Sedimentation and basin-fill history of the Neogene...

Sedimentation and basin-fill history of the Neogene clastic

succession exposed in the southeastern fold belt of the

Bengal Basin, Bangladesh: a high-resolution sequence

stratigraphic approach

M. Royhan Gani1, M. Mustafa Alam*

Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh

Received 12 July 2000; received in revised form 2 April 2001; accepted 13 March 2002

Abstract

The Tertiary basin-fill history of the Bengal Basin suffers from oversimplification. The interpretation of the sedimentary

history of the basin should be consistent with the evolution of its three geo-tectonic provinces, namely, western, northeastern

and eastern. Each province has its own basin generation and sediment-fill history related mainly to the Indo-Burmese and

subordinately to the Indo-Tibetan plate convergence. This paper is mainly concerned with facies and facies sequence analysis of

the Neogene clastic succession within the subduction-related active margin setting (oblique convergence) in the southeastern

fold belt of the Bengal Basin. Detailed fieldwork was carried out in the Sitapahar anticline of the Rangamati area and the

Mirinja anticline of the Lama area. The study shows that the exposed Neogene succession represents an overall basinward

progradation from deep marine through shallow marine to continental–fluvial environments. Based on regionally correlatable

erosion surfaces the entire succession (3000+ m thick) has been grouped into three composite sequences C, B and A, from

oldest to youngest. Composite sequence C begins with deep-water base-of-slope clastics overlain by thick slope mud that passes

upward into shallow marine and nearshore clastics. Composite sequence B characteristically depicts tide-dominated open-

marine to coastal depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of

incised channel, tidal inlet, tidal ridge/shoal, tidal flat and other tidal deposits is separated by shelfal mudstone. Most of the

sandbodies contain a full spectrum of tide-generated structures (e.g. herringbone cross-bedding, bundle structure, mud couplet,

bipolar cross-lamination with reactivation surfaces, ‘tidal’ bedding). Storm activities appear to have played a subordinate role in

the mid and inner shelf region. Rizocorallium, Rosselia, Planolites and Zoophycos are the dominant ichnofacies within the

shelfal mudstone. This paralic sedimentation of Neogene succession in the study area can serve as a good point of reference for

tide-dominated regressive shelf depositional systems. The top of the composite sequence B is marked by a pronounced erosion

surface indicating the final phase of marine regression followed by the gradual establishment of continental–fluvial

depositional systems represented by composite sequence A. In this composite sequence, stacked channel bars of low-sinuosity

braided rivers gradually pass upsequence into high-sinuosity meandering river deposits.A sequence stratigraphic approach has

been adopted to interpret the basin-fill history with respect to relative sea-level changes; and to subdivide the rock record into

several sequences and units (systems tracts and parasequences) based on identified bounding discontinuities, such as

transgressive erosion surface (TES), regressive erosion surface (RES), marine flooding surface (MFS), and incised valley floor

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0037 -0738 (02 )00182 -3

* Corresponding author.

E-mail addresses: [email protected] (M. Royhan Gani), [email protected] (M. Mustafa Alam).1 Present address: Geoscience (FO 21), University of Texas at Dallas, 2601 North Floyd Road, Richardson, TX 75038-0688, USA.

www.elsevier.com/locate/sedgeo

Sedimentary Geology 155 (2003) 227–270

(IVF). This approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove

vital to the oil companies engaged in exploration activities in the Bengal Basin. It is strongly recommended here that the

traditional lithostratigraphic classification of this part of the basin, which is based on the Assam stratigraphy, be abandoned or at

least revised. A tentative allostratigraphic scheme is presented, and it is suggested that to formalize the scheme further study,

both surface and subsurface, is needed.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Bengal Basin; Neogene; Lithofacies; Facies analysis; Bounding discontinuity; Sequence stratigraphy

1. Introduction

The Bengal Basin, covering Bangladesh and part

of eastern India, is one of the least studied and known

basins in the world. The geological evolution of the

basin began in the late Mesozoic with the break-up of

Gondwanaland and is ongoing (Alam, 1989). The

greater Bengal Basin is bounded by the Shillong

Plateau, the northeastern wedge of the Indian craton,

to the north; by exposures of the Indian craton to the

west; and by the Indoburman Ranges to the east (Fig.

1). The basin extends southward into the Bay of

Bengal.

The Bengal Basin is well known for the develop-

ment of one of the thickest (about 20 km) sedimentary

successions in the world. A considerable thickness of

the Neogene clastic strata is exposed in the Chitta-

gong–Tripura Fold Belt (CTFB) along the eastern

margin of the basin. However, only few publications

have presented facies analysis of these clastic rocks

exposed in the southern part of the CTFB (Alam,

1995; Alam and Ferdous, 1995, 1996; Alam and

Karim, 1997; Gani and Alam, 1999). The significance

of tidal influence in the shallow marine Surma Group

(Table 1) was first discussed by Alam (1995). More

recently Gani and Alam (1999) have suggested that

the entire Surma Group succession represents an

overall basinward progradation from deep marine to

coastal marine within the active margin setting of the

Indo-Burmese plate convergence.

This study is a follow-up of the work of Gani

(1999) and Gani and Alam (1999), and has six main

objectives: (1) to carry out a detailed lithofacies

analysis and palaeoenvironmental reconstruction of

the Neogene clastics (focusing on the traditional

Surma Group) exposed in the southeastern fold belt;

(2) to provide a full record of the entire exposed

succession for continuous tracking of the basin-fill

history from oldest to youngest; (3) to divide the rock

record on the basis of bounding discontinuities using

the conceptual framework of high-resolution sequence

stratigraphy; (4) to interpret the origin of, and genetic

relationships between, individual units in response to

relative sea-level changes; (5) to present a regional

correlation (depositional-strike parallel) of the identi-

fied bounding discontinuities between the two studied

anticlines; and (6) to propose a tentative revised

stratigraphic framework for the CTFB.

These objectives imply that this study is significant

in terms of local as well as international perspective.

For example, the study may shed light on the tidal

deposits of regressive shelves that are yet to be well

documented and modeled. The anatomy of the sand-

bodies presented here can be valuable to international

oil companies for accurate stratigraphic prediction of

reservoir facies and trapping styles. Furthermore, the

revised stratigraphic scheme proposed in this study

can be significant for a better understanding of the

sedimentary evolution of the CTFB region.

Fieldwork was carried out in two anticlinal struc-

tures within the CTFB, namely, the Sitapahar anticline

of the Rangamati area and the Mirinja anticline of the

Lama area (Fig. 2). The Mirinja structure, about 120

km long and extending southward into Mayanmar, is a

doubly plunging, elongated asymmetric anticline with

an axial trend of N35jW–S35jE and a plunge of 7jtoward N35jW. The eastern flank of the structure is

steeper and the axial plane dips 82j toward S56jW.

Detailed logging was carried out to document a nearly

3-km-thick Neogene clastic succession exposed along

the Lama–Fasiakhali road section in the western flank

of the structure. Rocks exposed in various creeks

along the Matamuhari River and on the eastern flank

of the structure have also been studied for cross-

checking. The Sitapahar structure, about 40 km in

length, is also a doubly plunging, elongated asym-

M. Royhan Gani, M. Mustafa Alam / Sedimentary Geology 155 (2003) 227–270228

metric anticline with an axial trend of N20jW–S20jEand a plunge of about 4j. The gentler eastern flank of

the structure has dips varying from 10j to 45j and the

steeper western flank dips at angles varying from 15j

to 70j (Ferdous, 1990). Fieldwork was carried out

along the Rangamati–Chittagong road section that

transversely cuts the northern part of the anticline,

and the greater than 3-km-thick Neogene succession

Fig. 1. Generalized tectonic map of the Bengal Basin and surrounding areas (modified from Uddin and Lundberg, 1998). Hinge zone separates

the shallow Indian platform to the northwest from the deeper Bengal foredeep to the southeast. (CTFB=Chittagong–Tripura Fold Belt.)

M. Royhan Gani, M. Mustafa Alam / Sedimentary Geology 155 (2003) 227–270 229

exposed on the eastern flank has been described in

detail.

2. Regional tectonics and stratigraphy

Understanding the tectonic setting of a region is a

necessary pre-requisite to understand the paleogeo-

graphic and sedimentation patterns of that region. The

tectonic framework and structural setting of the Ben-

gal Basin has been discussed by several authors

including some of the important recent accounts of

Murphy and Staff BOGMC (1988), Lohmann (1995),

Alam (1997), and Sikder (1998).

Various independent lines of evidence demonstrate

that the Indian plate was subducting below both the

Tibetan plate to the north and the Burmese plate to the

east during the greater part of the Tertiary. Gani and

Alam (1999) have reviewed the tectonic concepts of

southeast Asia and have offered a somewhat modified

tectonic setting for the CTFB. They have proposed

that the CTFB is a westward extension of the Indobur-

man Ranges and is closely related to the easterly

subduction of the Indian plate in an arc-trench tectonic

setting (Fig. 3). The CTFB region in the very early

Neogene was a remnant ocean basin and included the

migrated trench-slope bathymetry from the east. This

remnant ocean basin received sediment from two

directions—(i) collision-derived sediment from the

Barail and Naga Hills entering the basin from the

north along the trench axis and (ii) Indoburman arc-

derived sediment shed transversely to the trench from

the east. The trench axis has shifted further westward

and southward because of this heavy sediment load-

ing, giving rise to the Neogene accretionary prism

(Fig. 3) of Dasgupta and Nandy (1995). It is remark-

able that in the southeastern part of the Bengal Basin

the trench-slope bathymetry was flattened to shallow-

water conditions probably sometime in the Late Mio-

cene. At present the deep-water subduction zone

exists only southward from approximately 19jN lat-

itude as the Andaman–Sunda Trench.

Evans (1932) established a lithostratigraphic clas-

sification for the Tertiary strata exposed in the Lower

Assam Basin (see Fig. 1 for geographic location).

Since then in various published and unpublished

reports, without any detailed regional correlation,

this early stratigraphy has served as the basis for

stratigraphic correlation of the sediments within the

entire Bengal Basin. Later studies using micropa-

leontology (e.g. Banerji, 1984), palynology (e.g.

Baksi, 1972; Reimann, 1993), and seismic stratigra-

phy (e.g. Salt et al., 1986; Lindsay et al., 1991) have

partly refined this early scheme. Uddin and Lund-

Table 1

Traditional stratigraphic classification for the CTFB

For convenience, stratigraphic successions of the Mirinja and the Sitapahar anticline are compared at the right. The Barail Group (Oligocene) is

argued to lie in stratigraphically lower position below the Surma Group. (See text for discussion.)a Based on Evans (1932).b In this study.


berg (1999) have given a relatively in-depth sum-

mary of the stratigraphy of the Bengal Basin and

have discussed the depositional pattern of the Surma

Group in the eastern Bengal Basin using subsurface

lithofacies maps. Table 1 shows the old stratigraphic

nomenclature traditionally used for the CTFB region.

Fig. 2. Geological sketch map of part of the Chittagong–Tripura Fold Belt (CTFB) showing the distribution of the traditional stratigraphic units

(modified from Alam et al., 1990). Note the locations of the studied anticlines and the seismic profile in Fig. 29.


Fig. 3. Tectonic setting of the Bengal Basin within the regional context (from Gani and Alam, 1999). The Tertiary volcanic centers are marked

by solid dots and the bathymetric contours of the Bay of Bengal are shown in meters. (NAP=Neogene accretion prism.)


The base of the Surma Group as well as the contact

between the Bhuban and Bokabil formations in this

Group is not well defined in this region, highlighting

the need for a revised stratigraphic scheme.

3. Sedimentology and sequence stratigraphy of the

Neogene succession

Detailed sedimentological logging of the exposed

sections permits identification of the facies types and

bounding discontinuities within the Neogene clastic

succession in the studied anticlines. The criteria for

identifying the bounding discontinuities are primarily

based on the principles of high-resolution sequence

stratigraphy, and are briefly discussed here first.

Incised valley floor (IVF)—This surface erodes the

upper part of a progradational succession at best, and

shelfal mud at worst; and the lag deposits indicative of

channel erosion characteristically overlie it. The suc-

cession overlying this surface reveals deepening-

upward (transgressive) trend, in which typical estuar-

ine facies can be identified. The above criteria suggest

that this bounding discontinuity most probably devel-

oped as a result of channel incision at the time of sea-

level low; and hence is called an IVF.

Regressive erosion surface (RES)—It is developed

erosionally on shelfal mud and is overlain by shallow-

ing-upward shoreface deposits. This type of bottom-

truncated progradational shoreface deposits indicates

that the surface lying below has been generated

because of submarine erosion during the fall of relative

sea level; and hence the surface is designated as a RES.

Transgressive erosion surface (TES)—This surface

exclusively develops on valley-fill deposits experienc-

ing sea-level rise. It is overlain by either thin (5–15-

cm-thick) pebble horizon or 5–10-m-thick shoreface

sandbody before passing upward into shelfal mud. The

above criteria suggest that this surface has been devel-

oped because of submarine erosion during the rise of

relative sea level; and hence can be called a TES.

Transgressive surface (TS)—This surface is similar

to the TES except that it is non-erosional and is

directly overlain by shelfal mud. It should be noted

that subtle TSs are inferred to exist at the contact

between the fluvial and estuarine deposits within a

valley-fill succession. These will be discussed later in

the text.

Marine flooding surface (MFS)—This type of

bounding discontinuity is formed when shelfal mud

abruptly overlies a shallowing-upward shoreface suc-

cession. Therefore, it is the result of a short-term sea-

level rise; and hence is called a MFS.

On the basis of the earlier works (Alam, 1995;

Alam and Ferdous, 1995, 1996; Gani, 1999; Gani and

Alam, 1999), it can be concluded that the exposed

Neogene succession in the CTFB represents a basin-

ward progradation from deep marine base-of-slope

through shallow marine coastal to continental–fluvial

deposits; and that the entire succession may be

divided into three broad groups. In this study, we

have established three discrete composite sequences

based on the regionally correlatable bounding discon-

tinuities (Fig. 4). These composite sequences, desig-

nated as C, B, and A, from oldest to youngest,

represent three stages of basin evolution each having

its own sedimentation pattern and basin-fill architec-

ture. A comparison of the three composite sequences

with the traditional stratigraphic nomenclature is

shown in Table 1.

(i) Composite sequence C (1128 +m thick): The

basal composite sequence is conspicuously mud

dominated with minimal development of intervening

erosion surfaces. It begins with deep-water base-of-

slope clastics, encountered only in the Sitapahar

anticline (Fig. 4B), that grade upward into shallow

marine and nearshore deposits. Prograding parase-

quences separated by MFSs characterize this compo-

site sequence in the Sitapahar anticline, whereas in the

Mirinja anticline submarine channel deposits are con-

spicuous (Fig. 4A). The top of C is demarcated by a

pronounced erosion surface indicating a lowstand of

sea level.

(ii) Composite sequence B (1293+ m thick): The

middle composite sequence is sand dominated and has

several distinct and regionally traceable erosion sur-

faces. It is abbreviated in the Sitapahar anticline due to

truncation by the Manikchari fault (Fig. 4B). Tide-

dominated open marine shelfal to coastal depositional

settings under the control of cyclic relative sea-level

rise and fall typify this composite sequence, where

several incised valley-fill deposits are prominent. The

top of B is also a pronounced erosion surface of

lowstand sea level.

(iii) Composite sequence A (1080+ m thick): The

upper composite sequence represents the final phase


of basin-filling history with the establishment of

continental–fluvial depositional systems. Low-sinu-

osity braided to high-sinuosity meandering fluvial

deposits characterize a considerable portion of this

composite sequence.

On the basis of the overall regional tectonic setting

discussed in Section 2, it is assumed that the trend of

paleo-coastline in the CTFB was oriented north–

south. Alam (1995) documented a similar paleo-coast-

line trend from paleocurrent analysis of the Surma

Group in the Sitapahar anticline. The measured cur-

rent directions in Fig. 4 represent landward (eastward)

and basinward (westward) directions.

In the lithostratigraphic columns (Fig. 4) we have

shown the sequences of different hierarchies. Within a

composite sequence individual sequences represent a

single sea-level cycle (fall–rise–fall). As a result,

either IVF or RES bound them (both below and on

top). When completely developed and/or preserved

they show all the three systems tracts—lowstand,

transgressive and highstand—of an ideal sequence.

Each sequence is further subdivided into units based

on TES/TS and MFS. A unit therefore, in most cases,

represents a systems tract (separated by a TES/TS) or

a parasequence (separated by a MFS).

3.1. Composite sequence C

The lower composite sequence represents a basin-

ward progradation from deep marine (base-of-slope

deposits) to shallow marine clastics. The base-of-

slope deposits are observed only in the Sitapahar

anticline.

3.1.1. Mirinja anticline

Unit Ca: Monotonous laminated mud with paper-

thin discontinuous silt-streaks, and a few scattered

burrows (Zoophycos, Planolites and others unidenti-

fied) comprise the lower part of unit Ca. Micropa-

leontological study revealed several foraminiferal

tests (e.g. Rotalia, Quinqueloculina). Above the bush

cover, silt lenses, mostly with gradational bases and

sharp tops, increase in number within the mud. Ripple

cross-lamination within these lenses shows an east-

ward (landward) current direction. Rare thin ( < 5 cm

thick) and sharp-based siltstone to very fine sandstone

interbeds, either structureless or containing combined-

flow ripple-lamination, were observed. Just below

unit Cb, the number of burrows (Rizocorallium,

Rosselia, Planolites and Zoophycos) increases

abruptly.

This oldest stratigraphic interval is interpreted as

very outer-shelf mud grading upward into distal inner-

shelf silty mud indicating a shallowing-upward trend.

The quiet water shelf environment is occasionally

interrupted by storm events that deposited the sharp-

based thin siltstone and sandstone interbeds and lenses

(e.g. MacEachern et al., 1998). The trace fossil

association also substantiates this environmental inter-

pretation.

Unit Cb: This unit is 27 m thick and begins on an

erosional surface (Fig. 5A). The lower 10 m of this

unit has repetitive 10–20-cm-thick fining-upward

cycles (Fig. 5B). In each cycle the thickness of mud

intervals increases upward and the silt intervals show

parallel to truncated wavy lamination reflecting hum-

mocky cross-stratification. The lower part of the unit

passes upward (through a thin interval of wavy-

lenticular sand–mud) into a dominantly medium to

fine sandy succession, which contains trough cross-

bedding (Fig. 6), megaripple cross-stratification, thin

intercalated irregular mud laminae and scattered mud

clasts, up to 15 cm in size. The top 6 m of the unit is

composed of interbedded sandstone and mudstone.

The fine to very fine, ripple cross-laminated sandstone

interbeds abruptly decrease in thickness from 20 cm to

a few centimeters up-sequence. The laminated mud-

stone interbeds contain small sand lenses with oppos-

ing current directions.

The lower part of the unit is interpreted as waning

storm-generated fining-upward succession (Fig. 5B),

where storm influence decays upsequence. The mid-

dle part is thought to represent progradation of a tidal

sand sheet/sand wave (in the sense of Stride et al.,

1982), which may have been partially modified by

storm activity. Thin mud drapes along the foresets and

set boundary (Fig. 6), and occurrence of intercalated

mud laminae attest to this tidal influence. It is evident

that the lower and middle part of unit Cb indicate a

shallowing-upward trend. The unit rests erosionally

on inner shelf mud, indicating that this erosion surface

is probably a RES, and that the unit represents a

‘sharp-based’ shoreface sandbody (cf. Plint, 1988)

produced because of relative sea-level fall. Abrupt

thinning and muddier upward trend within the top 6 m

of unit Cb indicates progressive deepening (after a


Fig. 4. Details of the lithostratigraphic column of the exposed Neogene clastic succession with bounding discontinuities and tentative regional correlation. (A) The Mirinja anticline and (B) the Sitapahar anticline.

M. Royhan Gani, M. Mustafa Alam / Sedimentary Geology 155 (2003) 227–270 pp. 235–240

Fig. 5. (A) Detailed litho-log of the first shoreface sandbody (unit Cb in the Mirinja anticline) on shelfal mud; and (B) one representative storm-

generated fining-upward cycle (in the lower 10 m of the unit) characterized by: (a) scoured to load casted fine (to medium) massive sandstone

with mudstone clasts, (b) hummocky to parallel laminated siltstone, (c) mudstone with injection structures.


sea-level fall), the culmination of which is considered

as a TS.

The remaining part of composite sequence C is

mud dominated with three sand intervals. For con-

venience these three sand intervals are designated as

units Cc, Cd, and Ce, although this does not comply

with the criteria adopted for designating other units

in Fig. 4. Laminated mudstone with rare continuous

to discontinuous silt laminae and sharp-based lenses,

with rare low-angle micro cross-lamination, charac-

terize the muddy interval between units Cb and Cc.

A few massive to parallel laminated and sharp-based

20–50-cm-thick silt beds occur below the base of

unit Cc.

Unit Cc: This is a 25-m-thick deposit with basal

erosional relief traceable for more than 10 m. The

deposit is fine to medium sand with high matrix

content and structureless appearance. Conglomeratic

lags with some erratic lithic blocks up to 1 m long

occur above the erosional base. Shells and impres-

sions of bivalve fossils are occasionally encountered

within the deposit. Considering the overall rock con-

text of this part of composite sequence C, the unit Cc

is interpreted to represent a submarine gully/channel

filled chaotically and encased within the upper bathyal

mudstone. The laminated fissile mudstone above unit

Cc contains paper-thin silt streaks. The amount of silt

increases abruptly below unit Cd.

Unit Cd: This unit is 12 m thick and is charac-

terized by repetitive fining-upward cycles 10–20 cm

thick (Fig. 7) of ripple cross-laminated very fine

sandstone grading up into intercalated silt-mudstone

and shale. Each cycle has a sharp base with load and

injection structures. Numerous meandering trails

(Scolicia or Cochlichnus?), some crosscutting, are

preserved at the base of some of the sandstone beds

(Fig. 8). A 1.5-m-thick slumped bed including folded

thin sandstone beds occurs in the middle part of this

unit. The fining-upward cycles (Fig. 7) are not clear in

terms of their exact physiographic position of depo-

sition. However, in terms of their generative mecha-

nism it is probable that each cycle has been deposited

under a single pulse of waning current. Ripple lami-

nation within sandstone intervals resembles combined

flow ripples described by Myrow and Southard

(1991), and hence is indicative of storm activity.

However, the slump bed within this unit may be

indicative of slope instability of the upper bathyal

environment, hence suggesting some turbidity current

activity.

Above unit Cd, for some distance upward, thin

( < 10 cm) very fine, ripple cross-laminated sandstone

Fig. 6. Details of trough cross-bedding at the upper part of unit Cb in the Mirinja anticline.


beds with fining-upward trends and ending into

mudstone, are rarely present within the mudstone.

These beds are thought to have been deposited du-

ring unusual high tide and/or storm conditions.

Below unit Cd the mudstone is highly fissile with

silt streaks and lenses.

Unit Ce: This unit has been described earlier by

Gani and Alam (1999) and interpreted as a well-

developed and shelf-breaching submarine channel

filled with 15+-m-thick turbidities (Fig. 9). The

channel is laterally traceable along strike for over

200 m as multiple-stepped erosional feature (Fig.

Fig. 7. Repetitive fining-upward cycles in unit Cd in the Mirinja anticline. One of the cycles is represented by the scale-length (15 cm). See text

for explanation.

Fig. 8. Numerous meandering trails of trace fossil (Scolicia or Cochlichnus?) preserved at the base of a sandstone bed showing fining-upward

trend (Fig. 7) in unit Cd in the Mirinja anticline (coin for scale is 2.2 cm in diameter).


10) with relief of about 12 m. Orientation of the

channel wall is roughly east–west and current

direction within the channel-fill turbidites is toward

the west. Repetitive occurrence of thick-bedded

(average 60 cm) coarse silt to very fine sandstone

with characteristic water-escape structures, such as

internal convolution, network of dish and pillar

structures, water-escape pipes, characterizes the lo-

wer part of the channel-fill deposits. The upper part

of the channel-fill consists of an alternation of

thick-bedded (average 1 m), fine to medium, mas-

sive sandstone containing some water-escape struc-

tures at the top, and thin-bedded (average 15 cm)

muddy silt with a few water-disruption marks (Fig.

11). A few of these beds show partial amalgama-

tion. Some thin-bedded (10 to 20 cm) partial to

nearly complete Bouma sequences occur at the chan-

nel margins.

The genetic stratigraphic interpretation of ‘sharp-

based’ shoreface sandbodies (cf. Plint, 1988) has

been much debated in geological literature and will

be discussed later. In the present study the RES is

thought to have been generated by submarine ero-

sion due to forced regression (Posamentier et al.,

1992). Whether the deposits overlying the RES

should be considered as forced regressive systems

tract of Hunt and Tucker (1992) or lowstand sys-

tems tract of Posamentier et al. (1992) may be dif-

ficult to determine, and needs data from further

basinward (MacEachern et al., 1999). In our case,

for convenience, we have assigned the sandbody

resting on RES to a lowstand systems tract in a

broad sense. In this regard the combined lower and

middle part of unit Cb of composite sequence C

may be considered a lowstand systems tract, which

is overlain by a very thin and subtle transgressive

systems tract indicated by the deepening-upward

trend in the upper 6 m of unit Cb. However, as

the RES below unit Cb is not regionally correlatable

(Fig. 4), it is not considered here as a sequence

boundary. The rest of composite sequence C repre-

sents highstand systems tract without containing any

highstand parasequence of shoreline progradation.

The submarine channels within this part of the

composite sequence do not necessarily indicate

any sea-level fall; rather they, especially the upper-

most shelf-breaching and mature submarine channel

(Fig. 10), indicate littoral sediment capturing and

Fig. 9. Litho-log of the channel-fill turbidites (unit Ce in the Mirinja

anticline), measured from the N–S face (Fig. 10) of the channel.

See text for explanation.


by-passing into deep basin during highstand con-

dition (e.g. Kolla and Perlmutter, 1993).

3.1.2. Sitapahar anticline

In the Sitapahar anticline there is also no identifi-

able erosional sequence boundary within the compo-

site sequence C. A distinct progradation from base-of-

slope deep-sea clastics to shallow marine clastics is

reflected by the rock succession of this composite

sequence. Gani and Alam (1999) gave a detailed

description of the facies types and depositional envi-

ronments represented by units Ca and Cb. Only a brief

interpretation of the depositional scenario of these two

units is presented below.

Units Ca and Cb: Thin packages of turbidites

(mostly partial Bouma sequences), together with some

slump and debris-flow deposits contained within

thicker intervals of hemipelagic mudstone, character-

ize the base-of-slope deposits of unit Ca. Small-scale

submarine lobe, channel–levee complex (Fig. 12) and

Fig. 10. Outcrop attitude of the submarine channel Ce (in the Mirinja anticline) represented by block diagram.

Fig. 11. Alternation of thick-bedded massive sandstone and thin-bedded muddy siltstone at the upper part of the turbidite-fill channel Ce in the

Mirinja anticline. Note hammer for scale at the center of the photo (see also Fig. 9).


debris–mass complex represent some of the architec-

tural elements within this unit. The deposits of the unit

can tentatively be assigned to the ‘slope fan’ element

of lowstand systems tract in the sense of Van Wagoner

et al. (1988). However, at present there is no con-

clusive evidence for this interpretation; and the unit

may well be a part of the highstand systems tract

continuous with the overlying systems. The lower half

of unit Cb is composed of monotonously muddy slope

deposits that contain some localized intervals of very

thin-bedded turbidites. The upper half of the unit

represents shoaling-upward shallow marine sedimen-

tation with thicker shelfal mudstone passing strati-

graphically upward into distinct tidal ridge pro-

gradation (Fig. 13) followed by a coastal deposit. This

coastal deposit mainly reflects progradation and retro-

gradation of tidal flat strata coupled with some ripple

cross-laminated to structureless (may be storm-

reworked), fine-grained sandstone intervals. The top

of the unit is marked by a distinct MFS.

Unit Cc: This is a 34-m-thick unit characterized

by a coarsening-then fining-upward trend. The lower

part consists of interlaminated very fine sandstone

and mudstone that are gradationaly overlain by

structureless (to bedded) fine sandstone. The top

10 m of the unit reveals a muddier-upward trend

with interlaminated very fine sandstone and mud-

stone, passing upward into laminated mudstone.

Although the sedimentary structures within sand-

stone are not very clear, the sedimentation pattern

of this unit may be explained by progradation of a

sand bar (tidal?) on shelfal mud followed by trans-

gression without any development of nearshore de-

posits. Since the top part of unit Cc indicates a

deepening-upward trend the upper boundary of this

unit is interpreted as a MFS.

Unit Cd: Thick laminated mudstone with occa-

sional sandstone beds (a few centimeters thick) char-

acterize the lower 90 m of this unit, which passes up-

sequence into a 72-m-thick interval of monotonously

repetitive alternations of thin bedded sandstone and

mudstone. The two end-member interbeds are ripple-

laminated very fine sandstone 5–10 cm thick and

laminated mudstone 2–3 cm thick that are gradational

into each other through a 3–5-cm-thick wavy silt-mud

interval. This type of sedimentation pattern distinctly

reflects cyclic variation in sand–mud content within

the deposit. The top 20 m of unit Cd reflects a

muddier upward trend with the cyclic pattern gradu-

ally disappearing upward. A subtle MFS can be

recognized at the top. The monotonous alternation

of mudstone and sandstone in the upper part of this

Fig. 12. Small-scale channel– levee complex grading up into hemipelagic mudstone at the upper part of the base-of-slope deposits of unit Ca in

the Sitapahar anticline. (Person at left for scale.)


unit is interpreted as tidal rythmites in which the

cyclic variation of lithology is due to the neap-spring

variation in tidal cycle in the distal offshore setting.

Although similar types of tidal rythmites are common

in nearshore deposits, they have rarely been described

from a distal offshore depositional setting (e.g. Wil-

liams, 1989).

Unit Ce: The lower part of this unit consists of

monotonous laminated mudstone with sporadic silt

laminae, lenses, and a few sharp-based fine sandstone

beds (10 cm thick). In the upper part, laminated

mudstone gradually passes upward into interbedded

very fine sandstone 25 cm thick and mudstone 2–4

cm thick. This succession is overlain by repeated

fining-upward cycles, and is ultimately truncated by

an erosion surface. The individual fining-upward

cycles ( < 20 cm thick) belong to the continuum of

flaser-wavy-lenticular association (Reineck and Wun-

derlich, 1968). The nature of the laminated mudstones

at the lower part of Ce indicates that they are shelfal

muds, which are overlain by a coarsening-upward

facies succession. Although the sedimentary struc-

tures within the sandstone beds of this succession

are not discernable, the rhythmic alteration of thin

mudstone beds suggests that the succession may

reflect tidal ridge progradation on a regressive shelf

(e.g. Meckel, 1975; Dalrymple, 1992). The sedimen-

tary structures within the fining-upward cycles over-

lying this shallowing-upward succession justify tidal

flat progradation (e.g. Weimer et al., 1982; Alam,

1995).

This complete progradational succession of unit

Ce is erosionally overlain by an upward fining facies

succession with granule to pebble-sized mud intra-

clasts at the base. The lower 8 m of this succession

shows structureless medium (to coarse) sandstone

with thin mud interlaminae, whereas the upper 28 m

is characterized by an alternation of 10–20-cm-thick

very fine sandstone beds and 5-cm-thick mudstone

beds. This fining-upward succession is again trun-

cated by a 3-m-thick erosionally based upward

fining deposit of fine sandstone with mud intercala-

tion. A considerable thickness of this deposit may

have been eroded by the incised valley of the next

sequence. These two fining-upward successions (at

the top part of unit Ce) with heterolithic facies,

erosional bases, and basal lags give evidence for

tidal channel deposition (Shanmugam et al., 2000).

It may be argued that these deposits have been

generated by autocyclic lateral migration of tidal

channels (cf. Kumar and Sanders, 1974) on the

coastal plain at the final phase of gradual marine

regression.

Fig. 13. (a) Detailed litho-log of the first progradational sandbody

on shelfal mudstone at the upper part of unit Cb in the Sitapahar

anticline; and (b) one representative fining-upward cycle from the

lower 6.5 m of this sandbody. (From Gani and Alam, 1999).


The upper part of unit Cb and units Cc, Cd and Ce

represent progradational and shallowing-upward suc-

cessions each separated by a MFS. These deposits are

attributed to a single highstand parasequence set

consisting of four parasequences (sensu Van Wagoner,

1985; Van Wagoner et al., 1988).

3.2. Composite sequence B

This composite sequence develops five sequences

bounded by erosional surfaces (IVF or RES) of

predicted relative sea-level fall. The depositional

environments vary from shallow marine shelfal to

coastal settings punctuated by cyclic marine regres-

sion and transgression. Each sequence is divided into

units on the basis of bounding discontinuities of

relative sea-level rise.

3.2.1. Mirinja anticline

3.2.1.1. Sequence B1. Unit B1a: The unit begins on

a scoured surface overlain by 5-cm-thick intraforma-

tional lags consisting of numerous small mud rip-up

clasts ( < 5 mm in size). This is a monotonously

interstratified wavy-lenticular sandstone–mudstone

succession, which shows several muddier-upward

trends. Ripple cross-laminated (with some bipolarity)

very fine sandstone with thin mud drapes characterizes

the sandier portion, whereas the muddier portion is

laminated mudstone with a few sand lenticles. Sharp-

based, 3–5-cm-thick, very fine sandstone beds with

wave-rippled tops are rarely found within the unit.

The unit is thought to have been deposited in a

mixed wave- and tide-influenced shoreface setting

under the aegis of a slowly shifting coastline that

produced several muddier-upward trends. The setting

was probably away from a clastic input source, which

is supported by the lack of cross-bedded sandstone,

and the ripples that show predominantly eastward

(landward) current directions indicating a shelf source

of sediment transport. Since the shoreface deposits of

unit B1a erosionally rest on shelfal mud of the

composite sequence C, this scoured surface is

regarded as a RES. The generation of RES on tide-

dominated shelves is not well documented in the

geologic literature. The RES at the base of unit B1a

is believed to have been generated by subaqueous

tidal scouring in a shelfal setting during forced

regression (e.g. Reynolds, 1994). The unit grades up

into laminated mudstone at the top indicating a TS.

Unit B1b: The lower part of this unit is a muddy

succession with intercalated silt laminae and lenses

showing some sharp-based, low-angle, ripple cross-

lamination. The amount of silt increases upward and

silt becomes very fine sand before being dissected by

an erosion surface mantled by a 15-cm-thick, matrix-

supported conglomerate bed consisting of mud and

sand clasts. The sedimentation pattern of the rest of

the unit is chaotic compared to other units in this

anticline; yet two distinct intervals can be recognized.

The lower interval shows an indistinct upward coars-

ening trend. Interbedding of massive to ripple cross-

laminated fine sandstone (with thin mud laminae/

drapes) and wavy-lenticular sand–mud beds is typical

of most part of this interval, which is overlain by a 5-

m-thick fine to medium sandy succession character-

ized by low-angle accretion surfaces veneered by mud

clasts and rare wood fragments. This sandy succession

contains a 25-cm-thick set of trough cross-bedding

with tidal bundles (Fig. 14), flaser bedding, and

double mud drapes (mud couplets). On the other hand,

the dominant sedimentary structure in the upper

interval is ‘tidal-bedding’ of Reineck and Wunderlich

(1968) along with subordinate ripple cross-lamination

(with mud drapes, flasers) within the fine sandstone

interbeds. The uppermost 5 m of the unit shows a

repetition of distinct fining-upward cycles (average 20

cm thick) reflecting the continuum of flaser-wavy-

lenticular association.

The nature of the muddy succession at the lower

part of unit B1b indicates a distal offshore setting with

a slight upward shallowing trend before it is dissected

by an erosion surface, above which two sedimentation

intervals have been described. The lower interval

consists of coarsening-upward heterolithic facies suc-

cession beginning and ending with ripple cross-lami-

nated fine sand and trough cross-bedded medium

sand, respectively. This justifies that the succession

is a progradational sand bar deposit. Flaser structures,

thin mud drapes, mud couplets, and tidal bundles

indicate the tidal activity within the bar deposit.

Numerous modern and ancient examples of these

sedimentary structures in subtidal setting exist (e.g.

Alam, 1995; Visser, 1980; Allen, 1982; Nio and Yang,

1988; Shanmugam et al., 2000). Low-angle accretion

surfaces veneered by mud clasts found in the proximal


bar deposits indicate a lateral migration of the tidal

bar. It is notable that the distal bar facies is missing or

at least abbreviated at the erosive base of the lower

interval. Therefore, it is likely that this erosion surface

is a RES resulting from tidal scouring due to forced

regression (similar to the RES below unit B1a).

However, this RES is not considered a sequence

boundary, since it is not regionally correlatable (see

discussion in Section 4.1). The sedimentary structures

and the small-scale fining-upward cycles in the upper

interval suggest tidal flat progradation (e.g. Alam,

1995; Klein, 1971). Therefore, these two sedimenta-

tion intervals together indicate a single continuous

shoaling-upward succession from lower shoreline to

coastal environments.

3.2.1.2. Sequence B2. Unit B2a: A typical fining-

then coarsening-upward facies succession (Figs. 4 and

15) characterizes most of unit B2a. The upward fining

succession begins on a distinct erosion surface with

basal lags consisting of mud clasts of variable sizes (a

few are as large as 1 m) dispersed in medium sand-

stone. In the lower 50 m of the succession poorly

sorted medium to fine sandstone with dispersed mud

pebbles are structureless in nature with rarely devel-

oped indistinct parallel lamination. This part of the

unit passes upward into finely laminated very fine

sandstone with well-developed parting planes. The

fining-upward succession culminates with laminated

siltstone and mudstone containing a few thin horizons

of organic-rich material and coal fragments. In the

upward coarsening succession the interlaminated silt-

mud portion grades up into very fine sand with ripple

cross-lamination, wavy mud interlaminae/drapes, and

rare flaser. This succession is truncated by an erosion

surface above which bi-directional ripple cross-lami-

nated very fine sandstone passes upward into repeated

fining-upward cycles (average 20 cm thick), within

each of which bipolar ripple cross-laminated very fine

sandstone containing mud drapes grades up into

laminated mud. A pronounced second erosion surface

veneered by mud pebbles (up to 25 cm long) appears

at the topmost part of unit B2a. A 7-m-thick deposit

above this erosion surface contains fine sandstone

with parallel to low-angle lamination and 5-mm-thick

stringers of medium sandstone.

The pronounced erosion surface with basal lags at

the base of unit B2a, and the typical tripartite facies

zonation of this unit (Fig. 15) can suitably be

described by an estuarine depositional model with

possible incision at the channel base. Incised valley

fills truncating the preceding highstand deposits with

Fig. 14. Tidal bundles (arrow) in the middle cross-bedded set of the photograph, from the middle part of unit B1b in the Mirinja anticline. Scale

bar at top left is 30 cm.


fining-then coarsening-upward trends are character-

istic in stratigraphic record (e.g. Leckie and Singh,

1991) and the bases of these fills are key sequence

boundaries in the Exxon model. Majority of the

deposits of unit B2a are typically structureless with

dispersed mud clasts that may be indicative of very

rapid aggradation and wave reworking of the valley-

fill deposit at the time of rising sea level. Intuitively,

therefore, either the valley acted as a zone of sediment

bypass during sea-level low and preserved little or no

fluvial deposits at the very lower part, or these low-

stand deposits were rapidly reworked during the

ensuing transgression (e.g. Reinson, 1992; MacEach-

ern and Pemberton, 1994) giving rise to the apparently

structureless sandstone of the valley-fill, the base of

which could then be regarded as a co-planar erosion/

transgression surface (IVF/TES). Laminated siltstone

and mudstone with horizons of organic materials and

coal fragments characterize the ‘central basin mud-

stone’ of the estuary.

The coarsening-upward portion of the valley-fill is

thought to represent a tide-influenced estuary mouth

complex. As the valley is transgressively filled the

two erosion surfaces at the top part of unit B2a can be

considered as TESs similar to those described by

Zaitlin et al. (1994). The first TES can be argued as

a tidal scour ravinement surface overlain by indistinct

fining-upward deposits of a possible tidal inlet. The

second TES is a wave ravinement surface above

which wave- and storm-influenced deposits occur.

Parallel to low-angle lamination and lenses of medium

sandstone within fine sand attest to this storm and

wave activity. The top of unit B2a is a TS, which can

be regarded as a low energy maximum flooding sur-

face (e.g. Zaitlin et al., 1994). The valley-fill pattern

with crude tripartite facies zonation probably indicates

a wave-influenced estuary (Dalrymple et al., 1992;

Reinson, 1992) in which tidal currents were also

active in shaping the deposits. The whole of unit

B2a represents a transgressive systems tract with little

(or no) preservation of lowstand systems tract at the

very lower part.

Unit B2b: This unit is a monotonous laminated

mudstone interval with occasional silt to very fine

sand streaks, lenses and interlaminae. A 2-m-thick

zone of extensive burrowing (tracemakers include

Zoophycos, Rizocorallium, Planolites) is encountered

at the middle part of this unit; and also a 50-cm-thick

black mudstone interval of complete bioturbation with

some preserved burrows is observed near the top of

the unit. The unit is interpreted as shallow marine

shelfal mudstone deposit, which is supported by the

observed trace fossil association (cf. Pemberton et al.,

1992).

Fig. 15. Detailed litho-log of unit B2a (in the Mirinja anticline)

showing a fining-then coarsening-upward facies succession inter-

preted as a transgressively filled wave-influenced estuary deposit.

See text for explanation.


3.2.1.3. Sequence B3. Unit B3a: This unit is char-

acterized by fining-then (indistinct) coarsening-

upward trend (Fig. 16) developed on a distinct erosion

surface overlain by dispersed pebble lags. The lower

12 m of the unit consists of interbedded fine to

medium and very fine sandstone intervals with occa-

sional thin (2–5 cm) mud layers. Fine to medium

sandstone contains 25-cm-thick sets of trough cross-

stratification (westward current direction) with thick

mud drapes along the foresets. Bi-directional ripple

cross-lamination with mud drapes/interlaminae char-

acterizes the very fine sand interval. This lower 12 m

is overlain by a middle 13-m-thick zone dominated by

flaser bedded fine sand that gradually fines upward

into lenticular bedding, completing the fining-upward

trend of unit B3a. The upper 12 m of the unit reflects a

somewhat coarsening-upward trend in which a wavy

very fine sand–mud interval is overlain by a 2-m-

thick trough cross-laminated medium sandstone bed

including some thin mud interlaminae. The unit is

abruptly truncated by an erosion surface overlain by a

4-cm-thick pebble horizon of mud and sand clasts

(Fig. 17).

Unit B3a begins on an erosion surface with basal

lags, which cut deep into the shelfal mud, and is

overlain by fining-upward succession containing

cross-bedded sandstone. Therefore, the base of unit

B3a is speculated to be an IVF. Although the vertical

facies succession (Fig. 16) does not match with the

tide-dominated estuary-fill complex (Dalrymple,

1992), it has some resemblance with the Girond-type

macrotidal and open-ended estuary succession

described by Allen (1991). In this case, the base of

the unit would be a co-planar surface of IVF/TES that

passes upward from inner estuary tidal channel depos-

its (fining-upward trend) to a weakly developed cen-

tral estuary mud flat, and then to an estuary mouth

flood tidal delta (upward coarsening trend). Cross-

bedded sandstone with mud drapes and bi-polar ripple

cross-lamination supports the interpretation of tidal

channel of inner estuary. Lenticular bedding character-

izes the central estuary mud flat whereas the estuary

mouth flood tidal delta is characterized by cross-

stratified sand with some thin mud interlaminae. The

base of the unit does not include any fluvial deposits,

and it rests erosionally on shelfal mudstone. Consid-

ering this point it may also be argued that the tide-

dominated fining-upward succession resting on an

erosion surface at the lower part of this unit reflects

deposits of a tidal channel/inlet (connected to the open

sea) cut into the shelfal mudstone because of relative

sea-level fall. This type of regressive channel-fill fines

upward from cross-stratified to cross-laminated sand-

stone of in-channel deposits to flaser-wavy bedding of

tidal flat progradation (e.g. Dalrymple et al., 1990).

Fig. 16. Detailed litho-log of unit B3a in the Mirinja anticline

showing fining-then (indistinct) coarsening-upward trend. See text

for explanation regarding the origin of this sand body.


However, in this case the upward-coarsening trend at

the upper part of unit B3a remains problematic. In

terms of this alternative explanation, it could represent

a forced regression deposit, and hence be underlain by

a RES generated by tidal inlet scour during relative

sea-level fall—a conceptual framework suggested by

Posamentier et al. (1992). However, we prefer to

assign the deposit of unit B3a to an incised valley-

fill deposit. As the valley has been filled up trans-

gressively the erosion surface at the top of this unit

can be termed a TES produced by wave or current

ravinement at the time of rapid ensuing relative sea-

level rise (cf. Nummedal and Swift, 1987). The sur-

face is characteristically overlain by a thin pebble-

strewn horizon. This type of thin transgressive lag

deposit is very commonly described in geological

literature (e.g. Walker and Eyles, 1988; Arnott et al.,

1995; MacEachern et al., 1998).

Unit B3b: The entire unit is a monotonous finely

laminated mudstone with occasional silt streaks and

lenses. A few Planolites burrows have been identi-

fied. The amount of silt to very fine sand with micro-

ripple cross-lamination increases just below the base

of sequence B4. The unit is thought to represent

highstand shelfal mudstone, which shoals upward to

upper offshore deposits truncated below the next

sequence.

3.2.1.4. Sequence B4. Unit B4a: This unit is similar

to unit B3a in terms of both its upper and lower

bounding discontinuities and its fining-then coarsen-

ing-upward trend (Fig. 18). The fining-upward part of

the unit begins on an erosion surface and passes

upward from fine to medium, apparently structureless

sandstone with thin interstratified mud pebble hori-

zons to indistinctly parallel to trough cross-stratified

fine to very fine sandstone, then to interbedded 1-m-

thick fine to very fine parallel laminated sandstone

and 5-cm-thick laminated mudstone. In the later

deposit some typical mud-draped ripple cross-lamina-

tion with bi-directional reactivation surfaces are

present. The fining-upward trend culminates with

the appearance of a thick carbonaceous muddy silt

interval. The upper part of the unit reflects an upward

coarsening trend in which wavy-lenticular ‘tidal’ bed-

ding is overlain by planar cross-laminated fine sand-

stone with some mud interlaminae that is truncated by

an erosion surface overlain by a 15-cm-thick con-

glomerate bed containing lithic (sand and mud) clasts.

The base of unit B4a is also interpreted as an IVF

similar to that of unit B3a, and the facies succession

(Fig. 18) resembles the tripartite facies subdivision of

a ‘partially closed’ to ‘open-ended’ estuary-fill

described by Reinson (1992). The lower part contains

structureless to trough cross-stratified sandstone with

Fig. 17. Distinct TES (just above the 15 cm long scale) abruptly overlain by a 4-cm-thick pebble horizon (arrowhead) (on top of unit B3a in the

Mirinja anticline) and shelfal mudstone (see also Fig. 16).


no mud interlaminae that indicates a probable incised

fluvial deposit. This deposit fines upward to hetero-

lithic strata containing mud-draped ripple cross-lami-

nation with reactivation surfaces indicating tidal

influence within the channel, and the conversion of

fluvial valley into an estuary with the rise of relative

sea level. The carbonaceous muddy silt in the middle

part is speculated as a stagnant swampy deposit or

central estuarine mudstone. The coarsening-upward

upper part is thought to represent estuary mouth tidal

bar deposit. Considering the aforesaid interpretation,

the lower part of the valley-fill represents lowstand

deposits while the remaining part of the unit is a

transgressive systems tract, although interestingly,

there is no distinct boundary between these two

systems tracts. It is worth noting that the base of the

channel has deeply incised the underlying offshore

sandy mudstone. MacEachern and Pemberton (1994)

have described a similar case of valley incision in the

Viking Formation of Alberta. The erosion surface on

top of unit B4a is regarded as a TES for reasons

similar to that of unit B3a.

Unit B4b: This is a monotonous silty mudstone

unit. A few ripple cross-laminated very fine sand beds

(up to 5 cm thick) are found in the lower part of the

unit. A meters-thick zone of numerous sand-filled

burrows including Rizocorallium, Diplocraterion

(fan shaped), spiral Rosselia, with both protrusive

and retrusive spreitens, and some with crosscutting

relationships, is observed at the upper part of the unit

(Fig. 19). The unit is interpreted as a highstand shelfal

mud similar to unit B3b.

3.2.1.5. Sequence B5. This is the youngest and

thickest sequence of the composite sequence B.

Unit B5a: This unit is also composed of a fining-

then coarsening-upward facies succession. The lower

part, resting on an erosion surface with basal lags, is

characterized by intersecting sets of trough cross-

stratification (eastward current direction) contained

within 40-cm-thick co-sets. Foresets are strewn with

numerous mud clasts, whereas co-sets are draped by

thin (2 cm) mud laminae. This lower part is com-

posed of medium sand that passes upwards into fine

sand with characteristic bi-directional cross-bedding

in which foresets are frequently mud-draped (some

with couplets) and contain occasional back-flow

ripples at the toes (Fig. 20). The middle portion of

the unit is mud-dominated with some flaser to wavy

bedding. Interbedded 30-cm-thick (average) fine

sand with trough cross- to ripple cross-lamination

containing reactivation surfaces and 7-cm-thick

(average) laminated mudstone with sand lenses char-

acterize the uppermost coarsening-upward portion of

the unit.

Fig. 18. Detailed litho-log of unit B4a in the Mirinja anticline

showing a tripartite facies succession interpreted as a ‘partially

closed’ to ‘open-ended’ estuary-fill.


The succession resembles a transgressively filled,

tide-dominated estuary, as described by Dalrymple

(1992). The sedimentary structures of the fining-

upward succession at the lower part of the unit

plausibly indicate an inner estuary tidal channel

deposit. As the valley probably preserves no fluvial

sediments, the lower boundary cutting deep into the

shelfal mud is inferred as an amalgamated IVF/TES.

The middle part of the estuary-fill shows an intertidal

deposit, which is capped by trough cross-stratified,

upward coarsening heterolithic facies interpreted to be

the outer estuary sand bar deposit. The upper boun-

dary of B5a is TS (low energy maximum flooding

surface of Zaitlin et al., 1994). Above this, the TS

sandy siltstone passes upward into sandy/silty mud-

stone and then into laminated mudstone indicating the

continued gradual marine transgression establishing

an offshore setting.

Unit B5b: The lower part of this unit is a finely

laminated silty mud with the amount of silt slightly

increasing upward. A 60-cm-thick fine sandstone bed

with a highly scoured base and wavy to hummocky

lamination is encountered within this lower part. The

upper part of the unit is a coarsening-upward facies

succession (Fig. 21); the lower half is characterized by

ripple cross-laminated very fine sand with some wavy

mud laminae and the upper half shows repeated

fining-upward cycles. In each of these fining-upward

cycles (average 1 m thick), parallel to trough cross-

laminated fine sandstone grades up into flaser, wavy,

and lenticular bedding. Unit B5b is capped by a 4-m-

Fig. 19. Numerous sand-filled burrows (arrowhead) including Rosselia (spiral), Diplocraterion (fan shaped), Rizocorallium, with both

protrusive and retrusive spreitens, and some with cross-cutting relationships, at the upper part of unit B4b in the Mirinja anticline. (Lead-pencil

for scale is 13.5 cm long.)

Fig. 20. Details of tide-generated conspicuous trough cross-bedding

at the lower part of unit B5a in the Mirinja anticline.


thick upward fining fine sandy facies with erosional

base and huge basal lags. The unit abruptly terminates

into very finely laminated silty mud.

The lower silty mud facies of this unit is inter-

preted as shelfal mud. The upper sandy facies depicts

a progradational shoreface succession. The lower

shoreface deposit shows minimal tidal influence,

whereas the upper shoreface is presented by several

repeated subtidal (parallel to trough cross-stratified

sand) to intertidal (tidal beddings) progradational

successions, which is ultimately capped by a tidal

creek deposits. The top of unit B5b is interpreted as a

MFS.

Unit B5c: The unit begins with thick laminated

mudstone containing frequent very fine sand to silt

lenses and interlaminae. The upper part of the unit is

characterized by a coarsening-then (thin) fining-

upward succession. Burrowing is prevalent in most

of the lower portion of the upward coarsening suc-

cession giving these rocks a churned appearance with

some preserved relict laminae. This lower portion

passes upward from sandy silty mud to muddy silty

sand. The upper portion of the coarsening-upward

succession contains ripple cross-laminated fine sand

with thin mud interbeds grading up into trough cross-

stratified medium sand. Some occasional bipolar rip-

ple cross-lamination with mud drapes, and flaser

structures have also been identified in this portion.

The uppermost thin fining-upward succession (5 m

thick) of the unit is composed of wavy sand–mud

interlaminae grading up into laminated silty mud.

The lower silty mud of this unit is interpreted as

offshore mud. The coarsening-upward succession of

the unit is thought to represent a tide-influenced (tidal

structures are not prominently developed) bar progra-

dation. Upper offshore and distal bar facies shows

pervasive bioturbation, and the proximal bar facies

contains trough cross-stratified medium sand. The thin

upward fining succession at the uppermost part of the

unit is believed to indicate a deepening-upward trend,

the culmination of which is a MFS.

Unit B5d: It is a considerably thick succession

characterized by a distinct overall coarsening-upward

trend. The lower part contains laminated to blocky

silty mud. The lower half of the upward coarsening

succession is composed of heterolithic facies contain-

ing abundant characteristic sedimentary structures,

e.g. wavy bedding, trough cross-lamination with

mud drapes/mud couplets, bipolar ripple cross-lami-

nation with reactivation surfaces (Fig. 22), and flaser

structure. This lower part is capped by a 3-m-thick

Fig. 21. Detailed litho-log of the upper part of unit B5b in the

Mirinja anticline. This upward-coarsening facies succession in-

dicates a progradational parasequence of tide-dominated shoreface

to coastal deposits abruptly terminated by a MFS. See text for

explanation.


laminated mudstone interval. The upper half of the

coarsening-upward succession passes from wavy-len-

ticular sand–mud interval into a coarse sand interval

characterized by trough cross-lamination (some bi-

directional) including frequent mud drapes/interlam-

inae with subordinate development of planar lamina-

tion and bipolar ripple cross-lamination.

The unit distinctly reflects a shoaling-upward

facies succession beginning with offshore mud. The

upward coarsening deposit of this unit shows two

phases of sedimentation separated by a 3-m-thick mud

interval speculated to be an indication of local marine

transgression of autocyclic origin. The lower sedi-

mentation phase indicates deposition in a slowly

shifting intertidal (wavy beddings) to subtidal (trough

cross-stratification with mud couplets) environments.

The upper phase of sedimentation may represent tidal

ridge/delta front progradation in a tide-dominated

deltaic setting very similar to that described by Cole-

man and Wright (1975).

Each of the units B5b, B5c, and B5d represents a

prograding parasequence separated by a MFS, and

together they make a highstand progradational para-

sequence set. Considering the sedimentation and sea-

level history of sequence B5 it can be justified that the

very lower part of this sequence preserves very little or

no lowstand deposit that passes upward into thin

transgressive systems tract deposits (unit B5a and

the very lower part of B5b). The remaining part of

the sequence represents a highstand parasequence set

(cf. Van Wagoner et al., 1988) that is progradational.

3.2.2. Sitapahar anticline

The facies analysis of composite sequence B in the

Sitapahar anticline has been done mainly to compare

the nature of sequences already established in the

Mirinja anticline. Therefore, rather than detailing out

the facies types and their interpretation we will high-

light the gross depositional scenario and bounding

discontinuities of composite sequence B in the Sita-

pahar anticline to give a tentative correlation with

their counterparts in the Mirinja anticline described

earlier.

Sequence B1. The base of sequence B1 is an IVF

that can be correlated with the RES at the base of

sequence B1 in the Mirinja anticline. Both of these

surfaces indicate a fall in relative sea level. It is worth

noting that since two other channelized erosion surfa-

ces exist below sequence B1 (at the top part of unit Ce)

in the Sitapahar anticline some degree of caution has

been practised (as hinted by MacEachern and Pem-

berton, 1994) to choose a correct IVF, i.e. a sequence

boundary. It seems reasonable to us that the erosion

surface at the base of unit B1a is a genuine lowstand-

Fig. 22. Typical ‘tidal’ bedding with bi-directional ripple cross-lamination at the middle part of unit B5d in the Mirinja anticline.


induced surface to be considered as sequence boun-

dary. Unit B1a is interpreted as inner estuary fluvial

channel-fill facies which begins with a distinct 5-m-

thick conglomerate bed passing upward into a very

thick, fine to medium, structureless sand interval with

occasional thin mudstone interlaminae. The top part of

the channel-fill reflects a rapid deepening, indicated by

the appearance of thicker laminated mud intervals

within sandstone, culminating at the top by TS. The

lower part of unit B1a is thought to represent lowstand

systems tract and the rest is transgressive systems tract.

However, no distinct boundary has been identified

between these two systems tracts.

The remaining part of sequence B1 is a highstand

systems tract consisting of two parasequences that are

represented by units B1b and B1c. Unit B1b is

thought to represent a tide-influenced shoreline pro-

gradation on shelf mudstone. Two hummocky cross-

stratified sandstone beds 30 cm thick have been

encountered within the offshore mud. Progradational

shoreline facies at the upper part of the unit are

characterized by sandier upward trends in which sand

bar/dune cross-bedded facies is absent. Bipolar ripple

cross-lamination with flaser bedding indicates tide

influence. The top of unit B1b is regarded as a MFS

because the unit is abruptly overlain by shelfal mud-

stone of the next parasequence. Unit B1c is again a

progradational shoreline parasequence that is also

interestingly devoid of any cross-bedded facies.

Wavy-lenticular sand–mud facies, typical upward

fining (15–50-cm-thick) tidal flat facies, massive

channel (?) sand facies, are rather randomly associated

(like in unit B1b of the Mirinja anticline) under slowly

shifting coastline. It should noted that the speculated

RES at the lower part of unit B1b in the Mirinja

anticline is not represented in the Sitapahar anticline.

Sequence B2. The base of sequence B2 is an IVF

correlatable with the IVF at the base of the same

sequence in the Mirinja anticline. The incised valley

fill of unit B2a is very thin compared to its counterpart

in the Mirinja anticline. This thin valley-fill passes

upward from intraformational lags to structureless fine

to very fine sand with some parallel lamination. This

part is overlain by a heterolithic very fine sand–mud

interval. Tidal activity (with the gradual rise of rela-

tive sea level) is reflected by the development of

typical wavy (‘tidal’) bedding. Mid-estuary sand flat

or estuary mouth complex is not developed or is

reworked by the development of TES at the middle

of unit B2a. The deposits above the TES are fine to

medium sandstone with dispersed mud clasts. Since

the shelfal mud of unit B2b abruptly overlies this unit,

Fig. 23. Typical ‘tidal’ bedding with bi-directional ripple cross-lamination in unit B2b in the Sitapahar anticline (from the tide-dominated

shoreline progradation).


the top of unit B2a is interpreted as a maximum MFS.

Above the shelfal mud of B2b the remaining part of

the unit is a sandier upward succession interpreted as

tide-dominated shoreline progradation (Fig. 23) devel-

oped at the time of highstand condition. It is important

to note that the upper part of sequence B2, sequence

B3, and the lower part of sequence B4 are missing

because of the stratigraphic break at the Manikchari

fault (Fig. 4B).

Sequence B4. Above the fault only the upper part

of sequence B4 is observed that reflects a sandier

upward trend. Rizocorallium and Zoophycos trace-

makers are present in the offshore mudstone. The

sedimentary structures in the lower shoreline muddy

sandstone are obliterated by extensive burrowing that

renders this rock a churned appearance.

Sequence B5. The lower boundary of this sequence

is an IVF similar to that in the Mirinja anticline. The

lower part of unit B5a is interpreted as an incised

fluvial channel deposits containing 4-m-thick basal

lags of mud clasts (up to 60 cm long), sand pebbles

and quartz pebbles. The valley-fill distinctly fines

upward from coarse sandstone to very fine sandstone

with large- to small-scale trough cross-stratification.

Bidirectional trough cross-bedding at the top part of

this fill records evidence for tidal activity resulting

from the initial phase of sea-level rise, and gradually

passes upward into 10-m-thick silty mudstone. This

succession is erosionally truncated by another small

(10 m thick) channel speculated to be an inner estuary

tidal meander although no sedimentary structure

except some parallel lamination is observed within

the medium sandstone of the channel-fill. The top of

unit B5a is interpreted as TS. The rest of sequence B5

is a highstand systems tract consisting of three para-

sequences—units B5b, B5c, and B5d. Units B5b and

B5c are very thin and characteristically sharp-based

on offshore mudstone indicating that high-frequency

relative sea-level falls during the overall rise. There-

fore, the parasequence designation of these two units

may not be appropriate in a strict sense. The upper-

most unit B5d is a distinct shoaling/coarsening-

upward tide-dominated shoreline progradation. The

offshore facies characteristically consists of a thick

interval of interlaminated siltstone and mudstone with

delicate parallel-, wavy-, to ripple-lamination. Shore-

face facies shows several meter-thick upward coars-

ening successions each containing laminated mud

grading up into bipolar ripple cross-laminated fine

sand. These successions are thought to represent the

progradation of several small-scale bars. The topmost

part of unit B5d develops a typical wavy-lenticular

tidal bedding of intertidal deposits. The top of

sequence B5 is a pronounced erosion surface indicat-

ing a large fall of relative sea level.

3.3. Composite sequence A

This upper composite sequence of the Neogene

clastic succession in the CTFB represents the final

stage of progradational basin-fill history. The entire

sequence, essentially fluvial deposits, is exclusively

sandstone dominated with a characteristic yellowish-

brown colour. A detailed account of the lithofacies

types with their depositional connotation was given

by Alam (1996) and Alam and Ferdous (1995). Only a

brief description of the composite sequence A, which

shows similar patterns in both the anticlines, is pre-

sented below.

The sequence begins on a pronounced erosion

surface interpreted as an IVF overlain by a 2–5-m-

thick characteristic conglomerate bed consisting of

mud clasts (up to 15 cm long) with some quartz

pebbles (Fig. 24). The lowermost 40–50 m of the

composite sequence indicates an upward fining single

channel cycle that quickly evolves into an estuary

channel within a few meters up-sequence because of

the rise of relative sea level. The lower portion of this

cycle contains parallel to bidirectional cross-bedded

medium sandstone with some mud-draped foresets.

In the upper part, sandstone is ripple cross-laminated

with thin wavy mud laminae, the frequency of which

increases upward. The channel-fill culminates with

meter-thick laminated mudstone. Mud drapes, fre-

quent wavy mud interlaminae, and bipolar current

directions within dunes and ripples indicate the

influence of tides in the upper part of the channel

deposits. Most of the remaining parts of the compo-

site sequence consist of monotonous medium sand-

stone characterized by trough cross-, planar cross-,

and parallel-bedding (Fig. 25) with some ripple

cross-lamination and mud interlaminae, thought to

represent various types of channel bar deposits of a

large-scale braided river system. One characteristic

50+-m-thick zone of laminated mudstone with some

laminae and lenses of very fine sand are encountered


within this composite sequence in the Mirinja anti-

cline. This muddy interval may be indicative of a

lacustrine depositional environment that prevailed in

a low area of the flood plain. Sand–mud ratios

decrease at the uppermost part of composite sequence

A indicating a probable transition into a meandering

channel system. It is noteworthy that the pronounced

erosion surface at the base of composite sequence A

represents a large fall of relative sea level speculated

to be the final phase of marine regression from the

Fig. 25. Two sets of trough cross-bedded sandstone overlain by parallel bedded sandstone, at the middle part of the composite sequence A in the

Mirinja anticline. Note hammer at the middle of the photo for scale.

Fig. 24. Pronounced erosion surface (interpreted as an IVF—arrowhead) overlain by a thick intraformational conglomerate bed, on the top of

composite sequence B in the Mirinja anticline.


entire CTFB most probably associated with a tectonic

upheaval.

4. Discussion

4.1. Depositional and basin-fill history in relation to

sequence stratigraphy

The study presents a conventional facies analysis

that is set forth in a high-resolution sequence strati-

graphic framework to give a comprehensive scenario

of the stratigraphic architecture and depositional pro-

cesses of the Neogene clastic succession in the CTFB.

The entire lithostratigraphic column presented in Fig.

4 has been divided in a hierarchical order into com-

posite sequences, sequences, systems tracts, and para-

sequences on the basis of the key bounding

discontinuities of sequence stratigraphy. The nearly

complete documentation of the exposed rock succes-

sion with its interpreted facies and bounding disconti-

nuities, and its tentative regional correlation provide a

detailed understanding of the progressive basin-fill

history of southeastern Bengal Basin.

The identified bounding discontinuities can be

grouped broadly into two categories—those related

to marine regression associated with relative sea-level

fall and those related to marine transgression and

relative sea-level rise. In this study, incised valley

floor (IVF) and regressive erosion surface (RES) are

the surfaces generated by a fall in relative sea level

and form the sequence boundary. IVF is a prominently

developed surface forming a type I sequence boun-

dary according to the Exxon model (cf. Van Wagoner

et al., 1988). The RES was generated subaqueously by

either tidal scouring (e.g. below sequence B1 in the

Mirinja anticline) or wave scouring (e.g. below unit

Cb in the Mirinja anticline) associated with forced

regression. There has been considerable debate about

the origin and systems tract designation of sandbodies

resting on this type of erosion surface (e.g. Hunt and

Tucker, 1995; Kolla et al., 1995). For a recent dis-

cussion on the topic see MacEachern et al. (1999). In

general, when sea-level falls the landward develop-

ment of an initial subaerial exposure surface and/or an

incised valley floor passes seaward into a correlative

marine erosion surface (i.e. RES) and correlative

conformity surface. The architecture of the generated

sandbodies (above the RES) varies in the different

settings and combinations depending on the rate and

magnitude of the relative sea-level fall. With a limited

landward and seaward database, such as the present

study, it is better to assign these sandbodies generated

on a RES to a more general term like lowstand

systems tract. It is notable that a nonerosional regres-

sive surface (i.e. sharp but not erosional contact

associated with sharp-based sandbody) was described

in sequence B5 of the Sitapahar anticline.

Transgressive erosion surface (TES) and transgres-

sive surface/marine flooding surface (TS/MFS) within

the studied succession are related to a rise in relative

sea level. TES was formed subaqueously by either

wave scouring (wave ravinement surface, most com-

mon in the present study) or tidal scouring (tidal

ravinement surface). TESs are overlain either by thin

sandbodies with dispersed mud clasts (upper part of

unit B2a in both the anticlines) (Fig. 15) or by a few

centimeters thick mud and sand pebbles (top of units

B3a and B4a in the Mirinja anticline) (Fig. 17) and

pass upward into shelfal mudstone. TS and MFS are

both nonerosion surfaces across which there is evi-

dence of abrupt increase in water depth. The term

MFS is used in the case where this apparent deep-

ening caps a prograding parasequence (cf. Van Wag-

oner et al., 1988); in other cases, the term TS is used.

It is acknowledged that generation of MFSs could be

due to autocyclic processes (Walker, 1990; Miall,

1997).

In the present study, the Neogene clastic succession

of CTFB has been grouped into three composite

sequences—C, B and A, from oldest to youngest

(Fig. 4), that record a progressive basin-filling history

from deep marine to continental–fluvial environ-

ments. It is important to note that the correlation

between the two anticlines represents a depositional-

strike parallel correlation, and that the eastward and

westward current directions indicate landward and

basinward directions, respectively. The internal sed-

imentation patterns of composite sequence C shows

the least similarity (compared to the other two com-

posite sequences) between the two anticlines. The

deep marine base-of-slope deposits are encountered

only in the Sitapahar anticline (unit Ca). These depos-

its are interpreted as ‘mud/sand-rich slope apron’

according to the classification scheme of Reading

and Richards (1994). Small-scale depositional lobes,


channel–levee systems, and slump–debris mass char-

acterize the slope apron deposits. For a detailed

account of these deep-sea clastics see Gani and Alam

(1999). The base-of-slope clastics gradually pass

upward into shallow marine sandbodies through a

thick zone of muddy slope and shelf deposits. The

first neritic sandbody in both anticlines (unit Cb in the

Mirinja anticline and the upper part of Cb in the

Sitapahar anticline) (Figs. 5 and 13) is dissimilar in

nature. The sandbody in the Mirinja anticline (unit

Cb) rests on a RES and characteristically evolves

upward from storm-dominated shelf deposits to tide-

dominated shelf deposits (this type of combination

within a single progradational sandbody is not com-

mon in the geological record). The lower part of this

sandbody develops several typical storm-generated

and thin-bedded fining-upward cycles (Fig. 5). The

sandbody in the Sitapahar anticline (upper part of unit

Cb) (Fig. 13) is gradationally based and is character-

ized by a distinct coarsening- and thickening-upward

trend of tidal sand ridge progradation on shelfal

mudstone (Fig. 13). The rest of the composite

sequence C in the Sitapahar anticline reflects the

development of several progradational parasequences

separated by MFSs. The equivalent succession in the

Mirinja anticline does not show any parasequence;

rather, two submarine channels encased within the

upper slope to shelfal mudstone characterize its devel-

opment. The lower channel is characterized by a

chaotic fill of rapid clogging, while the upper channel

is a mature one and depicted with thick-bedded

turbidites (Figs. 9 and 10). Both the channels are

interpreted as highstand deposits with sediment trans-

port into the deep basin even at high sea level. It

should be noted that the RES at the base of unit Cb in

the Mirinja anticline is not observed in the Sitapahar

anticline, and is interpreted as a local phenomenon

(possibly due to a high-frequency sea-level fall).

Therefore, this RES is not treated as a sequence

boundary. It is also worth noting that in composite

sequence C shale predominates over sandstone, sug-

gesting that during this period the basin had high

accommodation space.

A pronounced relative sea-level fall terminated

the deposition of composite sequence C and initiated

the development of composite sequence B and a new

basin-fill history. This composite sequence in both

anticlines shows similar pattern of paralic sedimen-

tation under repeated rise and fall of relative sea

level. The base of composite sequence B is repre-

sented by an IVF in the Sitapahar anticline and a

pronounced RES in the Mirinja anticline. Sequence

B1 in the Sitapahar anticline begins with a character-

istic very thick and structureless fill of an incised

valley. This sequence gives a record of all the three

systems tracts of a typical sequence. The lower part

(unit B1a) is thought to be a lowstand deposit that is

overlain by the development of a thin transgressive

systems tract. The rest of the sequence is a highstand

systems tract that includes one parasequence of tidal

bar progradation in the Mirinja anticline, whereas it

includes two parasequences of tide-influenced shore-

line progradation in the Sitapahar anticline. The base

of this sequence in the Mirinja anticline is a RES,

which is thought to have been generated by tidal

current scouring—a rare case in geological literature.

Another similar RES (within unit B1b in the Mirinja

anticline) is not present in the Sitapahar anticline

indicating that this erosion surface may be autocyclic

in origin and probably represents a bedload parting

zone (Harris et al., 1995). Thereupon this RES is not

considered a sequence boundary. It is apparent that

sequence B3 is missing in the Sitapahar anticline

because of the Manikchari fault. Sequences B2, B3,

and B4 in the Mirinja anticline represent repetitive

development of incised valley complexes. It is sur-

prising to note that the highstand systems tracts

within these sequences include only shelfal mud-

stone. It can be argued that either the highstand

parasequences are eroded away beneath the incised

valleys, or the landward shifted shoreline facies

because of marine transgression did not get time to

prograde to the study area (i.e. the intervening still-

stand periods were very short). However, the surviv-

ing thin parasequence (of sequence B4) below the

IVF of sequence B5 in the Sitapahar anticline sup-

ports the former contention. The youngest sequence

B5 of composite sequence B reflects the develop-

ment of all three systems tracts of a typical sequence.

The lowstand systems tract is represented by fluvial-

incised valley-fill at the very lower part of the

sequence; the overlying transgressive systems tract

is likewise thinly developed. The highstand systems

tract, on the other hand, is distinctly developed with

three progradational parasequences separated by

MFSs.


The most conspicuous depositional element in

composite sequence B is the incised valley complex

(Figs. 15, 16, 18 and 26). All the valley complexes

except unit B1a of the Sitapahar anticline show a

characteristic fining-then coarsening-upward facies

succession. Among these successions, the valley-fill

of unit B5a in the Mirinja anticline is thought to

represent a tide-dominated estuary-fill (cf. Dalrymple,

1992), and all other fills show a tripartite vertical

facies separation indicating a mixed wave- and tide-

dominated estuary-fill (cf. Reinson, 1992). The boun-

dary between the lowstand systems tract (if present)

and transgressive systems tract within the valley-fill

complexes is cryptic. Although the incision of some

of the paleovalleys deep into the shelfal mud indicates

a pronounced fall of relative sea level, a 3D architec-

ture should be established for unequivocal interpreta-

tion of an incised valley complex, but this is beyond

the scope of the present study. The TES (e.g. Fig. 17)

capping some of the estuary-fill complexes are prob-

ably the most unequivocal bounding discontinuity

related to the rise in relative sea level.

The shallow marine sedimentation in the studied

rock succession is overwhelmingly tide-dominated.

Shoreface progradation is characterized by various

combinations of tidal sand sheet, sand ridge (e.g.

Fig. 13), and sand bar/shoal deposits, whereas coastal

sedimentation is typified by tidal flat deposits. Sedi-

mentary structures such as tidal bundles, mud cou-

plets, bidirectional reactivation surfaces, bipolar

trough cross-bedding and ripple cross-lamination,

and ‘tidal’ bedding suggest this tide-dominated envi-

ronment (e.g. Figs. 14, 20, 22 and 23)). However,

subordinate wave reworking is probably recorded in

the development of occasional structureless sandstone

beds dispersed with mud clasts. Only the lower part of

unit Cb in the Mirinja anticline characteristically

shows a repetitive occurrence of tempestites (Fig. 5)

indicating a storm-dominated shelf. Very rare and

isolated occurrences of hummocky cross-stratification

and occasional thin sharp-based sandstone in the

shelfal mudstone suggest a very subordinate storm

activity. The shelfal mudstone characteristically con-

tains a trace fossil association of Rizocorallium, Pla-

nolites, Techiechnus, Rosselia, and Zoophycos. In

composite sequence B sandstone predominates over

shale indicating an increased sediment supply (i.e. the

basin was supply dominated during this period). It is

worth noting that the tide-dominated sedimentation

patterns on transgressive shelves are well documented

and understood, whereas those on regressive shelves

are yet to be modeled (Dalrymple, 1992). In this

regard the paralic sedimentation of Neogene succes-

sion (especially the upper part of units Cb and Ce in

the Sitapahar anticline; units B1b, B5b, and B5d in the

Mirinja anticline) (Figs. 13 and 21) can serve as a

Fig. 26. Large outcrop view of incised valley deposits cropping out at the Matamuhari River section of the Mirinja anticline that is probably

equivalent to the incised valley of unit B2a of this anticline. (Person at lower middle of the photo for scale.)


good point of reference for the tide-dominated regres-

sive shelf depositional systems.

Again a pronounced fall in relative sea level

terminated the sequential development of paralic sed-

imentation in composite sequence B and brought forth

the continental–fluvial sedimentation of composite

sequence A, which represents the final phase of a

progressive basin-fill history. A 2–5-m-thick con-

glomerate bed at the base of composite sequence A

is present in both anticlines (Fig. 24). The very lower

part of this composite sequence represents a coastal

braid plain environment that passes upward into a

monotonous stacking of various sand bars of a braided

river system. The upper part of the composite

sequence reflects a gradual transition to a meandering

river system with a low sand–mud ratio.

The analytical approach presented here to interpret

the rock succession is a kind of high-resolution

sequence stratigraphy that is increasingly used as an

effective tool in hydrocarbon reservoir delineation

(e.g. Posamentier and Chamberlin, 1993). The explo-

ration strategies so far taken over the eastern Bengal

Basin give importance only to the structural traps

(fault-bounded anticlines). The facies and facies

sequence analysis of the present study sheds light

on a new type of play concept. The bounding dis-

continuities identified within the rock succession can

help both in the exploration and exploitation strat-

egies by elucidating the location and geometry of

stratigraphic traps, outlining seal architecture, and

delineating flow units. For example, some of the

incised valley fills, abruptly bounded both below

and above by highstand and transgressive marine

mudstones, respectively, act as good stratigraphic

traps. Several MFS-bounded parasequences are

abruptly overlain by transgressive shelfal muds,

which can act as seals as well as good source rocks.

The present research also hints toward another pro-

spective and new type of hydrocarbon play (i.e. the

deep-sea clastic play), which demands proper evalua-

tion. The hemipelagic mudstone encountered in unit

Ca in the Sitapahar anticline is highly carbonaceous.

The organic matter entrapped in deep-marine shale is

generally believed to be oil-prone. The submarine

channels of units Cc and Ce in the Mirinja anticline,

and the turbidite packets encased within the basinal

mudstone of unit Ca in the Sitapahar anticline can

serve as good examples of stratigraphic traps.

4.2. Palaeogeographic reconstruction in relation to

tectonic setting

One of the serious misconceptions about the basin

evolution has arisen from the fact that the entire

Bengal Basin has been indiscriminately designated

as a ‘foreland basin’. It is important to realize that the

present-day Bengal Basin occupies at least three

different tectonic provinces (Fig. 27)—(i) passive to

extensional cratonic margin in the west, (ii) collision-

related orogeny in the northeast, and (iii) subduction-

related orogeny in the east (CTFB region). These

tectonic provinces have given rise to much complexity

in the evolution of the Bengal Basin (for an overview

see Alam et al., 2003). Gani and Alam (1999) have

presented a somewhat refined tectonic evolution for

the eastern Bengal Basin, and suggested a subduction-

related (oblique subduction) active margin setting for

the CTFB, which is probably related more to the

evolutionary history of the western margin of the

Burmese plate than to the eastern margin of the Indian

plate.

Since the present study deals with the Neogene

rock succession of the CTFB, and the sedimentation

and tectonics are very much interrelated, it is impor-

tant to understand the early Neogene palaeogeography

of this part of the basin. During the very Early

Miocene the paleogeographic setting of the CTFB

included the westward migrating trench-slope bathy-

metry, and the sediment source was largely from the

newly uplifted Indoburman subduction complex to the

east (Figs. 3 and 27). The schematic palaeogeographic

(Early Miocene) map of the Bengal Basin and its

surroundings (Fig. 27) indicates that the tectonic

setting of the CTFB was different from other parts

of the basin. The remnant ocean basin had been

closing from northeast to southwest because of obli-

que subduction. As a result, the trench-slope bathy-

metry of the subduction zone had also been smoothing

out toward the south. It is notable that the slope apron

deposit at the lowest part of the studied rock succes-

sion was developed on a late-stage trench-slope

bathymetry (Gani and Alam, 1999), and that the

deep-water embayment (Fig. 27) has been smoothed

to shallow water conditions probably sometime in the

Late Miocene. The block diagram (Fig. 28) represents

the gross sedimentation pattern in the CTFB during

the Early Miocene.


The basic sequence model of Exxon was originally

developed in a passive to extensional plate margin

setting, and should be used with caution in case of

active margin setting (e.g. Miall, 1997) similar to the

CTFB overlying the zone of convergence. In an active

margin setting several distinct processes, of which

tectonic basin subsidence and source-area uplift are

most important, lead to local and regional changes in

relative sea level and thereby control the architecture

of the basin fill (e.g. Fulford and Busby, 1993). Large

Fig. 28. Conceptual Early Miocene paleogeographic model showing the sedimentation pattern (at highstand condition) within the active margin

setting of the Indo-Burmese oblique plate convergence (province 3 of Fig. 27).

Fig. 27. Schematic paleogeographic (Early Miocene) representation of the Bengal Basin and surroundings incorporating the plate tectonic

model. The positions of the three tectonic provinces of the Bengal Basin are shown by encircled numbers: (1) western Bengal Basin, (2)

northeastern Bengal Basin, and (3) eastern Bengal Basin (CTFB).


differences in the thickness and facies of the same

sequence (within composite sequences C and B) in the

two anticlines that are aligned along the strike are

believed to be due to these processes within the active

margin setting. The nature of sea-level change (e.g.

either eustatic or relative), which has given rise to

several regional bounding discontinuities in the

studied rock succession, and also the chronological

nature of these discontinuities (i.e. either synchronous

or diachronous) remain unsolved, and will require

further research. It is to be noted that the coupled

regressive/transgressive cycles of sequences B2, B3

and B4 in the Mirinja anticline are similar to the

coupled uplift-collapse cycles over relatively short

periods within the subduction zone described by Flint

et al. (1991).

4.3. Revision of traditional stratigraphic classification

In Section 2, the status of the traditional strati-

graphic nomenclature, which is based on the work of

Evans (1932) in the lower Assam Basin, has been

presented briefly. Traditionally, without any detailed

regional correlation, the lithostratigraphic scheme of

Evans (1932) has been casually extended for the entire

Bengal Basin mainly on the basis of loosely defined

lithologic similarity of the rock types. The danger of

this kind of regional simplification has been pointed

out by Das Gupta (1977) who regarded the Evans’

(1932) classification as merely local stratigraphy.

Brunnschweiler (1980) and Reimann (1993) strongly

argued the uncritical use of oversimplified conven-

tional stratigraphy. However, as no alternatives were

suggested the indiscriminate use of this traditional

stratigraphic terminology in various recent published

and unpublished works is still creating confusion and

dispute among regional geologists. In the previous

section, it was stated that the Bengal Basin includes at

least three discrete tectonic provinces, each with its

own tectonic and sedimentary history. Therefore,

separate stratigraphic classification scheme should

be established for each of the three tectonic provinces;

and only then intrabasinal (i.e. within these three

provinces) and interbasinal (e.g. between Bengal

Basin and Assam Basin) correlation of the strati-

graphic succession should be attempted.

In this section, a new tentative stratigraphic scheme

for the CTFB region is proposed. However, without

any regional marker bed or precise biostratigraphic

zonation it is difficult to undertake such an approach.

In fact, paleontologic dating is very difficult because

of high rates of sedimentation, the young age of the

rock succession, and the marginal marine nature of

many of the sediments (Harms, pers. comm., 2000).

We have adopted a sequence stratigraphic framework

giving emphasis to bounding discontinuities in order

to correlate the sediments and document the basin-fill

history of the Neogene clastic succession. However,

with the present database it is probably unwise to

propose a regional sequence stratigraphic nomencla-

ture for classification purposes, because, for example,

the bounding discontinuities are not yet clearly

Table 2

Revised allostratigraphic scheme for the Chittagong–Tripura Fold Belt (CTFB)

Age (approx.) Allogroup Thickness (approx.) (m) Brief description

Recent Alluvium

Plio–Pleistocene Kaptai 1100–1600 Large-scale, low-sinuosity braided river deposits grading

upward into high-sinuosity meandering river deposits.

Very high sand–shale ratio. Characteristically

yellowish-brown very coarse to fine sand.

Late Miocene Mirinja 1200–1600 Alternating nearshore sand and shelfal mud with regional

erosion surfaces related to relative sea level cycles.

High sand–shale ratio. Predominance of tidal

structures is obvious.

Middle Miocene Sitapahar 1000–1500 Slope mud predominant; sandy deposits ranging from

base-of-slope turbidites to nearshore tidal sands.

No major erosion surface.

Oligo–Early Miocene Chittagong 2000 + Speculated submarine fan complex in trench setting;

mud dominated.


defined in terms of their external causes. This diffi-

culty can be avoided if we apply the concept of

allostratigraphy in order to replace the traditional

lithostratigraphy. Allostratigraphic units are inter-

preted to represent more natural subdivisions of the

rock record than conventional lithostratigraphic units

(Walker, 1990; Miall, 1997). Therefore, the allostrati-

graphic scheme formally adopted by the North Amer-

ican Commission of Stratigraphic Nomenclature

(NACSN, 1983) has been incorporated in establishing

a tentative allostratigraphic classification for the

CTFB (Table 2) that recognizes four Groups (more

precisely allogroup) corresponding to the composite

sequences of Fig. 4. A brief description of each group,

from oldest to youngest, is given below.

Chittagong Group: Exists in the subsurface and

has not yet been reported from the outcrop. The group

represents speculated large-scale submarine fan com-

plexes (Gani and Alam, 1999) beneath the slope apron

deposits of the composite sequence C. The group

probably extends in age from the Oligocene to the

Early Miocene and is equivalent to the traditional

Barail Group and the lower part of the Surma Group.

Sitapahar Group (composite sequence C, equiv-

alent to the traditional middle Surma Group): Is

probably Middle Miocene in age and varies in thick-

Fig. 29. Subsurface seismic profile (see Fig. 2 for location) of the offshore Sangu structure (BOGMC, 1997). The composite sequence model

established for the Neogene clastic succession in the studied structures is compared with this profile.


ness from 1000 to 1500 m. The Rangamati–Chitta-

gong road section of the Sitapahar anticline is sug-

gested informally as the type section for this group.

The type section contains the oldest 1128+ m of the

rock succession (composite sequence C) exposed in

the eastern flank of the anticline. This group repre-

sents a progressive basin filling from deep marine

slope apron to shallow marine near shore deposits. No

major lowstand erosion surface has been recognized

in this group, where a low sandstone–shale ratio

indicates that the basin was accommodation domi-

nated during this time. The Sitapahar Group can be

farther divided into several alloformations based on

the bounding discontinuities (mainly the MFSs).

Mirinja Group (composite sequence B, equivalent

to the traditional upper Surma Group): Is probably

Late Miocene in age and varies in thickness from

1200 to 1600 m. The Lama–Fashiakhali road section

of the Mirinja anticline is suggested informally as the

type section for this group. In the type section, the

group is represented by the 1293 m of thick shelfal to

coastal succession (composite sequence B) exposed in

the western flank of the anticline. This group can

conveniently be divided into several alloformations

corresponding to the individual sequences, which in

turn can be divided further into allomembers based on

the TES and TS/MFS. The high sand–shale ratio of

this group indicates that the basin was supply domi-

nated during this period.

Kaptai Group (composite sequence A, equivalent

to the traditional Tipam Group and Dupitila Forma-

tion): Is probably Plio–Pleistocene in age and varies

in thickness from 1100 to 1600 m. The Kaptai–

Chandraghona road section in the western flank of

the Sitapahar anticline is suggested informally as the

type section for this group. The lower part of the

group represents braid plain coastal to fluvial deposits,

whereas the uppermost part represents deposits of a

meandering river system. The development of a 100–

200-m-thick patchy clay deposit (traditionally known

as the Girujan Clay) divides the group into the above-

mentioned two parts. Further study is needed to justify

such a subdivision.

The subsurface seismic profile of the offshore

Sangu structure (BOGMC, 1997) (Fig. 29) is used

to show the justification of the above-described strati-

graphic classification in a regional context. Except for

the lowermost group in our proposed stratigraphic

scheme (Table 2) the other three groups, i.e. the

composite sequences C, B and A, are clearly identi-

fied in this seismic profile. Composite sequence C

indicates a rather stable depositional scenario without

the development of any major erosion surface. Com-

posite sequence B shows several distinct erosion

surfaces most of which are interpreted as incised

valleys developed at the time of lowstand conditions.

Coastal plain to delta top fluvial channel deposits are

represented by the uppermost composite sequence A,

which is experiencing transgression because of the

Holocene sea-level rise. In the seismic profile, which

lies basinward from the study area (see Fig. 2 for

location), composite sequences C and B are thicker

and composite sequence A is thinner than their

equivalent composite sequences in the studied anti-

clines. However, it is suggested that further rigorous

studies, both from surface and subsurface, including

biostratigraphic age dating, are needed to formalize

the given stratigraphic scheme and to standardize the

interpreted sequences.

5. Conclusions

(1) The Neogene clastic succession (3000+ m

thick) as exposed in the CTFB represents an overall

basinward progradation from deep marine through

shallow marine to continental–fluvial environments

deposited within the subduction-related (oblique sub-

duction) active margin setting of the Indo-Burmese

plate convergence.

(2) A high-resolution sequence stratigraphic frame-

work has been adopted to interpret the basin-fill

history in response to relative sea-level changes, and

to subdivide the rock record into several sequences

and units (systems tracts and parasequences) based on

the identified bounding discontinuities, including

transgressive erosion surface (TES), regressive ero-

sion surface (RES), transgressive surface (TS), marine

flooding surface (MFS), and incised valley floor

(IVF).

(3) The entire succession can be broadly divided

into three composite sequences—C, B and A, from

oldest to youngest, on the basis of two regionally

correlatable pronounced lowstand erosion surfaces.

(4) In composite sequence C, slope apron deep-sea

clastics shoals upward into shallow marine and near-


shore clastics through a thick zone of slope mudstone.

Composite sequence B characteristically depicts tide-

dominated open-marine to coastal depositional sys-

tems with repetitive occurrences of incised valley,

tidal inlet, tidal ridge/shoal, and tidal flat deposits that

are separated by shelfal mudstone under the control of

cyclic fall and rise of relative sea level. At the final

phase of marine regression, composite sequence A

gradually establishes coastal plain through alluvial

plain fluvial depositional systems characterized by

stacked braided river sand bars that pass upsequence

into meandering river deposits.

(5) The bounding discontinuities identified within

the rock succession provide an insight into the explo-

ration and exploitation strategies for hydrocarbons

both in terms of location and geometry of stratigraphic

traps, outlining seal architecture and flow units.

(6) It is strongly recommended that the tradi-

tional lithostratigraphic classification practiced for

the CTFB be revised. A tentative stratigraphic

scheme incorporating the concept of allostratigraphy

is suggested. Further study including precise bio-

stratigraphic zonation is needed to formalize the

scheme.

Acknowledgements

The first author would like to thank D. Zafrul

Hassan for giving logistic support during the field-

work in the Mirinja anticline; and Abdul Hoque for

field assistance and Dr. Manzoor Hasan for logistic

support during the fieldwork in the Sitapahar

anticline. We thank Drs. Katherine Bergman, Tim

Cross and G. Shanmugam for their extensive reviews,

critical and constructive comments as well as many

thoughtful suggestions, which significantly improved

the paper.

References

Alam, M., 1989. Geology and depositional history of Cenozoic

sediments of the Bengal Basin of Bangladesh. Palaeogeogr.,

Palaeoclimatol., Palaeoecol. 69, 125–139.

Alam, M., 1997. Bangladesh. In: Moores, E.M., Fairbridge, R.W.

(Eds.), Encyclopedia of European and Asian Regional Geology.

Chapman & Hall, London, pp. 64–72.

Alam, M., Alam, M.M., Curray, J.R., Chowdhury, M.L.R., Gani,

M.R., 2003. An overview of the sedimentary geology of the

Bengal basin in relation to the regional tectonic framework

and basin-fill history. Sediment. Geol. (this volume).

Alam, M.K., Hasan, A.K.M.S., Khan, M.R., Whitney, J.W., 1990.

Geological map of Bangladesh. Geological Survey of Bangla-

desh, Dhaka, Scale 1:1,000,000.

Alam, M.M., 1995. Tide-dominated sedimentation in the Upper

Tertiary succession of the Sitapahar anticline, Bangladesh. In:

Flemming, B.W., Bortholoma, A. (Eds.), Tidal Signatures in

Modern and Ancient Sediments. Int. Assoc. Sedimentol., Spec.

Publ. 24, pp. 329–341.

Alam, M.M., 1996. Low-sinuosity braided-fluvial sedimentation

model: a comparison of the Lichubagan Sandstone Formation

and the Jamuna (Brahmaputra) River, Bangladesh. Indian J.

Earth Sci. 23, 13–19.

Alam, M.M., Ferdous, M.H.S., 1995. Lithofacies analysis and dep-

ositional environment of the Lichubagan Sandstone Formation

(Upper Tertiary) in the Sitapahar anticline, south-eastern Folded

Belt of Bangladesh. J. Indian Assoc. Sedimentol. 14, 9–18.

Alam, M.M., Ferdous, M.H.S., 1996. Neogene clastic succession of

the Sitapahar anticline southeastern Bangladesh: an example of

marine-continental transition. Dhaka Univ. J. Sci. 44 (1), 43–52.

Alam, M.M., Karim, S.M.R., 1997. Facies and environmental anal-

ysis of the Neogene (Surma Group) clastic succession in the

Mirinja anticline, Folded Belt of the Bengal Basin, Bangladesh.

Bangladesh Geosci. J. 3, 19–32.

Allen, J.R.L., 1982. Mud drapes in sandwave deposits: a physical

model with application to the Folkestone Beds (early Cretaceous

Southeast England). Philos. Trans. R. Soc. London, Ser. A 306,

291–345.

Allen, G.P., 1991. Sedimentary processes and facies in the Gironde

estuary: a model for macrotidal estuarine systems. In: Smith,

D.G., Reinson, G.E., Rahmani, B.A. (Eds.), Clastic Tidal Sed-

imentology. Can. Soc. Pet. Geol., Mem. 16, pp. 219–226.

Arnott, R.W.C., Hein, F.J., Pemberton, S.G., 1995. Influence of the

ancestral Sweetgrass Arch on sedimentation of the Lower Creta-

ceous Bootlegger Member, north-central Montana. J. Sediment.

Res. B65, 222–234.

Baksi, S.K., 1972. On the palynological biostratigraphy of the Ben-

gal Basin. In: Ghosh, A.K., et al. (Eds.), Proceedings of the

seminar on Paleopalynology and Indian Stratigraphy. Dept. of

Botany, Univ. of Calcutta, Calcutta, pp. 188–206.

Banerji, R.K., 1984. Post-Eocene biofacies, palaeoenvironments

and palaeogeography of the Bengal Basin, India. Palaeogeogr.,

Palaeoclimat., Palaeoecol. 45, 49–73.

BOGMC, 1997. Petroleum Exploration Opportunities in Bangla-

desh. Bangladesh Oil, Gas and Mineral Corporation (Petroban-

gla), Dhaka, March, 1997.

Brunnschweiler, R.O., 1980. Lithostratigraphic monster in modern

oil exploration. Proc. 3rd Offshore Southeast Asia Conference,

Singapore, pp. 1–7.

Coleman, J.M., Wright, L.D., 1975. Modern river deltas: variability

of processes and sand bodies. In: Broussard, M.L. (Ed.), Deltas:

Models for Exploration. Houston Geol. Soc., Houston, pp. 99–

149.

Dalrymple, R.W., 1992. Tidal depositional systems. In: Walker,


R.G., James, N.P. (Eds.), Facies Models: Response to Sea-level

Change. Geol. Assoc. of Canada, St. John’s, Newfoundland,

pp. 195–218.

Dalrymple, R.W., Knight, R.J., Zaitlin, B.A., Middleton, G.V.,

1990. Dynamics and facies model of a macrotidal sandbar com-

plex, Cobequid Bay–Salmon River estuary (Bay of Fundy).

Sedimentology 37, 557–612.

Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies

models: conceptual basis and stratigraphic implications. J. Sedi-

ment. Petrol. 62, 1130–1146.

Das Gupta, A.B., 1977. Geology of Assam–Arakan region. Q. J.

Geol., Min. Metall. Soc. India 49 (1/2), 1–55.

Dasgupta, S., Nandy, D.R., 1995. Geological framework of the

Indo-Burmese convergent margin with special reference to

ophiolitic emplacement. Indian J. Geol. 67 (2), 110–125.

Evans, P., 1932. Tertiary succession in Assam. Trans. Min. Geol.

Inst. India 27, 155–260.

Ferdous, M.H.S., 1990. Sedimentology of the sediments exposed in

the Sitapahar Anticline, Kaptai-Chandraghona area, Rangamati,

Bangladesh. M.Sc. Thesis, Univ. of Dhaka, Dhaka, Bangladesh,

141 pp.

Flint, S.S., Turner, P., Jolley, E.J., 1991. Depositional architecture of

Quaternary fan-delta deposits of the Andean fore-arc: relative

sea-level changes as a response to aseismic ridge subduction. In:

Macdonald, D.I.M. (Ed.), Sedimentation, Tectonics and Eu-

stasy: Sea-level Changes at Active Margins. Int. Assoc. Sedi-

mentol. Spec. Publ. 12, pp. 79–89.

Fulford, M.M., Busby, C.J., 1993. Tectonic controls on nonmarine

sedimentation in a Cretaceous fore-arc basin, Baja California,

Mexico. In: Frostick, L.E., Steel, R.J. (Eds.), Tectonic Controls

and Signatures in Sedimentary Successions. Int. Assoc. Sedi-

mentol. Spec. Publ. 20, pp. 301–333.

Gani, M.R., 1999. Depositional history of the Neogene succession

exposed in the southeastern fold belt of the Bengal Basin,

Bangladesh. M.Sc. Thesis, Univ. of Dhaka, Dhaka, Bangladesh,

61 pp.

Gani, M.R., Alam, M.M., 1999. Trench-slope controlled deep-sea

clastics in the exposed Surma Group in the southeastern fold

belt of the Bengal Basin, Bangladesh. Sediment. Geol. 127,

221–236.

Harris, P.T., Pattiaratchi, C.B., Collins, M.B., Dalrymple, R.W.,

1995. What is a bedload parting? In: Flemming, B.W., Bortho-

loma, A. (Eds.), Tidal Signatures in Modern and Ancient Sedi-

ments. Int. Assoc. Sedimentol. Spec. Publ. 24, pp. 3–18.

Hunt, D., Tucker, M.E., 1992. Stranded parasequences and the

forced regressive wedge systems tract: deposition during base-

level fall—Reply. Sediment. Geol. 81, 1–9.

Hunt, D., Tucker, M.E., 1995. Stranded parasequences and the

forced regressive wedge systems tract: deposition during base-

level fall. Sediment. Geol. 95, 147–160.

Klein, G., 1971. A sedimentary model for determining paleotidal

range. Geol. Soc. Am. Bull. 82, 2585–2592.

Kolla, V., Perlmutter, M.A., 1993. Timing of turbidite sedimentation

on the Mississippi Fan. Am. Assoc. Pet. Geol. Bull. 77, 1129–

1141.

Kolla, V., Posamentier, H.W., Eichenseer, H., 1995. Stranded para-

sequences and the forced regressive wedge systems tract: dep-

osition during base-level fall—Discussion. Sediment. Geol. 95,

139–145.

Kumar, N., Sanders, J.E., 1974. Characteristics of shoreface storm

deposits: modern and ancient examples. J. Sediment. Petrol. 46,

145–162.

Leckie, D.A., Singh, C., 1991. Estuarine deposits of the Albian

Paddy Member (Peace River Formation) and lowermost Shaf-

tesbury Formation, Alberta, Canada. J. Sediment. Petrol. 61,

825–849.

Lindsay, J.F., Holliday, D.W., Hulbert, A.G., 1991. Sequence strat-

igraphy and the evolution of the Ganges–Brahmaputra delta

complex. Am. Assoc. Pet. Geol. Bull. 75, 1233–1254.

Lohmann, H.H., 1995. On the tectonics of Bangladesh. Swiss As-

soc. Petrol. Geol. Eng. Bull. 62 (140), 29–48.

MacEachern, J.A., Pemberton, S.G., 1994. Ichnological aspects of

incised valley fill systems from the Viking Formation of the

Western Canada sedimentary basin, Alberta, Canada. In: Dal-

rymple, R.W., Boyd, R., Zatlin, B.A. (Eds.), Incised Valley

Systems: Origin and Sedimentary Sequences. SEPM Spec. Publ.

51, pp. 129–157.

MacEachern, J.A., Zatlin, B.A., Pemberton, S.G., 1998. High-reso-

lution sequence stratigraphy of early transgressive deposits, Vik-

ing Formation, Joffre field, Alberta, Canada. Am. Assoc. Pet.

Geol. Bull. 82, 729–756.

MacEachern, J.A., Zatlin, B.A., Pemberton, S.G., 1999. A sharp-

based sandstone of the Viking Formation, Joffre Field, Alberta,

Canada: criteria for recognition of transgressively incised shore-

face complexes. J. Sediment. Res. 69, 876–892.

Meckel, L.D., 1975. Holocene sand bodies in the Colorado delta

area, northern Gulf of California. In: Broussard, M.L. (Ed.),

Deltas: Models for Exploration. Houston Geol. Soc., Houston,

pp. 239–265.

Miall, A.D., 1997. The Geology of Stratigraphic Sequences

Springer, Berlin, 433 pp.

Murphy, R.W., Staff BOGMC, 1988. Bangladesh enters the oil era.

Oil Gas J., pp. 76–82. Feb. 29.

Myrow, P.M., Southard, J.B., 1991. Combined-flow model for ver-

tical stratification sequences in shallow marine storm-dominated

beds. J. Sediment. Petrol. 61, 202–210.

Nio, S.D., Yang, C.S., 1988. Recognition of Tidally-Influenced Fa-

cies and Environments International. Geoservices, Leiderdrop,

230 pp.

North American Commission on Stratigraphic Nomenclature

(NACSN), 1983. North American stratigraphic code. Am. As-

soc. Pet. Geol. Bull. 67, 841–875.

Nummedal, D., Swift, D.J.P., 1987. Transgressive stratigraphy at

sequence-bounding unconformities: some principles derived

from Holocene and Cretaceous examples. In: Nummedal, D.,

Pilkey, D.H., Howard, J.D. (Eds.), Sea-level Fluctuations and

Coastal Evolution. SEPM, Spec. Publ. 41, pp. 241–260.

Pemberton, S.G., MacEachern, J.A., Frey, R.W., 1992. Trace fossil

facies models: environmental and allostratigraphic significance.

In: Walker, R.G., James, N.P. (Eds.), Facies Models: Response

to Sea-level Change. Geol. Assoc. of Canada, St. John’s, New-

foundland, pp. 47–72.

Plint, A.G., 1988. Sharp-based shoreface sequences and ‘‘offshore

bars’’ in the Cardium formation of Alberta: their relationship to


relative changes in sea level. In: Wilgus, C.K.et al. (Ed.), Sea-

level Changes: An Integrated Approach. SEPM, Spec. Publ. 42,

pp. 357–370.

Posamentier, H.W., Chamberlin, C.J., 1993. Sequence stratigraphic

analysis of Viking Formation lowstand beach deposits in the

Joarcam field, Alberta. In: Posamentier, H.W., Summerhayes,

C.P., Haq, B.U., Allen, G.P. (Eds.), Sequence Stratigraphy

and Facies Associations. Int. Assoc. Sedimentol., Spec. Publ.

18, pp. 469–486.

Posamentier, H.W., Allen, G.P., James, D.P., Tesson, M., 1992.

Forced regression in a sequence stratigraphic framework: con-

cepts, examples, and exploration significance. Am. Assoc. Pet.

Geol. Bull. 76, 1687–1709.

Reading, H.G., Richards, M., 1994. Turbidite systems in deep-water

basin margins classified by grain size and feeder system. Am.

Assoc. Pet. Geol. Bull. 78, 792–822.

Reimann, K.U., 1993. Geology of Bangladesh Borntracger, Berlin,

160 pp.

Reineck, H.E., Wunderlich, F., 1968. Classification and origin of

flaser and lenticular bedding. Sedimentology 11, 99–104.

Reinson, G.E., 1992. Transgressive barrier island and estuarine sys-

tems. In: Walker, R.G., James, N.P. (Eds.), Facies Models: Re-

sponse to Sea-level Change. Geol. Assoc. of Canada, St. John’s,

Newfoundland, pp. 179–194.

Reynolds, A.D., 1994. Sequence stratigraphy from core and wire-

line log data: the Viking Formation, Albian, South central Al-

berta, Canada. Mar. Pet. Geol. 11, 258–282.

Salt, C.A., Alam, M.M., Hossain, M.M., 1986. Bengal Basin: current

exploration of the hinge zone of southwestern Bangladesh. Proc.

6th Offshore Southeast Asia Conference, Singapore, pp. 55–67.

Shanmugam, G., Poffenberger, M., Alava, J.T., 2000. Tide-domi-

nated estuarine facies in the Hollin and Napo (‘‘T’’ and ‘‘U’’)

Formations (Cretaceous), Sacha field, Oriente basin, Ecuador.

Am. Assoc. Pet. Geol. Bull. 84, 652–682.

Sikder, A.M., 1998. Tectonic Evolution of Eastern Folded Belt of

Bengal Basin. Ph.D. Thesis, Univ. of Dhaka, Dhaka, Bangla-

desh, 175 pp.

Stride, A.H., Belderson, R.H., Renyon, N.H., Johnson, M.A., 1982.

Offshore tidal deposits: sand sheet sand bank facies. In: Stride,

A.H. (Ed.), Offshore Tidal Sands: Processes and Deposits.

Chapman & Hall, Edinburgh, pp. 95–125.

Uddin, A., Lundberg, N., 1998. Unroofing history of the eastern

Himalayas and the Indo-Burman ranges: heavy mineral study of

Cenozoic sediments from the Bengal Basin, Bangladesh. J.

Sediment. Res. 68, 465–472.

Uddin, A., Lundberg, N., 1999. A paleo-Brahmaputra? Subsurface

lithofacies analysis of Miocene deltaic sediments in the Hima-

layan–Bengal system, Bangladesh. Sediment. Geol. 123, 239–

254.

Van Wagoner, J.C., 1985. Reservoir facies distribution as controlled

by sea-level change. Abstract and Poster Session, SEPM Mid-

Year Meeting, Golden, Colorado, pp. 91–92.

Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R.,

Sarg, J.F., Loutit, T.S., Harderbol, J., 1988. An overview of the

fundamentals of sequence stratigraphy and key definitions. In:

Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier,

H., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea Level Changes:

An Integrated Approach. SEPM, Spec. Publ. 42, pp. 39–45.

Visser, M.J., 1980. Neap-spring cycles reflected in Holocene sub-

tidal large-scale bedform deposits: a preliminary note. Geology

8, 543–546.

Walker, R.G., 1990. Facies modeling and sequence stratigraphy. J.

Sediment. Petrol. 60, 777–786.

Walker, R.G., Eyles, C.H., 1988. Geometry and facies of stacked

shallow-marine sandier upward sequences dissected by erosion

surface, Cardium Formation, Willesden Green, Alberta. Am.

Assoc. Pet. Geol. Bull. 72, 1469–1494.

Weimer, R.J., Howard, J.D., Lindsay, D.R., 1982. Tidal flats and

associated tidal channels. In: Scholle, P.A., Spearing, D. (Eds.),

Sandstone Depositional Environments. Am. Assoc. Pet. Geol.

Mem. 31, pp. 191–245.

Williams, G.E., 1989. Late Precambrian tidal rythmites in South

Australia and the history of the Earth’s rotation. J. Geol. Soc.

(London) 146, 97–111.

Zaitlin, B.A., Dalrymple, R.W., Boyd, R., 1994. The stratigraphic

organization of incised-valley systems associated with relative

sea-level change. In: Dalrymple, R.W., Boyd, R., Zaitlin, B.A.

(Eds.), Incised Valley Systems: Origin and Sedimentary Se-

quences. SEPM, Spec. Publ. 51, pp. 45–60.


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