Kachchh Basin, between latitudes 22°30′–24°30′N, and longitudes 68°–72°E, covers an area of about 35,000 km² inland and 36,000 km² 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′56″N and longitude 68°45′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′00″–23°32′00″N and longitudes 68°56′00″–68°59′00″E. In the present paper, petrological and geochemical characteristics of Kachchh lignites are presented, and based on that, their evolution is discussed.

Location map (a) and regional geological map (b) of northern part of Gujarat showing Kachchh Basin. (after Biswas 2005; Biswas and Raju 1973)

Regional geological map of northern part of Gujarat showing Matanomadh and Panandhro lignite mines of Kachchh Basin is shown in Fig. 1b. Kachchh Basin is a peri-cratonic rift basin located in the western continental margin of India (Biswas 1992). It came into existence in the late Triassic, during the rifting of eastern Gondwanaland, and it is mainly formed by Mesozoic and Cenozoic sediments. The basin opens towards west and it has a general slope towards west-southwest. Cenozoic outcrops of Kachchh are exposed to the west of Deccan Trap forming semi-circular outline. Kachchh Basin mainly includes a group of E–W trending ‘uplifts’ surrounded by Little Rann and plains of the Great Rann. Seven uplifts (Patcham, Khadir, Bela, Chorar, Wagad, Kachchh Mainland, and Saurashtra) occur in three sub parallel trending E–W lines. These uplifts are bounded by monoclonal flexures/faults from one side and gently dipping peripheral plains from the other side except for the Wagad uplifts. The Mesozoic strata have suffered fault-related uplifts and consequent folding. These uplifts are surrounded by the sediments of Tertiary period which form peripheral plains. These plains conceal the Mesozoic structures and the depressions occurring between them comprise the Cenozoic Sub-Basins. The Mesozoic rocks are exposed in Kachchh Mainland, Wagad, Bela, Khadir Chorar hills, and the Patcham uplifts. The Mesozoic rocks of Kachchh are divided into Upper and Lower Series. The upper series is plant bearing while the lower one has ammonites. The Mesozoic rocks are classified into a four-fold subdivision, namely, Pachcham, Chari, Katrol, and lower part of Umia. The upper non-marine Umia is named the Bhuj Formation which is of Middle Cretaceous in age (Merh 1995). A complete sequence of rock, from Triassic to Recent, is found in this basin. Biswas (1992) has noted that the Mesozoic sequence comprises Late Triassic (Rhaetic) continental, Middle to Late Jurassic marine, and Late Jurassic to Early Cretaceous fluvio-deltaic sediments. Further, Mesozoic sediments fill a major part of the basin 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 marine- to 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 lignite-bearing 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.

Litho column showing various rock units in Kachchh Basin (after Biswas 1992) along with lignite seam profiles at Matanomadh and Panandhro

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) was followed for inertinite macerals. The vitrinite/huminite reflectance measurement was carried out at the National Metallurgical Laboratory, Jamshedpur following ISO 7404-5:2009. A minimum of 200 measurements were taken on each sample.

4.1 Petrographic composition

Macroscopically, these lignites are generally stratified, matrix-rich, brown to black, inhomogeneous, and often contains patches of resin and pyrite specks. There are eight lignite seams in Matanomadh field and the top seam (Seam-I) is overlain by carbonaceous clay while Seam-VIII is underlain by dark grey carbonaceous clay. However, in Panandhro, out of the thirteen reported thin lignite seams only five lignite seams are mineable. Here, the roof of Seam-I comprises of carbonaceous shale while the floor of Seam-V comprises of dark grey carbonaceous clay. Petrographically, these lignites are dominantly comprised of huminite group macerals followed by the liptinite and inertinite groups while mineral matter is moderately high. The variation in the concentration of group macerals in various seams of Matanomadh and Panandhro lignite mines is shown in Fig. 3 while the characteristic macerals are shown in photomicrographs (Fig. 4).

Vertical variation of petrographic constituents in the lignite seams at Matanomadh and Panandhro area

Characteristic photomicrographs in the lignites of Kachchh Basin: a textinite (T); b ulminite and pyrite (Py) grains; c phlobaphinite (Ph);d densinite (D) with oxidation cracks; e densinite (D) and framboidal pyrite (Py); f densinite (D) and pyrite (Py) occurring as framboids and also in cavity; g resinite (R) and sporinite (Sp) observed under uv light; h megacutinite (Cu) as observed under uv light; i fungal bodies (F) occurring in densinitic background; j fusinite band (Fu) and densinite (D)

4.1.1 Matanomadh lignite

Maceral composition and mineral matter content in Matanomadh lignites are furnished in Tables 2, 3, 4, 5 and 6. Huminite varies from 41.8% to 58.6%, mean 52.2% (60.9%–78.2%, mean 69.4% on mmf basis) while liptinite varies from 11.0% to 22.0%, mean 16.3% (15.4%–29.9%, mean 21.0% on mmf basis). The inertinite content is relatively low (mean 6.9%, 9.6% on mmf basis). The mineral matter content varies from 15.1% to 37.0% (mean 24.6%). Huminite group is composed of ulminite-A (mean 12.6%; 15.6% on mmf basis), ulminite-B (mean 10.0%; 13.2% on mmf basis), attrinite (mean 12.1%; 18.0% on mmf basis), densinite (mean 13.3%; 17.3% on mmf basis), and phlobaphinite (mean 3.8%; 4.9% on mmf basis). Liptinite is represented by sporinite, cutinite, resinite, and liptodetrinite while other macerals occur in low concentration. Similarly, inertinite is mainly represented by fusinite, funginite, and inertodetrinite. Argillaceous mineral matter occurs in moderately high concentration (10.2%–29.4%, mean 17.8%) while sulphides and occur in low concentration.

Microlithotype analysis of Matanomadh lignite is given in Table 7. The analysis reveals that among the monomacerite, humite dominates (mean 46.8%) followed by liptite (mean 4.9%), and inertite (mean 1.6%). The bimacerites are contributed by clarite, huminertite, and durite. Carbominerites have a dominance of carbargilite (mean 17.1%), while carbopyrite and carbankerite occur in low concentrations.

4.1.2 Panandhro lignite

Maceral composition and mineral matter content in Panandhro lignites are given in Tables 2, 3, 4, 5 and 6. In this lignite, huminite is the most dominant maceral group (mean 60.3%; 74.9% on mmf basis). Liptinite is next in dominance (mean 11.9%; 15.1% on mmf basis), followed by inertinite (mean 7.9%; 10.0% on mmf basis). The mineral matter occurs in moderate concentration (mean 19.9%) (Tables 2, 3, 4, 5 and 6). The huminite group is mainly composed of ulminite-A (mean 13.3%; 16.6% on mmf basis), ulminite-B (mean 5.2%; 6.5% on mmf basis), attrinite (mean 13.2%; 16.2% on mmf basis), densinite (mean 21.0%; 26.2% on mmf basis), and phlobaphinite (mean 7.0%; 8.6% on mmf basis). The liptinite group is mainly represented by sporinite, resinite, and liptodetrinite while other liptinitic macerals occur in very low concentrations. The inertinite group is represented chiefly by fusinite, funginite, and inertodetrinite. The mineral matter includes sulphides (mean 4.9%), carbonates (mean 0.8%), and argillaceous mineral matter (mean 14.1%).

Microlithotype analysis of Panandhro lignite is given in Table 8. The analysis reveals that among the monomacerite, humite dominates (mean 59.0%), while liptite (mean 3.5%), and inertite (mean 1.1%) occur in low concentrations. Among the bimacerites, clarite, huminertite, and durite, occur in low concentration (Table 8). The carbominerites include carbargilite (mean 13.0%), carbopyrite (mean 5.0%), and carbankerite (mean 0.4%).

Details of the reflectance values of individual lignite samples of various seams are furnished in Table 9. The huminite reflectance measurement shows that R _{o, min} ranges from 0.20% to 0.25%, R _o,max from 0.41% to 0.48%, and R _{o, mean} from 0.31% to 0.35%.

4.2 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.