Geological Field Report On Jaintiapur Area, Sylhet, Northeastern Bangladesh

3rd Year
Geological Field Report on Jaintiapur

Area, Sylhet, Northeastern
Bangladesh.
- Report Submitted in Requirement of Partial Fulfillment

of the Syllabus for the 3rd Year B.S.(Honor’s)
Submitted By-
MAINUL ISLAM
Group-06
Roll: 2739
Reg No: 2012-414-880
Date:12-05-2016
Department of Geology,
University of Dhaka.
Abstract
This report aims at geologically characterizing the area, which was investigated by the students of 3rd year,
Department of Geology, University of Dhaka. It deals with the physiography, geomorphology,
structure, stratigraphy, petrography and its interpretation, correlation with standard geologic succession,
economic geology of Janitiapur-Tamabil, Sylhet, Northeastern Bangladesh, along with the facies
analysis and interpretation of paleo-environment of depositional history.
The investigated area lies in between 25°05´ N to 25°11´ N and 92° E to 92°11´15´´ E, latitude and longitude
respectively. It is a hilly region with irregular topography. Highest elevation of the investigated area is 301
feet above the MSL. The drainage pattern of the area is mainly dendritic.
Structurally, the area is an outer reflection of a monocline that trends nearly E-W.
The area exposes both fossiliferous and non-fossiliferous thick sequence comprising of a
succession of limestone, sandstone, shale, siltstone and claystone. Lithostratigraphically, the
sedimentary sequence of the area is divided from bottom to top as Sylhet Limestone Formation,
Kopili Shale Formation, Barail Group, Bhuban Formation, Boka Bil Formation, Tipam
Sandstone Formation, Girujan clay Formation, Dupi Tila Formation, Dihing Formation and
Alluvium. The age range of these formaions is Middle Miocene to Pleistocene. The constituent rocks of the
region are of Sedimentary origin. It appears from the rock records that the depositional
conditions in the basin varied quite considerably and were at time cyclic in nature. Analysis of
different facies associations that observed in different formation and application of different facies model
indicates that the Sediments of different formations were deposited in continental, continental
fluviatile , deltaic, shoreline, shoreline marine.
The petrographic analysis of the collected samples is included in this report which is performed
with a view to define and classify sandstones, to reconstruct their provenance tectonic relations,
to illustrate the diagenatic changes and to find out stratigraphic implications. The limestone
exposed in the investigated area and the gravels carried by the rivers are economically valuable.
Moreover, it is to be mentioned that the adjoining areas have high prospect for hydrocarbon exploration.
Acknowledgement
The author would like to express his deep gratitude and thanks to our honorable and respectable
teacher and our field leader Professor Dr. Muhammed Azizul Huque, Department of Geology, University
of Dhaka, for his systematic work procedure, supervision and guidance during the field work and
valuable suggestion about preparing the field report, specially the sedimentology section.We
acknowledge hereby his valuable contribution on stratigraphy, tectonics and structural part and
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preparation of a geological map of the investigated area. The author expresses his deepest sense of
gratitude as well as heartfelt thanks to Mr. Saiful Islam and S.M. Mainul Kabir, Assistant Professor and
Lecturer of Department of Geology, University of Dhaka for his valuable advice and
extraordinary care.

My thanks are also for the authority of Jaintiapur Upzilla Parishad for the accommodation of our
teachers and students in their rest house . I wish to thank the local people who helped us in arrangement of
transport and in other purposes.
I extend my deepest thanks to the committee of food, transport and first aid for their great service
during the field work and my classmates for their supportiveness and friendly co-operation
during the field work.
My special thanks goes to my group mates for their helpfulness during the field work.
Staffs of the Geology Department and the cooks who were exceedingly helpful in the field also
deserve thanks.
Lastly I want to thank the laboratory and office assistants for their contribution in completion of
the field report.
Finally I have no hesitation to admit the fact that this report would never have been completed
successfully without the valuable contribution of the people mentioned above.
Table of Contents
Introduction ..................................................................................................5
1.1 Purpose and
Scope .........................................................................................5
1.2 Location, Extent and
accessibility ...................................................................6 1.3 Methods of
Investigation ................................................................................7 1.4 Previous
Work................................................................................................9 1.5 Physical
Features..........................................................................................10
1.5.1Topography and Relief...............................................................................10
1.5.2Drainage and Water
Supply .........................................................................12
1.5.3Climate......................................................................................................13
1.5.4Vegetation.................................................................................................13
1.5.5PopulationandCulture………………….........................................................14
2
Regional Geology..........................................................................................16
2.1 Tectonic Setup ............................................................................................16
2.2 Regional Structure .......................................................................................18
2.3 Stratigraphic Setup ......................................................................................19
Structure..........................................................................................................23
3.1 Fold: ..........................................................................................................23
3.1.1Anticlinal Fold ..........................................................................................23
3.1.2 Drag Fold ................................................................................................24
3.2 Faults..........................................................................................................25
3.2.1 Dauki Fault
System ...................................................................................25 3.2.1 Local Fault
...............................................................................................26 3.3
Joint ...........................................................................................................27 3.4
Unconformity...............................................................................................29
3.4.1
Disconformity .......................................................................................................................29
3.4.2 Angular Unconformity ........................................................................................................30
3.4.3 Local Unconformity .............................................................................................................30
Stratigraphy .................................................................................................................................32
4.1 General Stratigraphic
Succession ............................................................................................32 4.2 Lithological
Description ..........................................................................................................33 4.2.2 Attitude
of beds .....................................................................................................................43 4.3
Stratigraphic Correlation..........................................................................................................44
Sedimentology ..............................................................................................................................4
5 5.1 Sedimentary Structures and Features- Paleo current data ................................................45
5.1.1: Depositional
Structure:.........................................................................................................45 5.1.2: Post
depositional deformed structures: .............................................................................46 5.1.3: Post
depositional chemically formed structure ....................................................................47
5.2 Grain size

Analysis ..................................................................................................................48 5.2.1 Sample
No.– 01 (Dupitila Group) ........................................................................................48 5.2.2 Sample
No.–02 (Surma ) ....................................................................................................51 5.3 Study of
Light and Heavy Minerals..........................................................................................54 5.3.1 Slide
No. 1 (Barail Sandstone) ..............................................................................................54 5.3.2
Slide No. 2 (Tipam Group) ....................................................................................................58
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Economic Geology .......................................................................................................................60
Summary and
Conclusion ...........................................................................................................62
List of the Tables…………………………………………………………………………………
54
List of the Pictures……………………………………………………………………………….55
List of the Maps………………………………………………………………………………….56
References ...........................................................................................................;.......................63
CHAPTER ONE
Introduction
The theoretical knowledge is of no value unless it is applied to the field. Fundamental to all geological knowledge
is geologic surveying. It is usually carried out for the systematic examination of any region together
available geologic information.
Geology deals about the earth’s history is not fulfill only by theoretical and laboratory work. So to fulfill the
knowledge about this subject comprehensive practical experience is essential for geology student.
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For this, a geological fieldwork was carried out by the Geology Dept., University of Dhaka in the month of
December 2015 in Jaintiapur-Tamabil, Sylhet, Northeastern Bangladesh by the students of
3rdyear B.S (Honors) [Session-2012-13].
1.1 Purpose and Scope

The principal tasks of the field geology is studying systematic sampling and geological mapping
covering aspects of petrology, sedimentology, stratigraphy and structural geology in order to
develop independent working ability.
The field work is done where the rocks and their necessary structural and stratigraphical features
are easily observed and studied in their natural environmental condition by some methods to
examine and interpret structures and materials at the outcrops.
Fieldwork was done for the following work
1. Producing Geological map.

2. Identification of lithology.
3. Identification Sedimentary structures.
4. Construction of Stratigraphic Column.
5. Sampling.
6. Grain size analysis.
7. Study of major structures and other structural features.
1.2 Location, Extent and accessibility

Jaintiapur is one of the Thana of Sylhet district. It is about 45-kilometer north-east of Sylhet
town. The investigated area is the northeastern part of Sylhet district near the India-Bangladesh
border. It lies roughly between latitudes 25°5´ N to 25° 11´ N and longitudes 92°0´ E to 92°11´
15´´E and cover the survey of Bangladesh Topographic sheets No. 83c/4 and 33c/8 of scale 1t
63,360.The area includes Ballaghat, Sripur , Tama Bil , Jaintiapur and Shari Ghat.
The area comprises about 147 square km, 14 km in the east-west direction from Balla Ghat to Afifanagar and 11
km in north-south direction from Tama Bil to Shari Ghat. Jaintapur is linked with Sylhet toun by a metalled road.
It is about 45-km northeast of Sylhet town.
The metalled road goes up to Jaflong through Sripur and TamaBil. TamaBil is about 60 km northeast from
Sylhet and Jaflong is 45 km from TamaBil. Jaintapur can be reached by bus which goes to Jaflong. The Shari
River is connected with Jaintapur by mud track. A mud track also runs from Jaintapur to Mahismara Bil along the
Nayagang River.The exposures can be found easily along mud track or foot track.
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Sylhet town is well connected with Dhaka City by road, rail and air.
Map 1: location of Jaintiapur
1.3 Methods of Investigation

The investigation was done by traverse method. This work was done along road section. The
investigation was also done along the bank of the rivers.
FIELD INVESTIGATION METHOD
At first a definite place was plotted on the base map and some preliminary information were put
on the map. This place is called the “reference point”. By pacing the distance from one
station to another was measured and plotted every pacing distance on the way. Every place of this work and
well exposures are worked by a station and each station was located on the base map.
To locate the stations and any position of the studying area the Global Positional System (GPS)
method was used for more confirmity.
Structural attitude (dip & strike) of the bed was measured by Clinometer and the readings were noted
down on a field note book. Samples were taken from every exposure and was collected in sample bags.
Lithology was noted down in every station and photograph of notable features were taken.
For microfossil study in the departmental laboratory samples were collected from different
formations.
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The different geological equipments that were used in the survey works are-
1. A base map of investigated area- Locating different outcrops in the investigated area.
2. Clinometers- For measuring the attitude of the beds.
3. Hammer- For breaking the rocks and digging for bedding planes.
4. Pocket lens-to examine the grains
5. HCl acid- Identifying the nature of rocks.
6. Sample bag- For collecting the rocks of the investigated area
7. Field note book- To note the collected data.
8. Measuring tape
9. Haver sack
10. Camera
11. Wooden pencils, color pencils, diagonal scale, pocket knife etc.
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Figure : Clinometer, Hammer, Haversack, Grain size Scale, Pocket lens
LABORATORY METHOD
Determination of depositional condition, provinces and detail mineralogical
composition of the rock was not possible in the field. These examinations were carried out in the
laboratory by analyzing collected samples in the following ways-
Mechanical Analysis
Mechanical analysis of the collected samples were done by sieving and plotting the
obtained data histograms, cumulative curves and other necessary graphs were drawn for each
sample to represent the grain size frequency distribution graphically and the grain size statistical
parameters were read for the interpretation of the depositional condition of the sediments of the
area.
Thin Section Study
The thin sections of the collected samples were studied under microscope for the study of
mineralogical composition of the rocks of the area.
1.4 Previous Work

A large number of exploration works and drilling has been carried out since 1933 in Sylhet.
Burma Oil Company had been the pioneer.
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A geological study of the eastern and northeastern part of the Surma Basin has been carried out
by M. A. Maroof Khan of Geological Survey of Bangladesh during 1964-66. J.F Holtrop and J
Keizer published a correlation of Surma Basin wells in 1960. K.M. Wallid
and Dr.Reimann carried out Palynostratigsaphic analyses of Oligocene outcrop samples. M.

Hoque studied the development of the Surma Basin and its relation of Hydrocarbon
accumulation.
Khan published a geologic map of one-inch equals to two miles scale, which embraced the whole
Tertiary succession of the area.
Haque (1982) developed a scheme of palynologic zone of a Cenozoic succession in the Surma
Basin. He also reviewed the exposed and subsurface stratigraphy of the Surma Basin.
D. K. Guha also investigated the area. Students and teachers of geology study the area every
year.
1.5 Physical Features
1.5.1 Topography and Relief
The investigated area is bounded from west to east by Khashi-Jaintia hill range and is
bordered on the northeast by abrupt scarp of the 4000 to 6000 feet high Shillong plateau. The
region is almost hilly. Numerous low to moderately elevated hillocks are present here.
Map 2: Satellite Map of investigated area
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The average elevation of the area is about 60 to 340 feet. Maximum elevation is found at Lalakhal
area and minimum in northwestern region. The hilly area does not comprise continuous heap of rocks but also
furrowed by numerous vallies giving the landscape of a rugged look. The area embraces two major types of
landforms. The investigated area exhibits moderately hilly topography. The hills having low to moderate
elevation are almost East-West trending. Four prominent hillocks are
Map 3: Contour Map of Bangladesh
found in the studied area. These are locally known as “ Tila‟. The most prominent is the Sonatila in the
northwestern part of the area located on the bank of Dauki River. It is about 214 feet in height.
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Towards east another prominent Tila is located in the Tamabil region with the highest peak of 200 feet.
Sripur Tila marks middle portion of area.
Dupitila is in the southwestern part of the studied area and it should be specially mentioned because this is
the hillock after which the formation of Dupitila named. The height of this hillock is about 301 feet.
The rest of the areas are flat alluvial land. A large plain covering several sq. miles between Jaintiapur and Dupitila
is locally known as Boga Bil, Bally Bil. These bills lie mainly on the valley of the Hari River used for
cultivation during dry season. During the flood these low-lying area totally undergo into water
1.5.2 Drainage and Water Supply

The area is well drained by network of locally important streams. The important rivers of the area are the Hari
River, Dauki River etc. Dauki originating from southeastern part of Shillong plateau encroaches
southeastern part of the Dauki town, India and flows into Bangladesh in north-south direction.
Piang River is the important tributary of Dauki. The Hari River is originated from Khashi-Jaintia hills, flows
southward and enters into Bangladesh near Bagchara. The Ragapani and Nayagang are important
tributaries of Hari River flowing in the central portion of the area. The streams are both structurally and
lithologically controlled and dendritic in pattern. Major streams are relatively fewer and are of
perennial type but minor rivers are large in number and intermittent in type. Many Khal, nala and bil are
also present.
Figure 1: Drainage & Water Supply
River is the important tributary of Dauki. The Hari River is originated from Khashi-Jaintia hills, flows
southward and enters into Bangladesh near Bagchara. The Ragapani and Nayagang are important
tributaries of Hari River flowing in the central portion of the area. The streams are both structurally and
lithologically controlled and dendritic in pattern. Major streams are relatively fewer and are of
perennial type but minor rivers are large in number and intermittent in type. Many Khal, nala and bil are
also present.

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Some marshy lands or swamps exist in the southern side of the area, most of which contain water
throughout the year. These and the ponds provide water for irrigation and domestic purposes.
The dip tube wells and the dug wells are source of drinking water to the people. During the dry
season people suffer from insufficient supply of drinking water as little or no rainfall causes
lowering the water table. But during monsoon water supply becomes adequate as heavy rainfall
lets to the filling of aquifers.
1.5.3 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 Jaintiapur 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 dusty
wind and often cyclonic storm,
(iii) Characterized by pleasant cool and dry weather begins from November and ends in
February.
The average annual rainfall (according to M.A.M KHAN) is more than 150 inches in the area.
1.5.4 Vegetation
The climatic condition of this investigated area is tropical to sub-tropical. A lot of precipitation and sufficient heat
favor the luxuriant growth of evergreen forest. Hillocks and slopes of this area are covered by thick
vegetation.
Important trees of the investigated areas are Shimul, Champa, Chapalish, Teak betel nuts etc. Tall
grasses and Bamboo also grow in this hilly region. High rainfalls, moistures wind together with vast alluvial
plains is responsible for cultivation and dense vegetation.
The total cultivable land is about 63,932 acres. Bills, khals and other lowlying areas are used for
Boro cutivation. Hari River bank was under watermelon cultivation. Orange and pineapple
gardens are present in some areas. This area is suitable for tea cultivation. Huge amount of tea are produced in
this area.
When we investigated this area, we saw a lot of tea gardens. A series of tea gardens are situated in hillocks and
valleys from Jaflong to Afifanagar. A lot of fruits such as jackfruit, papaw, banana are also grown here.
Other seasonal crops like tobacco, oilseeds and vegetables such as pumpkins, beams are also
grown in this area.
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Figure: vegetation

1.5.5 Population and Culture
The total population of this area is about 98.370 (source U.N.O. Office, Jaintiapur) and the
Jaintiapur Upzilla covers an area of 99.98 sq miles. The lifestyle of these people are not so easy, they are living
along the foot of the hills and in plain land. These people are socially and economically undeveloped. They have
no adequate facilities of civilization. Their education rate is 21%. Most of the people depend on
agriculture. Other depends on gardening, fishing, teaching, weaving etc, some of them are
engaged in gravel and sandstone quarrying, trade and commerce. A little percentage of the
population is employed in government services. Most of the people are Muslims, some are
Hindus, Christian and Buddhists. The migratory Khashia and Shaotalis are the tribal people. In
the tea garden there are some Oriyas, Nunayas and other people from Chotto Nagpur plateau,
India who brought before 1947 and settled here. The Khashias have their own language and mainly
Christian. They live in a group of 10-30 families. They work hard and the women work with men.
The people of investigated area have the culture almost similar to the other parts of Bangladesh,
except the tribal people, they have their own culture. The people in this area have their culture
according to their respective religion.
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Hindus influenced the culture of the area during the region of Jaintia raja. So this area has a long
historical background. Once upon a time Jaintiapur was a part of Oohomia Promilla-Empire of
Assam.
Fig: population and culture
CHAPTER TWO
Regional Geology
2.1 Tect onic Setup
The structure and tectonics of Bangladesh and adjoining areas have been studied by a number of
investigators including Bakhtine (1966), Sengupta (1966), Raju (1968),Holtrop & Keizer (1970),
Alam (1972), Desikachar (1974), Grahamet al.(1975), Guha (1978), Khan (1980), Matinet al.(1983),
Banerjee (1984), Le Dainet al.(1984), Alam(1989),Rahmanet al. (1990), Saltetal.(1986).
The overall structure and tectonics of the Bengal Basin is briefly discussed below on the basis of
the results of these investigations. The Bengal Basin is bordered on the north by the Pre-Cambrian Shillong
Plateau and to the west by the Indian Platform. To the east rises the Arakan-Yoma-Naga folded
system, and to the south it plunges into the Bay of Bengal. The Bengal Basin is an
exogeosyncline – that is, one in which thick detrital sediments within the craton were derived from uplift
beyond the margin of the craton. The Bengal foredeep is a part of the exogeosyncline. The Bengal exogeosyncline
14
is one ofthe world‟s largest, and is part of the Bengal Geosyncline. The latter includes the Bengal Basin and the
Bay of Bengal (Alam 1989).
The major structures described below are: 1) shelf zone, 2) hinge zone, 3) Bengal foredeep, 4)
Mobile belt, and 5) Sub-Himalayan Fore deep.
1) Shelf zone is a major tectonic element of Bangladesh lying in the western and shelf. The Indian shield and
Shillong massif are connected by the Rangpur platform. The width of the platform is 100 km.
Here, the slope is fairly smooth according to the seismic data.
The sedimentary deposits of this area form monoclinal beds with dips of 1– 2°. Towards the northern
portion of the platform the plunge of the basement is about 3– 4° and the depth of the basement is
over 2000 m.
Southern slope of the Rangpur platform is gently plunging towards the southeast and extends to
the Calcutta-Mymensing hinge zone. The thickness of sedimentary rocks is increasing.
towards the southeast. The thickness of the sediments over the shelf is about 8000 m and they are
marked by several unconformities.
The basement complex near the western margin of the shelf is marked by a series of buried ridges
and normal gravity faults. The east-west trending Dauki fault separates the stable shelf and the Shillong massif .
The shelf experienced the first marine transgression during the Late Cretaceous. The second major one was in the
Miocene generated by the uplift of the Himalayan and Burman ranges.
2) Hinge zone is a narrow zone of 25 km in width. Here, the monoclinal dip is 5– 6°. The bed dips over 20° in
the hinge - line (Guha 1978). The hinge zone in the northeast seems to be connected with the Dauki fault by a
series of east-west trending faults. It is also marked by deep basement faults probably started with the
breakup of Gondowanaland. Parallel to the hinge zone is the Bengal foredeep, which consist of several smaller
troughs and structural highs.
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Map 4: Tectonic Setup
the Indo - Burman ranges mark the eastern boundary of the Bengal foredeep. The total thickness
of the sediments here is high which exceeds 12,000 m.
According to gravity surveys and drilling data reported by Bakhtine (1966), Guha (1978), Khan
(1980), Matin et al . (1983), the Bengal foredeep can be further subdivided into five sub -zones: 1) Faridpur
trough, 2) Barisal gravity high, 3) Hatia trough, 4) Sylhet trough, and 5) South Shillong shelf
zone.
3) Mobile belt: The eastern side of the Bengal Basin is bordered by a mobile belt known as Tripura - Chittagong
fold belt, which extends north south as part of the Indo - Burmese mobile belt. In Bangladesh, this belt is
represented mainly by the hills of the Chittagong Hill tracts, Chittagong and Sylhet, which
appear to be analogous to the Sub-Himalayan or Siwalik ranges. They are characterized by the
presence of long narrow folds composed of thick sandy shales of the Neogene age, which are
4000– 8000 m thick (Alam 1989). The structure of this belt is of three categories:
a) On the west, they show box like forms, b) the hills of the middle portion are of disturbed asymmetric
structures, and c) those on the eastside have more highly disturbed and complicated structures.
4) The Sub Himalayan fore deep is a continuous east - west Indo – Gangetic geosynclinal belt extending
along the south foot of the Himalayas. Part of it cuts into Bangladesh in the northwest corner. The
16
sediments of this unit include coarse to fine clastics that are derived directly from the Himalayan uplift and are
essentially of fluvial mollasse in character. The north margin of this fore deep is strongly folded and faulted
(Alam 1989).
Tectonically, the structure of the Surma Basin and its adjoining areas are more active which is evidenced by the
subsidence of the Surma Basin is about 30 to 40 ft within the last several hundred years. The Surma Basin is
subsiding at present day at a rate of 21mm per year in central part and 1.5 to 2.5 mm per year in northern part. The
forced responsible for the development of the structure of the area are due to the under thrusting of
the Indian plate towards NNE direction (Paul, 1988).
2.2 Regional Structure

Jaintiapur Structure lies in between two contrasting structural set ups, the uplifting Shillong massif in the north and
the subsiding Surma basin in the south. It is bounded by the Khashi-Jaintia hills and Shillong Massif in the North,
Goyain trough in the south, Atgram anticline structure in the east. The area forms a narrow east-west elongated
strip and is characterized by intermittent swamps between the hills.
The Surma basin, a sub basin of Bengal basin is bounded on the north by the Shillong
plateau, east and southeast by the Chittagong-Tripura fold belt of the Indo-Burman ranges, and
west by The Indian Shield platform. To the south and southwest it is open to the main part of the
Bengal Basin. The published Bouger anomaly map show gradual higher values (negative)
towards the center of the basin. The Aeromagnetic interpretation map by Hunting (1980) indicates a gradual
deepening of basement towards the center of the basin and also reveals subsurface synclinal
features and faults within the basin. Its topography is predominantly flat with some north-south trending ridges
of twenty to several hundred meters elevation present in the north-eastern border. It is actively
subsiding (Johnson & Alam, 1991).
Dauki fault is a E-W trending regional fault lying at the boundary of India-Bangladesh. Geophysical evidence
and outcrop study confirms that the development of the Surma basin is a direct response to the
vertical movement of the Dauki fault that bounds the basin in the north.
2.3 Stratigraphic Setup

The area where study was done is mostly of unfossiliferous, detrital sedimentary rocks except
limestone. Most of the area exposes Tertiary rocks but few places were covered by gravel beds.
The rock represents pronounced lithologic variations both laterally and vertically. Good
exposures outcrop mostly along the river and stream sections. The excellent section along the
Hari River deserves a proposal to be the type section for the Neogene sequence of Bangladesh.
Hardly distinguishable contact between the contrasting lithology, absence of adequate fossil,
together with abrupt and frequent change of facies obscures the accurate dating and classification
of rock types of the area. However the lithostratigraphic classification of sediments of the area has been
established based on gross lithology and also by correlating them with the Tertiary
lithostratigraphic units of Assam in India.
A considerable volume of Tertiary sediments were laid down in this trough of the Bengal basin.
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Simultaneous upliftment of the Shillong massif together with the subsidence of the Surma basin
is responsible for deposition of about 20,000 feet thick sequence of sediments here. The
depositional history of the area was affected by different phases of the tectonic events of the
Himalayan orogeny. The north eastern part of the Indian plate movement along north east
direction caused folding and upliftment of Arakan-Yoma and was responsible for the upliftment
and 250 km. eastward migration o the Shillong massif plates. Consequently the sea regressed and
drained out from Assam- Arakan region. As
a result numerous streams with their tributaries made their appearance. The erosional and depositional
process cumulated by tilting have been continuing to give rise the present physiography of the
studied area.
Eocene was a period of stable slowly subsiding continental shelf condition in the Bangladesh area
and was not yet in influenced strongly by the continental collision between India and Asia that began in late
Palaeocene.
During middle to late Eocene time, the area was marked by an extensive marine transgression caused by
conspicuous basin ward subsidence; the whole area was under the sea. The Sylhet limestone was
deposited under open, marine warm climatic condition. Deposition of highly fossiliferous limestone
indicates shallow, marine environment.
This kind of deposition was followed by the accumulation of a very limited thickness of Kopili
shale which is indicative of changing environment from shallow marine to a clay receiving basin.
Such environmental change occurred as the collision event began to replace the area.
During Oligocene epoch different parts of the basin was devoted to marine regression. The rate
of rising of the Himalaya increased. As a consequence at the very beginning of this period
flowing of many streams initiated. These rivers carried huge amounts of sediments and deposited leading to the
development of the formation of a delta. Lithological characteristics of Barail group of rocks
suggest delta to near-shore environment of deposition.
Sea was withdrawn from the investigated area after the deposition of the Barail, evidenced by a
regional unconformity represented by laterite band between the Barail and the lower part of
Surma. There might have prevailed tropical to sub-tropical and humid climatic condition under which iron-
rich laterite formed during prolong exposure of Barail. During Miocene epoch the major orogenic
upliftment of Himalaya took place. The sand, silt and clay particles carried and deposited by numerous
streams caused the development of the mega delta. Gradually the delta advanced to the south as the
shoreline retreated. Under such environment deposition of Surma group of rocks took place. The grain
size and shape infers low energy condition of deposition and long transportation.
During late Miocene to early Pliocene time, southward movement of delta still continued.
Subsequently land environment prevailed beyond it. Tipam sediments were deposited under
continental fluvial environment in high energy condition. Massive bedding and poor to moderate
bedding suggest rapid deposition. Right after formation of Tipam Girujan
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clay was deposited under lacustrine environment in a locally developed lake in the fluvial system.
During Pliocene Dupitila was deposited under continental fluviatile environment. Presence of
quartz granules signifies that Dupitila was deprived of, winnowing process of marine transgression and
regression.
Map 5: Geological Group Formation Exposed in Bangladesh
19
After deposition of the Dupitila the area has undergone through a major tectonic activity.
Movement along the Dauki fault caused the tilting of the entire tertiary strata. The area suffered
upliftment up to Pleistocene period which is evident by the presence of the conglomeratic beds.
The gravels were carried by the Pleistocene rivers and deposited horizontally over the incline
Tertiary succession as bed load deposits. The gravels represent time gap between the deposition
of Dupitila formation and the recent alluvium.
The alluvium deposits suggest that the investigated area again went under the sedimentation and
fluvial system during sub-recent to recent time.
CHAPTER THREE
Structure
20
The investigated area and adjoining area lie within the Sylhet trough of Bengal Foredeep. Sylhet
trough is situated south of Shillong Massif and corresponds with vast low land of Surma basin. The northern limit
of this subsiding trough is bounded by Dauki fault. Faulting along the Shillong shelf zone coincided with
rapid subsidence of Surma basin is during Miocene and later time. The basin has been
encountered two short periods of erosion and non-deposition. One is due to the time of uplift and
folding in the east and continued subsidence in west at the end of Oligocene; the other is due to the uplift of
Shillong plateau accompanied by faulting in late Miocene and later time. This area dominantly shows E-W
structure due to the relationship to the fault zone that trends E-W along the border of the Shillong
Massif. Numerous major and minor geological structures have been found in the studied area.
The dominant structures of the investigated area are Folds, Faults,Unconformity and Joints.
3.1 Fold:
3.1.1 Anticlinal Fold

The studied area represents an asymmetrical E-W to NW-SE trending anticlinal major F1 type
fold. The axis runs from Dauki to Sripur and truncates against Dauki fault to the east. There is some superposed F2
type fold on the major fold. The trend of the superposed fold is NE-SW.
Our work is mainly carried out in southern flank as the northern flank is not well exposed in Bangladesh, which
continues in Meghalaya of India. Only in Sripur and Tamabil - Jaflong road cut area, little part of
northern limb is exposed in Bangladesh.
Amount of dip in southern flank varies from 20° to 50° towards south, in Dupitila and Shari River almost vertical
beds are found. The northern flank dips more steeply.
Evidence favoring the concept of anticlinal fold:

1. From Sripur-Tamabil to Dauki fault bed dips in opposite direction. It is not only in few faulted block as
illustrated by some authors assuming homocline.
2. Older rocks are found in the axis of the fold. If it is not an anticlinal fold, younger rocks like Tipam and Dupitila
is usual to found in Jaintia – Tamabil road cut section.
21
Figure 2: Micro Fold
3. At Rangapani river section, we found bed dips at different direction such as NE, SE and SW
following a distinctive phase. So, we can tell that they were occurred at the later phase of superposed
folds. It makes easier in deducing anticlinal folds.
3.1.2 Drag Fold

Prominent drag folds are observed in Shari river section near Affifanagar and Lalakhal Tea estate. Drag
folds in the silty shale bed are formed due to movement of the competent sandstone bed in the opposite
direction with each other. Drag folds are also observed at Nayagang river section.
Figure 3: Drag Fold
3.2 Faults
The major regional fault in the area is the Dauki fault. Besides, few other local faults were observed in different
formations. In fact our studied area comprises a zone extensively affected by movement of Dauki fault
system, which is responsible for the present physiography of the area. The local faults were evidenced
by displacement of different formations.
22
Figure 4: Micro Fault
3.2.1 Dauki Fault System
The Dauki fault is a series of faults that trends east-west and it is considered as the westward
continuation of the Naga-Disang thrust system. The Shilong Massif acts as the up thrown block.
The fault is poorly exposed and gravity data suggests that it is a deep-seated fault. The 5 km
wide zone of faulting can be characterized by extensive fracturing and steep dips. The Dauki
fault was initially described by (1964 Evan's) as a tear or transcurrent fault. But Murthy, 1969
explain that it is an up thrust along which there had been differential vertical movement of
basement block. Recent field studies in the southern Shilong plateau indicates that the Dauki
fault comprises a system of east-west trending faults, each of which changes in attitude upward
from nearly vertical at lower levels to high to low angle reverse at higher level. The Dauki fault
is exposed along the southern margin of the Shilong plateau for about 170 km from Jadukata
River ( ( lat. 250 14/ 30// N; long. 910 13/ 00// E) in the west of haflong (lat. 24 0 44/ 00// N;
long.930 02/ 30// E) in the East where it pass through the haflong-dia=sang thrust.
West of Jadukata River, the Dauki fault has been traced for a distance of 70 km below the alluvium by
geophysical methods up to Dalu (lat. 250 13/ 00// N; long . 910 13/ 00// E ).
Evidence those support the existence of the fault are as follows:

i) Sudden topographic changes and high relief difference was noted within few hundred meters.
Lower elevation in Bangladesh and higher elevation in India. The present height of the Shilong
plateau is due to repeated uplift along the Dauki system of faults over a long span of time. According
to Evans (1964), the amount of structural relief on both sides of the Dauki faults range up to 13,000
meters.
ii) Faulting is also evidenced by the presence of fault breccias and mylonites in the fault zone in the Sylhet
Limestone.
i i i ) Irregularities in the attitude of beds .
iv) The straight course of the Dauki River.
23
v) Terraces at river bank indicate major faulting.
vi) Omission of Tura Sandstone formation also indicates the presence of Dauki fault.
3.2.1 Local Fault
The force that activated the Dauki fault also activated some other faults through which small blocks of
rocks such as the Sylhet limestone might be up thrown in to the surface but this is merely a speculation as
evidences are rare.
A thrust Fault developed between Kopili and Barail at lat. 250 10/ 33.9// N and long. 920 03/ 05.2// E
along Sripur Tamabil road.
Minor normal faults were observed at latitude 25°06ʹ37.5˝ N and longitude 92°03ʹ05.2˝E.
Fig : Fault
3.3 Joint
As joints are typical associated structure of faults, the joints observed in our investigated area can be assumed as
the result of Dauki fault movement. Moreover if we review the regional setting of the area it can be seen that
multidirectional forces with variable intensities were responsible for
the development of the investigated area. As a consequence, joints having different
orientation and extension are scatteredly available throughout the investigated area. Generally
joints are the common features formed in more resistant part of sandstone, siltstone and silty shale of different
formations.
24

Figure 6: Joints
The location of some remarkable joints is mentioned: -

- Large scale vertical to sub-vertical joints is found in the Sylhet limestone near the Dauki River
- Highly jointed Barail sandstone is observed along the road-cut section in Sripur.
- Some inclined joints with low dip are also found in the Surma Group in Tetulghat.
- Very closely spaced parallel joints seem to be Strike joints are found in DupiTila
sandstone near the area.Sharighat
- Numerous sub vertical joints were observed in shale of Surma Group at Rangapani
section.

25
Fig: joint
26
3.4 Unconformity
Another recognizable structural feature, unconformity is a surface of erosion or non-deposition
that separates younger strata from older strata. In the working area two types of unconformity have been
recognized.
3.4.1 Disconformity
A major unconformity exists between Barail and Surma group. Near the eastern bank
of Nayagang stream (in the north of Jaintiapur) at lat. 25 0 08/ 25.4// N and long 920 7/ 17.9// E.It
is represented by a thin band of lateritic conglomerate, as observed in the field.Laterite also found in
Mahishmara. The band of laterite is of red to dark brown color and is composed of pebbles, cobbles,
granules and other ill sorted materials. The nature of the unconformity is non depositional i.e. the laterite might
have formed by the hardening of the weathering products of the Barail group of rocks (hematite cemented
sandstone) during prolong exposure before deposition of the Bhuban sediments. According to the
field investigation it can be categorized as disconformity. This is because the laterite band was continuous
along the contact and parallel to the strike of both of the formations.
FIG: Laterite bed found bwtween Surma and Barail

Group
3.4.2 Angular Unconformity

Gravel beds overlie most of the hillocks. This gravel bed makes an unconformity with the Barail and
Surma group of sediments. The underlying beds are inclined and the recent gravel beds are laid
27
horizontally over it, suggest an angular unconformity. In the field such kind of unconformity is noticed at Sonatila
and near the dry Rangapani river section in Sripur, where the gravel beds lie over the Barail sandstone. In the
Uzaninagar village near Jaintiapur, gravel beds are found to make angular unconformity with the Surma Group.
Figure 8: Angular unconformity
3.4.2: Local Unconformity

A local Unconformity is observed at the bank of Lalakhal between Tipam and Surma group.
Lithology change suggests the unconformity between these two groups.
28
Figure9 : Unconformity Between Dihing and Barial
CHAPTER FOUR
29
Stratigraphy
4.1 General Stratigraphic Succession
Table 1: Stratigraphic succession of the studied area [Paul, 1988 and field investigation]
Thickness in
meters
Age Group Formation Lithology
Recent Alluvium Unconsolidated sand, silt and clay.
Sonatila gravel Well rounded, smooth cobble to boulder

sized gravel with high sphericity.
Pleistocene
Coarse grained, yellowish sandstone with

subordinate Claystone containing quartz
Pliocene Dupitila 300+
pebbles.
Sandstone
Girujan Clay Whitish colored, massive sticky Claystone

containing ferruginous specks sparsely.
400
Mio-Pliocene Tipam Tipam Yellowish brown, medium to coarse grained

sandstone cross bedded sandstone.
1200
Gray colored, moderately hard, fine to very

fine grained sandstone with subordinate
Miocene Surma 1600
bluish gray laminated shale.
Pink colored, fine to very fine grained, very

well sorted Sandstone with subordinate
Oligocene Barail Renji 1180
laminated shale.
Kopili Black, fissile, splintery shale with high clay

shale content.
50+
Sylhet limestone Light colored, very hard and compact,

massive fossiliferous limestone.
Eocene Jaintia 35+
The stratigraphy of the area has been differentiated into a number of formations. Sylhet
limestone formation is found as the oldest in the normal sequence.The normal sequence of the
studied formation given below according to the law of superposition.
1) Alluvium
2) Dihing Formation
3) Dupi Tila formation.
4) Girujan clay.
5) Tipam sandstone.
6) Surma group
30
7) Barail sandstone
8) Kopili shale
9) Sylhet limestone
The names of the formations are established by Evans (1932) for the Tertiary
successions of Assam. Although it is difficult to correlate formations separated by hundred of
kilometer without the support of palaeontological data and also because of frequent facies
changes.
4.2 Lithologic description
The oldest rock exposed in Bangladesh is Tura Sandstone of Early Eocene age in
Takerghat area in Sunamganj district in Sylhet division.But this are not found in our investigated
area. The Sylhet Limestone Formation is the oldest rock exposed in our studied area, the middle
unit of the Jaintia Group of sediments. They are overlain by, from older to younger, the Barail
Group, Surma Group, Tipam Group, Dupi Tila and Dihing (Sonatila Gravel) sediments.
Sylhet Limestone
The term “Sylhet limestone” as a rock unit was first used by Khan (1963).The formation is
exposed on the east bank of the Dauki River near the Bangladesh-Meghalaya border. The
outcrop forms an inlier surrounded by recent deposits and rock of the Barail group.It is the oldest
(Eocene) rock of the investigated area.
The block has been subjected to severe forces which produced the crush breccia.
Slickenside preserved in Limestone.The grey, fossiliferous Limestone offers a variety of fossils

from disk shaped Discosyclina to elongate lens shaped Nummulites. The hard limestone is highly
jointed and fractured.The brecciated limestone occurs due to large Dauki Fault. The assemblages
of dominantly large microfossils indicate a shallow water, continental shelf zone and a clear
water environment is also documented by total lack of planktonic remains(Sarwar,1679). A fault
found in our investigated area due to the Dauki fault.
Though much work had not been done in the area, the similarity of nummulites
assemblage with the Eocene of Garo Hills and Assam makes it likely that this unit is of Middle
Eocene age.
31
Figure 10: Nummilitic Fossils Present in Limestone
Kopili Shale
The name of Kopili shale was given by Evans (1932) to the beds forming the upper stage
of the Jaintia group after the Kopili river of Garo Hill in India. It gives a minor outcrop
only on the west bank of the Rangapani River. It is dark gray to black coloured, very much
fissile, thickly badded to paper laminated, highly jointed shale. Interbedded sandstone with
argillaceous matrix is present. It is conformably overlain on the Sylhet limestone. Base of the
Kopili shale is not seen. The top of the Kopili shale are exposed beside Rangapani River in
Sripur tea garden. It also found on the east bank of the Dauki River near the Bangladesh-
Meghalaya border. The approximate thickness is about 30m. Fossil evidence suggests that the
Kopili shale is of late Eocene age (Evans 1932).
32
Figure 11: Kopili Shale Formation
Barail Group:
The Oligocene is represented by the Barail group, named by Evans (1932) after the Barail Range
in nearby Assam, India where the unit has its type locality.
The Barail Group (Renji Formation) is well exposed in Mahishmara , Sonatila and
near Sripur which is about 3.5 miles NW of Jaintiapur. Most of the exposures are covered by
Holocene deposits. The Barail forms high ridges than the adjacent Surma Group .The thickness
found in our investigated area is about 1160m.
In the neighboring Assam Barail sediments are divided into an arenaceous Laison Formation, an
argillaceous Jenum Formation, and an arenaceous Renji Formation. In Bangladesh most of the
Barail is deeply buried. A series of outcrops in the area between Jaintiapur in the east and the
Dauki Nala in the west was described by Khan (1978) as Jenum Formation. Reimann (1983)
who mapped the north-eastern fringe of the Surma basin.
33
The river Nayagang and exposures along the Jaintiapur – Tamabil Road provide a fairly
good section of the Jenum Formation.
The Jenum Formation composed of sandstone, siltatone and siltyshale. The sandstone is mainly
pink in colour , weathered to light yellow and gray, very fine to medium grained sometimes
crossbedded and thin to thick badded Argillaceous and Ferroginous materials. The block jointed
sandstone is found in many places. On the east bank of the Dauki River near the Bangladesh-
Meghalaya border the block jointed sandstone confiused with bedding plane.
The carbonaceous matters are found in sandstone in east bank of Dauki River and in sonatils
Chara.
The siltstone is light gray to yellow in colour. It is thin to thick bedded, fairly hard and compact
and well jointed.
The Unconformity boundary between Bhuban and Barail group is represented by thin
bands of Lateritic Conglomerate containing which is well exposed in Nayagang river section.
Small blocks of Laterite are widely spreaded on the hilltops and slopes of the hillocks situated on
the northern side of the horticulture garden of the investigated area. Laterite blocks are normally
formed on the surface of the iron-rich residual deposits. It is porous, reddish brown color, has a
hard protective ferruginous incrustation on the exposed surface, which is generally irregular and
rough. Sometime it is pisolitic . The pisolites have a concentric structure
and are cemented together by ferruginous and clay minerals. The vesicles of Laterite are filled up
with secondary mineral.
On the basis of lithology this formation is corrected with that of Renji of Assam instead
of Jenam which appear to be absent in this area. In Assam, the Renji formation is considered to
be of Oligocene age (Evans, 1932) on the basis of fossils.
34
Figure: Barail group
Surma Group:
The Surma Group has been named after the Surma seires of Assam, India (Evans 1932). The
sediment of the Surma group unconformably overlies the Barail Group. Good exposures of this
unit were observed in the east of Jaintiapur and in the Shari River. The change from the Barail
Group to the Surma Group rather sharp and is marked by the decrease of the interbedded
sandstone in shales and siltstones and the general predominance of argillaceous material. The
Surma Group is made up of bedded, laminated siltstone, shale, silty shale, claystone and
sandstone, mud clast found in Surma Sandstone in Afifanagar . Although some sandy shale is
also present. Shale of this unit is
profusely jointed and fractured and even small fault were observed in Tetulghat. It is also
exposed near Jaintiapur i.e. Afifanagar, Ujaninagar, Kamarbari, and East Gaurishankar. Most of
the sediments are covered by recent alluvium. The dip direction of the beds of this formation is
south-west and the amount of dip ranges from 42° to 50°. The thickness found in our
investigation is about 1650m.
It is composed of yellowish gray sandstone, bluish gray shale, sandy shale, and siltstone.
Sandstone is fine to medium grained, subangular and moderately sorted. The sandstone is hard
35
and is resistant to weathering and forms the cliffs. It shows micro cross
lamination,lamination,trough cross bedding and wavy bedding .
The shale is bluish gray in colour and weathered to gray and yellowish gray. The shale is well
laminated, hard and jointed .The Surma Group is generally subdivided into two formations
namely the Bhuban and the Bokabil but in the field it is difficult to distinguish between the two
units and there subdivision becomes impractical. The contact of Surma Group with the overlying
Tipam Formation is conformable. The Surma Group is overlain unconformably by Dihing
Formation at latitude 25°07/59.7//, longitude 92°07/51.1//. This is an angular unconformity.The
contact between Surma Group and Tipam Sandstone found in Afifanagar.
Figure: surma Group
Tipam Group:
The Tipam Group has been named after the Tipam Series (Mallet, F.R., 1876) given after the
Dihing River in Assam, India. The Tipam Group is subdivided into two formations from older to
younger- the Tipam Sandstone and the Girujan Clay.
Tipam Sandstone: The name has been used after the Tipam hills in Assam, India (Mallet
1876). The formation constitutes the lower part of Tipam Group and is conformably overlain by
Girujan Clay and the contact found in the eastern bank of the Shari River is gradual. The river
Shari gives an excellent exposure.
The top of Tipam Sandstone Formation form a conformable contact with the Girujan Clay
Formation is exposed at latitude 25°06/15.7//, longitude 92°08/54.4// in the bank of Shari River
36
where it consists of alternation of usually bedded to thick bedded and also laminated , fine
sandstone and mudstone. The base of Tipam Sandstone is exposed also in the Shari river bank
conformably overlying the Surma Group at the station- 9 (25°06 /36.0//N latitude, 92°10/48.9//E
longitude) where it consists of brown, fine to medium grained, massive sandstone. Overall
lithology of Tipam Sandstone consists of gray-brown to pale-gray, coarse-grained, cross bedded,
and massive sandstone. Intercalations of gray shale, conglomerate horizons, pebbles, laterite bed,
mud ball , wood fragments and petrified trunks, coal lenses also occur.
Figure: Tipam Sandstone
Girujan Clay:
The name has been given after the Girujan Clay stage of Tipam Series in Assam, India.Top
of this formation is exposed at latitude 25°05/54.5//, longitude 92°08/40.6//E by the Shari River
bank having a conformable contact with the overlying Dupi Tila Formation and base of this
formation is exposed at latitude 25°06/15.7//N, longitude 92°08/54.4//E having a conformable
contact with the underlying Tipam Sandstone Formation. The formation develops conformably
and gradationally from the underlying Tipam Sandstone Formation. It entirely consists mainly of
gray to bluish gray clay and mottled clay.
37
Figure: Garujan Clay
Dupi Tila Sandstone:

The Dupi Gaon is the Type locality of the Dupi Tila Formation. The formation is
exposed latitude 25°05’38.8”N, longitude 92°07’04.0”E at Sharighat behind the Sharighat

Primary school. The lithology is dominantly sandstone and siltstone with interbeds of claystone.
At latitude 25°05’50.8”N, longitude 92°08’39.0”E, the bedding plane is not well defined. The
lithology is dominantly fine to coarse grained, brown to yellowish brown, cross bedded
sandstone containing wood log/coal and quartz pebbles, clay galls etc. The Dupi Tila Sandstone
Formation conformably overlies the Girujan Clay Formation.
Except fossil wood no other fossils are identified in Dupi Tila Formation. In Assam it is
considered to be Mio-Pliocene in age (Lexique, 1957).
38
Figure: Dupitila sandstone
Dihing Formation:
The Dihing formation of Pleistocene age has unconformable contact with the Surma Group at
latitude 25°07’59.7”N, longitude 92°07’51.1”E, Uzaninagar and with Barail Group at latitude
25°10’42.2”N, longitude 92°00’58.5”E,Sonatila Chara . The formation consists of yellow and
gray, medium-grained, occasionally pebbly sandstone and clayey sandstone with interbeds of
mottled clay, and boulders of grainitic rock. The rocks are in most part poorly consolidated.
39
Figure: Dihing Formation
Alluvium:
Unconsolidated, loose material brought down by rivers and deposited in its beds of
alluvial fans or weathered material. Alluvium consists of sand, silt, clay in various proportions.
River born alluvium are mainly sand, and coarse grained material and weathered alluvium are
consists mostly of clay and silt. They cover various rock formations unconformably and of
Recent in age.
4.2.2 Attitude of beds

Table 2: Attitude of Beds
Station No. GPS Attitude of Bed
40
Station 01 N25°10’55.2’’
E92 °01’4.5’’
Station 02 N25°10’53.3’’ DD- S60°E, AD- 32°
E92°01’5.9’’ DD- S52°E AD- 52°
Station 03 N25°10’53.5’’ DD- S10°E
E92°01’5.9’’ AD- 20°
Station 04 N25°10’42.4’’ DD- N40°W
E92°02’3.3’’ AD- 50°
Station 05 N25°10’40.4’’ DD- N76°W
E92°02’3.9’’ AD- 13°
Station 06 N25°10’46.9’’ DD- Due North
E92°02’1.9’’ AD- 28°
Station 07 N25°10’51’’ DD-N30°W
E92°01’59.5’’ AD- 15°
Station 08 N25°08’26.5’’ DD- Due North
E92°07’20.4’’ AD- 42°
Station 09 N25°08’28.26’’ DD- S20°E, AD- 40°
E92°07’21.83’’ DD- Due South ,AD - 45°
Station 10 N25°7’30.5’’ DD-S40°W
E92°11’15.1’’ AD- 35°
Station 11 N25°07’17.85’’
E92°11’15.62’’
Station 12 N25°06’25.10’’ DD- S25°W
E92°10’5.6’’ AD- 40°
Station 13 N25°06’47.2’’
E92°10’55.9’’
Station 14 N25°06’48.038’’
E92°10’54.92’’
Station 15 N25°06’24.94’’
E92°10’32.71’’
Station 16 N25°06’21.04’’
E92°10’333.686’’
Station 17 N25°06’21.83’’
E92°10’32.78’’
Station 18 N25°05’49.93’’ DD- N11°W
E92°07’4.19’’ AD- 45°
Station 19 N25°06’44.51’’ DD- S10°E
E92°06’37.911’’ AD- 80°
Station 20 N25°05’44.49’’
E92°06’37.5’’
Station 21 N25°07’59.87’’ DD- S35°W
E92°07’51.67’’ AD- 45°
4.3 Stratigraphic Correlation
41
stratigraphic correlation of the area of investigation and its correspondence to classification of Tertiary rock
Stratigraphy units of Assam are given below:
Table 3:Stratigraphic correlation of Surma Basin with Assam Valley & Chittagong Hill Tract
Age North Eastern part of Assam Valley, India Mathur Eastern part of Bangladesh,
Surma Basin, Sylhet And Evans, 1964 Chittagong – Chittagong Hill
Tracts
Group Formation Series Stage Group Formation
Holocene Alluvium Alluvium and Alluvium
high level
terraces
Late Miocene DupiTila DupiTila Upper
to Mid Sandstone Sandstone DupiTila
Miocene
Lower
DupiTila
Mid Miocene Tipam Girujan Clay Tipam Girujan Clay Tipam Girujan Clay
Tipam Tipam Tipam
Sandstone Sandstone Sandstone
Early Surma Bokabil Surma Bokabil
Miocene Bhuban Bhuban
Oligocene Barail Renji Barail Jenum
Eocene Jaintia Kopili Shale Jaintia Kopili Shale
Sylhet Sylhet
Limestone Limestone
Tura Theria
Sandstone (not
found in
investigated
area
CHAPTER FIVE
Sedimentology
5.1 Sedimentary Structures and Features- Paleocurrent data
42
sedimentary structures are large-scale features of sedimentary rocks, that are best studied in out
crop in naked eye or hand lens. Different types of sedimentary structure that are encountered in the investigated
area are given below:
5.1.1: Depositional Structure:
(Stratification, Bed forms or Bedding plane markings)
i)Bedding or Lamination: Bedding or Lamination define stratification. Bedding is produced by
change in pattern of sedimentation, may be defined as change in sediment grain size
color,composition.
ii) Ripples: Ripples are developed in sand size sediments; sandstones .The ripple marks in the
studied area are current ripples. These are characterized by length less than 60cm and ripple
index less than 5(mostly 8-15). Unidirectional current produces these, so they are asymmetric
with a step lee side and gentle toss side.
iii) Cross stratification: It is an internal sedimentary structure of many sedimentary rocks and
consists of an angle to the principle bedding. It is common in study area.
a) Cross lamination and cross bedding: Cross lamination forms either a single set or many set with one
bed. On size alone stratification is divided into cross lamination and cross bedding where the set height is
less than 6cm and greater than 6cm respectively. Tabular cross stratification is straight crested and whereas
trough cross stratification is curved crested.
Figure 19: Cross Strata
b) Flaser & lenticular bedding: Flaser bedding is where there sand contains mud streaks usually in
troughs.Lenticular bedding is where mud dominates and cross laminated sand occurs
in lenses, both are found in SURMA GROUP in Shari river section.
43
Figure 20: Lenticular Bedding
iv) Mud cracks: Shrinking cracks with polygonal structures in fine-grained sediments through
desiccation and dewatering on exposure. It was found in Shari river section.
v) Rain spots: Rainfalls are small depression with rims, forms through the impact on soft exposed surface
of sediments. Sometimes they may be asymmetrical and indicate wind direction. Found in Shari
river section.
5.1.2: Post depositional deformed structures:

i) Load cast: Load is formed through differential sinking of one bed into another. Load casts are
common on soles of sandstones beds overlying mud cast, occurring as bulbons structure and may
be on the way to become ball and pillow structure found at Shari river section in Surma group.
ii)Ball and Pillow Structure: Ball and Pillow Structures are found in shale or mudstone consists of
hemispherical or kidney shaped sandstone which has originated from overlying sandstone layer and have
sinked into the softer rock as a result of loading. Found in rocks of Surma group at Shari river section.
iii)Soft rock deformation folding: The soft rock shale and mudstone be deformed in such a way the folds, in
cases overturned of penecontemporaneous type have formed.
iv) Slumping: Slump structure may involved many sedimentation units are commonly faulted; typically occur in
mudstones and sandy shales, less commonly in sandstones. It is observed in the rocks of Barail group in Tamabil
area.
5.1.3: Post depositional chemically formed structure
Concretions: Concretions probably the most common kind of sedimentary structures, formed by
precipitation of mineral matter around some kind of nucleolus such as a shale fragment, masses range
from peripheral to pipe shaped, common in sandstones and shale. Found in Tipam sandstone, DupiTila formation,
Barail group etc. in different section.
Sand vein: Vein in sandstone may be formed during earthquake. By releasing energy pore pressure
developed tremendously, then water & grain fluids become same type & injected through weak zone.
It clue about earthquake.
44
Figure 24: Sand Vein
Black magic: In the time of limestone deposition, heavy minerals are deposited on each beds of
limestone.
Fault breccia : At the great depth due to faulting formed angular grains powder etc. At high temperature and
pressure, it forms milonite.
Trace fossil: It is used as environmental detector. The rate of sedimentation is known form it.
Protrusion: Gas bubbles create protrusion.
LABORATORY ANALYSIS
The samples that were collected from the different sections are examined in the laboratory by
means of grain size analysis-sieving method, study of rock in thin section under microscope and
micropalaeontological study.
5.2 Grain Size Analysis:

Petrography is the study of rocks in thin section by means of a petrographic microscope. Petro
graphic study is done by grain size analysis, thin section study and occurrence of microfossils.
Grain size analysis is useful for classification, depositional history analysis etc. Thin section study is useful for
heavy mineral analysis, composition and textural analysis and microstructure analysis. Study of occurrence
of microfossil is useful to determine whether the sediments are deposited by transportation or
deposited in situ.
Grain size analysis data

Sample no: 01
Name of the sample: Dupitila sandstone.
Time of sieving: 20min
Amount of sample: 100gm
Diameter in mm Weight retain in gm Weight percent in gm Cumulative weight

percent in gm.
0.50 3.51 3.51 3.51
0.25 39.20 39.20 42.71
0.125 29.13 29.13 71.84
45
0.063 13.16 13.16 85.00
PAN 14.99 14.99 99.99
Total = 100 gm
Sieve loss= (100 – 99.99) gm
= 0.01
Total coarse sand (wt %) = 36.99
Total fine sand (wt %) = 62.15
Histogram of Dupitila Sandstone

45
40
35
Weight percent Retained
30
25
20
15
10
0
0.5 0.25 0.125 0.063 PAN
Grain Size in phi Scale
Figure 4.1: Histogram of Dupitila sandstone
Description of histogram:
46
By using grain size analytical data the histogram has been drawn. Here an ordinary
arithmetic graph paper is used. Histogram is drawn by taking weight percent retain in the vertical
scale verses grain size in mm in the horizontal scale. The histogram shows a bimodal grain size
distribution having modal class between 1.00 mm to 0.5 mm and 0.25mm to 0.125mm is called
primary maxima and secondary maxima respectively.
Bimodal distribution of grain size may result due to any of the following regions-
 Abnormal variation in depositional energy.
 Lack or abundance of certain grain size in the source materials.
 Mixing of materials from two or more sources.
 Different modes of deposition.
 Improper sampling.
Cumulative Weigth Percentage (gm)

120
100
80
Cumulative Weigth Percentage

(gm)
60
40
20
0
>.50 0.25 0.125 0.063 <.063
Figure 4.2: Cumulative curve of Dupitila sandstone
Cumulative curve:
47
By taking cumulative weight percent in vertical scale and grain size in mm in the
horizontal scale the cumulative curve has been drawn. The figure gives an “S” shaped curve
from which quartile and percentile have been determined in order to calculate the grain size
parameter according to Trask Method.
25 percentile (25p) or 1st quartile (Q1) = 0.72 mm.

50 percentile (50p) or 2nd quartile (Q2) = 0.177mm.
75 percentile (75p) or 3rd quartile (Q3) = 0.096875mm.
Median (Md) = 50p = 0.175mm.
Co-efficient of sorting, So = Q1/Q3 = 0.72/0.096875 = 2.68
Co-efficient of skewness, Sk = Q1Q3/Md2 = (0.7x 0.096875)/(0.177)2 = 2.21
From Trask Method,
1. Median (Md) is 0.175 indicates that the velocity of the transporting media was weak which could
transport fine sand sized particles probably by saltation and suspension.
2. Sorting (So) is 2.68 indicates well sorted. It means that, depositional media get enough time to
well sorted.
3. Skewness (Sk) is 2.21 which means that the coarse admixture exceeds the finer admixture and the
distribution of the grains are termed as finely skewed.
Sample no: 02
Name of the sample: Surma Group.
Time of sieving: 20min
Amount of sample: 100gm
Cumulative weight
Diameter in mm Weight retain in gm Weight percent in gm
percent in gm
0.50 5.51 5.51 5.51
0.25 37.20 37.20 42.71
0.125 37.13 27.13 69.84
0.063 13.15 13.15 82.99
PAN 16.99 16.99 99.98
Total = 100 gm
Sieve loss= (100 – 99.98) gm
= 0.02 gm.
48
Total medium sand (wt %) = 0.25
Total fine sand (wt %) = 18.745
Histogram of Surma Sandstone

40
35
30
Weight Percent Retained
25
20
15
10
0
0.5 0.25 0.125 0.063 PAN
Grain Size in phi Scale
Figure 4.3: Histogram of Tipam sandstone
Description of histogram:
By using grain size analytical data the histogram has been drawn. Here an ordinary
arithmetic graph paper is used. Histogram is drawn by taking weight percent retain in the vertical
scale verses grain size in mm in the horizontal scale. The histogram shows a bimodal grain size
distribution having modal class between >1.00 mm and 0.125mm to 0.0625mm is called primary
maxima and secondary maxima respectively.
Bimodal distribution of grain size may result due to any of the following regions-
 Abnormal variation in depositional energy.
 Lack or abundance of certain grain size in the source materials.
49
 Mixing of materials from two or more sources.
 Different modes of deposition.
 Improper sampling.
Cumulative Weigth Percentage (gm)

120
100
80
Cumulative Weigth Percentage

(gm)
60
40
20
0
>.50 0.25 0.125 0.063 <.063
Figure 4.4: Cumulative curve of Tipam sandstone
Cumulative curve:
By taking cumulative weight percent in vertical scale and grain size in mm in the
horizontal scale the cumulative curve has been drawn. The figure gives an “S” shaped curve
from which quartile and percentile have been determined in order to calculate the grain size
parameter according to Trask Method.
25 percentile (25p) or 1st quartile (Q1) = 1.5mm
50 percentile (50p) or 2nd quartile (Q2) = 0.9mm.
75 percentile (75p) or 3rd quartile (Q3) = 0.68mm.
50
Median (Md) = 50p = 0.9mm.
Co-efficient of sorting, So = Q1/Q3= 1.5/0.68 = 2.206
Co-efficient of skewness, Sk = Q1Q3/Md2 = (1.5x 0.68)/ (0.9)2 = 1.259
From Trask Method,

1. Median is 0.9mm indicate that the velocity of transporting media was weak, which could
transport fine sand sized particles, probably by saltation and suspension.
2. Sorting is 2.206, which indicates well sorted. It means that depositional media get enough time to
well sorting.
3. Skewness is 1.259, where positive means coarse admixture and the distribution of grain are
termed as finely skewed.
Thin Section Analysis:

Four slides have been prepared to examine under microscope, the textural and
mineralogical composition of Sylhet limestone, Barail sandstone, Tipam sandstone and siltstone
that were not properly identified within hand specimen with pocket lens. Heavy minerals were
also identified by studying this thin section under microscope.
The cross polar observation under microscope and the finding of petrographic analysis is given
in the following papers:-
Name of the slide: Barail sandstone

Framework grains:
The percentages of the framework grains of the rock sample are as follows:
1) Quartz—more than 80%
2) Feldspar—about 7%
3) Muscovite—about 5%
4) Other dark colored minerals about 1%
Matrix:
There is little or no matrix.
Cement:
The cementing material is Hematite, which gives the rock sample its reddish colour.
Pore space:
There is approximately no pore space because of well cementing.
51
By means of usual composition, texture, determination from the thin section analysis indicate the
grain size of the rock sample is very fine to fine, sub angular to sub rounded shaped and well
sorted.
Figure: Barail Sandstone under Plane Polarized Light
52
Figure: Barail Sandstone Under Cross Polarized Light
Name of the slide: Siltstone
Framework grains:
1) Quartz- about 75%
2) Feldspar- about 3%
3) Chlorite- about 3%
4) Biotite- about 2%
5) Dark colored minerals- about 2%
6) Lithic grains- about 1%
Matrix:
The matrix is formed of clay size materials, which are about 5% of the total rock.
Cement:
The cementing material at the rock sample is about calcite or calcareous and argillaceous which
comprise about 10% of the total rock.
53
Pore space:
There is no pore space within the rock sample.
Fig : Siltstone under Plane Polarized Light
Fig 4.12: Siltstone Under Cross Polarized Light
Name of the slide: Tipam sandstone

Framework grains:
1) Quartz- about 55%
54
2) Plagioclase feldspar- about 20%
3) Muscovite- about 5%
4) Other dark colored minerals- about 2%
5) Amount of chert/lithic grains- about 3%.
Matrix:
Silt and clay sized materials forms the matrix, which is about 15%.
Cement:
The cementing material is ferruginous which gives the rock yellowish colour.
Pore space:
There is about 1% pore space within the rock sample.
Texture determination that is grain size shape and sorting from thin section by means of usual
composition is indicative of medium to fine grained angular to sub angular and moderately
sorted.
Fig : Tipam Sandstone Under Cross Polarized Light
55
Fig : Tipam Sandstone Under Cross Polarized Light
CHAPTER SIX
Economic Geology
56
There is no commercially exploitable deposit in the investigated area. Economically important
mineral deposits are very rare in Bangladesh. Tertiary rock of the investigated area does not
possess any economically important mineral deposits.
Sandstone:
The sandstones of jenam, Tipam and Dupi Tila formation are very loosely cemented. These sandstones do
not satisfy the minimum standard requirements to be used as building materials.
Gravels and Boulders:

The gravels transported by the Dauki River and Rangpani River are most economically important geological
element of the area. Rounded to sub-rounded boulders. Pebbles are composed mainly gneiss,
quartzite, granite. Hundred of tones this hard rock is transported daily by tracks country wide for using in building
of road, multistoried structure, tail way balasts etc. The estimated of this hard sock is about one million
cubic feet (khan M, 1978). The gravels are generally 3 to 8 feet long and 2 to 4 feet thick. In Dauki rivers its
extends is about one and a half mile long, fifty feet wide and about 4 feet thick but in Rangpani river its
extent is small.
Figure 25: Gravels and Boulders
Sylhet Limestone:
A very small faulted block of the Eocene Sylhet limestone is exposed in the investigated area at the eastern
bank of the Dauki River. The Chhatak cement factory uses Sylhet Limestone and produces cement of
excellent quality comparable to any cement of the world. This factory has quarried out all Limestone.
Now its reserve is too small to warrant further exploration. It provides the local people with lime making
and other domestic usages.
57
Figure 26: Stone Crushing Industry
Sand :

A huge amount of sand is transported by the Dauki River and Rangapani River, which are being
used throughout the country as building material. This sand known as the Sylhet sand is excellent
in quality.
Gas and Oil :
In Bangladesh, there are 22 natural gas fields and 1 oil (well-7 of Sylhet gas field) field has been
discovered so far. Most of the gas field and oil field occurred in the Surma basin. This gas and oil occurred in
sandstone reservoir of Bhuban and Boka Bil formation. Surma group of Mio-pliocene age and lie at depth
ranging from surface. These gas and oil reservoirs are situated in the folded belt with gentle anticlinal
fold forming traps (Prof. Badrul Imam, 1984). Sothere has a possibility to find hydrocarbon in the investigated area
for which detailed geological investigation of the Jaflong- Lalakhal area is highly desired.
Bangladesh Atomic energy suggests that a small amount of Uranium (Ur) might be found in the
investigated area.
CHAPTER SEVEN
Summary and Conclusion
The investigated area is situated on the Surma basin of the mobile belt of Bengal basin. The area is a hilly region.
The average elevation of the area is about 100 feet to 125 feet above MSL. The area is drained by numerous rivers,
streams, khal etc. those flow in more or less in meandering pattern and carries huge amount of sediments.
The prominent structures of the area is a faulted anticline, a major fault named Dauki fault which runs along the
northern margin of the area and has been considered as the west ward continuation of the Naga – Disang thrust
58
fault system. Besides these few small scale faults, folds and local unconformity have been identified in the
area.
The investigated area consists of thick sequence of sediments from Eocene to Recent. Because of
the exposures of these sedimentary rock units this area is called the geological museum of Bangladesh. These
sediments are divided into several Groups which are from bottom to top Jaintia, Barail, Surma
and Tipam, thjese Group of sediments are overlained by Dupi Tila , Sonatila Gravel and
alluvium. The principal rock types are limestone, sandstone, siltstone, shale, and conglomerate.
Fossils are found only in the Sylhet limestone. The total thickness of these rock units are about
4110 meters.
The depositional environment of these rocks is shallow marine, marine, fluvial and lacustrine.
List of Tables
Table 1: Stratigraphic succession of the studied area_______________________________32
Table 2: Attitude of Beds
___________________________________________________________43
Table 3: Statigraphic
Correlations____________________________________________________ 44
List of Maps
59
Map 1: Location Map
__________________________________________________________________ 6 Map 2:
Satellite Map of investigated area _____________________________________________10
Map 3: Contour Map of
Bangladesh___________________________________________________ 11 Map 4: Tectonic
Setup ________________________________________________________________ 17 Map 5:
Geological Group Formation Exposed in Bangladesh _________________________ 21
References
-- Ahmed, A. : Oligocene stratigraphy and sedimentation in the Surma basin, Bangladesh, M. S.
Thesis, 1983.
-- Dean, Dennis R. (1992). James Hutton : the history of geology. Ithaca: Cornell University
Press. pp. 30–83. ISBN 9780801426667.
-- Alam, M.M. et al (2003): An overview of the sedimentary geology of the Bengal Basin in relation
to the regional tectonics and Basin fill history, Sedimentary Geology, 155, pp 227-270.
-- Gould, Stephen Jay (2001). Time's arrow time's cycle : myth and metaphor in the discovery
of geological time (11th print. ed.). Cambridge, Mass: Harvard University Press. p. 61. ISBN
9780674891999.
-- Billings,M.P: Structural Geology.
-- Geological note book wrote during field work.
-- Evans, P. (1933): Tertiary succession in Assam, than, geol. Inst. India, v-27.
60

-- Evans , P. (1964): The tectonic frame work of Assam, Geol. soc. India, Jour; vol.5, pp – 80 – 8 5 .
-- Dalrymple, G. Brent. : The age of the Earth. Stanford University Press, 1994. pp. 14, 43
Dalrymple, 1994, p. 14-17.

-- Stacey, Frank D. (2000). "Kelvin's age of the Earth paradox revisited". Journal of
Geophysical Research 105 (B6): 13155–13158. Bibcode:2000JGR...10513155S.
doi:10.1029/2000JB900028.
E.P. Evans: "The Authorship of the Glacial Theory", Issue 368, July 1887. Accessed on
February 25, 2008.
-- Hiller, K. and Ellahi, M., 1984: Structural development and hydrocarbon entrapment in the development in the
Surma Basin, Bang. (Northwestern Indo – Burman Fold Belt), 5th offshore South East Asia conf. session-
6, logging, Singapore.

-- Holtrop, J. F. and Keizer, j. (1970): Some aspect of the Stratigraphy and correlation of the Surma
Basin Wells, East Pakistan, ESCAFE minerals Resources Development Series,no.6.
-- Imam, M.B, 2005: Mineral Resources of Bangladesh.
-- Khan,F.H : Geology of Bangladesh
-- Khan, M.A (1978): Geology of the eastern and north eastern part of sadar subdivision Sylhet
district in Bangladesh. Record of G. S. B. vol.2, part-iv.
-- Mannan,A (2002): Stratigraphic evolution and Geochemistry of the Neogene Surma Group,
Surma Basin, Sylhet, Bangladesh, Dept. of Geology, University of Oulu.
-- Llana-Funez, S.; Marcos, A.; Bastida, F. (20 March 2014). "Deformation structures and
processes within the continental crust: an introduction". Geological Society, London, Special
Publications 394 (1): 1–6. Bibcode:2014GSLSP.394....1L. doi:10.1144/SP39

-- Monsur M. H. : An Introduction to the Quaternary Geology of Bangladesh. A complimentary
research of IGCP 347. Quaternary Stratigraphic Correlation of the Ganges-Brahmaputra
Sediments (1995) .
-- Paul, H.P (1988) : Structure and tectonics of north Eastern part of the Surma Basin, Sylhet, Bang.
M.sc Thesis, Geology Dept. Dhaka University.

Reimann, K.U.(1993) : Geology of Bangladesh.

Wallid, K. M. (1982) : Paleostratigraphy of the Tertiary Sediments as Exposed along the Jaflong
Tamabil road cut section in Sylhet district, Bang. Unpublished M. Sc. Thesis, Geology Dept.
Dhaka University.
61
-- Ball, Philip (2009). "In Retrospect: the physics of sand dunes". Nature 457 (7233): 1084.
Bibcode:2009Natur.457.1084B. doi:10.1038/4571084a.
62

Geological Field Report On Jaintiapur Area, Sylhet, Northeastern Bangladesh

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3rd Year

Geological Field Report on Jaintiapur

- Report Submitted in Requirement of Partial Fulfillment

5.2 Grain size

1.1 Purpose and Scope

Fieldwork was done for the following work

1. Producing Geological map.

1.2 Location, Extent and accessibility

Map 1: location of Jaintiapur

1.3 Methods of Investigation

FIELD INVESTIGATION METHOD

2. Clinometers- For measuring the attitude of the beds.

4. Pocket lens-to examine the grains

5. HCl acid- Identifying the nature of rocks.

6. Sample bag- For collecting the rocks of the investigated area

7. Field note book- To note the collected data.

Thin Section Study

1.4 Previous Work

and Dr.Reimann carried out Palynostratigsaphic analyses of Oligocene outcrop samples. M.

1.5 Physical Features

1.5.1 Topography and Relief

Map 2: Satellite Map of investigated area

Map 3: Contour Map of Bangladesh

1.5.2 Drainage and Water Supply

Figure 1: Drainage & Water Supply

Fig: population and culture

2.2 Regional Structure

2.3 Stratigraphic Setup

Map 5: Geological Group Formation Exposed in Bangladesh

3.1.1 Anticlinal Fold

Evidence favoring the concept of anticlinal fold:

3.1.2 Drag Fold

Figure 3: Drag Fold

3.2.1 Dauki Fault System

Evidence those support the existence of the fault are as follows:

i i i ) Irregularities in the attitude of beds .

iv) The straight course of the Dauki River.

3.2.1 Local Fault

The location of some remarkable joints is mentioned: -

FIG: Laterite bed found bwtween Surma and Barail

3.4.2 Angular Unconformity

Figure 8: Angular unconformity

3.4.2: Local Unconformity

Recent Alluvium Unconsolidated sand, silt and clay.

Sonatila gravel Well rounded, smooth cobble to boulder

Coarse grained, yellowish sandstone with

Girujan Clay Whitish colored, massive sticky Claystone

Mio-Pliocene Tipam Tipam Yellowish brown, medium to coarse grained

Gray colored, moderately hard, fine to very

Pink colored, fine to very fine grained, very

Kopili Black, fissile, splintery shale with high clay

Sylhet limestone Light colored, very hard and compact,

4.2 Lithologic description

Slickenside preserved in Limestone.The grey, fossiliferous Limestone offers a variety of fossils

Figure: surma Group

Figure: Tipam Sandstone

Dupi Tila Sandstone:

exposed latitude 25°05’38.8”N, longitude 92°07’04.0”E at Sharighat behind the Sharighat