Chin.J.Geochem.(2010)29:021–032 DOI: 10.1007/s11631-010-0021-1
Geochemical signatures of Mesoproterozoic siliciclastic rocks of the Kaimur Group of the Vindhyan Supergroup, Central India Meenal Mishra* and Shinjana Sen Department of Geology, Faculty of Science, Banaras Hindu University, Varanasi-221005, India * Corresponding author, E-mail:
[email protected]
Received October 27, 2008; accepted December 30, 2008 © Science Press and Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2010 Abstract The Upper Kaimur Group of the Vindhyan Supergroup in Central India, primarily consists of three rock types-DhandraulSandstone, Scarp Sandstone and Bijaigarh Shale. Mineralogically and geochemically, they are quartz arenite, sublitharenite to litharenite and litharenite to shale in composition, respectively. The A-CN-K ternary plot and CIA and ICV values suggest that the similar source rocks suffered severe chemical weathering, under a hot-humid climate in an acidic environment with higher PCO2, which facilitated high sediment influx in the absence of land plants. Various geochemical discriminants, elemental ratios like K2O/Na2O, Al2O3/TiO2, SiO2/MgO, La/Sc, Th/Sc, Th/Cr, GdN/YbN and pronounced negative Eu anomalies indicate the rocks to be of post-Archean Proterozoic granitic source, with a minor contribution of granodioritic input, in a passive margin setting. The sediments of the Upper Kaimur Group were probably deposited in the interglacial period in between the Paleoproterozoic and Neoproterozoic glacial epochs. Key words Geochemistry; siliciclastics; Upper Kaimur Group; Mesoproterozoic; Vindhyan Supergroup; Central India
1 Introduction The Vindhyan Basin of Central India is one of the largest and best preserved Proterozoic sedimentary basins throughout the world, covering a vast area of 178000 km2. It is exposed along the Son valley in the central part of India. It comprises mostly undeformed and unmetamorphosed sediments with a thickness of ~4 km, dominantly carbonate-rich sediments in the lower part, followed by siliciclastics in the upper part. The Kaimur Group of the Vindhyan Supergroup is of special significance because it consists dominantly of siliciclastic rocks lying unconformably over the carbonate-rich Semri Group–Lower Vindhyans. Therefore, the rocks from the Kaimur Group hold a strong evidenceregarding changing environment of deposition, climatic conditions, tectonics and weathering conditions, during Mesoproterozoic. The mineralogical and chemical compositions of terrigenous sedimentary rocks are indicative of several variables such as provenance, weathering conditions, transportation, diagenesis, climate and tectonism (Bhatia and Crook, 1986; Roser and Korsch, 1986, 1988; Cullers et al., 1988, 2000; Taylor and McLennan, 1985; McLennan and Taylor, 1991; Fedo et al., 1995, 1997; Cox et al., 1995; Nesbitt et al., 1996). In www.gyig.ac.cn www.springerlink.com
geochemical studies of the clastic sedimentary rocks, the major elements, selected trace elements like Th, Sc, Co, Cr, Zr, Hf, Y including rare earth elements (REEs) and their elemental ratios are sensitive indicators of the source rocks, tectonic setting, paleoweathering conditions and paleoclimate (Bhatia, 1983; Roser and Korsch, 1986; McLennan et al., 1993; Cullers et al., 1988, 2000; Condie, 1993). Geochemistry has proven itself to be a very important tool in the evaluation of depositional environment and basinal settings, especially of Precambrian sedimentary rocks where fossil records are meagre. They also provide us with valuable information on the crustal evolution during the Precambrian. The present study provides the first detailed report on the geochemistry of sandstone and shales from the Upper Kaimur Group in order to evaluate the provenance, tectonic setting, paleoweathering and paleoclimatic conditions during their deposition.
2 Geological setting The Vindhyan Basin overlies the stable Bundelkhand craton of Archean-Early Proterozoic age (Tandon et al., 1991). Vindhyan Supergroup has been broadly divided into four Groups-Semri, Kaimur, Rewa and Bhander, from the bottom to the top. The
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Semri Group, also called Lower Vindhyan Group, is gently deformed and mildly metamorphosed and consists of carbonate-rich sediments. They are overlain by siliciclastics of later three Groups, i.e., the Upper Vindhyans Group. The Vindhyans are bordered by the Aravalli-Delhi orogenic belt (2500–900 Ma) (Roy, 1988) in the west and the Satpura orogenic belt (1600–850 Ma) (Verma, 1991) to the south and east. The Bundelkhand massif (3.3–2.5 Ga) (Crawford and Compston, 1970; Mondal et al., 2002) occurs at the centre of the basin and divides it into two sub-basins-Son Valley in the east and Aravalli-Vindhyan in the west.
charya et al., 1986; Morad et al., 1991; Bhattacharya and Morad, 1993; Bose et al., 2001; Mishra and Sen, 2008a, b), whereas some workers have interpreted their depositional environment varying from beach to barrier bar or shoal to tidal flat and lagoon (Misra, 1969; Banerjee, 1974; Singh, 1980).
Fig. 2. Panoramic view of the Upper Kaimur Group sedimentary rocks exposed at Markundi Ghat, Sonbhadra district. The arrow pointing upwards indicates the coarsening upwards sequence. DS. Dhandraul Sandstone; SS. Scarp Sandstone; BS. Bijaigarh Shale.
Fig. 3. Mud cracks in the Bijaigarh Shale.
Fig. 1a. Detailed geological map of the Vindhyan Supergroup, in and around the Sonbhadra and Mirzapur districts (modified after Auden, 1933); b. location map of the study area, with map of India in the inset outlining the Vindhyan outcrops in Central India.
The Kaimur Group measured at, up to 400 m in thickness (Sastry and Moitra, 1984) lies unconformably over tilted, deformed and eroded Rohtas Limestone of the Semri Group (Misra, 1969) along the Son River valley (Fig. 1). The rocks of the Kaimur Group are largely of fluvial origin (Auden, 1933; Bhatta-
The Upper Kaimur Group is broadly divided into three formations-Bijaigarh Shale (bottommost) Scarp Sandstone (middle) and Dhandraul Sandstone (topmost) (Auden, 1933; Prakash and Dalela, 1982) (Table 1). The Upper Kaimur Group is a coarsening upward sequence (Fig. 2). Bijaigarh Shale dominantly consists of shales of sandy to clayey nature. It has a thickness of up to 50 m and its estimated age is 750 Ma using the Pb-Pb dating method (Balasubramanyan and Chandy, 1976), while 1670 Ma determined by using the Re-Os dating technique (Singh et al., 2002). Sedimentary structures like ripple marks and mud cracks are frequently observed (Fig. 3). Scarp Sandstone is pinkish red, olive green to gray and even black-coloured, medium-grained sandstone. Cross bedding, current bedding and lenticular bedding (Fig. 4) are common sedimentary structures observed. The age of the Scarp Sandstone has been determined to be 940 Ma, using the K-Ar dating method (Vinogradov et
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al., 1964). Dhandraul Sandstone overlying the Scarp Sandstone is dirty white to pure white-coloured, medium to coarse-grained sandstone. The beds are mostly tabular and laterally continuous for tens to hundreds of meters with sharp boundaries. The Dhandraul sandstone exhibits many sedimentary structures like large scale cross bedding with long, low-angle foresets which alternate with cosets of parallel laminated sandstone, ripple marks, flute and load casts. The study was carried out around Markundi Ghat and Churk, Son valley in the Sonbhadra district, India (Fig. 1). Table 1. Stratigraphy of the Vindhyan Supergroup showing in particular lithology and structure of the Kaimur Group exposed along Markundi ghat section
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ceous and ferruginous nature. Minor amounts of matrix are also present in some samples. Quartz is the most dominating mineral. 3.1 Bijaigarh shale It is a typical shale having ferruginous cement, which is locally pyritiferous. The grains of quartz are subangular to subrounded and contribute nearly 75% of the whole rock. Resistant minerals like muscovite and biotite are also abundant. Feldspars are nearly absent, which might have been replaced by kaolinization and illitization. 3.2 Scarp sandstone On the basis of petrography it is categorized varying from sublitharenite to litharenite. The interspaces between the grains are occupied by ferruginous cements (Fig. 5). Sometimes, the matrix of siliceous nature is also present. There are angular to subangular quartz grains of both monocrystalline and polycrystalline nature. However, the polycrystalline quartz is less abundant than monocrystalline quartz, thereby indicating its felsic source. The grains are moderately sorted with quartz having a low degree of sphericity with etched surfaces that show evidence of pressure solution activities. Plagioclase occurs in the samples that are in contact with the Bijaigarh Shale (increasing immaturity). Apart from that chert fragments, lithic fragments and resistant minerals like muscovite, zircon and tourmaline were also observed. Zircon, the most abundant heavy mineral in these rocks, occurs as an inclusion in quartz and also shows zoning in certain cases.
Fig. 4. Lenticular bedding in the Scarp Sandstone.
Fig. 5. Ferruginous cement in the Scarp Sandstone (Magn×20, PPL).
3 Petrography
3.3 Dhandraul sandstone
The rocks of the Upper Kaimur Group show variations and increases in maturity, both texturally and mineralogically from Bijaigarh Shale to Scarp Sandstone and to Dhandraul Sandstone. All the three rock types are grain supported with cements of sili-
It is supermature sandstone-quartz arenite having more than 95% of quartz grains. The cement is of siliceous nature. The grains are well sorted having a high degree of sphericity and roundness. Like the Scarp Sandstone, these sandstones contain both
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monocrystalline and polycrystalline quartz, with monocrystalline quartz higher in abundance. However, the high proportion of monocystalline quartz may be attributed to the disaggregation of the original polycrystalline quartz during high energy and/or long-distance transport from the source area (Dabbagh and Rogers, 1983). The surface of quartz in these rocks also shows etching. Muscovite and feldspars are usually absent, however, they have shown their presence in the samples collected close to the contact with the Scarp Sandstone. Chert fragments and lithic fragments (Fig. 6) are abundant. Heavy minerals like zircon and tourmaline are present. The former is more in abundance than in Scarp Sandstone.
Fig. 6. Lithic fragments in the Dhandraul Sandstone (Magn×20, cross polars).
4 Analytical techniques The unweathered fresh samples were collected along the Markundi-Ghat and Churk sections (Fig. 1). About 2 kg samples of representative sedimentary rocks were broken to thumb-nail sized pieces using a hardened-steel hammer, crushed as fine as -60 mesh in the hardened-steel mortar and after coning and quartering, they were powdered to -200 mesh in an agate mortar for chemical analysis. Eight samples of the Bijaigarh Shale, seven of the Scarp Sandstone and six of the Dhandraul Sandstone each, i.e., a total of twenty-one samples were analyzed for the major oxides and seventeen for trace elements and REE data. Major and trace elements and REE data for the selected samples of shales and sandstones are listed in Tables 2 and 3, respectively. The geochemical data were obtained using ICP-AES and XRF techniques for major oxides and ICP-MS techniques for trace elements and REE from the Activation Laboratories Ltd., Ontario, Canada and partly at Wadia Institute of Himalayan Geology, Dehradun, India. The precision is 6, indicating the rocks to be mature (Singh, 2001). Thus, various chemical indices such as CIA and ICV suggest that the source rocks of the Kaimur Group, have suffered an intense and high degree of weathering. The rocks were formed from compositionally mature sediments characterized by recycled input or intensive weathering of the first cycle sediments.
Fig. 10. A-CN-K ternary diagram of the Bijaigarh Shale, Scarp Sandstone and Dhandraul Sandstone from the Son valley. The average values of granite (Gr) is taken from Condie (1993) and UCC and PAAS are from Taylor and McLennan (1985).
6.2 Paleoclimate Chemical weathering is an important mechanism driving elemental fractionation away from parental bedrock signatures (Nesbitt and Young, 1982). The extent of fractionation depends on bedrock and local weathering conditions related to climate. Stronger chemical weathering is generally associated with warm and humid climates, whilst more arid climate is generally associated with relatively weak chemical weathering (Nesbitt and Young, 1982).
Studies of the relationship between climate and the degree of rock weathering have shown that higher rainfall corresponds to the increased loss of labile minerals and higher CIA values in the resulting sediments (Basu, 1981; James et al., 1981; Suttner et al., 1981; Dutta and Suttner, 1986; Girty, 1991; White and Blum, 1995). Consistent rainfall will continuously flush a weathering profile with unsaturated fluids for hydrolysis and removal of the products of ion exchange, and volumetrically more parent rock material will be subjected to decomposition over a given unit time. The CIA values of sandstones and shales of the Kaimur Group range from 72 to 87. This suggests that the source rocks of the these sedimentary rocks were subjected to severe and high chemical weathering and were generated and deposited under a climatic regime that included significant rainfall. Other factors that contribute to weathering are atmospheric carbon dioxide and surface temperature that enhances the rate of weathering. Higher temperatures significantly enhance the rate of mineral decomposition and the potential for minerals such as plagioclase and potassium feldspar to undergo hydrolysis. It is assumed that in the absence of plants, the enhancement of temperature and heavy rainfall must have contributed to a high degree of weathering. All the above observations point to a relatively aggressive weathering environment, where there is a higher degree of rainfall-likely in conjunction with elevated surface temperatures and atmospheric CO2, have worked to severely decompose the parent rock types of the Kaimur Group. Because of the increased erosional effects of rainfall on an unvegetated landscape, other factor(s) must have been combined with rainfall to efficiently break down parent rock materials at a pace faster than erosion. Therefore, the rocks of the Kaimur Group were affected by strong chemical weathering and were formed under warm-humid climatic conditions. During the Proterozoic there occurred two episodes of global glaciation: one in the Paleoproterozoic, the Huronian glaciation (2400–2100 Ma) and the other in the Neoproterozoic, the Cryogenian glaciation (800–635 Ma). In between these two glacial epochs there was a long interglacial period from 2050–750 Ma that included the Mesoproterozoic era (1600–1000 Ma) completely. The climate after the Paleoproterozoic glacial event till the ‘Snowball Earth’ of Neoproterozoic was tropical due to influx of greenhouse gases like CO2 and water vapour resulting from large-scale continental volcanism. The above discussion shows that the rocks of the Kaimur Group have escaped these glacial events and have been deposited under the tropical interglacial climatic conditions. This is also supported by the red colour of the Bijaigarh Shale and Scarp Sandstone, indicating an interglacial
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oxidizing environment. The siliciclastics of the Kaimur Group were deposited during the interglacial period under warm humid climate during the Mesoproterozoic era. 6.3 Provenance and tectonic setting The A-CN-K plot (Fig. 10) and various major element ratios like K2O/Na2O, Al2O3/TiO2, SiO2/MgO discussed in the earlier sections implied the rocks to be derived from granitic source. Moreover, the data on CaO-Na2O-K2O diagram (after Bhatia, 1983) cluster with in granitic field (Fig. 11). Taylor and McLennan (1985), McLennan and Taylor (1991) suggested that the REEs, Th, Sc and high field strength elements (HFSEs) are especially useful for monitoring source area composition (Cullers, 1994a, b, 2000; Cullers et al., 1988).
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The Th/Sc-Zr/Sc diagram (McLennan et al., 1993) shows that the samples of the Bijaigarh Shale, Scarp Sandstone and Dhandraul Sandstone are clustered around average granite with a minor input from granodiorite; confirming felsic source (Fig. 12). The possible input from granodiorite is also illustrated in the La-Th-Sc diagram (Fig. 13) (Cullers and Podkovyrov, 2000, 2002). The rocks are mainly clustered within the continental margin and passive tectonic setting field, indicating input dominantly from granite with lesser contribution of granodioritc input.
Fig. 13. The La-Th-Sc ternary diagram for (Bhatia and Crook, 1986) the rocks of the Kaimur Group shows a continental margin environment and indicates possible input from granodioritic source (Cullers and Podkovyrov, 2000, 2002). The fields are: A. oceanic island-arc; B. continental island arc; C. active continental margin; D. passive margin. Fig. 11. CaO-Na2O-K2O diagram (after Bhatia, 1983) shows the provenance of the sandstones and shales of the Kaimur Group to be average granite.
Fig. 14. The diagram shows the provenance of the rocks from the Kaimur Group to be post-Archean (Taylor and McLennan, 1985; Slack and Stevens, 1994). Fig. 12. Th/Sc-Zr/Sc diagram (McLennan et al., 1993) shows that the samples of the Bijaigarh Shale, Scarp Sandstone and Dhandraul Sandstone are clustered around average granite with minor input granodiorite. The fields are: AG. average granite; AGT. average granodiorite; AV. average felsic volcanic; B. average basalt.
The REE signatures also corroborate felsic nature of the provenance since the negative Eu is regarded as evidence for a differentiated source, similar to granite (Taylor and McLennan, 1985; Slack and Stevens, 1994). In addition, the Eu/Eu* values for these rocks
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are less than 0.85, characteristic of sediments recycled from upper continental crust. The sediments with pronounced negative Eu anomalies
