International Journal of Coal Geology, 9 {1988) 305-314 305 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Occurrence of faunal coal balls in Gondwana sediments (Permian) of Arunachal Himalaya, India

ANAND-PRAKASH 1, TRILOCHAN SINGH 2 and SURESH C. SRIVASTAVA'

1Birbal Sahni Institute o[ Palaeobotany, Lucknow-226007, India eWadia Institute of Himalayan Geology, Dehradun, India

(Received May 22, 1986; revised and accepted June 16, 1987)

ABSTRACT

Anand-Prakash, Singh, T. and Srivastava, S.C., 1988. Occurrence of faunal coal balls in Gond- wana sediments (Permian) of Arunachal Himalaya, India. Int. J. Coal Geol., 9: 305-314.

The nature and genesis of fossiliferous inorganic concretions present within the coal and car- bonaceous beds of Garu Formation, Arunachal Pradesh have been studied in detail. These con- cretionary bodies are described here as "coal balls" due to their close similarity with the true coal balls reported from European and American coals. Further, the presence of coal balls in this area also indicates that the sediments of the Garu Formation were deposited in shallow marginal swamps subjected to marine incursions during the Lower Permian.

INTRODUCTION

The term coal ball has been used for a long time to describe irregularly dis- persed inorganic concretions of variable size and shape found in some coals and associated sediments of Europe and America. The first systematic study of coal balls was carried out by Stopes and Watson in 1909. Later, Jeffrey (1917) studied coal balls in connection with the origin and development of coals. Further, S.E. Moore (1940) defined coal balls in general and has also given a detailed description of the various types of concretions found in coal seams and associated sediments.

During the past two decades much work concerning the origin and genesis of coal has been carried out. In this connection, coal balls were also studied in detail, hence the understanding regarding the origin and development of coal balls has been constantly growing and has contributed to the concept of in situ (autochthonous) formation of coal. Evans and Amos (1961) suggested that certain Iowa coal balls were formed by the introduction of salt water through springs from the floor of the coal seam. This probably represents a specialized

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condition for the formation of coal balls. Regarding the environment in which coal balls could have formed, L.R. Moore (1968) related the occurrence of coal balls with marine animal remains with the seaward extension of swamps, which were subjected to marine incursions, possibly across an offshore barrier. Eggert and Phillips (1982) studied the environment of deposition of the coal balls from Fountain and Parke counties, Indiana and suggested near-shore condi- tions for the formation of these coal balls. Mamay and Yochelson (1962) pro- posed a detailed classification for coal balls occurring in Pennsylvanian coal seams.

So far, the occurrence of coal balls, particularly the fossil-bearing coal balls, has not been reported from Indian coals. However, pyrite concretions ( sulphur balls) have been observed in a number of collieries in the Pench-Kanhan coal- field of Satpura Gondwana Basin, Madhya Pradesh, India. The calcium car- bonate, iron oxide and iron sulphide (pyrite) concretions have also been reported from a number of Lower Gondwana coalfields ( Ghosh, 1971; Chandra et al., 1971) but in all these cases the concretions contain no fossils except a fossil wood (Dadoxylon) from a siderite sphere in Gondwana Coal measures (Rao, 1936).

Recently, the present authors have studied a number of concretions contain- ing well-preserved marine animal remains in the coal and associated carbon- aceous sediments of the Garu Formation, Arunachal Himalaya. In the present communication, these concretions are being designated as true coal balls for the first time from Indian extrapeninsular Gondwana sediments. An at tempt has also been made to interpret the palaeoenvironmental conditions under which these sediments have been deposited on the basis of the nature of the coal balls.

GENERALGEOLOGY

The Permian Gondwana sediments in Arunachal Pradesh are tectonically disposed in a linear and narrow belt all along the foothills of the Lesser Him- alaya. The Gondwana sediments are upthrust against the Siwalik rocks along its southern boundary, while the northern boundary is marked by a plane of discordance which is revealed mostly as a thrust against Miri and/or Bomdila Group of rocks (Fig. 1 ). These sediments are affected by the Himalayan tec- tonism and are often folded in nature. Gondwana sediments exhibit; (a) con- tinental facies with typical Glossopteris flora and (b) marginal facies with typical marine fauna. The present interest lies in the sediments of marginal facies which have been referred to as "Garu Formation" by Kumar and Singh (1974) characterized by a succession of siltstone, coal, carbonaceous shale, sandstone and pebbly mudstone units.

The lithological succession of the Garu Formation is given in Table 1 and shown in Fig. 2.

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Lithologieal succession of the Garu Formation (after Kumar and Singh, 1974, improved by the authors)

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Black carbonaceous shale, bands and lenses of coal with concretions ( = coal balls), white micaceous sandstone and brown ferruginous shale.

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309

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PLATE 1

1. A prominent coal ball in the carbonaceous shale unit of the Bomte Member, Garu Formation near Garu Village.

2. Coal balls in carbonaceous shale, aligned both along and across the bedding plane. Direction of the pen indicates the bedding plane.

3. Irregularly dispersed crinoid and brachiopod shells on the surface of the coal ball. 4-7. Coal balls of different size and shape:rounded, subrounded and elongated.

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PLATE 2 1. Brachiopod fossils (Chonetes sp. ) placed in the central part of a coal ball. 2. Concentration of fossils, mainly brachiopods, in a coal ball showing irregular distribution. 3. A concretion showing concentric rings. 4. A gastropod genus Platyteichum sp. forming the nucleus of a coal ball.

5-6. Gastropod fossils (Platyteichum sp.) isolated from coal balls. 7. A concretion showing pyrite mineralization in patches.

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The concretions studied here have been recorded from the coal and carbona- ceous shale beds of the Borate Member of the Garu Formation. Most of these concretions have yielded Lower Permian marine fossils such as Chonetes sp. and Platyteichum sp. These concretions are here referred to as coal balls on the basis of their physical, chemical, and faunal characteristics.

NATURE AND OCCURRENCE OF COAL BALLS

The coal balls occur generally in the coal and carbonaceous beds of the Borate Member of the Garu Formation. These sediments are present in a linear belt between Garu and Tatamari villages and also beyond that in an east-west di- rection in the West Siang District of Arunachal Pradesh (Fig. 1). The coal balls occur most frequently between Gensi and Tatamari where the Gondwana sediments attain their maximum thickness. The frequency of coal balls de- creases towards east. They have also been observed in the Gondwana sedi- ments near Renging in the East Siang District.

The concretions can be clearly marked on the outcrop surface by their rounded to subrounded, oval-elongated shape (Plate 1, figs. 1, 4-7) and hard nature which protects them from weathering. Their size generally varies from 1 cm to 30 cm, in some cases it ranges upto 50 cm. The coal balls are seen embedded in the sediments both along and across the bedding plane (Plate 1, fig. 2 ), and are distributed unevenly throughout the exposed surface.

The coal balls are generally composed of calcium carbonate, magnesium car- bonate, iron carbonate and iron-oxide with varying amounts of clay and silt. A small amount of phosphate and iron sulphide is also present. Most of the coal balls are massive and calcareous in nature. They have yielded well-preserved marine animal fossils, viz. brachiopods, bivalves, gastropods, cephalopods, cri- noidal remains and bryozoans (Singh, 1978a,b,c,d). These fossils either form a nucleus (Plate 2, fig. 4) or are concentrated in the central part of the coal balls ( Plate 2, fig. 1 ), but mostly the fossils are dispersed in an irregular pat- tern in the entire coal ball (Plate 1, fig. 3 and Plate 2, fig. 2). Although it is difficult to isolate fossils from the coal balls, some well preserved fossils have been extracted (Plate 2, figs. 5, 6). The fossils indicate an Early Permian age (Singh, 1981). Some concretions, however, exhibit concentric rings (Plate 2, fig. 3) and pyrite mineralizations (Plate 2, fig. 7). Such concretions rarely contain fossils.

DISCUSSION

Studies of coal and their associated sediments have been focussed on either their behaviour, genesis and utilization or to their origin and development. Coal balls were studied in order to understand the palaeoenvironmental con- ditions of the basin during and after the deposition of sediments. The post-

depositional conditions are mainly reflected by diagenesis and lithification ~,l sediments.

On the basis of the present knowledge, it is widely accepted that most of the Carboniferous coals of Europe and America are autochthonous in origin. whereas those of the southern hemisphere (Gondwanaland) are allochtho- nous in origin. Amongst these the coal balls, particularly the fossil-bearing ones, generally occur in in situ coals. Almost all types of coal balls have been recorded from the autochthonous Carboniferous coals of Europe and America. Most contain well-preserved plant and animal remains of both fresh water and marine origin. Coal balls occur rarely in Gondwana coals because of their drifted origin. In most of the cases the concretions are devoid of fossils. Generally, these are simple concretions of calcium and magnesium carbonates and iron sulphide, and not coal balls in the true sense because of the general absence of fossils. However, some fossils have been reported rarely from these coal balls. Thus in general, the absence of coal balls in most of the Gondwana coals, in- eluding those of India, may be attributed to their drifted origin and continental nature of the depositional basins. In the same way, the Gondwana coals of India also do not contain true coal balls because concretions from Indian Gondwana sediments are generally devoid of fossils. To the best of our knowl- edge the occurrence of true faunal coal balls is reported here for the first time from Indian Gondwana coal and associated sediments.

Most of the concretions recovered from the coal and associated sediments of the Garu Formation, Arunachal Himalaya are massive and calcareous in nature. They are generally rounded to subrounded and are composed of ho- mogeneous, fine grained material. No concentric rings have been observed in such concretions (Plate 2, fig. 1). They contain in most of the cases well- preserved marine animal remains which are either grouped together or are irregularly distributed. There is no observable sign of compression and degra- dation in these fossils. They appear to have been formed during the peat-form- ing stage and are identified here as true coal balls which are placed in Group C of the classification scheme of Mamay and Yochelson (1962), which desig- nates coal balls containing only animal remains. The other type of concretions shows concentric rings and seldom contains fossils, and is thus distinguishable from the above coal balls.

The presence of the coal balls with marine animal remains, therefore, raises an important question, of whether the Gondwana coals of Arunachal Himalaya have also been formed under conditions similar to those of European and American coals? If so, then the Gondwana coals of Arunachal Pradesh cannot be compared with the peninsular Gondwana coals of India, at least as regards their genesis and mode of formation. Further, the Indian Gondwana coals are believed to have been tormed in continental basins under fluvial conditions and are allochthonous in origin, unlike Arunachal coals which are assumed to

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have formed in marginal basins in a lagoonal environment under marine influence.

The environment in which the Gondwana coals of Arunachal Pradesh have formed and occurrence of marine animal remains suggest the possibility of marine incursions across the coal-forming swamps. This Gondwana basin of Arunachal Himalaya probably represented a linear lagoon marginal to the sea shore. The basin also seems to have been fed by some channels supplying fresh water and small amounts of land-derived sediments to the swampy areas, as a variety of pollen and spores belonging to land plants is present in the coal balls, similar to those described from the coal and carbonaceous beds containing these concretions ( Srivastava and Dutta, 1977; Singh, 1979 ). Under these con- ditions the vegetal matter slowly accumulated in the basin forming peat mainly derived from plants growing in shallow-water marginal conditions.

At this stage, both before and during peat formation, the peat swamps were invaded by the sea at frequent intervals. This possibility has also been sug- gested by L.R. Moore (1968) for the formation of coal balls in European coals. Thus, the necessary salts (calcium and magnesium carbonates) required for the formation of the coal balls were supplied by the sea water during these periodic marine incursions. Another main constituent of the concretions is iron sulphide (pyrite) which seems to have been formed by sulphur bacteria during the diagenesis of peat. Furthermore, the coal balls seem to have formed before the compaction of the vegetal matter as the coal and carbonaceous shale layers are often seen folded around the coal balls. In summary, a critical as- sessment of the coal balls indicates seaward extension of the peat swamps which were periodically inundated by the sea probably across the offshore barriers. Under these conditions marine animals might have reached the swamps mainly during periods of storms and high tides. However, afterwards the sea water probably withdrew rapidly from the swamps and the peat formation continued almost unaffected. The availability of salt water in the swamps, therefore, cre- ated favourable conditions for the coal ball petrifaction and preservation of the animal remains in them.

As regards the palaeogeographical aspects, at present the Gondwana sedi- ments of Arunachal Pradesh are located far south of the actual Tethyan Hi- malayan zone. This poses the problem as to how the sea water could reach areas, where Gondwana sediments were being deposited during Permian times a considerable distance from the sea. A narrow sea arm could have existed during the Permian in this area connected with the Tethys sea in the north. Similarly, the presence of marine beds at Umaria and Manendragarh, Madhya Pradesh, have been explained by the existence of a sea arm during the Permian, deep inland into the peninsular areas of the Indian Shield.

AC KNOWLEDGEMENTS

This paper forms a part of the collaborative research programme of the Wadia Institute of Himalayan Geology, Dehradun and Birbal Sahni Institute of Pa-

laeobotany, Lucknow. The authors are thankful to the IXrectors of’ these In

stitutions for constant encouragement and inspiration.

REFERENCES

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Eggert, D.L. and Phillips, T.L., 1982. Environments of deposition-coal-balls, cuticular shale, and grey-shale floras in Fountain and Parke counties, Indiana. Dept. Nat. Res.. Geol. Surv. Rep. 30, State of Indiana: 43 pp.

Evans, W.O. and Amos, D.H., 1961. An example of the origin of coal balls. Proc. Geol. Assoc., 72: 445.

Ghosh, T.K., 1971. Microstructure of some coal balls from the Lower Gondwana coalfields 01 India. Q. J. Geol. Sac. India, 43: 241-242.

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Singh, T., 1978b. A new species of spiriferoid genus Subansiria, Subansiri district, Arunachal Pradesh. In: V.J. Gupta (Editor), Contributions to Himalayan Geology. Hindustan Publish- ing Corporation (India), Delhi, pp. 162-164.

Singh, T.. 1978c. Brachiopods from Permian Formation of Siang district, Arunachal Pradesh. In: V.d. Gupta (Editor), Contributions to Himalayan Geology. Hindustan Publishing Corpora- tion (India), Delhi, pp. 171.-188.

Singh, T., 1978d. Lower Permian cephalopods from Eastern Himalaya, India. Himalayan Geol.. 8(l): 178-193.

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Stopes, M.C. and Watson, D.M.S., 1909. On the present distribution and origin of the calcareous concretions known as ‘coal balls’. Philos. Trans. R. Sot. London, Ser. B, 200: 167~-218.