Petrographic characteristics and paleoenvironmental history of Eocene lignites of Cambay basin, Western India

Lignite samples collected from Vastan, Rajpardi and Tadkeshwar lignite mines of Cambay basin (Gujarat) were subjected to organic petrographic investigations and geochemical analyses and the data, thus generated, is used to reconstruct the paleodepositional history of these lignite sequences. The lignites of Cambay basin dominantly comprise huminite maceral group (71.6%–86.3%) followed by liptinite (10.1%–19.3%) and inertinite (3.6%–11.0%) maceral groups. The mineral matter varies from 9.0% to 20.0%. The petrography based facies model indicates that these lignites have high values of gelification index (GI) and low tissue preservation index revealing a continuous wet condition in the basin and a relatively slower rate of subsidence during the decay of organic matter. On several occasions, during the formation of seams in Tadkeshwar, Rajpardi and Vastan mines, the value of GI exceeded 10 which indicates a forest permanently flooded and the cause of pronounced degree of degradation. However, few sections in Tadkeshwar seam had relatively drier spells of environmental conditions due to fluctuation in the water table as revealed by moderately high content of inertinite macerals. This is specially indicated by the occurrence of funginite which normally thrives in the upper oxygenated peatigenic layer and indicates prevalence of oxic conditions during plant deposition. Such conditions prevailed during a transgressive phase but there were intermittent fluvial activities also giving rise to supratidal flood plain as reflected in the form of associated carbonaceous shales in the basin.


Introduction
In Gujarat, the lignite deposits are found mainly in Cambay Basin (Tadkeshwar, Rajpardi and Vastan lignite mines), Kachchh Basin (Panandhro and Matanomadh lignite mines) and Saurashtra Basin (Bhavnagar lignite mine). Cambay Basin is located in the northwest part of the Indian craton, covering an area of 59,000 km 2 , with a length of 425 and 13 km wide in the central part. It extends from 21°to 25°N latitudes and 71°15 0 to 73°30 0 E longitudes in the state of Gujarat and partly in Rajasthan (Pandey et al. 1993). The lignite seams in this region lie unconformable on the weathered trap on lithomargic clay. 1 There is almost 200 m thick muddy succession in Vastan lignite field which has been differentiated into three stratigraphic units-succession-A, succession-B and succession-C. They directly lie over the Deccan traps. The 45 m thick succession-A is the lignite bearing and consists of greenish grey shales, carbonaceous shales and shell carbonates apart from lignite (Garg et al. 2008). Mc Cann (2010) considers the shell rich beds (carbonate facies) as Chenier ridge. Further, he believes that the carbonaceous shale litho-facies and the greenish grey shale litho-facies are part of mudstone facies. Prasad et al. (2013) found the carbonaceous shale litho-facies to be associated with the lignite litho-facies and linked them to marsh land; and considered the greenish grey shale facies formed under bay environment. They have shown that successions B and C essentially comprise of mud which are deposited in supratidal mudflats under dry condition.
Geologically, these deposits belong to Eocene age (Saxena 1979;Singh and Singh 2005;Balasubramanyan 2006;Sahni et al. 2006;Singh et al. 2010aSingh et al. , b, 2012aSingh et al. , b, 2016a. Lignites, along with mud rich sediments and siderites from Rajpardi, Vastan, and Tadkeshwar, were deposited in a predominantly low-energy, near-shore/coastal setting as confirmed by McCann (2010). Presence of shales along with lignite seams also indicates intermittent fluvial activities (Singh et al. 2010a, b;Prasad et al. 2013). Home et al. (1978) have discussed the cyclic occurrence of coal and marine and nonmarine biota successively in the younger beds. It would be interesting to see the position of Cambay basin closer to equator during the Early Eocene period. The northward drift of Indian Plate during Cretaceous to Eocene period is shown in Fig. 1a. Considerable work on sedimentation, stratigraphy, structures and depositional environments of the Tertiary sediments has been carried out by ONGC in Surat and Bharuch districts. Significant contributions, in this respect, have also been made by Rao (1969), Sudhakar and Basu (1973), Dutta et al. (2011). Some important references on the study of coals and lignites of Gujarat include those of Saxena (1979); Pareek (1981); Singh andSingh (2003, 2005); Sahni et al. 2006;Joshi (2007); Singh et al. (2010a); Thakur et al. (2010); Dutta et al. (2011);Singh (2012) and Rao et al. (2013). Studies carried out so far have shown that a significant Tertiary hydrocarbon belt extends through the Sanchor-Mehsana-Ahmedabad-Tarapur-Broach-Surat regions of Gujarat which continues further to the Barmer region of Rajasthan (Mathur et al. 1968;Biswas 1987Biswas , 1992. Nevertheless, further study is required to understand the hydrocarbon potential of this region and its controls.
Lignite, when exposed to the atmosphere develops cracks and splits into small pieces due to moisture loss. It contains resin patches, pyrite and minerals such as marcasite, siderite and chalcopyrite. The focus of the present research work encompasses the lignite deposits of the Cambay basin of Gujarat in a view to understand the petrological and geochemical attributes of the lignite deposits of Gujarat, because very little petrological data exists about these lignites as compared to the wealth of literature available on Indian Gondwana coals.

Geology of Cambay basin
The Cambay Basin, situated in the northwestern part of Indian craton, is a narrow elongated rift-graben which came in to existence as a result of rifting in the early Paleocene. Saurashtra craton lies on the west of Cambay Basin while Aravalli is located in the northeast, and Deccan lies in the southeast. It extends from a little north of Sanchor to the south in Cambay Gulf and ultimately opens into Arabian Sea. On the west and east, it is bounded by basin margin faults. It extends upto Viramgram and Gogha in the west while Himmatnagar-Baroda and Rajpipla lie near its eastern margin (Pandey et al. 1993). Based on the major basement fault, Cambay Basin is sub-divided into five blocks (Mathur et al. 1968) which are listed from north to south as Sanchor Patan Block, Mehsana-Ahmedabad Block, Tarapur-Cambay Block, Jambusar-Broach Block, and Narmada Tapti Block.
The Quaternary sediments cover the major part of the Cambay basin. The beddings generally trend towards N-E and S-W with gentle dip. Tertiary sediments are not profound in this basin but occur in Gogha-Bhavnagar and Jhagadia-Tarkeswar areas. The Tertiary rocks in this region consist of sandstone, conglomerate, clay limestone, lignite ranging in age from early Eocene to early Miocene. The major litho units include Babaguru Formation underlain by the Tadkeshwar Formation which, in turn, is underlain by Nummulitic Formation. The Tadkeshwar Formation comprises the lignites in form of lenses having variable thickness and lateral extent. The lignite producing mines of Cambay Basin are mainly located in the southern part of Narmada-Tapti block. The younger Cambay Shale/Tadkeshwar Formation comprises the lignite bearing horizons and the shales are good source rock for oil and gas in the basin. The lignite seams are often associated with greenish grey shales, carbonaceous shales and shell-carbonates (Garg et al. 2008). At the bottom lies the grey clay-bed, which is continuous with varying thickness and is overlain by carbonaceous clay-bed. Sand lenses, sandstone, carbonaceous clay and lignite are reported to occur embedded in grey clay. The carbonaceous clay bed is conformably overlain by a thick lignite seam, which is a marker bed. The lignite seam is soft, friable and brown containing yellow resin and pockets of clay. The lignite seam is conformably succeeded by upper carbonaceous clay. The grey, plastic, bentonitic clay lies over upper carbonaceous clay (GMDC 1989). There is an increase in the proportion of sand towards the upper portion of the bentonitic clay and gradually converts into sandy clay. Two unconformities are reported in the area, one between the Traps and Vagadkhol Formation, and the other between the lower clays and upper sands of the Tadkeshwar Formation. Borehole data indicates the presence of two persistent lignite seams designated respectively as top (0.15-8.8 m thick) and bottom (1.0-3.4 m thick) seams/horizons. In addition to them, a few non mineable and uneconomic local seams also occur in this area. The sub-surface occurrence of coal/lignite in Gujarat extends from Tharad in the North to Mahisagar Fig.1 a Shifting of Indian plate during last 71 million years (left); location of India during Eocene period (right); after Chatterjee and Bajpai (2016). b Geological map of Cambay Basin and location of lignite fields (redrawn from, GSI 2012) River in the South. Along the Saurashtra coast and to the east of the Gulf of Cambay, the basin spreads as a thin strip comprising 75-150 m thick beds of greenish and whitish grey and black clay/shales with lignite seams. The Cambay lignites have been studied by Bajpai et al. (2005), Singh (2012), Singh et al. (2010aSingh et al. ( , 2012a and Prasad et al. (2013). The geological map of Cambay basin is shown in Fig. 1b and the details of the geological succession of the basin are given in Tables 1, 2 and 3.

Method of study
In the present investigation, the lignite samples were collected from all the working mines of Cambay Basin (Tadkeshwar, Rajpardi and Vastan) of Gujarat (Fig. 1b). Every precaution was taken to collect fresh coal samples from the various sections in stratigraphic order. The coal sampling was done following the Pillar Sampling Method. The vertical seam profile was reconstructed and megascopic variations in the coals were studied. For petrographic studies and geochemical analysis the coal samples, were crushed to -18 mesh size and -70 mesh size respectively. For microscopic examination, the lignite samples were crushed and particulate mounts were polished. The maceral analysis was performed on the polished mounts under reflected light using a Leitz Orthoplan-Pol Microscope equipped with Wild Photoautomat MPS 45 in the Coal and Organic Petrology Laboratory, Department of Geology, Banaras Hindu University. The line-to-line and point-topoint spacing was maintained at 0.4 mm and more than 600 counts were taken on each sample. The methodology given by Taylor et al. (1998) was adopted; huminite macerals were termed and described as per ICCP-1994 (Sýkorová et al. 2005), liptinite macerals have been identified as per  ICCP system-1994 (Pickel et al. 2017) while ICCP (2001 was followed for the nomenclature of inertinite macerals. In the present study, the indices, gelification index (GI) and tissue preservation index (TPI), have been taken from Flores (2002) which is a modified version of Kalkreuth et al. (1991), for brown coals and the calculations are as per the following formulae: The micro and mega-petrographic profiles were constructed for various seams in order to understand the variation in the nature and quality of lignite from bottom to top of different coal seams. The megascopic characters and group maceral composition of these lignites have been used by Singh et al. (2016a, b) as provided in Tables 4, 5 and 6 while discussing the geochemistry of selected trace elements of these lignites. The set of data has also been used here for discussing their paleomire. The reflectance measurement has been performed on huminite macerals especially ulminite grains. Help has been taken from Jennifer et al. (2013) for rank study. The proximate analysis was carried out as per BIS 2003.

Result and discussion
Macroscopically, the lignites of Cambay basin are generally unstratified, matrix rich and inhomogeneous in nature as per ICCP (1993). One xylite rich band of 8-10 cm was observed in Rajpardi seam. Dominantly, the bands Humi-huminite, lipt-liptinite, inert-inertinite were of brown colour and the details are discussed by Singh et al. (2016a). Megascopic characteristics of the lignite seams of Cambay basin along with their petrographic composition (maceral groups), mineral matter (pyrite and other minerals) and ash yield are provided in Tables 4, 5 and 6. Pyrite and resin patches could be seen in most of the lithotype bands. Though pyrite is the main mineral present in most of the analysed samples but there is no definite pattern of variation from bottom to the top of lignite seams. Litho column showing various rock units in Cambay Basin along with lignite seam profiles at Vastan, Tadkeshwar and Rajpardi is shown in Fig. 2 while the vertical variation of petrographic components is shown in Fig. 3.

Petrography
The quantitative distribution of individual macerals in the Cambay lignites is provided in Tables 7, 8 and 9. In these lignites, huminite is the dominant maceral group which ranges from 59.3% to 77.0% with a mean of 68.7% (71.6%-86.3%, mean 77.92% on mmf basis) and comprises mainly of detrohuminite followed by telo-huminite and gelo-huminite respectively. Liptinite group is second in dominance and varies from 10.8% to 16.9%, mean 13.3% (10.1%-19.3%, mean 14.8% on mmf basis) and constitutes mainly of sporinite, resinite, liptodetrinite while other macerals of this group occur in small concentration.
Inertinite, which ranges from 5.1% to 9.9% (mean 7.9%) (3.6%-11.0%, mean 7.4% on mmf basis), is contributed chiefly by funginite, inertodetrinite and fusinite whereas other macerals of this group occur in very low concentration. Mineral matter ranges from 9.0% to 20.0%, mean 11.8%. There is no definite trend of variation of these petrographic constituents from bottom to top of the lignite seams.

Paleomires
The qualitative and quantitative petrographic data has been used to discuss the evolutionary history of the lignites of Cambay of Gujarat in this section. For this purpose, the data has been generated on the lignite samples collected from all the working mines of the Cambay (Vastan, Rajpardi and Tadkeshwar mines). The generated data is useful in understanding the coal facies, nature of flora, and depositional controls on the development of peat.
It is important to understand the variations in the petrographic constituents of Gujarat lignites along the lignite seam profiles, from bottom to top, in order to know their evolutionary history. In the Cambay basin, brown bands are more prevalent and are mostly unstratified and contain high matrix while only few black, thin bands (V-5, VL-6, R-1, and T-7; Tables 4, 5, 6; Fig. 2) are seen. The black bands in these lignites, are relatively more strongly gelified compared to the brownish bands. More gelification could be caused as a result of anaerobic processes (Iordanidis and Georgakopoulos 2003). Various shades of darkness could also result due to different degrees of plant decomposition (Hagemann and Wolf (1987). The black bands have very high detro-huminite content with little bit less amount of telo-huminite content. Since macerals directly relate to the plants and environment, they provide useful guide to characterize the paleomire. Certain macerals are indicative of paleo-depositional environment, through their presence or absence, (Teichmüller 1989). The characteristics imparted to coal differ with variation in the peat forming plant communities, nutrient supply, temperature, pH, bacterial activity, redox potential and types of deposition (Teichmüller et al. 1998a, b, c;Lin and Tian 2011). Increase in the clastic mineral matter relates to the surface inundation in the basin (Singh and Singh 1996). Petrography based facies models have been used to discuss the evolutionary history of the paleomires of the Cambay basin. Using petrological tools a number of researchers have successfully performed paleoecological reconstructions and contributions have been made by Cohen and Spackman (1972), Styan and Bustin (1983), Cohen et al.  Deng andSun (2011), andSingh et al. (2012a, b, c, d). Detro-huminite is throughout high in the lignites of Cambay basin wherein densinite is more consistent. With decrease in the densinite content, there is concomitant increase in the respective attrinite content. Detro-huminite is normally generated in relatively aerobic condition (Teichmüller et al. 1998b). Moreover, the activities of physical breakdown of plant to particulate matrix are observed at the margin of the peat beds (Kuder et al. 1998). To characterize the paleomires of the Australian Permian coals, Diessel (1986) introduced two indices, gelification index (GI) and tissue preservation index (TPI). Less humified structured and strongly humified unstructured tissue derived macerals indicate the degree of humification and the vegetation type (Diessel 1992). Accordingly, a high TPI value suggests a high subsidence rate of basin and dominance of wood derived tissues, and a low TPI suggests a low subsidence rate with high humification. The degree of gelification of huminite macerals is revealed by GI which distinguishes between gelified and ungelified macerals. For gelification to take place a continuous presence of water is necessary and fluctuation in water table is likely to affect the degree of gelification. However, these indices should be calculated and used carefully. Researchers like Calder (1993) and Collinson and Scott (1987) believe that the palynological and paleobotanical data provide higher precision in the determination of paleoenvironment. Several researchers have objected the usage of such indices for Tertiary or low   T1   T2   T3   T4   T5   T6   T7   T8   T9   T10   TL1   TL2   TL3   TL4   TL5   TL6   TL7 Ulminite ( rank coals (Lambersen et al. 1991;Crosdale 1993;Dehmer 1995;Wust et al. 2001;Scott 2000;Moore and Shearer 2003;Amijaya and Littke 2005). Few researchers have tried a combination of petrographic, organic geochemical and/or isotope data (Bechtel et al. 2002(Bechtel et al. , 2003(Bechtel et al. , 2007a(Bechtel et al. , b, 2008Mayr et al. 2009;Savard et al. 2012;Singh et al. 2012c) for the reconstruction of paleoenvironment. Kalkreuth et al. (1991), Petersen (1993), and Flores (2002) further modified these indices to make them useful for the low rank coals too.
In the present investigation, the indices have been taken from Flores (2002) which is a modified version of Kalkreuth et al. (1991), for brown coals.
The lignites of Cambay basin are characterized by high GI and low TPI values indicating a continuous wet condition in the basin and a relatively slow rate of subsidence during the decay of organic matter. Few sections in Tadkeshwar seam suffered relatively drier spells as revealed by moderately high content of inertinite macerals. This is especially revealed by the occurrence of funginite which normally thrives in the upper oxygenated peatigenic layer and indicates prevalence of oxic conditions. This indicates fluctuation in the water cover in the peat swamp. Hoekel (1989) believes that intensive gelification of plant tissues is a function of acid ground water due to marine influence.         (Iordanidis and Georgakopoulos 2003). The formation of Vastan lower seam began and continued further under a limno-telmatic environment and during the formation of upper part of this seam, there was a shifting towards telmatic environment for a shorter period as indicated by a sudden increase in the TPI values and again there was a shifting to a limno-telmatic environment. Similarly, the formation of Vastan upper seam also began under limno-telmatic environment; it shifted to telmatic environment for a relatively shorter period, as indicated by its thin band, and then returned to limno-telmatic condition which persisted for most part of the time. However, during the formation of upper part of this seam, there was again a small shift to telmatic environment but soon it returned back to limno-telmatic condition. Similarly, the formation of Tadkeshwar Lower seam as well as Tadkeshwar Upper seam formed under limnotelmatic to telmatic condition while Rajpardi seam evolved under limno-telmatic environment where the values of TPI are \1. This is also in concurrence with the study of Jasper et al. (2010). There exists a strong sympathetic correlation between ash content and sulphur content (r = 0.92 in Rajpardi; r = 0.91 in Vastan; r = 0.88 in Tadkeshwar) supporting the above view that clastic sedimentation has increased the tree density and tissue preservation in the Cambay Basin. This is also evident from a low to moderate but positive correlation between TPI and clastic minerals (r = 0.42 in Tadkeshwar, r = 0.17 in Vastan, and r = 0.10 in Rajpardi seams). A similar observation was also noticed in Nagaland coals of north-eastern India (Singh et al. 2012b). However, a low but negative correlation value exists between GI and TPI values (r = -0.09 for Tadkeshwar lignite, and r = -0.02 for Vastan lignite of Cambay Basin). Possibility of a eustatic rise of 70-140 m in the sea level during Early Paleogene period, as compared to the present level, is discussed by Haq et al. (1987). This is indicated by global transgressions during 58.5-52.8 Ma. The signatures are seen in the Late Paleocene-Early Eocene sedimentary  Diessel (1992) discusses that a wet forest may also produce a peat having similar petrographic composition as that of marsh peats provided a favourable condition prevails in the basin for microbial destruction. Variation of GI, TPI, GWI and VI values with depth in the lignite seams of Cambay Basin is shown in Fig. 7.
A ternary model proposed by Singh et al. (2012a), initially proposed for understanding the paleomires of the Eocene lignite deposit of Rajpardi, has been used in the present investigation. This model is based on maceral composition as well as clastic mineral content. Since the clastic minerals relate directly to the water cover in the basin, it provides a better understanding related to the fluctuations of the water cover in the basin. The data plots of Cambay lignites on this model (Fig. 8) indicate their origin in wet moor with moderate to high flooding where the tissue preservation was moderate. The results obtained through the petrography-based ternary models compare well with the ones shown by GI, TPI, GWI and VI indices. The presence of framboidal pyrite indicates increasing activity of sulfate-reducing bacteria in the carbonate and sulphate rich waters of the basin at the time of peat formation (Kuder et al. 1998;Teichmüller et al. 1998a). Bacterial activity is the source of energy for such chemical reduction from sulphate to sulfides (Neavel 1966). Under these situations adsorption of iron is seen on clays and pyrite is found adjacent to clay zones (Cabrera et al. 1995). Presence of clay rich layers in the lignites of Cambay basin is confirmed by the X-ray diffraction study showing marine influence in this basin where the pyrite framboids formed through bacterial activity. The petrography based facies models used in the present investigation for the reconstruction of the paeoenvironment of Cambay basin points to a coastal marshy setting for the evolution of these Coal facies determined from Gelification Index (GI) and the Tissue Preservation Index (TPI) in relation to depositional setting and type of mire for the lignites of Cambay Basin, Gujarat (modified from Diessel 1986) lignites. This environmental condition prevailed during transgressive phase, but there were intermittent fluvial activities giving rise to supratidal flood plain as indicated by the presence of associated carbonaceous shales, shales and green shales in the basin. The marshy coal plain to bay model of Prasad et al. (2013) supports the view and also explain the cyclicity of various lithofacies. While discussing the depositional paleoenvironment of Vastan lignite sequence, Sahni et al. (2006) have also shown that there was a large supply of fresh water into the bay. According to Singh et al. (2016b), a similar depositional setting also existed during the formation of the nearly located lignite deposits of Rajasthan. Further, based on petrographic facies models, their study point out that the Barmer lignites of Rajasthan evolved under wet forest swamp to clastic marsh having telmatic to limno-telmatic condition; there prevailed a moderate rate of subsidence and a very slow fall in ground water table. This also holds well in case of Cambay basin lignites. Further, Vastan lignites and the associated sediments (Cambay Basin) have yielded rich pollen assemblage Tapaswi 2000, 2001). Marine ostracods are reported by Bhandari et al. (2005) and marine fish remains and mammalian fossils by Nolf et al. (2006), and Rana et al. (2005a, b, c). Such environment is possible as a result of sea water influx into the peat swamp owing to eustatic rise which has been evidenced by the views of Haq et al. (1987), Hardenbol et al. (1998), Sluijis et al. (2008, and Prasad et al. (2013).
(2) The lignites of Cambay basin have high GI and low TPI values indicative of a continuous wet condition in the basin and a relatively slow rate of subsidence during the decay of organic matter. Few sections in Tadkeshwar seam suffered relatively drier spells as revealed by moderately high content of inertinite macerals as indicated by the occurrence of funginite which normally thrives in the upper oxygenated peatigenic layer and indicates prevalence of oxic conditions during plant deposition. GI value [10 indicates a forest permanently flooded and pronounced degree of degradation which happened on several occasions during the formation of seams in Tadkeshwar, Rajpardi and Vastan seams of the Cambay basin. The petrography based facies models indicate the prevalence of such environmental condition during transgressive phase. However, there existed intermittent fluvial activities giving rise to supratidal flood plain which are reflected in the form of associated carbonaceous shales in the basin.
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