The microconstituents of Godavari coals, South India

International Journal o f Coal Geology, 3 (1983) 31---61 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

31

T H E M I C R O C O N S T I T U E N T S OF G O D A V A R I C O A L S , S O U T H I N D I A

G.K.B. NAVALE, B.K. MISRA and ANAND-PRAKASH Birbal Sahni Institute o f Palaeobotany, 53 University Road, P.O. Box 106, Lucknow 226007 (India)

(Received March 16, 1982; revised and accepted January 19, 1983)

ABSTRACT

Navale, G.K.B., Misra, B.K. and Anand-Prakash, 1983. The micro-constituents of Godavari coals, south India. Int. J. Coal Geol., 3: 31--61. The present paper deals in detail with the coal typology, rank and correlation of some Indian coal seams. Petrological evaluation of several known coal seams reveals that there are two main seams, the King (lower) seam and Queen (upper) seam, with distinctly different characteristics. All the other seams, locally known by different names elsewhere in the field, are correlatable with either the King or Queen seams. The coals are of inferior quality because of the predominance of mixed coal types associated with dispersed mineral matter. However, some seams show characteristics which are amenable to quality improvement with suitable preparation methods. The coals appear to have been formed rapidly in a fluctuating environment (aerobic to anaerobic) under relatively cold to gradually warming conditions.

INTRODUCTION The extensive use of coal first b e c a m e widespread in the eastern part o f I n d i a as it c o n t a i n s large deposits o f Permian coals. With the progress of geological p r o s p e c t i n g in India, m o r e and m o r e L o w e r G o n d w a n a (Permian) deposits o f b i t u m i n o u s coals were discovered in o t h e r parts o f the c o u n t r y , particularly in the s o u t h eastern region. The Godavari Valley (Fig. 1), the area o f coals investigated, is in the s o u t h e a s t e r n part o f I n d i a and has m a n y w o r k a b l e coal seams associated with L o w e r G o n d w a n a (Permian) sediments. The discovery o f coal in the Godavari Valley was m a d e as early as 1 8 4 1 , w h e n a search for coal was m a d e by Walker near K o t a ( 1 8 ° 5 4 ' N , 7 9 ° 5 8 ' E ) . F u r t h e r e x p l o r a t o r y geological w o r k was carried o u t b y m a n y geologists and King's ( 1 8 7 2 ) discovery o f coal at Yellandu was c o n s i d e r e d to be a significant c o n t r i b u t i o n for the coal i n d u s t r y in s o u t h India. R e c e n t l y , R a o ( 1 9 7 1 ) has carried o u t detailed geological investigations o f the Godavari Valley coalfields (Table I). T h e b a s e m e n t r o c k s in the area u p o n which the G o n d w a n a sediments lie u n c o n f o r m a b l y are A r c h a e a n gneisses, non-fossiliferous Pakhal limestones

0166-5162/83/$03.00

© 1983 Elsevier Science Publishers B.V.

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33 TABLE I General geological succession, after King (1881 ) Local groups

Survey classification

Approximate European classification

Alluvium Fossil wood and implement gravels Traps of Jangaom Valley Chikiala Sandstones, Kota beds, Maleri beds Kamthi sandstones

Alluvium Older alluvial deposits of the Godavari Lower Deccan traps Rajmahal ? Upper Gondwanas Kamthi, Lower Damuda Gondwanas Barakar Talchir ? Kurnul ? Lower Series Vindhyans Kudapah Upper Series Transitions Gneisses of the main or eastern region of the Peninsular area

Recent Post-Pliocene

Barakar sandstones Talchir beds Sullavai sandstones Pakhal series Gneiss

Upper Cretaceous Middle Jurassic with Triassic affinities Triassic and Upper Palaeozoic

? Lower Palaeozoic ? Pre-Palaeozoic Azoic

It is succeeded gradationally by the Barakar Formation. The latter is characterized by an alternating sequence of coal seams, coarse, massive felspathic sandstones and carbonaceous shales. The Barakar sediments are overlain by the Kamthi F o r m a t i o n which essentially consists of massive sandstones and occasional thin coal seams. The Kamthi F o r m a t i o n is succeeded by Upper G ondw ana sediments represented by the Maleri, Kota and Chikiala beds, chiefly consisting of sandstones, intercalations of red and white clays, fine-grained grey sandstones and thin limestone beds. General information concerning palynological and petrological aspects of L o wer Go n d wa na (Permian) coal seams of India is available (Bharadwaj, 1971, 1972, 1974; Navale, 1971, 1974, 1979a; Pareek, 1969). These investigations have also established that the coals differ in their properties owing to the changes in environmental conditions and plant communities (Navale, 1971, 1979b). However, the coals from Godavari Basin, which comprise innumerable coal seams, have not been studied in detail for an over-all evaluation, particularly with reference to the source material and coal seam characteristics for correlation and utilization, although some preliminary work has already been done on certain coalfields (Ramana Rao, 1962, 1965; Ghosh, 1962; Pareek et al., 1964; Ramana Rao and Moiz, 1966; Moiz

34 and R a m a n a Rao, 1976). The earlier p r e s u m p t i o n t h a t all the coals are u n f i t f o r utilization m a y p r o v e t o be i n c o r r e c t s h o u l d be t a k e n o f a full assessment o f t h e coals, T h e r e f o r e , this p a p e r certain aspects o f t h e physical c o n s t i t u e n t s o f G o d a v a r i coals, their n a t u r e and c o m p o s i t i o n f o r c o r r e l a t i o n o f the coal seams utilization prospects.

low-grade advantage deals with evaluating a n d their

MATERIAL AND METHOD T w e n t y representative coal samples f r o m d i f f e r e n t coal seams f r o m the K o t h a g u d e m , Yellandu, R a m a g u n d a m , R a m k r i s h n a p u r a m and Belampalli coalfields o f t h e Godavari Valley (Table II) have been utilized for the present petrographic study. F o r p e t r o g r a p h i c s t u d y , samples c o l l e c t e d vertically o f t e n d o n o t r e p r e s e n t t h e entire thickness o f the seam at a n y o n e place while the samples collected laterally are also n o t exactly f r o m the same level because o f the following reasons: (a) a c o m p l e t e seam is seldom e x p o s e d o n a single mine face; and (b) t h e seams have a t e n d e n c y t o split and are n a m e d d i f f e r e n t l y in various coalfields. T h e thickness o f the split seams varies in places, hence, the l o c a t i o n o f samples b e c o m e s difficult. TABLE II List of the coal samples from the Godavari Valley Serial no.

Seam

Incline/Pit no.

Sample no.

1. 2. 3. 4. 5. 6.

King Seam King Seam Green Seam Green Seam Top-1 Seam Top-2 Seam

2 6, 7 9, 10 9, 10 9, 10 Pit no. 20

7. 8. 9. 10. 11. 12.

King Seam D-Seam D-Seam Queen Seam Queen Seam Queen Seam

Pit no. 20 Pit no. 20 Pit no. 22 Pallampalli Incline Pit no. 22 Pit no. 22

10 11 13A 9 12 12B

Yellandu (Singreni)

13. 14. 15.

Seam-1 Seam-2 Seam-3

Incline-3 Incline-3 Incline-7

16 17 18

Ramagundam

16. 17.

Seam-3 Seam-4

RK-1 RK-1

19 20

Ramakrishnapuram

18. 19. 20.

Top Seam Bottom Seam Salarjung Seam

Shantikhani Incline Shantikhani Incline "85 Dip"

24 26 29

Bellampalli (Tandur)

1 2 6 6B 7C 8

Coalfield

Kothagudem

35 Particulate pellets from crushed and sieved (+ 18 mesh) coal samples were prepared by embedding in Carnuba wax (8 samples) and in Araldite CY 230. The pellets were polished following standard polishing methods. Maceral and microlithotype analyses were carried out in accordance with I.C.C.P. (1963, 1971) procedures under incident light using a lower-power oil-immersion objective {20) on a Leitz MPV-1 microscope. An automatic point counter was attached to control the pellet movement. For rank evaluation, reflectance measurements (max. reflectance with polarizer) were obtained (vitrinite maceral) at the 548 nm wave band according to I.C.C.P. (1971) recommendations. An EMI S-20 photomultiplier and a sensitive light-spot galvanometer were utilized for recording reflectance measurements. Silicon carbide (Ro 7.7% in oil) and a glass prism (Ro 1.23% in oil) standards served for calibration of the photometric unit. The correction for 0% reflectance value was also made as per I.C.C.P. recommendations. MACROSCOPIC CHARACTERISTICS The Godavari coals are mostly dull in lustre. They contain more fusain and dull bands than semibright and bright bands. The dull coal bands are, generally, of variable thickness and they are of lenticular shape. The coals are hard and compact with a granular texture. Generally, thick dull bands alternate with very thin or thin to medium, bright coal layers (2--15 mm and rarely even up to 5 cm thick, Ramana Rao, 1965). Many intimate alternations of broad dull and thin bright or semibright layers are c o m m o n in coal seams forming either durain, clarodurain or duroclarain bands. Fusain layers are sometimes compressed and matted. The coal seams are prone to splitting, c o m m o n l y causing a single seam to become several seams over a long distance, separated by shale and sandstone partings. The Godavari coals, contrary to other Damuda coals, contain more dull and intermediate coal types, interspersed with clastic mineral matter. MICROSCOPIC CHARACTERISTICS All the coal seams of the various coalfields of the Godavari Valley mainly consist of three maceral groups: vitrinite, exinite (liptinite) and inertinite. Among these maceral groups inertinite group of macerals is, in general, the most conspicuous, followed by the vitrinite and exinite groups. However, in a few cases vitrinite macerals dominate over the inertinite group.

Vitrinite group A considerable part of the vitrinite in the Godavari coals occurs as finely divided irregular shreds, fragments and sheets associated with trimacerite, durite and vitrinertite macrolithotypes (P1. 2-11,13,15). Thick vitrinite microbands occur less commonly associated with clarite (P1. 1-3); rarely

37 vitrinite occurs as individual bands (P1.1.1,2). Both types of vitrinite -- collinite and telinite -- are found in the coals. The former being the dominant variety. The collinite chiefly consists of telocoiiinite (P1. 1-1), desmocollinite (P1. 1-3--7), and corpocollinite (Pl.l-4). Bark tissues with telinitic or suberinitic cell walls and phlobaphenite-filled lumens are common. Wood tissues with thick- or thin-walled cells are also frequently observed. Microvitrinites are quite c o m m o n in the Godavari coals, apparently suggesting that smaller macrofragmental vegetal source material made a significant contribution to its formation. The vitrinites generally show a dark grey to light grey colour. Rarely grey to bright grey vitrinites are also met with. Shaly matter, clay, quartz and carbonate minerals are c o m m o n l y associated with the vitrinites. The Godavari coals show a large variation in their vitrinite content which ranges from very small (7.50 to 12.50%) to high (48.50 to 47.50%) proportions. In the Kothagudem coalfield the vitrinite content ranges from 12.50 to 43.00%. In the YeUandu coalfield the variation is extreme, the lowest vitrinite content being 7.50% and the highest 48.50%. In the Ramagundam and Ramakrishnapuram coalfields the range is between 40.00--46.00% and 27.50-33.00%, respectively. The vitrinite proportion in the Bellampalli coalfield is from 29.00 to 47.50% (Table IIIA).

Exinite (liptinite) group The exinite group of macerals in the coal chiefly consists of maceral sporinite, whereas cutinite and resinite macerals, though present, are relatively less significant. Of the sporinite, the major fraction is tenuisporinite (P1.1-3,6,7; P1.2-10--12). Crassisporinite is negligible in all the coal seams. Distinctly consistent proportions of sporinite are observed in the Yellandu, Ramagundam and Belampalli coalfields. Coal seams in these coalfields frequently have densely packed microspores in clarite bands. Their typical association seems to be governed by the proportions of vitrinite and inertinite macerals. Megaspores are less c o m m o n to c o m m o n in these coals. They appear as elongated streak-like structures with distinct lumens with different sculptural features (P1.2-16). The cutinites are long and slender, sometimes even folded, with serrated margins on one side. Resin bodies, though present in small amounts in some coal seams, usually occur as infillings in cell cavities of telinite, rendering discernible cellular structure to the tissues. In sectional view, resin bodies usually appear as small spherical, elliptical and spindle-shaped bodies or like rodlets. PLATE 1. l--Telocollinite (vitrite). 2--Telocollinite band (vitrite) flanked by duroclarite. 3--Telocollinite band flanked by duroclarite. 4--Elliptical corpocollinite bodies in a clarite band. 5--Alternating broad bands of vitrinite with thin bands of degradosemifusinite and fusinite forming vitrinertite. 6--Clarite band consisting of desmocollinitic groundmass and microsporinite. 7--Duroclarite band associated with (top right hand corner) fusite. 8--Fusite band consisting of pyrofusinite.

¢.o O0

39 The exinitic constituents are consistent in the Godavari coals. In various seams of the K o t h a g u d e m coalfield, t h e y range from 10.00--16.50% but in the Yellandu coalfield t hey form 11.00--21.00%. In the Ramagundam and Ramakrishnapuram coalfields the exinite c o n t e n t ranges from 12.00 to 18.00% and 13.50 to 15.50%, respectively. The Bellampalli coal seams contain 13.50--16.50% of exinite (Table IIIA). Thus, the Godavari coals are characterized by a relatively higher a m o u n t of liptinitic fraction than other equivalent Indian G ondw anan coals. Inertinite group

Fusinite, semifusinite, inertodetrinite, fusinized resin or sclerotinite and micrinite are c o m m o n l y observed macerals of the inertinite group in the Godavari coals. Semifusinite. Of the coals studied, semifusinite is subdominant to d o m i n a n t in the total fraction of inertinite macerals. It occurs as isolated fragments, in thin to thick microbands associated, generally, with fusinite and vitrinite. Cellular structures in semifusinite are usually n o t well preserved (P1.1-5; P1.2-13). However, occasionally, well-preserved cellular structures have also been observed. Cell walls in the semifusinite are relatively thicker than in the fusinite. Compressional effects on semifusinite have also been observed in the form of broken cell walls (Bogen structure) and folded structures. Cell cavities, cracks and fissures developed in the semifusinite are c o m m o n l y filled with carbonate minerals or shaly matter. Most of the semifusinite present in the coal appears to be degrado-semifusinite while rank semifusinite is n e x t in abundance. Occasionally, primary semifusinite has also been observed. Whitish grey to white colours characterize the semifusinite of the Godavari coals. Transitional forms with well-preserved cellular structure, between vitrinite and semifusinite and vitrinite-semifusinite to fusinite, have also been observed. Such transitional forms deserve critical observation for the assessment o f active and inert fractions of the coal. Fusinite. Fusinite is a dom i nant to subdominant maceral of the inertinite group in the Godavari coals. It is observed as irregular, isolated fragments in thin to thick microbands. Degradofusinite (P1.1-5, 7; P1.2-14,15) with poorly preserved cellular structures is very c o m m o n . However, pyrofusinite (P1.1-8; P1.2-9,10) and rank fusinite with well-preserved structures are also f o u n d in

PLATE 2. 9--Pyrofusinite (fusite) with partial development of bogen-structure. 10--A part of clarodurite band. 11 and 12.--Duroclarite bands consisting of micrinite, macrinite and inertodetrinite. 13--Vitrinertite formed by alternating bands of degradosemifusinite and fusinite with vitrinite. 14--Vitrinertite formed by the association of vitrinite, fusinite and sclerotinite (pseudosclerotinite). 15--Vitrinertite associated with scierotinite-Coronasclerotes durus. 16--Durite with megasporinite. 17--Durite with scierotinite (pseudosclerotinite). 18--Secondary pyrite developed over siderite.

Duroclarite

Clarodurite Carbominerite

32.50

18.00

14.50 13.00

10.00

I

5.50 -15.00 9.50

5.50

Fusite Inertodetrite Vitrinertite Durite

14.50

Vitrite

27.50 i 3 . 5 0 J 31.00 16.50 . . 11.50q 16.50 | 0.50 | 0 . 5 0 | 39.50 3.00 | 7.501 13.00

Clarite

B. M I C R O L I T H O T Y P E S

Vitrinite Vitrodetrinite Sporinite (Megaspores) Cutinite/resinite Semifusinite Fusinite Fusinized resin Sclerotinite Micrinite-macrinite Inertodetrinite Mineral matter

A. M A C E R A L S

No. 2 Incline King Seam Sample -1

22.50 7.00

16.50

13.50 10.00 4.00 7.00

0.50

12.00

I

4.00| 7.00_] 9.25

39.00

30.50

49.75

29.00q 2.00_] 31.00 10.00 . 16.00q 22.75

6 and 7 Incline King Seam Sample- 2

Kothagudem coalfield

.

30.00

9"50"l

31.50

13.50__] 23.00 8.00

16.00| 6.50[ 3.50 25.00

0.50

8.50

. 15.001 21.00| 1.50| 0.50} 49.00 3.50| 7.501 9.00

29.501 0.50J 12.00

7.00-] 25.00 9.50

18"001

11.00 I 26.00 7.50_] 7.50 8.50

11.00

12.50

21.501 17.50 I 0.5 _ 49.00 2.00 I 7.50.J 7.50

30.50 _ 13.00

43.00

17.50

°01

14'001 24.00 10.00-] 11.00

6.00 5.50 6.00 11.00

7.00

23.50

8.501 15.50| 1.50| 1.00} 36.00 2.50 I 7.00_] 7.50

42.001 1.00J 13.50

o

19.00_] 45.50 17.00

26"501

1 0 . 5 0 | 18.50 2.00-] 5.00 11.50

-2.50

1.00| 9.50J 28.00

22.00~ 46.50

14.001

12.50 _ 13.00

9 and 10 Incline 9 and 10 Incline 9 and 10 Incline Pit no. 20 Green Seam Green Seam Top-1 Seam Top-2 Seam Sample -6 Sample -6 B Sample -7 C Sample-8

Frequency distribution of macerals (A) and microlythotypes (B) in the coal seams of the Godavari Valley

TABLE III

O

Vitrite Clarite Semifusite Fusite Inertodetrite Vitrinertite Durite Duroclarite Clarodurite Carbominerite

B. M I C R O L I T H O T Y P E S

Vitrinite Vitrodetrinite Sporinite (Megaspores) Cutinite/resinite Semifusinite Fusinite Fusinized resin Selerotinite Micrinite-macrinite Inertodetrinite Mineral matter

A. M A C E R A L S

TABLE III (continued)

16.001 23.501 1.00[

11.50"1 16.501 0.50 L

10.00 19.00 4. O01 5.501 14.00 4.50] 2.50 10.50 22.001 37.50 15.50_] 6.50 5.50 40.50 6.001 17.003 6.50

8.5o_I

9.501

6.50 l

4.00[ 17.5OA 15.50

15.00

16.00

2.001 5.50-] 10.50

7.50

23.00

24.50

62.00

No. 20 Pit King Seam Sample-10

37.00

36.00

Pellampalli Incline Queen Seam Sample-9

Yellendu coalfield

1.00

q 2800. 8.50j 36.50

2.00_] 3.50 6.00

1.501

2501

16.00 31.00

16.50

17.007 25.5o 8.50~

1.50] 1.50 12.00

4.507

36.00

10.501 16.50

6.50 21.50

14.50

5.50j

6.00

2.001

3.00 /

6.00

1

6.00j

1.001

O,1oo,

10.501

11.00 10.50

19.00

8.001

38.50

No. 22 Pit Queen Seam Sample-12 (Coal 1-3)

48.50

26.50

No. 20 Pit 'D' Seam Sample-11

1

6.00

37.00

5.50

31.007 5200 2100j

2.00_J 3.50 8.00

3.001

1.007

11.00 14.00

14.50

8.50j

2.501

10.501

21.50

27.00

No. 22 Pit Queen Seam Sample-12B (Coal 5-11)

7.00

18.507 16.00j

2.50] 0.50 24.50

5.507

38.00

34.50

10.001 18.00

4.00 11.50

12.50

lO.OOj

2.5o 1

0.50|

14°°I

11.001

32.00 i 32.50 0.50 f 17.00

No. 22 Pit 'D' Seam Sample-13A

Vitrite Clarite Semifusite Fusite Inertodetrite Vitrinertite Durite Duroclarite Clarodurite Carbominerite

B. M I C R O L I T H O T Y P E S

Vitrinite Vitrodetrinite Sporinite (Megaspores) Cutinite/resinite Semifusinite Fusinite Fusinized resin Sclerotinite Micrinite-macrinite Inertodetrinite Mineral matter

A. M A C E R A L S

TABLE III (continued)

/

15.50 23.00 2.001 5.00| 8.50 1.50 J 5.50 4.50 18.00 1 3.50 J 21.50 21.50

32.00

16.00

39.00 1 1.00] 40.00 12.00 -~.~0]

No. 3 Incline Seam-1 Sample-16

27.50 20.50 3.50 7.50 1.50 2.00 3.00 12.50 4.50 17.50

]

0.50 | 2.00_J 19.00

17.00

12.50

46.00 _ 14.50 -4.50 1 13.50 -20.50

No. 3 Incline Seam-2 Sample-17

Ramagundam coalfield

6.5oj

2501

13.00

0007 7.50_]

2.002 1.50 10.00

23.50

7.oo I 11.50

19.50 21.00

15.50J 21.00

3.001 5.00 13.00

5.0o I

1.501

9.00 5.50

14.00 17.50 (1.00 carbonate}

850j

37.OO

9.5o

10-00/ 0.75 / 0.75 / 36.00 1.50 /

1.501

14.501

7.501

13.50

18.00

10.501 -

33.00

RK-1 Seam-3 Sample-19

42.00

26.00

No. 7 Incline Seam-3 Sample- 18

2.001

19.00 3.00

3.00_1 3.50 18.00 52.50

7.501 12.50

6.00 4.50

10.00

16.50 / 4.00 / 47.00 2.50 / 1.o0 / 13.ooj

10.00l

15.50

27.50

RK-1 Seam-4 Sample-20

Ramakrishnapuram coalfield

Vitrite Clarite Semifusite Fusite Inertodetrite Vitrinertite Durite Duroclarite Clarodurite Carbominerite

B. M I C R O L I T H O T Y P E S

Vitrinite Vitrodetrinite Sporinite (Megaspores) Cutinite/resinite Semifusinite Fusinite Fusinized resin Sclerotinite Micrinite-macrinite Inertodetrinite Mineral matter

A. M A C E R A L S

TABLE III (continued)

22.50

12.50 24.50 4.007 4.50[ 11.50 3.003 8.00 5.50 19.007 9.00_1 28.00 10.00

0.50[ 4.50J 13.50

47.50 -16.50 -8.001 9.50[ --

Shantikhani Top Seam Sample-24

3.50 10.00 1.501 7.50[ 12.00 3.00 J 7.00 20.50 25.001 18.50_] 43.50 3.50

29.00 -15.00 -14.001 14.50| 1.00[ 2.50 | 45.00 2.50 ] 10.50 J 11.00

Incline Bottom Seam Sample-26

Belampalli coalfield

9.00 9.00 1.507 7.50 4.00] 2.003 7.50 6.50 34.50 7 53.50 19.001 7.00

10.501 I0.00| 1.50[ 1.00[ 32.50 2.50[ 7.003 17.00 (3.00 carbonate)

13.50

37.50

"85 Dip" Salarjung Seam Sample-29

o0

44 some of these coals. The cell walls of the fusinized tissues are comparatively thinner than those of the semifusinite, though fusinized tissues with thick cell walls have also been observed. Folding and bogen structures in the fusinite are rare. Cell lumens are mostly filled with carbonate minerals or with black shaly matter (P1.2-10). Thus, the fusinite present in the coal is dominantly of a hard variety. Occasionally, primary fusinite has also been observed. The degradofusinite of the Godavari coals generally shows white to brightly white colours, whereas the rank fusinite and pyrofusinite varieties have brightly white to yellowish white or sometimes even yellowish colours. Accretionary carbonate minerals in cracks and fissures of semifusinite and fusinite disrupting and displacing the original structures of tissues have been commonly observed in most of the coal seams. Micrinite is a common maceral of the coals studied, especially in the exinite-rich bands (P1.1-3,6; P1.2-11,12). Fine-grained micrinite has been found dispersed in the cavities of telinites and, rarely, in the cavities of spores. Micrinite grains also occur as small isolated patches or lenses within the vitrinite. Macrinite occurs as irregular and homogeneous bodies, white to bright white in colour. It is c o m m o n l y associated with durite bands where spores occasionally outline the individual macrinite grains. Inertodetrinite forms a conspicuous maceral in almost all the coal seams (P1.1-3,6, 7; P1.2-11,12}. It has been found associated with all the macerals, but the most c o m m o n association is with durite and trimacerite bands. It is also c o m m o n in bands which are rich in mineral matter. Inertodetrinite has colours varying from white, bright white to yellowish, and it has high relief with variable reflectivity. The conspicuous occurrence of inertodetrinite, which is relatively less common in Damodar Valley coals, is one of the characteristic features of the Godavari coals. Sclerotinite, including the fusinized resin bodies described earlier by Pareek (1966) and highly reflecting resin bodies recorded by Razvi and Ramanna Rao (1969), is c o m m o n in these coals, particularly in seams of the Kothagudem, Ramakrishnapuram and Belampalli coalfields. It has been found either as discrete, round or oval bodies up to 600 microns in size or in the form of irregular structures. The sclerotinites typically exhibit two distinct structures. Type one shows perforations or vesicles of varying dimensions (P1. 2-14,15,17), whereas type two is generally oval or elliptical in shape and displays radiating or irregularly arranged, straight to slightly curved cracks. Along these cracks {dessication cracks), oxidation rims are quite commonly seen (PI. 2-11 ). Sclerotinite is mostly found in dull coals and is c o m m o n l y associated with durite and trimacerite bands. Some sclerotinite bodies, Coronasclerotes australis and Coronasclerotes durus described by Stach and Pickhardt (1957), are also c o m m o n in the Godavari Valley coals (P1. 2-15). It may be mentioned that a critical microscopic evaluation is desirable in separating the fusinized resin bodies which resemble sclerotinite. Inertinite macerals constitute a dominant to subdominant proportion in

45 these coals. Like vitrinite, a wide range of variation is shown in their distribution. In the Yellandu coalfield the inertinite content is the highest (62.00%) while in the Ramagundam it is the lowest (16.00%). The coal of the Ramagundam coalfield generally shows a more subdominant inertinite content (16.00--26.00%) than any other coalfield in this valley. The Kothagudem and Yellandu coalfields contain 36.00--46.50% and 18.50-62.00% inertinite macerals, respectively. The Ramakrishnapuram coal has 36.00--47.00% of inertinite, whereas in the Belampalli coalfield the content ranges between 22.50 and 32.00% (Table IIIA). Mineral matter The mineral matter, which is a conspicuous constituent of the Godavari coals, has not only influenced the quality of the coal, but also reflects upon the depositional conditions prevailing during the accumulation of the source material in the basin. In general, the Godavari coals are characterized by relatively high proportions of mineral matter. Fine clay and other minerals are c o m m o n l y found dispersed in the groundmass of the coal constituents. The types of minerals found in these coals can be grouped under: (1) clay minerals; (2) silica minerals; (3) carbonate minerals; and (4) sulphide and other mineral groups. Clay minerals. Predominate over other minerals. They are mostly associated with vitrinite and inertinite macerals. Clays are found as fine inclusions in clarodurite, durite, vitrinertite and vitrite bands. Generally, they are irregularly distributed and are also found interbedded with the coaly matter, in varying thickness, forming shaly coal or carbargillite. Occasionally, these minerals form infillings of telinitic cell cavities and in extreme conditions they are also seen as nodules or ball-like structures. Silica minerals. Quartz is the only recognizable silica mineral and it occurs in the form of discrete grains embedded in the macerals. The grains are mostly clastic in origin and are generally angular to subangular in shape. They are usually small but larger grains (up to 100 microns) have also been encountered in places. Fine quartz clasts are also found in cracks and fissures of the coalified plant tissues. Carbonate minerals. Mostly secondary calcium carbonate is the dominant mineral in this group. It occurs as large clastic grains, specially in the Kothagudem coals, and also as smaller anhedral grains, as well as coarse to fine branched and anastomosing stringers. It is also c o m m o n as infillings, mostly in the cell cavities of semifusinite and fusinite. The mineral is very c o m m o n as stringers and crack fillings in vitrite, clarite and trimacerite microlithotypes. Another carbonate mineral observed is siderite which

46 c o m m o n l y occurs as nodules and concretions, particularly in vitrite and clarite. These are also found in association with semifusinite and fusinite.

Sulphide minerals. Sulphide minerals and other minor mineral groups are rarely found in the Godavari coals. Very rarely, small pyritic grains or specks have been seen in the coal seams either individually (P1.2-18) or associated with siderite. The association of the pyritic grains with siderite concretions probably indicates a replacement origin of the former. No attempt has been made to study the mineralogical composition of the coal seams in detail. However, during the organic microcomponental analyses of the coal seams, more than 90% of the mineral matter encountered falls within the mineral groups described above. In almost all the seams of the Godavari Valley, mineral matter is ubiquitous. The lowest content of mineral matter recorded is 6.00% from a sample from the Yellandu coalfield and the highest is 32.00% from the Ramagundam coalfield. The coal seams of the Kothagudem coalfield are characterized by the persistent presence of carbonate minerals which, in some seams, a m o u n t to half of the total mineral content (7.50--28.00%). A similar trend is shown by the clay minerals. Quartz is the third c o m m o n mineral in this coalfield. The coals of the Yellandu coalfield are associated with more clay minerals and quartz, but an occasional high incidence of carbonate minerals has also been observed. Here the mineral content ranges from 6.00 to 15.50%. The Ramagundam coalfield is characterized by a persistently high mineral content (14.00- -32.00%) in which the major fraction is of clay minerals, and quartz and carbonate minerals play a secondary role. The coals of the Ramakrishnapuram coalfield are characterized by clay, carbonate and quartz or clay and quartz associations (10.00--17.50%). This association is close to that of the Kothagudem coalfield. In the Belampalli coalfield, the coal seams contain (11.00--17.00%) more clay minerals with subordinate quartz, but the association of clay minerals with carbonate minerals has also been recorded in one seam (Table IIIA). MICROLITHOTYPES The microlithotypes in the Godavari Valley coals vary in seams of different coalfields depending upon the general maceral composition. As in other Lower Gondwana coals, the microlithotypes may broadly be grouped under vitrite, clarite, inertite, durite, vitrinertite and trimacerite ("Intermediate types") which includes the trimaceral association, namely, clarodurite and duroclarite. These are basically trimacerite- and inertite-dominant coal types and they contain subordinate quantities of durite and clarite microlithotypes. These latter occasionally become quite significant in some of the coal seams of the Godavari Valley. The following are the characteristic features of the microlithotypes present.

47

Vitrite group Vitrite in the Godavari coals generally occurs as alternating microlayers associated with clarite, vitrinertite and trimacerite microlithotypes (P1.11,2,3,5). Broad vitrite bands are only rarely observed. Unlike Carboniferous coals, vitrite in these coals is only seldom pure. It is usually associated with clastic (quartz) or precipitated (carbonate) epigenitic mineral matter. Irregular or dispersed distributions of clay minerals within the vitrite bands seem to be a c o m m o n feature. The vitrite microlithotypes in the Godavari coals hardly ever exceed 20.00%, except in one seam each of the Kothagudem and Ramagundam coalfields, where the contents are 23.50% and 27.50%, respectively. Likewise, no vitrite content could be recorded in one seam from both the Kothagudem and Yellandu coalfields. Vitrinite ranges from 8.50 to 23.50% in the Kothagudem, 4.00 to 16.00% in the Yellandu, 15.50 to 27.50% in the Ramagundam, 6.00 to 12.50% in the Ramakrishnapuram and 3.50 to 12.50% in the Belampalli coalfields (Table III B).

Clarite group Most of the clarite microlithotype in the Godavari coals is invariably sporinite clarite (microspores; P1.1-3,6). A few megaspore-rich clarite bands have also been recorded. The clarites in these coals are characterized by medium to high sporinite contents, as has already been recorded by Ramana P~ao and Razvi (1965) from the Kothagudem. The authors have observed rich sporinite contents in the clarite bands of a few coal seams from the Kothagudem, Yellandu, Ramagundam and Belampalli coalfields. The microlithotype is, invariably, associated with mineral matter. In general, the clarite contents of coals from the Kothagudem and Ramakrishnapuram coalfields is low and ranges from 0.50 to 11.00% and 4.50 to 5.50%, respectively. Persistent and relatively high proportions of clarite have been recorded from the Yellandu (11.50--31.00%), the Ramagundam (12.00--23.00%) and Belampalli (9.00--24.50%) coalfields. One sample of the King seam from the Yellandu coalfield contains no vitrite and clarite.

Inertite group The inertite microlithotype group in the Godavari Valley coals is represented by semifusite, fusite and inertodetrite microlithotypes (P1.1-7,8; P1.2-9,10). Semifusite and fusite occur as thin or thick micro bands and also as fragmented masses. The inertodetrite microlithotype is only occasionally observed. The inertite microlithotype forms the second dominant group after trimacerites in these coals. Its lowest recorded frequency is 2.50% while the

48 highest is 31.50% (Table IIIB). In the Kothagudem (10.00--31.50%) and Yellandu (2.50--24.50%) coals, the microlithotypes show a high frequency while in the Ramagundam (8.50--12.50%), the Ramakrishnapuram (9.50-12.50%) and Belampalli (7.50--12.50%) coals the inertite content is persistently low.

Vitrinertite group In the Godavari coals the vitrinertite microlithotype is present in all the coal seams but only seldom attains significance (P1.1-5,7; P1.2-13--15). Both vitrinite-rich as well as inertinite-rich vitrinertites have c o m m o n l y been recorded. The vitrinertite shows higher contents in the Belampalli (7.00--8.00%) and Kothagudem coals (3.50--15.00%), while in the Yellandu (0.50--5.50%), the Ramagundam (1.50--5.50%) and Ramakrishnapuram (3.50--5.00%) coals, it is persistently low (Table IIIB).

Durite group The durite microlithotype, though present in all the coal seams of the Godavari Valley, does not show a persistent character as do trimacerite and clarite (P1.2-16,1 7). Its lowest content is 3.00% in the Ramagundam and the highest is 40.50% in the Yellandu coalfields. In coal seams of the Kothagudem and Yellandu coalfields, it ranges from 7.00 to 25.00% and 6.00 to 40.50%, respectively. In the Ramagundam coals, it is persistently low and ranges from 3.00 to 10.00%, while in the Ramakrishnapuram it ranges from 13.00 to 18.00%. The Belampalli coals are similar to the Kothagudem coals in their durite contents (5.50--20.50%, Table III B).

Trimacerite group The trimacerite microlithotype group is persistently dominant in almost all the coal seams of the Godavari Valley. Vitrinite-rich trimacerite (duroclarite) is quite conspicuous (P1.2-11,12). The lowest recorded contents of the trimacerite group is 17.00% from the Ramagundam Coalfield, the highest (53.50%} is from the Belampalli coals. The duroclarite fraction in the Godavari coals ranges from 6.00 to 34.50%. It is impersistent in the Kothagudem (9.50--26.50%) and Yellandu (6.00-31.00%) coalfields. In the Ramagundam (12.50--18.00%), the Ramakrishnapuram (21.50 33.00%} and Bellampalli (19.00--34.50%) coals, duroclarite is relatively persistent and marked by a gradual increase in the lower and upper limits of its fractions (Table IIIB). The clarodurite microlithotype of the trimacerite group occurs commonly, though it is less in a m o u n t than the duroclarite (P1.2-10). Association of mineral matter with this microlithotype is more common than with duro-

49 clarite. Like duroclarite, the clarodurite fraction in the Kothagudem (7.00-22.50%) and Yellandu (8.50--21.00%} coals is relatively variable. In the Ramagundam (3.50--7.50%) and Ramakrishnapuram (15.50--19.00%) coals, clarodurite is remarkably persistent. However, in the Bellampalli coals (9.00--19.00%), as in the Kothagudem and Yellandu coals, it shows considerable variation (Table IIIB). RANK (MATURITY) The Godavari Valley coals show a reflectance range of from 0.58 to 0.85%. The highest ~1 V-step frequencies fall in the range of 0.65--0.70% (for 8 samples), 0.70--0.75% (for 11 samples) and 0.75--0.80% (for one sample). Calculated maximum reflectance in oil similarly ranges from 0.67 to 0.75% Ro max. A slight decrease in reflectance in a few samples (8, 10, 20 and 26} is attributable to their very high low-reflecting desmocollinitic fraction associated mainly with trimacerite and durite microlithotypes and also probably to the embedding medium, i.e. Carnuba wax (16, 20, 24, 26 and 29). However, the higher value for sample 11 appears to be exceptional. After plotting the reflectance values on a standard curve (KStter, 1960}, ranges of volatile matter (37.30--40.00%) and carbon contents (80.00--81.60%) were obtained. The results indicate that most of the samples have attained High Volatile Bituminous A Stage, while a few samples have reached only the High Volatile Bituminous B Stage. Thus, in general, the coals are of low rank. The Godavari coal shows normal maturity in contrast to the other Lower Gondwana coals of India in which local and abrupt increases in rank are observed owing to igneous intrusions, severe faulting and folding and other associated features. In the Godavari coals gradual changes in reflectance of vitrinites are observed, unlike in the coals of Damodar and other basins (Navale and Misra, 1980). It may reasonably be presumed that as the Godavari coals are least disturbed and almost free from igneous intrusions, folds and faults, the coal katagenesis appears to have been governed mainly by temperature and time factors coupled with the depth of burial. The geothermal gradient apparently remained relatively low throughout the geological past to cause the high rank to these coals. SEAM CHARACTERISTICS (Table III A, B) Coal seams of the Godavari Valley are characterized mainly by the fine state of maceral division. They have high inertinite, low vitrinite and a slightly higher exinite contents. The seams are characterized by the conspicuous presence of dispersed mineral matter intimately associated with the organic microconstituents. This nature and the mode of quantitative composition of petrographic entities have resulted in a mixed type of microlithotypes, represented by the dominant trimacerite and durite associations, contrasted with

50 subdominant vitrite-clarite and inerite fractions. These associations are exhibited by the most frequent and persistent broad dull bands which are interspersed with fine impersistent semibright to bright layers. The sedimentary mineral matter shows a preferential association with those of organic inerts. Thus, frequent changes in coal types observed vertically as well as laterally (coal-shaly coal-coalyshale-carbonaceous shale-shale) seem to be a very c o m m o n feature of the coal seams. A critical appraisal of the quantitative composition of macerals along with maturation of coal seams of the Godavari Valley reveals that they possess two contrasting types of petrographic characters. Evaluation based on certain arbitrary associations (vitrite-clarite, durite-trimacerite) of microlithotype groups, also appears to be satisfactory for typifying the coals. These arbitrary associations show somewhat similar trends of variation to those shown by macerals. The contrasting features are quite compatible with the quantitative petrographic composition and maturity (Figs. 2 and 3) displayed by the lower {King) and upper (Queen) seams in both the Kothagudem and Yellandu coalfields. In general, lower coal seams of the Godavari Valley have dominant inertinite and subdominant vitrinite contents. They are characterized by average higher reflectance. Their vitrite-clarite fraction is low while inertite, durite-trimacerite microlithotypes show high contents. The upper seams, on the contrary, contain high vitrinite (vitrite-clarite) and relatively low inertinite (inertinite-inertite) contents, while durite-trimacerite fractions are also lower than the fractions of the lower seams. The upper seams are marked by the lower reflectance values. Thus, sample nos. 1, 2 (King Seam} and 6, 6B (Green Seam) of the Kothagudem, 10 (King Seam), 11, 13A (D-Seam) of the Yellandu, 20 (Seam-4) of the Ramakrishnapuram and 26 (Bottom Seam) of the Belampalli coalfields show petrographic characters assignable to the lower seam. These samples have reflectance values normally ranging from 0.70 to 0.73% Ro max, but the two samples 20 (0.67%) and 11 (0.76%) show values lower as well as higher than the normal range, respectively. Average frequencies of microlithotypes for all the nine lower seam samples show 16.80% of vitriteclarite, 19.00% of inertite and 52.00% of the durite-trimacerite association. The inertite and durite-trimacerite fractions are higher, while vitrite-clarite trimacerite are lower than the corresponding average contents of the upper seams. Of the nine samples, only sample no. 11 is rather peculiar in its petrographic characters and exhibits maximum values in every aspect. Petrographic characters referable to upper seams are shown by sample nos. 7C, 8 (Top-1 and Top-2 seams of the Kothagudem) 9, 12, 12B {Queen seam of the Yellandu), 16, 17, 18 {Seams 1, 2 and 3 of the Ramagundam), 24 and 29 (Top and Salarjung seams of the Belampalli). The reflectance values range from 0.68 to 0.72 Ro-max, which is lower than the reflectance for the lower seam. Average frequencies of the microlithotypes for all the 11 samples show higher vitrite-clarite (28.30%) and lower inertite {12.13%)

51 and durite-trimacerite {41.80%} fractions than the corresponding values for the lower seams. Only one sample (8-Top-2 seam} shows all the abnormal features for the seam. It is apparent from the foregoing observations that two major seams -- lower and upper -- are present in the Godavari Valley. These may be referred to as the King Seam (Lower) and the Queen Seam (Upper) equivalents due to their petrological and rank (Fig. 2 and 3, reflectance value) similarities. T he King Seam includes thin individual seams or splits of the main seam having identical petrographic characters and maturity 60174

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(rank). Similarly, all the samples of the main seam, thin seams or the split seams exhibiting petrographic features and maturity closely comparable to the Queen Seam (Upper) are designated as the Queen Seam equivalents.

53 CORRELATION OF COAL SEAMS

Although the occurrence of two major coal seams has been ascertained by general petrographic data, precise correlation of the individual seams by any single factor becomes difficult. Nevertheless, plotting of various graphs of macerals and microlithotypes with mineral matter and on mineral-matterfree basis has led to some reasonable associations, especially, on mineralmatter-free basis. These data are utilized in categorizing the coal types and correlation of the coal seams in the individual collieries as well as among the various collieries of the coalfields of the Godavari Valley.

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VII and VE/I (Figs. 4 and 5) The V/I and VE/I ratio curves show definite relationships among samples o f the same seam collected from different inclines or pits and also among the seams o f different collieries. The relationship is also evident between, so far, unrelated coal seams locally named in various collieries and coalfields. In the Ko th ag udem coalfield, sample no. 1 and 2 of the King Seam are similar except for a slightly higher inertinite c o n t e n t in the latter. Sample no. 2 shows better relation with the Green Seam samples (6, 6B). Samples 7C and 8 (Top-1 and Top-2) possess no mutual relationships or with other samples from the coalfield. In the YeUandu coalfield, the King Seam sample (10) shows no relation with the King Seam (1, 2) of the K o t h a g u d e m coalfield as the form er contains the lowest vitrinite and highest inertinite fractions. However, it is similar to the Top-2 seam of the Kothagudem coalfield. Although this relation-

55 ship is not justified according to the field data, the reason such abnormal association may be because these t w o samples have been collected from typically dull coal bands. Of the t w o samples (11 and 13A) collected from different pits representing the D-seam, sample 11 is again peculiar and shows no similarity to any of the samples of the colliery, while sample 13 A (D-seam) exhibits a close relationship with the King Seam sample (1) of the Kothagudem coalfield. The upper seam i.e., the Queen Seam, is represented by samples 9, 12, 12B, collected from two different inclines. The first two samples are nearly identical in their maceral composition, whereas 12 and 12B, in spite of being from the same incline, show differences in their maceral contents, because sample 12 is from a bright band while 12B is from a relatively dull band. The Queen Seam samples {9, 12) show close similarities to Top-1 seam (7C) of the Kothagudem coalfield. Sample 12B somewhat resembles the sample 13A of the D-seam. In the Ramagundam coalfield three seams, 1, 2 and 3, are represented by sample nos. 16, 17 and 18, respectively. Seam-1 (16) and Seam-2 (17) appear similar to each other, while Seam-3 (18) is different and shows relationship with sample 11 of the D-seam of the Yellandu coalfield. Seam-3 and Seam-4, represented by the samples 19 and 20 in the Ramakrishnapuram coalfield, do not show any mutual relationships. Seam-3 (19) appears closely similar to the sample 9 and also somewhat similar to the sample 12B of the Queen Seam (Yellandu coalfield). Seam-4 (20) shows good relationship with the Green Seam (6B, 6) and also with the King Seam (2) of the Kothagudem coalfield. In the Belampalli coalfield two samples-24 (Top Seam) and 26 (Bottom seam) were collected from incline-3 and a third sample 29 (Salarjung Seam) was collected from pit No. 85. All three samples are mutually dissimilar. The top-seam (24) shows close similarity with the Seam-2 (17) of the Ramagundam coalfield and also, distant relationship with the D Seam (11) of the Yellandu coalfield. The Bottom seam (26) is like Seam-4 (20) of the Ramakrishnapuram coalfield while its distant relationship with the Green Seam (6B, 6) and the King Seam (2) of the Kothagudem coalfield is also noted. The Salarjung Seam (29) shows close similarity with the Top-1 Seam (7C) of the Kothagudem, and the Queen Seam (9, 12) of the Yellandu coalfield.

Vitrite-clarite/Inertite and Vitrite-clarite/Durite-trimacerite (Fig. 6) These microlithotype ratio curves, in most cases, show definite trends of associations already depicted by maceral plottings, but the association and the relations between the microlithotypes is not clear. To ascertain the probable proximity of seam samples on the basis of maturity, vitrinite and vitrite-clarite +51 vitrinertite, durite and trimacerite data were plotted separately against the reflectance values of individual samples in Figs. 3 and 6. The figures show almost the same associations as

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Fig. 6. Groupings of the coalseams, based on mierolithotypes, in the Godavari Valley.

obtained by the petrographic components. The rank (maturity) groupings also justify the previous results, especially for the coal seams which are presumably similar, though locally named in different coalfields separately, where no King, Queen, Green and D-seams are known to occur. They all fall with King and Queen seams in rank lines. The tentative correlation based on maceral, microlithotype and maturation of the coal seams (Fig. 7) present in the various coalfields of the Godavari Valley indicates that almost all the lower seams, i.e. Seam-4 (Ramakrishnapuram) and the Bottom Seam (Belampalli) including Green Seam (Kothagudem) and D-seam (Yellandu), were more or less simultaneously deposited along with the King Seam of the Kothagudem and Yellandu coalfields in a single phase of deposition. The Bottom Seam of the Belampalli coalfield (Tandur Colliery) is similar to the Ross Seam (local name for the b o t t o m seam in the area) in its petrographic characters (Pareek et al., 1964) and in turn appears to be equivalent to the King Seam. Similarly, Top seams 1 and 2 of the Kothagudem, seam 3 of the Ramakrishnapuram and Salarjung and Top seam of the Belampalli coalfields were, apparently, simultaneously deposited with the Queen Seam of the Kothagudem and Yellandu coalfields. However, a short time interval during their deposition is possible. Thus, the Top Seam of the Kothagudem and the

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BELAMPALL{

RAMAGUNDA~

Fig. 7. A tentative correlation of the coal seams in the different coal fields of the Godavari Valley {diagrammatic). Salarjung seam {local name for top seams in the area) of the Belampalli coalfields are the equivalents of the Queen Seam. The position of D-seamsample 11 (Yellandu), Top Seam (24) of the Belampalli and three seams (16, 17 and 18) of the Ramagundam coalfields is not yet clear. QUALITY OF THE COAL The results of proximate analysis from various sources indicate that the Godavari coals are high volatile, low rank bituminous coals and are noncoking in nature. They have an average ash content of 15.00--20.00% some times even up to 26.00%, volatile matter (on air-dry basis) 24.00--30.00%, fixed carbon 46.50--48.50% and low sulphur 0.30--2.70%. The calorific value varies from 9750--11730 BTU/lb. Ramana Rao (1962), and Ramana Rao and Rao (1965) recorded petrographically a decrease in mineral matter and ash contents with the reduction in grain size of the Tandur (Belampalli) and Kothagudem coals, respectively. We have recorded a good amount of carbonate minerals as crack, fissure and cavity fillings along with quartz grains in the King and Queen seams of the Kothagudem and Yellandu coalfields and also in the third seam of the Ramagundam and Ramakrishnapuram and Salarjung Seam of the Belampalli coalfields. As has already been observed by Ramana Rao (1962) and Ramana Rao and Rao (1965) probably a greater part of the quartz and carbonate minerals, barring shaly and clayey associations present in these coals, are amenable to easy separation by reducing the grain-size alone. However, detailed cleaning tests on different grain-size fractions may prove fruitful for suitable preparation.

58 Ramana Rao (1965) obtained coherent and coarsely vesicular crucible cokes from vitrain bands of the Kothagudem coals, and observed that the coking property decreased with the increase of trimacerite, durite and mineral contents. As has already been stated, the Godavari coals contain low amounts of reactive constituents (vitrite and clarite), high mineral matter contents, and attained only a low maturity (rank); this renders these coals, by and large, unsuitable for coking purposes. However, a few coal seams, viz. Top-1 Seam (7C) of the Kothagudem, part of D-seam (11) of the Yellandu, three seams (16, 17, 18) of the Ramagundam and Top-Seam {24) of the Belampalli coalfields, show potentiality for use as blending coals with highrank and low-ash coals. COMPARISON WITH OTHER LOWER GONDWANA BASINS OF INDIA A comparison of the characteristic features of the Godavari Valley coals with the coals of the Damodar Basin (the largest and economically most important Lower Gondwana coal basin, comprising Raniganj, Jharia, Karanpura, Bokaro, Giridih, Ramgarh coalfields) shows that the latter differ conspicuously in having the following features: (1) Dominance of vitrinite macerals. (2) Inertinite is present as a subdominant constituent. (3) Sclerotinite (including fusinized resin bodies) is relatively low. (4) Vitrite and clarite microlithotypes are the dominant microlithotype groups. (5) Mineral matter is relatively low in quantity. (6) Vitrain bands are abundant and by and large pure. (7) Clarain bands are common. (8) Dull bands are relatively less conspicuous than the bright and semibright layers. (9) Higher maturity (rank) of the coals, rendering their coking potentiality economic. However, the coals of other Lower Gondwana basins of India, namely Son-Mahanadi (Talchir, Ib-river, Chirimiri-Sohagpur, Korba and other coalfields), Satpura Gondwana Basin (Pench-Kanhan coalfield, Bharadwaj et al., 1974) and Wardha-Godavari (Wardha coalfield, Anand-Prakash and Khare, 1976; Figs. 4 and 5), very closely resemble the Godavari coals in all the characteristic petrological features described earlier. It is remarkable to find a close similarity in nature, composition and distribution pattern of macerals, microlithotypes and mineral matter. Even the rank corresponds with the coals of the above basins except in some coals of the Pench-Kanhan coalfield where igneous intrusions have changed the general character of the coals. From these resembling features, it may be reasonable to presume that the coal seams of the Godavari-Wardha, Satpura and Son-Mahanadi coal basins have formed under a similar set of geophytological and environmental conditions.

59 CONDITIONS OF DEPOSITION The positive evidence has come from palaeobotanical, palynological and petrological studies (Surange, 1966; Bharadwaj, 1971, 1974; Navale, 1978) that the main Permian coal deposits in India were initially formed under a cold-temperate climate with alternating dry and humid seasons. Only during the later stages (Upper Permian) did the climate become warmer and warm-temperate. The stunted broad-leafed flora which characterized Glossopteris foreSts was somewhat similar to that usually found today in subarctic regions. Low temperature is also indicated by the coal-forming plants having marked seasonal growth with intervening dormant periods. Besides this, the occurrence of fresh feldspar grains in the sediments associated with coal seams also suggests low-temperature conditions and rapid burial of the source material. There is also evidence that the Permian coals of India were formed in faulted, slowly sinking continental platform basins, distributed in a fairly widespread area under shallow, fresh-water conditions. Because the formation of coal deposits was preceded by a glacial period, the bulk of the strata which followed were laid down mostly as thick series of fluviatile and lacustrine and rarely as deltaic deposits. Therefore, it is reasonable to presume that the glaciation has influenced the subsequent history of the coal deposition. The glaciated terrain might have acted as a ready source of detrital material during the deposition of the coal. Post-glacial readjustments seem to have locally affected the pattern of subsidence in the coal basins, affecting the lateral continuity of the seams as well. The above-mentioned environmental conditions prevailed during the formation of coal in Lower Gondwana coal basins, in general, and in the Godavari Basin in particular. But a critical evaluation of Permian coals reveals that there existed two or three distinct phases of formation of coal in the basins. However, petrological studies of the Godavari coal seams suggest that the coals formed under one particular set of geophytological and environmental conditions. In a relatively shallow basin, the vegetable peat, being microfragmental in nature, was partly covered by water, i.e. having open-water peat conditions, and was frequently eroded. The presence of large amounts of inertinite, trimacerite and durite also indicates periodic lowering of water in the basin, facilitating fusinization, alternating with the wet periods, which also supports the formation of a trimacerite and durite association. Profuse clastic grains in the seams may be due to some factors or combination of factors, namely, a temporary high rate of local subsidence, physiographic relief etc., and suggest an allochthonous source material for the peat during coal formation. Association of mineral matter with almost all the microlithotypes indicates that partly they were also wind-blown from the surrounding areas; this is substantiated by the ubiquitous presence of inertodetrinite. These conditions probably prevailed during the formation of coals in Son-Mahanadi, Pench-Kanhan and Wardha coal basins also as they resemble the coals of the Godavari Basin in almost all the coal compositional features (Figs. 4 and 5).

60 CONCLUSION T h e 20 coal s a m p l e s r e p r e s e n t i n g 14 local coal s e a m s in various coalfields o f t h e G o d a v a r i Valley h a v e b e e n g r o u p e d into l o w e r {King S e a m ) a n d U p p e r ( Q u e e n S e a m ) s e a m c o m p l e x e s b a s e d on their distinct c o m p o s i t e p e t r o l o g i c a l characters. T h e L o w e r seams are c h a r a c t e r i z e d b y a higher a m o u n t o f inertinite macerals, d o m i n a n t t r i m a c e r i t e durite m i c r o l i t h o t y p e , and relatively higher r a n k t h a n the u p p e r seams. T h e u p p e r seams have m o r e vitrinite a n d less t r i m a c e r i t e - d u r i t e association. T h e u p p e r seams have slightly l o w e r m a t u r i t y (rank). T h e G o d a v a r i Valley coals w e r e d e p o s i t e d in slowly sinking faulted basins, u n d e r s h a l l o w - w a t e r c o n d i t i o n s , w h i c h i n t e r m i t t e n t l y e x p e r i e n c e d slow as well as high rates of s e d i m e n t a t i o n . T h e b u l k of t h e s t r a t a a s s o c i a t e d w i t h t h e coal seams were laid d o w n as fluviatile, lacustrine and, rarely, as deltaic deposits. T h e vegetal s o u r c e m a t e r i a l was, p r i m a r i l y , m i c r o f r a g m e n t a l in n a t u r e a n d witnessed periodic d r y a n d w e t c o n d i t i o n s ; this is r e f l e c t e d in t h e u l t i m a t e coal c o m p o s i t i o n s . Because o f the high a m o u n t o f s e d i m e n t a r y m i n e r a l m a t t e r associated with the organic m i c r o c o n s t i t u e n t s , t h e coals h a v e f r e q u e n t l y c h a n g e d f r o m shaly coal, c o a l y shale t o shale. H o w e v e r , a f e w coal seams ( Q u e e n S e a m or its equivalents} viz., T o p - 1 S e a m o f t h e K o t h a g u d e m , D - S e a m o f the Yellandu, all t h e t h r e e s e a m s o f t h e R a m a g u n d a m , and the T o p s e a m o f the Belampalli coalfields are of b e t t e r q u a l i t y f o r e c o n o m i c utilization. T h e G o d a v a r i coals are o f l o w r a n k a n d r e s e m b l e t h e coals o f t h e Son-Mahanadi, P e n c h - K a n h a n a n d W a r d h a coal basins in c o m p o s i t i o n a l features, suggesting similar d e p o s i t i o n a l c o n d i t i o n s .

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