Abstract
The Bengal Basin has a long geological history, and its westernmost part is included in India in the geographical province of West Bengal. The Palaeogene sedimentary strata of the Bengal Basin are palynofloristically less explored to date. The present work deals with the palynomorph analysis of middle-late Eocene lignite sediments from Panagarh-Domra sector in undivided Bardhaman district, West Bengal, India (23°30ʹ34ʹʹ N, 87°27ʹ19ʹʹ E). The palynoassemblage consists of 54 pollen and non-pollen palynomorph types, where angiosperm pollen grains are predominant (84%), followed by pteridophytic (9%) and fungal morphs (7%). Among the angiosperms, pollen grains of Dipterocarpaceae are the most dominant (27%), having a close resemblance to those of extant genera Dipterocarpus, Shorea and Hopea. Furthermore, the climatic parameters during the time of deposition in the study area have been quantified using the Coexistence Approach (CA). The study suggests that a warm, humid tropical climate was prevalent, supporting the dipterocarp-dominated vegetation harbouring both wet and dry deciduous and evergreen elements during the middle to late Eocene in the western Bengal Basin, the remnants of which perhaps are still present in the form of a tropical dry deciduous forest possibly after experiencing a gradually declined precipitation in the western part of West Bengal.
Keywords
INTRODUCTION
The Bengal Basin is one of the largest geosynclinal basins of the world, with its long geological history similar to other pericratonic basins of the east coastal region of India that started evolving during the late Jurassic-early Cretaceous time (Prasad & Pundir, 2020). This basin characteristically has a thin wedge-shaped tongue that gradually thins towards the west. This western wedge and the eastern deeper part of the West Bengal part of Bengal Basin differ in structural form (Roy Barman, 1992), and the distance from the coastline might have influenced the sedimentary environments through time. While the other Cenozoic basins of India are palynologically well explored, data, especially from the Palaeogene sedimentary strata of the Bengal Basin are quite meagre. The pioneering palynostratigraphic work was done by Baksi (1971) in the deeper basin area from Eocene to Plio-Pleistocene, and later Baksi and Deb (1980) worked on the palynoassemblage of the late Cretaceous sedimentary sequence in the same region. However, most of the palynological investigations in the Bengal Basin were concentrated on the Quaternary sediments (Das, 2014; Roy & Barui, 2015; Sen & Bannerjee, 1990; Vishnu-Mitre & Gupta, 1972). A few works are available dealing with the older sediments, also, for instance, Bera and Banerjee (1995), Batabyal and Bera (2011) worked on Eocene lignite deposits from the westernmost part of the basin, while Mandal and Vijaya (2011) studied the early Eocene palynoassemblage from the north-central part of the basin. Evidently, there is a gap in documentation of the palynostratigraphic history of the western Bengal Basin, especially in the westernmost part. So, it is essential to explore the palynostratigraphy at both the eastern deep and western wedge parts of West Bengal part of the Bengal Basin, to compare the palynoassemblage from both regions to get the complete picture of the vegetation history of the basin through time. In the present work, we intend to explore vegetation patterns during the middle to late Eocene in the western Bengal Basin by palynoassemblage analysis and quantify different climatic parameters using the Coexistence Approach (CA).
Fossil history of Diptercarpaceae, a major timber-producing plant family, dates back to the late Cretaceous, and it is a prominent member of tropical rainforests of India since the K-Pg transition (Bansal et al., 2022; Khan et al., 2020). The palynological analysis of the Eocene sediments of India shows their significant occurrence throughout the country. This family is also well represented in the petrified Neogene (late Miocene to Pleistocene) wood flora of the Bengal Basin (Bera & Banerjee, 2001; Biswas et al., 2019; Roy & Ghosh, 1981). Moreover, a luxuriant dipterocarp-dominated forest still occurs in the western parts of West Bengal. The present work thus aims to explore whether there is a continuous existence of this family from the Palaeogene to the present day in this part of the Bengal Basin.
MATERIAL AND METHODS
Geological setting
The Bengal Basin is situated in West Bengal, Assam, Tripura, Meghalaya, Manipur, Nagaland, and Meghalaya, in the Indian province, while the south-central regions fall within Bangladesh (Banerji, 1984; Roy Barman, 1992). This basin, being situated in close vicinity of tectonically active Eurasian, Indian and Indo-Burma plates and the most active deformation fronts such as the uplifting Himalaya and the Indo-Burmese orogenic belts, has always experienced intense seismic activity (Nath et al., 2014). As a result, sedimentation occurred throughout the entire basin in different phases, namely, (a) Permo-Carboniferous to early Cretaceous, (b) Cretaceous to mid-Eocene, (c) Mid-Eocene to early Miocene, (d) Early Miocene to mid-Pliocene and (e) Mid-Pliocene to Quaternary (Alam et al., 2003). The western Bengal Basin, that is, the West Bengal part of the Bengal Basin, represents a sedimentary wedge where the thickness of the sediments gradually increases eastward (Roy Barman, 1992). The present study area (23°30ʹ34ʹʹ N, 87°27ʹ19ʹʹE) is situated in the westernmost part, that is, the West Bengal part of Bengal Basin (Figure 1). Among the six boreholes drilled by the Geological Survey of India in and around Panagarh-Domra sector, undivided Bardhaman district, West Bengal, India, the lignite sample was collected from the borehole PGD-6 at Tilokchandrapur at a depth of 225.5 m (Figure 1).
Map and litholog of PGD- 6 bore core showing the sampling site in western Bengal Basin, India (green shaded area showing tropical dry deciduous forest cover, after Bera et al., 2023).
Collection and maceration of lignite sample
The lignite specimen is macerated following the standard procedure of Bruch and Pross (1999) with a little modification, and the isolated palynomorphs are preserved in 25% glycerine, then mounted in glycerine jelly, observed and photographed using a transmitted light microscope (Axioskop 40). All the specimens and the slides are deposited in the Palaeobotany-Palynology Laboratory, Herbarium cum Museum, Department of Botany, University of Calcutta (CUH).
Climate analysis using Coexistence Approach (CA)
Plant fossils are a reliable tool for quantitative palaeoclimate reconstruction that is used all over the globe by comparing the variability in past climatic parameters (Liu et al., 2011). The CA (Mosbrugger & Utescher, 1997; Utescher et al., 2014) is one of the widely accepted methods for quantifying Palaeogene climate. Considering that the climatic requirements of the fossils are similar to those of their nearest living relatives (NLRs), CA reconstructs the climatic range (‘coexistence interval’, CI) in which the majority of the NLRs of the palynoassemblage concerned could coexist. The climatic requirements of the NLRs are provided by the Palaeoflora database (Utescher et al., 2024). Mean annual temperature (MAT) and mean annual precipitation (MAP), temperature of the coldest month (CMT), temperature of the warmest month (WMT), wettest month precipitation (MPwet), driest month precipitation (MPdry) are considered for the study. The mean annual range of precipitation (MARP = MPwet–MPdry) is also calculated.
Here, we are using 17 taxa from the mid-late Eocene palynoassemblage for which climate data are available. This exceeds the minimum number of taxa required for a reliable application of the CA (Mosbrugger & Utescher, 1997).
RESULTS
A total of 54 pollen and non-pollen palynomorphs are recovered from the lignite sample, among which 42 species belong to angiosperm (26 dicots and 16 monocots), while the non-pollen palynomorphs are represented by six pteridophyte and six fungal morphs (Table 1).
Recovered pollen and non-pollen palynomorphs from the Eocene lignite from PGD-6 bore core, western Bengal Basin, India. Modern generic names of the identified palynotaxa and their habitat/environment are given following Sarkar (1990), Stuchlik (1994) and Traverse (1988).
Angiosperm pollens
The recovered pollen grains fall under 16 dicot and four monocot families. The palynoassemblage is dominated by Dipterocarpaceae (27%) and followed by Arecaceae (18%). We observed five different pollen types having similarity with three modern dipterocarpaceous genera. While Albertipollenites cutchensis, Foveotricolpites alveolatus and Dipterocarpuspollenites retipiletus show remarkable similarity with modern Dipterocarpus indicus pollen, Intrareticulites brevis is found to be similar with the pollen grains of modern Shorea. Similarly, Tricolpites reticulatus is significantly similar with modern Hopea pollens. The members of Dipterocarpaceae not only dominate the palaeoflora in diversity but also in abundance. The other dry deciduous elements of the palynoassemblage include Madhuca (Sapotaceae), Tectona (Lamiaceae), Diospyros (Ebenaceae) and Terminalia (Combretaceae). Arecaceae is the most dominant (18%) family among the monocots, having affinity with the modern genera Arenga, Cocos, Phoenix, Nypa, Calamus, and so on (Table 1, Plate 1, Figures 2 and 3).
Frequency distribution of the recovered palynomorphs from the mid-late Eocene Bengal lignite.
Frequency distribution of the palynomorph families from the mid-late Eocene Bengal lignite.
Non-pollen palynomorphs (NPPs)
The pteridophytic spores and fungal morphs occupy 9% and 7% of the assemblage respectively. Among the pteridophytic spores, Polypodiaceae members (3%) dominate the assemblage, while fungal spores having affinity with modern Fungi Imperfecti are found most frequently (5.5%) (Table 1, Plate 2, Figures 2 and 3).
Age of the lignite deposit
According to Ghosh et al. (1963), Verma and Rawat (1963) and Singh et al. (2014) significant presence of polycolpates (Retistephanocolpites, Pseudonothophagidites, Polycolpites) and diporates (Psilodiporites) are an indicator of the Eocene age, and these are also found to occur in the present palynoassemblage. Earlier, Baksi (1971) worked in the deeper basin area from Eocene to Plio-Pleistocene time and proposed seven palynoassemblage zones and his palynoassemblage zone III of mid-late Eocene age was characterised by the occurrence of ornamented tricolpate & tricolporate (Polycolpites & Polycolporites), Caesalpiniaceae and allied groups, ornamented Triporites, Palmaepites, Araceaepites, Santalaceaepites, Olacaceaepites, Araliaceoipites. Comparing our study with Baski’s, we found that due to the frequent occurrence of polycolpates (Pedaliaceae), tricolporates (Fabaceae, Araliaceae), triporates (Betulaceae), Araceae and palm pollen grains, we suggest the same age, that is, mid-late Eocene, for the presently studied lignite palynoassemblage.
Climate analysis
The reconstructed CIs for this palynoassemblage range between 26.8°C and 27.2°C for MAT and 1842–3151 mm for MAP, 22.2°C–26.1°C for CMT and 27.3°C–28.1°C for WMT, 346–389 mm for MPwet and 3–69 mm for MPdry. The CIs for MAT, MAP, CMT and MPwet show Nyssaceae as an outlier (Table 2, Figures 4, 5 and 6).
The coexistence intervals of MAT and MAP of the palynoassemblage (vertical red lines indicate common range for all the NLRs). Nyssaceae is an outlier.
The coexistence intervals of CMT and WMT of the palynoassemblage (vertical red lines indicate common range for all the NLRs). Nyssaceae is an outlier.
The coexistence intervals of MPwet and MPdry of the palynoassemblage (vertical red lines indicate common range for all the NLRs). Nyssaceae is an outlier.
Climate parameters during mid-late Eocene and present day in the western Bengal Basin.
DISCUSSION
The early to middle Eocene beds of western India show a significant presence of marine forms such as dinocysts and foraminifera (Dutta et al., 2011a, b; Khanolkar & Sharma, 2019; Rao et al., 2013; Singh et al., 2014, 2021; Uddandam et al., 2023). They are also found present in the mid-late Eocene sediments from the deeper basinal part of the Bengal Basin (Baksi, 1971). Interestingly, no such marine forms are found present in the middle to late Eocene lignite from the western margin of Bengal Basin (Tables 1 and 3), possibly due to the long distance from the ancient coastline during deposition in the area.
Vegetation composition in the western Bengal Basin, West Bengal, India, through ages.
The family Dipterocarpaceae has been an important element of the palaeovegetation of India through the ages. It is believed to have originated during the mid-Cretaceous in tropical Africa and subsequently dispersed to India during the late Maastrichtian-Palaeocene (Bansal et al., 2022). They are well-known in both the Palaeogene and Neogene fossil floras of Southeast Asia and Africa (Bancroft, 1935; Feng et al., 2013; Gregory et al., 2009; Joshi & Mehrotra, 2007; Khan et al., 2016 and references therein). Several dipterocarp mega and microfossil records from various regions of Asia (Bera et al., 2016; Biswas et al., 2019; Dutta et al., 2011b) reveal that a remarkable species richness of this family existed in the Asian palaeoflora since the Late Palaeocene. Subsequently, Khan et al. (2020) reported the occurrence of a dipterocarp leaf fossil from the latest Maastrichtian (Late Cretaceous) to earliest Danian (early Palaeocene) sediments of Deccan Intertrappean beds of Central India. Palynological analysis of early Eocene lignite sediments from Matanomadh, Kutch Basin and Surat (Gujarat, India), early Eocene sediments from Garo Hills, Meghalaya, and middle Eocene lignite sediments in Kutch Basin show significant occurrence of dipterocarp pollen grains (Dutta et al., 2011a; Khanolkar & Sharma, 2019; Rao et al., 2013; Singh et al., 2014, 2021; Uddandam et al., 2023).
Dipterocarpaceous fossil woods were also recovered from Vastan and Tadakeswar lignite mines (early Eocene), Gujarat, India (Rust et al., 2010) and late middle Eocene Pondaung Formation, Myanmar (Litch et al., 2014). Earlier, Bera et al. (2016), by analysing the terpenoid content of the resin droplets embedded in the same Eocene lignite sediments collected from the PGD-6 bore core of the present study, revealed their dipterocarpaceous source.
Furthermore, frequent occurrence of petrified dipterocarp woods from the Neogene sediments of the western Bengal Basin, India, suggests that the dipterocarps were also dominant elements of the Neogene forests of the Bengal Basin (Bera & Banerjee, 2001; Biswas et al., 2019; Roy & Ghosh, 1981). Besides, the Neogene beds of Bengal Basin harboured a number of other tropical dry deciduous elements (Bera & Banerjee, 2001; Ghosh et al., 2021; Roy & Ghosh, 1981; Sen et al., 2012) (Table 3).
The modern Dipterocarpaceae family is an important component of the lowland tropics and sub-tropics of south-east Asia, which are found to occur in the south-east Asian seasonally dry tropical forests (SASDTF) (Hamilton et al., 2019; Olson et al., 2001). The present study area now falls under the Burdwan Forest Division, identified as ‘Rarh’ Bengal, which is bordered by the Ajay river in the north and the Damodar river in the south, covering ~15,925.52 ha forest area (Dutta et al., 2020). Presently, a Dry Peninsular Sal forest (having Shorea robusta, Tectona grandis, Terminalia spp., Madhuca latifolia, etc.), Northern Dry Mixed Deciduous forest (with Diospyros melanoxylon, Schleichera oleosa, Boswellia serrata, etc.), Dry Deciduous scrub (with Terminalia arjuna, Butea monosperma, etc.) are found to grow in this Tropical Wet-Dry Savanna Region (Champion & Seth, 1968; Chorley et al., 1984; Dutta et al., 2020; Roy et al., 2015) in and around the bore core location (Figure 1).
In the present study, the CIs of different climatic parameters show that the mid-late Eocene was slightly warmer throughout the year than the present, with slightly lower summer temperature and higher winter temperature in the western margin of the Bengal Basin (Table 2). Previously, Ghosh et al. (2021) postulated that during the late Miocene-early Pliocene, the area experienced lower MAT and summer temperature than today, but the winters were warmer than the present day. Subsequently, the MAT, summer and winter temperatures rose during the Pliocene to early Pleistocene in the area (Ghosh et al., 2021).
Furthermore, a considerably higher value of MARP resulting from the significant difference between MPwet and MPdry values points towards a clear-cut seasonality of precipitation during the mid-late Eocene. While the MAP was higher than now during the mid-late Eocene, a decreasing trend of MAP was observed during the late Miocene-early Pliocene to the present time (Ghosh et al., 2021). Thus, a high MAP during the mid-late Eocene might have favoured the growth of the moist deciduous elements in the area of deposition and perhaps, the wet deciduous forest started to transform towards a dry deciduous type in the western margin of Bengal Basin.
In the rich angiosperm-dominated palynoassemblage of the Eocene Bengal lignite deposit, the Dipterocarpaceae members are found to be most diverse and abundantly occurring. Other dry deciduous forest elements such as Tectona grandis, Diospyros, Madhuca and Terminalia were also found present in the Eocene palynoassemblage. Furthermore, a continuum of tropical forest containing both dry and wet deciduous and evergreen elements, including dipterocarps and associates, was found to occur in the area of deposition from Palaeogene, through Neogene up to the modern times. Thus, we suggest that the present-day dry deciduous forest in the western part of West Bengal might be a modified form of an ancient dipterocarp-dominated forest that existed during the mid-late Eocene time.
CONCLUSIONS
The palynoassemblage analysis reveals the significant occurrence of dipterocarps along with other tropical dry deciduous elements in the western Bengal Basin during the mid-late Eocene. The warmer temperature regime and high precipitation might have favoured the development of a tropical wet deciduous forest in the area during the time of deposition. With the gradual decrease in precipitation, this ancient forest may have modified into a modern-day dry deciduous forest in the western part of West Bengal.
Footnotes
Acknowledgements
This work is dedicated to the Late Dr Ruby Ghosh (Scientist E, Birbal Sahni Institute of Palaeosciences, Lucknow, India). The authors acknowledge the UGC-CAS VII, Department of Botany, University of Calcutta, DST-FIST, DBT-BUILDER for the necessary facilities and Mr S.K. Bandopadhyay, Ex-Director, Geological Survey of India, for providing the lignite specimen. The authors are indebted to PD Dr Angela A. Bruch (Senckenberg Research Institute, Frankfurt, Germany) for her expert guidance in the Coexistence Approach.
Authors’ contribution
S. Basak and S. Bera conceived the idea. S. Basak and D.K. Paruya collected and analysed the data. S. Basak led the writing, and S. Bera supervised the overall work.
Data Availability Statement
The datasets analysed during the work are available in The Palaeoflora Database - Documentation and Data (Version 2024) [Data set]
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
The authors received no financial support for the research, authorship and/or publication of this article.
