Cretaceous—Eocene sedimentation, tectonism and biofacies in the bengal basin, India

Palaeogeography, Palaeoclimatology, Palaeoecology, 34 (1981): 57--85

57

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

CRETACEOUS--EOCENE SEDIMENTATION, TECTONISM AND BIOFACIES IN THE BENGAL BASIN, INDIA

RANJIT K. BANERJI

Geology Faculty, Indian Institute of Technology, Bombay (India) (Received October 24, 1979; revised version accepted September 15, 1980)

ABSTRACT Banerji, R. K., 1981. Cretaceous--Eocene sedimentation, tectonism and biofacies in the Bengal Basin, India.Palaeogeogr., Palaeoclimatol.,Palaeoecol., 34: 57--85. The Bengal Basin, located in the eastern part of the Indian subcontinent, is one of the thickest sedimentary basins in the world. The basin started downwarping in the Early Cretaceous and the sedimentation has since been continuous. From an initial transgressive marine phase, lasting almost up to end of the Eocene, the basin has evolved through a subsequent regressive marine and a later continental deltaic phase. In the absence of detailed information available from the deeper basinal parts, an analysis of the sedimentary and tectonic history and biofacies of the regions forming the western and northwestern shelves of the Bengal Basin pertaining to the initial phase of its evolution is attempted. This is achieved through the synthesis of all the geological information available on the basis of recent exploration and research conducted on the sequences developed in West Bengal, Meghalaya, Mikir Hills and Assam, India. The subsidence of the Bengal Basin was the result of differential adjustments of the crystalline basement along certain well-established trends and was initiated soon after the Gondwana tectonism, sedimentation and volcanicity. The earlier sediments were not typically marine but were more paralic to estuarine in nature. The entire Bengal Basin, except the area now comprising Assam, came under the influence of the first regional marine transgression during the Late Campanian--Maestrichtian. The coarse clastic sediments were deposited under shallow coastal marine conditions. A more widespread marine transgression took place during the Early--Middle Eocene which covered almost the entire shelf area including Assam. During this transgression, the sediments belonging to the orthoquartzite--carbonate association were formed in the shelf. The sea started receding in the Late Eocene during which the entire Bengal shelf Came under the influence of heavy unloading of fine clastics. The top of this fine clastic shaly sequence is an important sedimentary datum in the Bengal shelf which marks the end of the first phase in the evolutionary history of the basin.

INTRODUCTION

The Bengal Basin (Fig.l) in eastern India contains a thick sedimentary record of continental to marine clasticand carbonate sediments ranging in age from Early Cretaceous to Holocene. Due to an extensive cover of alluvial deposits concealing most of the older sequences and the outcrops occurring in 0031-0182/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

58

I 88"

[

I ~"

SIS"

ABBREViATiONS C~)- GARO HILLS; ( ~ - KHASI HILLS; (~)- JAN'TIA HLLLS; B-BOLPUR-1; B(I- BURDWAN-t O- BEBAGRAM

G - GALASI - I

Gb- GHA~AL

p - PALEOGENE

Q-QUATERNARY TO REC£NT

J - JALANGI

M - MEMARi

N-NEOGENE

Fig.1. Map of the Bengal Basin, showing different geological provinces, major tectonic elements and important locations. the isolated and generally inaccessible parts of this region, a comprehensive geological investigation of this-basin was not possible. Most geologists have worked independently and in isolation. There has not been any single coherent approach towards a proper understanding of the sedimentation history, biofacies and structural evolution of the Bengal sedimentary basin. A considerable amount of subsurface data is being accumulated through various agencies working for the hydrocarbon exploration in different parts o f this basin. The present paper attempts a critical evaluation of all the available geological information on the basis of the research conducted by the author in the last decade.

59

The evolution of the Bengal Basin has undergone two successive phases. The first, a marine transgressive phase (with minor regressions) ending after the Late Eocene, was followed by a regressive phase (with minor transgressions) resulting in a continental deltaic deposition. This paper refers to the early geological history of the region constituting the shelf part of the Bengal Basin up to the end of the Late Eocene. The information available with regard to the first phase from the deeper basinal parts is rather scanty and incomplete. The Bengal Basin is a double-fronted asymmetrical basin with the flanks sharply dipping to the west and gently sloping to the east. The western flank borders the Precambrian crystalline massifs near long. 88°E (Fig.l). The important massifs forming the eastern part of the Indian crystalline shield are the Singhbhum and the Chotanagpur massifs (Fig.2) towards the west, the Shillong massif towards the north and the Mikir--Mishmi Hills massifs towards the northwest of the basin. The northern boundary of the basin, that runs parallel with lat. 25°N up to long. 93°E, is marked by the contact of the Cretaceous--Eocene rocks with the crystalline basement. Further to the east, the basin swings in a northeasterly direction up to lat. 28°N. The shelf zone extends between the Precambrian massifs on the west and northwest and the NE--SW-trending Bhagirathi tectonic hinge delineated near Calcutta at a depth of around 5000 m. To the east of the hinge, a foredeep named the Bp °

"~

N E P A

_;

L

91o"

i

./

B

H

;

u

T

A ~,'-'~,

JQRHAT

L r ) / /

:I

_

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}i

C H 03"A NAGPUR

i

/" ,3; ,,. . . . . . k0f_ " 5 /' ~ 88"

z'

/

~

BAY

170

7?0Kin

192"

Fig.2. Map of the Bengal Basin, showing the palaeotectonic trend lines during the Cretaeeous--Palaeogene.

60

Jessore depression occupies a greater part of Bangla Desh, where more than 11,000 m of sediments are expected (Sengupta, 1966). The shelf can be further classified into three major geological provinces, namely the West Bengal I shelf to the southwest, the Meghalayan 1 shelf to the north and the Mikir Hills and the Assam ~ shelves to the northeast. The basin merges with the physiographic basin of the Bay of Bengal towards the south. The sedimentary rock sequence is exposed in the Garo, Khasi and Jaintia Hills of Meghalaya and in the Mikir Hills. In the other shelf areas and in the Jessore depression, the sequence is concealed by the Recent alluvium. For further geological details about these areas, the reader may consult Krishnan (1953, 1968) for the structural and tectonic history of India; Basu (1962), Biswas (1959, 1963) and Sengupta (1966) for the West Bengal shelf; Biswas (1962), Chakraborty (1972), Chakraborty and Bakshi (1972), Medlicott (1869) and Samanta (1971) for Meghalaya and the Mikir Hills; Banerji (1980a, b), Dasgupta (1977), Evans (1932, 1959 and 1964), Mathur and Evans (1964) and Raju (1969) for the Assam shelf and adjoining areas; Khan and Azad (1963) and Leitz and Kabir (1978) for Bangla Desh and Chibber (1934) and Clegg (1941) for the Burmese region. The first phase of evolution of the Bengal Basin can be objectively envisaged in more than one stage. It begins with the stage of initial subsidence in the Early Cretaceous soon after the Gondwana tectonism, followed by the marine transgressions in the Late Cretaceous and in the Early--Middle Eocene and ends with the stage of the regressive marine cycle in the Late Eocene. CO NF I GUR ATI ON AND EVOLUTION OF THE BASIN

Tectonic setting prior to the downwarping of the Bengal Basin The process that led to the opening of the Bengal Basin is rather complex and it can be phased into more than one cycle. The first cycle, which was prior to the opening of the Bengal Basin, culminated in the development of a set of faulted troughs or grabens in the crystalline basement. The Gondwana rocks occupied these tectonic troughs (Krishnan, 1968) with the faulted boundaries arranged along certain specific linear trends (Table I). Some of the well-known linear trends and the locations of Gondwana troughs in eastern India are shown in Fig.2. The Gondwana sedimentation commenced in Late Carboniferous--Early Permian times and continued till the end of the Late Jurassic essentially under'the terrestrial, fluvial to lacustrine environments. The subsidence kept pace with the rate of sedimentation and the accumulation was accentuated by the differential mobility across these linear zones. A considerable thickness of sediments (over 1000 m) representing the equivalents of the Gondwana Group is present in the western marginal alluvium-covered areas of West Bengal. Through the faulted and fractured ~Named after the political states of the Indian Republic.

61 zones a large amount of basic lava (basalt, dolerite and andesite type) erupted soon after the Gondwana sedimentation. These lava flows occur in the West Bengal subsurface (Biswas, 1963) and are exposed near Rajmahal and in the southern flanks of the ShiUong Plateau.

Stage 1. Initial subsidence (Early Cretaceous) The next cycle coinciding with the initial subsidence of the Bengal Basin was the differential block adjustments in the basement rocks during the early parts of the Cretaceous. It is, however, beyond the scope of the present paper to discuss the cause and mechanism of these tectonic cycles affecting eastern India and more emphasis is placed on the type of sedimentation generally associated with such tectonic events. Due to these adjustments, the coastal marine and supra-littoral clastic deposition transgressed widely craton-ward over the formerly well
TABLE I

O~ t~

Tectonic events in relation to the sedimentation history in the Bengal Basin (shelf area) Approximate age of events

Tectonic events along:

Sedimentary events and deposits

Marine events

Late Eocene

Eastern Ghat trend (NE--SW)

uplift in distant provenance, sedimentation of fine clastics brought by rivers

change in depositional slope, regression, leading into brackish to transitional environments

Early to Middle Eocene

latitudinal trend (E--W)

sedimentation of orthoquartzite--carbonate association in Meghalaya

d o m i n a n t l y marine, fluctuating within littoral to neritic zones

Eastern Ghat trend (NE--SW)

opening of the Assam shelf, sand-carbonate sedimentation in the West Bengal, Mikir Hills and Assam shelves

major transgressive phase with minor oscillations

Dharwar trend (NW--SE)

argillites and m i n o r sandstone deposition on the flanks of Mikir Hills

marginal marine, first marine phase in northeastern Mikir Hill shelf

latitudinal trend (E--W)

uplift in Shillong Plateau, sedimentation of d o m i n a n t quartzose and felspathic sandstone, carbonaceous shale and minor carbonates in Meghalaya and West Bengal

coastal marine, littoral to inner-neritic zone

Eastern Ghat trend (NE--SW)

tilting of depositional slope towards SE to S, clays and fine sands in West Bengal, conglomerates and coarse sands in Meghalaya

coastal marine to supra-littoral; incipient marine transgression from SE and S

Late Cretaceous to Paleocene

Early Cretaceous

terrestrial fluvial, lacustrine

lower Gondwana sedimentation in the Damodar graben folding of crystalline rocks, uplift of the East Bengal Ridge

latitudinal trend (E--W)

Eastern Ghat trend (NE--SW)

Permian to Triassic

Palaeozoic? Preeambrian

open marine conditions to the east of NE--SW-trending East Bengal Ridge

fluvial--lacustrine

eruption of basic lava through the fractured zones, upper Gondwana sedimentation in West Bengal and western Meghalaya

submeridional trend (N--S)

Jurassic

fluvial, lacustrine

upper Gondwana sedimentation in the subsiding NW--SE-trending troughs near Mikir Hills

(NW--SE)

Dharwar trend

O~

64

WI86°E

88°1E

3" GONDWANA __:_ .BASIN ~ __:

.--

GONDWANA TROUGH

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CRETACEOUS

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TRENDING GRABEN

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I VERTICAL I

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EXAGGERATED

Fig.3. Schematic section across the western part of the West Bengal shelf, showing the genetic relationship between the Gondwana basin and the marine Cretaceous--Palaeogene basin.

interfingering with the marginal marine sediments was largely restricted to the western part. This suggestion is based on the study of isopachs and sedimentation patterns made for some of the southeastern coastal basins of India (Sastri et al., 1976). The sedimentation was essentially controlled by the river system which did not involve the transportation of sediments from a considerable distance as is evident from the textural analysis of the sediments (Banerji, 1980b). The microfauna, represented by the arenaceous foraminiferal species of Ammobaculites, Ammodiscus, Bathysiphon, Haplophragmoides, Pelosina, Trochammina and few ostracodes, wherever found, are indicative of estuarine, bay, or lagoonal to marginal shoreline facies. The sediments do not show any good evidence for progressive shoaling.

Synthesis (1) The areas now constituting the western parts of the present Bengal Basin have undergone a series of differential block movements affecting the crystalline basement along several linear zones. The troughs/grabens thus formed in the basement were the sites of Gondwana sedimentation during the Late Carboniferous to the Late Jurassic. (2) Through the fissures developed in the basement, the eruption o f basaltic to andesitic lava took place during the Late Jurassic. These lava flows covered the Gondwana sequence in most of the areas. (3) The NE--SW "Eastern G h a t " trend became active soon after the volcanic activity. This has resulted in a slow and localised subsidence in the areas to the east of the Gondwana depocentre. The diversified lithological

65 TABLE II Correlation of various stratigraphic units in the Bengal shelf Stratigraphic units in the shelf areas o f

Geologic 5

Oligocene

West Bengal

Meghalaya

Mikir Hills

Assam

Barail Fm. or BtLrdwan Fro. (sst. sh. coal)

Barail Fro. (sst. sh,)

Barail Fm. (sst. sh.)

Barail Fm. (sst. sh, coal)

sst.

1st. Narpuh Eocene

Middle

Sylhet (Ist. sh.)Fro.

Sylhet Fro.

j ~ ~ F m Early

~ ~ - \ c "

, (sst. sh.) Paleocene

/ ~ j ' ~

/ 3ararapani Fro. ,,~lst. sh.)

Sylhet Fro. Ba~l~ ~ , x

Umlatdoh 1st.

Sandstone

Lakadong sst. Therria ]st.

I Unit Mikir Fm. (sst.)

(sst. arkose) ................

I~

i

Tura Fro. ~ , , , , (sst. arkose, / ~ coal) / ~'~ Cherra /7// Fro. / "Langpar

v~, i

(sst') ~x,N ~1=:

Cretaceous

-

Late

Ghatal Fm. (sh. sst.)

M a h a d e o ~ sh.) ~ Fro. ~~ (sst. grits)

-

Bolpttr Fro. (sh. sst.)

Gumaghat Fro. (sst. congl. sh.)

Rajmahal Traps (basic lava (flows) Jttrassic to pre-Jurassic

Jorhat Fm. (argillite s, sst.) 9 '

.................................................

Gondwana Group (sst. sh. coal. congl.)

[11111111]1

Precambrian

Sylhet Traps (basic lava flows)

i

Dergaon Beds (sh. sst.)

? \

Archaean crystalline rocks (granites, gneisses, schists)

F m . = F o r m a t i o n ; sst. = s a n d s t o n e ; 1st. = l i m e s t o n e ; sh. = s h a l e ; c o n g l . = c o n g l o m e r a t e .

suites accumulated in the depressions partly reflected the provenance undergoing activation of variable intensities during the Early Cretaceous. (4) The tectonism responsible for the initial downwarp of the Bengal Basin was younger than and not related to the Gondwana tectonism. The locus of the initial deposition was largely restricted to the west of the present
Burdwan Fm. and related units

Kopili Fro.

OligoMiocene

Late Eocene

Sylhet Fm. (320 m)

Jalangi Fm. (700 m)

Late to Middle Eocene

Early Eocene to Late Cretaceous

(30 m)

Rock units

Geologic age

disconformity

sandstone with carbonaceous shale, lignitic coal in the upper part

.... transgressive onlap .............

dominantly carbonate, biosparites to biosparudites, carbonate bands separated by two clastic interbeds of varying thickness and comprising of sand, silt and clay, often glauconitic

brachyhaline foraminifera in the upper part, occasional estuarine forms in the middle, almost barren in the lower part

benthic forms in clastic bands, few arenaceous, spore pollen and hystrichosphaerids in clay bands

Assilina, Discocyclina, Operculina) in carbonate, small

highly fossiliferous, rich in larger foraminifera (Nummulites,

calcareous benthic forms (rotaliids, miliolids, larger forms) in the lower part

upper part

~

minor regression

brackish/marshy/lagoonal towards the upper part, otherwise largely fresh water

major marine transgression

carbonate bands indicate warm, open marine, littoral to inner-neritic, probable reefoidal at places; clastic bands indicate brackish/ marshy to littoral ; oscillatory

coastal shallow marine

estuarine/brackish/ marshy/lagoonal intertidal zone

( Trochammina, Haplophragmoides, Miliammina, etc.) in the

arenaceous foraminifera

major regression

Palaeoenvironments

surface of erosion and/or non-deposition

poorly fossiliferous, rare arenaceous foraminifera, spore and pollens

Microfaunal association

dark grey shales with varying a m o u n t of calcareous matter and minor sand and siltstone

fine sandstones and shales

D o m i n a n t lithology

Pre-Barail (Burdwan Formation) stratigraphy and palaeoenvironments in the West Bengal shelf

TABLE III

O~ O~

Upper Gondwana (1000 m)

(500 m)

Rajmahal Traps

Bolpur Fm. (170 m)

(12o m)

Ghatal Fm.

. . . . .

Precambrian i

Archaean

fresh-water estuarine

first incipient marine . . . . transgression

brackish, marshy, lagoonal, littoral

--

subaqueous lava flow

fresh water, estuarine lacustrine

crystalline rocks granite gneiss and schist

........................................................................................................................

--

...............................................................................................................

sandstone, clays and coal non-conformity

sp.

Marginulina

devoid of foraminifera, rare Characeae, ostracods

Dorothia oxycona, ]arvisi, L e n t i c u l i n a

brachyhaline, shallow marine and estuarine forms intercalated throughout; foraminifera:

.............................................................................................................

amygdaloidal/nonamygdaloidal basalt,. dolerite, gabbro, andesite tufts

non-conformity

red clay, claystone, greenish and light coloured sandstone

disconformity

interbedded calcareous dark grey shale, calcareous siltstone and sandstone

!.... non-conformity

LIJt, [lllll ............................ l

Jurassic

Early Cretaceous

Late Cretaceous

O~

68

Stage 2. First marine transgression in the Late Cretaceous and the opening o f the Bengal Basin The movement which resulted in the differential block subsidence in the western part was replaced b y a new set of tectonic adjustments in the craton soon after the cessation of the lava flow. The areas to the east and southeast of the original fractured zones filled with the Gondwana deposits and the lavas attained a greater degree o f mobility and the depositional floor started tilting towards the south and southeast. This was achieved through taphrogenic fragmentation and downwarping of the craton along the basin margin faults aligned to the Eastern Ghat trend (Sastri et al., 1977). The open sea (an extension of the southern palaeo-Indo-Pacific Ocean), which was present at a considerable distance from the Gondwana troughs during the Jurassic and the Early Cretaceous, started transgressing over a wide area of the craton due to this tilt during the Late Cretaceous. In the subcrop sequence of the West Bengal shelf area, the fresh-water estuarine deposits of the Early Cretaceous were followed b y the brackish, marshy, lagoonal and littoral shelf deposits in the Late Cretaceous (Biswas, 1963). A coastal marine sequence is developed in the Meghalayan shelf area (Krishnan, 1968). The marine influence became widespread throughout the Bengal shelf during Late Campanian times. This inference is based on the presence of typical planktonic foraminifera belonging to Late Campanian--Maestrichtian times.

Geographic limits o f the Cretaceous marine transgression The marine transgressive phase during the Late Cretaceous was effective up to the western limit of the West Bengal shelf and the southern margin of the Shillong Plateau (Fig.2) in Meghalaya. It has been suggested b y Dasgupta (1977) that the Shillong Plateau had attained its plateau character b y this time. The northern boundary of the Late Cretaceous marine basin was following an E--W trend on account of this elongated plateau. The outcrops of the Cretaceous rocks in the southern parts of Meghalayas have essentially maintained an E--W strike. No typical in situ occurrence of the marine Late Cretaceous sediments is known further to the north of this E--W trend. Towards the northeast, a localised encroachment by an arm of the sea t o o k place up to the flanks of the Mikir Hills where the subsidence was possibly continuing since Gondwana times. The Assam area, which was a part of the original Indian crystalline massif, remained as a highland and was thus spared from this marine incursion during the Late Cretaceous. Towards the eastern part of the Bengal Basin, a prominent linear gravity high exists near Barishal and Chandpur (Fig.2) and it follows further northeast (Raju, 1969). This linear feature is the result of the tectonic adjustments along the NE--SW "Eastern Ghat" trend. The Barishal--Chandpur gravity high, also known as the East Bengal Ridge, is an important subsurface feature in the eastern part o f the Bengal Basin (Banerji, 1979). During the Late Cretaceous it formed the eastern limit of the Bengal Basin. The ridge sloped

69 towards the west at a high angle into the Jessore Depression. It is, however, not possible to ascertain at this stage the eastern continuity of this ridge due to rather incomplete information available from the eastern part.

Sedimentary environments The outer fringe areas of the Late Cretaceous basin are represented by a sequence of rocks composed of a dominant quartzose and felspathic sandstone derived from the acid igneous and metamorphic suites of the Indian shield and some carbonaceous shale and impure carbonate. The principal rock types have been classified as formations and are named after the localities of their type areas. The diversified lithologies of the Mahadeo (also spelled as Mahadek by Nagappa, 1959), Langpar and Cherra Formations of the Meghalayan shelf (Table IV) and the Ghatal Formation of the West Bengal shelf (Table II) are the results of the oscillatory movements and instability along the basin margin during the Late Cretaceous. The Mahadeo Formation (Medlicott, 1869) in the Garo Hills represents atypical example of the coastal water sedimentation during the first incipient marine transgression. The lower part constitutes a very coarse arkosic to orthoquartzitic pebbly sandstone followed by alternating medium-grained quartzose sandstone and greenish grey shale in the middle part and the calcareous fine-grained glauconitic sandstone and shale towards the top. The lithological compositions of the lower and middle units are the product of high mechanical energy and rapid burial. The thin calcareous shaly beds of the uppermost unit containing a typical Late Campanian-Maestrichtian planktonic foraminiferal assemblage (Globotruncana stuarti, G. arca, G. contusa, Heterohelix striata, H. plummerae, Pseudotextularia elegans, etc.) alternate with more silty and sandy beds which have yielded mostly benthonic foraminifera (belonging to the families Nodosariidae, Orbitoidae and Rotaliidae) and a few mega-fossils (Stigmatopygus elatus, Alectryonia ungulata, Baculites vagina). This alternating biofacies indicates the marine conditions oscillating between shallow marginal, intertidal, littoral to innerneritic zones. The water salinity was near normal during this time. The Langpar Formation (Medlicott, 1869) is composed of the sandy impure carbonate, shale and minor sandstone and it rests conformably on the Mahadeo Formation in the Khasi and Jaintia Hills (Biswas, 1962). The microfaunal assemblage of this formation is represented by Late Cretaceous foraminifera such as Globotruncana stuarti, G. contusa, G. arca, Heterohelix moremani, H. plummerae, Pseudotextularia elegans, Dorothia oxycona and the Palaeocene foraminifera such as Globorotalia pseudo bulloides, G. uncinata and Globigerina triloculinoides. Its biofacies and lithological suites suggest the continuation of the marine cycle and the establishment of an open marine inner-neritic environment. The Cherra Formation (Medlicott, 1869) consists of essentially coarse felspathic sandstone. It is better developed in the southern parts of the Khasi Hills (Fig.l) where it appears to be the lithofacies variant of the Langpar

70 Formation. This sandy sequence is essentially devoid of any kind of marine fossils; the only record of the fossils is in the form of poorly preserved plant remains (Medlicott, 1869). It is thus concluded that a portion o f the Khasi Hills was n o t under the influence of marine incursion at the time span represented by the marine Langpar Formation elsewhere in the Meghalayan shelf. The Ghatal Formation (Biswas, 1963) in the subcrop sequence of the West Bengal shelf is represented by calcareous shale, shaly limestone, siltstone and fine sandy intercalations. Its variable lithofacies are indicative of unstable conditions in the depositional environment. The greater abundance of brachyhaline and estuarine foraminifera (Biswas, 1963) in the Ghatal Formation is due to some restrictions of marine conditions in the shelf part of West Bengal as compared to Meghalaya. In the subsided blocks (developed since Gondwana times) on the flanks o f the Mikir Hills, the Late Cretaceous is represented by a thin fossiliferous argillaceous sequence (Dergaon Beds, Table IV) containing an assemblage of typical Late Maestrichtian planktonic and benthonic foraminifera (Abathomphalus, Globotruncana, Heterohelix; rotalids and nodosariids). This m a y suggest a brief encroachment of the area by the Cretaceous sea up to the Mikir Hills whereas the Assam region did not experience any such marine activity.

Synthesis (1) During the Late Cretaceous, the NE--SW trend became more active throughout the east coast of India. The tilting of the deposition slope towards the south and southeast was initiated due to downwarping and taphrogenic fragmentation along the basin margin faults aligned to the NE--SW "Eastern G h a t " trend. A marine transgressive phase arising f~om the southern palaeoIndo-Pacific ocean covered a wider part of the craton including those parts fractured during the Gondwana times. This marine transgressive phase was active during Late Campanian--Maestrichtian times. (2) The limit o f the Bengal Basin during the Cretaceous period is drawn up to the flanks of the Shillong massif towards the north, the Singhbhum massif towards the west, and the NE--SW-trending East Bengal Ridge towards the east and southeast. The basin remained open towards the south. (3) The greater degree of oscillations in the shelf part of the Bengal Basin during the Late Cretaceous is indicative of the continued instability and tectonic adjustments. Thus all the geological provinces of the Bengal Basin did not come under the same influence of marine activity simultaneously. (4) During the Late Cretaceous, the marine transgressive phase was not effective in Assam. A small arm of the sea, however, could penetrate up to the Mikir Hills which resulted in the deposition of a thin marine fossiliferous shaly sequence -- the Dergaon Beds over the Jorhat Formation. (5) The marine sedimentation which commenced due to the first transgression in the Late Cretaceous was fairly continuous in the Meghalayan shelf up to the Paleocene. The microfaunal elements present in the Langpar Formation exemplify this transition. The terrestrial coarse clastic sedimentation

71 (Cherra Formation) was contemporaneous with this marine transition in more interior highlands.

Stage 3. Transgressive marine sedimentation during the Early Palaeogene The marine sedimentation which commenced during the Late Cretaceous continued throughout the early parts of the Palaeogene. The Langpar Formation represents a continued marine phase since the Late Cretaceous. The oscillatory movements and the resulting instability experienced in the shelf during the Late Cretaceous remained characteristics in the Early Eocene as well. A greater part of the shelf area came under the influence of a widespread marine transgressive phase during the Early--Middle Eocene. The shelf regained stability for a brief period during the Middle Eocene. Throughout the shelf this marine phase remained effective until the beginning of the Late Eocene. The course clastic sand and carbonate association deposited as a result of the marine transgressive cycle during the Early--Middle Eocene is well known in the shelf area. In the Meghalayan shelf, the lower sand sequence with thin lenticular interbeds of lignite and coal is k n o w n as the Tufa Formation (Fox, 1937; Table IV) and is dated as Early Eocene (based on microfauna, spore and pollen). The sand grains vary in size from moderately coarse to fine in the lower part and are relatively well sorted in the upper part. This formation has an erosional contact with the underlying gneissic and granitic basement and with the Late Cretaceous--Paleocene sequence (Mahadeo and Langpar Formations) at different places. The Tura Formation is progressively onlapped by the Sylhet Formation (Vredenburg, 1907; Table IV) which is essentially a carbonate sequence containing several clastic interbeds of sandstone, siltstone and clay (Fig.4). The carbonate rocks are of highly fossiliferous biosparite to biosparudite type and are hard, compact and massive in nature. The fossils include shells of pelecypods, gastropods, crinoids, corals, echinoids, bryozoans, foraminifera, algae and hystrichosphaerids. Among the foraminifera, the more c o m m o n forms are species of Nummulites, Assilina, Discocy-

clina, Dictyoconoides, Alveolina, Globigerina, Globorotalia, Bulimina, Cibicides, Lenticulina and Rotalia. Based on their distribution, several foraminiferal biostratigraphic zones are delineated within the Eocene sequence (Tables V and VI). It has been noticed by Dasgupta (1977) that towards the north and west of the Garo Hills, most of the carbonate bands tend to wedge out and pass into more and more sandstones. Based on the palaeontological data, Biswas (1962) suggests that the Tura Formation is the time-equivalent of the lower part of the Sylhet Formation. The sequences homotaxial to the Tufa and Sylhet Formations are referred to as the Jalangi and the Sylhet Formations in West Bengal (Table III), the Mikir Formation and the Garampani Limestone in the Mikir Hills and the Basal Sandstone unit and the Sylhet Formation in Assam, respectively (Fig.4, Table II). The Jalangi Formation (Biswas, 1963) is composed of sandstone

Paleocene

Eocene

Early

Middle

/J

/

Langpar

A / Cherra

Tura Fro. (400 m)

(200 m)

Sylhet Fro.

(600 m)

Kopili Fro.

Barail Fro.

Oligocene

Late

Lithounits

~eologic tge

Pre-Barail stratigraphy in the Meghalayan shelf

TABLE IV

abundance of larger foraminifera ( N u m m u lites, Assilina, etc.), molluscan shells, algae, etc.

light grey limestone with intervening sand, clay and marl bands, more sandy towards the west

poor in microfauna, plant fossils, spores and pollen

planktonic and benthonic foraminifera devoid of microfauna

orthoquartzites, kaolinitic clays, coaly shale and arkosic sands at the base Langpar Fm. : limestone with sandy shale Cherra Fm.: coarse sandstone

transgressive onlap -~------

arenaceous forums towards the top and calcareous benthos in the b o t t o m ; ostracods, spores and pollen

dark grey fissile shale, minor fine sands, ferruginous and lateritised

estuarine (oscillatory)

coastal marine

coastal marine to estuarine unstable phase

open marine, marginal marine stable phase

prodeltaic brackish to partly marine

brackish, estuarine (regression)

rare arenaceons foraminifera, spore and pollen

shale, coaly shale, sandstone erosional unconformity

Palaeoenvironments

Dominant fauna and flora

Dominant lithology

g r a n i t e s a n d gneisses

Archaean

s a n d s t o n e , shale coal, c o n g l o m e r a t e

n o n -con f o r t u i t y . . . . . . .

microporphyritic olivine b a s a l t

Preeambrian

Gondwana group (1000 m)

Sylhet Traps (500 m)

few p l a n t fossils

p o o r l y fossiliferous w i t h few b e n t h o n i c f o r a m i n i f e r a , few p l a n t fossils

~ p l a n t fossils

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~

barren

non -conformity ......................................................

conglomerate and coarse sandstone

Gumaghat Fm. (150 m)

non-conformity

Early

massive a r k o s i c a n d g l a u c o n i t i c sandstone

disconformity

Mahadeo Fm. (165 m )

.............................

lilill[liliililli11)

Jurassic

Creta ceous

Late

lacustrine, estuarine

......................................

s u b a q u e o u s fissure eruptions

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75 TABLE V S t r a t i g r a p h i c r a n g e s o f l a r g e f o r a m i n i f e r a l s p e c i e s in t h e E o c e n e s e q u e n c e o f t h e B e n g a l Basin x = definite occurrence ? = doubtful occurrence -- = probable/rare occurrence

Eocene Middle

Late ¢o

1 Alveolina elliptica nuttalli 2 Assilina regularia 3 Assilina regularia sijuensis 4 Assilina simsongi 5 Assilina spira corrugata 6 Lockhartia hunti 7 Nummulites acutus 8 Nummulites obtusus 9 Nummulites pinfoldi 10 Dictyoconoides cooki 11 Nummulites beaumonti 1 2 Nummulites c f . N. gizensis 1 3 Nummulites discorbinus

Eorupertia s p . Fabiana s p . Nummulites c f . N. perforatus Asterocyclina matazensis Discocylina javana Discocylina omphalus Nummulites stamineus Nummulites pengaroensis Discocyclina eamesi Discocyclina pygmaea Discocyclina sella Nummulites chavannesi Nummulites fabiani Discocyclina archiaci Discocyclina assamica Discocyclina angustae Discocyclina dispansa 31 Discocyclina sowerby 32 Discocyclina cf. D. trabayensis 3 3 Pellatispira inflata 3 4 Pellatispira madraszi 35 Pellatispira cf. P. irregularis 3 6 Pellatispira cf. P. orbitoidea 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Lw

Md

Lw

Md

Up

Lw

x x

x

---

x x

?

x x x x x x

x x x x x x

x x x

--

x

x

x

x

x

X

m

x

x x

x x

x

?

x ? x

x x x

Up

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x

x

x x x x

?

x

x

--

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x

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x

x

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x

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--

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x

x

X ?

x

x ? x x x

x x ?

?

x

N o t e : T h e r e is e v e r y l i k e l i h o o d t h a t t h e u p p e r r a n g e o f Nummulites pengaroensis m a y g o into the basal part of Oligocene.

76 TABLE VI S t r a t i g r a p h i c ranges o f p l a n k t o n i c f o r a m i n i f e r a in t h e E o c e n e s e q u e n c e o f t h e Bengal Basin X = definite occurrence ? = doubtful occurrence -- = probable/rare occurrence

Eocene Early to Middle

Late

..~

¢'a

"~.~ Z.

•~

1 Globorotalia spinuloinflata 2 Truncorotaloides topilensis 3 Truncorotaloides rohri 4 Globigerina praebulloides occlusa 5 Globorotalia spinulosa 6 Globorotalia lehneri 7 Hantkenina longispina 8 Globigerina officinalis 9 Pseudohastigerina micra 10 Globigerina linaperta 11 Globigerinita unicavus 12 Globigerina angiporoides 13 Hantkenina brevispina 14 Hantkenina suprasuturalis 15 Globigerinita howei 16 Chiloguembelina tenuis 17 Cribrohantkenina inflata 18 Globigerina angustiumbilicata 19 Globigerina tripartita tripartita 20 Glo bigerina pseudoampliapertura 21 Globorotalia gemma 22 Globorotalia cerroazulensis 23 Globorotalia opima nana 24 Globorotalia centralis 25 Hantkenina alabamensis 26 Pseudohastigerina micra 27 Globigerina corpulenta 28 Globigerina gortani 29 Globigerina ampliapertura 30 Globigerina yeguaensis 31 Globorotalia increbescens 32 Hantkenina liebusi 33 Hantkenina primitiva

Lw

Md

x x x

x x x

x

x x

Up

~.~

Lw

Md

Up

Lw

Md

X X X X X

X X X X X X

X ? X X

x x

x

X X

X X

x

x

x

x x

x ?

x

x x x

-x x

x

x

x

x x

x x

? x

x

x x

x x

x

x

x

x

x

x

x x

x x

x x

x x .9

x ? _

x x x

x

x

x

--

x

x

Up

--

x

x

x x x

77 with carbonaceous shale and coal and is represented largely by fresh-water sediments in its lower part and by brackish to shallow marine sediments towards the top. Three carbonate bands, separated by two coarse clastic interbeds of varying thicknesses, are identified within the Sylhet Formation of the West Bengal subsurface. A coarse- to medium-grained sandstone with minor shale and coal (Mikir Formation) is overlain by a foraminiferal limestone (Garampani Formation) in the Mikir Hills (Samanta, 1971). In Assam, the Basal Sandstone Unit (an informal rock unit) is dominantly arkosic and somewhat clayey and the Sylhet Formation has more clastic and argillaceous content than those of the corresponding Meghalayan outcrops.

Sedimentary environments It is suggested that such a kind of sandstone and carbonate sedimentation during the Eocene can be best exemplified by the sedimentation model proposed by Pettijohn (1957) for an orthoquartzite--carbonate (consanguineous) association. According to him (op. cit.} a consanguineous association is a natural group of sedimentary rocks which are related to one another by their origin. The orthoquartzite--carbonate facies is a product of sedimentation in a marginal to low-lying stable land surface within the limits of marine incursion. The water remains extremely shallow and regresses several times. The sediments are both calcareous and purely quartzitic. The quartz grains are mature, clean-washed and rounded to subrounded in nature. The repeated washing and winnowing of detrital sediments in a near-shore zone result in the removal of felspars. In the Meghalayan and Assam shelves the arkosic sands overlying the crystalline basement are gradually replaced by more quartzose sands towards the top of the Tura Formation and in the Basal Sandstone Unit, respectively. These quartzose sands show most of the typical characters of the orthoquartzite--carbonate association as enumerated. The carbonate sequence of the Sylhet Formation is characterised by the several interbeds and numerous scattered quartz grains. Many alternations of calcarenite, calcilutite, algal beds, gritty sand and pebble conglomerate generally characterise this type of the carbonate association and these features are also typical of the Sylhet Formation. Such an orthoquartzite--carbonate association generally rests unconformably on a crystalline basement with thin arkosic and conglomeratic material. During the Middle Eocene period, the depositional conditions appear to have been rather uniform and stable in the West Bengal shelf. The carbonate sedimentation has been relatively thick in Debagram (Fig.l) and in other shelf areas (Biswas, 1963). The faunal association is represented by corals, algae, bivalves, gastropods, bryozoans and echinoids which indicate a warm open marine condition of deposition, and the presence of shallow burrows in the sediments is suggestive of a tidal environment. The abundance of skeletal remains, scarcity of terrigenous material and evidences of bioturbation point to a lower rate of sedimentation. The over-all environmental conditions prevalent during the Middle Eocene in the West Bengal shelf are thus found

78 to be suitable for carbonate reef growth, especially on a gentle monoclinal feature within the depositional slope. Such a t y p e of structural configuration for the Bengal shelf has been suggested by Biswas (1963). There is thus a strong possibility of finding carbonate reefs in the subsurface Eocene level of West Bengal. During the later parts o f the Early Eocene, the Assam region first witnessed the activation of the Eastern Ghat trend, resulting in the differential subsidence of the crystalline basement. The sedimentation in these subsiding blocks was initiated with the influx of terrigenous coarse clastics deposited under the supra-littoral environment. These clastics were derived from the northwestern and western igneous massifs and the sediments were brought in by the river system. The weathering products of the acid igneous rocks filled all the erosional valley surfaces to almost a peneplanation level under the fluvial--lacustrine conditions. A typical orthoquartzitic--carbonate association appeared somewhat later in Assam due to the marine encroachment effective in the area n o t earlier than the Middle Eocene. The microfaunal study reveals that no marine Paleocene and Early Eocene beds are developed in the Mikir Hills--Assam region. The open marine sedimentation was here thus limited to a very short period within the Middle Eocene.

Synthesis (1) The marginal marine sedimentation which c o m m e n c e d with the initial marine transgressive phase during the Late Cretaceous continued into the Paleocene in some parts of Meghalaya and West Bengal. The next phase of marine transgression, which covered a large area, was effective in West Bengal and Meghalaya during the Early Eocene and in Assam--Mikir Hills during the Middle Eocene. (2) The sedimentation of the Tura Formation started under a marginal swampy environment (indicated by the coal seams), followed by a transgressive phase during the Early Eocene. (3) Almost contemporaneous with or slightly younger than this clastic association was the carbonate sequence which was being deposited in a somewhat stable shelf condition and remained largely devoid of any clastic influx. It is possible that while carbonate sedimentation was going on at one place, the equivalent sedimentation elsewhere was mostly of a coarse clastic type, indicating the areas of detritus unloading. Thus, the limestones of the Sylhet Formation and the sandstones of the Tura and Mikir Formations may be partly synchronous and facies variants. (4) The Basal Sandstone Unit o f Assam was partly deposited under fluvial-lacustrine conditions during the later part of Early Eocene, prior to any effective marine incursion in this area. (5) The West Bengal shelf remained unstable throughout Paleocene--Early Eocene times. The development of dominant fresh-water sediments of the Jalangi Formation along with some coal beds indicates a regression of the sea from this shelf part. This phase of positive movement in the West Bengal shelf

79 was soon followed by conspicuous negative movements accompanied by a renewed and widespread marine transgression (Table III}. An extensive deposition of shallow clear water, open marine shelf carbonate took place at this time. The reef-building activity was possibly concentrated on the gentle depositional slopes of the West Bengal shelf.

Stage 4. Regressive marine cycle during the Late Eocene The carbonate sequence of the Sylhet Formation is conformably overlain by a dominantly argillaceous sequence all over the shelf (Fig.4, Table II). This sequence, known as the Kopili Formation (Evans, 1932), comprises grey to dark grey, sometimes greenish grey, laminated to fissile shale, at places, carbonaceous to coaly in nature and containing thin bands of siltstone, fine-grained ferruginous sandstone, and thin argillaceous limestone as minor constituents. It attains a thickness of an average 600 m and maintains an almost uniform composition in the Meghalayan shelf. In some parts of West Bengal, its thickness is considerably reduced to less than 30 m (Table III). The overlying Barail sequence (fine-grained sandstone and carbonaceous shale with coal, poorly fossiliferous; Table II), deposited during the Oligocene-Early Miocene, has an off-lapping relationship with the underlying Kopili Formation and the contact between these two is either a surface of erosion or non-deposition formed due to the retreating sea during Late Eocene--Early Oligocene times (Banerji, 1980a). Microfaunal study helps in identifying two subdivisions within this shaly sequence. The lower half of the Kopili sequence is rich in calcareous foraminifera such as Nummulites chavannesi, N. pengaroensis, Discocyclina eamesi, D. javana, D. omphalus, Cibicides sp., Lenticulina sp., Rotalia sp., Globigerina

gortani, G. tripartita tripartita, Globorotalia cerroazulensis and G. centralis, etc. The planktonic forms generally constitute 10% of the entire foraminiferal assemblage. The upper half, on the other hand, is poorly fossiliferous and contains mostly the arenaceous foraminiferal species of Trochammina,

Haplophragmoides, Ammodiscus, Ammobaculites, Bathysiphon, Cyclammina, Saccammina and is rich in spores and pollen of Late Eocene age. The uppermost few metres of the Kopili Formation is often devoid of any marine fauna. The change in the faunal and floral constituents from the lower to upper subdivisions of the Kopili Formation is, however, gradual and no sharp line can be drawn. A gradual transition from an open-marine to brackishestuarine facies between the lower and upper subdivisions is suggested. The carbonaceous shale and the coal, wherever developed, may represent the deposition in a shallow lagoon or in an estuary. There is no major distinction between the over-all microfaunal composition of the 8ylhet Formation and the lower subdivision of the Kopili Formation. A higher dominance of large-sized benthic forms of foraminifera such as Nummulites, Assilina and Discocyclina, etc., is generally characteristic of the Sylhet Formation and the smaller-sized species which persisted in the Late

80 Eocene are only known from the Kopili Formation (Tables V and VI). Based on the time-stratigraphic index species of planktonic and larger foraminifera, the Sylhet--Kopili transition is marked at the base of the Priabonian (Late Eocene, International Planktonic foraminiferal Zone P15; Mehrotra and Banerji, 1973) and the youngest horizon of the Kopi]i Formation is placed in the Late P'riabonian (Zone P17) to Early Lattorfian (Oligocene, Zone P18).

Sedimentary environments As already pointed out, the Kopili Formation has a conformable contact with the underlying Sylhet Formation. While the deposition of the carbonates (of the Sylhet Formation) during the Middle Eocene was due to an over-all stability in the basin, the accumulation of the fine clastics of the Kopili Formation was a result of reincited instability and gradual uplift in the distant foreland shields. The supply of the fine clastics (from a distant provenance) and the gradual southwardly regression of the sea brought about this change in the depositional facies. The detritus supply was maintained under the influence of the river system which resulted in the formation of freshwater-estuarine rocks in some parts of West Bengal and Assam. In Meghalaya, the relatively greater thickness of the Kopili Formation indicates that the sediments were derived from somewhat rapidly eroding source areas and were poured into the subsiding depositional sites initially under shallow openmarine conditions (as indicated by the presence of calcareous microfauna in the lower part). The environment was progressively changing through the brackish marsh (indicated by the presence of an exclusively arenaceous microfaunal assemblage in the upper part) into an almost fresh-estuarine type (uppermost sequence being generally devoid of microfauna). This gradual shallowing of the basin margin and the withdrawal of the sea resulted in a development of prodeltaic conditions along the basin margin. This was the precursor for the sedimentary environments formed during the Oligocene for the overlying Barail sequence. The short-span unconformity surfaces are recognised faunisticaUy over the top of the Kopfli Formation in the Assam shelf (Banerji, 1980a).

Synthesis (1) During the later part of the carbonate sedimentation (the Sylhet Formation), a low-order instability and uplift in the distant parts of the craton induced a flow of fine clastics represented by the clays and fine sands which started depositing under marine to brackish marshy conditions during the Late Eocene. (2) No major change in the basinal configuration during the Sylhet--Kopili transition is suggested. (3) The Kopili sequence represents a transitional stage changing from true marine at the base to progressively terrestrial towards the top. Its top marks the end of the marine phase of sedimentation and forms an important datum almost throughout the Bengal shelf.

81 PALAEOGEOGRAPHY AND THE BASIN EVOLUTION A reconstruction of the regional palaeogeography of the Bengal Basin from its inception until the end of the Kopili sedimentation (latest Eocene) is attempted (Figs.5 and 6). Prior to the subsidence, the Bengal Basin was a land area comprising parts of the igneous and metamorphic shield {eastern extensions of the Indian shield) that possibly continued to the east as far as the NE--SW-trending East Bengal Ridge. Further east, a palaeobasin (an IndoBurmese Basin) existed up to the Shan Plateau of Burma where sediments derived from the Shan Plateau and partly from the East Bengal Ridge were deposited since the Palaeozoic. The southern continuation of the Bengal shield up to the southern palaeo-Indo-Pacific ocean is presently debatable as the geographic limit of this palaeo-ocean is so far not well understood. On this huge continental shield, several fault zones were reactivated during the Mesozoic which coincided with the well-known Gondwana tectonism of India. One of the oldest and most well documented trough systems formed along these fault zones is the Damodar graben (Fig.2), which came into its existence in the Permian and extended eastwards as a long linear trough from long. 81°E into the present Bengal Basin area in which the coal-bearing sediments belonging to the Gondwana System were deposited. The general slope of this trough was towards the east and possibly it cut through the East Bengal Ridge and continued into the Indo-Burmese Basin. During the latest Jurassic, large amounts of basic lava flows erupted from fractures and fissures developed in the basement. The NE--SW "Eastem G h a t " trend of the southeastern Indian shield was active in the Bengal Basin during the later part of the Early Cretaceous. Due to tectonic adjustments, local downwarping resulted in a fresh-water-estuarine deposition. Such a sedimentation was probably homotaxial and synchronous with the other semi-fresh-water sedimentation going on in the east coast basin of India. The further activation of the NE--SW trend has resulted in the progressive tilting of the palaeodepositional slope towards the southeast and to the south. A marine incursion t o o k place essentially from the south during the Late Cretaceous. The first marine phase could reach almost up to the present sedimentary basin margin in West Bengal and Meghalaya during the Late Campanian and covered these areas by a shallow sea. The basin to the east was limited up to a prominent p a l a e o - f e a t u r e - the East Bengal Ridge (Banerji, 1979). This marine phase in the shelf area was not only short lived but also oscillatory, and hence resulted in a variable litho- and bio-facies. The change in the eustatic level was possibly the result of the continued adjustments along several prominent tectonic elements within the basement. The second well-known marine transgressive phase commenced in the Early Eocene, but its effect was largely felt only during the Middle Eocene. During this period, the areas further northeast, i.e. the Assam shelf, opened up as a gulf. The sediments of the clastic--carbonate association were deposited all over the Bengal shelf. The fossiliferous carbonate sequence indicates warm

82

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ESTUARINE"[0 BRACKISH

PRESENT DAY GEOOR,&PHIC.&L ~ SHELF SEDIMENTS Ig~UNDARY (APPROXIMAIE) ~EC10NIC HINSE ZONE [-::-----~ BA$1NAL SEDIMENIS

F ~ ~

DIRECTION OF SEDIMENT SUPPLY CURREN/ DIRECTION

P"q~" J DIRECTION ~F MARINE 1 RANSGRESSION

Fig.5. Palaeogeographic maps of the Bengal Basin since its inception up to the end of Eocene period.

83 open stable marine conditions in the shelf. The East Bengal Ridge continued to separate the Bengal Basin from the eastern "Indo-Burmese basin". During the Late Cretaceous a gradual upwarping within the Indo-Burmese basin along the Arakan--Yoma axis (Clegg, 1941) resulted in progressive bifurcation of this basin into a western "Indian trough" and an eastern "Burmese trough" (Fig.6). The depositional facies in the Indian trough has not been found to be related with the Bengal Basin (Banerji, 1979). The East Bengal Ridge was acting as the eastern limit of the Bengal Basin during this period.

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Fig.6. Bengal and Burmese palaeobasins during the Palaeogene.

195°

84 During the Late Eocene, the Bengal shelf came under the impact of heavy unloading of the fine clastics which were brought in by a river system from a considerable distance. The distant craton which formed the provenance for these sediments was possibly undergoing some kind of upheaval and epiorogenic movements; the impulses so generated, however, failed to reach and affect the Bengal Basin to a great extent. A thick sequence of fine clastics (Kopili Formation) was formed in most of the shelf area. The original open marine conditions existing up to the Middle Eocene were gradually replaced by more brackish, lagoonal to estuarine facies during the Late Eocene. This change in the depositional environment marks the end of a true marine phase in the Bengal Basin. The latter sediments (the overlying Barail sequence) started depositing under a prodeltaic condition which led to the revival of estuarine sedimentation on the terrestrial depositional surface formed on the top of the Kopili Formation. It is, therefore, suggested that the top of the Kopili Formation is an important sedimentary datum in the Bengal shelf and it can easily be demarcated by the lithological and faunal discontinuity. An erosional unconformity separates the Barail sequence from the Kopili Formation in Assam. The sedimentation history during the first phase (transgressive marine with minor regressions) of the Bengal shelf thus concludes with the retreat of the sea after the deposition of the Kopili Formation during the Late Eocene--Early Oligocene times. ACKNOWLEDGEMENTS

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