Palaeoenvironmental reconstruction and evidence of marine influence in Permian coal-bearing sequence from Lalmatia Coal mine (Rajmahal Basin), Jharkhand, India: A multi-proxy approach

Highlights

• A comprehensive study on Indian Permian Gondwana sediments to decipher palaeoenvironments, climate and depositional conditions.

• Coal-bearing sequences of early Permian Glossopteris dominant terrestrial vegetation

• The climatic conditions were not stable throughout the deposition of the sequence as suggested by the geochemical evidences.

• The CO₂ level is suggested to be at a higher side during Artinskian compared to Sakamarian based on stomatal index.

• Marine influenced depositional setting of the coal-bearing sequence is identified based on geochemical evidences.

Abstract

Lalmatia coal-bearing sequences belong to the Barakar Formation (Permian) of Lower Gondwana. The well-developed coal-bearing sequences (approx. 52 m) exposed along the mine section have been studied to understand the depositional environment. Along with this, a cuticle study was performed to understand the palaeoclimate with the help of the stomatal index. The distribution of n–alkanes (n–C₁₅ to n–C₃₁) suggests major higher plant and algal dominant source input, although variations are seen in the relative input. Steranes identified include C₂₉ regular sterane and 4-Methylsteranes. The Pr/Ph ratio varied from 0.43 to 4.26. Warm temperate to subtropical palaeoclimate with fresh to brackish water bodies, and reducing oxygen-poor environment were inferred from the Rb/Sr, Sr/Cu, Th/U, Sr/Ba, V/(V+Ni). Mean δ¹³C value measured is −23.0‰ ± 0.60‰ for coal and −22.6‰ for shale. These values are well correlated with typical δ¹³C values of the Permian coal and shale. The stomatal index of Glossopteris leaves (280 million years) was taken into account, and it showed a value of 10.7 that assumes a higher level of atmospheric CO₂ during the Barakar Formation (Artinskian age). The result also shows that Permian taxon Glossopteris can be used for CO₂–proxy during Permian age. The geochemical evidence suggests that towards the middle part (middle coal seam) of the studied section, the depositional milieu was under a marine-influenced marginal condition. The geochemical studies and carbon isotopic ratios unequivocally suggest that fluctuating climatic conditions existed during the deposition of the Lalmatia coal-bearing sequence.

Introduction

Climate is the supreme factor controlling the floral composition as plant families respond to the change in environmental factors. The dominance or absence of plant communities is directly related to the temperature and moisture conditions and thus to the climatic variations. Hence, the determination of palaeofloral composition can provide clues to the change in environmental factors that occurred during sedimentation (Hautevelle et al., 2006). The latest developments in the macromolecular analysis of organic matter demonstrated the importance of biomarker compounds in the palaeoclimate reconstruction (Castañeda and Schouten, 2011). Although, the OM is affected by various alterations after the deposition causing transformations in the C=H bonds and functional groups, the carbon skeletal structures of biomarker compounds are indestructible. Hence, in the sedimentary record, these compounds are well upheld for geological ages (Peterson et al., 2007). Due to this source specificity and resistance to degradation, biomarkers provide highly convincing signatures reflecting past environments than other proxies (Zhang et al., 2014, Zhang et al., 2016a). Besides this, inorganic geochemical parameters also provide highly useful information regarding past environmental shifts. Accordingly, various methods are applied to differentiate past sedimentary realms. Elemental composition is an important parameter that can provide indications of geodynamic and palaeoclimatic conditions (Fu et al., 2018). However, quantitative analysis of major and trace elements is the most commonly accepted and applied method across basins (Liu and Zhou, 2007; Dai et al., 2016; Zhang et al., 2016b; Wang et al., 2017; Fu et al., 2018). Consequently, the occurrence and composition of trace elements in coal have been studied extensively (Dai et al., 2015, Dai et al., 2016, Dai et al., 2017, Dai et al., 2018). Similarly, one of the robust proxies which can reflect the variation of the carbon cycle in local to regional level is the stable carbon isotopic ratio (δ¹³C) (Chen et al., 2014; Aggarwal et al., 2019). Considering the morphological aspects, various studies have shown that the quantitative estimation of the stomatal number of fossil plant leaves (i.e, the stomatal index and density analysis) can be efficaciously correlated to the palaeoatmospheric fluctuations (McElwain, 1998; Wagner et al., 2005). Thus, in addition to the geochemical parameters microscopic investigation of cuticles is also considered as an essential tool to decipher past climates (Stace, 1965; Cutler, 1982; Kovach and Dilcher, 1984; Upchurch Jr., 1995).

Several studies have been done on the coal-bearing sequences of the Rajmahal Basin particularly on the morpho-taxonomical (Feistmantel, 1880; Singh et al., 1987; Bajpai and Maheshwari, 1991; Maheshwari and Bajpai, 1992; Srivastava and Pant, 2002), palynology (Tripathi and Ray, 2005; Tripathi et al., 2010), petrological (Roy et al., 1983; Madabhushi, 1990; Singh, 1992), molecular composition and hydrocarbon potential (Tewari et al., 2016, Tewari et al., 2017) aspects. Earlier studies have shown possible marine influences at various localities in the lower Gondwana basins of India as given in Fig. 1 (e.g. Shah and Sastry, 1975; Chatterjee and Hotton, 1986; Venkatachala and Tiwari, 1988; Ahmad and Khan, 1993). Reports of marine incursions during Gondwana are based on Eurydesma–Productus–Conularia. Apart from this, reports of marine incursions are reported from Daltonganj (Dutta, 1965), Umaria marine beds (Sinor, 1923), Subansiri (Sahni and Dutta, 1959), Mahendragardh (Ghosh, 1954), Badhaura of Western Rajasthan (Mishra et al., 1961; Shah, 1963) in India. The other evidence of marine incursion during Permian is from sedimentary nodules, invertebrate faunas, Acritarch (Leiosphaerid and Foveofusa), coastal–marine, and Cruzianaichno–facies (Seilacher, 1964). Recently, Goswami (2008) has pointed out marine incursions in Mahanadi Basin from trace fossils wave ripples and acritarchs. In the present study, multi–proxy analyses have been done to know the OM source, depositional conditions, palaeoclimatic scenario and its variations during the period of formation of Permian Lalmatia coal–bearing sequences. More importantly, this is the first attempt from the Indian context to study palaeoenvironment and marine influence based on multiple proxies (biomarker, plant fossils, stable carbon isotope, and geochemical records) of Gondwana sediments along with the stomatal index of Glossopteris leaves.