Palaeosols and palaeoclimate reconstruction of the Maastrichtian Lameta Formation, Central India

Abstract

Various geological studies have been done worldwide of Upper Cretaceous palaeosols, however similar extensive palaeosol studies are lacking in India. It is well established that palaeosols of any age are one of the best proxies for palaeoclimate reconstructions and for local to regional stratigraphic correlations of continental deposits. The Cretaceous deposits in Central India occur as isolated outcrops along the Narmada valley, known as the Lameta Formation. The Upper Cretaceous Lameta (~infratrappean) Formation of the Jabalpur area (Madhya Pradesh, Central India) preserves one of the most diverse Late Cretaceous reptilian assemblages (largely dominated by dinosaurs) and other significant biotic elements of the Indian subcontinent. The Lameta Formation in the Jabalpur region comprises excellent exposures, wherein the Mottled Nodular Bed (MNB) consists of sandy marl lithofacies together with palaeosol facies. Therefore, detailed pedofacies investigations on the basis of morphological, micromorphological and geochemical studies have been carried out in the MNB exposed at the Chui Hill (CH) and the Chhota Simla Hill (CSH) locations in Jabalpur city of Madhya Pradesh (Central India). These studies reveal the presence of various in-situ pedofeatures. For example, abundant bioturbation features, moderately developed blocky and granular microstructure, crystallitic and undifferentiated b-fabrics, Fe–Mn oxide/clay, silica to carbonate coatings, hypocoatings as well as quasicoatings, nodules/concretions and other textural as well as depletion pedofeatures. These palaeosols are interpreted as moderately to well-developed palaeosols that formed under tropical warm, humid climatic conditions rather than semi-arid conditions as deduced on the basis of diagnostic pedogenic characters. Tropical humid conditions are further supported by indices of base loss, salinization and hydrolysis in these palaeosols. The calculated mean annual precipitation (MAP) values range 779.44 ± 182 to1150.02 ± 235 mm/yr and CIA-K (Chemical Index of Alteration without potassium) values document significant chemical weathering. The present study will be important for future studies such as Upper Cretaceous palaeosol investigations along the east-west and central Narmada valley of peninsular India in order to utilize them as stratigraphic tools, taking into account worldwide reports of contemporaneous dinosaur-bearing Upper Cretaceous palaeosols in different parts of the world.

Introduction

The Cretaceous Period, which lasted for approximately 77 million years, has been considered as having been a time of classic greenhouse climate (Burgener et al., 2019 and references therein). This was, a significant period in dinosaur evolution (Khosla, 2017) and some of the last dinosaur species have been reported from this period. Also, the end of the Cretaceous witnessed large-scale Deccan volcanic activity, which is considered to have affected Earth's climate and environment in many ways (Dessert et al., 2001; Schoene et al., 2015). A detailed palaeoclimate record from Upper Cretaceous deposits has the potential to improve our understanding about the evolution of global climate changes as well as climatic conditions prevalent prior to the Cretaceous-Palaeogene extinction event.

Amongst various climate proxies, ancient/fossil soils (i.e. palaeosols) represent an important natural archive with a minimum sampling interval of 100 years (Birkeland, 1974, 1984; Bradly and Eddy, 1991). This is because they formed by the direct interaction between hydrosphere, lithosphere, biosphere and atmosphere (Birkeland, 1984; Retallack, 1990). They are, indeed, sensitive, reliable climate recorders and have been widely used to reconstruct past climate conditions at fine temporal and spatial scales throughout Earth's geological history (Retallack, 1990; Kraus, 1999; Sheldon and Tabor, 2009; Tabor and Myers, 2015, and many more). Further, in complex geological settings, palaeosol studies have proved beneficial for palaeoclimate reconstructions. Therefore, palaeosols are one of the best proxy tools for interpreting and reconstructing changes in past climate and they are helpful in sequence stratigraphic analyses in a variety of depositional environments (Retallack, 1990; Lander et al., 1991; Driese et al., 1994; Kraus, 1999; Singh et al., 2017 and many more).

In order to deduce the palaeoclimatic conditions, various researchers from different countries, for instance Spain, France, Korea, Romania, and Argentina (López-Martinez et al., 2000; Cojan et al., 2003; Paik et al., 2004; Therrien, 2004, 2005; Van Itterbeeck et al., 2004, 2005; Bojar et al., 2005; Kim et al., 2009; Therrien et al., 2009 and many references therein; Hechenleitner et al., 2016; Basilici et al., 2017) performed detailed palaeosol studies on Upper Cretaceous dinosaur-bearing deposits. To date, these strata in India have not yet been extensively investigated for palaeosols as palaeoclimate proxies.

In India, Upper Cretaceous deposits are best exposed as the Lameta Formation in the Jabalpur region of Central India (Fig. 1a). It preserves one of most diverse Late Cretaceous dinosaur faunas of India (Vianey-Liaud et al., 2003; Khosla and Sahni, 2003; Fernández and Khosla, 2015; Khosla and Verma, 2015; Khosla et al., 2015; Khosla, 2019). The Lameta Formation is also among the few Maastrichtian dinosaur-bearing formations for which a significant fossil record and stratigraphic sections are known (Buffetaut, 1987; Brookfield and Sahni, 1987; Tandon et al., 1995; Khosla, 2014; Khosla and Verma, 2015; Khosla et al., 2016 and many more). The Lameta Formation has been studied extensively in the context of sedimentology and palaeontology, but the palaeoenvironment and palaeoclimate conditions during the deposition of these sediments are still controversial (Matley, 1921; Chanda, 1967; Singh, 1981; Brookfield and Sahni, 1987; Sahni and Khosla, 1994a, b; Tandon et al., 1998; Tandon and Andrews, 2001; Shukla and Srivastava, 2008; Saha et al., 2010; Khosla, 2014; Srivastava et al., 2015; Khosla, 2017 and references therein). The Maastrichtian Lameta Formation is divided into four lithostratigraphic units (from the oldest to the youngest): Green Sandstone, Lower Limestone, Mottled Nodular Beds and Upper Calcified Sandstone (Tandon et al., 1990). The Mottled Nodular Bed (MNB) of the Lameta Formation is the most distinctive unit. It is well exposed in the Jabalpur region, particularly at the places Chui Hill, Chhota Simla Hill, Bara Simla Hill and Lameta Ghat. In general, it consists of sandy marl with palaeosols.

Sahni and Mehrotra (1974) interpreted the MNB as a fluvial deposit, whereas Kumar and Tandon (1978, 1979) considered it a coastal deposit. Singh (1981) interpreted the MNB as a supra tidal deposit, whereas Brookfield and Sahni (1987) interpreted it as a semiarid, floodplain deposit extensively modified by pedogenesis. Tandon et al. (1990, 1995) interpreted the MNB as a pedogenically-modified sheet wash deposit of a semi-arid alluvial plain. Later, Tandon et al. (1998) documented the MNB as multiple calcrete profiles, which formed part of an extensive regolith. Saha et al. (2010) interpreted it as coastal complex-shallow marine deposit on the basis of the presence of extensive development of crab burrows. Some researchers considered it as fluvial-lacustrine deposit based upon lithofacies architecture (Srivastava and Mankar, 2013, 2015).

Thus, a wide array of researchers since the early seventies has demonstrated that the Mottled Nodular Bed of the Lameta Formation in the Jabalpur region was deposited under varied depositional environments and climatic conditions. Therefore, interpretation of the climate and environmental conditions during deposition of these sediments has reached no consensus, perhaps due to the lack of detailed micromorphological studies of palaeosols in these sediments. Researchers have shown that the micromorphological studies of palaeosols in various sedimentary environments are far more reliable proxies for past climate reconstructions (McCarthy et al., 1997a, McCarthy et al., 1997b, 1998; Stoops et al., 2010; Singh et al., 2017 and references therein).

Therefore, detailed micromorphological studies together with morphological and geochemical studies of palaeosols have been done in the MNB unit of the Lameta Formation, exposed at the Chui Hill (CH) and Chhota Simla Hill (CSH) locations in the Jabalpur cantonment area of Madhya Pradesh (Central India) (Fig. 1b). Here, palaeoclimate conditions during deposition of the MNB unit have been documented on the basis of diagnostic pedofeatures and pedogenic processes. Thus, the present work on the Upper Cretaceous Mottled Nodular Bed deposits of the Jabalpur region provides additional palaeosol data from India to already published palaeosol studies of Upper Cretaceous dinosaur-fossil bearing strata for palaeoclimatic reconstructions.