PalaeoboTalliSI 49 (2000): 177-195

0031-0174120001177 - 195 $2.00

Petrography, genesis and deposition of

Tertiary coals from Northeastern India
BASANT K. MISRA

Birbal Salwi Institute of Palaeobotany, 53 University Road, Lucknow 226 007, India.

(Received 30 June 1999: revised version accepted 07 March 2000)

ABSTRACT

Misra BK 2000. Petrography, genesis and deposition of Tertiary coals from Northeastern India.
Palaeobotanist 49(2) : 177-195.

The coallields of AlUnachal Pradesh. Assam and Nagaland are disposed along a linear belt of overthlUsts
where the coal-bearing Oligocene (Tikak Parbat Formation. Barail Group) strata are folded and dispersed
into a number of thrust slices. Whereas. in Garo. Khasi and Jaintia hills of Meghalaya the coal seams are
associated with an almost undisturbed and sub-horizontal Late Palaeocene SylhetJTura formations ofJaintia
Group.

The coals are bright, non-banded. vitrinite-rich and have moderate to low amountS of liptinite and
inertinite macerals. Pyrite and calcite are the dominant associated minerals, besides generally low proportions
of clay and quartz. The coals have moderate to very high amounts of fluorescing macerals consisting chiefly
of perhydrous vitrinite. liptodetrinite and resinite. Macerals. cutinite. sporinite, suberinite. exsudatinite,
alginite and fluorinite are also present. The rank of the coals ranges between high volatile bituminous C and
B stages, on the basis of vitrinite reflectance (Garo Hills: 0'54-0'62%: Jaintia Hills: 0'68-0·81 % and Assam:
0'70-0'74%).

The coals from northeastern India were derived essentially from tropical deciduous mangrove-mixed
angiospermous forest vegetation. The coals, presumably, originated from autochthonous to hypo-
autochthonous eutrophic peat accumulations in lagoons and/or near-shore back swamps in Assam. Arunachal
Pradesh and Nagaland and in small isolated estuarine back swamps in Meghalaya. The vegetal maller. in
general. was subjected mainly to anaerobic biodegradation under neutral to mildly alkaline subaqueous
conditions and facilitated the precipitation of early diagenetic pyrite and calcite and ill situ release of plant-
bound minerals in the peal. Seasonal freshwater ponds or lakes developed occasionally on the ancient-peat
surfaces hosted the growth of aquatic and water-edge taxa whose remains are present in the coal seams.
Especially pteridophytic herbaceous and shrubby vegetation growing in the vicinity of ancient peat swamps
appear to be mainly responsible for the presence of structured and detrital inertinites. Occasional burning of
the wet peat surfaces has been inferred to be the reason for relatively high inertinite contents in the coals
from Meghalaya. It has also been presumed that whenever biodegradation of organic matter was severe the
normal supply of vegetal matter fell short to produce a peat layer. instead some minor and major authigenic
partings were developed within the pre-existing peal. The variation in the rank and thickness of the coal
seams was presumably controlled by the prevailing geothermal gradient and tectonic conditions in different
areas.

Key-words-Petrography. Coal genesis, Depositional environment. Tertiary coal. North-East India.

© Birbal Sahni Institute of Palaeobotany. India

178 THE I'ALAEOBOTANIST

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INTRODUCTION the sediments underlying the coal measures. In Meghalaya

the coal seams are thin (0'2-2 m, occasionally up to 3 m thick),

N ORTHEASTERN India is known as a repository of

country's important Tertiary coal deposits (Fig. J), from
Garo, Khasi and laintia hills of Meghalaya, in the west, to
few in number and laterally non-persistent. Eastwards from
Mikir Hills, in Nagaland, Assam and Arunachal Pradesh, the
coal seams are distinctly thicker (4- j 8 m) and relatively more
Mikir Hills, in central Assam, the coal bearing strata are Late in number. The coal-bearing strata in Meghalaya are either
Palaeocene in age. The coal deposits in Nagaland, Assam and undisturbed or have suffered only minor structural disturbance.
Arunachal Pradesh are associated with the Oligocene On the other hand, the coal-bearing sediments in Nagaland,
sediments. In Nagaland and Assam few thin seams with Assam and Arunachal Pradesh are exposed discretely along a
thickness between 0·5 to 1·5 m, especially in Makum, Dilli- narrow linear belt expressed in a series of complex and
leypore and Nazira (or Borjan) coalfields, are known from imbricate overthrusts, the "belt of Schuppen". The coalfields
 MISRA - PETROGRAPHY. GENESIS AND DEPOSITION OFTERTIARY COALS FROM NORTHEASTERN INDIA 179

of Nagaland (Nazira or Borjan and Changki- Valley), Assam Nagaland, Assam and Arunachal Pradesh
(Dilli-Jeypore and Makum) and Arunachal Pradesh (eastern
part of Makum and Namchik-Namphuk), economically the The coalfields of Nagaland, Assam and Arunachal
most important Tertiary coal resources of India, are aligned Pradesh (Figs 1-2) are aligned along an active mobile belt
along this belt. which experienced intense tectonic disturbance resulting into
The present contribution deals with the genesis of some a series of imbricate overthrusts. known as "zone" or "belt of
of the Tertiary coal deposits of northeastern India on the basis Schuppen." Thrusting along this belt accompanied folding and
of petrographic data along with the available mega- and micro- interlocked slicing of Tertiary strata. The most important
floral and sedimentological information. synclinal structure developed is in Makum and Namchik-
Namphuk coalfields. The anticlinal structure is reported from
GENERAL GEOLOGY AND Makum and Dilli-Jeypore coalfields. The Tertiary sedimen-
LITHOSTRATIGRAPHY tation in this belt commenced in an external trough near the
platform con'esponding to miogeosynclinal belt. Along this
belt rapid subsidence and detrital supply resulted in a >2000
The general geology, structure and lithostratigraphic to 6000 m thick pile of Tertiary sediments. However, during
sequence of coal-bearing areas in northeastern India described Barail (Oligocene) sedimentation intermittent phases of slight
here are based primarily on Raja Rao (1981). For the details emergence allowed the development of widespread coal-facies.
about the chemical properties of the coals and number and The Tertiary sediments in Nagaland, Assam and
thickness of coal seams present in Arunachal Pradesh, Assam. Arunachal Pradesh exhibit rapid lateral and vertical variations
Nagaland and Meghalaya, the reader is referred to Raja Rao (1981).

SCALE
.,
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l"'.-::> • • /'*3 l . (.
. f· '-. _.-' v p.., TEZPUR - JORHAT / " .r LEGEND
s s ~ ':\.0 • r.} (* 1. MAKUM COALFIELD
A NOWGONG~::::- Borh~llQ \ 2. JEYPORE COALFIELD
. <. .
=<>~~~.~H/ATI ~~\. /NAGALAND ./
3. NAZIRA IBORJAN)
COALFIELD

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.,. . ., . . . .. \.. . . _......
'-\'7 -'
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(
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. MEG H A··.. . LAY A .~.---....... ) "",0 " (' / 5. NANGWALBIBRA COLLIERY
*' 5 Khasi g. Jaintia*,4 "? 0SHILONG <'
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(' ',,' 'yo (WEST DARANGGIRI
COALFIELD)

_._._CH~~APUNJI.
.......... _ .Hi.lIs
-,. .) ..,
c..._ ('vMAN
. I PUR ".
'v" ._./

Fig. I-Map showing locations of some of the important Tertiary coal deposits in Northeaslem India (modified after Misra. 1992).
180 THE PALAEOBOTANIST

and range in age from Eocene to Pliocene. The lithostrati- strata were deposited under stable shelf condition along the
graphic sequence in these areas has been given in Fig. 3. periphery of the plateau. The coal measures are sub-horizontal
The coal-bearing sediments of Tikak Parbat Formation in attitude. The sedimentary exposures in the area range in
(Oligocene), in the 'belt of Schuppen' ,are preceded by marine age from Late Cretaceous to Eocene and show frequent
and followed by fluvial sedimentary sequences. The lithofacies variations. The main lithostratigraphic sub-divisions
predominance of fine-grained sediments comprising sandy- are given in Fig. 4.
shale, si Itstone, shale, mudstone and clay units are The workable coal seams in Khasi and Jaintia hills are
characteristic of deposition by tracti ve currents. Presence of associated with the Lakadong Sandstone Member (Late
arenaceous foraminifers, although not diagnostic of age, and Palaeocene) of Sylhet Limestone Formation, Jaintia Group.
sporadic limestone beds (Misra, 1981) indicate a definite In Garo Hills, the approximate equivalent of Lakadong
marine influence. The Barail Group comprises a wide spectrum Sandstone Member is the Middle MemberofTura Formation,
of lithofacies deposi ted under sha Ilow mari ne, lagoonal, deltaic which itself is considered partly equivalent to the Sylhet
and fluviatile environments. Limestone Formation. In Meghalaya, coal-bearing sediments
comprising essentially lithomargic clays, shales, argillaceolls
Meghalaya sandstones and coal seams, are sandwitched between
foraminiferal limestone units which implies a short duration
The major part of Meghalaya (Fig. 2), covered by episode of emergence between two marine incursions. The
Precambrian granites and gneisses, is a plateau-Shillong peat accumulation in Meghalaya, during this interphase
Plateau, which is a horst, uplifted during Early Cretaceolls. Its occurred in deltaic, estuarine and lagoonal environments (Raja
evolution is linked with the basaltic effusion (Shylhet Trap) Rao,1981).
during Jurassic-Early Cretaceous periods. The coal-bearing

50
HoJ u-I
0
~ 50
!
100Km
I
+
+

SCALE

LEGEND
IVv\v.l ALLUVIUM (RECENT) 1>;'/:::1 BARAIL GROUP (OLIGOCENE) I.. <I BASALT (JURASSIC?)

F==:l DIHING AND DUPITIIA GROUPS ITTTTTTl JAINTIA AND DISANG GROUPS 1+++1
METAMORPHIC AND IGNEOUS
~ (MIO - PLIOCENE) UlllllJ (PALAEOCENE - EOCENE) ~ ROCKS (ARCHAEAN)

V777:l TIPAM AND SURMA GROUPS I+t+l=l=H LANGPAR- MAHADEK FORMATION

I::?-::-:::I THRUST AND FAULT
~ (MIOCENE) ttttttt1 (CRETACEOUS)

Fig. 2-Map showing geological and structural selling in Northeastern India.

 MISRA - PETROGRAPHY. GENESIS AND DEPOSITION OFTERTIARY COALS FROM NORTHEASTERN INDIA 181

STATUS OF PETROLOGICAL 1976; Navale & Misra. 1979; Goswami, 1985, 1987; Ahmed.
INFORMATION 199Ia); West Daranggiri Coalfield, Garo Hills, Meghalaya
(Ghosh, 1964, 1969; Ahmed & Bharali, 1985; Ahme~, 1991 b;
Mishra & Ghosh, 1996) and Laitryngew Coalfield, Khasi Hills,
Petrographic study of Tertiary coals in India was initiated Meghalaya (Ghosh, 1964, 1969; Sen & Sen, 1969; Ahmed.
by Ganju (1955). The earlier studies were made on thin 1971). Most of these petrographic data (Fig. 5) are of only
sections of coals, and dealt with the morpho-petrography only. very limited use in the present context. because they do not
After about a decade, quantitative assessment of macerals and provide the requisite information for ascertaining the speci fie
microlithotypes, under incident light, on polished coal surfaces nature of coal genesis. The majority of the information referred
(particulate pellets) was introduced by Ghosh (1964. (969) to is based on the study of a few samples (Ghosh, 1969; Sen
and Sen and Sen (1969). Determination of coal rank through & Sen, 1969; Ahmed, 1971; Ahmed & Bharali, 1985; Navale
reflectance measurement, earl ier used only sporadically & Misra, 1979; Mukherjee, 1976). Whereas, in some cases
(Ghosh, 1969), was routinely established by Navale and Misra the data appears to be unreliable as the authors (Sen & Sen,
(1980) and Misra (1981). The use of fluorescence microscopic 1969) have recorded very low proportion of liptinite and
study was made only during the latter part of eightees (Misra, inertinite macerals or extremely high amount of liptinite
1992a, b, c; Misra & Navale, 1992; Misra et al., 1990). macerals (Goswami, 1987) not supported by fluorescence
Obviously, most of the earlier petrographic investigations, microscopic studies (Misra, 1992a, b, c). Besides, some of
in general, were primarily oriented towards the understanding the petrographic data are on mineral matter-free basis (Sen &
of coal microconstituents as such, under both normal Sen, 1969; Navale & Misra. 1979; Goswami, 1987). The
transmitted and incident light modes. However, since 1964 record of low to very low contents of inertinite macerals
some available petrograph ic data provide general information presumably resulted from the unrecorded fractions of fungal
about quality and grade of some of the important Tertiary coal remains (sclerotinite: spores, sclerotia, hyphae, etc.) that are
deposits of nOltheastern India, viz., Makum and Dilli-Jeypore frequent in these coals.
coalfields, Assam (Ghosh, 1969; Sen & Sen, 1969; Mukheljee.

Age GrouplFormation Thickness Lithology

(metres)

Pliocene Dihing Gr. 1.800 Mostly pebbly sandslone with thin greyish clay bands.

Pliocene Namsang Fm. 800 Fine to coarse grained sandstones with clay beds.

Miocene Tipam Gr.

Girujan Clay Fm. 1.800 Mottled clays with greyish sandstones.

Tipam Sandstone Fm. 2.300 Fine to coarse grained ferruginous bluish green
micaceous and felspathic sandstones. sandy shales,
sandy clays and clays.

Oligocene Barail Gr. Greyish to yellowish white massive fine grained

Tikak Parbat Fm. 600 sandstones. sandy shales. siltstones, mudstones,
sporadic dolomitic limestones and coal seams.

Baragolai Fm. 3.500 Greyish to bluish grey or yellowish mudstones.

massive sandstones. sandy shales. carbonaceous
shales. clays and thin coal seams.

Naogaon Fm. 3,000 Hard massive medium grained sandstones,

dark grey spli ntary shales wi th sandstone
alternations.

Eocene Dlsang Fm. Grey to dark grey splintary shales. sandy shales
with thin aiteillations of quartzitic sandstones.

Fig. 3-A generalized lithostratigraphic sequence of Teniary sediments in Arunachal Pradesh, Assam and Nagaland (afler Raja Rao, 1981).
182 THE PALAEOBOTANIST

Age Formation/ Thickness Lithology

Member (m)
Late Eocene Kopili Formation Ferruginous sandslOnes. grey siltstones. and pyritous
splimary shales.
Middle Eocene Sylhet Li mestone Formation 60-150 Bluish massive to thinly bedded limestones with marly
Prang Limestone (Siju interbands (Rich in foraminifers).
Limestone) Member
Early Eocene Nurpuh SandslOne Member 15-26 Coarse to medium grained sandstones with bands of
sandy limestones.
Umlotdoh Limestone Member 70-110 Grey to pinkish grey limestones. sandy limestones and
calcareous sandstones (Rich in foraminifers).
Late Palaeocene Lakadong Sandstone Member 35-250 Buff coloured medium to coarse grained arkosic
(= Mid. Member.ofTura sandstones with pebbly interbands, thin grey
Formation In Garo Hills) carbonaceous shales. clays and coal seams.
Palaeocene Lakadong Limestone Member 25-60 Grey to brownish grey limestones. siliceous limestones
(Rich in foraminifers).
ThelTia Sandstone Member 20-80 Buff coloured medium to coarse grained arkosic
S<lndSlOnes with thin b<lnds of pyrite-rich silty sandstones
(U nfossil i ferous).
Late Cretaceous Langpar Formation 10-50 Buff coloured c<llc<lreous-ferruginous s<lndstones <lnd
(Danian) e<ll1hy limestones.
Maastrichtian 160-335 Massive coarse grained gl<luconitic s<lndstones containing
d<lrk grey sh<lles <lnd calc<lreous horizons.

Fig. 4--Generalized lithostratigraphic sequence of Tertiary sediments in Meghalaya (after Raja Rao. 1981).

MEGASCOPIC CHARACTERS vitrinite macerals (41-79%, occasionally as low as 37%) with

low to moderate amounts of liptinite (5-19%) and inertinite
In general. the Tertiary coal seams of northeastern India (3-20%, rarely up to 34%) macerals. Associated minerals
are devoid of lithotype banding, unlike the Permian Gondwana comprise mainly pyrite, calcite and argillaceous matter (clay
coal seams. They are bright and appear to comprise entirely and quartz). Their contents vary between 4% to 20%, rarely
vitrain lithotype (Raja Rao. 1981; Misra. 1981, 1992a). up to 35% (Fig. 6). These coals are well known for their pyrite
However, certain coal seams ofLaitringew-Cherrapunjee area. contents (1-20%, max. 33%, Misra, 1992a, b).
Khasi Hills, Meghalaya are thinly banded, containing <I to 3 The Tertiary coals of northeastern India, under
cm thick alternating bands of bright and semi-bright bands, fluorescence mode (Fig. 7 on mineral matter-free basis), are
and also occasionally dull bands. The Wakting seam (main characterized by high to very high proportions of fluorescing
seam) of Nazira Coalfield, Nagaland has a thick dull band in macerals (47-88% max. up to 91%) comprising chiefly
its basal part (Raja Rao, 1981). The coals, especially from perhydrous or fluorescing vitrinite (25-62%, max. 78%) and
Nagaland. Assam and Arunachal Pradesh, show greasy to liptodetrinite (8-35%, max. 40'5%) with subordinate amounts
vitreous lustre and break with sub-conchoidal to conchoidal of other liptinite macerals (4-22%, max. 28%). The liptinite
fracture. They are blocky but crumble easily because of rapid fractions in these coals, excluding liptodetrinite, are formed
oxidation on aerial exposure. These coals commonly contain mainly by resinite (1-18%), and cutinite+suberinite (004-7'6%).
tiny pyrite specks and in certain seams concretions and The sporinite content is always low (0'3-2%, max. 3'3%).
encrustations of pyrite are also present, e.g. Bapung Coalfield, Alginite (up to I %) maceral is represented by alga
]aintia Hills, Meghalaya and Makum Coalfield, Assam, besides Botryococcus. In addition to these, bituminite, exsudatinite
criss-cross veins of secondary pyrite (Raja Rao, 1981; Misra, and f1uorinite are also commonly present (Misra. 1992a. b).
1981. 1992a, b, c; Ahmed, 1971; Ahmed & Bharali. 1985).
Maceral Characteristics
The vitrinite macerals in the Tertiary coals of northeastern
India comprise chiefly collodetrinite (earlier termed as
OPTICAL PROPERTIES OF COALS
desmocollinite) and collotelinite (telocollinite), besides low
amounts of collogelinite (corpocollinite) and telinite (Misra,
Maceral* Composition 1981, 1992a, b). Invariably. collodetrinite is the dominant
In comparison to most of the Permian Gondwana coals maceral in all the coal seams. Collodetrinite and some of the
of India, the Tertiary coals of northeastern India are rich in collotelinite display granular or spongy texture with relatively
* The terminology for description of macerals. excluding those of vitrinite group. is as per ICCP (197 I). For vitrinite macerals, revised terms of the ICCP
(1995) have been used.
 MISRA - PETROGRAPHY. GENESIS AND DEPOSITION OFTERTIARY COALS FROM NORTHEASTERN INDIA 183

weaker reflectance than the associated vitrinite. Frequent of the coal seams. Granular bituminite is commonly associated
occurrence of fungal spores, sclerotinite, and fine clusters and with trimacerites, c1arites and in collodetrinite. Pyrite is usually
knots of hyphae in vitrinite indicates that the vegetal mailer associated with almost all the liptinite macerals. •
forming the coal seams experienced high degree of Most of the coal seams of Assam, Arunachal Pradesh.
biodegradation. On the basis of thin section study, Ganju Nagaland and Meghalaya (Khasi and Jaintia hills) have very
(1955) had already reported the presence of highly degraded high total fluorescing maceral contents (normally 72-91 %).
woods due to fungal activity in the coals from Makum However, certain coal seams. especially from Nazira Coalfield
Coalfield, Assam. Framboids. crystal and granules of pyrite (52-61-4%) and some patchy seams of Jaintia Hi lis, and the
in vitrinite, especially in collodetrinite, and c1arite and main seam (25-54%) of West Daranggiri Coalfield (Garo
trimacerite are frequent (Misra, 1992a). Occasionally, fine Hills) have relatively lower proportions of total fluorescing
microgranules of pyrite are seen inside the lumens of macerals. In general, the coal seams from Jaintia Hills have
collotelinite, e.g. coals of Nazira Coalfield, Nagaland. higher I iptodetrinite content than those from the other areas.
Crushing and shattering of vitrinite and inertinite macerals, The coal seams of Garo and Jaintia Hills are also characterized
including microfaults and brecciation, are frequent in the coals by relatively higher amount of particulate liptinite macerals.
from Arunachal Pradesh, Assam, Nagaland and Garo Hills of excluding Iiptodetrinite, than that of Assam and certain coal
Meghalaya. seams (Sutunga area) of the Jaintia Hills.
The main inertinite macerals recorded are semifusinite,
fusinite (rank-, degrado- and pyro-inertinite types), Microlithotypes
inertodetrinite and sclerotinite with sporadic macrinite and
micrinite. Usually the inertodetrinite and sclerotinite together Misra (1981) carried out quantitative assessment of
constitute the major fraction of the total amount of inertinite microlithotypes in the coals of Makum Coalfield, Assam. He
present, especially in the coals from Assam and Nagaland found that the coals are rich in vitrite microlithotype (68-4-
(Misra, 1992a, b). The structured inertinites (semifusinite and 94-4%) with subordinate proportions of vitrinertite (3-4-
fusinite) embedded in vitrinite normally have empty cell- 25,6%) and c1arite (2,2-9,8%). Since the data was obtained
lumens. Occasionally, the lumens are filled with calcite and on mineral maller-free basis, therefore, it is incomplete.
framboids of pyrite and also small amount of argillaceous Goswami (1987) and Ahmed (1996a) and Ahmed (1991a)
mailer. In contrast to the coal seams of Assam and Nagaland, provided some qualitative information about the
those from Meghalaya have relatively higher proportion of microlithotypes (by plotting of maceral data on triangular
inertinite macerals consisting mainly of semifusinite and diagram for knowing the microlithotype composition of coals)
fusinite (Fig. 6). in the coals. respectively, from Makum and Dilli-Jeypore
Pyrite and calcite are the main minerals associated with coalfields. This information is only qualitative and not
the coals from northeastern India. Clay and qual1z (argillaceous quantitative and also on mineral maller-free basis. Goswami
matter) together are normally between 20% to 40% of total (1987) found clarite- V and duroclarite- V dominance in the
mineral matter content of the coals. microlithotype assemblage of the Makum coals, whereas other
Under fluorescence mode, perhydrous or fluorescing associated microlithotypes are vitrite and vitrinertite-V
vitrinite constitutes the bulk of the total fluorescing macerals Ahmed (1991a. 1996a) observed only two microlithotypes-
recorded in the coals from northeastern India (Misra, 1992a, vitrinenite- V and duroclarite- V in the coals from Oi lIi-Jeypore
b). Among the liptinite macerals. liptodetrinite and resinite Coalfield, whereas in the Makum coals only one (vitrinertite-
are the dominant followed by cutinite+suberinite and spori nite. V) was reportedly present. Presence of only one or two
Exsudatinite and fluorinite are common in almost all the coal microlithotypes in these coals is inconceivable, therefore the
seams. Tenuicutinite is common and crassicutinite is sporadic. observations are unrealistic and unreliable. However, on the
Suberinite occurs commonly in c1arite, trimacerite as thin to basis of author'S observation, the coal seams of Assam and
thick bands, fragments and shreds. It is also associated with Nagaland have the dominance of vitrite and sporinite-poor
liptodetrinite as highly degraded and amorphous bodies. clarite along with subordinate amounts of duroclarite and
Resinite is especially common in Dilli-Jeypore (Assam), vitrinertite microlithotypes. besides occasionally low
Nazira (Nagai and), and Bapung and Jaraiil (Meghalaya) proportions of carbopyrite. The c1arodurite and inertitite
coalfields. The liptodetrinite present in these coals is formed microlithotypes are uncommon. whereas durite is almost
chiefly either by resinite or resinite+cutinite+suberi nite. Well- absent. Some of the coal seams from Jaintia Hi lis, Meghalaya
preserved and partially degraded alginite (Botryococcus) has have almost the similar microlithotype composition as the
been observed in Khasi and Jaintia hills of Meghalaya. preceding ones. However, the coal seams of Meghalaya
However, highly degraded and disorganized alginite, on the generally have higher proportions of duroclarite and
basis of fluorescence properties, has been observed in most clarodurite than that of Assam and Nagaland. Associated
184 THE PALAEOBOTANIST

Area Mineral
Coalfield Ro Vitrinite Liptinite Inertinite Matter
Author % % % % %

Makum Coalfield, Assam

Ghosh (1969) 74.5-87.3 2.8-4.7 4.7-18.6 3.0-4.2
Sen & Sen (1969)* 87.8-964 0.9-1.5 2.7-10.7
Mukherjee (1976) 86.5-94.2 0.9-5.0 2.2-6.2 2.2-3.7
Goswami (1985)* 86.3-94.0 1.1-6.3 5.0-7.3
Goswami (1987)* 52.0-94.0 6.0-48.0 0.0-28.5
Ah med (l996b) 0.66-0.71 82.8-86.6 3.9-5.2 5.0-6.7 34-6.7
Mishra & Ghosh (1996) 0.62-0.73 71.1-88.2 3.0-14.0 1.3-164 2.0-18.57

DiIIi-Jeypore Coalfield, Assam

Goswami (1985)* 95.0-98.0 0.3-24 1.5-2.4
Ahmed (199Ia) 0.60-0.67 79.7-814 2.7-2.9 12.0-13.0 34-4.7
Mishra & Ghosh (1996) 0.60 734 14.1 8.3 4.2

Namchik-Namphuk Coalfield,
Arunachal Pradesh
Mishra & Ghosh (1996) 0'53-0,68 68"8-83'7 3'7-15-4 3'5-5'7 7'1-12,6

Jhanzi-Disai Coalfield, Nagaland

Mishra & Ghosh (1996) 0'72-0'74 86'8-87,9 1-4-33 0·6 8·2-11·2

Nazira Coalfield, Nagaland

Goswami (1985)* 72"5-78'8 3"0-9'7 \7'8-18'3

Coalfields of Khasi Hills, Meghalaya

Ghosh (1969) 80'2-89'4 1'3-2'8 2'9-12"8 4'2-6-4
Sen & Sen (1969)* 94'0 1'2 4'2
Goswami (1985)* 89"6-93"0 2'0-3"4 3"6-8-4
Mishra & Ghosh (1996) 0'54-0'63 52'1-74'9 7'9-11'6 5'8-18'1 5'0-20'1
Ahmed el al. (1997) 62"0-72"8 8"2-21'6 4'1-14'7 4'8-9"2

Coalfields of Jaintia Hills, Meghalaya

Ahmed (1971) 82'1-83"4 5'2 7'2-7,6 4'1-5'0
Ahmed (I 996a) 0'74-0'75 81'5-83"0 5'5-6'1 5'1-5-3 6'3-7-0
Mishra & Ghosh (1996) 0'54-0'85 50'6-87"8 2'5-12"0 6'6-25'3 4'2-21'7

Coalfields of Garo Hills, Meghalaya

Ghosh (1969) 84.0-85.0 5,0-6,0 6.0-7.0 3.0-4.0
Goswami (1985)* 86'9-92'5 1'5-5'2 6,0-7"8
Ahmed (l99Ib) 58'5-73"0 3-8-5'0 14-8-31'5 5'0-7-6
Mishra & Ghosh (1996) 0'37-0'54 60'0-79'2 3'1-9"6 3'4-16'7 5'5-27'0

Fig. 5-Maceral composition (volume %) of Tertiary coals from Northeastern India, under normal reflected light (data from other sources).

common microlithotypes are vitrinertite and inertite, and RANK

sporadic durite, especially in the coal seams of Garo Hills,
Meghal~ya, and occasionally minor amount of carbopyrite, The rank (Figs 5, 6) of the Tertiary coals was determined
by reflectance measurements (mean maximum) on vitrinite
(collotelinite) maceral, in oil (R o max, %), The coals of Assam
(R o max. 0,71-0'75%), and those of laintia Hills, Meghalaya
(Ro max, 0'64-0'69%, occasionally 0'72-0'86%) have attained
 MISRA - PETROGRAPHY. GENESIS AND DEPOSITION OFTERTIARY COALS FROM NOrrrHEASTERN INDIA 185

a rank corresponding to high volatile bituminous B to When compared with the chemical properties, the Ro max.
occasionally A stages. The coals of Nazira Coalfield, Nagaland % of the coals, in most cases, do not correspond with each
(R o max. 0'59-0'67%) and West Daranggiri Coalfield ofGaro other. Instead their volatile matter contents indicate relati vely
Hills, Meghalaya (R o max. 0'54-0'62%) have reached only lower rank than that ascertained by the Ro max. %. Whereas,
high volatile bituminous C stage. With few exceptions, on the basis of calorific values and moisture contents their
probably influenced by heating from some subsurface igneous rank seems to be higher than that estimated by the reflectance
bodies (Fig. 6: Sutunga area Ro max. 0,72-0'86%), the younger measurements (Misra. 1992b). The abnormality in the rank of
coal seams (Oligocene) of Assam and Nagaland have attained these coals as estimated by chemical properties and reflectance
higher maturity than the Late Palaeocene coals seams of measurements has been attributed to their higher sulphur
Meghalaya. As a result, there is an eastward increasing trend contents (Das Gupta, 1979; Raja Raa, 1981). It is well known
in coal rank from the West Daranggiri Coalfield, Garo Hills that the Late Palaeocene and 01 igocene coals of northeastern
of Meghalaya to Makum Coalfield, Assam, coinciding with India contain high amount of total sulphur present mostly in
the westward increasing age of the deposit (Fig. 8). This inverse the organic form (60-99%). The remaini ng inorganic fraction
relationship between the coal rank and age of the deposit is is associated as pyrite (Raja Rao, 1981; Misra, 1992a). The
evidently related to the greater depth of burial of the coal seams organic sulphur occurs as thiols, thiophenes, heterocyclic
(>2,000-6,000 m thick overlying sediments), prevailing high thionaphtyl, sulphides and disulphides, etc., and also partly in
geothermal gradient and intense tectonic activity in Assam ring structures. About 50% of the organic sulphur is reactive
and Nagaland. On the other hand, in Meghalaya the coal seams and a part of that displaces oxygen instead of carbon from
experienced shallow burial (±500 m thick overlying sediments) oxygenated groups. Consequently, they contain relatively low
and show least tectonic effect (Misra, 1992a, b; Raja Rao, amount of reactive oxygen for their rank. Low reacti ve oxygen
1981 ).

Area, Mineral
Coalfield & Ro max. Vitrinite Liptinite Inertinite Maller
Coal Seam % % % % %

Makum Coalfield, Assam

Seam no. 3 0'72-0'75 60,5-74,2 5,0-9,5 10'6-19'5 7-7-21'8
Seam no. I 0'73-0'75 60'9-76'9 5'4-8'1 9'1-20'1 52-183

DiUi-Jeypore Coalfield, Assam

Seam no. 6 0.71 528-66.0 8.4-16.2 13.8-16.0 4.0-22.8
Seam no. 4 0'71 54'0 10'2-14'0 6'4-19'8 12'-294
Nazira Coalfield, Nagaland
Main Seam 0'59-0'67 76'4-79'0 8'6-9'2 3'0-72 66-9'0

Bapung area, Jaintia Hills, Meghalaya

Top Seam 0.64-0.69 42.0-66.0 7.0-18.6 13.6-32.6 5.6-22.2
Middle Seam 0.66-0.68 53.0-63.4 11.8-13.6 6.2-200 13.8-20.6
Bottom Seam 0'64-0'69 42'6-68'2 4'0-15'6 6'8-35'0 5'4-22'4

Sutunga area, Jaintia Hills, Meghalaya

Top Seam 0.81-0.86 70.4-74.4 6.0-9.8 7.4-118 56-10.4
Middle Seam 0.72 68.8 108 88 116
Bottom Seam 0'73 58'0 9'2 15'8 17'0

Jarain area, Jaintia Hills, Meghalaya

Top Seam 0.68-0.69 46.6-59.6 10.8-14.0 7.4-19.6 166-35.2
Bottom Seam 0'68-0'69 54'8-56'0 8'4-11'0 5'0-10'0 26'8-28'0

West Daranggiri Coalfield, Garo Hills,

Meghalaya
Main Seam 0'54-0'62 36'8-64'8 7'6-13'2 15'4-34'4 10'6-262
Fig. 6--Maceral composition (volume %) and rank of Tertiary coals from northeastern India. under normal incident mode.
186 THE PALAEOBOTANIST

content in these coals has been ascribed to be responsible for recorded, besides salt glands of mangrove plant leaves
their high swelling index (B.S.S.) at relatively lower rank stage (Oudlzkusumites=Heliospermopsis) by Banerjee (1985).
than the normal humic coals. It is because of these properties Plant megafossil genera referable to families- Lallllea.
that they were termed as 'abnormal' coals by Iyenger and Lahiri Mallgifera and Parishia (Anacardiaceae), Saccopewlulll
(1958). This abnormal property of the coals has been (Annonaceae), Alstollia (Apocynaceae), Avicellizia
confirmed by both spectral fluorescence measurements on (Avicenniaceae), Santiria (Burseraceae), Caloplzyllum.
sporinite maceral and fluorescence alteration measurements Garcillia and Kayea (C1usiaceae), Termillalia (Combretaceae),
on vitrinite maceral in the coals from Garo and Jaintia hills, Bride/ia (Euphorbiaceae), Dalbergia and El1lada (Fabaceae),
Meghalaya (Misra, 1998). Apo/lollias (Lauraceae), Heynea (Meliaceae), Mel1lecyloll
(Memecylaceae), Myristica (Myristicaceae), Rhizophora
COAL FORMING PLANT COMMUNITIES (Rhizophoraceae), Nepheliul11 (Sapindaceae) and Plerygola
AND THEIR ENVIRONMENTAL (Sterculiaceae), and a gymnospermous plant, Podocarpus are
known to occur in the coal-bearing sediments of the area
SIGNIFICANCE (Misra, I 992c; Awasthi & Mehrotra, 1995).
From the plant fossil records, a humid tropical climate
The mega- and microplant remains recovered from the with high annual precipitation has been inferred (Misra, 1992c;
coal-bearing sequences of different areas are listed in Fig. 9. Awasthi & Mehrotra, 1995). This climate faci Iitated the growth
The letter 'P', in the list, denotes the record of pollen related of luxuriant coastal to near-shore, including mangrove, forest
with a taxon/family, and the symbol habitat/community. In the vegetation with prolific undergrowth during the deposition of
following text only significant taxa (at family and generic coal-bearing sediments in the area. It has also been presumed
levels) and those without established affinities have been that the climate and vegetation (unpublished palynological
mentioned. report by the author from the coalfields ofDilli-Jeypore, Assam
and Nazira, Nagaland) in the nearby areas of Assam and
Makum Coalfield, Assam
Nagaland were almost similar.
The microfloral assem blage recovered from the coal
Khasi Hills, Meghalaya
seams and associated sediments in the basal part of the Tikak
Parbat Formation, Barail Group is either dominated by the The palynological assemblage recovered from coal-bearing
angiospermous pollen or pteridophytic spores (Misra, 1981). Lakadong Sandstone Member, Sylhet Formation (Dutta & Sah.
Fungal remains including a variety of epiphyllous elements, 1970; Kar & Kumar, 1986) is highly rich in pteridophytic spores
excluding hyphae and mycelia, are abundant (25-69%, rarely (up to 70%). Significant taxa are-LycopodiulIlsporites and
below 25%) in the working seams (nos. I & 3). The assemblage Dalldotiaspora along with sub-ordinate amount of
from the coal seams generally has the dominance of Schizaeaceous and Cyatheaceous spores, etc. Angiospermous
angiospermous pollen (13-42%) in the top sections of the pollen are chiefly monocots, viz., Arecaceae (Proxapel1ites,
seams. Pteridophytic spores are sometimes in high amounts Spil/oll1ollosulcites and Spillozollcolpites), Liliaceae
(26-57%), i.e., more than the angiosperms. (Matallomadhiasulcites) and Potamogetonaceae
Pteridophytic spores recovered from the coal seams are (Retipilollapiles). Whereas, commonly present dicot pollen taxa
represented mainly by Parkeriaceae (Slrialriletes), Cyathea- belong to families Euphorbiaceae, Onagraceae. Labiatae.
ceae (Cyathidites) , Polypodiaceae (Polypodiisporiles and Clusiaceae (Kielmeyerapol!enites) , Bombacaceae. Gunneraceae,
Polypodiacaeasporites), Schizaeaceae (LygodiulIlsporites) Pellicieraceae, Fabaceae, Anacardiaceae, Meliaceae. Oleaceae,
and Matoniaceae (Dalldotiaspora). Spores of Striatriletes are Myricaceae, Sapindaceae, Rhizophoraceae, Lecythidaceae,
present in very high frequency (40-60%) in the basal part of Polygonaceae. Proteaceae and Droseraceae.
seam no. I in Baragolai and Tipang collieries. The All the five coal seams in Laitryngew and three coal seams
palynological assemblage. though rich in dicot pollen in Cherrapunjee areas (Kar & Kumar, 1986) have yielded a
comprises persistently high percentages of monocots, trilete dominant assemblage with varying proportions of
belonging to families Arecaceae and Agavaceae alone. The Lygodilill/sporites (22-45%) and Dalldotiaspora (18-40%).
most represented dicot families are - Rubiaceae, Anacardia- Angiospermous pollen are mainly those belonging to families
ceae, Alangiaceae, Oleaceae, Lecythidaceae, Meliaceae. Arecaceae, Liliaceae and Potamogetonaceae, Clusiaceae.
Rhizophoraceae, Onagraceae, Myrsinaceae, Sapotaceae, Gunneraceae and Droseraceae.
Nyssaceae. Ericaceae and Droseraceae. Some significant On the basis of the preceding microtloral evidences, Dutta
pollen genera, of unknown affinities, are Tricolpites (T levis), and Sah (1970) and Kar and Kumar (1986) inferred a near-
Meyeripollis, Po/yco/piles and Engelhardtioipollellites. shore shallow water coastal environment. The vegetal
Dctaplata and Palania are the common microplanktons accumulation for the formation of coal seams took place in a
stretch of freshwater coastal swamps.
 MISRA - PETROGRAPHY. GENESIS AND DEPOSITION OFTERTIARY COALS FROM NORTHEASTERN INDIA 187

Jaintia Hills, Meghalaya including one local seam. The 3'd and 5'" seams are also
characterized by the presence of fair amounts of dinoflagellate
The coal seams and associated sediments of Palaeocene cysts, viz., Cordosphaeridiul1l, Adllatosphaeridiul11,
Sylhet Formation in laintia Hills (Tripathi & Singh, 1984; Polysphaeridium and Homotriblium (Tripathi & Singh, 1984).
Singh & Tripathi, 1986; Mandai, 1986) have yielded higher In Lad-Rymbai (Bapung area) either three (top, middle
proportions of angiospermous pollen (29-66%) than the and bottom) or only two (top and bottom) coal seams are
pteridophytic spores (30-58%). The coal seams in Bapung present in different localities. They have yielded angiosperm
area have yielded almost equal proportions of pteridophytic dominant assemblage. The main pollen genera recovered are
spores and angiospermous pollen, besides fungal elements Proxapertites, Spinimonosu!cites, SpiniZOlloco!pites, etc. of
(Tripathi & Singh, 1984). The pteridophytic spores are mainly family Arecaceae. Subordinate amounts of associated dicot
Lycopodiumsporites (Lycopod iaceae), LygodiulIIsporites pollen belong to families Liliaceae, Bombacaceae, Clusiaceae,
(Schizaeaceae), Dandotiaspora (Matoniaceae) along with etc. Lycopodiaceous, cyatheaceous and polypodiaceous spores
spores of Polypodiaceae and Parkeriaceae. Angiospermous are the main pteridophyte representatives.
pollen are mostly represented by Proxapertites, The coal seam in the Sutunga area (MandaI, 1986) yielded
Palmaepollellites, Spinomonosulcites and Pallllidites of a fairly high amount of angiospermic pollen, of which monacot
Arecaceae. Other pollen associates are of the families pollen genera Proxapertites, Spillomollosulcites.
Liliaceae, Euphorbiaceae, Oleaceae, Myricaceae and Spillozollocolpites and Acallthotricolpites of family Arecaceae
Chenopodiaceae. There is a dominance of Lygodiumsporites constitute about 68·0% of the bulk. Other pollen forms belong
and sub-dominance of Dandotiaspora in the lower three seams, to families Bombacaceae, Gunneraceae, Oleaceae,

Area Fluorescing Liptinite Lipto- Total Total

Coalfield Vitrinite Sp, Cu, Re, detrinite Fluorescing Non-fluo.
Coal Seam % Sub, Alg, % Macerals Macerals
FIr, Ex (%) % %

Makum Coalfield, Assam

Seam no. 3 42'2-62'1 8'2-12'0 1.';':,-23'2 78'4-88·5 11'5-21'6
Seam no. I 47'4-59'7 7'2-17'1 15'7-22'3 78'0-88'6 I 1'4-22'0

DiIIi-Jeypore Coalfield, Assam

Seam no. 6 29.7-479 18.7-22.1 Recorded. 48.4-70.0 300-51.6
Seam no. 4 50'1-56'7 25'5-27'9 Flour ViI. 78'0-82'2 17-8-22'0

Nazira Coalfield, Nagaland

Main Seam 35'3-40'9 16'7-20'5 52'0-61'4 38'6-48'0
"

Laitryngew Coalfield, Khasi Hills, Meghalaya

Main Seam 31 '6-67'4 6'4-134 8'2-27-2 47'2-88-4 11'6-52'8

Bapung area, Jaintia Hills, Meghalaya

Top Seam 23.0-42.0 11.5-17.6 22.0-34.1 72.3-79.2 10.8-277
Middle Seam 41.0-60.0 11.8-19.7 11.0-25.6 82.1-86.8 13.2-179
Bottom Seam 30'7-50'0 15'0-20'7 20'2-35'0 74'1-88'3 11'7-25'9

Sutunga area, Jaintia Hills, Meghalaya

Top Seam 575-78.1 2.0-7.0 8.0-9.2 59.5-91.2 8.8-40.5
Middle Seam 345 4.5 14.0 53.0 47.0
Bottom Seam 67'5 7'4 81 83'0 17'0

Jarain area, Jaintia Hills, Meghalaya

Top Seam 37.0-48.5 12.0-14.0 27.3-34.1 77.0-90.5 9.5-23.0
Bottom Seam 25'5-50'6 26,0-40'5 84'8-90'5 9'5-15'2
13'9-18'8
West Daranggiri Coalfield, Garo Hills, Meghalaya
Main Seam 18'6-30'0
35'0-56'0 12'1-18'8 9'3-22'0 70'0-81'4

Fig. 7-Maceral composition (% on mineral maUer-free basis) of Tertiary coals from northeastern India. under incident blue light excitation.
188 THE PALAEOBOTANIST

Anacardiaceae, Clusiaceae, Fabaceae and Onagraceae. pollen-spore assemblage, Saxena et al. (1996) concluded that
Besides, certain unaffiliated forms like Relilribrevicolporiles the Nangwalbibra area (West Daranggiri Coalfield) was in
have also been recorded. Pteridophytic spores of the close proximity to the shoreline and the presence of a few
assemblage, in the coal seams, are mainly constituted by specimens of dinoflagellate cysts indicate shallow marine
Lycopodiumsporites (Lycopodiaceae), Lygodiul11sporiles conditions of deposition.
(Schizaeaceae), Cyathidites (Cyatheaceae), etc. Plant megafossil records (Bhattacharyya, 1983; Mehrotra,
Singh and Tripathi (1986) envisaged that the prevailing 1999) from Garo, Khasi and laintia Hills, Meghalaya include
climate during Palaeocene-Eocene epochs was humid tropical. several plant genera (Fig. 9): Heleropanax (Araliaceae),
Besides, the persistent presence of a variety of dinoflagellate Mangifera (Anacardiaceae), Polyalthia (Annonaceae), Nypa,
cysts even in some coal seams indicate that the coal-bearing Phoenix and Amesoneuron (Arecaceae), Bomvax
sediments "appear to have been deposited under brackish- (Bombacaceae), Bllrsera (Burseraceae), Calophyllum
water to shallow-marine conditions". However, no marine (Clusiaceae), Terlllinalia and Calycopleris (Combretaceae),
indications have been recorded from Sutunga (MandaI, 1986), Desl1lodiulll, Derris, Mil/ellia, POllgalllia, Pterocarpus,
Lad-Rymbai and larain areas. Nevertheless, coastal to near- Bauhinia, Trachylobium, Sine/ora and Alvizia (Fabaceae),
shore conditions are evident from the palynological Lislea. Neolislea, and Phoebe (Lauraceae) Barringtonia
assemblage. (Lecythidaceae), Artocarpus (Moraceae), Syzygill/ll
(Myrtaceae), Nelumbo (Nymphaeaceae). Osman thus,
Garo Hills, Meghalaya
Ligustrum and Anlholilhes (Oleaceae), Atalantia (Rutaceae),
Palynological assemblage recovered from the coal seams Schleichem (Sapindaceae), Clzrysoplzyl/wn (Sapotaceae),
and associated sediments (Singh et a1., 1975; Singh, 1977a, SOl'lneratia (Sonneratiaceae), Sterculia (Sterculiaceae),
b; Saxena et al., 1996) of Tura Formation, laintia Group in Grewia and Trillmjetla (Tiliaceae), and hema (Ulmaceae).
Nangwalbibra area, West Daranggiri Coalfield, is dominated Taxa like Terminalia, Nypa, BarringlOnia, Sonneralia,
by pteridophytic spores belonging chiefly to families Caloplzyl/ulII, Lilsea, Neolitsea, Phoebe, Osmanlhus,
Lycopodiaceae, Matoniaceae, Polypodiaceae, Osmundaceae, Ligustrum, Antltolithes and Derris are typical beach forest,
Schizaeaceae and Gleicheniaceae. The two important coal including mangrove elements and suggest estuarine conditions
seams (No.1 and 2 main seam) contain almost identical taxa. with large amount of swampy vegetation (Bande, 1992;
Pollen are represented most by the genera Spinomonosulciles, Mehrotra, 1999). Whereas, the occurrence of Nelwllvo nllcifer
Pro.wperlileS, Spinozonocolpiles (Arecaceae) and indicates the existence of ponds (Mehrotra, 1999).
Matanolnadhiasulcites (Liliaceae). Other significant The preceding mega- and micro-floral records indicate
angiospermous pollen associates belong to families that the climate during coal formation in Garo Hills was warm
Bombacaceae, Braccicaceae, Fabaceae, Gunneraceae, and humid (tropical) with much higher rainfall than today
Meliaceae, Onagraceae, Nyssaceae, Labiatae, Polygonaceae, (Mehrotra, 2000). The flora, in Meghalaya during Late
Myrtaceae, Myrs inaceae, Rh izophoraceae, R ubi aceae, Palaeocene Epoch, consisting of evergreen to moist deciduous
Lentibulariaceae (Ulricularia) and Droseraceae. Some coal forest vegetation. including coastal and mangrove plants, was
seams, older than the main seam, in the southern part of the responsible for the formation of coal seams (Singh & Sarkar,
area are characterized by the persistent presence of 1990; Mehrotra, 2000).
microforaminifera (Singh el al., 1975). Saxena el al. (1996)
also recorded some dinoflagellate cysts from the coal-bearing DISCUSSION
section of the Tura Formation.
Ambwani (1993) carried out palynological investigation The geological, sedimentological, palaeobotanical and
on coal-bearing sediments. comprising three thin coal seams. petrological information available on the coal-bearing areas
from Rekmangiri coalmine. The palynological assemblage of northeastern India are mostly of general nature, non-
consists of more or less similar microflora as that reported by sequential, of very restricted type and rather incomplete in
Singh el at. (1975), Singh (1977a) and Saxena et al. (1996), relation to the present context. For some areas, there is a
besides dinoflagellate cysts of Apectodiniul1l in the 3,d seam complete lack of petrological and palaeobotanical information.
(top seam). Therefore, an attempt to deduce the origin of these Tertiary
Singh et a1. (1975) and Singh (l977a, b) presume that coals would be only of a general nature, and in certain respects
the coal seams and associated sediments in the northern and imperfect till further data are available.
southern parts of the Nangwalbibra area were deposited under The fossil mega- and micro-floral evidences (Fig. 9)
warm humid climate near coastal area in a shallow freshwater establ ish that the vegetal matter for the formation of Tertiary
milieu, probably representing deltaic regime. However, coal deposits in northeast India was chiefly derived from
sediments below main coal seam in the southern part of the coastal/near-shore tropical semi-evergreen to evergreen
area locally experienced marine incursions. On the basis of deciduous forests, including back mangroves, mangrove
 MISRA - PETROGRAPHY. GENESIS AND DEPOSITION OFTERTIARY COALS FROM NORTHEASTERN INDIA 189

associates and mangrove plant communities (Dutta & Sah, Hills and Mawbehlarkhar area, East Khasi Hills (Meghalaya)
1970; Kar & Kumar, 1986; Singh & Tripathi, 1986; Singh & are, respectively of allochthonous origin. Where deposition
Sarkar, 1990; Misra, 1992a, b, c; Awasthi & Mehrotra, 1995; took place under oscillating conditions in shallow near-shore
Mehrotra, 2000). Herbs and shrubs, including pteridophytes, basins (lagoons or embayments) on unstable shelf. According
grew profusely as undergrowths in the forests and back to Ahmed (1991 a), the coal seams of Dilli-Jeypore Coalfield
mangroves, especially the taxa of Lycopodiaceae, (Assam) were fanned mostly from woody plants under shallow
Polypodiaceae and Schizaeaceae, besides other moisture and water and reducing conditions in neutral to weakly alkaline
shade loving cosmopolitan ferns belonging to families milieu. There were fluctuations in water table and the "coal
Cyatheaceae, Matoniaceae, Parkeriaceae, etc., were also swamp" was "dry oxygenated". Mishra and Ghosh (1996), on
commonly associated. Aquatic and water-edge (freshwater) the basis of gelification (GI) and tissue preservation index
angiospermous plants like NYlllphaea (waterlily), NelulIlbo (TPI), concluded that the Late Palaeocene and Oligocene coals
(lotus), Potamogeton, Utricularia, Drosera, etc., were also were "deposited in wet forest swamps and in marshy
associated. environment." They also infer marine influence during coal
The vegetation that formed Late Palaeocene coals in formation.
Meghalaya was dominated by arborescent and herbaceous Cohen (1984) and Mac Cabe (1984) observed that the
angiosperms, besides herbaceous pteridophytic taxa. Among deltaic model for the coal formation has been overplayed by
angiosperms, contribution of monocots, especially the palms, the geologists and that the peats of deltaic environments would
was much more significant than the dicots. Pteridophytes were produce only thin seams with too much of ash or mineral matter
mostly the plants of Lycopodiaceae, Matoniaceae and to be of economic value. However, presence of typical
Schizaeaceae along with Cheilanthaceae, Cyatheaceae, mangrove, mangrove associate and back mangrove taxa. viz.,
Osmundaceae, etc. However, during Oligocene in Arunachal Rhiwphora, Sonneratia, Avicenllia, Nypa, Barringtollia,
Pradesh, Assam and Nagaland an apparent change, both in Ca/ophyllum, Terlllina!ia, etc., along with several varieties of
quality and quantity of pteridophytic contribution, is witnessed other coastal/beach taxa (Fig. 9) and dinoflagellate cysts in
together with higher representation of dicots over monocots certain coal seams, cannot be rejected for the fear of over
in the coal-fonning vegetation. Important pteridophytic taxa emphasizing similar conditions of deposition. The good quality
recorded belong to families Parkeriaceae, Polypodiaceae and coal seams with thickness varying between 0·5 to 18 metres
Schizaeaceae. The pteridophytes registered further decrease testify for their economic significance.
and were represented mostly by the spores of Schizaeaceae Low amounts of clastic minerals in most of these coals
and Polypodiaceae. [presumably produced ill situ from the degradation of vegetal
The depositional models suggested for the Tertiary coal- matter under alkaline milieu (Renton et aI., 1979)] and thick
bearing sediments of northeastern India during Late seams either devoid of or only with minor parting bands.
Palaeocene and Oligocene epochs by earlier workers are broad Besides, clean to very clean nature of vitrinite macerals and
and of generalized nature. Pascoe (1964, p. 1580) thought presence of clay, carbonaceous clay/shale or other fine-grained
that the coals from MeghJlaya were formed "not far from the sediments as seam floor and roof, wherever present, preclude
coast, possibly in brackish water lagoons". From fossil floral the possibility of the formation of these Tertiary coals primarily
evidences, dominance of aquatic and 'marshy' taxa and from drifted vegetal matter (Raja Rao, 1981; Misra, 1992a,
presence of dinocysts, Biswas (1962) presumed that in Garo b).
Hills the vegetation "grew right within the basin of deposition" Bright non-banded coal seams with high to very high
and there was "periodic brackish water influence". On the basis vitrinite and poor sporinite contents (Figs 5, 6, 7) are indicative
of geological and sedimentological studies, Mathur and Evans of their formation from forest vegetation (Stach et aI., 1982;
(1964), Das Gupta (1979), Raja Rao (1981) and Misra (1981) Misra, 1992a, b). High frequency of collodetrinite and common
inferred coastal, deltaic, lagoonal, estuarine, shallow-marine occurrence of fungal remains in the coal seams imply the
and back-swamp depositional conditions for the coals in Assam accumulation of ancient peat under subaqueous conditions.
and Nagaland. Raja Rao (1981) and Misra (1981) suggested The vegetal matter appears to have been subjected to high
autochthonous origin of coals in Assam and Nagaland. degree of aerobic fungal and bacterial degradation in the
However, Bhandari et al. (1973) presumed that, in Assam and acrotelm (aerobic upper zone of a peat) and anaerobic bacterial
Nagaland, the coal seams formed under fluvial conditions on degradation in the catatelm (Clymo, 1987). Since syngenetic
a delta plain. On the basis of vitrinite-rich nature of coals, pyrite and calcite occurring together are the definite indicators
presence of framboida I pyrite and high organic sulphur content of anaerobic and alkaline milieu, and the ombrogenous-
(in the total sulphur), association of cun'ent-bedded and ripple- oligotrophic (raised and nutrition poor) peat bogs are acidic
marked sandstones with shale and coal, and absence of seat- in nature (TeichmUller, 1989; Cameron, 1989). the possibility
earth (clay bed), Ahmed (1991 b) and Ahmed et al. (1997) of these coals to have formed as raised-bogs is improbable.
concluded that the coals in West Daranggiri Coalfield, Garo
190 THE PALAEOBOTANIST

Instead, they appear to have originated from eutrophic (Raja Rao, 1981; map plates IV & VI). Here, isolated
peatswamps. exposures of coal-bearing strata intervened by Precambrian
In Garo, Khasi and laintia Hills of Meghalaya, the rocks are only 1 to 2 km apart from each other. Evidently,
exposures of coal seams, as published in maps (Raja Rao, most of these coal-bearing exposures represent original shapes
1981), are generally aligned parallel to Precambrian rocks. of the basins. The preceding geological facts imply that peat
They occur in isolated patches of various shapes and trends accumulation in Meghalaya took place in estuarine back-
with lengths ranging from approximately 1 to 7 km and width swamps (Misra, 1992c) rather than in the back-swamps on
between 1 and 5 km either directly over the Precambrians or delta plain as visualized by other workers. In a progradational
underlain by older sedimentary units (Raja Rao, 1981; map deltaic setting, swamps develop away from the existing
plates Ill, IV & VI-VIII). Besides, there are some smaller landmass not closure to it, whereas estuarine back-swamps
patches of coal-bearing sediments within the Precambrian form along and parallel to landmass and coastline, as is the
rocks themselves, e.g. Balphakram and Pendengru areas in case in Meghalaya. Possibly, it is because of the estuarine
Garo Hills and northwest of Langrin Coalfield, Khasi Hills sedimentation set up that the dinoflagellate cysts and

SCALE J

~
o5-~50 ./
~ ,
100Km
<'
r-- .....,.. /'-./''/
\
'--', DIBjjRfUGARH. 2 ~ (J)8 .

. ~ ~<c,~
Q.J..LlJ.lJJ-'- / ' " . -
( ITANAGAR
@ SIBSAGAR /'
~)
~
~ ~~ /
....j
../"' .
---'_'_'J'- •
JORHAT
W "," /
0'< ___

<c,, ," !
~ I
BRAHMAPUTRA RIVER
E::> C>-= :"'?":> ( LEGEND
.(..····7 ) ~ 0.55 - 0.60
KOHIMA
lj~~ SHIL LONG
~
@ / WBl > 0.60 - 0.65

~ ~. ~
@ > 0.65 - 0.70
/
.. :.:... .
/ anm
-'-._.
> 0.70 - 0.75
6~ .,.;-. 4
'--.- ..-- ..............
.......... "'-i VITRINITE REFLECTANCE
IRa ma •. %)

PALAEOCENE AND c=J OLIGOCENE COAL SEAMS; 1-3,8 MAKUM, DILlI-JEYPORE, NAZIRA AND NAMCHIK - NAMPHUK
COALFIELDS: GROUP OF SMALL COALFIELDS: L JAINT IA HILLS,S GARO HILLS AND 6 KHASI HILLS 7 MIKIR HILLS

Fig. 8- Normal reneclance (R o max. %) trend in the Palaeocene and Oligocene coal seams of Arunachal Pradesh. Assam. Nagaland and Meghalaya
(coalfield areas, especially of Meghalaya, are exaggerated for illustration) (afler Misra. I 992a).

.......
,/

Fig. 9-J..isl of angiospermous planl megafossils recorded from coal-bearing horizons of Assam and Meghalaya, with their living affinities. 'P' denotes
pollen record, and different symbols habilal/communilies.

• Mangrove plants: q, Mangrove associates: # Back mangrove plants: ° Trees !herbs /shrubs of coaslal or near-shore (lilloral!swampy) habita!. Data compiled
from: Bande (1992): Awasthi (1974, 1984): Singh el al. (1975): Singh (1977): Dutla and Sah (1970): Sah and Kar (1974): Tripathi and Singh (1984): Kar
and Kumar (1986): Kar (1985): Misra (1981): Mandai (1986). Bhatlacharyya (1983): Ambwani (1991. 1993): Awaslhi and Mehrolra (1995): Saxena ('I (II.
(1995) and Mehrotra (2000).
 MISRA - PETROGRAPHY, GENESIS AND DEPOSITION OFTERTIARY COALS FROM NORTHEASTERN INDIA 191

Families Assam Meghalaya

Makum Coalfield Garo, Khasi & Jaintia Hills

Agavaceae po
Alangiaceae P~#

Anacardiaceae Mangifera", Lannea", Parishia" Mangifera U P*~

p*Q
Annonaceae SaccoperalulI1° Polyallhia
Apocynaceae Alstonia"
Araliaceae lIe/eropanax
Arecaceae Nypa* P*~# Nypa*, Phoenix, Amesoneuron p*Q#
Avicenniaceae A vicel/nia*
Bombacaceae Bombax
po
Braccicaceae P
Burseraceac Sal1/iria" Bursera
Chenopodiaceae P
Clusiaceae Calophyl/ulI1°#' KayeaO, Mesua, Garcinia" Calophyl/um"#
P P
Combretaceae Tenninalia#o Terll1inalia#o, Calycopleris
Droseraceae P P
Ericaceae P"
Euphorbiaceae Bridelia" P
Fabaceae Dalbergir/", Enlada~ Desll1odillln, Derris#, Mil/ellia,
P PongOlnia, Albizia, Bauhinia, Plerocarpuso,
Trachylobilll1J, Sindorrl
P
Gunneraceae P
Haloragaceae P
Labiatae P
Lauraceae Apol/onias" Lis/ea, Phoebe. Neolistea
Lec yt hid aceae P~# Barril/glonia~#
pl'#
Lentihulariaceae P
Liliaceae P P
Meliaceae Heynea" p*#Q p*#¢
Memecylaceae /\1emecylon"
Moraceae Arrocarpus, Ficus
Myricaceae P
Myrsinaceae p" po
Myristicaceae Myrislica#o
Mynaceae Syzygiul/1° P
Nymphaeaceae Nelul/1bo"
P
Nyssaceae P P
Oleaceae P Osman/luIs", Liguslntl/1, Anlholithes P
Onagraceae P P
Pellicieraceae P
Polygalaceae P
Polygonaceae P
Potamogetonaceae P
Proteaceae P
Rhizophoraceae Rhizophora* p* p*
Rubiaceae p# p#
Rutaceae Aralanlia
Sapindaceae Nepheliul1I" Schleich era P
Sapotaceae P Chrysophylllll/1
Sonneratiaceae Sonnera/ia*
Stercul iaceae Pterygola Slerculia
Tiliaceae Grell'ia, Trill/n/ella
Ulmaceae Trema~
192 THE PALAEOBOTANIST

foraminifers are recovered commonly from the sediments, vegetal supply. but not significant enough to cause any apparent
besides the coal seams. change in the provenance. The shortfall in vegetal supply, in
In Garo Hills, the provenance was a positive landmass certain cases, was responsible for relatively high mineral matter
since the beginning of the Tura Formation, whereas in Jaintia content in certain coal seams of Meghalaya. The fact that no
Hills a peneplained surrounding landmass ensured the drastic vegetational change has been recorded during coal
precipitation of the Lakadong Limestone Member. Later the formation in Meghalaya, clearly suggests that minor sea-level
uplift of source area accompanied by basin subsidence resulted fluctuations did not cause extermination of the existing flora
in the deposition of the coal-bearing Lakadong Sandstone in the area. More or less si milar (continuance of existing flora)
Member on Early to Middle Palaeocene sediments. Periodic conditions appear to have been there in Assam as most of the
high-energy sedimentation and shifti ng of distributary channels common taxa are well recorded throughout the section.
encroaching upon the accumulating peats may have caused The dominance of freshwater taxa with those typical of
temporary or local cessation of peat formation and probably brackish water affinity (mangrove plants) in the coal seams is
also controlled the ultimate thickness of the coal seams in not some thing unusual when considering ancient, virgin
different localities and sections. Complete cessation of vegetal coastal and near-shore forest vegetation. Because intermixing
accumulation for peat formation in Meghalaya resulted with offlora in fossil record, i.e.. in swamp, is controlled by several
the beginning of rapid basinal subsidence due to activation of factors, e.g.. seaward distributary channels and seawater inlets
the existing tectonic lineaments. as is evident by the high- in the swamps, etc. Major complications arise from the
energy sedimenllltion (medium to coarse grained sandstones dispersal pattern of allochthonous spores and pollen. In this
and pebble beds) above the sediments of the Tura Formation regard a gross analogy of the ancient peatswamps. disregarding
in Garo Hills and towards upper part of the Lakadong specific details, can be made with the existing near-shore
Sandstone Member in Jaintia Hills. Latter, a marine marine influenced lakes on the eastern coastal margins of India.
transgression in both the regions caused the deposition of Presumably, certain landward swamps, especially in
fossiliferous Siju and Umlatdoh limestone members (Fig. 4). Meghalaya, remained unaffected by marine influx for a period
Palaeoshore-line parallel orientation of the coal deposits of time. Consequently, freshwater aquatic angiosperms
in Assam and Nagaland with relatively greater areal extent (Poralllogeroll. NYlllphaea. Nell/Illbo Urricularia. etc.)
than those of Meghalaya in spite of intense tectonic flourished in and around seasonal lakes/ponds developed over
disturbances. including moderate to intense folding and slicing the peat surface to be recorded, occasionally. in high
of strata along with coal seams implies the existence of frequencies in the coal seams (Dutta & Sah, 1970).
lagoonal swamps (Raja Rao. 1981). Tectonically least Contrary to the high frequency of pteridophytic spores
disturbed, non-persistent and thin coal seams «1 -3 m thick), present in the pollen-spore assemblages. their megafossil
in Garo, Khasi and Jaintia hills ofMeghalaya (Raja Rao. 1981), records are rather poor. The pteridophytes are normally slender
suggest peat accumulation in small isolated low lying estuarine and delicate seasonal plants. As per their lifecycle strategies.
or estuarine back-swamps caused by the encroachment of the they do not shed leaves and die out standing through drying
sea on undulating Precambrian (Garo hills) or Cretaceous and complete dehydration. Therefore, fossil remains of
(Khasi and Jaintia hills) sediments, under unstable basin pteridophytic plants are generally not found preserved. The
conditions. Thin and non-persistent coal seams occurring at preceding facts and a general correlation between high
close intervals (2-8 seams) also reflect frequent and relati vely pteridophytic content with high fungal activity and high
unstable conditions during Late Palaeocene coalforming inertinite contents in coal seam sections (Misra, 1992b) imply
episodes. In Assam and Nagaland, on the other hand, the basins that the pteridophytic plants played a primary role in the
enjoyed far more stable tectonic conditions during coal formation of inertinite. Since the cell-lumens of structured
formation of Oligocene Epoch. as is evident from thick and inertinites, in these coals, are invariably empty, i.e., without
extensive nalLIre of the coal seams. clastic mineral fillings, it has been presumed that the
Association of fine-grained sediments with coal seams pteridophytes and associated shrubby angiosperms also grew
indicates deposition mostly from suspended-load material by in nearby peat swamps.
sluggish low-energy channels with sediment source, quite The association of perhydrous vitrinite, liptodetrinite and
probably from an almost peneplained source area. This bituminite along with biogenic pyrite and calcite is
condition isquiteevident in Meghalaya where three limestone characteristic of subaquatic and sapropelic including calcium-
members occur alternating with arenaceous members including rich coals (Stach et al., 1982; TeichmUller, 1989) and the
that whi~h bears the coal seams (Raja Rao, 1981). On the Tertiary coals of northeastern India contain high proportions
preceding evidences. it has been presumed that short-lived of these macerals and minerals (Misra, 1992a, b). The
peat swamp formations in Meghalaya were controlled by minor syngenetic (biogenic) pyrite precipitation requires anaerobic
sea-level fluctuations caused by episodic and slightly increased bacterial growth (negative EIT-potential), neutral to mildly
rate of basin subsidence just perturbi ng the prevai I ing rate of alkaline milieu (pH 6'5-8) and stagnant water body (Stach er
 MISRA - PETROGRAPHY. GENESIS AND DEPOSITION OFTERTIARY COALS FROM NORTHEASTERN INDIA 193

al., 1982). The conditions favouring pyrite precipitation are dinoflagellates and phytoplanktons, in Garo and Jaintia Hills,
readily available near marine influenced zones in lagoons and Meghalaya, indicate definite influence of marine or brackish
estuaries where wave and current action is negligible (Cecil water.
ef al., 1979). Under such conditions, in the presence of The vegetal matter accumulating under suBaqueous
abundant organic matter, though acrotelm may still have conditions in lagoon or near shore back-swamps in brackish
aerobic influence, coalification in the catatelm proceeds by water milieu in Assam and Nagaland and in small isolated
putrefaction (fermentation) with the help of anaerobic bacterial estuarine swamps in Meghalaya experienced high aerobic
degradation instead of normal peatification or humification. biodegradation (fungal and bacterial) in the acrotelm and
Consequently, perhydrous vitrinite, bituminite and extensive anaerobic bacterial degradation in the catatelm.
liptodetrinite macerals are produced with the enrichment of Occasionally small freshwater lakes/ponds formed on the peat
proteinaceous, fatty-lipoid and other hydrogen-rich microbial, surface facilitated the growth of aquatic and water-edge plants.
algal and microfaunal degradational products (Stach ef al., Shrubby pteridophytes and angiosperms growing in the vicinity
1982; TeichmUller, 1989). Similar conditions with varying of the peat swamps were responsible for the major part of the
degree of putrefaction influence have been visualized for the structured and detrital inertinites. However, occasional
genesis of Tertiary coal seams of northeastern [nd ia (M isra, cindering of peat surfaces, especially in the coal seams of
1992a, b). Meghalaya, presumably produced more structured inertinites.
The variations in the seam thickness and rank of the coal seams
CONCLUSIONS appear to have been controlled by the then existing tectonic
and geothermal gradients in different areas.
The Tertiary coal seams of northeastern India are Acknowledgements-The aU/hoI' wishes to express his gratitude
generally bright non-banded in appearance, rich to very rich to Prof Ansh/( K Sinha, Director, Birbal Salmi IllStit/lte of Palaeo-
in vitrinite macerals along with subordinate amounts of botany, Lucknow for inviting and permiffing this paperfor publica-
inertinite and Iiptinite macerals. The vitrinite macerals are tion and to Monika Wolf, Krefeld (Gennany)jor critically reviewing
represented chiefly by collodetrinite, the vegetal degradational the manuscript. Assistance of VP Singh in the preparation of illus-
products. Structured vi tri ni te maceral, collotel i ni te, are trations is also thankfully acknowledged.
occasionally predominant. Main inertinite macerals are
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