Understanding the paleomires of Eocene lignites of Kachchh Basin, Gujarat (Western India): petrological implications

The present paper entails the results of the investigations carried out on the lignite deposits of Kachchh Basin. The lignite samples were drawn from five lignite seams from Panandhro lignite field (Seam-I to -V) and eight lignite seams (Seam-I to -VIII) from the Matanomadh lignite field which are currently operational. The petrographic analysis of the lignites indicates a dominance of huminite group of macerals which is mainly contributed by ulminite-A, ulminite-B, attrinite, densinite, and phlobaphinite. Liptinite (chiefly sporinite, cutinite, resinite, and liptodetrinite) and inertinite (chiefly fusinite, funginite, and inertodetrinite) groups occur in subordinated amount. The mineral matter occurs in moderate concentration. Though sulfur content is high in these lignites, there is no fixed trend of variation of sulfur from bottom seam to top seam. The investigation reveals a flooded forest swamp having high rate of degradation. However, there were a few drier periods indicated by relatively more inertinite macerals. The petrography-based models indicate that the Kachchh lignites of Gujarat evolved in coastal marshy setting under transgressive phase. However, there were few intermittent fluvial activities giving rise to supratidal flood plain. This led to the formation of the associated carbonaceous shales in the basin.


Introduction
Kachchh Basin, between latitudes 22°30 0 -24°30 0 N, and longitudes 68°-72°E, covers an area of about 35,000 km 2 inland and 36,000 km 2 offshore in western India. The Great Rann on the north and east, Little Rann on the southeast, Gulf of Kachchh on the south, and the rest by the Arabian Sea, flank the Kachchh mainland. The name 'Kachchh' is given because of its tortoise-like morphology, where the central portion of this area forms a table-land sloping on all sides. There are three hill ranges, trending east-west; the Banni is formed by the sediments deposited from northern border of the mainland and is covered by soil. This area receives fluctuating rainfall (up to 88 cm) (Merh 1995). The Rann is a dry-bed, saline desert throughout the year except during the monsoon season, which formally connects the Narmada rift with Sind and separates Kachchh from the mainland. The Rann is divided into two areas, Great Rann and Little Rann, which are covered by a layer of salt and a thin layer of fine clays (Fig. 1a). The Cenozoic rocks unconformably overlie the Deccan basalt and Mesozoic rocks with an aggregate thickness of around 300 m (Biswas 1992;Sarkar et al. 1996) and contain huge lignite deposits including those of Matanomadh. Based on the foraminiferal assemblage, the age of lignite is suggested as early Eocene to early-middle Eocene (Dutta et al. 2011). The lignite deposits of Kachchh occur at Panandhro, Matanomadh, Umarsar, Akrimota, Lefri, and Lakhpat-Dhedhadi in Lakhpat Taluka (IBM 2013). The Panandhro lignite field is located in Kachchh district of Gujarat, and is surrounded by villages such as Panandhro, Fulra, and Khanot. The lignite fields are bounded within latitude 23°45 0 56 00 N and longitude 68°45 0 00 00 E (Survey of India Toposheet No. 41 F/14). It is the third largest lignite deposits of India, with reserves of 95-100 Mt. Matanomadh lignite mine is spread over an area of 1314 h with a total resource of 3.6 Mt. This lignite field lies between latitudes 23°29 0 00 00 -23°32 0 00 00 N and longitudes 68°56 0 00 00 -68°59 0 00 00 E. In the present paper, petrological and geochemical characteristics of Kachchh lignites are presented, and based on that, their evolution is discussed.
whereas the Tertiary sediments are of a wide variety and include shallow marine-shelf sediments in the peripheral and intervening structural lows, bordering the Mesozoic uplift areas. The Quaternary sediments are mostly marineto fluvial, eolian, and lacustrine in nature. The Kachchh Mainland exposes the major stratigraphic succession and the outcrops of Late Tertiary and Quaternary sediments are seen at the border of the lower part of the Mesozoic sequences. Sedimentological evidences indicate that Late Triassic, Jurassic, and Early Cretaceous sediments were deposited in early syn-and post-rift stages respectively (Biswas 2005). The Naredi Formation is the lignite-bearing formation. The strata including the lignite seams has a dip towards the centre of the basin. The basin appears to be a syncline with its axis in the south-south west to north-east. In Panandhro field, the border of the basin is well-marked by steep escarpments. It is elongated in NW-SE direction with a width of 3 km along the southern fringe and tapers down to 600 m towards the northern end. The lignitebearing sedimentary succession of Matanomadh mine comprises nine lignite seams with individual seam thicknesses ranging from 0.15 to 4.88 m. However, in Panandhro mine 13 lignite seams have been reported varying in thickness from 0.10 to 10 m. The lignite encountered is dark brown in color; compact; uniform in texture; amorphous in nature; and impregnated with resin, pyrite, and marcasite. The seams are impersistent, contaminated, and exhibit splitting tendency (GSI 2012). The details of the geological succession of Kachchh Basin are given in Table 1, and the lithocolumn and the lignite seam profiles are given in Fig. 2 which shows various bands present in the lignite seams.

Method of study
Lignite samples were collected from five seams (Seam-I to Seam V) from Panandhro lignite field and eight seams (Seam-I to Seam VIII) from Matanomadh lignite field of Kachchh Basin which are currently being mined. The pillar samples were drawn as per Schopf (1960) in a way to represent full-seam thickness. The lignite samples were put together to form composite bands on the basis of similar megascopic characteristics and each composite band is taken as one composite sample and has been given unique sample number. The lignite samples were crushed, and reduced in quantity through quartering and coning, and subjected to various analyses. The -18-mesh samples were used for preparation of polished lignite mounts for petrography. For proximate and other chemical analyses, the samples were further reduced to -70-mesh fraction. Maceral analysis has been performed 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. White light was sourced from a 12 V/100 W halogen lamp while for fluorescence system, a Ploemopak with filter block I 2/3 having blue excitation filters (BP450-490), dinomatic mirror (RKP510) and suppression filter (LP520), was used. The line-to-line and point-to-point spacing was maintained at 0.4 mm and more than 600 counts were taken on each sample. The methodology described by Taylor et al. (1998) was adopted. Huminite macerals were termed and described as per ICCP-1994(Sýkorová et al. 2005, while ICCP (2001)

Chemical attributes
Chemical composition of these lignites is given in Table 10. The ultimate result indicates that Matanomadh lignites contain 66.9% carbon, 4.8% hydrogen, 1.0% nitrogen, and 19.4% oxygen (daf basis). The sulfur content of this seam is high and varies from 3.8% to 11.3% (mean 7.8% on daf basis). There is no definite trend of variation of these elements from Seam-I to Seam-VIII, however, Seam-V has a relatively high carbon and low nitrogen and oxygen content. The details of these elements are furnished in Table 10.
Panandhro lignite contains 66.1% carbon, 5.1% hydrogen, 1.0% nitrogen and 21.2% oxygen on daf basis. The sulfur content in these seams is high and varies from 3.7% to 11.6% on daf (mean 6.6%). There is no definite trend of variation in the distribution of these elements from Seam-I to Seam-V, though a little fluctuation is noticed.

Depositional environment
It is important to understand the variations in the distribution of the petrochemical constituents of Kachchh lignites along the lignite seam profiles from bottom to top in order to reconstruct their evolutionary history. The vertical variation of petrographic components is shown in Fig. 5.
The black bands, in Kachchh lignites, are strongly gelified compared to the brownish bands. Owing to the sensitivity of the macerals towards varied environmetal conditions, they are useful to characterize the paleomire. Teichmüller (1989) has demonstrated that the presence or absence of macerals is indicative of the paleo-depositional environment. Nevertheless, these macerals acquire various characteristics through peat-forming plant communities, nutrient supply, bacterial activity, types of deposition, temperature, pH, and redox potential (Teichmüller et al. 1998a, b;Lin and Tian 2011). Surface inundation in the basin relates to increase in the clastic mineral matter (Singh and Singh 1996). Facies models mainly based on petrographic elements have been used to discuss the evolutionary history of the paleomires of Kachchh Basin. A number of researchers (Cohen and Spackman 1972;Styan and Bustin 1983;Cohen et al. 1987;Calder et al. 1991;Grady et al. 1993;Hawke et al. 1996;Singh and Singh 1996;Shearer and Clarkson 1998;Jasper et al. 2010;Singh et al. 2010a, b;Suárez-Ruiz et al. 2012;Singh et al. 2012aSingh et al. , b, 2014 have attempted paleoecological    2013) have made a detailed discussion on 'coal rank' and 'coal type' and relation of latter to the environment of peat formation, the climate of peat formation, and the decompositional history of the mire. Systematic discussion on the paleoenvironmental interpretations and their signatures in maceral was initiated by Diessel (1982). He emphasized the presence of diagnostic macerals and proposed facies diagrams based on them. Subsequently, Diessel (1986) introduced two indices, gelification index (GI) and tissue preservation index (TPI) to characterize the paleomires of Australian Permian coals. Further, Diessel (1992) showed that less humified structured and strongly humified unstructured tissue derived macerals manifest the degree of humification and the vegetation type. High TPI values relate to a high subsidence rate of the basin and dominance of wood-derived tissues, while a low TPI value is seen as a result of a low rate of subsidence and high humification owing to the predominance of herbaceous vegetation in the peat swamp. GI values indicate the degree of gelification of huminite macerals and distinguishes gelified macerals from the ungelified ones. A continuous presence of water in the peat swamp is essential for gelification. Fluctuation in water table affects the gelification because more inertinites will form during dry periods due to increasing oxidation. Nonetheless, scientists have advised to take care while using these indices. Vertical variation of these indices along the lignite seams profiles is   Calder (1993) and Collinson and Scott (1987) believe that palynological and paleobotanical data would provide high precision for paleoenvironment. Several researchers have raised serious remarks for using these indices for low-rank coals (Lambersen et al. 1991;Crosdale 1993;Dehmer 1995;Wust et al. 2001;Scott 2002;Moore and Shearer 2003;Amijaya and Littke 2005). Several scientists have used a combined petrographic, organicgeochemical, and/or isotope data for understanding and reconstructing the environment of paleomire (Bechtel et al. 2002(Bechtel et al. , 2003Singh et al. 2013). Kalkreuth et al. (1991), Petersen, (1993), and Flores (2002 modified these indices to broaden their usage for low-rank coals. In the present study, the indices have been taken from Flores (2002) which is a modified version of Kalkreuth et al. (1991) for brown coals. The calculations are as per the following formulae: Very high GI values of more than 10 in several sections and even [100 in few sections of Kachchh Basin, indicate a permanently flooded forest swamp having high degradation. This type of environment is prevalent in limnotelmatic swamps where low to moderate subsidence rate occurs and there is a slow fall in the ground-water table. This type of marsh is recognized as treeless open-marsh and limnic-plant communities (Iordanidis and Georgakopoulos 2003). Intensive gelification of plant tissues in the SE Asian coals is also reported by Hoekel (1989) which is attributed to be a function of acid ground water due to  Liptinite Sporinite 6.0 4.7 6.1 4.9 4.4 9.6 5.9 6.3 6.4 4.7 4.9 8.3 9.5 6.6 5.8 5.    Calder et al. (1991) gave more credence to the influence of ground water and how it characterized the environment of a paleomire. They have used the ground water index (GWI) and vegetation index (VI) for the reconstruction of paleoenvironment. Mires form in rheotrophic to ombrotrophic hydrological conditions. Ombrotrophic to mesotrophic paleoenvironments have low GWI values (\1) whereas values more than 1 prevail in the rheotrophic hydrological condition. Drowning of peat is indicated by GWI values more than 5. Following formulae were used in the present study:  The GWI and VI values of investigated lignites of Kachchh Basin are suggestive of mesotrophic to rheotrophic hydrological conditions (Fig. 7), having the dominance of herbaceous to marginal aquatic vegetation. Variation of GI, TPI, GWI, and VI values with depth in the lignite seams of Kachchh Basin is shown in Fig. 5. It is evident from the figure that in Panandhro area maximum gelification occurred during the formation of Seam-II while in Matanomadh area high gelification was observed during the formation of Seams I, II, and V. Further, in Panandhro area, GWI was high during the formation of seams I, II, and V while in Matanomadh area the GWI was high during the formation of seams I and VI. Singh et al. (2012a) proposed a ternary model for understanding the paleomires of the Eocene lignite deposit of Rajpardi. This is based on maceral composition as well as clastic minerals. While the former is sensitive to varying environmental conditions, the latter directly relates to the water cover in the basin and would provide a better clue to the paleoenvironment. It is evident from the plots of Kachchh lignites, on this model, (Fig. 8) that these lignites evolved under wet moor with moderate to high flooding having variable levels of tissue preservation. Yet another facies model has been used to understand the paleomires of Kachchh lignites. Singh et al. (2010a) initially proposed this ternary facies model, based on microlithotype and carbominerite composition, for Vastan lignites located closely in the Cambay Basin in Gujarat. The plots of the lignites of Kachchh (Matanomadh and Panandhro) Basin, in this model also, point toward a wet-moor environment with moderate flooding where increasing bacterial activity prevailed (Fig. 9). The results obtained through these ternary models compare well with the ones shown by GI, TPI, GWI, and VI indices. The physical break down is prominent at the margins of peat      beds (Kuder et al. 1998). Framboidal pyrite indicates increasing activity of sulphate-reducing bacteria which thrive in carbonate-and sulphate-rich waters (Kuder et al. 1998;Teichmüller et al. 1998a). These conditions lead to adsorption of iron on clays and pyrite is seen adjacent to clay zones (Cabrera et al. 1995). Clay-rich layers in the lignite of Kachchh Basin have been confirmed by X-ray diffraction. This is indicative of marine influence in the basin which led to the formation of pyrite framboids. Haq et al. (1987) have noted that there was a eustatic rise of 70-140 in the sea level during Early Paleogene. This has also been indicated by global transgressions during 58.5-52.8 Ma (Hardenbol et al. 1998). Recently, Srivastava and Singh (2017) have discussed about the initial marine transgression during Late Paleocene sedimentation in Kachchh Basin. Prasad et al. (2013) also believe that the lignite-bearing sequences in western India evolved in consequence of extensive transgressive event. The investigation of the Kachchh lignites of Gujarat points that these lignites of Gujarat evolved under coastal marshy setting which prevailed in transgressive phase but there were intermittent fluvial activity which gave rise to supratidal

Conclusions
1. The lignites are dominantly rich in huminite group macerals with subordinate liptinite and inertinite groups. The mineral matter is moderately high. In these lignites, huminite group is mainly contributed by ulminite-A, ulminite-B, attrinite, densinite, and phlobaphinite. Sporinite, cutinite, resinite, and liptodetrinite chiefly contribute to liptinite group while fusinite, funginite, and inertodetrinite are the main inertinite macerals. 2. The sulfur content in the seams of Matanomadh and Panandhro fields is high but there is no definite trend of variation of sulfur or other elements from bottom to the top of the seam.

Very high GI values ([100 in few sections) in
Kachchh Basin, indicate a permanently flooded forest swamp having high degradation. Such environment prevails in limno-telmatic swamps where low to moderate subsidence rate occurs and there is a slow fall in the ground water table. This is supported by GWI and VI values of the investigated lignites, indicating mesotrophic to rheotrophic hydrological conditions. Nevertheless, few sections of Matanomadh and Panandhro lignites underwent spells of relatively drier periods, as indicated by relatively more inertinite macerals. Further, maximum gelification in the Panandhro field was observed during the formation of Seam-II while it was high during the formation of Seams I, II, and V in the Matanomadh field. The Panandhro field also witnessed a high GWI during the formation of seams I, II, and V while the Matanomadh field witnessed high GWI during the formation of seams I and VI. 4. The investigation of the Kachchh lignites of Gujarat indicates that the Gujarat lignites formed under a coastal marshy environment during a transgressive phase. However, there was intermittent fluvial activity, which gave rise to a supratidal flood plain which led to the formation of the associated carbonaceous shales.