Petrography, heavy mineral study and tectonic setting of Walash Naopurdan Series Sandstones, Qalander area, Northeastern Iraq

The Petrography and Heavy mineral study of Walash-Naopurdan Series Sandstone (Middle-Late Eocene) at Qalander area is carried out. Thin section study of twenty seven samples showed that these sandstones consist of quartz, feldspar, rock fragments (lithics), matrix, and cement. The rock fragments are of sedimentary, igneous, and metamorphic types as an indication of multiple source rocks originated from the complex thrust zone of northeastern Iraq. The Heavy mineral analysis also showed different types, dominated by opaques and transparent minerals as a further indication of multiple source rocks. The main constituents of the studied sandstones (quartz, feldspar, rock fragment and matrix) are used in their classification which showed that the sandstones are of lithic greywacke type.The provenance indicating triangular diagrams (QtFL and QmFLt) showed that the Walash-Naopurdan Series Sandstone is of recycled orogen sources shared between transitional, mixed, dissected and transitional arcs tectonic settings. These represent a complex convergent boundary with dissected transitional volcanic arc which was subjected to uplift and erosion.

Many studies such as those of ( Underwood and Bachman ,986) ( Dickinson 1985), ( Uddin, et al, 2007) and   ( Pindell, et al. 2009). have indicated the importance of the lithology and heavy minerals content of sandstones as a tool to predict their provenance and tectonic setting (Table 1). Crook (1970b) has recognized three major classes of greywacke and has attributed their differences to provenance: the quartz-poor (under 15 percent quartz) being of volcanic provenance, the quartz-rich (over 65 percent and commonly 80 percent) being of sedimentary provenance, and the intermediate class (15 -65 percent quartz) being of mixed provenance. So the greywacke sandstones of Walash-Naopurdan Series at Qalander area (35, 5 percent quartz) are intermediate class.
Recycled orogens refers to several tectonic settings such as tectonically uplifted complexes, subduction zones, thrust sheets situated behind volcanic arc islands and suture belts, collisions represented by foreland fold -thrust belts. (Dickinson and Suczek, 1979). These areas are dominated by sedimentary strata with subordinate volcanic rocks and their metamorphic derivates exposed to deformation due to uplifting and erosion by orogenic uplifting of fold belts and thrust sheets. Recycled orogens of greywacke sandstones in present study refers to tectonically uplifted complexes subduction zones and thrust sheets situated behind volcanic arc islands.

Geology of the study area
Topographically, the area consists of complicated and rugged mountains with scarps and valleys; the drainage of the studied area is of irregular dendritic type where the valley slopes are generally very steep, this complication is probably attributed to the tectonic setting of the area which is dominated by folding and thrusting. ( Al-Chalabi 2004).Geologically the study area composed of Naopurdan Group and Walash volcanic Group. (Jassim and Goff. 2006) (Fig.2):Stratigraphically the lithofacics that occur in the study area are: A. Naopurdan Shaly Rock Group (Oligocene): This group is composed mainly of gray shale beds with green sandstone and coralline limestone, tuffaceous slaty shales and felsitic volcanics and some conglomerates, (Buday and Jassim, 1987). (5 samples taken from green sandstone). B. Walash Group: According to ( Buday 1980), the lithological sequence of this group in ascending order is: 1. Red mudstone, cherty siltstones and shales 2. The lower volcanics, which are mostly closely associated with basal red beds and composed of basic and acidic lavas, often pillow lava and associated pyroclastics. 3. Lower sediments which are composed of red mudstone, red and grey shale, sandstone, conglomerates and limestone (8 samples taken from sandstone). 4. The upper volcanic which are composed mainly of basic and intermediate lavas with andesite and pyroclastic sediments. 5. The sequence overlain by the upper sediments which consists of red mudstone in addition to conglomerate (mainly agglomerate), siltstone and greywackes sandstone (14 samples taken from sandstone). The age of Walash Series is Middle-Late Eocene (Koiy, 2006) which is thrusted over Red Bed Series (Early-Middle Miocene).

Sampling and Methodology
Twenty seven sandstone samples were collected from massive and blocky sandstone rock beds (12 m) which are alternated with claystone. The samples were taken along traverse starting in ascending order.
The thin sections were studied petrographically under transmitted polarizing microscope and the percent of different constituents were obtained by counting 400 grains using point counter (Table 2). Similarly, the heavy mineral percentages were obtained by counting 300 grains using grain slides prepared according to Hutchison's (1974) method for the heavy minerals fraction separated from the sandstones by bromoform as heavy liquid (Table 4).

Results and Discussion
The studied Walash-Naopurdan sandstones consist of quartz, feldspars and lithics of various rock types.

Quartz.
Quartz forms the main constituent in the studied sandstones ranging between 23-49% with an average of 35.5% (Table 2). Two types of quartz grains were recognized, monocrystalline and polycrystalline. Monocrystalline quartz is angular to sub-rounded in shape, usually have straight extension and occasionally with wavy extension indicating metamorphic origin (Folk, 1974) (Plate1-1). The amount of monocrystalline quartz is between 16-32 % with an average of 23.8%. Polycrystalline quartz grains contain some aggregate crystals of quartz which have sub-angular to sub-rounded shape (Plate 1-2). The amount of polycrystalline quartz is between 7-18 % with an average of 11.6%.The amount of polycrystalline quartz is less than monocrystalline quartz which is due to the lesser stability of the former, while increasing amount of monocrystaline quartz is due to the remoteness of their source rocks. The contacts between quartz crystals are mostly elongate indicating plutonic source rocks; while fewer contacts are of sutured type as an indication of metamorphic origin (Folk, 1974).

Feldspar.
Feldspar grains are present in miner amounts ranging between 5-18% and averaging 10%. Two type of feldspars exist in the studied sandstones, Na-and K-feldspars.
The Na-feldspar (plagioclase) is sub rounded and angular in shape with albite twining (plate 1-3) , partially altered to clay minerals.
The amount of Na-feldspar ranges between 4 -14% with an average of 7.8%, albite type indicating its derivation from granitic acidic igneous source rocks (Thoreau, 1982).
The K-feldspar is orthoclase which has platy form and dusty appearance due to the decomposition to clay minerals (Plate 1-4). The amount of K-feldspar ranges between 1-4% with an average of 2.2%, Feldspars are less stable than quartz and can be altered to clay minerals (Boggs, 1997). The presence of orthoclase and plagioclase indicates plutonic igneous or metamorphic source rocks (Pittman, 1970).

Rock fragments (Lithics).
Most of the rock fragments are terrigenous grain that were derived from older source rocks and survived destruction. They are very important for studying the source rocks and more reliable than individual minerals such as quartz and feldspar that could be derived from different source rocks (Boggs, 1997). The amount of total rock fragments are between 17-33% averaging 25.5%. They consist of the following types:

-1 Sedimentary rock fragments.
Sedimentary rock fragments are the most abundant lithics ranging in amount between 11-24% with an average of 17.7%.They are of different types including chert, carbonates and clastics. The chert lithics are probably derived from the radiolarian chert beds of Qulqula Series exposed in the thrust zone of northeastern Iraq. The carbonate lithics are mostly composed of sparry calcite (Plate 1-5); they indicate carbonate source rocks (Al-Juboury, 1994). The clastics are abundant and exist as sandstone grains (Plate1-6), siltstone (Plate 1-7), clay (Plate 1-8). The sandstones are of medium grain size and consist of quartz in clay matrix; while the siltstones are smaller in size and consist of tiny dark colored quartz crystals in clay matrix.

-2 Igneous rock fragments.
These clasts are usually rare ranging in amount between 3-8% with an average of 5.7% (Table 1).They are mostly dark colored grains of basalt (Plate 1-9). The existence of these lithics is attributed to igneous source rocks (Al-Juboury, 1994).
The serpentine lithics are derivatives of peridotites exposed in ophiolite complexes of northeastern Iraq. The schist lithics shows foliation with fine grained texture (Plate 1-10).The metamorphic rock fragments are derived from the thrust zone area which represents exposed uplifted fold belts and thrust sheets.

Matrix.
The matrix represents ligament materials that are filling void between particles which generally consists of formed clay and micritic materials (Dickinson, 1970). It is produced as a result of disintegration and decomposition of unstable constituents especially rock fragments and feldspars. The matrix amount is relatively high ranging between15-28% and averaging 20.3% (Table 2).
The matrix are of two types, first is of autochthonous origin which was created by diagenesis, the second is allochthonous in origin which was transported from the source area and deposited in the basin (Tucker, 1985). The matrix has been considered as an essential characteristic of greywacke and was noted by some as the essence of the greywacke problem. Wherever the matrix is mainly friable clay material (Plate 1-11), this means that most of this matrix is autochthonous (Selley, 1982).

Cement.
Cement represents binding material which is deposited from chemical solutions present between grains. It ranges between 5-12% with an aver age of 8.7% (Table 2). The main kinds of cements in the studied sandstones are carbonates (Sparry calcite cement) (Plate 1-12). 6. Sandstone rock fragments (quartz grain in clay matrix) crossed nicols. 7. Siltstone rock fragments with small size of quartz in clay matrix. 8. Clay rock fragment in sandstone(crossed nicols). 9. Igneous Rock fragments usually found as dark color grains of (basalt). 10. Metamorphic rock fragments (schist) that shows oriented crystals. 11. Autochthonous friable clay matrix which is deposited in the basin. 12. Sparry calcite cement between sand grains (crossed nicols).

Classification of Sandstones
The data given in Table 3 are plotted on the equilateral triangle of Pettijohn et.al., (1973) in order to name the type of the studied sandstones. This triangle depends on the main constituents of the sandstone, i.e., quartz, feldspar, rock fragment and matrix. It showed that most of the studied samples are situated in lithic greywacke field ( Figure  3).

Heavy Minerals
The heavy minerals are very important group of minerals which exist in clastic sedimentary rocks because they are provenance indicators (Markevich, et al., 2007).
The general properties of heavy minerals depend on many factors including the source rocks. Usually this group of minerals is divided into two types, the opaques and the transparent minerals. Twenty seven grain slides were used in this study for the identification and counting heavy minerals in the sandstones of Walash-Naopurdan Series.

Opaques
The studied grain slides showed that the opaque heavy minerals comprise the highest percentage with an average of %69.45 (Table 4).
The identification of opaques depended on Kerr's (1959) classification in such a way that the black colored grains are magnetite (Plate 2-1), reddish weak brown is Goethite (Plate 2-2), and dark with metallic luster is chromite (Al-Chalabi. 2004) (Plate 2-2). These opaques are usually derived from mafic igneous rocks (Pettijohn, 1975). Chromite is an evidence of the proximity to the source rocks (Dill, 1998).

Transparent Heavy Minerals
This group of heavy minerals is usually positively identifiable under the polarizing microscope because they can transmit light in thin sections. These heavy minerals can be classified according to their stability into three subgroups: ultrastable, metastable and unstable heavy minerals (Folk, 1974).

Ultra Stable Heavy Minerals
They include zircon, rutile and tourmaline. Their presence is an evidence of metamorphic rocks which were derived from acidic igneous source rocks (Chaodong et al., 2005), or metamorphic and igneous mafic source rocks (Ruiz et al., 2007).

Zircon
Zircon is found as subrounded grains or prisms with pyramidal endings (Plate 2-3). It can be derived from metamorphic and acidic igneous source rocks. The rounded grains are abundant in reworked sediment (Kerr, 1959;Pettijohn, 1975).
Zircon is characterized by its colorless, pale yellow or pale pink colors and high relief. The amount of zircon is between 2-8 % with an average of 4.44 % (Table 4).

Tourmaline
Tourmaline is a group of minerals with complex chemical compositions and various colors. The variation in colors is due to the complexity of chemical composition and they all show paleochroism. The subrounded grains are usually brown to yellowish brown representing dravite (Elyas 1988;Ismail, 1996).
The percentage of tourmaline is between 3-6 % with an average of 4.1 % (Table 4). It is derived from metamorphic and igneous source rocks while the rounded grains are derived from reworked sediment source (Kerr, 1959;Pettijohn 1978).
Rutile can be identified by its high relief, red or brown to yellowish orange color , pleochroism from yellow to reddish brown (Kerr, 1959). The amount of rutile is between1-6% with an average of 3.5 % (Table 4).

Metastable Heavy Minerals
This subgroup includes garnet, epidote and kyanite which were recognized in the studied sandstones.

Garnet
Garnet includes two mineral groups, pyralspite and grandite (ugrandite) with a general composition X 3 Y 2 (SiO 4 ) 3 , where X site is occupied by Ca 2+ with possible extensive substitution by Mg, Fe, and Mn; while Y site is occupied by Al 3+ (Nesse, 2000). Garnet with a general formula (Mg,Fe,Ca) 3 Al 2 (SiO 4 ) 3 is an intermediate composition between the two groups. Garnet is characterized by distinct optical properties such as high relief, isotropy, pitted appearance, subrounded form and pale brown to dark brown color (Plate 2-5). The subrounded form of garnet is attributed to its crystallization in cubic system (Al-Sayegh, and Al-Jubouri, 2002).

Epidote.
The amount of epidote is between 4-12% with an average of 7.6% (Table 4). It has a dusty appearance, subrounded form, pistachio green to yellowish green colors and weak pleochroism (Plates 2-6).

Unstable Heavy Minerals
The unstable heavy minerals recognized in the studied sandstones are pyroxene, hornblende, and serpentine. Pyroxenes: are colorless or pale green in color, prismatic or irregular in shape, and have eroded surfaces which are evident on the corroded outer rim of the grains (Plate 2-7). The amount of pyroxenes is between 1-3% with an average of 1.6% (Table 4). Pyroxene minerals are derived from mafic igneous rocks.
Amphibole: group is represented by hornblende grains which are prismatic or subrounded in forms, green in color, and have rhombohedral cleavages (Plate 2-8). The amount of hornblende is between 1-3% with an average of 1.3% (Table 4). It is derived from metamorphic and acidic igneous source rocks (Folk, 1974;Pettijohn, 1975).
Serpentine: minerals are yellowish green in color and have fibrous forms (Antigorite -Lizardite) (Plate 2-9). The amount of serpentine is between 2-7% with an average of 3.97% (Table 4). Serpentine forms as a result of hydration of peridotite and also can be derived from mafic igneous rocks, or hydrothermal metamorphic source rocks (Pettijohn, 1978).