geological field report on jaintiapur-tamabil area, sylhet
DESCRIPTION
Report Submitted in Requirement of Partial Fulfillment of the Syllabus for the 3rd Year B.S.(Honor's) in Department of Geology, University of DhakaTRANSCRIPT
-
Geological Field Report on Jaintiapur-
Tamabil Area, Sylhet, Northeastern
Bangladesh. Report Submitted in Requirement of Partial Fulfillment of the Syllabus for the 3rd Year B.S.(Honors)
Submitted By-
Pritam Saha Group-03 (Dauki)
Roll-2319
Date-19.07.2012
Department of Geology,
University of Dhaka.
-
i
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 2505 N to 2511 N and 92 E to 921115 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 an anticline 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.
-
ii
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.
Our sincere appreciation goes to Dr. Syed Humayun Akhter, Professor of Department of
Geology, University of Dhaka for his incomparable inspiration which enhanced our spirit during the field work. We acknowledge hereby his valuable contribution on stratigraphy, tectonics and structural part and preparation of a geological map of the investigated area.
The author expresses his deepest sense of gratitude as well as heartfelt thanks to Md. Mostafizur Rahman, 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 and a local school. 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.
-
iii
Table of Contents Abstract ..............................................................................................................................................i
Acknowledgement..............................................................................................................................ii
Table of Contents .............................................................................................................................. iii
List of Tables ......................................................................................................................................v
List Of Illustrations ..............................................................................................................................v
List of Figures .................................................................................................................................... vi
List of Maps....................................................................................................................................... vi
Introduction .......................................................................................................................................1
1.1 Purpose and Scope ....................................................................................................................1
1.2 Location, Extent and accessibility ...............................................................................................2
1.3 Methods of Investigation ...........................................................................................................2
1.4 Previous Work...........................................................................................................................3
1.5 Physical Features.......................................................................................................................4
1.5.1 Topography and Relief.........................................................................................................4
1.5.2 Drainage and Water Supply .................................................................................................6
1.5.3 Climate...............................................................................................................................7
1.5.4 Vegetation..........................................................................................................................7
1.5.5 Population and Culture........................................................................................................8
Regional Geology................................................................................................................................9
2.1 Tectonic Setup ..........................................................................................................................9
2.2 Regional Structure ...................................................................................................................11
2.3 Stratigraphic Setup ..................................................................................................................12
Structure..........................................................................................................................................15
3.1 Fold: .......................................................................................................................................15
3.1.1 Anticlinal Fold ...................................................................................................................15
3.1.2 Drag Fold ..........................................................................................................................16
3.2 Faults......................................................................................................................................16
3.2.1 Dauki Fault System ............................................................................................................16
3.2.1 Local Fault ........................................................................................................................17
3.3 Joint .......................................................................................................................................17
3.4 Unconformity..........................................................................................................................18
-
iv
3.4.1 Disconformity ...................................................................................................................18
3.4.2 Angular Unconformity .......................................................................................................18
3.4.3 Local Unconformity ...........................................................................................................19
Stratigraphy .....................................................................................................................................20
4.1 General Stratigraphic Succession ..............................................................................................20
4.2 Lithologic Description ..............................................................................................................21
4.2.1 Lithologic Log of Studied Section ........................................................................................27
4.2.2 Attitude of beds ................................................................................................................34
4.3 Stratigraphic Correlation..........................................................................................................36
Sedimentology .................................................................................................................................40
5.1 Sedimentary Structures and Features- Paleocurrent data ..........................................................40
5.1.1: Depositional Structure:.....................................................................................................40
5.1.2: Post depositional deformed structures: .............................................................................42
5.1.3: Post depositional chemically formed structure ..................................................................42
5.2 Grain size Analysis ...................................................................................................................43
5.2.1 Sample No. 01 (Barail Group) ..........................................................................................44
5.2.2 Sample No. 02 (DupiTilla) ................................................................................................48
5.2.3 Sample No. 03 (Surma Group) .........................................................................................52
5.3 Study of Light and Heavy Minerals............................................................................................56
5.3.1 Slide No. 1 (Barail Sandstone) ............................................................................................56
5.3.2 Slide No. 2 (Surma Group) .................................................................................................56
5.3.3 Slide No. 3(DupiTilla Formation).........................................................................................57
5.4 Provenance .............................................................................................................................57
5.4.1 Condition of deposition: ....................................................................................................58
Economic Geology ............................................................................................................................60
Summary and Conclusion ..................................................................................................................63
References .......................................................................................................................................64
Appendices
Appendix-A Stereographic Solution for Structural attributes data (Fold,Fault,Joint)
Appendix-B Graphic Litholog (Grain Size, Sedimentary Structure and Broad Lithology)
Appendix-C Geological Map
-
v
List of Tables Table 1: Stratigraphic succession of the studied area__________________________________________ 20
Table 2: Attitude of Beds _________________________________________________________________ 34
Table 3:Stratigraphic correlation of Surma Basin with Assam Valley & Chittagong Hill Tract__________ 36
Table 4:Stratigraphic correlation between the stratigraphy of Surma Basin and the studied sections __ 38
Table 5: Terminology of bed thickness______________________________________________________ 40
Table 6:Grain Size Analysis Data of Barail Group _____________________________________________ 44
Table 7: Barail Sandstone ________________________________________________________________ 47
Table 8: Grain Size Data of DupiTilla Sandstone ______________________________________________ 48
Table 9: DupiTilla _______________________________________________________________________ 51
Table 10: Grain Size Data of Surma Group ___________________________________________________ 52
Table 11: Surma Group __________________________________________________________________ 55
List Of Illustrations Illustration 1: Lithological log of Shari river section ___________________________________________ 27
Illustration 2: Lithological log of Shari River Section ___________________________________________ 28
Illustration 3: Lithological Log of Mahishmara-Nayagang Section ________________________________ 29
Illustration 4: Lithological log of Rangapani River Section, Sripur Tea Garden ______________________ 30
Illustration 5: Lithological log of Tamabil Road Section & Dauki River Section _____________________ 31
Illustration 6: Lithological log of Tamabil Road Section & Dauki River Section _____________________ 32
Illustration 7: Lithological log of Sonatila Chara (Tamabil Road Section & Dauki River Section) _______ 33
Illustration 8: Histrogram of Barail Sandstone________________________________________________ 45
Illustration 9: Cumulative Curve of Barail Sandstone __________________________________________ 46
Illustration 10: Histogram of DupiTilla Sandstone_____________________________________________ 49
Illustration 11: Cumulative Curve of DupiTilla Sandstone ______________________________________ 50
Illustration 12: Histogram of Surma Group __________________________________________________ 53
Illustration 13: Cumulative Curve of Surma Group ____________________________________________ 54
-
vi
List of Figures Figure 1: Drainage & Water Supply _________________________________________________________ 6
Figure 2: Micro Fold _____________________________________________________________________ 15
Figure 3: Drag Fold ______________________________________________________________________ 16
Figure 4: Micro Fault ____________________________________________________________________ 16
Figure 5: Local Fault _____________________________________________________________________ 17
Figure 6: Joints _________________________________________________________________________ 17
Figure 7: Joints _________________________________________________________________________ 18
Figure 8: Angular unconformity ___________________________________________________________ 18
Figure 9: Unconformity Between Dihing and Barail ___________________________________________ 19
Figure 10: Nummilitic Fossils Present in Limestone ___________________________________________ 21
Figure 11: Kopili Shale Formation __________________________________________________________ 22
Figure 12: Contact Between Barail and Kopili Shale ___________________________________________ 22
Figure 13: Barail Sandstone _______________________________________________________________ 23
Figure 14: Contact Between Surma & Barail (Laterite) _________________________________________ 23
Figure 15: Contact between Tipam & Surma _________________________________________________ 24
Figure 16: Girujan Clay ___________________________________________________________________ 25
Figure 17: DupiTilla Sandstone ____________________________________________________________ 25
Figure 18: Dihing _______________________________________________________________________ 26
Figure 19: Cross Strata ___________________________________________________________________ 41
Figure 20: Lenticular Bedding _____________________________________________________________ 41
Figure 21: Flaser Bedding ________________________________________________________________ 41
Figure 22: Iron Concretion________________________________________________________________ 42
Figure 23: Hummocky Structure ___________________________________________________________ 42
Figure 24: Sand Vein ____________________________________________________________________ 43
Figure 25: Gravels and Boulders ___________________________________________________________ 60
Figure 26: Stone Crushing Industry_________________________________________________________ 60
List of Maps Map 1: Location Map _____________________________________________________________________ 2
Map 2: Satellite Map of investigated area ____________________________________________________ 4
Map 3: Contour Map of Bangladesh_________________________________________________________ 5
Map 4: Tectonic Setup ___________________________________________________________________ 10
Map 5: Geological Group Formation Exposed in Bangladesh ___________________________________ 13
Map 6: Mineral Resources map of Bangladesh _______________________________________________ 61
-
1
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
earths 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. For this, a geological fieldwork was carried out by the Geology Dept., University of Dhaka in the month of
December 2011 in Jaintiapur-Tamabil, Sylhet, Northeastern Bangladesh by the students of 3rd year B.S (Honors) [Session-2010-11].
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.
-
2
8. Interpretation of Geological history.
9.Identification of economic importance.
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 255 N to 25 11 N and longitudes 920 E to 9211
15E 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.
Sylhet town is well connected with Dhaka City by road, rail and air.
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.
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
Map 1: Location Map
-
3
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.
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.
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.
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 196466. 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.
-
4
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. 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 2: Satellite Map of investigated area
-
5
Map 3: Contour Map of Bangladesh
-
6
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. 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.
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
Figure 1: Drainage & Water Supply
-
7
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.
-
8
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.
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.
-
9
CHAPTER TWO
Regional Geology
2.1 Tectonic 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), Graham et al. (1975), Guha (1978), Khan (1980),
Matin et al. (1983), Banerjee (1984), Le Dain et al. (1984), Salt et al. (1986), Alam (1989), Rahman et al. (1990). 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 is one
of the worlds 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 northwestern
portion of it. The margin has a northeast-southwest trend along which the basement complex slopes steeply downward to form a hinge zone. The thickness of the sediments over the shelf is
about 3000m and they are marked by several unconformities (Alam 1989). The northern portion is known as the Rangpur platform and the southern is the Bogra 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 12. Towards the northern portion of the platform the
plunge of the basement is about 34 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
-
10
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 56. 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.
3) The Bengal
foredeep, which is a large elongated
trough, occupies the
vast area between the hinge-line and
Arakan-Yoma-Naga folded
system. This is the deeper part of the Bengal
Basin where the basement is
deeply subsided here and the subsidence is
directly related with the uplift
of Himalayas-Burmese mountain chain.
It is about 450 km wide in the
south of Bangladesh and narrowing
towards the northeast. The
Basement is probably 1215
km deep. The
folded belts of Map 4: Tectonic Setup
-
11
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.
4) 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 40008000 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.
5) 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 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
-
12
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.
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.
-
13
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 characterist ics 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
Map 5: Geological Group Formation Exposed in Bangladesh
-
14
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 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.
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.
-
15
CHAPTER THREE
Structure
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.
Figure 2: Micro Fold
-
16
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.
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.
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 Evan's (1964) 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. 240 44/ 00// N; long. 930 02/ 30// E) in the east where it pass into the Haflong-Disang thrust.
Figure 3: Drag Fold
Figure 4: Micro Fault
-
17
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. iii. Irregularities in the attitude of beds . iv. The straight course of the Dauki River.
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 250637.5 N and longitude 920305.2 E.
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.
Figure 5: Local Fault
Figure 6: Joints
-
18
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 Sharighat area.
- Numerous sub vertical joints were
observed in shale of Surma Group at Rangapani section.
3.4 Unconformity Another recognizable structural feature, unconformity is a surface of eros ion 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. 250 08 25.4 N and long. 920 07 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.
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 horizontally over it, suggest an
Figure 7: Joints
Figure 8: Angular unconformity
-
19
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.
3.4.3 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.
Figure 9: Unconformity Between Dihing and Barail
-
20
CHAPTER FOUR
Stratigraphy
4.1 General Stratigraphic Succession
Table 1: Stratigraphic succession of the studied area
Age Group Formation Lithology Thickness
(m)
Recent Alluvium Unconsolidated sand, silt and clay
Pleistocene Dihing Well rounded, smooth cobble to
bolder sized gravel with high sphericity
Pliocene
Mio-Pliocene
Tipam
DupiTila Sandstone
Coarse grained, yellowish sandstone with subordinate clay stone containing quartz pebbles
Girujan Clay Whitish color massive sticky clay
stone containing ferruginous specks sparsely
432
Tipam Sandstone Yellowish brown, Medium to
course grained cross bedded sandstone
1004
Miocene Surma Alteration of gray colored,
moderately hard, fine to very fine grained sandstone and bluish gray laminated shale
1309
Oligocene Barail Renji Pink colored, medium to coarse grained, very well sorted
sandstone with subordinate laminated shale
950
Eocene Jaintia Kopili Shale Black, fissile, splintery shale with high clay content
110
Sylhet Limestone Light colored, very hard and
compact, massive fossiliferous Limestone
25
[Paul, 1988 and field investigation]
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 is given below according to the law of superposition.
-
21
1. Alluvium 2. Dihing formation
3. DupiTila formation 4. Girujan clay
5. Tipam sandstone 6. Surma group 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 paleontological 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, DupiTila 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
breccias. 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 brecciaed 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 plank tonic remains(Sarwar,1979). 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.
Figure 10: Nummilitic Fossils Present in Limestone
-
22
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 colored,
very much fissile, thickly bedded to paper laminated, highly jointed shale. Inter bedded 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).
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) mapped the north-eastern fringe of the Surma basin.
The river Nayagang and exposures along the Jaintiapur Tamabil Road provide a fairly good section of the
Figure 11: Kopili Shale Formation
Figure 12: Contact Between Barail and Kopili Shale
-
23
Jenum Formation.
The Jenum Formation composed of sandstone, siltstone and silty shale. The sandstone is mainly pink in color, weathered to light
yellow and gray, very fine to medium grained sometimes cross bedded and thin to thick bedded
Argillaceous and Ferruginous 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
confused with bedding plane. The carbonaceous matters are found in sandstone in east bank
of Dauki River and in Sonatila Chara.
The siltstone is light gray to yellow in color. 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.
Surma Group
The Surma Group has been named after the Surma series
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
Figure 13: Barail Sandstone
Figure 14: Contact Between Surma & Barail (Laterite)
-
24
Barail Group to the Surma Group rather sharp and is marked by the decrease of the interbedded sandstone in shale and siltstones and the general predominance of argillaceous material. The
Surma Group is made up of bedded, laminated. siltstone, shale, silty shale, clay stone and sandstone, mud clast found in Surma Sandstone in Afifanagar. Although some sandy shales are
also present. Shale of this unit is profusely jointed and fractured and even small fault were observed in Tetulghat. It is also exposed near Afifanagar, Jaintiapur. 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, sub angular and moderately sorted. The sandstone is hard 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 2507/59.7//, longitude
9207/51.1//. This is an angular unconformity. The contact between Surma
Group and Tipam Sandstone found in
Afifanagar.
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.
Figure 15: Contact between Tipam & Surma
-
25
The top of Tipam Sandstone Formation form a conformable contact with the Girujan Clay Formation is exposed at latitude 2506/15.7//, longitude 9208/54.4// in the bank of Shari River
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 (2506/36.0//N latitude, 9210/48.9//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.
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 2505/54.5//,
longitude 9208/40.6//E by the Shari River bank having a conformable contact
with the overlying DupiTila Formation and base of this formation is exposed at latitude 2506/15.7//N, longitude
9208/54.4//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.
DupiTila Sandstone
The Dupi Gaon is the Type locality of the Dupi Tila Formation. The formation is exposed latitude 250538.8N,
longitude 920704.0E at Sharighat behind the Sharighat Primary school. The lithology is dominantly sandstone and siltstone with inter beds of clay stone. At latitude
250550.8N, longitude 920839.0E, 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 gall etc. The DupiTila Sandstone Formation conformably overlies the
Girujan Clay Formation.
Except fossil wood no other fossils are identified in DupiTila
Formation. In Assam it is considered to be Mio-Pliocene in age (Lexique, 1957). Organic specks are also present in
DupiTila sandstone.
Figure 16: Girujan Clay
Figure 17: DupiTilla Sandstone
-
26
Dihing formation
The Dihing formation of Pleistocene age has unconformable contact with the Surma Group
at latitude 250759.7N, longitude 920751.1E, Uzaninagar and with Barail Group at latitude 251042.2N, longitude
920058.5E,Sonatila Chara. The formation consists of yellow and gray, medium-grained,
occasionally pebbly sandstone and clayey sandstone with inter beds of mottled clay, and boulders of granitic rock. The rocks are in
most part poorly consolidated.
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.
Figure 18: Dihing
-
27
4.2.1 Lithologic Log of Studied Section
4.2.1.1 Shari River Section
Illustration 1: Lithological log of Shari river section
-
28
Illustration 2: Lithological log of Shari River Section
-
29
4.2.1.2 Nayagang-Mohismari Section
Illustration 3: Lithological Log of Mahishmara-Nayagang Section
-
30
4.2.1.3 Rangapani River Section, Sripur Tea Garden
Illustration 4: Lithological log of Rangapani River Section, Sripur Tea Garden
-
31
4.2.1.4 Tamabil Road Section & Dauki River Section
Illustration 5: Lithological log of Tamabil Road Section & Dauki River Section
-
32
Illustration 6: Lithological log of Tamabil Road Section & Dauki River Section
-
33
Illustration 7: Lithological log of Sonatila Chara (Tamabil Road Section & Dauki River Section)
-
34
4.2.2 Attitude of beds Table 2: Attitude of Beds
Station
No.
Latitude Longitude Attitude
Camp 250545.9 N 920704.8 E
1 250548.3 N 920703.7 E 70, S20E
2 250516.7 N 920839.1 E
3 250554.5 N 920840.6 E Contact between DupiTila and Girujan Clay
4 250617.0 N 920853.3 E Contact between Girujan Clay and Tipam
Sandstone
5 250622.8 N 920859.1 E Laterite in Tipam Sandstone
6 250620.9 N 921052.8 E 75, S34W; 38, S14W; 74, S14W
7 250633.7 N 921038.7 E
8 250638.7 N 921050.1 E Contact between Surma Group and Tipam Sandstone
9 250640.4 N 921049.6 E 50, S17W
10 250644.0 N 921049.1 E
11 250645.6 N 921051.9 E 45, S8W
12 250645.6 N 921003.9 E 45, S9W
13 250645.2 N 921053.9 E
14 250647.5 N 921056.8 E
15 250653.2 N 921057.9 E
16 250654.3 N 921056.7 E 49, S15W
17 250654.9 N 921057.4 E
18 250651.5 N 921116.4 E
19 250717.7 N 921115.2 E 46, S28W
20 250718.2 N 921111.0 E
21 250723.6 N 921125.6 E
22 250731.0 N 921115.1 E 46, S24W; 33, S26W; 39, S20W
23 250759.7 N 920751.1 E 46, S26W; Gravel bed over Surma
24 250808.7 N 920759.0 E 48, S34E
25 250815.6 N 920806.3 E 58, S20E; 59, S32E
26 250824.8 N 920805.2 E 40, S26E
27 250825.5 N 920757.4 E 25, S20E; Laterite contact between Surma
and Barail
28 250834.9 N 920749.8 E 49, S25E; 45, S23E
29 250859.2 N 920755.7 E 30, S20E
30 250856.5 N 920754.2 E 48, S12W
31 250806.1 N 920729.5 E 46, S20E
32 250833.0 N 920718.2 E 32, S28E
33 250827.5 N 920723.3 E 23, S8E; 28, S6E
34 250825.2 N 920717.7 E 44, S20E
35 251036.4 N 920434.4 E 20, N20W
36 251042.8 N 920439.9 E 19, N22E
-
35
37 251054.8 N 920422.2 E 30, N80E; 30, N76E
38 251041.9 N 920428.6 E 8, N85E
39 251034.2 N 920421.8 E 12, S70W
40 251039.2 N 920412.6 E 10, S84W
41 251041.1 N 920409.4 E 20, S26E
42 251043.8 N 920335.1 E 20, N13W; 5, N45E; Minor syncline
43 251049.5 N 920309.6 E 10, N70W
44 251033.3 N 920312.2 E 15, S10W; 20, S20W
45 251034.7 N 920308.5 E 20, S37W
46 251034.3 N 920304.6 E 25, S24W
47 251043.9 N 920231.1 E 7, N7W
48 251050.7 N 920212.6 E 30, N20E
49 251059.1 N 920156.0 E 54, N50E; 56, N26E; 40, N20E; 52,
N50E
50 251056.2 N 920158.9 E 24, N14E
51 251054.7 N 920158.1 E 50, N11E; 27, N11W
52 251052.1 N 920200.5 E 22, N8E; 34, N19E; 23, N13E
53 251050.4 N 920200.4 E 12, N14E
54 251047.5 N 920200.2 E 6, N32E
55 251043.7 N 920158.5 E 3, N3W
56 251041.4 N 920155.4 E 13, N20W
57 251037.7 N 920151.8 E 10, S38E
58 251039.4 N 920147.2 E 22, S10W
59 251039.4 N 920144.4 E 25, S33E
60 251041.5 N 920139.5 E 10, S29E
61 251042.5 N 920134.2 E 41, S14E
62 251038.5 N 920132.9 E 20, S62E; 15, N40E
63 251033.6 N 920133.4 E 26, S22W
64 251028.9 N 920131.8 E 31, S6E
65 251026.1 N 920125.4 E 36, S10E; 26, S10W
66 251024.7 N 920117.1 E 12, S15W
67 251018.8 N 920111.8 E 17, S17W
68 251042.8 N 920054.9 E 28, S52W
69 251045.4 N 920059.4 E 10, S31E; 15, S15E
70 251037.4 N 920111.5 E 12, S35E; 24, S12E
71 251032.5 N 920111.9 E 22, S16E
72 251031.0 N 920110.8 E 30, S10W; 15, S14E
73 251029.4 N 920110.6 E 23, S13E
74 251026.8 N 920111.1 E 32, S10W
75 251024.4 N 920111.5 E 36, S6W
-
36
4.3 Stratigraphic Correlation 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
Surma Basin, Sylhet
Assam Valley, India
Mathur And Evans,
1964
Eastern part of
Bangladesh,
Chittagong
Chittagong Hill Tracts
Group Formation Series Stage Group Formation
Holocene Alluvium Alluvium and high
level
terraces
Alluvium
Late Miocene
to Mid Miocene
DupiTila Sandstone DupiTila
Sandstone
Upper DupiTila
Lower DupiTila
Mid Miocene
Tipam Girujan Clay Tipam Girujan Clay
Tipam Girujan Clay
Tipam Sandstone Tipam
Sandstone
Tipam
Sandstone
Early Miocene
Surma Bokabil Surma Bokabil
Bhuban Bhuban
Oligocene
Barail Renji Barail Jenum
Eocene Jaintia Kopili Shale Jaintia Kopili Shale
Sylhet Limestone Sylhet Limestone
Tura Sandstone (not found in
investigated area)
Theria
Sylhet Limestone:
Gray to dark gray colored, very hard and compact, non-porous, calcium carbonate, highly fossiliferous, crystalline limestone suggests that the investigated rock unit can be correlated with
the Sylhet limestone (Evans 1932) of Eocene age. Kopili Shale:
Gray to dark colored, compact, fissile, laminated shale suggest that the investigated rock unit can
be lithostratigraphically correlated with the Kopili Formation of Assam (Evans 1932) of Eocene age.
Barail Group:
-
37
The red to pink colored, fine grained, well sorted, bedded sandstone with subordinate gray colored, non-fissile, laminated shale suggest that the investigated rock unit can be correlated with
that of Barail Group of Assam (Evans 1932) of Oligocene age.
Surma Group:
Repetitive sequence of gray, hard, silty, well sorted, argillaceous sandstone; gray nodulated clay stone and gray, silty, laminated shale suggest that the investigated rock unit can be correlated
with that of Surma Group of Assam (Evans 1932) of Miocene age. Tipam Group:
Tipam Sandstone:
Yellowish to reddish brown colored, medium to coarse grained, very poorly sorted, cross
bedded, massive sandstone suggest that the investigated rock unit can be correlated with the Tipam Sandstone of Assam (Evans 1932) of Pliocene age. However this unit is designated as of middle Miocene age by Khan (1978).
Girujan Clay:
Bluish gray colored, moderately hard, sticky, massive, non- laminated pure clay stone (without any sand and silt) suggest that the investigated rock unit can be correlated with the Girujan Formation of Assam (Evans1932) of Pliocene age.
However this unit is designated as of middle Miocene age by Khan (1978).
Dupi Tila Sandstone:
Reddish brown-to-brown colored, loose, friable, coarse grained, containing pebble grains sandstone with subordinate clay stone suggest that the investigated rock unit can be correlated to
Dupi Tila Formation of Plio- Pleistocene age. Evans (1932) named DupiTila Formation after a hill halfway between Sylhet and Jaintiapur. This is the only type section of the investigated rock units of Bangladesh.
Sona Tila Gravel:
The gravels of off white to yellow color, various sizes with high sphericity and roundness.
Igneous and sedimentary composition is the characteristics of the gravel beds. It was named as SonaTila Gravel beds by Monsur (1995).
-
38
It can be correlated with the Dihing Formation of Assam by its lithology. Table 4:Stratigraphic correlation between the stratigraphy of Surma Basin and the studied sections
Age North Eastern
part of Surma
Basin, Sylhet
Shari River
Section
Nayagang-
Mohismari
Section
Ranga-
pani
River
Section
Tamabil
Road
Section &
Dauki
River
Section
Gro
up
Form
atio
n
Gro
up
Form
atio
n
Gro
up
Form
atio
n
Gro
up
Form
atio
n
Gro
up
Form
atio
n
Holo-cene
Alluvium Alluvium Alluvium
Pleisto-cene
Dihing Dihing
Dih
ing
Late
Mio-cene to
Mid
Mio-cene
Dupitila
Sand Stone
Dupitila
Mid
Mio-cene
Tipam Girujan
clay
Tip
am
Girujan
clay
Tipam sand stone
Tipam sand stone
Early Mio-
cene
Surma
Surm
a
Surm
a
Oligo-cene
Barail Renji
Bar
ail
Renji
Bar
ail
Ren
ji
Bar
ail
Ren
ji
-
39
Eocene Jaintia Kopili Shale
Jain
tia
Kopili
shal
e
Jain
tia
Kopili
shal
e
Sylhet lime
stone
Sylh
et
lim
e st
one
Tura sand stone
-
40
CHAPTER FIVE
Sedimentology
5.1 Sedimentary Structures and Features- Paleocurrent data
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.
Parallel lamination is defined by grain size or mineralogical composition or color change; can be
produced in several ways. These are common sedimentary structures found throughout the area. Descriptive terms for bed and lamina thickness is given:
Table 5: Terminology of bed thickness
1m----------
.3m---------
.1m---------
.03m-------- 1cm---------
.3cm -------
Very thickly bedded -------------------------
Thickly bedded ------------------------- Medium bedded
------------------------- Thinly bedded
------------------------- Very thinly bedded -------------------------
Thickly laminated -------------------------
Thinly laminated
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.
-
41
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. 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.
c) Haring bone cross bedding: It refers to bipolar cross bedding where cross
bed dips in opposite directions, produced by reversal of current and
indicate deposition by tidal current. It was found rocks of SURMA GROUP in Shari river section.
iv) Massive beds: Massive have no apparent internal
structure. It was found in different rocks of different formation.
v) Mud cracks: Shrinking cracks with polygonal structures in fine-grained sediments through
desiccation and dewatering on exposure. It was found in Shari river section.
vi) 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.
vii) Mottled bedding: Some bedding is found to be mottles and indicate irregular stratification. It was
found in DUPITILA FORMATION in Shari river section.
Figure 19: Cross Strata
Figure 20: Lenticular Bedding
Figure 21: Flaser Bedding
-
42
viii) Sole marking: Sole marking are bedding plane features with characterize the under surface of some sandbeds. It was found in SURMA GROUP of 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) Flame structure: Flame structures are wavy or flame shaped tongue of mud or shale projected upward into an overlying
layer commonly sandstone. The crests of some flame are bent over or overturned. It
was fond in Surma group at Jaintia group.
iii)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.
iv)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.
v) 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
i) 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.
ii) Hummocky structure: It is a unique structure formed in marine storm condition in
fine-grained sandstones (as pocket conglomerate) and contain a strong weathered
wave base. U.S gives this name. John harms in Manathod Oil Company also known HCS
Figure 22: Iron Concretion
Figure 23: Hummocky Structure
-
43
(hummocky cross stratification).
iii) 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.
Black magic: In the time of limestone deposition, heavy minerals are deposited on each beds of limestone.
iv) Fault breccia: At the great depth due to faulting formed angular grains powder etc. At high temperature and pressure, it forms
milonite. v)Trace fossil: It is used as environmental detector. The rate of sedimentation is known form it.
vi) Protrusion: Gas bubbles create protrusion.
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 textura l analysis and microstructure
analysis. Study of occurrence of microfossil is useful to determine whether the sediments are deposited by transportation or deposited in situ.
Sandstone samples were first disintegrated and sieved to separate into different s ize grades. Histograms are constructed on an ordinary arithmetic graph paper from the grain size analysis
data by plotting the weight percentage into the vertical axis versus grade scale into the horizontal axis. Cumulative curve has been drawn in an ordinary arithmetic graph paper taking cumulative
weight percent in the vertical scale and grain size in phi scale in the horizontal scale. The different percentiles have been grade from the cumulative curve in order to calculate the grain size parameters or statistical parameter according to Folk and Ward (1957) method.
According to folk and ward:-
Graphic mean, Mz = 16+84+50
3
It is the best graphic measure for determining overall size in the sample.
Inclusive graphic standard deviation, I = 84-16
3 +
95- 5
66
Figure 24: Sand Vein
-
44
This is the best measure for sorting of the grain.
Inclusive graphic skewness, SKI = 16+84- 250
2(84-16) +
5+ 95- 250
2(95-5)
It is the best statistical measure of skewness.
Graphic kurtosis, KG = 95-5
2.44(75- 25)
The kurtosis is measured to determine the nature of the curve.
Samples of sandstone of DupiTila formation, Tipam sandstone formation and Surma group have been analyzed for grain size analysis.
5.2.1 Sample No. 01 (Barail Group)
Name of the sample- Barail Group
Time of Sieving: 20 min Initial weight of Sample- 50 gm
Table 6:Grain Size Analysis Data of Barail Group
Mesh no
Grain size in (mm)
Grain size in (micron)
Grain size in phi scale
Weight retained in (gm)
Weight percent
Cumulative percent
25 0.71 710 0.5 0.11 0.22 0.22
35 0.5 500 1 0.42 0.84 1.06
45 0.35 350 1.5 2.03 4.06 5.12
60 0.25 250 2 6.94 13.86 18.98
80 0.177 177 2.5 6.86 13.72 32.7
120 0.125 125 3 25 50 82.7
170 0.088 88 3.5 0.21 0.42 83.12
230 0.0625 62.5 4 0.69 1.38 84.5
Pan 7.72 15.44 99.94
Sieve Loss= (50-49.98)gm=0.02gm
-
45
Illustration 8: Histrogram of Barail Sandstone
0.22 0.844.06
13.86 13.72
50
0.42 1.38
15.44
0
10