Cathodoluminescence petrography for provenance studies of the sandstones of Ora Formation (Devonian–Carboniferous), Iraqi Kurdistan Region, northern Iraq

Journal of African Earth Sciences 109 (2015) 195–210

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Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci

Cathodoluminescence petrography for provenance studies of the sandstones of Ora Formation (Devonian–Carboniferous), Iraqi Kurdistan Region, northern Iraq Muhamed F. Omer ⇑,1 Department of Geology, College of Science, Salahaddin University, Erbil, Iraq

a r t i c l e

i n f o

Article history: Received 18 February 2015 Received in revised form 21 May 2015 Accepted 22 May 2015 Available online 30 May 2015 Keywords: Cathodoluminescence Sandstone petrography Provenance Ora Formation Northern Iraq

a b s t r a c t Advanced techniques such as scanning electron microscope hot-cathodoluminescence (SEM-CL) and electron microprobe analysis (EMPA) as well as petrography are applied to investigate the paragenetic sequence and provenance of intertidal–tidal flat sandstones of the Ora Formation which is exposed in northern Iraq. The Formation consists mostly of supermature quartzarenites with subordinate immature subarkoses and sublitharenites. The mean framework composition of the thinly bedded sandstones of Ora Formation is Q95.7F2.8R1.4 where 90% of the quartz grains are monocrystalline and texturally supermature. Meanwhile the mean composition of the thickly bedded sandstones is Q85.4F9.7R4.8 with 84% of the quartz grains are monocrystalline and texturally immature. The provenance of the Ora sandstones is dominantly craton interior and less recycled orogeny. The Ora Formation has undergone intensive and complex episodes of eogenesis, mesogenesis, and telogenesis. Compaction and quartz cementation is more dominant than other diagenetic processes evident from tight grain supported fabric which predated authigenic illite formation; this is also evident from close packing of detrital framework that resulted in reduction of primary porosity. Albitization has postdated eogenesis of K-feldspar cements. Cathodoluminescence study indicated four main distinctive fabrics in quartz grains, (1) healed fractures, (2) mottled textures, (3) low-intensity dark CL streaks and patches, and (4) shocked quartz. The results indicate the dominance of brown to dark blue CL for quartz of low-temperature metamorphic origin; bright blue colors for the felsic plutonic and high-temperature metamorphic quartz; with considerable amounts of detritus that has originated from felsic and mafic volcanic rocks and are characterized by red, violet colors luminescence. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The provenance of siliciclastic sedimentary rocks is an important discipline as it may help revealing the tectonic evolution of the sediment provenance. Most previous research was focused on sandstones because they retain the original components of the source area; and much of such research has attempted to interpret the source rock lithology from detrital mineral assemblages (e.g., Girty, 1987; Wanas and Abdel-Maguid, 2006; Vdancy, 2013; Zaid, 2013; Blamey et al., 2014). Provenance studies using detrital mineral assemblages are complicated by the variable stability of different component. Rock fragments may be lost during weathering of the source area or during transport. For example, feldspars may also be destroyed or altered during diagenesis by albitization of ⇑ Corresponding author. Tel.: +964 770 134 83 22. 1

E-mail address: [email protected] Current address: Warsaw University, Geology Faculty, Poland.

http://dx.doi.org/10.1016/j.jafrearsci.2015.05.021 1464-343X/Ó 2015 Elsevier Ltd. All rights reserved.

plagioclase or K-feldspar (McBride, 1985; Milliken, 2005; González-Acebrón et al., 2012). In very mature sandstones, the proportion of diagnostic unstable components is very low, and the interpretation of source area based on these components is generally not possible. Over the years, many petrographic, isotopic, and geochemical techniques have been developed and used in provenance studies of detrital quartz. One of such recent developments is the application of scanning electron microscope -cathodoluminescence (SEM-CL) technique which has proven to be a particularly useful tool for studying sedimentary rocks because it may provide more direct information of the provenance of single mineral grains (e.g., Milliken, 1994; Seyedolali et al., 1997; Götze and Zimmerle, 2000; Kwon and Boggs, 2002; Götte and Richter, 2006; Boggs, 2009). Seyedolali et al. (1997) identified many features and CL characteristics for the determination of quartz provenance. Kwon and Boggs (2002) illustrated that SEM-CL provides more accurate results on provenance than routinely used point counting. Such CL features; also provide detailed

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M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210

information about compaction, brittle deformation or pressure solution in the diagenetic history of sandstones (Houseknecht, 1991; Milliken and Laubach, 2000). The aim of this research is to study the diagenetic evolution and provenance of individual quartz grains as well as feldspar grains in the sandstones of Ora Formation. 2. Geological setting Ora Formation is located in northern Iraq close to the Iraqi– Turkish border (Fig. 1). Its outcrops are located within the Northern Thrust Fault Zone of northern Iraq. Geologically, northern and northeastern Iraq is part of the Alpine Mountain Belt of the Near East, represented by the Taurus-Zagros Fold Belt which was developed as a result of collision between the Afro-Arabian and the Eurasian continents (Sharland et al., 2001). Iraq is tectonically situated in the northeastern sector of the Arabian Plate which is in a collision state with the Eurasian (Iranian) Plate; this collision

resulted in the creation of the Zagros Foreland Basin which is divided into four tectonic zones. These zones are (1) Imbricate Zone, (2) High Folded Zone, (3) Low Folded Zone, and (4) Mesopotamia Foredeep (Fouad, 2010: Fig. 2). The thrust zone occurs as a narrow strip in the extreme north, just south of the border between Iraq and Turkey, and in the northeast along the border between Iraq and Iran. The Ora Formation is one of the Paleozoic formations which are exposed in the northern part; within the imbricate zone (Fig. 2) with thickness reaching to 75 m in its type locality (A in Fig. 1). The Ora Formation was first described and introduced by Wetzel (1952) in the Northern Thrust Zone of Iraq. It consists of black micaceous and calcareous shale with olive green silty marl, thin to medium bedded sandstone and lenses of detrital limestone. Fine-grained sandstone beds occur intermittently throughout the formation. The formation has been drilled in western Iraq in wells Khlesia-1 and Akkas-1 and in water well K5-1 (Fig. 2). In the Khlesia-1 well, the formation is 493 m thick (depth interval

Fig. 1. Location and geological map showing Ora Formation and other Paleozoic rocks (modified after Sissakian (2000)).

M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210

197

Fig. 2. Main tectonic zones in Iraq (after Fouad (2010)).

1757–2250 m) and contains abundant sandstone interbeds up to 10 m thick; while in the K5-1 well the formation is 403 m thick (depth interval 940–1343 m) and comprises black shale and thin beds of black limestone. The Paleozoic sedimentary sequences of northern Iraq fall into three major sedimentary cycles dominated by siliciclastic, or mixed siliciclastic-carbonate units. These units are separated by relatively major breaks indicating mainly the effect of the Caledonian and Hercynian orogenies. Based on Buday (1980), these cycles from older to younger are: 1. The Cambro-Ordovician (Silurian?) Cycle. 2. The Devonian (Late Devonian–Lower Carboniferous) Cycle. 3. The Upper Carboniferous–Upper Permian Cycle. The Paleozoic cycles commence with the Ordovician Khabour Formation as the oldest exposed rocks in Iraq (Bellen et al., 1959); the age of this formation was restricted to Upper Tremadocian (Lower Ordovician) according to ichnotaxa of Cruziana (Omer, 2012). In western Iraq, it was succeeded by the Silurian Akkas Formation which is only known from the subsurface; the entire Silurian and Early Devonian succession is missing in outcrop sections of northern Iraq. Khabour Formation is unconformably overlain by the Late Devonian to Early Carboniferous cycle represented by the Pirispiki, Chalki, Kaista, Ora and Harur formations. The uppermost cycle is late Permian in age and comprises the Chia Zairi Formation (Fig. 1). According to Jassim and Goff (2006), the upper part of the Kaista Formation is now included in the Ora Formation and the name Ora Shale has been changed to Ora Formation (Fig. 1). Palynological studies indicate that it is of Late Devonian–Early Carboniferous age (Barzinjy, 2006; Aqrawi et al., 2010). It was probably deposited in shallow-marine to near-shore environments.

3. Sampling and methodology Fieldwork was carried out on two outcrops of Ora Formation in northern Iraq (Fig. 1). Both lithostratigraphic sections were measured and described in the field (Figs. 1 and 3). A total of thirty-two fresh samples were collected from the thin- to medium-bedded sandstones of Chalky Nasara section (longitude 43°100 0000 E, latitude 37°170 4500 N), in the core of Chiazinar fold where Paleozoic formations are successively well exposed (Fig. 1). The second locality was Ora section (longitude 43°220 5100 E, latitude 37°170 5700 N) from which eighteen samples were collected. Fifty polished thin sections were prepared from sandstone samples from Ora and Chalky Nasara sections and studied under a standard Nikon Eclipse LV 100 Pol petrographic microscope with automatic stage at Warsaw University. 400–500 grains were counted in each polished section using the Gazzi-Dickinson method to minimize the dependence of rock composition on the grain size. Framework parameters were adapted from Ingersoll and Suczek (1979). The feldspar-rich thin sections were stained by HF and sodium cobaltnitrate that helps easy identification of potassium feldspar (Chayes, 1952). Scanning electron microscopy (SEM) was performed, using a P |GMA™|VP-ZEISS with EDX, BRUCKER X Flash 6/10. The microscope was operated at 20 kV electron acceleration voltage, using AsBÒ detector and backscattered electron (SEM-BSE) modes. Cameca SX100 (5WDS) electron microprobe analysis (EMPA) was used for quantitative determination of the composition of coexisting feldspars. The analyses were carried out at 15 kV and 10–20 nA probe current, and electron beam of 5 lm diameter. SEM-CL image analysis is used to recognize microcracks, healed fracture, zonation, or deformation features (Seyedolali et al., 1997; Kwon and Boggs, 2002; Bernet and Bassett, 2005; Boggs and Krinsley,

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Fig. 3. The studied outcrops of Ora Formation showing (a) sandstones intercalated with black to grey shale in Ora section; the red arrows show the location of collected samples from the thinly bedded sandstones. The lower contact with Pirispiki Formation is conformable; (b) close-up view of (a) showing fissility of the black shale and current ripples in the thinly bedded sandstone units; (c) channel-fill dominated by lenticular bedding with isolated lenses in the Chalky Nasara section; and (d) quartzarenite sandstones interbedded with black shale in the Chalky Nasara section. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2006). X-ray diffraction (XRD) analyses were performed for studied samples using Philips (PW3710) diffractometer (Cu Ka radiation, 35 kV, 28.5 mA). All these studies were performed at Warsaw University-Poland. A total of 200 quartz grains were counted using hot cathode microscope HC1-LM at the Institute of Paleobiology, Polish Academy of Sciences for visual and spectroscopic CL-analyses (Neuser et al., 1996). The microscope was connected to a triple-grating spectrograph of EG & G Princeton Research Instruments for recording the high-resolution spectra. The spatial resolution of spectroscopic analyses was about 30 mm. Electron energy of 14 kV and a beam current density of 0.1 lA mm2 were used for both CL microscopy and spectroscopy. 4. Lithofacies and depositional environment A total of fifty samples were collected from thin to medium bedded and laminated sandstones of the two studied sections (Fig. 1). It comprises vertically variable alternations of thin to medium bedded sandstones, siltstone and black to grey shale that were deposited in tidal flat environment (Behnam, 2013). The lower part of the formation consists of well-bedded dolomitic limestone which is fossiliferous, pale brown to yellowish white, occasionally pale grey in color. The dolomitic limestone is overlain by medium bedded sandstone (Fig. 3a). The thinly bedded sandstones of up to 10 cm thickness that showed ripple currents (Fig. 3a–c) were intercalated with very thin laminated, fissile, black to grey shale; while the thickly bedded sandstones were up to 30 cm thick (Fig. 3a). Lenticular bedding was observed in both sections (Fig. 3c). The Ora Formation abruptly

overlies the siliciclastic sandstone of Pirispiki Formation (Fig. 3a) and is overlain by the Harur Formation which show changes from dominant shale into dominant limestone. The boundaries to theses formations are uniform and conformable. The depositional environment of the Kaista Formation which is now considered the lower part of the Ora Formation was studied by Omer (1993) who recognized four major microfacies and classified it into sub-microfacies. These are (1) Mudstone (fossiliferous and non fossiliferous), (2) Wackestone (bioclast, echinoderms and algal), (3) Packstone (oncoid and lithoclast), and (4) Grainstone (pelletical and bioclast) sub-microfacies. The results suggested that lower part of the Ora Formation was deposited in intertidal to subtidal and shoal environments. The sandstone/shale ratio as well as the thickness of sandstone-dominated successions systematically decreases toward the outer shelf. There are many fining upward cycles of the tidal channel and intertidal flat deposits. The sedimentology and facies analysis in Ora section have been studied by Behnam (2013) and subdivided it into three facies: (1) cross-bedded quartzarenite facies, (2) sand–mud interbedded facies, and (3) black shale facies. The succession of this unit consists of repeated fining upward cycles of medium to very fine sandstone overlain by shale interbedded with thin layers of sandstone or siltstone. The deposition of the Ora Formation was influenced by tidal action, possibly under shallow tide-dominated shelf environment. The sandstone beds are interpreted as subtidal and lower intertidal deposits; meanwhile the shale beds are interpreted as upper intertidal deposits. Behnam (2013) concluded that the lateral and vertical distribution of these facies is strongly influenced by the paleogeographic position.

M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210

5. Results 5.1. Sandstone petrography The sedimentary structures of the Ora Formation sandstones are mostly massive, rippled cross lamination, and occasionally planar lamination. The studied sandstones of the Lower to Upper Ora Formation are fine- to medium-grained, subrounded to rounded and moderately sorted, while subangular to angular grains are much less common (Fig. 4a). The common grain contacts in these sandstones are long, concave–convex and suture types with rare point contacts (Fig. 4b). The results of point counting are presented in Table 1. The detrital composition of the sandstones is dominated by monocrystalline quartz. The proportion of the monocrystalline quartz (Qm) ranges from 48.3% to 61.4% and 51.2% to 62.3% in Chalky Nasara and Ora sections, respectively (Table 1). Monocrystalline quartz (Qm) is seen in three variants: non-undulose, slightly undulose (<5°), and undulose (>5°) (Scholle, 1979; Basu, 1985; Tortosa et al., 1991). In medium grained sandstones, the monocrystalline quartz ranges in size from 0.20 mm to 0.30 mm, and show subrounded to rounded shape (Fig. 4a). In fine-grained sandstones it ranges from 0.10 mm to 0.19 and is subrounded in shape. In very fine-grained sandstones it ranges from 0.05 mm to 0.80 mm and are mostly subangular. Quartz grains with straight extinction dominates the lower part of the Ora Formation, while the slightly undulose (>5°) type dominates the upper part of the formation (Fig. 4b). Polycrystalline quartz (Qp) is composed mainly of three or more crystals per grain, with straight to undulose extinction. Some polycrystalline grains display sutured internal boundaries between composite crystals indicating a probable early stage in the development of metamorphic polycrystalline quartz. On the basis of a petrographic study Behnam (2013) suggested a plutonic provenance for quartz grains in the Ora Formation. K-feldspar (orthoclase, microcline and microperthite, Fig. 4c) is more abundant than plagioclase in both sections of the Ora Formation (Table 1). The average grain size of feldspars range between 0.07 mm and 0.17 mm. Many plagioclase grains have distinctive albite twinning (Fig. 4d). The results of the point counts reveal a generally high proportion of quartz in the sandstones of Ora Formation. These sandstones are characterized by very low proportion of matrix, and much less if compared with cement (Table 1). The mean matrix content of the sandstone is 1.2% and 1% in Chalky Nasara and Ora sections, respectively. The matrix is generally composed of fine-grained quartz (<0.03 mm in grain size), schist, rock fragments and detrital clay minerals (Fig. 4e and f). Point counting of thinly-bedded sandstones showed an average framework composition of Q95.7F2.8R1.4; while the thick beds have an average framework composition of Q85.4F9.7R4.8. Mica makes up 0–4% of the framework grains in both sections. It is dominated by elongate muscovite flakes which were often buckled and bent around harder detrital grains (Fig. 4g). Muscovite and chlorite are more common in the thinly-bedded sandstones compared with the thickly-bedded sandstones. Muscovite is slightly more abundant than chlorite and biotite (Table 1). Heavy minerals form minor amounts (<2%) of the sandstones in the two studied sections (Table 1). They include zircon, tourmaline, rutile, epidote, sphene, staurolite and opaque minerals (Fig. 4h). Based on the Folk et al. (1970) scheme, the Ora sandstones are classified into mature quartzarenite, subarkose and immature sublitharenite (Fig. 5a). The Q–F–L diagram of Dickinson et al. (1983) indicated that the Ora Sandstones fall mainly in the field of ‘‘Craton Interior’’, while few samples plot in the field of ‘‘Recycled Orogen’’ (Table 1, Fig. 5b). This indicates that these sandstones were derived from the stable parts of the craton, with possible minor contribution

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from recycled orogen (Dickinson et al., 1983). Because the original composition of sandstones may have been significantly altered by diagenetic processes, the use of such provenance indicators have to be applied with caution (McBride, 1985). 5.2. Diagenesis Petrographic analysis of the Ora sandstones indicates diagenetic modification including compaction, cementation, dissolution, and alteration of feldspars. The diagenetic processes occurred during early eogenesis, middle mesogenesis and late stage of telogenesis close to fractures and faults. The characterization of the paragenetic history is based on petrographic observations and cathodoluminescence investigation (Fig. 6). 5.2.1. Compaction Compaction is subdivided into two categories, mechanical and chemical compaction (Boggs, 2006). Strong mechanical compaction resulted in disaggregation of rock fragments (e.g., siltstone); as well as rupturing of mica grains and intensive grain annealing of detrital quartz grains (Fig. 7a). Furthermore, pressure solution appears to be an important factor which was only observed between detrital grains (Fig. 7b). 5.2.2. Quartz cement Quartz is the main cementing material in the sandstones of Ora Formation, which occurs in the form of syntaxial overgrowths on detrital quartz grains in all samples (Fig. 7c). Quartz cement ranges between 9.6% and 16.9% with a mean of 12.3% in Ora section, while in Chalky Nasara section it ranges between 7.3% and 18.3% with a mean of 11.6% (Table 1). Porosity can be reduced when quartz cement completely or partially fills pore spaces (Laškova, 1987; Kilda and Friis, 2002). Some of the studied sandstones showed complete cementation by quartz as a mosaic of interlocking overgrowth (Fig. 4a); other quartz overgrowths adjacent to feldspar show rhombohedral embayments which are related to the feldspar dissolution. When sediments are subjected to different conditions during diagenetic processes, various silica sources for quartz cementation might be activated. Such diagenetic conditions include dissolution of feldspars (Hawkins, 1978), pressure solution (Bjørlykke et al., 1986; Dutton and Diggs, 1990; Dutton, 1993; Walderhaug, 1994), replacement of quartz and feldspar by calcite (Burley and Kantorowicz, 1986) and transformation of clays (Rodrigo and Luiz, 2002). The dissolved silica in sandstones during diagenesis can be derived from the extensive dissolution of feldspar and lithic fragments, kaolinization and chemical compaction (Zhang et al., 2008; Umar et al., 2011) which is most probably the case in the sandstones of Ora Formation. Cathodoluminescence investigations of quartz cement for the sandstones of Ora Formation revealed that the quartz cement is characterized by dark brown color, low SEM-CL intensity (non-luminescence) and forms small euhedral overgrowths. Their thickness range between 10 and 20 lm but in places occasionally grew to a larger size (>20 lm thick) and developed euhedral crystals which occlude primary porosity, and they exhibit no zonation (Fig. 10e). The timing of this cementation predated the formation of illite. The observed fluid inclusions are very small and located at the boundary of detrital grains and their overgrowth. Similar phenomena have been described by Omer and Friis (2014). Minor amounts of calcite and ferruginous cement were observed in thin sections (Table 1). The ferruginous cement in Ora sandstone is found around detrital quartz grains and as coating of cavities and hence it is considered later than quartz (Fig. 7d) suggesting that the precipitation of iron-oxide was the latest authigenic event. Ismail (1996) suggested that the ferruginous cement

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Fig. 4. Photomicrographs of framework grains of Ora sandstones under cross-polarized light (XPL). (a) packed subrounded nature of moderately sorted sandstone, with slightly undulose extinction (U); tidal flat environment (Chalky Nasara section, sample 25); (b) long contact (black arrow) and suture contact (red arrow) between monocrystalline quartz grains showing slight undulose extinction (U) (Chalky Nasara section, sample 20); (c) microcline with well-developed grid twinning (red arrow), surrounded by chlorite (albitized K-feldspar (K), Ora section, sample 9); (d) albitized polysynthetic plagioclase grain (Pg) showing well developed overgrowths (red arrow) (Chalky Nasara section, sample 5); (e) fine grained matrix surrounding detrital quartz grain (Qtz) (red arrow) (Chalky Nasara section, sample 2); (f) quartzarenite sandstones and scare schist rock fragment (red arrow); (g) immature sublitharenite sandstones rich in muscovite flakes oriented parallel to detrital quartz grains indicating a low compaction effect (Chalky Nasara section, sample 4); (h) compacted sandstone with dominant rounded grains of zircon as an indication of recycling (red arrow) (Chalky Nasara section, sample 7). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

may have been derived from weathering of iron-rich rocks, where the iron materials are transported as hydrated colloidal oxides within water and then precipitated as coatings around grains and/or pores. Minor calcite cements was observed in the

sandstones of the Ora Formation. It commonly occurs in association with detrital carbonate components suggesting that these carbonate grains have acted as nuclei for the formation of calcite cement.

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M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210 Table 1 Modal analysis of selected sandstones of the Ora Formation. Quartz Qm Cha Samp. 1 53.5 2 61.0 3 51.3 4 48.3 5 54.4 6 56.4 7 55.8 8 52.3 9 53.2 10 54.1 11 55.2 12 53.8 13 52.9 14 54.2 15 55.1 16 54.7 17 53.5 18 56.0 19 55.1 20 56.0 21 56.7 22 57.3 23 59.1 24 56.9 25 58.4 26 57.0 27 61.4 28 57.2 29 60.1 30 58.3

Mica

Cements

Porosity

Qp

K

P

R.F.

Mat

Mu

Ch

Bio

Sil

Cl

Ca

Fer

HM

Pyr

PP

SP

TPC

7.7 5.5 4.6 7.6 5.9 7.4 4.5 6.2 5.9 6.1 5.8 6.2 5.5 6.3 5.3 5.0 5.2 6.1 5.2 5.2 5.6 4.2 6.1 5.7 6.1 5.3 5.8 5.0 4.7 5.2

2.6 0.7 1.8 0.2 1.3 2.2 2.4 2.1 2.0 2.4 2.2 1.9 2.3 2.5 2.0 1.5 2.4 4.4 4.8 5.1 3.1 2.7 2.0 2.5 2.0 3.1 1.9 2.0 1.7 1.6

0.8 2.1 0.4 0.6 3.9 1.0 0.8 1.1 1.6 0.9 1.6 1.7 1.3 0.9 1.2 1.9 1.2 2.7 2.0 1.8 0.7 0.4 0.2 0.3 0.5 0.2 0.6 0.3 0.8 0.9

5.1 0.4 0.8 2.2 1.3 1.1 0.4 4.9 5.8 5.7 4.9 5.3 6.2 5.4 5.8 6.1 5.9 3.5 2.6 3.1 0.9 0.7 0.9 1.2 1.4 1.6 1.1 0.8 1.0 1.3

1.2 0.9 0.4 1.4 1.6 1.8 2.2 1.6 1.4 1.1 1.0 1.7 2.0 1.7 1.9 2.1 1.6 1.2 1.0 1.2 1.0 0.7 0.4 1.1 0.9 1.1 0.9 0.3 1.0 0.8

5.4 0.4 2.5 4.9 2.1 2.4 2.4 4.8 2.3 3.1 2.0 1.8 2.0 1.6 2.2 2.0 2.4 1.5 2.2 2.0 1.9 1.0 1.4 1.9 1.5 1.7 2.2 1.1 1.6 2.3

3.8 0.7 5.3 2.6 1.7 1.1 1.2 2.1 1.9 2.0 1.8 1.1 1.0 0.7 0.8 1.1 1.0 0.4 0.4 1.1 1.0 0.9 0.5 0.7 0.7 1.0 1.0 0.2 1.0 0.8

2.5 0.0 0.8 0.2 2.0 2.7 0.2 2.5 1.5 1.3 2.0 1.6 0.9 1.4 1.1 1.5 1.9 0.9 0.9 1.5 1.6 1.0 0.2 1.4 0.4 0.9 1.3 0.8 1.8 1.0

8.4 16.7 12.5 8.4 9.6 10.2 16.1 9.4 10.1 11.7 10.4 9.6 7.4 8.2 7.3 9.4 9.3 9.0 10.4 11.1 12.5 14.7 15.1 13.9 14.7 13.6 12.8 18.3 14.8 14.9

3.4 3.7 7.5 13.9 5.9 4.1 6.3 4.9 5.7 6.2 6.2 8.8 9.2 9.7 9.0 7.9 5.8 4.6 4.4 4.2 5.3 6.1 5.7 6.2 5.3 6.6 4.9 6.3 5.2 7.1

0.6 0.0 0.6 0.1 0.0 0.9 0.0 0.8 0.0 0.1 0.0 0.0 0.0 0.4 1.1 0.0 1.1 1.8 1.0 0.6 0.0 0.8 0.2 0.6 0.9 0.3 0.0 0.0 0.4 0.2

0.5 0.0 3.4 0.4 1.1 2.0 0.6 1.4 1.9 1.7 2.4 1.6 1.2 0.0 1.1 0.0 2.6 1.6 1.6 1.0 1.3 1.5 1.9 2.1 1.6 1.0 0.8 0.9 1.3 1.0

1.7 3.3 2.5 2.4 1.3 1.8 4.9 1.6 1.3 1.0 1.8 1.2 1.7 1.3 1.8 1.3 1.6 2.0 2.7 1.8 2.6 3.1 2.2 1.6 1.2 1.8 1.3 1.8 1.0 0.9

1.6 0.2 0.6 3.2 2.5 1.2 0.0 1.2 1.6 0.8 1.0 0.0 2.7 1.6 1.4 1.7 1.3 1.0 0.8 0.9 1.5 1.2 0.5 0.0 0.0 0.9 1.0 0.6 0.4 0.3

0.2 0.7 0.5 0.6 1.3 1.6 0.4 0.5 0.6 0.2 0.4 0.5 0.9 0.3 0.6 0.7 0.8 1.2 1.0 0.8 1.1 1.0 0.9 1.0 1.3 0.7 0.9 0.8 1.1 0.8

1.0 3.3 3.9 2.4 3.5 2.0 1.3 2.1 2.9 2.7 1.4 2.0 2.5 3.2 2.1 2.6 2.2 2.0 2.7 2.6 2.9 2.4 2.2 2.0 2.6 2.3 2.1 2.8 2.0 2.3

464 418 477 486 491 441 489 422 491 412 497 403 457 432 493 438 408 440 459 468 445 481 403 476 436 405 451 484 416 429

Range

48.3– 61.4

4.2– 7.7

0.2– 5.1

0.2– 3.9

0.4– 6.2

0.3– 2.2

0.4– 5.4

0.2– 5.3

0.0– 2.7

7.3– 18.3

3.4– 13.9

0.0– 1.8

0.0– 3.4

0.9– 4.9

0.0– 3.2

0.2– 1.6

1.0– 3.9

Mean

55.6

5.7

2.3

1.1

2.9

1.2

2.2

1.3

1.2

11.6

6.3

0.4

1.3

1.8

1.0

0.7

2.4

Ora Samp. 1 54.1 2 60.1 3 62.3 5 54.2 7 51.2 9 52.2 10 53.0 11 54.1 12 61.2 13 57.0 14 59.5 15 56.2 16 62.0 17 58.3 18 61.3

6.3 4.4 5.8 6.1 5.9 5.9 6.3 6.2 5.2 5.9 5.3 5.5 5.8 5.0 4.5

3.0 0.9 1.1 1.3 2.0 2.0 2.2 1.9 2.0 2.5 2.0 4.1 1.4 2.2 1.2

0.6 2.9 2.2 3.9 1.6 1.6 1.6 1.7 0.2 0.3 0.5 0.2 0.6 0.3 0.4

4.2 0.7 1.0 1.3 6.8 6.8 4.9 5.3 0.9 1.1 1.4 1.6 1.0 0.8 1.0

1.1 1.0 0.9 1.6 1.4 1.4 1.0 1.7 0.4 1.1 0.9 1.1 0.9 0.3 0.8

4.7 3.2 3.7 2.1 3.0 2.3 2.0 1.8 1.1 1.9 1.5 1.7 2.2 1.1 1.6

2.9 0.9 1.4 1.7 1.9 1.9 1.8 1.1 0.5 0.7 0.7 1.0 1.0 0.2 1.0

2.3 1.1 1.0 2.0 1.6 1.5 2.0 1.6 0.2 1.4 0.4 0.9 1.3 0.8 1.8

10.1 12.6 9.8 9.6 10.1 10.1 11.4 9.6 14.6 13.9 14.7 13.6 12.8 16.9 14.8

4.4 5.1 5.3 5.9 5.7 5.7 6.2 8.8 5.9 6.2 5.3 6.6 4.9 6.5 5.2

0.2 0.8 0.4 0.0 0.0 0.0 0.0 0.0 0.2 0.6 0.9 0.3 0.0 0.0 0.4

0.6 0.9 0.4 1.1 1.9 1.9 2.4 1.6 1.8 2.1 1.6 1.3 0.8 0.9 1.3

1.5 2.0 1.2 1.3 1.3 1.3 1.8 1.2 2.2 1.6 1.2 1.8 1.3 1.8 1.0

1.4 1.0 0.3 2.5 1.6 1.6 1.0 0.0 0.5 0.0 0.0 0.9 1.0 0.6 0.4

0.3 0.6 0.7 1.3 0.6 0.6 0.4 0.5 0.9 1.5 1.3 0.7 0.9 0.9 1.1

1.8 1.8 2.4 3.5 2.9 2.9 1.8 2.0 2.2 2.0 2.6 2.2 2.1 2.8 2.0

Range

51.2– 62.3

4.4– 6.3

0.9– 4.1

0.2– 3.9

0.7– 6.8

0.3– 1.7

1.1– 4.7

0.2– 2.9

0.2– 2.3

9.6– 16.9

4.4– 8.8

0.0– 0.9

0.4– 2.4

1.0– 2.2

0.0– 2.5

0.3– 1.5

1.8– 3.5

Mean

57.1

5.6

1.9

1.2

2.5

1.0

2.2

1.2

1.3

12.3

5.8

0.2

1.3

1.5

0.8

0.8

2.3

455 426 480 469 492 435 483 497 463 471 465 432 486 492 485

Cha: Chalky Nasara section; Ora: Ora section; Samp.: Sample number; Qm: Monocrystalline quartz; Qp: Polycrystalline quartz; K: Potash feldspar; P: Plagioclase feldspar; R.F.: Rock fragment; Mat: Matrix; Mu: Muscovite; Ch: Chlorite; Sil: Silica cement; Cl: Clay cement; Ca: Calcite cement; Fe: Ferrugenous; HM: Heavy minerals; Pyr: Pyrite; PP: Primary porosity; SP: Secondary porosity; TPC: Total point counts.

5.2.3. Authigenic clay The main authigenic clays consist of coatings of illite platelets arranged tangentially to, and in places, partially detached from grain surfaces and as pore filling (Fig. 7e and h). These coats are usually thicker in the sandstones containing abundant mud intraclasts than in sandstones with few mud intraclasts. The coating in laminated sandstones, preferentially occur in the fine-grained laminae. In some thin sections, illitic coatings are texturally similar to smectite, suggesting a smectitic precursor (Morad et al., 1994) that would have been partially to completely

illitized during diagenesis. SEM study revealed that illite growth often had fibrous habit, exists as pore filling and grain-coating aggregates (Fig. 7e, f, and h) postdating the quartz overgrowth phase and predating calcite cementation (Umar et al., 2011; Zaid, 2013). Illitization which becomes a progressive process under burial conditions at a temperature range of 90–130 °C requires potassium-rich pore water in sandstones (Morad et al., 2000). The honeycomb-like texture produced on clays as coating consists of mixed illite/smectite. Mixed clays occur as a minor pore lining to locally pore-filling precipitate that has

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ragged-platy morphology. SEM analysis shows that mixed illite/smectite layers postdate the quartz cementation (Fig. 7e). 5.2.4. Dissolution The loss of carbonate cement and unstable detrital grains may be an indication of free flow of acid formation water through most of the sandstones. Carbonate dissolution is evident in the form of patches of discontinuous carbonate cement and oversized pores (Fig. 7g) that take place at mesogenic stage. 5.2.5. Pyritization Pyritization has taken place at early eogenesis stage under reducing conditions of marine environment which lead to the formation of authigenic pyrite as cement of trace amounts (Fig. 7h). Pyrite is an accessory diagenetic mineral which is common in marine sandstones, The pyrite cement formed under reducing and alkaline conditions and probably precipitate by reaction of aqueous iron with hydrogen sulfide produced form the decay of the organic matter (Kirchner, 1985), and may form as a result of production of hydrogen sulfide by sulfate reducing bacterial (Mozer, 2010). 6. Provenance analyses 6.1. Albitization of feldspar

Fig. 5. (a) classification of the sandstones of the Ora Formation according to Folk et al. (1970); (b) QtFL tectonic discrimination diagram of Dickinson et al. (1983) applied on the detrital components of the Ora sandstones (Qt is the total quartz, F is the total feldspar, L is the total rock fragments; the dotted lines mark the boundaries between the major fields of tectonic provenances).

Two types of feldspars were identified in Ora sandstones, type-1 is untwined, turbid K-feldspar of medium (0.25 mm) and fine (0.15 mm) grain sizes (Fig. 4c); type-2 is polysynthetic twinned plagioclase containing fluid inclusions which commonly exhibit optical continuity between polysynthetic twins and their authigenic overgrowths (Fig. 4d). Electron microprobe analysis carried out on fresh feldspar crystals with little or no replacement textures show that they are abundantly perthitic orthoclase or microcline that were altered to albite (Fig. 8a). The composition of alkali feldspar in the sandstones of Ora Formation ranges between Ab9.79An0.32Or89.88 and Ab3.60An0.08Or96.32. Some of the feldspars in subarkosic sandstones have been partially or extensively albitized (Fig. 9a). The composition of plagioclase feldspar in the Ora sandstones varies between Ab91.57An0.41Or8.02 and Ab99.91An0.00Or0.09. Representative analyses of feldspars are shown in Table 2. Diagenetic albitization of feldspars significantly affects the use of feldspars in provenance interpretations (Asiedu et al., 2000, 2009). Similarly, albitization of feldspars under burial conditions (>2.5 km depth) as indicated by Dutta and Wheat (1993) should also have similar significance on provenance interpretations. Albitized K-feldspars of Ora sandstones show relicts of the blue luminescent original color (Fig. 9c). Detrital feldspars show bimodal distribution (Fig. 8a), even in a single thin section. Dutta and Wheat (1993) have suggested that such computational bimodality may indicate the primary origin of feldspars. 6.2. Cathodoluminescence of quartz

Fig. 6. Paragenetic history of the sandstones of Ora Formation, Northern Iraq.

Mature quartz dominated sandstones normally have been modified significantly from source to the basin of deposition and therefore the petrography of such sandstones is not always a dependable provenance indicator (e.g., Götze and Zimmerle, 2000; Augustsson and Bahlburg, 2003). Since the CL properties of quartz grains depend on condition of their formation (temperature and pressure as well as geochemistry) and do not change during transport and later diagenesis they can be implied as provenance indicator (Zinkernagel, 1978; Marshall, 1988; Götze and Zimmerle, 2000; Götze et al., 2001; Augustsson and Bahlburg, 2003). CL studies of single quartz grains in the sandstones of the Ora Formation showed brown, dark blue, bright blue, violet, red

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203

Fig. 7. SEM-CL images and photomicrographs of quartzarenite (a) heavily compacted sandstone showing pervasive fracturing partly due to compaction (red arrow) and partly by pressure solution effect that produced post-compaction quartz cement representing non-luminescent phase (Ora section, sample 1; CL image); (b) pressure solution and suture contacts between detrital quartz grains. (Ora section, sample 1; XPL); (c) sandstone showing thin iron oxide films surrounding quartz grains as early cement followed by authigenic quartz cement overgrowth filling the open spaces indicated by arrows (Chalky Nasara section, sample 2; XPL); (d) ferruginous cement (F) filling intergranular pore spaces between euhedral quartz grains that postdated quartz cementation (Chalky Nasara section, sample 3; XPL); (e) quartz overgrowth coated (QO) coated by fibrous illite (il) and mixed-layer clay illite/smectite (il/sm) (Chalky Nasara section, sample 4; SEM image); (f) authigenic illite showing fibrous habit and filling pores inhibiting growth of quartz cement (Chalky Nasara section, sample 7; SEM image); (g) intensive dissolution of carbonate forming secondary porosity (S) indicated by an arrow (Chalky Nasara section, sample 17; XPL); (h) extensive illitization of neoformed muscovite surrounding elongated and corroded quartz grains (Qtz) and authigenic pyrite (Py) (Chalky Nasara section, sample 1; XPL). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210

Fig. 8. (a) Ternary orthoclase (Or)–albite (Ab)–anorthite (An) composition diagram of Pichler and Schmitt-Riegraf (1993) applied to the detrital feldspar of the sandstones of Ora Formation; (b) ternary anorthite (An)–albite (Ab)–orthoclase (Or) provenance diagram of Trevena and Nash (1981) applied to the detrital feldspars of the sandstones of Ora Formation. A = Authigenic; G = Granophyre; M = Metamorphic; P = Plutonic; V = Volcanic.

and green luminescence. Four characteristic CL features of detrital quartz were observed in the sandstones of Ora Formation comprising healed fractures (Fig. 10a and c), mottled textures (Fig. 10d), low-intensity dark CL streaks and patches (Fig. 10e and f), and shocked quartz. The healed fractures appear as distinct thin black lines (5–10 lm thick) with weak or lacking CL (Fig. 10c). These fractures exist in more than one set are sparsely distributed and widely spaced; these lines do not appear in BSE images. Integrated SEM-CL study applied on randomly selected 200 grains in each of 25 quartzarenite samples from Ora Formation

are shown in Table 3 and Fig. 11. The results showed three different quartz types accounting for 56% of metamorphic sources, 43% of plutonic origin, and 1% of volcanic provenances. 7. Discussion Two styles of sandstone diagenesis were recognized in the Ora Formation. The first one concerns the texturally supermature sandstones, which are fine to medium grained, well- to sub-rounded and well-sorted. The second one concerns the texturally immature

M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210

205

Fig. 9. BSE images showing (a) partially albitized detrital K-feldspar of type 1 feldspar (1, 2, 3, 4 and 5 are analyzed spots); Si is silica cement (Chalky Nasara section, sample 18); (b) albitized plagioclase showing relics of the original feldspar; the illite cement has postdated albitization (arrow) (Ora section, sample 5); CL image (c) albitized Kfeldspar with relicts of the original blue following exfoliation lines (yellow arrow) (Ora section, sample 15). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

sandstones that consist of very fine to fine-grained, rounded to subangular and poorly sorted grains, and enriched in mica. The framework grains in the supermature sandstones were dominated by single quartz grains (monocrystalline) with minor amounts of feldspar and rock fragments and occasional micas and detrital clays; their average composition is Q95.7F2.8R1.4. Meanwhile the average composition of immature sandstones is Q85.4F9.7R4.8. These sandstones have undergone intensive and complex episodes of diagenesis, including eogenesis, mesogenesis, and telogenesis. The paragenetic sequence and provenance is inferred with respect to time by OM, SEM, hot-CL and EMP analysis in thin section study. Minor amounts of pore-filling, calcite replacement, and quartz overgrowths have taken place during early diagenesis. Mechanical compaction started to reduce pore spaces during burial. Compaction is dominated by tight grain supported fabric of sandstones as it is evident from close packing of detrital framework which caused reduction of primary porosity (Fig. 4a). The mechanical compaction continued from early to middle stages, and can also be observed from annealing of detrital quartz grains (Fig. 7a) and physical breakdown of some feldspar grains. The compaction is also evident from the concave–convex and sutured contacts of neighboring clastic grains (Figs. 4b and 7b) and certain degree of stylolitization. The chemical compaction was activated during this stage allowing pressure solution to provide the necessary silica for quartz cementation. Pressure solution at grain contacts is indicated by the frequent occurrence of sutured boundaries and the parallel orientation of elongate detrital quartz grains leading to the formation of pervasive microstylolites (Figs. 4b and 7b). The thin shale laminae interbedded with thinly

bedded sandstones within the Ora Formation have probably acted as an extra source for quartz cement; similar case has been ˇ yzˇiene˙ et al. (2006) in the sandstones of Cambrian observed by C Deimena Group in Lithuania. The loss of SiO2 requires addition of K2O and Al2O3 during shale diagenesis which is an open system process (Land et al., 1997; Lynch et al., 1997). Based on this assumption, the amount of silica liberated through shale diagenesis of Ora Formation would be enough for the creation of quartz overgrowths in the associated sandstones. Where post-compactional quartz cementation occurred, quartz was precipitated as syntaxial overgrowths; in some cases so massively that all remaining porosity was lost (Bernet et al., 2007). The precipitated post compactional quartz cement which show slight blue color was observed in some cases and is considered as an indication of late diagenesis (Fig. 10e); this means that such cement was probably not coeval and genetically not linked. Additional silica sources for post compactional quartz cement in quartzarenites can be produced during alteration and dissolution (Worden and Morad, 2000). The current study revealed that most primary porosity was blocked by compaction and post compactional quartz cementation (Figs. 4h and 10e), a phenomenon which was also observed by Bernet et al. (2007). The quartz overgrowths are well developed and continued to late diagenesis stage. The minor amounts of ferruginous cement filling pores have post-dated quartz cement during mesogenesis stage (Fig. 7d). Authigenic pyrite was produced at early eogenesis stage under reducing conditions of marine environment (Fig. 7h). Albitization of K-feldspars and plagioclase can produce many diagenetic minerals such as calcite and illite causing significant

0.08 4.78 95.14

0.08 3.60 96.32

0.00 5.06 94.94

0.08 4.47 95.45

0.11 5.65 94.25

1.46 4.60 93.94

changes of the original framework composition of sandstone (Boles, 1982; Morad et al., 1990). Thus, identifying diagenetic albitization can hinder provenance interpretations. Albitization of K-feldspar and plagioclase is a wide diagenetic process in the studied sandstones and is possibly simultaneous in both. The optical continuity of polysynthetic twining of the overgrowth and the plagioclase grains could be an indication that the polysynthetic cements has pre-dated or was simultaneous with the albitization process (Morad et al., 1990). Albite cements has more likely postdated illite cements (Fig. 9b). Thus albitization postdated eogenesis K-feldspar cements and pre-dated the hydrothermal metamorphism as an indication of mesogenesis process. The textural evidence presented in this study reveals that albitization is guided by planes of weaknesses such as fractures, cleavage traces and twining planes (Fig. 4d); this is supported by the idea that the mechanism of albitization is probably a dissolution and precipitation process (Boles, 1982; Morad, 1988; Saigal et al., 1988; Ramseyer et al., 1992). Albitization of K-feldspar normally happens at greater depth and high temperatures (Morad et al., 1990) suggesting that the diagenetic albitization of the sandstones of Ora Formation has taken place under such conditions. Dominance of Na-rich feldspars ranging in composition between Ab91.8 and Ab99.9 is a reflection of the widespread albitization processes in the Ora Formation. This assumption is further supported by the following indications.

0.32 4.36 95.32

1. The predominance of albite among plagioclase feldspars with average composition of Ab98.07An0.01Or1.91 (Fig. 8a). 2. BSE image show evidence of partial albitization (Fig. 9a). The greater compositional dispersion of luminescent plagioclase grains when compared to nonluminescent plagioclase grains could be the result of partial albitization of these luminescent grains. However, in some plagioclase grains, different luminescent colors are observed. The lack of CL of most feldspar grains and evidences of partial albitization are shown by CL (Fig. 9c). 3. The predominance of chemically pure albite forming >99% (Figs. 4d and 9b) is an indication of diagenetic origin (Boles, 1982; Saigal et al., 1988). 4. Relicts of the original K-feldspar or plagioclase are shown in Fig. 4c; such phenomenon has also been observed by González-Acebrón et al. (2010).

0.32 9.79 89.88 Or.: Ora section; Ch.: Chalky Nasara section; No.: Number sample.

0.08 5.81 94.11 End-member compositions % An 1.48 0.00 Ab 6.02 99.69 Or 92.50 0.31

0.00 99.91 0.09

0.00 99.81 0.19

0.05 99.59 0.36

0.03 99.78 0.19

0.24 98.74 0.68

0.26 99.06 0.68

1.45 98.26 0.29

0.01 98.07 1.91

0.00 4.73 95.27

19.99 20.00 19.95 20.01 20.04 20.08 20.07 20.10 20.10 19.90 19.84 Total

19.86

20.00

20.16

19.94

19.74

19.91

19.92

20.17

20.04

11.88 4.06 0.02 0.00 0.19 3.84 0.01 11.92 4.11 0.02 0.00 0.22 3.72 0.00 11.86 4.13 0.03 0.00 0.17 3.74 0.00 11.89 4.14 0.03 0.00 0.20 3.74 0.01 11.90 4.13 0.00 0.00 0.14 3.85 0.01 11.87 4.12 0.05 0.00 0.19 3.83 0.02 11.87 4.14 0.02 0.00 0.18 3.85 0.01 11.91 4.17 0.00 0.01 0.39 3.60 0.02 11.90 4.14 0.01 0.00 0.23 3.77 0.05 Cations based on 32 oxygens Si 11.68 11.91 Al 4.14 4.09 Fe 0.02 0.00 Ca 0.00 0.00 Na 0.26 3.89 K 3.74 0.01 Ba 0.00 0.00

11.95 4.09 0.00 0.00 3.83 0.00 0.00

11.98 4.10 0.00 0.00 3.92 0.01 0.00

12.24 4.10 0.01 0.00 3.79 0.01 0.00

11.88 4.08 0.02 0.00 3.95 0.01 0.00

11.80 4.08 0.02 0.01 3.81 0.03 0.00

11.92 4.09 0.00 0.01 3.86 0.03 0.00

11.93 4.13 0.00 0.06 3.79 0.01 0.00

12.20 4.21 0.02 0.00 3.66 0.08 0.00

11.88 4.15 0.01 0.00 0.19 3.80 0.02

100.71 101.44 101.13 101.85 101.47 101.07 101.93 101.09 100.05 Total

101.72

101.63

101.13

101.14

102.08

100.04

101.26

100.97

101.47

101.37

100.75

Ch.24

65.42 19.14 0.15 0.02 0.63 16.01 0.06 65.13 19.24 0.21 0.02 0.50 16.05 0.00

Or.15 Ch.22

65.42 19.31 0.22 0.00 0.57 16.15 0.19 65.21 19.18 0.00 0.00 0.41 16.56 0.11

Ch.20 Ch.19

64.66 18.98 0.33 0.00 0.54 16.34 0.23 65.54 19.11 0.08 0.00 0.53 16.18 0.31 65.26 19.28 0.14 0.00 0.50 16.60 0.16 68.78 20.60 0.12 0.00 11.50 0.34 0.04

Or.13 Ch.18 Or.11

69.32 20.39 0.00 0.30 11.37 0.05 0.04

Ch.16 Or.7

69.15 20.10 0.02 0.05 11.52 0.12 0.00 69.13 20.33 0.13 0.05 11.51 0.12 0.01 64.84 19.39 0.03 0.07 1.11 15.43 0.22 68.32 19.84 0.11 0.01 11.66 0.03 0.07

Or.5 Or.4 Ch.6 Ch.5

69.73 20.15 0.05 0.01 12.02 0.07 0.07 69.23 20.15 0.01 0.00 11.71 0.03 0.01

Or.3 Or.2

69.45 20.21 0.00 0.00 11.46 0.02 0.00 65.10 19.27 0.07 0.00 0.72 16.24 0.67

Or.1 Ch.2

SiO2 Al2O3 FeO CaO Na2O K2O BaO

69.71 20.23 0.00 0.00 11.72 0.06 0.00

Ch.1

64.26 19.08 0.12 0.00 0.68 15.91 0.00

No. Sec.

Table 2 Representative EMP analysis of detrital feldspar grains from the sandstones of Ora Formation.

64.87 18.72 0.13 0.00 0.53 16.39 0.07

M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210

Ch.26

206

As has been explained earlier the source rocks for the studied sandstones are of two types, plutonic and metamorphic. Trevena and Nash (1979, 1981) have probably provided the most comprehensive studies on detrital feldspar composition as provenance indicator. The discrimination diagram of Trevena and Nash (1981) applied to the studied sandstones show that the feldspars in the Ora Formation are of plutonic and metamorphic origin (Fig. 8b). The late diagenetic events started by the dissolution and replacement of silica by carbonate cement (Fig. 7g). This process typically occurs when the pore waters are undersaturated with respect to carbonate (i.e., the interstitial fluid is strongly alkaline), so the major part of silica overgrowth and, to a lesser extent, parts of the detrital quartz grains dissolved. The presence and growth of thick illite coatings have inhibited quartz overgrowths and rare mixed layer clays (illite–smectite) partially embedded and grown on authigenic quartz as an indication of its later origin than quartz cementation which is considered as mesogenesis stage (Fig. 7e and f). The occurrence of ultrastable heavy minerals (zircon, tourmaline and rutile) as predominant group of heavy minerals suggests plutonic source rocks; whereas subordinate amounts of epidote, sphene and staurolite indicate metamorphic predecessor (Morton, 1985; Morton et al., 1992). The dominance of ultrastable

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207

Fig. 10. CL images of detrital quartz grains showing (a) brown quartz grains which dominate the studied sandstones representing metamorphic sources as well as some quartz grains of plutonic origin displaying dark blue, bright blue, violet, red colors luminescence; a healed fracture within quartz grain is shown by red arrow (Chalky Nasara section, sample 1); (b) volcanic quartz showing red luminescence; quartz cement (Qc) phase filling pore spaces between quartz grains of metamorphic (M Qtz). plutonic (P Qzt) and volcanic (V Qtz) origin (Chalky Nasara section, sample 2); (c) plutonic quartz grain showing thin healed fractures (red arrow) (Ora section, sample 11); (d) quartz grains of metamorphic origin showing various characteristics such as mottled (red arrow) and nearly uniform texture (Chalky Nasara section, sample 7); (e and f) plutonic quartz displaying various features such as filled fractures (F), dark CL streaks (red arrow) and patches (P); thin rims of inherited quartz cement (white arrows) around detrital quartz grains and post-compactional quartz cement (Post Co.) are also shown (Ora section, sample 3). M = Metamorphic; P = Plutonic; V = Volcanic; Qtz = Quartz. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

minerals indicates intensive chemical weathering of the source rocks and the recycling of earlier detrital material; this in turn indicates a high degree of maturity of sandstones (Morton, 1985). These features are characteristic of clastic passive margin facies (Burnett and Quirk, 2001). The presence notable amounts of fresh detrital muscovite and chlorite suggests derivation from low to medium-grade metamorphic source rocks (Fig. 4g) (Blatt, 1982; Boggs, 1992). Detrital quartz-dominated mature sandstones normally modify significantly from source to the basin of deposition; the absence of lithics renders the petrography of quartz-dominated sandstones not always a reliable provenance indicator (e.g., Götze and Zimmerle, 2000; Augustsson and Bahlburg, 2003). The CL properties of quartz grains that depend on changes in temperature and pressure as well as geochemistry of the depositional environment during the growth of such quartz grains and post-dated geological

events (Zinkernagel, 1978; Matter and Ramseyer, 1985), can be implied as provenance indicator (Götze and Zimmerle, 2000). Based on such facts, the CL signal of assumingly unchanged single quartz grains since deposition in the source area can be used as provenance indicator (Zinkernagel, 1978; Marshall, 1988; Götze et al., 2001; Augustsson and Bahlburg, 2003). Walderhaug and Rykkje (2000) observed variations in the CL color of quartz from violet to blue brown in plutonic rocks and from yellow brown to violet brown and violet in metamorphic rocks. Most of the studied quartz particles show brown, dark brown or brownish CL colors (Fig. 10a); such colours are indicative of typical Low-grade metamorphic origin (Richter et al., 2003). Zinkernagel (1978) reported that brown luminescence is characteristic of quartz in metamorphosed igneous rocks, metasedimentary rocks, some contact metamorphic rocks, and regional metamorphosed rocks. Accordingly to these observations, the CL

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M.F. Omer / Journal of African Earth Sciences 109 (2015) 195–210

Table 3 Point counting percentages of individual quartz grain for sandstones Ora Formation based on hot (CL) study. No.

Pl. Qtz

Vol. Qtz

Met. Qtz

T.P.C.

Or.1 Or.8 Or.10 Or.11 Or.14 Or.16 Ch.1 Ch.2 Ch.4 Ch.5 Ch.7 Ch.9 Ch.12 Ch.16 Ch.18 Ch.19 Ch.22 Ch.25 Ch.26 Ch.27 Ch.28 Ch.29 Ch.30 Ch.31 Ch.32

30.8 40.4 39.6 37.1 52.5 53.7 30.0 29.8 35.8 38.8 36.0 34.9 40.1 34.6 36.2 38.8 40.2 50.3 53.8 57.4 54.3 55.1 57.1 52.0 52.9

0.9 1.3 0.4 0.3 1.2 0.9 0.6 0.5 1.6 0.0 3.1 1.2 0.0 0.0 0.0 0.8 0.2 0.0 0.3 1.1 0.0 0.5 1.9 0.4 0.2

68.1 57.8 59.6 62.3 46.0 45.2 69.3 69.5 62.5 61.2 60.9 63.8 59.7 65.2 63.1 60.2 59.1 49.5 45.7 41.2 45.0 44.2 40.3 47.6 46.2

200 200 200 197 200 200 200 200 200 200 200 200 200 200 200 199 198 200 195 200 200 198 200 200 200

Average

43%

1%

56%

CL: Cathodoluminescence; Or: Ora section; Ch: Chalky Nasara section; Pl.: Plutonic; Vol.: Volcanic; Met.: Metamorphic; Qtz: Quartz; T.P.C.: Total point counted.

characteristics of quartz grains in the sandstones of Ora Formation suggest derivation from multiple source areas or lithologies. Major source lithologies are low- to high-grade metamorphic rocks and plutonic rocks, but notable scarce amounts of detritus were also derived from volcanic rocks which is evident from the red CL color (Fig. 10b; Table 3). The contribution from plutonic rocks is documented by blue to violet CL colors and the healed fractures and black patches in the quartz grains (Fig. 10a, c, and f). The fracture-healing material in Ora sandstones is silica. Healed fractures are particularly common in quartz of plutonic origin and in some metamorphic quartz, but are uncommon in volcanic quartz (Boggs and Krinsley, 2006). Some quartz grains display irregular,

indistinct pattern of non-differential CL that gives the grain a mottled texture (Fig. 10d). Irregular mottled texture is particularly common in metamorphic quartz as the only characteristic feature. The mottled texture indicates irregular distribution of activator ions or defect structures within the grain. These mottled textures may be caused by incomplete recrystallization or annealing that accompany metamorphism or it may be the result of deformation during metamorphism (Boggs and Krinsley, 2006; Boggs, 2009). The presence of irregular shaped bands up to 100 lm thick and black patches characterizes many plutonic quartz grains. Black streaks and irregular patches of dark CL are associated with fractures in many grains (Fig. 10e and f). Zoning in quartz crystals which characterizes volcanic sources and reflects complex growth history and rate of crystal growth or chemical variation in the melt during crystallization (Watt et al., 1997) have not been observed in the studied sandstones. An addition from low-to-high grade metamorphic rocks is shown by brown color and the irregular mottled texture (Fig. 10d). Such dominance can be expected from active margins with no volcanism or from passive margin. Source rocks of sediments at non-volcanic convergent margins, might come from older crustal material near or from unroofed rocks that were produced as a result of subduction (Schwab, 1986). Sediments deposited along passive continental margins are expected to be very rich in quartz grains of metamorphic and plutonic, and recycled affinities, even though such sediments may have been transported across tectonic boundaries (Potter, 1994). Depositional basins at passive margins are often fed by sources from the interior of continents, the rocks of which were produced at ancient active plate margins. The dominance of metamorphic over the plutonic source rocks based on CL characteristics, might suggest a source area not far from the studied Ora Formation such as the Bitlis Metamorphic Belt of Southern Turkey. The Bitlis Massif Turkey is an allochthon emplaced over the Arabian continental margin during the Early Miocene (Rigo de Righi and Cortesini, 1964; Perinçek, 1979; Yılmaz et al., 1993; Robertson et al., 2006). The Bitlis Massif is situated 1000 km north of the Arabian–Nubian Shield, which is characterized by juvenile continental crust, 0.55–0.83 Ga in age. The Bitlis complex consists of metamorphic and igneous rocks including ophiolite (Göncüoglu, 1998; Ustaömer et al., 2012). Granitoids also occurs within Bitlis Massif which contains accessory minerals such as garnet, zircon, tourmaline, epidote, rutile, sphene, apatite, biotite, muscovite, chlorite, and opaques (Ustaömer et al., 2012). All these accessary minerals were recognized in the sandstones of Ora Formations as heavy minerals (Fig. 4h). The existence of quartz and feldspars in Ora Formation with metamorphic and igneous characteristics and the presence of the same heavy minerals as in the granitoids of Bitlis Massif support the idea that these minerals might have been originated from the Bitlis massif which is 165 km to the northwest of the studied Ora Formation.

8. Conclusions Petrographic studies of the Devonian–Carboniferous sandstones of Ora Formation provided new insights into the chemical and structural properties of detrital quartz and feldspar grains. Accordingly, the following conclusions can be drawn as follows:

Fig. 11. Ternary provenance discrimination diagram of Bernet and Bassett (2005) applied to the sandstones of Ora Formation based mainly on cathodoluminescence analysis of 200 quartz grains per sample; the metamorphic quartz includes low- and high-grade metamorphic types.

1. The sandstones of Ora Formation are quartzarenite to subarkose and sublitharenite. The detrital grains are dominated by monocrystalline quartz with slightly undulose extinction. 2. Framework mineralogy and quartz types petrography suggests that the metamorphic and igneous rocks of Bitlis Massif in southeastern Turkey may have contributed as source rocks of the sandstones of Ora Formation.

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3. The quantity of quartz cement increases with increasing the amount of interlayered shale lamina within sandstones and consequently with decreasing the thickness of sandstone bed. 4. Diagenetic changes in the sandstones followed two styles depending on texture and mineralogy. Low CL intensity (non-luminescence) of dark brown color of quartz cement surrounding detrital quartz grains are considered an early quartz cement diagenesis and slightly blue color of post compactional quartz cement of late diagenesis. The primary porosity of the sandstones is reduced due to intense mechanical compaction and due to filling of early quartz cement. 5. The occurrence of thick illitic coating that fills the pore spaces shows fibrous habit which has inhibited the growth of quartz cement and postdated quartz overgrowth of mesogenic stage. 6. Petrographic evidences including cathodoluminescence and microprobe analysis, suggests a diagenetic albitization process. Albitization occurs in the lower part of the formation, thus postdated K-feldspar cements and predated the calcite cements which are probably the product of albitization. The composition of plagioclase feldspar in the Ora sandstones varies between Ab91.6An0.4Or8.0 and Ab99.9An0.0Or0.1. The feldspars in the sandstones of Ora Formation are of metamorphic and plutonic origin as indicated by Or–Ab–An provenance diagram. 7. The CL characteristics of quartz grains of the Ora Formation sandstones also suggest derivation from multiple provenances. The major sources are metamorphic and plutonic rocks with notable contribution from volcanic rocks. 8. The paragenetic events of Ora Formation has taken place in the following order: mechanical compaction – pressure solution – quartz cementation – pyritization – K feldspar authigenesis – albitization of feldspars – calcite cementation – clay cementation – ferruginous cementation and dissolution.

Acknowledgements The author would like to thank Erasmus Mundus Action Program of European Union for funding this research project as a postdoctorate fellowship. I am grateful to Professor Bagiñski and the authorities at the Faculty of Geology, the University of Warsaw for their assistance and making all research facilities including SEM and EMPA available for this project. Thanks also to Professor Stolarski at the Polish Academy of Sciences for making the hot-CL available for this study. Thanks also to Professor Henrik Friis (Aarhus University, Denmark) for constructive criticism and review which helped to improve the manuscript significantly. References Aqrawi, A.A.M., Goff, J.C., Horbury, A.D., Sadooni, F.N., 2010. The Petroleum Geology of Iraq. Scientific Press Ltd., Beaconsfield, Bucks, UK, 424 p. Asiedu, D.K., Suzuki, S., Shibata, T., 2000. Provenance of sandstones from the Wakino Sub group of the Lower Cretaceous Kanmon Group, northern Kyushu, Japan. Island Arc 9, 128–144. Asiedu, D.K., Suzuki, S., Shibata, T., 2009. Provenance of Early Cretaceous Hayama Formation, Okayama Prefecture, Inner Zone of South west Japan: constraints from modal mineralogy and mineral chemistry of de rived detrital grains. Okayama Univ. Earth Sci. Rep. 16, 29–42. Augustsson, C., Bahlburg, H., 2003. Cathodoluminescence spectra of detrital quartz as provenance indicators for Paleozoic metasediments in Southern Andean Patagonia. J. S. Am. Earth Sci. 16, 15–26. Barzinjy, D.N., 2006. Sedimentology and Palynology of Kaista and Ora Formations in Zakho Area, Iraqi Kurdistan Region, Unpublished M.Sc. Thesis, Salahaddin University, 111 p. Basu, A., 1985. Reading provenance from detrital quartz. In: Zuffa, G.G. (Ed.), Provenance of Arenites: NATO ASI Series, C148. D. Reidel Publishing Company, Dordrecht, pp. 231–247. Behnam, W.M., 2013. The Sedimentology of Ora FORMATION (Upper Devonian– Lower Carboniferous) in North and West Iraq. Unpublished Ph.D. Thesis, Baghdad University, 168 p.

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