Geochemistry of an intercalated unit of arkose and shale of the Dhandraul Formation belonging to the Vindhyan Supergroup, Eastern India: Insights from provenance, depositional environment, and geodynamic set-up

doi:10.21203/rs.3.rs-2587457/v1

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Geochemistry of an intercalated unit of arkose and shale of the Dhandraul Formation belonging to the Vindhyan Supergroup, Eastern India: Insights from provenance, depositional environment, and geodynamic set-up

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In this study, we have presented a hitherto unreported mappable member of an intercalated unit of arkose and shale of the Dhandraul Formation of the Kaimur Group exposed in Kaimur district, Bihar, India which was not reported by earlier workers in the Vindhyan basin. It is a contribution for understanding the possible provenance, source area weathering, depositional environment, and geodynamic set-up of this member. Based on field characteristics and petrography study, three lithofacies units have been identified viz. (i) coarse to medium-grained arkose, (ii) intercalated sequence of arkose and shale, and (iii) shale. Geochemically, these lithofacies predominantly occupied the field of arkose and shale except samples fall in the field of sub-arkose. These lithofacies display relatively an enrichment of SiO₂ and Al₂O₃ and show low concentrations of MgO, CaO, Na₂O, K₂O, and TiO₂. The values of ∑REE in arkose are varying from 78.74 to 128.81 ppm whereas the values of fractionation indicate (La/Sm)_N (3.73-4.22), (La/Yb)_N (7.33-15.59), (Gd/Yb)_N (1.32-2.30), and Eu/Eu* (0.58-0.66). In shale, ∑REE ranges from 354.02 to 382.11 ppm while the fractionation contents of (La/Sm)_N, (La/Yb)_N, and (Gd/ Yb)_N, and Eu/Eu* are ranging from 3.82 to 4.82, 7.65 to 11.85, 1.38 to 1.73, and 0.56 to 0.81. On the basis of rock fragments and paleocurrent direction, the possible sources are presumed to be the Chhotanagpur Gneissic Complex and the Mahakoshal Group of rocks, which lie towards the south and southwest. In the binary and ternary plots, most of the samples of arkose and shale have mostly occupied the field of passive margin tectonic setting except one sample comes in the field of the continental island arc. Based on mineralogical and textural maturity, sedimentary structures and the overall sequence of these lithofacies from sandstone to shale member of the Dhandraul Formation show fining upward sequence which attributes the deposition in a shallow coastal fluvial-marine environment in a transgressive phase. This discovery has opened a new opportunity to relook at the depositional environment of the Dhandraul Formation in other parts of the Vindhyan basin.

Geochemistry

Dhandraul Formation

Kaimur Group

Vindhyan Supergroup

India

The Vindhyan Basin of Central India is one of the largest and best preserved Proterozoic sedimentary basins throughout the world, which occupies a vast area of more than ~104,000 km² in Central India and attains a huge thickness of more than 5 km (Malone et al. 2008; Quasim et al. 2017; Mandal et al. 2019; Yadav et al. 2022b, c). This basin comprises of approximately 4000m thick sequence of largely undeformed rocks comprising shales, sandstones, limestones, and dolostones with subordinate felsic volcanics, lying on top of the deformed metasediments of both Bijawar/Mahakoshal Group and the Archaean gneissic basement (Bose et al. 2001; Ray and Chakraborty 2006). It covers a long period of sedimentation history spanning (~1800-500 Ma) during the Proterozoic eon (Quasim et al. 2017; Yadav et al. 2022c). The rocks of the Vindhyan Supergroup are divided into three outcrop sectors, namely, the Bundelkhand sector, the Son valley sector, and the Rajasthan sector (Chakraborty et al. 2010; Mandal et al. 2019).

In the Sone Valley, Central India, the Kaimur Group has been divided into lower Kaimur Group (LKG) and upper Kaimur Group (UKG). The Sasaram sandstone, the Ghurma shale, and the Markundi sandstone belong to the LKG whereas the UKG is represented by the Bijaigarh shale, the Scarp sandstone, and the Dhandraul sandstone (Auden 1933; Prakash and Dalela 1982; Quasim et al. 2017). This Group is a 400m thick, sandstone dominated units which overlies the Semri carbonates. The shallow coastal fluvio-aeolian environments to the shoreface environment are responsible for the formation of the Kaimur Group (Chakraborty and Bose 1992; Bhattacharya and Morad 1993; Bose et al. 2001; Sen 2010). The age of the Kaimur Group is about 1.1 Ga (Malone et al. 2008; Sen et al. 2014).

Major contributions in relation to the regional geology, stratigraphy and/or sedimentology, weathering, provenance, climatic conditions, depositional environments, and tectonic settings of the UKG sandstones exposed in Sone Valley have been attempted by several workers (Dickinson and Suczek 1979; Srivastava and Mehrotra 1981; Dickinson 1985; Bhatia and Crook 1986; Roser and Korsch 1988; Akhtar et al. 1994; Gupta et al. 2003; Chakraborty 2006; Mishra and Sen 2010 2012; Sen et al. 2014; Quasim et al. 2017; Yadav et al. 2022c). Besides, very few works have been made to establish the geology, stratigraphy, mineralization, depositional environments, and tectonic settings of the UKG sandstones exposed in the state of Bihar, Eastern India (Prakash et al. 1979; Prakash and Kumar 1980; Verma et al. 1984; Singh and Sinha 1998; Yadav and Das 2019, 2021c).

Based on the above-mentioned information, an intercalated unit of arkose and shale of the Dhandraul Formation of the Kaimur Group has not been yet reported by the earlier workers in the Vindhyan Basin (Table 1). The present paper provides the first occurrence of a mappable member of an intercalated unit of arkose and shale of the Dhandraul Formation of the Vindhyan Supergroup in the eastern part of the Vindhyan Basin exposed in Kaimur district, Bihar. The main objectives of this study are (i) to report the results of geochemical analysis of arkose and shale from the Dhandraul Formation, (ii) to understand the possible provenance, depositional environment, and geodynamic set-up of this member, (iii) infer the weathering conditions at the source area based on the petrography study and whole rock geochemistry, and (iv) integration of all data set to depict the paleo-environmental conditions of the eastern part of the Vindhyan Basin.

The Vindhyan Basin is the repository of the thickest Proterozoic succession in India which developed during the Palaeo to the Neoproterozoic time. Rocks of this basin are mainly represented by shales, sandstones, limestones, and dolostones with subordinate felsic volcanics. The basin is in sickle shape and extends from Sasaram, Bihar in the east through Uttar Pradesh, Madhya Pradesh, to Chittorgarh, Rajasthan in the west, which is extended below the Deccan Trap of the Malwa plateau in the south and the alluvium of the Indo-Gangetic Plain in the north (Bose et al. 2001; Chakraborty et al. 2010; Sen 2010; Mishra and Sen 2010; Sen et al. 2014; Quasim et al. 2017; Mandal et al. 2019; Yadav and Das 2021c; Yadav et al. 2022c). The Vindhyan Supergroup is broadly separated from base to top into four groups; Semri, Kaimur, Rewa, and Bhander (Paikaray et al. 2008; Quasim et al. 2017; Sallstedt et al. 2018). The Semri Group, also called the lower Vindhyan Group, is gently deformed and mildly metamorphosed and consists of carbonate-rich sediments. They are overlain by siliciclastics of the later three Groups, i.e., the upper Vindhyans Group (Sen et al. 2014; Quasim et al. 2017).

In the Sone Valley, the Vindhyan Supergroup overlies the slightly metamorphosed Bijawar Group of rocks which provided the age 2780 ± 365 Ma by Rb-Sr method Crawford and Compston (1970). Sarangi et al. (2004) proposed a Pb-Pb age of 1721 ± 90 Ma for Kajrahat limestone and the Deonar Formation, chiefly constituted of Porcellanites has yielded late Palaeoproterozoic ages i.e. 1628 ± 8 Ma by zircon U-Pb method (Rasmussen et al. 2002) and 1631 ± 1 Ma and 1631 ± 5 Ma by U-Pb, zircon and ⁸⁷Sr/⁸⁶Sr isotope (Ray et al. 2002). Ray et al. (2003) dated the Rohtasgarh Limestone belonging to the Rohtas Formation of the Semri Group of Sone valley and reported the age to be 1601 ± 130 Ma. K-Ar dating yielded 940 Ma age of Scarp sandstone which was determined by Vinogradov et al. (1964) and the age of Majhgawan kimberlite has been dated at 1140 ± 12 Ma (Crawford and Compston 1970).

In Bihar, rocks of the Vindhyan Supergroup are spread over an area of 3350 km² and occupy the eastern tip of the Vindhyan Basin (Fig. 1a & b; Table 1). Four formations belonging to the Semri Group are observed in the Rohtas and Kaimur districts of Bihar viz. Kheinjua Shale Formation, Fawn Limestone Formation, Glauconitic Sandstone Formation, and Rohtas Limestone Formation (Sarkar 1981; Singh and Thakur 1999). The Kaimur Group comprises Ghaghar, Bijaigarh, Scarp, and Dhandraul Formations with cappings of laterites (Verma et al. 1984; Yadav and Das 2019; Maurya et al. 2020; Yadav and Das 2021c; Yadav et al. 2022c). A low-energy environment with carbonate precipitation is mainly responsible for the formation of the Semri Group of rocks while the Kaimur Group is deposited in a high-energy clastic sedimentation regime. The general strike of the lithounits of the Vindhyan basin in the Bihar area varies from NE-SW to ENE-WSW with a varying dip of 5° to 15° towards the northwest. Vertical joints trending NE-SW and NW-SE, small-scale folds, fault, and slickensides are also recorded in the eastern part of the Vindhyan basin. The average palaeocurrent direction as recorded from the current bedding and ripple marks points towards NW (Yadav et al. 2022c).

The geology of the mapped area is mainly composed of coarse to medium-grained sandstone of the Ghaghar Formation, carbonaceous shale intercalated with fine sandstone belonging to the Bijaigarh Formation, sandstone with thinly bedded siltstone of the Scarp Formation, medium to coarse-grained sandstone (quartz-arenite) and intercalated unit of arkose and shale of the Dhandraul Formation belonging to the Kaimur Group and laterite cappings. Besides, the Quaternaries are represented by the Mohanpur Formation composed of silty clay with kankar, (Fig. 2; Yadav and Das 2019). The studied area falls in Toposheet no. 63P/9 which comes under the easternmost part of the Vindhyan basin in the Kaimur district of Bihar (Fig. 2).

Sandstone of the Dhandraul Formation belongs to the Kaimur Group is the youngest and dominant Formation of the Vindhyan Supergroup in Bihar which has been mapped in the study area and is mostly occupied by high hills and plateau tops. The important plateau areas where the Dhandraul sandstone is exposed are Bahadag, Barap, southwest of Dohar, north of Jawarh, Jamuninar, Karar, and Tala (Fig. 2). This sandstone shows gradational contact with the underlying Scarp sandstone. It is milky white and sometimes it looks dirty white, medium to coarse-grained but at places gritty in size, massive to highly jointed rock, highly mature, well sorted, and composed of rounded to sub-rounded grains of quartz as essential mineral along with feldspar, muscovite, tourmaline and opaque as accessories (Fig. 3a). Three sets of prominent faults with attitudes N30ºE/vertical, N70ºE/vertical and N70ºW/vertical and filled with ferruginous material mainly rich in hematite are observed in this unit (Fig. 2). Palaeocurrent directions are preferably collected from tabular cross-bedding and trough cross-bedding which range from 150º to 320º (Fig. 2).

Intercalated unit of arkose and shale is mapped for the first time towards the southeast of Jamuninar village which shows the gradational contact with sandstone of the Dhandraul Formation and is mainly categorized into three litho facies units on the basis of lithological variations viz. i) coarse to medium-grained arkose, ii) intercalated sequence of arkose and shale, and iii) shale (Fig. 3b to 3d). The first unit is coarse to medium-grained arkose, moderately sorted, poorly mature, and composed of sub-rounded to sub-angular grains of quartz along with altered feldspar, muscovite, and opaque (Fig. 3b). Quartz grains are mostly sub-rounded to sub-angular and constituting ~ 70% of the framework grains and having ~ 30% of altered feldspar. The framework grains are bounded by ferruginous cement. Above this unit, an intercalated sequence of arkose and shale of variable thickness (Fig. 3c) is observed. Shale is the youngest litho facies of this member which is exposed below the laterite and it is mainly buff colour, fine to very fine-grained, thinly laminated (Fig. 3d), and composed of micaceous and clayey minerals. The overall sequence of these litho-facies shows Fining Upward (FU) sequence which indicates the deposition in a shallow coastal fluvial-marine environment in a transgressive phase.

Three detailed lithologs of an intercalated unit of arkose and shale and sandstone of the Dhandraul Formation to the southeast of Jamuninar were systematically prepared to examine the lithofacies variation (Fig. 8). Detailed petrography studies of an intercalated unit of arkose and shale lithofacies were carried out with available thin sections. During the sample preparation process, the outer or weathered portions of all the samples were removed to minimize alteration problems. After that, about 500 grams of sample was collected and crushed with an iron mortar and then pulverized with an agate mortar and finally processed through -80 mesh and -200 mesh. Two sets of samples (original and duplicate) were prepared through coning and quartering. To the adoption of the above methodology, seven fresh samples from the arkose (4 nos.) and shale (3 nos.) were collected for the analysis of major oxides, trace elements, and REE by X-Ray Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometer (ICPMS) instruments at the Chemical Laboratory, Geological Survey of India (GSI), Eastern Region, Kolkata. Analytical precision and accuracy of data received from the above-mentioned instruments are analogous with international reference standards. Both XRF and ICPMS instruments have National Accreditation Board for Testing and Calibration Laboratories (NABL) accreditation ISO/IEC 17025:2017 (Certificate No.: TC-5956) and analytical work for the samples had been carried out in compliance with NABL guidelines. The chemical results of these lithofacies are furnished in Table 2.

Three lithofacies units have been identified based on field characteristics and petrography study viz. i) coarse to medium-grained arkose, ii) intercalated sequence of arkose and shale, and iii) shale (Fig. 4a to 4h). Coarse to medium-grained arkose is moderately sorted, poorly mature, and composed of sub-rounded to sub-angular grains of quartz and altered feldspar as essential minerals along with muscovite, zircon, and opaque as accessories (Fig. 4a to 4d). Quartz grains are mostly monocrystalline, sub-rounded to sub-angular, and constitute more than 70% of the framework grains which are bounded by ferruginous cement (Fig. 4a to 4d). Monocrystalline quartz is predominantly noticed among the framework grain constituents and the contacts between the framework grains are mainly observed point to point, concavo-convex, and suture (Fig. 4a to 4d). In a few samples, elliptical rock fragments (RF) of quartzite are also observed in between the framework grains of quartz (Fig. 4c) and this rock fragment might have been derived from the Mahakoshal Group of rocks. Apart from that, broken and fractured grains of quartz are seen within it which indicates the effects of brittle deformation (Fig. 4d). The second unit is medium-grained arkose which shows poorly matured in terms of mineralogical and textural characteristics, and consists of sub-angular grains of quartz, altered feldspar, plagioclase, muscovite, and sericite which are embedded in the arenaceous matrix (Fig. 4e & 4f). The last variety of this lithofacies is shale which is composed predominantly of fine to very fine-grained quartz, altered feldspar along with the micaceous and clayey minerals (Fig. 4g & 4h).

6.1 Geochemical characteristics of an intercalated unit of arkose and shale

The analytical results of major oxides (wt.%), trace elements (ppm), and REE (ppm) of an intercalated unit of arkose and shale of the Dhandraul Formation are given in Table 2. The arkose of intercalated sequence shows higher SiO₂ content (83.05–86.12 wt.%), Al₂O₃ (2.86–8.12 wt.%), Fe₂O₃ (4.51–10.73 wt.%), MgO (0.17–0.41 wt.%), CaO (0.06–0.12 wt.%), Na₂O (0.16–0.18 wt.%), K₂O (0.32–2.22 wt.%), and TiO₂ (0.06–0.30 wt.%). The values of ∑REE in arkose are varying from 78.74 to 128.81 ppm whereas the values of fractionation indicate (La/Sm)_N (3.73–4.22), (La/Yb)_N (7.33–15.59), (Gd/Yb)_N (1.32–2.30), and Eu/Eu* (0.58–0.66). In shale lithofacies of this member, the content of SiO₂ (71.16–78.24 wt.%) is comparatively low than arkose whereas the Al₂O₃ content varies from 10.89 to 13.44 wt.%. The other elements like Fe₂O₃ (2.67–5.25 wt.%), MgO (0.92–1.67 wt.%), CaO (0.10–0.21 wt.%), Na₂O (0.08–0.14 wt.%), K₂O (3.45–4.78 wt.%), and TiO₂ (0.49–0.72 wt.%) display low contents. The values of ∑REE in shale show higher in comparison with arkose which ranges from 354.02 to 382.11 ppm. The fractionation contents of (La/Sm)_N, (La/Yb)_N, and (Gd/ Yb)_N, and Eu/Eu* are ranging from 3.82 to 4.82, 7.65 to 11.85, 1.38 to 1.73, and 0.56 to 0.81 (Table 2). Based on the Eu anomaly, it can be stated that the felsic igneous rocks like granites are a possible source for the formation of this member in the study area.

The source rock composition, transport history, provenance, paleoweathering, and geodynamic set-up of ancient sedimentary rocks and modern sediments were inferred by using the geochemical data (Bhatia and Crook 1986; Roser and Korsch 1988; Akhtar et al. 1994; Nesbitt et al. 1996; Gupta et al. 2003; Chakraborty 2006; Mishra and Sen 2010; Sen et al. 2014; Quasim et al. 2017; Yadav et al. 2022c). An intercalated unit of arkose and shale of the Dhandraul Formation displays quite an enrichment of SiO₂ and Al₂O₃ and shows low concentrations of MgO, CaO, Na₂O, K₂O, and TiO₂ (Table 2). In these lithounits, quartz is the main source for the enrichment of silica whereas the Al₂O₃ and K₂O contents may be related to the presence of K-feldspars and clay minerals. The low contents of CaO and Na₂O may be attributed to the dominance of K-feldspar over plagioclase within these lithofacies while opaque minerals and rutile are the main sources of TiO₂. The enrichment of Fe₂O₃ may be indicated by the presence of iron oxide heavy minerals and ferruginous cement. The depletion of Na₂O (< 1%) in an intercalated unit of arkose and shale can be attributed to a comparatively smaller amount of Na-rich plagioclase which is also supported by the petrographic studies. The higher ratio of K₂O and Na₂O (Arkose = 1.88–12.33 wt.%; Shale = 34.14–44.38 wt.%) are increasing from arkose to shale lithofacies which indicates gradual dominance of K-feldspar over plagioclase. Since Al₂O₃ primarily resides in feldspars and TiO₂ in mafic minerals, the Al₂O₃/TiO₂ ratio (Arkose = 24.29–47.67 wt.%; Shale = 18.67–22.22 wt.%) indicates that the sediments of these rocks come from the felsic source.

The data of major oxides along with the average upper continental crustal value (UCC) are plotted against immobile Al₂O₃ to depict the mobility of elements during weathering and transport (Fig. 5a to h). The negative correlation between SiO₂ and Al₂O₃ is expected in sedimentary rock (Fig. 5a), which is mainly controlled by aluminous clay and quartz content respectively. Positive correlations between K₂O, MgO, TiO₂, and P₂O₅ versus Al₂O₃ are observed (Fig. 5c, e, g & h), which implies that the presence of K-feldspar, thin veins of ankerite, rutile, and apatite minerals. No correlation between CaO and Na₂O versus Al₂O₃ (Fig. 5b & d) is attributed that the depletion of CaO and Na₂O relative to K₂O indicates weathering of plagioclase to be dominant in the sandstones compared to K-feldspar. Besides, no correlation between MnO and Al₂O₃ is noticed within these lithofacies (Fig. 5f). In the binary plot of log (SiO₂/Al₂O₃) versus log (Na₂O/K₂O), all samples of an intercalated unit of arkose and shale fall mostly in the field of arkose except few samples occupy the field of sub-arkose (Fig. 5i; Pettijohn et al. 1972). Majority of the samples fall in the fields of arkose and shale (Fig. 5j), while plotted in the binary plot of log (SiO₂/Al₂O₃) vs. log (Fe₂O₃/K₂O).

6.2 Inference for provenance composition

The geochemical and sedimentological tools have been used to establish the provenance determination, especially for sedimentary rocks of the Kaimur Group in the Sone Valley, Central India (Bhatia and Crook 1986; Nesbitt et al. 1996; Gupta et al. 2003; Chakraborty 2006; Paikaray et al. 2008; Sen 2010; Mishra and Sen 2010, 2012; Sen et al. 2014; Quasim et al. 2017, 2019). Besides, the signature of REE, Eu/Eu*, and negative Eu values have also corroborated that the possible source rocks for the formation of the rocks of the Kaimur Group are granite (Mishra and Sen 2010, 2012). The REE patterns have also been used to infer sources of sedimentary rocks since basic rocks contain low LREE/HREE ratios whereas more silicic rocks usually contain higher LREE/HREE ratios and negative Eu anomalies (Wronkiewicz and Condie 1989; McLennan and Taylor 1991). In the Sone valley, lithological, sedimentological, structural, and critical evaluation of isopach, lithofacies, and stratum contour maps studies were carried out in detail that has suggested that the prevalence of north and north-westerly paleocurrent direction, throughout the Vindhyan sedimentation, implies a predominance of the southerly and south-easterly source area during the entire Vindhyan period (Bhattacharya and Morad 1993; Bose et al. 2001; Gupta et al. 2003; Chakraborty 2006; Mishra and Sen 2012). The Chhotanagpur Gneiss Complex (CGC) and the Mahakoshal Group are the possible source rocks for the origin of the Kaimur Group, which are situated on the southern and southeastern sides of the Vindhyan basin (Mishra and Sen 2012; Sen et al. 2014; Quasim et al. 2017, 2019; Yadav et al. 2022c).

All the samples of arkose and shale are plotted in the binary diagrams of TiO2 vs. Zr and TiO2 vs. Ni which are attributed that the felsic igneous rocks are mainly responsible for the formation of these lithofacies (Fig. 6a & 6b). Arkose and shale samples occupy the granites and gneisses and felsic igneous provenance fields while plotted in the discriminant function-1 (DF-1) vs. discriminant function-2 (DF-2) diagram (Fig. 6c, P1-Mafic Igneous Provenance; P2-Intermediate Igneous Provenance; P3-Felsic Igneous Provenance; and P4-Granites and Gneisses; Roser and Korsch 1988). REE plot of these lithofacies displays flat LREE and HREE and negative Eu anomaly (Eu/Eu* = 0.58–0.66 in arkose; 0.56–0.81 in shale) implying towards the source of felsic igneous rocks (Fig. 6d).

6.3 Weathering intensity-implication for source area composition

Nesbitt and Young (1982) proposed the Chemical Index of Alteration (CIA) which mostly used a chemical index to assess the degree of chemical weathering in the source area. The CIA is defined as CIA = [Al₂O₃/(Al₂O₃ + CaO*+Na₂O + K₂O)]×100 in molecular proportions and CaO* is the amount incorporated in the silicate fraction of the rock. The intensity of chemical weathering in the source region is determined by the value of CIA (Nesbitt and Young, 1982). According to Fedo et al. (1995), the CIA value of 50–60 shows incipient weathering, CIA 60–80 intermediate weathering, and CIA > 80 extreme weathering. In the study area, the CIA values of arkose and shale have varied from 74.24 to 83.63 and 72.21 to 77.78 respectively which indicate intermediate to extreme degree of chemical weathering (Table 2). Furthermore, paleoweathering conditions can also be detected using the Al₂O₃-CaO + Na₂O-K₂O(A-CN-K) ternary diagram (Fig. 7a) of Nesbitt and Young (1984). The majority of samples of arkose and shale fall in the granite line except a few samples of arkose occupy the line trend of granodiorite, suggesting the protolith of the area to be of intermediate to felsic source rocks (Fig. 7a).

The classification of the Chemical Index of Weathering [CIW = (CIA-K) = Al₂O₃/(Al₂O₃ + CaO*+Na₂O)* 100] was proposed by Harnois (1988) to understand the intensity of weathering. All the values of CIW are quite similar to CIA values except that in the CIW index, K₂O is not considered. Therefore, the values of CIW are usually higher than CIA values. The values of CIW of arkose and shale are varying from 92.26 to 96.78 and 97.78 to 98.29 which are attributed to the intermediate to extreme degree of chemical weathering (Table 2). The arkose and shale samples are plotted in CIA verse CIW bivariate plot (Fig. 7b) which reflects predominantly intermediate silicate weathering except for two samples of arkose occupying the field of extreme silicate weathering. Chemical weathering can also be calculated by the Plagioclase Index of Alteration (PIA), especially when the plagioclase weathering needs to be monitored. It is defined as PIA = [(Al₂O₃-K₂O)/(Al₂O₃ + CaO*+Na₂O-K₂O)*100], where CaO* (CaO* = CaO total subtract CaO contained in non-silicate minerals) is CaO only reside in silicate fractions (Fedo et al. 1995). The values of PIA of arkose (91.37–95.62) and shale (96.56–97.51; Table 2) and plot of CIA vs. PIA (Fig. 7c) are suggested the intermediate to extreme degree of plagioclase weathering. A bivariate plot of SiO₂ against total Al₂O₃ + K₂O + Na₂O as proposed by Suttner and Dutta (1986) was used to identify the maturity and climatic conditions of arkose and shale lithofacies. Based on this plot, the semihumid condition is responsible for the formation of arkose whereas the semiarid condition is depicted for the formation of shale (Fig. 7d).

6.4 Implication for depositional environment

The tidal flat-lagoon facies comprising both siliciclastic and carbonate rocks are the most common in the Vindhyan Basin (Auden 1933). He also believed that the Semri Group represents a product of a shallow-marine environment whereas the younger terrigenous formations of the Vindhyan Supergroup represent either an estuarine or fluvial-deltaic environment. The depositional environments of the Kaimur Group exposed in the Sone Valley, Central India were discussed by several workers (Bose et al. 2001; Gupta et al. 2003; Chakraborty 2006; Ahmad et al. 2009; Mishra and Sen 2010; Sen et al. 2014; Quasim et al. 2017, 2019). In addition, very less work on the depositional environment of the Kaimur Group of rocks has been done in the eastern part of the Vindhyan Basin in the state of Bihar (Singh and Sinha 1988; Yadav and Das 2019; Yadav et al. 2022c).

In the present paper, three litho-logs have been prepared of an intercalated unit of arkose and shale and sandstone of the Dhandraul Formation to the south and southeast of Jamuninar from which some inferences on depositional environments have been made (Fig. 8). In sandstone lithofacies, sedimentary structures like tabular cross-bedding, trough cross-bedding, ball and pillow structures, and parallel lamination are observed (Fig. 8) whereas trough cross-bedding, ball and pillow structures and parallel lamination are noticed in an intercalated unit of arkose and shale lithofacies (Fig. 8). Paleocurrent data were collected from tabular cross-bedding and trough cross-bedding which varies from 240º to 320º. Based on the grains size, sorting, mineralogical and textural maturity, sedimentary structures, bimodal distribution of palaeocurrent patterns, and overall sequence of these litho-facies from sandstone to shale shows Fining Upward (FU) sequence which indicates the deposition in a shallow coastal fluvial-marine environment in a transgressive phase (Fig. 8).

6.5 Interpretation for geodynamic set-up

The geodynamic set-up of sedimentary rocks has been determined by using the ternary Qt-F-L and Qm-F-Lt, diagrams (Dickinson and Suezek 1979; Dickinson 1985; Dutta 2005; Sen et al. 2014). The Qt-F-L plot emphasizes the grain stability and maturity, relief in the provenance, transport mechanism, and the source rock composition whereas the Qm-F-Lt diagram deals with the source rock composition. Three main types of provenance were identified like continental blocks, magmatic arcs, and recycled orogen by Dickinson and Suezek (1979). Besides, the major element’s composition and the ratio of trace elements of clastic sedimentary rocks are also considered to be used to predict the tectonic settings (Bhatia 1983; Bhatia and Crook 1986; Roser and Korsch 1986; Kroonberg 1994; Quasim et al. 2017). Bhatia (1983) also discriminated tectonic settings of Paleozoic greywacke sandstones on the basis of major element data and oceanic island arc, continental island arc, active continental margin, and passive margin settings.

In the binary plot of SiO₂ vs. K₂O/Na₂O and the ternary plot of K₂O-CaO-Na₂O, all the samples of arkose and shale have occupied the field of passive margin tectonic setting (Fig. 9a & 9b). The majority of the sandstone samples occupy the passive margin setting while plotted in the ternary diagram of SiO₂/20-K₂O + Na₂O-TiO₂ + Fe2O₃(t) + MgO (Fig. 9c). In Th-Sc-Zr/10 ternary plot, most of the samples fall in the passive margin field except one sample comes in the field of the continental island arc (Fig. 9d).

In the eastern part of the Vindhyan basin in the state of Bihar, a mappable member of an intercalated unit of arkose and shale of the Dhandraul Formation of the Kaimur Group belonging to the Vindhyan Supergroup has been reported for the first time. Based on the compressive field data, petrography studies, and interpretations of the geochemical data, provenance, depositional environment, and geodynamic set-up of this intercalated unit of arkose and shale is attempted. It is envisaged that granites of the Chhotanagpur Gneissic Complex and metasedimentary rocks of the Mahakoshal Group present in the south and southwest direction are the probable source rocks for these lithofacies. The A-CN-K plot and CIA, CIW, PIA, and ICV values suggest that the source rocks are subjected to severe extents of chemical weathering under semi-humid to semi-arid climates. These lithofacies display low contents of MgO, CaO, Na₂O, K₂O, and TiO₂. REE plot shows the flat LREE and HREE, and values of (La/Sm)_N, (La/Yb)_N and (Gd/Yb)_N, and Eu anomaly are also attributed to the presence of felsic igneous rocks in the provenance. Most of the samples of arkose and shale lithofacies come in the field of passive margin tectonic setting except one sample fall in the field of the continental island arc while plotted in the binary and ternary plots. On the basis of lithologs, a shallow coastal fluvial-marine environment in a transgressive phase is depicted for the formation of these lithofacies from sandstone to shale member of the Dhandraul Formation.

Acknowledgments

The authors are sincerely obliged to the Director General, Geological Survey of India for according necessary permission to publish this manuscript. We are also extremely grateful to the Deputy Director General, Geological Survey of India, SU: Bihar for his constant cooperation, encouragement, guidance, and valuable suggestions during the finalization of the manuscript. This paper is the outcome of the Annual Field Season Programme (FSPMIS ID: M1AGS/STM/NC/ER/SU-BR/2017/20848) of the Geological Survey of India, State Unit: Bihar, Patna.

Conflict of interest statement

On behalf of all authors, the corresponding author states that there is no conflict of interest.

Data availability statement

The datasets generated during the current study are available from the corresponding author on reasonable request. All the aforesaid mentioned datasets are not publicly available due to the policy of the parent organization i.e. the Geological Survey of India but are available from the corresponding author on reasonable request.

Author contributions

Pawan Kumar Yadav: Conceptualization, Methodology, Data curation, Writing- Original draft preparation, Visualization, Investigation, Writing- Reviewing and Editing. Manorama Das: Preparation of chemical plots, Digitization.

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Table 1: Stratigraphy of the Vindhyan Supergroup, Son Valley (modified after Mandal et al., 2019)
Sgr	Gr	Fm	Eastern part of Son valley sector (existing)	Western part of Son valley sector (existing)	Stratigraphy of Son valley sector proposed by Mandal et al. (2019)		Proposed stratigraphy of eastern part of Vindhyan basin in Bihar (present study)
V I N D H Y A N S U P E R G R O U P	U P P E R V I N D H Y A N G R O U P	B H A N D E R	Upper Bhander Sandstone	Upper Bhander Sandstone	Upper Bhander Sandstone		Not exposed
			Sirbu Shale	Sirbu Shale	Sirbu Shale
			Lower Bhander Sandstone	Lower Bhander Sandstone	Lower Bhander Sandstone
			Bhander Limestone	Bhander Limestone	Bhander Limestone
			Ganurgarh Shale	Ganurgarh Shale	Ganurgarh Shale
		R E W A	Rewa Sandstone	Rewa Sandstone	Rewa Sandstone		Not exposed
			Rewa Shale	Rewa Shale	Rewa Shale
		K A I M U R	Dhandraul Sandstone	Dhandraul Sandstone	Dhandraul Sandstone		Intercalated unit of arkose and shale	Dhandraul Formation
							Dhandraul Sandstone
			Scarp Sandstone/ Mangeswar Sandstone	Scarp Sandstone/ Mangeswar Sandstone	Scarp Sandstone/ Mangeswar Sandstone		Scarp Sandstone
			Bijaigarh Shale		Bhagwar Shale/Silicified Shale		Bijaigarh Shale
			Ghaghar Sandstone/Upper Sandstone/Quartzite
			Susunia Breccia
			Silicified Shale
			Sasaram Sandstone (Lower Quartzite)		Sasaram Sandstone (Lower Quartzite)		Sasaram Sandstone or Ghaghar Sandstone (Lower Quartzite)
	UNCONFORMITY
	L O W E R V I N D H Y A N OR S E M R I G R O U P	R O H T A S	Rohtas Limestone	Bhagwar Shale	Rohtas Limestone	Rohtas Limestone
				Rohtas Limestone
			Rampur Shale	Rampur Shale	Rampur Shale	Not exposed
		K H E I N J U A	Chorhat Sandstone	Chorhat Sandstone	Chorhat Sandstone	Glauconitic Sandstone Formation
			Koldaha Shale	Koldaha Shale	Koldaha Shale	Kheinjua Shale Formation
			Porcellanite	Porcellanite	Porcellanite	Not exposed
		K A J R A H A T	Kajrahat Limestone	Kajrahat Limestone	Kajrahat Limestone	Not exposed
			Arangi Shale	Arangi Shale	Arangi Shale	Not exposed
		DEOLAND				Not exposed
UNCONFORMITY
MAHAKOSHAL GROUP						Not exposed

Sgr - Supergroup; Gr - Group; Fm - Formation

Table 2: Analytical results of major oxides (wt.%) , trace elements (wt.%) and REE (ppm) of arkose and shale of the Dhandraul Formation belonging to the Kaimur Group.
Sample No.	KB/12	KB/13	KB/14	KB/15	KB/10	KB/5	KB/6
Major elements (wt.%)	Arkose				Shale
SiO₂	84.51	86.12	83.05	84.19	71.16	78.24	73.58
Al₂O₃	7.56	5.10	8.12	2.86	13.20	10.89	13.44
Fe₂O₃(T)	4.51	5.82	5.34	10.73	5.25	2.67	4.06
MnO	0.01	0.05	0.40	0.80	0.02	0.02	0.02
MgO	0.41	0.22	0.26	0.17	0.92	0.97	1.67
CaO	0.12	0.06	0.09	0.07	0.16	0.10	0.21
Na₂O	0.18	0.16	0.18	0.17	0.14	0.09	0.08
K₂O	1.35	1.55	2.22	0.32	4.78	3.45	3.55
TiO₂	0.30	0.21	0.18	0.06	0.66	0.49	0.72
P₂O₅	0.07	0.06	0.09	0.03	0.10	0.08	0.11
Total	99.02	99.35	99.93	99.40	96.39	97	97.44
Trace and REE (ppm)
Ba	210	104	301	119	515	405	508
Co	<5	<5	8	<5	13	<5	9
Cr	40	53	26	44	71	18	42
Cu	20	15	17	<5	22	<5	<5
Ga	8	5	7	<5	17	10	9
Nb	7	6	7	5	15	11	12
Ni	12	8	11	12	9	5	5
Pb	234	365	684	204	17	5	7
Rb	81	51	78	16	215	125	148
Sc	5	5	5	5	15	8	10
Sr	56	48	38	18	112	34	35
Th	11	7	15	8	21	9	16
V	31	25	18	15	71	36	59
Y	30	25	14	5	39	22	27
Zn	12	7	9	20	21	18	23
Zr	163	175	219	143	295	315	299
La	25.34	18.35	27.67	16.06	76.55	80.12	78.98
Ce	48.91	34.21	55.82	36.53	155.80	145.23	145.34
Pr	6.04	4.31	6.22	3.77	18.66	14.78	21.66
Nd	20.57	14.70	22.43	14.04	73.31	67.11	65.87
Sm	4.28	3.03	4.31	2.40	12.59	10.45	11.34
Eu	0.81	0.56	0.82	0.41	2.21	2.56	2.89
Gd	3.83	2.68	3.38	1.98	11.58	9.79	10.56
Tb	0.63	0.45	0.52	0.29	1.81	0.98	1.54
Dy	3.65	2.55	2.93	1.44	11.55	10.45	12.34
Ho	0.73	0.51	0.58	0.25	2.36	1.56	1.45
Er	2.14	1.52	1.74	0.68	6.65	4.56	5.68
Tm	0.35	0.24	0.29	0.11	1.17	0.89	0.78
Yb	2.33	1.60	1.81	0.69	6.75	4.56	5.67
Lu	0.36	0.25	0.28	0.10	1.13	0.98	1.43
∑REE	119.98	84.96	128.81	78.74	382.11	354.02	365.53
∑LREE	57.05	40.95	61.46	36.68	183.32	320.25	180.74
∑HREE	14.02	9.80	11.53	5.53	42.99	33.77	39.45
LREE/HREE	4.07	4.18	5.33	6.63	4.26	9.48	4.58
Eu/Eu*	0.61	0.60	0.66	0.58	0.56	0.77	0.81
(La/Sm)N	3.73	3.81	4.04	4.22	3.82	4.82	4.38
(La/Yb)N	7.33	7.74	10.33	15.59	7.65	11.85	9.39
(Gd/Yb)N	1.32	1.35	1.51	2.30	1.38	1.73	1.5
Zr/Y	5.26	7.00	12.17	9.53	4.15	8.75	5.07
La/Th	2.30	2.62	1.84	2.01	3.65	8.90	4.94
La/Sc	5.07	3.67	5.53	3.21	5.10	10.02	7.90
Th/Sc	2.20	1.40	3.00	1.60	1.40	1.13	1.60
Rb/Sr	1.45	1.06	2.05	0.89	1.92	3.68	4.23
Na2O/K2O	0.13	0.10	0.08	0.53	0.03	0.03	0.02
K2O/Na2O	7.50	9.69	12.33	1.88	34.14	38.33	44.38
K2O/Al2O3	0.18	0.30	0.27	0.11	0.36	0.32	0.26
Al2O3/SiO2	0.09	0.06	0.10	0.03	0.19	0.14	0.18
SiO2/Al2O3	11.18	16.89	10.23	29.44	5.39	7.18	5.47
Fe2O3(T)/K2O	3.34	3.75	2.41	33.53	1.10	0.77	1.14
Al2O3/TiO2	25.20	24.29	45.11	47.67	20.00	22.22	18.67
Fe2O3(T)+MgO	4.92	6.04	5.60	10.90	6.17	3.64	5.73
K2O+Na2O	1.53	1.71	2.40	0.49	4.92	3.54	3.63
Al2O3+ Na2O+ K2O	9.09	6.81	10.52	3.35	18.12	14.43	17.07
CIA	82.08	74.24	76.53	83.63	72.21	74.95	77.78
PIA	95.39	94.16	95.62	91.37	96.56	97.51	97.15
CIW	96.18	95.86	96.78	92.26	97.78	98.29	97.89
ICV	6.62	7.90	8.51	12.28	11.32	7.34	9.64

No competing interests reported.

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Geochemistry of an intercalated unit of arkose and shale of the Dhandraul Formation belonging to the Vindhyan Supergroup, Eastern India: Insights from provenance, depositional environment, and geodynamic set-up

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Abstract

Figures

1. Introduction

2. Regional Geological Setting

3. Geology Of The Study Area

4. Methodology And Analytical Techniques

5. Petrography

6. Results And Discussion

6.1 Geochemical characteristics of an intercalated unit of arkose and shale

6.2 Inference for provenance composition

6.3 Weathering intensity-implication for source area composition

6.4 Implication for depositional environment

6.5 Interpretation for geodynamic set-up

7. Conclusion

Declarations

References

Tables

Additional Declarations

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