Nagham Omar1*
Tom McCann1
Ali I. Al-Juboury2Mohammed A. Al-Haj3
Arkan O. Sharazwri4
John S. Armstrong-Altrin5- 1Institut für Geowissenschaften, Geologie, University of Bonn, Bonn, Germany
- 2Petroleum Engineering Department, College of Engineering, Al-Kitab University, Kirkuk, Iraq
- 3Geology Department, College of Science, Mosul University, Mosul, Iraq
- 4Department of Petroleum Geosciences, Faculty of Science, Soran University, Soran, Erbil, Iraq
- 5Universidad Nacional Autónoma de México, Instituto de Ciencias Del Mar y Limnología, Unidad de Procesos Oceanicos y Costeros, Ciudad Universitaria, Ciudad de México, Mexico
The Lower Jurassic-Lower Cretaceous successions from the Ranya and Warte areas in the northeastern Iraq were investigated to determine the provenance and tectonic setting. Fifty-seven shale samples from six Jurassic-Lower Cretaceous formations, i.e., Sarki, Sehkaniyan, Sargelu, Naokelekan, Barsarin, and Chia Gara, were analyzed using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and X-ray fluorescence (XRF). Mineralogical results showed that the shales comprise mainly calcite, dolomite and quartz, with traces of feldspar, illite/mica, kaolinite and chlorite. Geochemically, a range of elemental ratios, along with light rare earth elements heavy rare earth elements (HREE), revealed that felsic and intermediate igneous rocks constituted the predominant source rocks, whilst Eu/Eu* anomalies indicated a felsic provenance. The obtained mineralogical and geochemical data suggested that the sediments were derived from the igneous rocks in the Sanandaj–Sirjan Zone of Iran. Additionally, the tectonic discrimination diagrams revealed a passive margin setting, which is consistent with the tectonic province of the Ranya and Warte shale successions of northeastern Iraq.
1 Introduction
The determination of tectonic setting and the provenance history of the Jurassic-Lower Cretaceous shales from northeastern Iraq is the novelty of this paper. Detailed studies covering the entire Jurassic-Lower Cretaceous successions in the northeastern Iraq were attempted. In this study, we integrated the mineralogical and geochemical data of the Jurassic-Lower Cretaceous shales from the Warte and Ranya areas, northeastern Iraq.
The chemical composition of shales provides important indication to assess the paleotectonic setting and provenance of the clastic rocks (McLennan et al., 1993; Lee, 2009; Moradi et al., 2016; Al-Juboury et al., 2021; Ramos-Vázquez and Armstrong-Altrin, 2021). The chemical and mineralogical composition of siliciclastic rocks is influenced by a number of variables, such as the composition of the source rocks, the extent of weathering in the source area, the depositional setting, and post-depositional modifications (Nesbitt et al., 1980; Dickinson et al., 1983; Roser and Korsch, 1988; McLennan et al., 1993; Nesbitt et al., 1996; Ekoa Bessa et al., 2021; Al-Juboury et al., 2024; Rasool et al., 2025). The signature of the source materials is preserved because components such rare earth elements (La and Y ppm) and transition trace elements (Th, Sc, Hf, and Ti ppm) are transported to the sedimentary basin without undergoing considerable fractionation (Agbenyezi et al., 2022; Yao et al., 2025). These immobile elements are widely used in provenance studies (Dickinson and Suczek, 1979; McLennan et al., 1983; Bhatia and Crook, 1986; Roser and Korsch, 1986; McLennan and Taylor, 1991; Mahanta et al., 2021; Mehrabi et al., 2021), and to assess the relative contribution of felsic and basic sources (Wronkiewicz and Condie, 1987; Armstrong-Altrin et al., 2013; Mudoi et al., 2022). The fine-grained nature and impermeability of shales help to retain the original geochemical signatures of source rocks (Cullers, 1995; Mahanta et al., 2021; Kuznetsova et al., 2024) enabling scientists to use the geochemical signature of shales to determine the paleotectonic setting of a sedimentary basin (Dickinson and Suczek, 1979; McLennan et al., 1983; Bhatia and Crook, 1986; Roser and Korsch, 1986; Verma and Armstrong-Altrin, 2013; Moradi et al., 2016; Mahanta et al., 2021; Tobia and Mustafa, 2022). The shales of the Early Jurassic-Early Cretaceous successions in two northeastern Iraqi outcrop sections—the Ranya and Warte sections—which include six Jurassic-Early Cretaceous formations (Sarki, Sehkanyian, Sargelu, Naokelekan, Barsarin, and Chia Gara) are the subject of the current study. This study is focused on mineralogical investigations using X-ray diffraction (XRD) and scanning electron microscopy (SEM), as well as geochemical (major, trace and rare earth elements) analyses by X-ray fluorescence (XRF) spectroscopy. The Lower Jurassic-Lower Cretaceous successions of Iraq have previously been studied in terms of their depositional environment, hydrocarbon potential, paleoclimate, paleosalinity, paleoweathering, and paleoredox conditions, but little attention has been paid to investigating their source areas and tectonic setting (Mohialdeen et al., 2013; Abdula, 2016; Mustafa and Tobia, 2020; Omar et al., 2020; Tobia and Mustafa, 2022; Omar et al., 2022; Omar et al., 2023). The present work constitutes a pioneering study that combines the mineralogy and geochemistry data of the Lower Jurassic-Lower Cretaceous shales from northeastern Iraq with the aim of determining their provenance history and tectonic setting.
2 General geology
The regions under investigation, namely, the Ranya and Warte areas, are situated in the Zagros Basin of northeastern Iraq (Figure 1), at the northeastern edge of the Arabian Plate, which signifies the point where the continental regions of the Eurasian margin collide with the Arabian Plate (Beydoun et al., 1992; Stampfli and Borel, 2002; Liu et al., 2018). The two study areas, which are around 70 km apart, are situated in the Imbricated Zone and the High Folded Zone, two distinct structural zones of the Zagros Basin (Figure 1).
Figure 1. Regional plate tectonic map of northern Iraq showing the study areas and the structural zones of the Zagros Basin within the Arabian Plate.
The Zagros Basin comprises three structural zones, the Thrust Zone, the High Folded Zone, and the Low Folded Zone (or Foothills). The Low Folded Zone is defined by a comparatively thin layer of sedimentary deposits and an absence of notable folding, conversely, the High Folded Zone features a thick and folded sedimentary layer, with the degree of folding becoming more pronounced as one move towards the north-east (Jassim and Goff, 2006). The latter Zone of Jassim and Goff (2006) falls within the Imbricated Zone as described by Zainy et al. (2017). According to Zainy and coworker, the Imbricated Zone can be subdivided into two subzones, including the Balambo-Tanjero Subzone and the northern (Ora) Thrust Subzone (i.e., the Thrust Zone) (Zainy et al., 2017). The Early Jurassic-Early Cretaceous-age sequence of the Warte area is exposed within the Imbricated Zone, whilst the Early Jurassic-Early Cretaceous-age succession of the Ranya area lies within the High Folded Zone (Figures 1, 2).
Figure 2. Geological map of the study areas (modified from Delizy and Shingaly (2022)).
The relative tectonic stability of the area during the Early Jurassic is reflected by the deposition of several carbonate and evaporite successions as represented by the Alan, Mus and Adaiyah formations in the Mesopotamian Basin of central Iraq and the correlative Sarki and Sehkaniyan formations in the Zagros Basin of northern Iraq. In the Middle Jurassic, deposition of the organic-rich shales of the Sargelu Formation and their equivalents on the Arabian Plate occurred (Murris, 1980; Beydoun, 1991; Sadooni, 1997; Kameran et al., 2023). In the Late Jurassic, the Naokelekan and Barsarin formations were deposited on the eastern margin of the Zagros foreland basin (Murris, 1980; Numan, 2000), while the Najmah Formation was deposited on the western basin margin. The Chia Gara Formation was deposited in the Late Jurassic-Early Cretaceous. From the Late Jurassic through into the Cretaceous, the tectonic setting of the Arabian plate margin changed from extension, to compression (Numan, 1997; Numan, 2000). In the Zagros Thrust Zone of northeastern Iraq, ophiolite complexes are abundant along the Iraq-Iran border (Figure 3). These resulted from the Neo-Tethys Ocean opening and subducting, followed by an oblique collision between the Iranian microcontinent and the African-Arabian plate in the Late Cretaceous-Early Tertiary (Ismail and Carr, 2008). The majority of the Iraqi ophiolites are Cretaceous in age (Ali et al., 2019), therefore, they could not be a possible source for the Jurassic sediments of the area of study, while the Iranian ophiolites were dated of Jurassic age (Figure 3). Within the structural zones that are related to the Zagros Basin, the Jurassic-age rocks are frequently encountered as isolated outcrops in eroded cores and limbs of anticlines (Numan, 2000).
Figure 3. The Zagros Ophiolitic Belt forms part of a 3,000 km long series of Upper Cretaceous-age ophiolites which extend from Cyprus to Oman, and continuing further east into Asia (modified after Ali et al. (2019)).
As noted above, the Lower Jurassic-Lower Cretaceous successions of both the Ranya and Warte areas comprise six formations (Sarki, Sehkanyian, Sargelu, Naokelekan, Barsarin, and Chia Gara). Some of these formations are considered to be part of the recognized megasequences in the region. Sharland et al. (2001) examined the sequence stratigraphy of the Arabian Plate region. Five main deformational phases were responsible for the evolution of the Arabian Plate, with sedimentary successions being deposited during each tectonic phase, with each megasequence reflecting the tectonic conditions which were active during deposition (Sharland et al., 2001; Jassim and Goff, 2006; Zainy et al., 2017; Othman and Omar, 2023). The studied Early Jurassic-Early Cretaceous successions of northern Iraq, were deposited during AP6-AP8 (Sharland et al., 2001). This period was mostly characterized by a passive margin setting, interrupted by shorter periods of extensional tectonics (Sharland et al., 2001). In particular, post-rift thermal sagging dominated the Arabian plate during the Lower Jurassic–Lower Cretaceous period, creating a passive margin toward the northwest and northeast. The ongoing evolution of the Neo-Tethys Ocean saw continued spreading to the northeast, while the Mediterranean Sea in the north began to open during the Late Jurassic. The rifting associated with the formation of the Mediterranean Sea is thought to have created local basins in the areas, which is now northeastern Iraq, including the areas of this study (Fadhel and Al Rahim, 2019).
3 Materials and methods
For an in-depth mineralogical and geochemical investigations, fifty-seven samples were collected from the Ranya and Warte sections of the Lower Jurassic-Lower Cretaceous shale successions of the Sarki, Sehkaniyan, Sargelu, Naokelekan, Barsarin, and Chia Gara formations (Figure 4).
Figure 4. Stratigraphic columns of the Jurassic-Lower Cretaceous successions in both studied sections showing the lithological components and samples location.
Representative bulk (random) samples were analyzed at the Geological Institute of Bonn University in Germany using X-ray diffraction (XRD) with a D8 Advance diffractometer from Bruker AXS GmbH. Ni-filtered Cu Kα radiation, 30 kV/40 mA, divergence aperture V20, detector aperture: 0.2 mm, scatter aperture V20, goniometer speed: 0.04 °/min, 2 s/step, measurement range 4 °–70 ° two-theta are the source information used for X-ray diffraction system.
X-ray fluorescence (XRF) spectroscopy analysis of the major, trace and rare earth elements was conducted at the Bureau Veritas Laboratory, Canada and the Geological Institute of Bonn University in Germany. The analyzed elements include, Si, Al, Fe, Mn, Mg, Ca, Na, K, Ti, P, S, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Zr, Co, Sc, Th, U. The accuracy and analytical precision were determined using analyses of reference material (STD OREAS45E_IG and STD SO-18), and duplicate samples were measured in each analytical set. Duplicate measurements were conducted for 10% of the analyzed samples (precision 2%–3%).
At the Premier Corex Group Laboratory in Houston, Texas, in the United States, an FEI Quanta FEG 650 FE-SEM instrument fitted with two Bruker EDS XFlash 5030 energy dispersive X-ray spectroscopy (EDS) detectors and an FEI R580 Everhart-Thornley (ETD) electron detector was used to perform scanning electron microscopy (SEM) analyses on a subset of samples based on the obtained XRD results.
A Mapping/HyperMap Spectrum is used for EDS elemental map generation. Fresh fracture surfaces were prepared by breaking rock fragments as close to perpendicular to the bedding planes as possible and were subsequently used for analysis. All samples were mounted on aluminum stubs using a conductive, high-viscosity adhesive. Prior to SEM examination, the samples were sputter-coated with a 10 nm layer of iridium using a Leica EM ACE600 sputter coater. The analyses were conducted under high vacuum conditions with an accelerating voltage set at 10 kV.
4 Results
4.1 Mineralogy
The dominant minerals in the shale samples, as indicated by XRD analysis, are quartz and calcite. Dolomite was also distinguished, in addition to traces of feldspar and some clay minerals, including, illite/mica, chlorite, and kaolinite (Figure 5).
Figure 5. Representative X-ray diffractograms showing the main components in the shales: (A) Sample eight from the Sarki Formation of the Warte section; (B) Sample 13 from the Sargelu Formation of the Warte section; (C) Sample six from the Chia Gara Formation of the Ranya section. C, Calcite; Qz, Quartz: I/M, Illite/mica; F, Feldspar; Do, Dolomite; K/C, Kaolinite/chlorite.
In addition, SEM confirmed the presence of illite, illite/mica- and illite/smectite-mixed layers, and kaolinite. Calcite was found to be present in several forms, including star-shaped Mg-calcite microcrystals, euhedral (hexagonal) crystals, and columnar calcite crystals (Figure 6). The dominance of carbonate minerals (calcite and dolomite) was determined in various age-successions from other parts of Iraq especially in carbonate-rich clastic rocks (Abdullah et al., 2025; Mahmood et al., 2025).
Figure 6. Representative SEM images showing the main components in the studied shales: (A) Shale from Chia Gara Formation; (B) Shale from Sarki Formation. C, Calcite; Qz, Quartz; I, Illite/mica; I/S, Illite/smectite mixed layers; K, Kaolinite; F, Feldspar; Py, Pyrite; Mg, calcite (arrow).
Quartz grains occur mainly in a subhedral form with overgrowths. Feldspar was detected mainly as euhedral crystals while pyrite framboids were also recognized. The framboidal forms of pyrite were previously reported to reflect anoxic-euxinic conditions in ancient marine settings (Liao et al., 2010; Tian et al., 2014). A variety of clay minerals were noted, including, kaolinite as degraded hexagonal plates (Figure 6), illite/mica either as platelets or as fibers and laths, and illite/smectite mixed layers present as matted crenulated flakes (Figure 6).
4.2 Geochemistry
Tables 1, 2 list the results of the principal oxides, trace, and rare earth elements of the shale samples from the six formations (Sarki, Sehkanyian, Sargelu, Naokelekan, Barsarin, and Chia Gara) of the Ranya and Warte areas. The shales of the Early Jurassic-age Sarki and Sehkaniyan formations contain greater amounts of SiO2 and Al2O3 (wt.%) in comparison with the Middle and Late Jurassic-age formations (Table 1). The average K2O/Al2O3 ratios for the Early Jurassic-age formations are also higher than those of other Jurassic formations. The initial composition of ancient sediments can be predicted based on this ratio value (Crook, 1974; Islam et al., 2015; Obasi et al., 2020). The ratios of feldspars and clay minerals are 0.3–0.9 and 0.0–0.3, respectively (Cox and Lowe, 1995). According to the XRD results, illite/mica and kaolinite/chlorite are the main clay minerals detected in the current study (Figure 5). The Al2O3/TiO2 ratios of the studied shale samples are as follows: 14.8 (Lower Jurassic, Sarki and Sehkaniyan), 19.8 (Middle Jurassic Sargelu), and 8.6 (Late Jurassic–age Naokelekan and Barsarin formations). The Al2O3/TiO2 ratios of the Middle Jurassic shales are very similar to that of the Post Archean Australian Shale (PAAS) (19.0) (Taylor and McLennan, 1985), whereas the lowest values were found in the Late Jurassic shales. Titanium, which is mostly found in phyllosilicates (Condie et al., 1992), may actually reveal the composition of the source rocks (McLennan et al., 1993), and is immobile in comparison to other elements throughout a number of deposition-erosion cycles. The concentrations of some of the trace elements and ratios used in the current study, along with rare earth elements are listed in Table 2. The LREE/HREE ratios show slight variations among the shale samples. The ratio is in general high in the Lower Jurassic shales (7.6) when compared to the Middle and Upper Jurassic shales (6.1 and 6.0), while it increases to 9.3 in the Upper Jurassic to Lower Cretaceous shales of the Chia Gara Formation. The Eu/Eu* ratios vary widely among the shale samples. The Average Eu/Eu* ratio in the Lower Jurassic shales is 0.91, increasing to 2.25, and 2.14, respectively, in the shales of the Middle Jurassic and the Upper Jurassic to Lower Cretaceous periods.
Table 1. Major oxide concentrations (wt.%) of the studied samples from various Jurassic-Lower Cretaceous formations.
Table 2. Rare earth and trace elements (ppm) and elemental ratios of shales.
5 Discussion
5.1 Proxies for source rock determination
Bivariate and ternary diagrams, which combine various elements and elemental ratios (e.g., in the form of major metal oxides) as well as individual elements (i.e., trace and rare earth elements) have been used to examine the provenance of shales (Dai et al., 2016; Akkoca et al., 2019; Armstrong-Altrin et al., 2020). Some of these diagrams, for example, those published by Floyd and Leveridge (1987) and Roser and Korsch (1988), are considered to be most useful, and are still utilized by researchers to discriminate the provenance signatures of sandstone and shale suites (e.g., K2O vs. Na2O, La/Th vs. Hf, and K vs. Rb). According to Floyd and Leveridge (1987), the La/Th vs. Hf plot can be a helpful tool for distinguishing arc compositions and sources, as well as important information about the composition of the source area. Arc settings dominated by felsic composition have uniformly low La/Th ratios (<5) and contain 3–7 ppm of Hf (Floyd and Leveridge, 1987). Zircon is released as the arc gradually unroofs sedimentary basement rocks which increase the Hf content (Wang et al., 2010). The La/Th vs. Hf plot (Figure 7) shows that the Lower Jurassic formations (Sarki, Sehkanyian) varying from felsic igneous to sedimentary sources, whilst the Middle to Upper Jurassic formations (Sargelu, Naokelekan, Barsarin and Chia Gara) reflect mainly felsic to intermediate igneous sources.
Figure 7. Hf versus La/Th diagram for the shales of the Lower Jurassic-Lower Cretaceous formations (after Floyd and Leveridge, 1987).
Th/U is useful to interpret the sedimentary recycling histories and derivation from older sedimentary rocks (McLennan et al., 1990; Asiedu et al., 2000). The Th/U ratio in upper crustal rocks vary from 3.5 to 4.0 (McLennan et al., 1993). An increase in the Th/U ratio may occur during deposition as a result of oxidation of U to soluble U6+. This oxidation renders uranium mobile in an oxidizing environment, allowing it to be leached from sediments or redeposited elsewhere as secondary uranium minerals, including uranium oxides, phosphates, or silicate minerals (McLennan et al., 1990; McLennan et al., 1993; Cumberlanda et al., 2016). Therefore, the ratios of Th/U may be important in interpreting sedimentary recycling histories (McLennan et al., 1990). Moreover, the ratios of Th/U in sedimentary rocks, above 4.0, may indicate intense weathering in the source area, sediment recycling, and its derivation from older sedimentary rocks.
In general, the Th/U ratios in the investigated samples are <4.0 (Table 2), with higher values in the Lower Jurassic shale samples, which refer to variation in weathering at the source area from moderate to weak. In addition, the data obtained from the TiO2 vs. Zr plot (Figure 8) corresponds to the results obtained from the La/Th vs. Hf plot (Figure 7).
Figure 8. TiO2 vs. Zr diagram of the Lower Jurassic-Lower Cretaceous shales (Hayashi et al., 1997).
These suggested source rocks for the studied formations are also included in the Co/Th vs. La/Sc (Figure 9A) and Th/Sc vs. Zr/Sc plots (Figure 9B), where most of the Lower Jurassic shales lie near the felsic igneous or granite field, the Middle to Upper Jurassic shales align to intermediate or andesite sources.
Figure 9. (A) Co/Th vs. La/Sc diagram for the shales of the Lower Jurassic-Lower Cretaceous formations (after Khudoley et al., 2001; Gu et al., 2002). (B) Th/Sc vs. Zr/Sc diagram for the shales of the Lower Jurassic-Lower Cretaceous formations (after McLennan et al., 1993).
This reveals the felsic to intermediate igneous signature of the Lower Jurassic formations (Sarki, Sehkanyian), and the contrasting intermediate to felsic signature of the Middle to Upper Jurassic formations (Sargelu, Naokelekan, Barsarin, Chia Gara), with no evidence of a mafic signature. The Al2O3/TiO2 ratio of the Ranya and Warte shales has been used to assume the composition of the parental materials (Hayashi et al., 1997; Ramos-Vázquez et al., 2018). The Al2O3/TiO2 ratio values for the Ranya and Warte shales range from 12.7 to 29 with a mean value of 18.2, indicating intermediate to felsic source rock composition for both formations. Based on the type of the igneous rocks using TiO2 vs. Al2O3 diagram (Figure 10), the Lower Jurassic to Upper Jurassic formations (Sarki, Sehkanyian, Sargelu, Naokelekan, Barsarin and Chia Gara) reflect more a granitic signature. Derivation from felsic and intermediate igneous sources may indicate a significant contribution of detritus from continental settings or rifted continental margin (Vdačný et al., 2013).
Figure 10. Scatter plot of Al2O3 vs. TiO2 Lower Jurassic-Lower Cretaceous shales (after Amajor, 1987).
Figure 11 displays the chondrite-normalized REE patterns for the Lower Jurassic-Lower Cretaceous shales (Taylor and McLennan, 1985). In general, the shales have lower REE levels than the North American Shale Composite (NASC) and Post Archean Australian Shale (PAAS) (Table 2 and Figure 11). The studied shales are more enriched in LREE than HREE and show almost consistent patterns similar to PAAS and NASC (Figure 11). The Chia Gara shales are more enriched in the LREE and the Naokelekan shales are more enriched in HREE as compared to the other shales.
Figure 11. Average chondrite-normalized rare earth element (REE) patterns for the shales from the Lower Jurassic-Lower Cretaceous formations. The REE patterns of this study are compared with the average PAAS and NASC (Taylor and McLennan, 1985).
Moreover, the ratios of LREE/HREE have been utilized to infer source of sedimentary rocks (Condie et al., 1992). Generally, the felsic rocks contain higher LREE/HREE ratios, whereas the intermediate and mafic rocks contain lower LREE/HREE ratios. The ratio of LREE/HREE (average of 7.6) in the shales from the Lower Jurassic Sarki and Sehkaniyan formations is higher than the average ratio in both Sargelu (Middle Jurassic) and Naokelekan and Barsarin formations (Upper Jurassic) which have an average of 6.0. This may reflect the higher contribution of felsic than intermediate igneous rocks to the Lower Jurassic sediments. Furthermore, the Eu/Eu* ratio values of clastic rocks have been used by many authors to identify the source area (Tawfik et al., 2018; Tobia et al., 2019; Cai et al., 2022). The Eu/Eu* ratio of the Ranya and Warte shales have an average of 1.89 (Table 2). This would point to their derivation from a source area dominated by felsic rocks (Tobia et al., 2019; Cai et al., 2022).
However, significant inconsistency is noticed from the samples within the same formation. For example, the Eu/Eu* ratio of the Sargelu Formation from the Warte shales ranges from 0.65 to 3.64, and varies between 0.78 and 3.85 from the Chia Gara Formation from the Ranya shales. These differences could be clarified by the heterogeneous source rocks, where sediments from a single sedimentary section, might derive from various lithologies with distinctive REE signatures. Slight influence from intermediate or mafic rocks can modify the whole REE pattern (Armstrong-Altrin et al., 2020). The observed variations can also be explained by the sediment sorting and transport. Physical transport and depositional processes could fractionate the REE-bearing minerals like zircon, monazite, and apatite, creating intra-formation inconsistency (Garzanti et al., 2021). Post-depositional alteration should also be considered, for example, weathering or diagenesis can restructure the REEs, mainly the Eu, which is sensitive to the redox conditions, where Eu anomalies could reveal plagioclase fractionation in the source or selective removal of Eu2+ under reducing conditions (Tobia et al., 2019; Cai et al., 2022).
Additionally, a recent study on the paleoenvironment conditions which were prevalent during the deposition of the Sargelu, Naokelekan, and Najmah formations (Zey Gawara area, Kurdistan Region of northern Iraq) suggested that the source area of the Middle to Upper Jurassic formations (i.e., Sargelu and Naokelekan) was dominated by felsic igneous rocks (Mina and Abdula, 2023). The presence of detrital quartz and feldspars in addition to illite and kaolinite identified by XRD and SEM analyses may also suggest the contribution from felsic (acidic) igneous rocks. Quartz, feldspar and mica are the common minerals in the felsic rocks. Additionally, quartz is less soluble in surface environments than other detrital minerals and can be abundant even in sediments mainly sourced by mafic igneous rocks (Garzanti et al., 2021). Kaolinite typically develops under acidic conditions as a result of complex weathering processes or hydrothermal alteration of feldspar, and other aluminosilicate minerals. In contrast, illite is among the most abundant clay minerals in fine-grained mudstones, forming through silicate weathering—primarily of feldspar, due to the dissolution of muscovite (Deer et al., 1992). Kaolinite might also form by intense supergenic weathering in hot and wet conditions.
5.2 Tectonic setting discrimination
The sedimentation, diagenesis and composition of sediments are largely influenced by tectonic setting in which those sediments are formed (Pettijohn et al., 1972; McLennan et al., 1983; Zimmermann and Spalletti, 2009; Armstrong-Altrin et al., 2012; Armstrong-Altrin et al., 2015; Basu et al., 2016; Cusack et al., 2020; Jafarzadeh et al., 2022; Al-Juboury et al., 2024). Different tectonic settings produce clastic sedimentary rocks with distinct chemistry and detrital component characteristics (Dickinson and Suczek, 1979; Dickinson et al., 1983; McLennan et al., 1983; Roser and Korsch, 1986; Floyd and Leveridge, 1987; Al Harbi and Khan, 2008; Basu et al., 2016; Ren et al., 2023). Some information on the tectonic setting and the source of sediments can be attained from the elemental composition, including Si, Al, Ti, Fe, Mn, K, Mg, Ca, Na and P, as well as through bivariate plots (Roser and Korsch, 1986; Rollinson, 1993; Zaid and Al Ghatani, 2015; Ghasemlooytakantapeh et al., 2023). Numerous researches have effectively employed the two discriminant diagrams based on key elements provided by Verma and Armstrong-Altrin (2013) for the tectonic discrimination of siliciclastic deposits (Armstrong-Altrin et al., 2015; Tawfik et al., 2015). These can be classified as either low-silica (35%–63%) or high-silica (63%–95%) based on the variations in SiO2 values. The diagrams outlined in Figure 12 shows three different tectonic settings, including island arc, rift and collision. In the current study, the SiO2 contents of the shale samples are low (generally <50%; Table 1), so the low-silica type diagram is selected to infer the tectonic setting. On this plot, almost all the studied shale samples are plotted in the collision setting, except for a few samples from the Middle to Upper Jurassic fall within the rift field (Figure 12). While most of the shale samples plot within the collision field, the passive margin origin does not contradict the obtained result from Figure 12. Tectonic discrimination diagrams for shales often misclassify mature passive margin sediments (Bhatia and Crook, 1986; McLennan et al., 1993). Additionally, the attained mineralogical and geochemical data suggested that felsic and intermediate igneous rocks constituted the predominant source rocks reflecting the composition of ancient continental crust being weathered under a quiescent, passive margin setting.
Figure 12. Discriminant function (DF) multidimensional diagram for low-silica clastic sediments (fields after Verma and Armstrong-Altrin (2013)). The subscript m2 in DF1 and DF2 represents the low-silica diagram based on log-ratios of major elements. Discriminant function equations are DF1 (Arc-Rift-Col) m2 = (0.608 × In [TiO2/SiO2] adj) + (−1.854 × In [Al2O3/SiO2] adj) + (0.299 × In [Fe2O3 t/SiO2] adj) + (−0.550 × In [MnO/SiO2] adj) + (0.120 × In [MgO/SiO2] adj) + (0.194 × In [CaO/SiO2] adj) + (−1.510 × In [Na2O/SiO2] adj) + (1.941 × In [K2O/SiO2] adj) + (0.003 × In [P2O5/SiO2] adj) − 0.294. DF2 (Arc-Rift- Col) m2 = (−0.554 × In [TiO2/SiO2] adj) + (−0.995 × In [Al2O3/SiO2] adj) + (1.765 × In [Fe2O3t/SiO2] adj) + (−1.391 × In [MnO/SiO2] adj) + (−1.034 × In [MgO/SiO2] adj) + (0.225 × In [CaO/SiO2] adj) + (0.713 × In [Na2O/SiO2] adj) + (0.330 × In [K2O/SiO2] adj) + (0.637 × In [P2O5/SiO2] adj) − 3.631.
A discrimination diagram, which was proposed by Roser and Korsch (1986), has also been used to identify the tectonic setting of the study area. The results suggest that all samples were derived from a passive margin setting with two samples plot in the active continental margin field (Figure 13). Based on these results, it would appear that the shales were derived from a passive margin setting.
Figure 13. Tectonic discrimination diagram of Roser and Korsch (1986) for the shales from the Lower Jurassic-Lower Cretaceous formations. ARC, oceanic island arc margin, ACM, active continental margin; PM, passive margin.
Although that felsic and intermediate rocks might also occur in arc and intraplate volcanic settings, their presence in a sedimentary succession does not reflect solely an active margin. The provenance of the Jurassic shales of northeastern Iraq has not been only controlled by the rock type but also supported by geochemical and mineralogical signatures, including trace-element ratios (e.g., Th/Sc, Zr/Sc, La/Sc) and rare earth element (REE) patterns. These datasets from the shales show enriched LREE profiles, flat to slightly fractionated HREE, and low Eu anomalies, indicative of felsic–intermediate continental crust sources, for example, the Sanandaj–Sirjan Zone (Shakerardakani et al., 2015; Barber et al., 2019; Moghadam et al., 2019). Additionally, the shales include a mixture of old detrital zircons (Proterozoic–Paleozoic) with only rare Triassic–Jurassic grains, which correlates well with an erosion of long-lived continental crust rather than a volcanic arc (Koshnaw et al., 2019; Jones et al., 2020; Omer et al., 2021). Consequently, the felsic–intermediate geochemical signatures reflect the composition of ancient continental crust being weathered under a quiescent, passive margin setting, rather than direct evidence for an active magmatic arc.
Three ternary plots of the trace and rare earth elements (Th-Sc-La), (Sc-Zr-Th), and (Co-Zr-Th), introduced by Bhatia and Crook (1986), have been selected for the shale samples of the Ranya and Warte sections. The results indicate that most of the studied samples were deposited in a passive margin and continental island arc (Figure 14). The proposed passive margin, as noted above, being the tectonic province of the Ranya and Warte shale successions, correlates well with the geology of northern Iraq. The sedimentary successions of northern Iraq, including the Ranya and Warte successions, were deposited within megasequences AP6-AP8, a period when the region was a passive margin, interrupted by periods of extensional tectonics (Numan, 1997; Sharland et al., 2001; Jassim and Goff, 2006; Zainy et al., 2017; Othman and Omar, 2023). Sharland and coworkers reported that the interval spanning the Early Jurassic to the Early Cretaceous was characterized by tectonic instability (Sharland et al., 2001). At this time the tectonic regime of the Arabian Plate which dominated by extensional activity changed to where compression was dominant (Numan, 1997; Numan, 2000; Zainy et al., 2017). This change was related to the collision and subduction of the Neo-Tethys oceanic crust beneath the Iranian Plate. This change, from extension through to subduction-related compression occurred during the Late Jurassic and Early Cretaceous. Accordingly, the Early Jurassic-age Sarki, Sehkanyian formations and the Middle Jurassic-age Sargelu Formation were deposited in an extensional environment, whereas the Late Jurassic-age Naokelekan, Barsarin formations and the Late Jurassic-Early Cretaceous-age Chia Gara Formation were formed during a compressional period.
Figure 14. Discrimination diagrams of Bhatia and Crook (1986) using: (A) Th-Sc-La; (B) Th-Sc-Zr/10; (C) Th-Co-Zr/10 elements, illustrate the tectonic setting fields, A. Oceanic Island Arc; B. Continental Island Arc; C. Active Continental Margin; D. Passive Margin.
While specific source is difficult to identify, it has been noted that large volumes of igneous rocks were emplaced beneath Eurasia during Late Paleozoic subduction with this being followed by post-collisional low-pressure and high-temperature magmatism and metamorphism (Stampfli and Borel, 2002; von Raumer et al., 2009). This Late Paleozoic magmatism is characterized by the presence of A-type granites in the Pontides-Caucasus region as well as NW Iran (Okay et al., 2001; Topuz et al., 2010; Rolland et al., 2011). As a result of subsequent subduction and collision, related to the evolution of NeoTethys, the Late Paleozoic-age rocks are rarely exposed, although some plutons have been described from NW Iran (Advay and Ghalamghash, 2011; Saccani et al., 2013). Omer et al. (2021) in their study of the Ordovician-age Khabour Formation (northern Iraq), noted the presence of Proterozoic-age detrital zircons and suggested that they were derived from rocks associated with major magmatic events with the assembly of northern Gondwana, and involving unroofing of late-stage granitic rocks. The zircon signatures from Iraqi successions as documented by Jones et al. (2020) and those noted by Koshnaw et al. (2017), Koshnaw et al. (2019) do not exhibit the distinctive Mesozoic and Late Paleozoic Eurasian signatures when compared to those documented by Zhang et al. (2016) from the Zagros Orogen of Iran. Thus, the rare Triassic-Jurassic-age zircon grains within the Tanjero Formation and the lower Red Beds derived possibly, from igneous sequences emplaced during rifting in the Neo Tethys Ocean, e.g., granites and gabbros in the Sanandaj–Sirjan Zone of Iran (Shakerardakani et al., 2015; Barber et al., 2019; Moghadam et al., 2019), which is a part of the Zagros orogenic belt that formed as a result of the collision between the Arabian and the Eurasian Plates (Alavi, 1994; Mohajjel et al., 2003).
Recent studies further support this interpretation. Moghadam et al. (2024) indicated that the Sanandaj–Sirjan Zone magmatism comprised of I-, S- and A-type granitoids with zircon U–Pb ages from ∼180 Ma to the Eocene, indicative of extensive crustal reworking. Detrital-zircon populations from the central Sanandaj–Sirjan Zone display prominent Neoproterozoic (∼0.6 Ga), 0.9–1.1 Ga and Paleoproterozoic age groups, attesting to a well-developed continental basement (Shakerardakani et al., 2021). Jurassic plutonism in the Ghorveh–Dehgolan batholith (∼160–140 Ma) records a transition from arc-related I-type to extensional A-type granites, signalling a shift away from a simple convergent-margin configuration (Yajam et al., 2015). Furthermore, detrital-zircon data from the NW Zagros foreland record a mixture of ancient Neoproterozoic–Paleozoic and younger Mesozoic–Cenozoic sources compatible with unroofing of the Sanandaj–Sirjan Zone and adjacent suture units (Koshnaw et al., 2021). Collectively, these lines of evidence indicate that the felsic–intermediate signatures in northeastern Iraq reflect erosion of Sanandaj–Sirjan Zone igneous rocks built upon ancient continental crust rather than direct input from an active arc, and that a passive-margin depositional setting is consistent with the provenance inferred in this study.
6 Conclusion
Mineralogical and geochemical data from the Lower Jurassic-Lower Cretaceous shale samples of the Ranya and Warte areas in northeastern Iraq revealed information about their provenance history and tectonic setting. The shales of the Ranya and Warte areas are dominated by calcite, dolomite and quartz with traces of feldspar, illite/mica, kaolinite and chlorite. The occurrence of detrital quartz and feldspar with the clay minerals illite and kaolinite indicates their felsic (acidic) igneous origin. Moreover, the major, trace, and rare earth element ratios, showed that the Jurassic-Lower Cretaceous shales were mainly derived from felsic to intermediate igneous sources, whilst the REE patterns and Eu/Eu* anomalies suggested a felsic-dominated provenance. The felsic–intermediate signatures are indicative of erosion of the Sanandaj–Sirjan Zone igneous rocks of Iran.
In addition, the tectonic discrimination diagrams proposed a passive margin setting, being the tectonic province of the Ranya and Warte shale successions, northeastern Iraq.
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
NO: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Validation, Writing – original draft. TM: Conceptualization, Data curation, Methodology, Supervision, Writing – review and editing. AA-J: Conceptualization, Data curation, Formal Analysis, Methodology, Supervision, Writing – review and editing. MA-H: Formal Analysis, Software, Writing – original draft. AS: Investigation, Software, Writing – original draft. JA-A: Formal Analysis, Writing – review and editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This research was funded by the Arab German Young Academy of Sciences and Humanities (AGYA) that is funded under the German Federal Ministry of Education and Research (BMBF) grant (01DL20003).
Acknowledgements
The authors would like to thank Mr. Ahmed H. Al-Obeidi, North Oil Company, Kirkuk, Iraq, for preparing some of the figures.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Keywords: Iraq, Jurassic, provenance, shale, tectonic setting
Citation: Omar N, McCann T, Al-Juboury AI, Al-Haj MA, Sharazwri AO and Armstrong-Altrin JS (2026) Provenance and tectonic setting of the Lower Jurassic-Lower Cretaceous shales from the northeastern Iraq: mineralogical and geochemical approaches. Front. Earth Sci. 13:1716704. doi: 10.3389/feart.2025.1716704
Received: 30 September 2025; Accepted: 16 December 2025;
Published: 23 January 2026.
Edited by:
Ramanathan Alagappan, Independent Researcher, Tiruchirapalli, IndiaReviewed by:
Rosa Sinisi, National Research Council (CNR), ItalyMonalisa Mallick, National Geophysical Research Institute (CSIR), India
Copyright © 2026 Omar, McCann, Al-Juboury, Al-Haj, Sharazwri and Armstrong-Altrin. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Nagham Omar, czZuYW9tYXJAdW5pLWJvbm4uZGU=