rocks and provenance of sediments (He et al., 2017).Whereas, Boggs (2009) illustrated that the heavy minerals are commonly used to identify source rocks.Pyroxene, epidote, amphibole, and chlorite are the prevalent transparent heavy minerals in the sandstone samples of the studied formation, accounting for more than 60% of the non-opaque heavy minerals, indicating that these minerals could not have been transported over long distances from the source rocks (Ali, 2021).
According to the heavy mineral analysis of the studied sandstone sediments, and based on the proportions of zircon, rutile, and tourmaline, the sediments are mineralogically immature, furthermore, the relationship between metastable minerals (garnet and staurolite) and ultra-stable minerals (zircon, tourmaline, rutile), in the Tanjero Formation suggests that most of the Tanjero sandstone sediments are immature (Fig. 3A).This is confirmed by the ternary diagram, which shows that the samples of sandstone are plotted near the moderately stable zone (Fig. 3B) (Kasper et al., 2008), which means that the studied sediment has moderate stability and its mineral contents could not have been transported over long distances and were accumulated near the source rocks (Prothero and Schwab, 2014).The percentage and content of heavy minerals depends on hydrodynamic conditions such as sediment influx from weathered parent rocks, wave energy and velocity, and water or air currents (Rao et al., 2001;Pandey, 2017).The relationship between sediment composition and tectonic setting, as well as their significant indicators for the tectonic environment was clarified by Pettinjohn et al..(1973).The sediments' heavy mineral assemblages could be used to differentiate some tectonic settings, where these suites of heavy minerals might be utilized as criteria for distinguishing the island arc, intra-oceanic, and deep marginal sea zone sediments (Nechaev and Isphording, 1993;Tobia and Kafy, 2016).However, recent research indicates that studying heavy mineral assemblages may provide information on plate tectonic settings that is more understandable and easier to interpret than data from modal or chemical analyses (Mohammed et al., 2018).Heavy mineral assemblages may be reasonably dependable and unambiguous markers of the major plate tectonic settings associated with continental edges.Plotting the percentage value of heavy minerals on a right angle ternary diagram, GM, MT, and MF, respectively, displayed the abundant mineral constituents of marine mafic magmatic rocks, the abundant mineral constituents of basic metamorphic rocks, and the accessory mineral constituents of metamorphic and granitic rocks, which are also the most resistant mineral phases.MT is less definite than the other two components in distinguishing sediments from different tectonic environments (Fig. 4).(Nechaev and Isphording, 1993;Mohamed and Dar, 2005).The percentage analytical values plotted in the ternary diagram (Fig. 4 and Table 4) demonstrate that the suites of heavy minerals distinctly reflect the immature sandstone samples of the Tanjero Formation in the studied area because all samples fall into the active continental margin field, where MF > GM in the studied samples, which are distinguished by a relatively high percentage of heavy minerals originating from intermediate and basic igneous rocks.These source rocks are represented by the Zagros Mountains and its derivative formations.The studied samples are in the tectonic zone of the tectonically active edge, which is caused by the edge of the northeastern Arabian plate colliding with the western edge of the Iranian plate, and this tectonic situation was reflected in the types of heavy minerals deposited in the studied Tanjero Formation (Nechaev and Isphording, 1993).

Conclusions
The sandstone sediments of the studied Tanjero Formation possess a diversified assemblage of heavy minerals, which are dominated by opaque minerals and some non-opaque minerals like pyroxene, epidote, amphibole, chlorite, rutile, and garnet.The variety of these minerals, which is obvious by the presence of a high percentage of opaque minerals, is greater than that of non-opaque minerals, indicating that the sediment source is from basic and ultrabasic igneous and metamorphic rocks.The presence of a significant percentage of unstable and meta-stable mineral contents confirms that they are derived directly from the nearby main source rocks, as well as ultra-stable and meta-stable mineral relationships show that the Tanjero sandstone sediments are immature and have moderate stability, and were often imparted over short distances and accumulated near the source area.The tectonic and structural setting of the area was indicated by the MF-MT-GM ternary diagram, which reflected the active continental margin in the sediments, which was MF greater than GM.This indicates that the heavy minerals of the formation were derived from the active tectonic plate resulting from the collision of the Iranian and Arabian plates.

Fig. 1 .
Fig.1.Geological map of the Tanjero Formation outcrop distribution in the Sulaimanyia area showing the location of the studied area (modified from Jassim et al.,1987).

Fig. 2 .
Fig. 2. ZTR index for the sandstone samples of the Tanjero Formation

Table 2 .
Heavy minerals (opaque and non-opaque) in the sandstone of Tanjero Formation

Table 3 .
Heavy mineral abundance (%) in sandstone samples of the Tanjero Formation.

Table 4 .
Heavy mineral data of the studied sandstone sediments of the Tanjero Formation.