Geological Field Report along the Ghagra-Rangamati

road cut section of the Sitapahar anticline,

Chittagong-Tripura Fold Belt of Bengal Basin.

Report Submitted in requirement of partial fulfillment of the syllabus for

4th year B.S (Hons.), Department of Geology, University of Dhaka.

Submitted by-

Pritam Saha

Group-03

Roll- JN 030

Exam Roll- 1920

i

ABSTRACT

The Bengal Basin in the northeastern part of Indian subcontinent, between the

Indian Shield and Indo-Burman Ranges, comprises three geo-tectonic provinces

are

(1) The Stable Shelf;

(2) The Central Deep Basin (extending from the Sylhet Trough in the

northeast towards the Hatia Trough in the south); and

(3) The ChittagongTripura Fold Belt.

Due to location of the basin at the juncture of three interacting plates, viz., the

Indian, Burma and Tibetan (Eurasian) Plates, the basin-fill history of these

geotectonic provinces varied considerably. Precambrian metasediments and

PermianCarboniferous rocks have been encountered only in drill holes in the stable shelf province. After Precambrian peneplanation of the Indian Shield,

sedimentation in the Bengal Basin started in isolated graben-controlled basins on

the basement. With the breakup of Gondwanaland in the Jurassic and Cretaceous,

and northward movement of the Indian Plate, the basin started down wraping in the

Early Cretaceous and sedimentation started on the stable shelf and deep basin; and

since then sedimentation has been continuous for most of the basin. Subsidence of

the basin can be attributed to differential adjustments of the crust, collision with

the various elements of south Asia, and uplift of the eastern Himalayas and the

Indo-Burman Ranges. Movements along several well-established faults were

initiated following the breakup of Gondwanaland and during down wraping in the

Cretaceous. By Eocene, because of a major marine transgression, the stable shelf

came under a carbonate regime, whereas the deep basinal area was dominated by

deep-water sedimentation. A major switch in sedimentation pattern over the

Bengal Basin occurred during the Middle Eocene to Early Miocene as a result of

collision of India with the Burma and Tibetan Blocks. The influx of clastic

sediment into the basin from the Himalayas to the north and the Indo-Burman

Ranges to the east rapidly increased at this time; and this was followed by an

increase in the rate of subsidence of the basin. At this stage, deep marine

sedimentation dominated in the deep basinal part, while deep to shallow marine

conditions prevailed in the eastern part of the basin. By Middle Miocene, with

continuing collision events between the plates and uplift in the Himalayas and

Indo-Burman Ranges, a huge influx of clastic sediments came into the basin from

the northeast and east. Throughout the Miocene, the depositional settings

continued to vary from deep marine in the basin to shallow and coastal marine in

the marginal parts of the basin. From Pliocene onwards, large amounts of sediment

ii

were filling the Bengal Basin from the west and northwest; and major delta

building processes continued to develop the present-day delta morphology. Since

the Cretaceous, architecture of the Bengal Basin has been changing due to the

collision pattern and movements of the major plates in the region. However, three

notable changes in basin configuration can be recognized that occurred during

Early Eocene, Middle Miocene and Plio-Pleistocene times, when both the

paleogeographic settings and source areas changed. The present basin

configuration with the GangesBrahmaputra delta system on the north and the Bengal Deep Sea Fan on the south was established during the later part of

Pliocene and Pleistocene; and delta progradation since then has been strongly

affected by orogeny in the eastern Himalayas. Pleistocene glacial activities in the

north accompanied sea level changes in the Bay of Bengal.

iii

ACKNOWLEDGEMENT

Behind every successful field investigation there lies the valuable contribution of a

number of people who are to be acknowledged.

At first I take the opportunity of expressing my gratitude to the team leader Dr.

Mustafa Alam, Professor, Department of Geology, University of Dhaka for his

arrangement of the field tour and supervision in the field. We are very thankful to

him. He always made us alert for accurate investigation and was conscious for our

comfort.

My sincere appreciation goes to Dr. Subrata Kumar Saha, Associate Professor,

Department of Geology, University of Dhaka for his distinguished lectures.

My heartful thank also goes to Mr. Saiful Islam, Assistant Professor, Department

of Geology, University of Dhaka for his valuable lectures, careful guidance and

helpful manner during field trip.

I extend my deep thanks to the authority of Chittagong Hill Tracts Development

Board for the well arrangement of our accommodation and to some of the local

people who helped us in making arrangement of the transport and conducting the

field work.

I also wish to express my satisfaction to the food, committee, transport and first aid

for their great services and classmates especially my group members staff officials

for their supportiveness during field work and while preparing the report.

iv

TABLE OF CONTENT

Abstract i

Acknowledgement iii

Table of content iv

List of Figures v

List of Maps vi

1. Introduction 1

1.1 General Statement 2

1.2 Purpose and Scope 2

1.3 Location, Extent and Accessibility 3

1.4 Previous Work 4

1.5 Method of Investigation 5

1.6 Physiography 6 1.6.1 Topography and Relief 6 1.6.2 Drainage and Water Supply 9 1.6.3 Vegetation and Cultivation 10 1.6.4 Population and Culture 10 1.6.5 Climate 10

2. Structures and Tectonics 12

2.1 General Statement 13

2.2 Tectonic Frame work of Bangladesh 17

2.3 Regional Structure 21 2.3.1 Fold 21 2.3.2 Fault 21 2.3.3 Joint 22 2.3.4 Unconformity 22

3. Stratigraphy 24

3.1 General Statement 25

3.2 Criteria for identifying the bounding discontinuity 25 3.2.1 Incised Valley Floor (IVF) 25 3.2.2 Regressive Erosional Surface (RES) 26 3.2.3 Transgressive Erosional Surface (TES) 26 3.2.4 Marine Flooding Surface (MFS) 26 3.2.5 Transgressive Surface (TS) 26

3.3 Composite Stratigraphy 26 3.3.1 Composite Sequence C 27

v

3.3.2 Composite Sequence B 27 3.3.3 Composite Sequence A 27

3.4 Detail Description 28 3.4.1 Composite Sequence C 28

3.4.1.1 Sitapahar Anticline 28 3.4.1.1.1 Units Ca and Cb 28 3.4.1.1.2 Unit Cc 31 3.4.1.1.3 Unit Cd 32 3.4.1.1.4 Unit Ce 32

3.4.2 Composite Sequence B 33 3.4.2.1 Sitapahar Anticline 33

3.4.2.1.1 Sequence B1 34 3.4.2.1.2 Sequence B2 35 3.4.2.1.3 Sequence B4 36 3.4.2.1.5 Sequence B5 36

3.4.3 Composite Sequence A 39 3.4.3.1 Chittagong Group 40 3.4.3.2 Sitapahar Group (composite sequence C, equivalent to the traditional middle Surma Group) 40 3.4.3.3 Mirinja Group (composite sequence B, equivalent to the traditional upper Surma Group) 41 3.4.3.4 Kaptai Group (composite sequence A, equivalent to the traditional Tipam Group and Dupi Tila

Formation) 41

4. Geological History 42

4.1 Tectonic History 43

4.2 Depositional History 45 4.2.1 Unit C 45

4.2.1.1 Subunit C1 45 4.2.1.2 Subunit C2 46

4.2.1.2.1 Division C2a 46 4.2.1.2.2 Divisions C2b and C2c 47

5. Economic Geology 49

6. Conclusion 51

References 53

Geological Map 55

LIST OF FIGURES

Figure 1: Satellite image of investigated area 4 Figure 2: Small scale fault 22 Figure 3: Unconformity between Lower Dupitilla And Tipam 23 Figure 4: Mud Gall 23 Figure 5: Nodular Structure 29

vi

Figure 6: Onion Structure interbedded with Silty sandstone. 29 Figure 7: (a) Detailed litho-log of the upper part of unit Cb in the Sitapahar anticline; and (b) one representative

fining-upward cycle (From Gani and Alam, 1999) 30 Figure 8: Leisingang Ring Structure 31 Figure 9: Channel base 31 Figure 10: Rhythemtic Structure 32 Figure 11: Tidal Sequence 35 Figure 12: Zoophycos Trace Fossil 36 Figure 13: General litho-stratigraphic column of the lower part of the Surma Group exposed in the Sitapahar

anticline with detailed sedimentological logs of salient portions (From Gani and Alam, 1999). 38 Figure 14: Fining Upward Sequence 39 Figure 15: Schematic paleogeographic (Early Miocene) representation of the Bengal Basin and surroundings

incorporating the plate tectonic (From Gani and Alam,1999) 43 Figure 16: Conceptual Early Miocene paleogeographic model showing the sedimentation pattern (at highstand

condition) within the active margin (Gani and Alam,1999) 44 Figure 17: 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(Gani and Alam,1999) 48 Figure 18: Construction Stone (Calcareous Concretion) 50

LIST OF MAPS

Map 1: Location Map 3 Map 2: Contour Map of Bangladesh 8 Map 3: Drainage map of Rangamati 9 Map 4: 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. 15 Map 5: (a) Regional tectonic setting of the Bengal Basin showing location of the study area within the

ChittagongTripura Fold Belt(CTFB). NAP D Neogene accretion prism. The Tertiary volcanic centers are

marked by solid dots (modified from Dasgupta and Nandy, 1995, and Khan, 1991). 16 Map 6: 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) 17 Map 7: Tectonic Framework of Bangladesh 18

1

C H A P T E R 1

1. INTRODUCTION

1.1 General Statement

1.2 Purpose and Scope

1.3 Location, Extent and Accessibility

1.4 Previous Work

1.5 Method of Investigation

1.6 Physiography

1.6.1 Topography and relief

1.6.2 Drainage and water supply

1.6.3 Vegetation Cultivation

1.6.4 Population and Culture

1.6.5 Climate

2

1.1 GENERAL STATEMENT

The Bengal basin covering Bangladesh and part of eastern India, is one of the least

studied and yet well-known basins in the world. The geological evolution of this

basin began in the Late Mesozoic through the breakup of the Gondwanaland and

ongoing. The greater Bengal basin is bounded by the Shillong Plateau to the north,

by exposures of the Indian craton to the west, and Indo-Burman ranges to the east.

These hills are part of the Frontal Folded Belts of Arakan-Yoma.

On this report it has been attempted to present an overview on the tectonic setup,

stratigraphy, drainage, geomorphology, depositional history and economic geology

of the field area, Rangamati as well as the southeastern part of Bangladesh.

1.2 PURPOSE AND SCOPE

Field study primarily means obtaining geologic knowledge. It employs methods

and techniques to examine the structure, physical features, stratigraphy and also

materials exposed at the outcrop. The eastern and more importantly the

southeastern part of Bangladesh is much complex and exhibits much of the

tectonic evolution of the Bengal basin. The area consists one of the major tectonic

element of the Bengal basin i.e. The Frontal Fold belt. The rocks exposed here

ranges from the age of Miocene to Pliocene. On the contrary the area has the

drawback of lacking petroleum. Nonetheless the area is a paramount for geologic

study. Huge piles of sediments are exposed to be studied and enhance the

knowledge of the complexities of the nature. Our main purpose was to observe and

study the sedimentation history, structure, stratigraphy, fossils, depositional

environment, and economic geology of the area and also to make a geological map

of the studied area.

The purposes can be described briefly as

Preparation of a geological map of an area.

Determination of lithology and sedimentary structures.

Construction of stratigraphic column.

Study of formation of the column.

Study of sedimentary structures and other structural features.

Interpretation of the geologic history of the area.

Prediction of the economic importance of the area.

3

It is to be noted that, though the investigated area offered very good scope, to study

properly the time schedule was very short, very detail investigations were not

possible.

1.3 LOCATION, EXTENT AND ACCESSIBILITY

The area is located about 75 Km northeast of Chittagong town and is situated in the

southern part of Chittagong-Tripura folded belt. The investigated area lies between

2237.315 N to 2238.077 N latitude and 9206.307' E to 9211.271 E longitude and is included investigated area the survey of Bangladesh toposheet no. 84 B/2

and 84 B/3. It is situated about 250 km south-east of Dhaka city. The area is

connected to the Chittagong city by a metalloid road. It is also connected with

Banderban hill district by a jeepable road.

MAP 1: LOCATION MAP

4

The Sitapahar anticline is in the investigated Rangamati district. The anticline is

about 70 Km (N-S) long and 12 km (E-W) wide. It covers about 550 Sq. kms of

Rangamati district. The total extent of the investigated area is just around 840 sq.

kilometers.

Kaptai reservoir against the river Karnaphuli literally encloses the Rangamati town

as a whole, hence, the accessibility of the area is clearly easy.

FIGURE 1: SATELLITE IMAGE OF INVESTIGATED AREA

1.4 PREVIOUS WORK

Many geologists surveyed on the Sitapahar anticline in the eastern part of

Bangladesh. Shell attempted to know about the sub-surface structural configuration

with seismic surveys. So they attempted to drill a deep exploration well on the

Sitapahar anticline which was about 1377m in depth (in 1988).

There are not many publications that presented facies analysis on these clastic

rocks exposed in southern part of CTFB (Alam, 1995; Alam and Ferdous, 1995,

5

1996; Alam and Karim, 1997; Gani and Alam, 1999). The significance of tidal

influence in shallow marine Surma Group was first discussed by Alam (1995).

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. Gani and

Alam, latter in another publication (2000), have shown a detailed lithofacies

analysis and gave an interpretation about the origin of, and genetic relationships

between, individual units in response to sea-level changes.

1.5 METHOD OF INVESTIGATION

Method of investigation is all the same everywhere and sometimes it is technical

and talent to determine the location, stratigraphy, lithology, environment of

deposition of different section. We used the traverse method. The whole study as

accomplished into two phases, which are field study in the field and laboratory

analysis of the samples collected from the field. The investigation in the field was

performed by adopting traverse method and the equipments used are as follows:

a. Base map (R.F scale 1:25,000). b. Clinometer. c. Hammer (chisel hammer). d. Pocket lens. e. Acid bottle with dilute HCl. f. Sample bag. g. Field notebook. h. Pens, pencils, colour pencils, diagonal scale etc. i. Camera, binocular. j. Haver sack.

TAKING LOCATIONS AND BEARING:

The base map helped to determine the location of the suitable sections. Arial

distance was measured by taking pacing. Our instant positions were plotted on the

map by the help of the clinometer and a remarkable point on the base map. Bearing

of the section was measured by clinometer.

6

LITHOLOGIC INVESTIGATION:

The lithology of the area was studied by observing good exposures emphasizing on

the following aspects color, texture, composition and sharp contact of various rock

strata. Presence of carbonate was determined by using dil. HCl.

STRUCTURAL INVESTIGATIONS:

Attitude of the bed i.e. dip direction and amount of dip measured with the help of

clinometers. Hammer was used for cleaning and finding beds in rough, disturbed

and vegetated outcrops.

STRATIGRAPHICAL INVESTIGATIONS:

The stratigraphic succession was made by observing the position of different rock

units in the field, their lithology as well as thickness. Unconformity or time gap

between two different type of lithology is marked by erosional surface, laterite

band and soil.

COLLECTING SAMPLES:

Samples were collected in sample bags from different rock strata

of different sections with proper labeling for further study in lab.

CONSTRUCTION OF GEOLOGIC MAP:

The attitude of strata and lithology were plotted on the base map with proper

reference points in order to prepare a geological map of the area.

TAKING PHOTOGRAPHS:

Important photographs of sedimentary structure, physical feature i.e. (fossil) and

structural features (fault, unconformity, joint) of the area were taken by camera

where possible, rough sketches of rock units was also made.

1.6 PHYSIOGRAPHY

1.6.1 TOPOGRAPHY AND RELIEF

The topography of the investigated area is rugged with elevations ranging from

16m to 493m,supported the fact that this is a hilly region. Numerous low to

7

moderately elevated hillocks are present here. Highest elevation of the hill range is

Phoromain whose elevation is 493m and minimum in the low lying lakes surrounding Rangamati town. The relief of the area is high which decrease towards

east.

Topographically the are a is rugged terrain. Numerous hills and hillocks flourish

the surface of the field area. Most of the hills are associated with a correspondent

depressed zone or valleys.

8

MAP 2: CONTOUR MAP OF BANGLADESH

9

1.6.2 DRAINAGE AND WATER SUPPLY

Karnaphuli, the main channel of water system in the area, crosses Shubhlong range

and Silchari where it penetrates the Sitapahar range (Khan 1991). The Karnaphuli

river and its numerous tributaries and distributaries are the main components of the

drainage system in the investigated area. The Karnaphuli river originates from the

high Arakan-Yoma and cuts across the hill tracts to fall into the Bay of Bengal

(khan 1991).

MAP 3: DRAINAGE MAP OF RANGAMATI

10

There are numerous streamlets in the investigated area. The important chara or

creeps that drain the area are Shumba Chara, Shundari chara, Manikchari chara,

Ghagra chara, hatimara chara, Tippara chara etc.

Even though the investigated area is hilly terrain, groundwater prospect of the area

is fairly good. Shallow hand tube well can reach to groundwater and thus there is

no problem of drinking water. The Kaptai reservoir feeds the investigated area

throughout the season, particularly in dry season.

1.6.3 VEGETATION AND CULTIVATION

The investigated area is covered by forest with evergreen vegetation, which is due

to the good and suitable climatic condition of this region. Throughout the field area

bushes are abundant with some fruit trees like mango, jackfruit, guava are also

numerous. Besides this tall trees like Garjan, Jarul, Shal, Shegun etc are not

uncommon. The major agricultural activity of this area is shifting cultivation, they

also grow pineapple, ginger, turmeric etc. Generally, shifting cultivation is

preferred in high steep hill ranges where pineapples, ginger, turmeric etc. in low

hill ranges. Both Sedentary and Jhuming cultivations are practiced here.

1.6.4 POPULATION AND CULTURE

The area is sparsely populated and presents and interesting disparity with the

general demographic picture in Rangamati. The people live on the top of the hills

and also on foot of the hills. The life is very hard in the investigated area as the

major part of the population of the area live mostly along the foot hills and most of

the tribal people live on the hilly region.

Total population of the Rangamati district is about 400000. Population density is

about 6000 per sq. km. Most of the people are poor and maintain their lives by

handicrafts and also cutting woods.

1.6.5 CLIMATE

The area can be characterized by tropical to sub-tropical climatic condition. The

temperature of the area ranges from 90F to 65F. Three distinct seasons are felt in

Rangamati and adjoining areas. (i) The summer starts from march and with high

temperature and moderate precipitation, it lasts till May, (ii) In June the monsoon

begins and continue up to October, with dark cloud in sky and heavy rainfall with

11

dusty wind and often cyclonic storm, (iii) Characterized by pleasant cool and dry

weather begins from November and ends in February.

12

C H A P T E R 2

2. STRUCTURES AND TECTONICS

2.1 General Statement

2.2 Tectonic Frame work of Bangladesh

2.3 Regional Structure

2.3.1 Fold

2.3.2 Fault

2.3.3 Joint

2.3.4 Unconformity

13

2.1 GENERAL STATEMENT

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. The basin extends

southward into the Bay of Bengal. The Bengal Basin is well known for the

development 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

Chittagong 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 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 identified

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

14

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 the anticlinal structure within the CTFB, namely, the Sitapahar

anticline of the Rangamati area. Rocks exposed in various creeks along the

Matamuhari River and on the eastern flank of the structure have also been studied

for crosschecking. The Sitapahar structure, about 40 km in length, is a doubly

plunging, elongated asym- metric anticline with an axial trend of N20WS20E and a plunge of about 4 degree. The gentler eastern flank of the structure has dips

varying from 10 degree to 45 degree and the steeper western flank dips at angles

varying from 15 degree to 70 degree (Ferdous, 1990). Fieldwork was carried out

along the RangamatiChittagong road section that transversely cuts the northern part of the anticline, and the greater than 3-km-thick Neogene succession exposed

on the eastern flank has been described in detail.

15

MAP 4: 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.

16

MAP 5: (A) REGIONAL TECTONIC SETTING OF THE BENGAL BASIN SHOWING LOCATION OF THE STUDY AREA WITHIN THE CHITTAGONGTRIPURA FOLD BELT(CTFB). NAP D NEOGENE ACCRETION PRISM. THE TERTIARY VOLCANIC CENTERS ARE MARKED BY SOLID DOTS (MODIFIED FROM DASGUPTA AND NANDY,

1995, AND KHAN, 1991). THE BATHYMETRIC CONTOURS OF THE BAY OF BENGAL ARE SHOWN IN METERS.

17

MAP 6: 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)

2.2 TECTONIC FRAME WORK OF BANGLADESH

Tectonic Framework refers to the basic structural frame on which Bangladesh

stands. It is essential to have a clear conception about the tectonic framework of

Bangladesh in order to evaluate the prospect of Mineral Resources including oil

and Natural Gas.

18

MAP 7: TECTONIC FRAMEWORK OF BANGLADESH

Bangladesh is divided into two major tectonic units: i) Stable Pre-Cambrian

Platform in the northwest, and ii) Geosynclinal basin in the southeast. A third unit,

a narrow northeast-southwest trending zone called the hinge zone separates the

above two units almost through the middle of the country. This hinge zone is

currently known as palaeo continental slope. [Sifatul Quader Chowdhury]

Stable Pre-Cambrian Platform in Bangladesh, Stable Pre-Cambrian Platform

refers to the stable shelf of the Bengal Basin. It is the part of the basin that lies on

the west and northwest of the line joining Calcutta and Mymensingh. Rangpur

Saddle represents Indian Platform and connects the Indian Shield with the Shillong

Massif and the Mikir Hills. Shillong Massif is a large thrust block of the Indian

19

Shield. In Rangpur Saddle the basement is the most uplifted and is covered with

thin sedimentary deposits. In Madhyapara area of Dinajpur the basement is only

130m deep from the ground surface and is overlain by Dupi Tila Sandstone and

Madhupur Clay of Plio-Pliestocene age. Rangpur Saddle can be divided into 3

parts- Rangpur Saddle, Northern Slope of Rangpur Saddle and Southern Slope of

Rangpur Saddle.

Bogra Shelf represents the southern slope of Rangpur Saddle which is a regional

monocline plunging towards southeast gently to Hinge Zone. This zone marks the

transition between the Rangpur Saddle and the Bengal Foredeep from depositional

as well as structural point of view.

Calcutta-Mymensingh Gravity High reflects a tectonic element known as Hinge

Zone and more recently as palaeo continental slope in the framework of

Bangladesh. The Hinge Zone is a narrow strip of about 25 km wide complex

flexure zone, which separates the Bengal Foredeep from the shelf zone. It trends

approximately N 30 E along the Calcutta-Pabna-Mymensingh gravity high and

extends upto the western tip of Dauki fault. Hinge Zone is connected with Bengal

Foredeep by deep basement faults that probably started with the breakup of

Gondwanaland. Since then they have been repeatedly reactivated. In the northeast

of Bangladesh the Hinge Zone turns to the east and seems to be connected with the

Dauki Fault, probably by a series of east-west trending faults. [ASM

Woobaidullah]

Geosynclinal Basin the geosynclinal basin in the southeast is characterised by the

huge thickness (maximum of about 20 km near the basin centre) of clastic

sedimentary rocks, mostly sandstone and shale of Tertiary age. It occupies the

greater Dhaka-Faridpur-Noakhali-Sylhet [Silet]-Comilla [Kumilla]-Chittagong

areas. The huge thickness of sediments in the basin is a result of tectonic mobility

or instability of the areas causing rapid subsidence and sedimentation in a

relatively short span of geologic time. The geosynclinal basin is subdivided into

two parts ie fold belt in the east and a foredeep to the west.

Bengal Foredeep occupies the vast area between Hinge Line and Arakan Yoma

Folded System and plays the most important role in the tectonic history of Bengal

Basin. Tectonically, Bengal Foredeep can be divided into two major regions- (a)

Western Platform Flank and (b) Eastern Folded Flank. The Western Platform flank

is further subdivided into (a) Faridpur Trough (b) Barisal-Chandpur High (c)

Hatiya Trough (d) Madhupur High and (e) Sylhet Trough.

20

Sylhet Trough situated on the southern side of the Shillong Massif and

corresponds to the vast low lands of Surma Valley with numerous swamps (haors)

where absolute elevation marks even below the sea level. It is a sub-basin of the

Bengal Foredeep in the northeastern part of Bangladesh and is characterised by a

very pronounced, vast, closed negative gravity anomaly up to 84 mgl (Milligal).

Shillong Massif forms the northern boundary of Sylhet Trough while the great

Dauki Fault separates the trough from the Massif. The Trough is bounded on the

east and southeast by the sub-meridional trending folded belt of Assam and Tripura

as the frontal deformation zone of Indo-Burman Ranges.

Indian Platform bounds the trough from the west while it is open in the southwest

to the main part of Bengal Basin. It is an oval shaped trough about 130 km long

and 60 km wide. Sub-meridional trending anticlinal folds of Chittagong-Tripura

Folded Belt gradually plunge northward to the Sylhet Trough. In cross-section the

Sylhet Trough is sharply asymmetrical with comparatively gentle southern and

steep faulted northern slope. Dauki Fault with 5 km wide fault zone forms the

contact between Shillong Massif and Sylhet Trough. The evolution of Sylhet

Trough includes (i) a passive continental margin (Pre-Oligocene) to (ii) a foreland

basin linked to the Indo-Burman Ranges (Oligocene and Miolene) to (iii) a

foreland basin linked to south-directed over thrusting of Shillong Plateau

(Pliocene-Holocene). The anticlinal folds of Habiganj, Rashidpur, Bibiana, Maulvi

Bazar, Katalkandi, Fenchuganj, Harargaj, Patharia, Beani Bazar (Mama Bhagna)

and Kailas Tila, which occupy the southern rim of Sylhet Trough have sub-

meridional trend in contrast to sub- latitudinal trending Chhatak, Jalalabad, Sylhet,

Dupi Tila and Jatinga structures. These two structural trends form a syntaxial

pattern at the northeastern tip of Sylhet Trough. The Neogene sediments have

excellent development in Sylhet Trough while the Paleogenes are at greater depths.

Folded Belt represents the most prominent tectonic element of Bengal Foredeep

with general sub-meridional trending hills parallel to the Arakan Yoma Folded

System. Folded belt extends within Bangladesh for 450 km (N-S) and about 150

km wide covering an area of 35,000 sq km of on-shore area. A large number of

narrow, elongated N-S trending folds of the eastern part of Bangladesh (Sylhet and

Chittagong Divisions), Tripura, southern part of Assam, Mizoram and Myanmar

territory adjacent to S-E of the Chittagong Hill Tracts occupy the Folded Belt west

of the Arakan Yoma Folded System. The folds are characterised by ridge forming,

box-like in cross section, high amplitude with variable width and lie en-echelon

with the adjacent structures. The elevation of these elongated anticlinal folds in

Bangladesh ranges from 100 -1,000m. Some of the structures are faulted and

thrusted and the intensity of folding increases gradually from west to east.

21

Consequently, the structures of the eastern part are tightly folded, faulted and

thrusted with narrower synclines between them.

The Neogene sedimentary sequence developed here are largely unfossiliferous and

consists mainly of the alteration of shales, clays, claystones, siltstones and

sandstones with occasional intra-formational conglomerates which can be

subdivided into nine formations on the basis of lithology.

It is recently reported that turbidity facies assemblage have been recorded in

Bhuban rocks in Sitapahar anticline representing Bouma sequence with 5 well

defined cycles which need confirmation by extensive further investigation.

2.3 REGIONAL STRUCTURE

2.3.1 FOLD

The Sitapahar anticline is the most prominent structure of the investigated area

which axis is trending NNW-SSE direction along the main structure of the

Chittagong Tripura Folded Belt (CTFB). This doubly plunging anticlinal structure

is about 400km long and 12-15 km wide and major part of the western flank is

more steeper than the eastern flank.The western flank dips in an angle ranging

from 400 to85

0 and the eastern flank shows dip ranging from 4-85

which indicate

that the anticline is an asymmetric anticline. From the attitude of the beds the

anticline is suspected as a plunging anticline.

2.3.2 FAULT

A regional fault is believed to run along the axis of the structure which trend is

North direction. The eastern side of the fault is acted as foot wall and the western

part acts as a hanging wall which indicate the presence of reserve fault at Manik

Chari (Ferdouse 1999) which is at a few kilometer east of Ghagra. A normal fault

is found at Sundari Chara. The indication of the regional fault are characterized by

the following terms:

1. Satellite image

2. Steep dip

3. Associate with drag fault

4. Thickness variation

5. Lithologic similarity

22

FIGURE 2: SMALL SCALE FAULT

2.3.3 JOINT

Joints are the very common structural features in the investigated area. These are

widely distributed in the area. Joints are well developed in relatively resistant

rocks. Vertical joints are the prominent structural features.

2.3.4 UNCONFORMITY

Unconformity is an erosion and non- depositional surface. A regional

unconformity is present in between Surma group and Tipam Sandstone , which is

an angular unconformity. It is located in Ghagra. A local unconformity also found

in Surma group.

23

FIGURE 3: UNCONFORMITY BETWEEN LOWER DUPITILLA AND TIPAM

FIGURE 4: MUD GALL

24

C H A P T E R 3

3. STRATIGRAPHY

3.1 General Statement

3.2 Criteria for identifying the bounding discontinuity

3.2.1 Incised Valley Floor

3.2.2 Regressive Erosional Surface

3.2.3 Transgressive Erosional Surface

3.2.4 Marine Flooding Surface

3.2.5 Transgressive Surface

3.3 Composite Stratigraphy

3.3.1 Composite Sequence C

3.3.2 Composite Sequence B

3.3.3 Composite Sequence A

3.4 Detail Description

3.4.1 Composite Sequence C

3.4.1.1 Sitapahar Anticline

3.4.1.1.1 Units Ca and Cb

3.4.1.1.2 Unit Cc

3.4.1.1.3 Unit Cd

3.4.1.1.4 Unit Ce

3.4.2 Composite Sequence B

3.4.2.1 Sitapahar Anticline

3.4.2.1.1 Sequence B1

3.4.2.1.2 Sequence B2

3.4.2.1.3 Sequence B4

3.4.2.1.5 Sequence B5

3.4.3 Composite Sequence A

3.4.3.1 Chittagong Group

3.4.3.2 Sitapahar Group

3.4.3.3 Mirinja Group

3.4.3.4 Kaptai Group

25

3.1 GENERAL STATEMENT

The exposed Neogene succession represents an overall basinward progradation

from deep marine through shallow marine to continentalfluvial 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. (1) Unit C (lower unit, 71 m thick) is characterized by several thin

packets of turbidites and slumped beds contained within a muddy sequence. The

unit is thought to have been deposited in a setting not far basinward from the base-

of-slope. (2) Unit B (middle unit, 291 m thick) is a monotonous muddy slope

deposit that contains some localized zones of very thin-bedded turbidites. (3) Unit

A (upper unit, 208 m thick) represents the progradation of the first shoreface sand

body on a 176-m-thick shelfal mud. Detailed bed by bed measurements have been

carried out in all these units. A general litho-stratigraphic column of the lower part

of the Surma Group exposed in the Sitapahar anticline have been given in Fig 8.

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-coastline trend from paleocurrent analysis

of the Surma Group in the Sitapahar Anticline. The measured current directions

represent landward (eastward) and basinward (westward) directions.

3.2 CRITERIA FOR IDENTIFYING THE BOUNDING DISCONTINUITY

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.

3.2.1 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 succession overlying this surface reveals

deepeningupward (transgressive) trend, in which typical estuarine facies can be

identified. The above criteria suggest that this bounding discontinuity most

26

probably developed as a result of channel incision at the time of sealevel low; and

hence is called an IVF.

3.2.2 REGRESSIVE EROSIONAL SURFACE (RES)

It is developed erosionally on shelfal mud and is overlain by shallowing- upward

shoreface deposits. This type of bottomtruncated 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.

3.2.3 TRANSGRESSIVE EROSIONAL SURFACE (TES)

This surface exclusively develops on valley-fill deposits experiencing sea-level

rise. It is overlain by either thin (515- cm-thick) pebble horizon or 510-m-thick

shoreface sandbody before passing upward into shelfal mud. The above criteria

suggest that this surface has been developed because of submarine erosion during

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

3.2.4 MARINE FLOODING SURFACE (MFS)

This type of bounding discontinuity is formed when shelfal mud abruptly overlies

a shallowing-upward shoreface succession. Therefore, it is the result of a short-

term sealevel rise; and hence is called a MFS.

3.2.5 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.

3.3 COMPOSITE STRATIGRAPHY

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 basinward progradation from deep marine

base-of-slope through shallow marine coastal to continentalfluvial deposits; and

that the entire succession may be divided into three broad groups. These composite

sequences, designated as C, B, and A, from oldest to youngest, represent three

27

stages of basin evolution each having its own sedimentation pattern and basin-fill

architecture. A comparison of the three composite sequences with the traditional

stratigraphic nomenclature is shown.

3.3.1 COMPOSITE SEQUENCE C

The basal composite sequence is conspicuously mud dominated with minimal

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

ofslope clastics, encountered only in the Sitapahar anticline, that grade upward into

shallow marine and nearshore deposits. Prograding parasequences separated by

MFSs characterize this composite sequence in the Sitapahar anticline, whereas in

the Mirinja anticline submarine channel deposits are conspicuous. The top of C is

demarcated by a pronounced erosion surface indicating a lowstand of sea level.

3.3.2 COMPOSITE SEQUENCE B

The middle composite sequence is sand dominated and has several distinct and

regionally traceable erosion surfaces. It is abbreviated in the Sitapahar anticline

due to truncation by the Manikchari fault. Tide dominated open marine shelfal to

coastal depositional settings under the control of cyclic relative sea-level rise and

fall typifies this composite sequence, where several incised valley-fill deposits are

prominent. The top of B is also a pronounced erosion surface of low stand sea

level.

3.3.3 COMPOSITE SEQUENCE A

The upper composite sequence represents the final phase M. Royhan Gani, M.

Mustafa Alam / Sedimentary Geology 155 (2003) 227270 233 of basin-filling

history with the establishment of continentalfluvial depositional systems. Low-

sinuosity braided to high-sinuosity meandering fluvial deposits characterizes 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-coastline trend from paleocurrent analysis of the Surma Group in the

Sitapahar anticline. In the lithostratigraphic columns we have shown the sequences

of different hierarchies. Within a composite sequence individual sequences

represent a single sea-level cycle (fallrisefall). 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 tractslowstand, transgressive and

28

highstandof 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 TES/TS) or a parasequence (separated by a MFS).

3.4 DETAIL DESCRIPTION

3.4.1 COMPOSITE SEQUENCE C

The lower composite sequence represents a basinward progradation from deep

marine (base-of-slope deposits) to shallow marine clastics. The base-ofslope

deposits are observed only in the Sitapahar anticline.

3.4.1.1 SITAPAHAR ANTICLINE

In the Sitapahar anticline there is also no identifiable erosional sequence boundary

within the composite sequence C. A distinct progradation from base-ofslope 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 environments represented by units Ca and Cb. Only a

brief interpretation of the depositional scenario of these two units is presented

below.

3.4.1.1.1 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, characterize the base-of-slope deposits of unit Ca. Small-scale

submarine lobe, channellevee complex and debrismass complex represent some

of the architectural 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 conclusive

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 sedimentation with thicker shelfal mudstone

passing stratigraphically upward into distinct tidal ridge progradation followed by

a coastal deposit. This coastal deposit mainly reflects progradation and

retrogradation of tidal flat strata coupled with some ripple cross-laminated to

29

structureless (may be stormreworked), fine-grained sandstone intervals. The top of

the unit is marked by a distinct MFS. Flame Structure is observed in this unit.

FIGURE 5: NODULAR STRUCTURE

FIGURE 6: ONION STRUCTURE INTERBEDDED WITH SILTY SANDSTONE.

30

FIGURE 7: (A) DETAILED LITHO-LOG OF THE UPPER PART OF UNIT CB IN THE SITAPAHAR ANTICLINE; AND (B) ONE REPRESENTATIVE FINING-UPWARD CYCLE (FROM GANI AND ALAM, 1999)

31

3.4.1.1.2 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

mudstone, passing upward into laminated

mudstone. Although the sedimentary

structures within sandstone 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

transgression without any development of

nearshore deposits. Since the top part of unit

Cc indicates a deepening-upward trend the

upper boundary of this unit is interpreted as a

MFS.

FIGURE 9: CHANNEL BASE

FIGURE 8: LEISINGANG RING STRUCTURE

32

3.4.1.1.3 UNIT CD

Thick laminated mudstone with occasional sandstone beds (a few centimeters

thick) characterize the lower 90 m of this unit, which passes upsequence 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 510 cm thick and laminated

mudstone 23 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 sandmud

content within the deposit. The top 20

m of unit Cd reflects a muddier upward

trend with the cyclic pattern gradually

disappearing upward. A subtle MFS

can be recognized at the top. The

monotonous alternation of mudstone

and sandstone in the upper part of this

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 near shore

deposits, they have rarely been described from a distal offshore depositional setting

(e.g. Williams, 1989).

3.4.1.1.4 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 24 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

Wunderlich, 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

FIGURE 10: RHYTHEMTIC STRUCTURE

33

facies succession. Although the sedimentary structures 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 sedimentary structures

within the fining-upward cycles overlying 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 intraclasts at the base.

The lower 8 m of this succession shows structureless medium (to coarse) sandstone

with thin mud inter laminae, whereas the upper 28 m is characterized by an

alternation of 1020-cm-thick very fine sandstone beds and 5-cm-thick mudstone

beds. This fining-upward succession is again truncated by a 3-m-thick erosionally

based upward fining deposit of fine sandstone with mud intercalation. 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 auto-cyclic lateral migration of tidal channels (cf. Kumar

and Sanders, 1974) on the coastal plain at the final phase of gradual marine

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

progradational and shallowing-upward successions 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).

3.4.2 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

regression and transgression. Each sequence is divided into units on the basis of

bounding discontinuities of relative sea-level rise. 3.4.2.1 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 highlight the gross depositional scenario and bounding

discontinuities of composite sequence B in the Sitapahar anticline to give a

34

tentative correlation with their counterparts in the Mirinja anticline described

earlier.

3.4.2.1.1 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

surfaces 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

Pemberton, 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-induced surface to be considered as sequence boundary. Unit B1a is

interpreted as inner estuary fluvial channel-fill facies which begins with a distinct

5-mthick 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

progradation on shelf mudstone. Two hummocky crossstratified 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 mudstone of the next

parasequence. Unit B1c is again a progradational shoreline parasequence that is

also interestingly devoid of any cross-bedded facies. Wavy-lenticular sandmud

facies, typical upward fining (1550-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 be noted that the speculated

RES at the lower part of unit B1b in the Mirinja anticline is not represented in the

Sitapahar anticline.

35

FIGURE 11: TIDAL SEQUENCE

3.4.2.1.2 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 sandmud

interval. Tidal activity (with the gradual rise of relative 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,

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 developed at the time of highstand

condition. It is important to note that the upper part of sequence B2, sequence B3,

36

and the lower part of sequence B4 are missing because of the stratigraphic break at

the Manikchari fault.

3.4.2.1.3 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.

FIGURE 12: ZOOPHYCOS TRACE FOSSIL

3.4.2.1.5 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 parasequences - units B5b, B5c, and B5d. Units B5b and B5c

37

are very thin and characteristically sharp-based on offshore mudstone indicating

that high-frequency relative sea-level falls during the overall rise. Therefore, the

parasequence designation of these two units may not be appropriate in a strict

sense. The uppermost unit B5d is a distinct shoaling/coarseningupward 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. Shoreface facies shows several meter-thick upward

coarsening 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 indicating a large fall of relative sea level.

38

FIGURE 13: GENERAL LITHO-STRATIGRAPHIC COLUMN OF THE LOWER PART OF THE SURMA GROUP

EXPOSED IN THE SITAPAHAR ANTICLINE WITH DETAILED SEDIMENTOLOGICAL LOGS OF SALIENT PORTIONS (FROM GANI AND ALAM, 1999).

39

3.4.3 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 color. A detailed account of the litho-facies types

with their depositional connotation was given by Alam (1996) and Alam and

Ferdous (1995). Only a brief description of the composite sequence A is presented

below.

The sequence begins on a pronounced erosion surface interpreted as an IVF

overlain by a 25-mthick characteristic conglomerate bed consisting of mud clasts

(up to 15 cm long) with some quartz pebbles. The lowermost 4050 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, frequent 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

composite sequence

consist of monotonous

medium sandstone

characterized by trough

cross-, planar cross-, and

parallel-bedding with some

ripple cross-lamination and

mud interlaminae, thought

to represent various types

of channel bar deposits of

a large-scale braided river

system. Sandmud ratios

decrease at the uppermost

part of composite sequence

A indicating a probable

FIGURE 14: FINING UPWARD SEQUENCE

40

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

entire CTFB most probably associated with a tectonic upheaval. An

allostratigraphic framework has been adopted, giving emphasis on bounding

discontinuities, to analyze the sedimentation and basin-fill history of the Neogene

clastic succession. It has been recognized that allostratigraphic units are more

natural subdivisions of the rock record than conventional lithostratigraphic units

(Walker, 1990; Miall, 1997). Therefore the allostratigraphic scheme formally

adopted by the North American Commission of Stratigraphic Nomenclature North

American Commission on stratigraphic Nomenclature (NACSN), 1983) has been

incorporated in establishing a separate stratigraphy for the CTFB. Table 3 shows

such a tentative stratigraphic classification for the CTFB that recognizes four

Groups (more precisely allogroups) equivalent to the composite sequences.

3.4.3.1 CHITTAGONG GROUP

The group is speculated to exist in the subsurface and has not yet been reported

from the outcrop. The Group is thought to represent largescale submarine fan

complex as envisioned by Gani and Alam (1999) and to exist beneath the slope

apron deposits of the composite sequence C. The Group probably ranges in age

from the Eocene (?) or Oligocene to the Early Miocene and is equivalent to the

traditional Barail Group and the lower part of the Surma Group. 3.4.3.2 SITAPAHAR GROUP (COMPOSITE SEQUENCE C, EQUIVALENT TO THE

TRADITIONAL MIDDLE SURMA GROUP)

It is probably Middle Miocene in age and ranges in thickness from 1000 m to 1500

m. However, recent study based on Dinoflagellates (Uddin and Uddin, 2001)

indicates that the exposed sediments of the CTFB are probably not older than Late

Miocene/ Early Pliocene. The RangamatiChittagong road section of the Sitapahar

anticline could be the type section for this Group. In the type section it is

represented by the oldest 1128+ m of the rock succession (composite sequence C)

exposed in the eastern flank of the anticline (Gani and Alam, this volume). This

Group represents a progressive basin filling from deep marine slope apron to

shallow marine nearshore deposits, and would represent the period of closing of

the suture between the Bengal Basin and the subduction zone lying off the Burma

Block. No major erosion surface of sea level lowstand condition has been

encountered within this Group, in which a low sand/shale ratio indicates that the

basin was accommodation-dominated during this time. The Sitapahar Group may

41

be further divided into several alloformations depending on the bounding

discontinuities (mainly the marine flooding surfaces). 3.4.3.3 MIRINJA GROUP (COMPOSITE SEQUENCE B, EQUIVALENT TO THE TRADITIONAL

UPPER SURMA GROUP)

It is probably Late Miocene in age and ranges in thickness from 1200 to 1600 m.

The LamaFashiakhali road section of the Mirinja anticline could be the type

section for this Group. In the type section the Group is represented by the 1293 m

thick shelfal to coastal succession (composite sequence B) exposed in the western

flank of the anticline (Gani and Alam, this volume). The Group may conveniently

be divided into several alloformations corresponding to the individual sequences,

which in turn can be divided further into allomembers depending on the

transgressive erosion surface and transgressive surface/marine-flooding surface.

The high sand/shale ratio of this Group indicates that the basin was supply-

dominated during this period. 3.4.3.4 KAPTAI GROUP (COMPOSITE SEQUENCE A, EQUIVALENT TO THE TRADITIONAL

TIPAM GROUP AND DUPI TILA FORMATION)

It is probably Plio-Pleistocene in age and ranges in thickness form 1100 to 1600 m.

The stratigraphic succession in the western flank of the Sitapahar anticline along

the KaptaiChandraghona road section could tentatively serve as a type section for

this Group. The lower part of the Group represents braided stream coastal to fluvial

deposits, whereas the uppermost part represents deposits of meandering river

system. A 100200 m thick and rather patchy clay deposits (traditional Girujan

Clay) sometimes divides the Group into the above-mentioned traditional units.

42

C H A P T E R 4

4. GEOLOGICAL HISTORY

4.1 Tectonic History

4.2 Depositional History

4.2.1 Unit C

4.2.1.1 Subunit C1

4.2.1.2 Subunit C2

4.2.1.2.1 Division C2a

4.2.1.2.2 Divisions C2b and C2c

43

4.1 TECTONIC HISTORY

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

(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.

FIGURE 15: SCHEMATIC PALEOGEOGRAPHIC (EARLY MIOCENE) REPRESENTATION OF THE BENGAL BASIN

AND SURROUNDINGS INCORPORATING THE PLATE TECTONIC (FROM GANI AND ALAM,1999)

Since the present study deals with the Neogene rock succession of the CTFB, and

the sedimentation and tectonics are very much interrelated, it is important to

44

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 bathymetry, and the sediment source was largely

from the newly uplifted Indo-Burman subduction complex to the east. The

schematic palaeogeographic (Early Miocene) map of the Bengal Basin and its

surroundings 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 oblique subduction. As a result, the trench-slope

bathymetry 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

succession was developed on a late-stage trench-slope bathymetry (Gani and Alam,

1999), and that the deep-water embayment has been smoothed to shallow water

conditions probably sometime in the Late Miocene. The block diagram represents

the gross sedimentation pattern in the CTFB during the Early Miocene.

FIGURE 16: CONCEPTUAL EARLY MIOCENE PALEOGEOGRAPHIC MODEL SHOWING THE SEDIMENTATION

PATTERN (AT HIGHSTAND CONDITION) WITHIN THE ACTIVE MARGIN (GANI AND ALAM,1999)

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 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

45

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-

ollapse cycles over relatively short periods within the subduction zone described

by Flint et al. (1991).

4.2 DEPOSITIONAL HISTORY

An overview of depositional environment of the investigated area is described

below

4.2.1 UNIT C

The overall sandshale ratio of this unit is 30:70. For the convenience of

discussion the unit is further divided into two subunits which are discussed below

in stratigraphic order.

4.2.1.1 SUBUNIT C1

At the base of this subunit there exists a 1.5-m-thick slump bed with isolated and

enclosed slump blocks (up to 1 m long) within a muddy matrix of distorted nature.

The lower part of this subunit is not organized and develops two Bouma sequences

of the types Tab and Tac. The upper part shows repetitive occurrence of eleven

medium-bedded Bouma Tabd sequences. Thickness of individual beds varies from

12 to 50 cm. The base of the Ta division (fine sand) is sharp to slightly erosional

with associated rip-up mud clasts and some micro-loading. Almost all the Td

divisions characteristically show convolution and micro-injection structures. The

uppermost two sand beds are amalgamated along an erosional surface. The beds of

this overall thickening-upward subunit C1 show onlap termination onto local basin

relief in the southwards direction. The subunit C1 shows no evidence of major

channeling and is interpreted here as a small-scale depositional lobe in which the

thickening-upward trend results from a compensation cycle (Mutti and Sorrino,

1981) due to the progressive smoothing of subtle depositional relief or the

progressive lateral shifting of the turbidity current axis. The same types of small-

scale (5 to 25 m thick) thickening-upward depositional sandstone lobes with onlap

46

termination within an active margin setting have also been described from the

geological record (Cazzola et al.,)

4.2.1.2 SUBUNIT C2

This subunit is further divided into three divisions: C2a, C2b and C2c.

4.2.1.2.1 DIVISION C2A

This division begins with an even larger (nearly 7 m thick) slump bed containing

dispersed slump blocks within mud. The lower part of this slump bed contains

large blocks (>1 m), some with preserved bedding characteristics, whereas the

upper part (Fig. 4b) is more muddy, enclosing a few small channelized sand bodies

(85 cm long, 20 cm thick) that nearly retain their original attitude. These

channelized sand bodies are filled with very fine to coarse sand with some mud

clasts and coal streaks, and both their lower and upper boundaries are concave with

abrupt thinning of channel margins. The top 2.5 m of division C2a is characterized

by the alternation of massive sandstone beds, with normal grading in the

lowermost sand bed, and wavylenticular fine sandmud beds. This upper part of

C2a (above the slump bed) begins with a distinct scoured base containing

numerous rip-up mud clasts. It contains three sand beds that show an overall

fining- and thinning-upward trend. Two alternative explanations can be offered for

the generation of these wavylenticular beds (mentioned above), which does not

accord with the divisions of the Bouma sequence. The origin of these beds can be

easily comprehended when the top part of division C2a is interpreted as a small

channel levee system. A similar small-scale (6 m thick) association of massive

sand beds and wavylenticular beds from the lower part of the Halifax Formation,

Nova Scotia, has been interpreted as a channellevee system (Stow et al., 1984).

The structural scheme of some parts of the wavylenticular (rarely flaser) beds is

thought to match with the fine-grained turbidite model of Stow and Shanmugam

(1980). Alternatively, some of the tractional characteristics of ripple cross-

lamination (coupled with? bi-directional cross-lamination observed in one

dislocated block) of the wavylenticular beds suggest similarities with the variant

model (bottom-current-reworked turbidite) suggested by Stanley (1987). It is to be

noted that the paleo-Bengal Basin was a deep-water embayment opening to the

south (see tectonic discussion). In this type of basin internal waves and tides

commonly play a major role in bottom-current reworking processes (e.g. McCave

et al., 1980).

47

4.2.1.2.2 DIVISIONS C2B AND C2C

These divisions are self-explanatory in the graphic log (Fig. 3). The random

association of Bouma sequences Ta, Tabc and Tc with debris-flow (mostly mud-

flow and rarely grain-flow types) deposits characterizes these divisions. Two small

channels (50 cm wide, 15 cm deep) with conglomeratic fill were encountered in

division C2b. The orientations of the channel walls are eastwest. Pebbly

mudstone beds in division C2c, formed by cohesive freezing, contain large (up to

40 cm) rolled-up sandstone clasts, and show considerable flowage of matrix mud.

Ta divisions in C2b and C2c are frequently load-casted with some dish structures

and vertical water escape pipes. The sand beds in division C2c pinch out laterally

after about 150 m in a northwards direction. Division C2b is characteristically

dominated by highly carbonaceous, massive, homogeneous, nonburrowed, and

grayish black mudstones of hemipelagic origin. This thick sequence of

hemipelagite indicates basin starvation, probably due to a short-term sea-level rise.

The turbidite beds within the divisions C2b and C2c are thought to be mainly of

unconfined sheet-flow origin. Two down-to-basin synsedimentary, high-angle

normal faults with sand intrusion along the fault planes appear to have affected

parts of divisions C2a and C2b.

48

FIGURE 17: 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(GANI AND ALAM,1999)

49

C H A P T E R 5

5. ECONOMIC GEOLOGY

50

Except the Chittagong hill tract concretions, the economic importance of the

respected area is not mentionable.

FIGURE 18: CONSTRUCTION STONE (CALCAREOUS CONCRETION)

Ferruginous and calcareous concretions, used as construction material, are

available in the investigated area. The boulders are used as raw material for river

bank protection. A large amount is exported to Chittagong port for construction

Sitapahar structure has an excellent possibility to be a hydrocarbon reservoir but

need a detail exploration. Hydrocarbon prospect of the area is yet to be assessed.

51

C H A P T E R 6

6. CONCLUSION

52

The Neogene clastic succession (3000+ m thick) as exposed in the CTFB

represents an overall basinward progradation from deep marine through shallow

marine to continentalfluvial environments deposited within the subduction-related

(oblique subduction) active margin setting of the Indo-Burmese plate convergence.

A high-resolution sequence stratigraphic framework 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 erosion surface (RES), transgressive surface (TS),

marine flooding surface (MFS), and incised valley floor (IVF).

The entire succession can be broadly divided into three composite sequencesC,

B and A, from oldest to youngest, on the basis of two regionally correlatable

pronounced lowstand erosion surfaces.

In composite sequence C, slope apron deep-sea clastics shoals upward into shallow

marine and near-M. Royhan Gani, M. Mustafa Alam / Sedimentary Geology 155

(2003) 227270 267 shore clastics through a thick zone of slope mudstone.

Composite sequence B characteristically depicts tide-dominated open-marine to

coastal depositional systems 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.

The bounding discontinuities identified within the rock succession provide an

insight into the exploration and exploitation strategies for hydrocarbons both in

terms of location and geometry of stratigraphic traps, outlining seal architecture

and flow units.

A tentative stratigraphic scheme incorporating the concept of allostratigraphy is

followed.

53

REFERENCES

Alam, M.M., 1991. Paleoenvironmental study of the Barail succession exposed in northeastern Sylhet, Bangladesh. Bangladesh J.Sci. Res. 9, 25

32.

Alam, M.M.,Gani,M.R.,2004. Fluvial facies architecture in small-scale river systems in the Upper DupiTila Formation, northeast Bengal Basin,

Bangladesh. Journal of Asian Earth Sciences 24 (2004) 225236

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-ll history.Sedimentary Geology

155, 179208.

Boggs.S.,1995,Principles of sedimentology and stratigraphy,Prentice Hall,Inc,Englewood cliffs,New jersey,774p.

Billings,M.P.,1997,Structural geology,3rdedition,Prentice-Hall,India,606p. Evans, P., 1932. Tertiary succession in Assam. Trans. Min. Geol.Inst. India

27, 155 260.

Folk,R.L.,1971,Petrology of Sedimentary Rocks,Hemphills Book Store,Austin,Texas,170p.

Hiller, K., Elahi, M., 1988. Structural growth and hydrocarbon entrapment in the Surma basin, Bangladesh. Circum Pacific Council for Energy and

Mineral Resources Earth Sci.Series, vol. 10, 657669.

Holtrop, J.F., Keizer, J., 1970. Some aspects of the stratigraphy and correlation of the Surma Basin wells, East Pakistan. ECAFE Miner. Res.

Dev. Ser. 36, 143154.

Imam,B.,2005,Energy Resources of Bangladesh,University Grant Commission of Bangladesh,Dhaka,280p.

Johnson, S.Y., Alam, A.M.N., 1991. Sedimentation and tectonics of the Sylhet trough, Bangladesh. Geol. Soc. Amer. Bull. 103,15131527.

Khan, M.R., Muminullah, M., 1980. Stratigraphy of Bangladesh.Petroleum and Mineral Resources of Bangladesh. Seminar and Exhibition, Dhaka, pp.

35 40.

Khan, F.H., 1991. Geology of Bangladesh. The Univ. Press, 207 pp Pettijohn,F.J.,1984,Sedimentary Rocks,1stIndian edition,CBS publishers and

distributors,New Delhi,India,628p.

54

Rahman,J.J.,Faupl,P.,2003. The composition of the subsurface Neogene shales of the Surmagroup from the Sylhet Trough, Bengal Basin,

Bangladesh. Sedimentary Geology 155 (2003) 407417

Saha,S.K.,2008. A study on Microfossils from Eocene sediments of Sylhet trough,North-eastern Bengal Basin, Bangladesh. Dhaka Univ.J.Sci.

56(2):231-236(2008) July

55

GEOLOGICAL MAP