Geological Field Report Based On The Fieldwork Along Jaintlapur And Adjoining Areas Of Sylhet

CHAPTER: 1

• INTRODUCTION:

terminate on the north at the
Shillong Plateau and on the East in the Tripura and Chittagong Hills. These
hills are parts of the Frontal Folded Belts of Arakan-Yoma.
The investigated area belongs to the
northern part of Surma basin, a major tectonic part of Bengal Basin.
The studied area consists of one broad homoclinal fold having almost E-W trend
and average elevation of 100 feet above mean sea level. It is bounded by the
major fault Dauki in the north. Two rivers Dauki and Hari control the drainage
system of the area.
This report has attempted to discuss
on the tectonic setup, stratigraphy, drainage, geomorphology, depositional
history, and economic geology of the investigated area, Jaintiapur as well as
the N-W part of Bangladesh.

PURPOSE AND SCOPE:
Fieldwork program simplifies the
knowledge of geology. The investigated area, the N-E part of Bangladesh is
the “geological museum”. The major tectonic lineament Dauki Fault is studied
here. The rocks from Eocene to Recent age are best exposed here, where we got
the highly fossiliferous limestone. On the other hand this area is highly
prospective for hydrocarbon. So our purpose was to study the tectonic and
structural settings, stratigraphy, fossils, depositional environment, economic
geology of the area and also to make a geological map of the studied area.
THE
PURPOSES ARE~
To know the Regional tectonic setting
of the whole area
To study the major structural
features
To study the rocks ( lithology and
sedimentary structures)
To study the fossil remains
The arrangement of the structural
relations particularly with their relations to one another
To construct stratigraphic columns
To construct a geological map of the
studied area
To observe the hydrocarbon
potentiality on the basis of the source rock, reservoir rock and cap rock.
LOCATION, EXTENT AND ACCESSIBILITY:
The area
is mapped between latitudes 25°5´N and 25°11´ N and longitudes 92°E
and 92°12´E. It covers the Survey of Bangladesh Topographic Sheet 83C/4
and 8. This area includes Tamabil, Jaintiapur, Sripur, Afifanagar, Lalkhal Tea
State Dupigaon, and adjoining
areas. The investigated area is the
N-E part of Bangladesh,
near the Bangladesh-India border. The whole area is well connected by pitched
surface road with the Sylhet town. It is about 43 km NE of the Sylhet town. The
base camp is also well connected by metalloid road with the Sylhet-Jaflong
highway. The investigated sites are:
Along Jaflong-Jaintiapur section.
Dupigaon-Hari section.
Afifanagar-Hari section.
Jaintiapur traverse.
PREVIOUS WORK
A good volume of works has been done
on the studied area and its surrounding area as well as the whole Sylhet
district, as it has been supposed to highly prospective of hydrocarbons.
Based on the geophysical surveys
(gravimetric, magnetic, seismic) drilling has been resumed in the early fifties
and exploration since then has been extremely successful; Pakistan Shell Oil
Company, Pakistan Petroleum Limited, OGDC and Petrobangla respectively hence
forth drilled 17 wells with a total footage of 140891 feet.
Khan M.A.M (1978) published a report
and a reconnaissance geology map in the scale of 1 inch 2 miles of the eastern
and northern Surma basin.
Elahi, M. (1984) published the
structural development and Haque (1988) studied the sedimentology and
petrography of the exposed rocks of Surma basin. Paul, H. (1988) reviewed the
structure and tectonics of the north-eastern part of Surma basin. Mr. Maruf
Khan of Petrobangla investigated the area in the recent part.
METHOD OF INVESTIGATION
Method of investigation is all the
same everywhere and sometimes it is technical and talent to determine the
location, stratigraphy, lithology, environment of deposition of different
section. We used the traverse method. The whole study as accomplished into two
phases, which are field study in the field and laboratory analysis of the
samples collected from the field.
CHAPTER: 2
PHYSICAL FEATURES:
TOPOGRAPHY AND RELIEF:
The investigated area is bounded by
in the NE by the abrupt scarp of the 4000 to 6000 feet high Shillong Plateau
and in the E by Khashi-Jaintia
Hill Range.
To the east of the Dauki
River there is a 5 miles
long hilly area up tot he Sripur known after Jaflong. There is a number of
hillocks from Dauki
River to north of
Chatughat. SonaTila is the highest hillock in this area , which is about 200
feet high from the MSL. In the south east there is a continuous hilly area from
Jaintiapur, DupiTila, and Naya khal. The trend of these hillcks is almost east
west. They are parallel to the strike of the beds and hence termed as strike
hills. A number of cliffs are also found in the investigated area. Scarf’s are
found along the Hari
River. There are also
swamps in the area and the valleys act as swamps in the rainy season.
DRAINAGE AND WATER SUPPLY
The Dauki, the Hari, the Nayagang are
the prominent river in the investigated area. These rivers are mainly
meandering type and matured in stage. The drainage system is dendritic type.
Besides these main rivers there are some khals, bills, and nalas. From March to
September these are well fed by huge water and become almost dry in the rest of
the year. The Dauki River originates somewhere in the southern part of
Shillong Plateau in India
and flows into Bangladesh
in the north-south direction in this area. The Hari
River originates from Khasia- Jaintia Hills of India
and flows southward and then enters Bangladesh,
then joins to Surma
River.
Water
supply in this area is very bad, though it has a good drainage. There is no
permanent water supply. In the monsoon there is some water in the rivers, khals
and subsurface. But in the dry season the people of the area suffers a lot of
water. When we visited this area (March13-March19, 2004) the area was like a
totally dry land. The tube wells, wells, and even deep pumps did not get water
table. So we had to by our water for a lot of cost.
VEGETATION AND CULTIVATION:
The investigated area is covered by
forest with evergreen vegetation, which is due to the good and suitable
climatic condition of this region. The average rainfall of this area is 150
inches annually.
Mixed bamboo and deciduous forests of
tropical evergreen are found in this area. The important trees are Shimul,
Chapalish etc. There are several tea gardens in this area which are on the body
of hills. Pineapples, bananas, betel leaves, cane grow well in this area. The
low and plain lands are well cultivated by watermelon and paddy.
CHAPTER: 3
STRUCTURE AND TECTONICS:
REGIONAL SETUP:
Basin
covers entire Bangladesh,
part of West Bengal, Tripura, Monipur, Mizoram states of India and parts of the north-western Burma. The
interpretation of the tectonics of this basin has been suffering from both
oversimplification and over complification. The Bengal
basin is the result of prominent mantle up warping, intercontinental break,
newly created oceanic crust and its convergence at the previously stable
continental margin of miogeoclinal character.
For the
details study f Bengol basin we must go back to the pale tectonics of India. India has been separated from Australia along
a pale continental shelf by the process of mantle activated rift. The initial breakup
of the Gondwana land occurred in the Early Cretaceous time with separation of India from combined Australia
and Antarctica northerly direction. New sea
floor was created what is now the Bay
of Bengol and sea floor
that has subducted beneath the Sunda Arc. The northward flight of the Indian
plate was maintained along two driving lines, that is over fracture zone and
90˚E Ridge transform. The magnetic anomaly numbers on the western side of the
90˚E Ridge transform is the indication of NNE motion of the crustal segment of
this region. The 90˚E transform became ridge due to massive intrusion of magma
from the hot spot located near Gulf of Cambay
(Bose 1973). It is believed that Chaqos Laccadive Ridge is related to Deccan volcanism and is the extension of Western Ghat
Fault of India.
The Bengal Basin
started originating as the oceanic crust (Indian plate) converged beneath the
Burmese plate. Tectonically Bengol
Basin could be divided
into two margins:
1. Active margins
2. Passive margins.
Passive margin is characterized by Moho upwarping. It has the following
parts:
NSP-Northern
slope platform
SP– Stable
platform (Buried ridge)
NGH– Nawabgonj Gaibandha high (Burried ridge)
JH– Jaripur
high
SSP– Southern part of platform
Hinge zone–
Characterized by series of down to the basinal thrust faults.
The platform flank comprises series
of horsts and gravens of Gondwana period and northward slopping basement
flanking the Sub-Himalayan Foredeep.
Active margin is characterized by the
deeper basin. It contains Sylhet trough, Faridpur trough, Hatia trough zone of
elevated oceanic crust (Barisal-Chadpur gravity high) and Eastern Tertiary
Folded Belt. Which were formed due to the migration of detachment zone again
and again of the obducted Burmese and subducted Indian plate. There is a trend
TT3 along which the folded belt abruptly changes the topography and forms a
trough again. Near the Dauki fault a homoclinal fold is formed by the dragging
effect of this mega fault. All the anticlines and synclines of the folded belt
have a trend due E-W. There are some other E-W trending small folds near the
Jaintiapur area, Sylhet that was also due to dragging effect of Dauki fault.
The whole Bengal
basin is separated by two distinct crustal domains along a paleocontinental
margin, almost passing in a NNE-SSW direction. The north and northwest domain
of the margin including part of Sylhet trough is underlain by the continental
crust whereas the south and southern domain including Faridpur and Hatia
troughs underlain by the oceanic crust.
The sedimentation of Bengal basin was
characterized by Assamese Gulf and Burmese
Gulf. Assamese Gulf
carried the sediments of Himalayan and Burmese Gulf
carried the sediments of Arakan-Yoma. Bengal
basin is the one of the thickest sedimentary basins of the world. The initial
collision between India
and Eurassian plate occurred in the Late Paleocene time. This was a time of
change in relative plate motion in the entire Indian Ocean.
The Eocene limestone then deposited along entire continental shelf of northeast
India.
The back-arc basin is represented by Central Burma or Irrawaddy
basin and fore-arc basin is represented by Arakan-Yoma Folded Belt and its
western extension up to Chittagong-Tripura Hills. According to Brunnschweiler
(1974) thick sediments were deposited during Mio-Pleistocene in Irrawaddy Basin
with the interception of the convergence of Indian plate and the Tertiary
sediments deposited in the fore-arc basin, the region was uplifted during
Miocene orogeny and followed by Pleistocene Orogeny to form the present
Arakan-Yoma mega-anticlinorium and its western extension covering
Chittagong-Tripura Folded Belt.
STRUCTURE AND TECTONICS OF FIELD AREA:
We got the practical knowledge of the
structures of the studied area. These structures can be divided into two
groups:
Major structures
Minor structures
MAJOR STRUCTURES:
The major
structures of the studied area are:
Homoclinal Fold
Fault
Burried River
Unconformity
HOMOCLINAL
FOLD:
The major structure of Jaintiapur and
its adjoining areas is a homoclinal fold. The strike of this fold is almost E-W
and dips towards south. The amount of dip increases towards the dip direction.
This homoclinal fold is the result of the dragging effect of the Dauki Fault.
The anticline dips 5˚ to 8˚ in SonaTila section and 35˚ to 40˚ in Nayagang
section. Homoclinal fold is best studied along the Hari River.
It extends from Tamabil to Kanaighat to the south.
FAULT:
This area is bounded by a major
fault, the Dauki fault. But we could not study this fault. We found some
evidences of this fault, which we were, studing Sylhet limestone. These are:
STRUCTURAL CRITERIA:
Presence of fault breccias,
milonites, fault gause
Abrupt change of topography
STRATIGRAPHIC CRITERIA:
Different types of rocks on both
sides of the fault.
This is a thrust fault without the
vertical movement of 10-11 km.
But the major fault that we studied
is the fault between the blocks of contact plane between Bhuban and Bokabil and
the block contact between Bokabil and Tipam. It is a right lateral movement. It
was observed along the Hari
River in between the
areas Afifanagar and Bargang. The displacement is about 30m. Probably the fault
line is followed by the Hari
River.
UNCONFORMITY:
A prominent unconformity was observed
between the rocks of Barail and Bhuban along the Nayagang River
section. The unconformity surface is a laterite bed, 1m thick. The type of this
unconformity is an angular one. There is another unconformity observed between
the recent gravel deposits and Surma rocks. It was found in all over the Jaintiapur,
Jaflong Sripur areas.
MINOR STRUCTURES:
1.
Small folds
2.
Small faults
3.
Joints
SMALL FOLDS:
There are several small folds in the
study area. They are detached folds. These small folds are the result of the
dragging effect of the Dauki fault. We observed such fold in the Rangapani
section. The trend of these folds is almost E-W.
SMALL FAULTS:
A small local fault was observed
between the contact plane of DupiTila, Girujan Clay and the contact line
between the Tipam, Girujan Clay. But we did not find the evidence clearly. The
continuation of beds indicates a right lateral fault could have there. It was
found along the Hari
River near Bargul.
JOINTS:
Joints have been observed in almost
every formations. Mainly strike, slip joints are common.
ATTITUDE OF BEDS:
Kopili Shale:

Barail:

Bhuban:

Bokabil:

Tipam Sandstone:

Girujan Clay: 35°N75°E
DupiTila Sandstone:

CHAPTER-6
ECONOMIC GEOLOGY:
Economically the area is rich enough.
Most of the rocks exposed here contain some economic values. The whole Sylhet
anticline is very much important because it contains gas fields and an oil
field of the country. The natural gas and oil history of Bangladesh is
based on this area. On the other hand this area is also famous for the building
stones. The economic importance of the rocks of the studied area are described
below:
Sylhet Limestone: this is one of the exposed limestone of Bangladesh. It is now almost
exhausted for the quarry done in the past. If further extension of this
formation is found, it could be used for production of lime.
Kopili Shale:
this formation is very good source rock for the hydrocarbons.
Barail and Surma Group: The rocks of Barail and Surma group are very appropriate for
reservoir rock of the hydrocarbons.
Tipam and DupiTila: The sandstone of Tipam and DupiTila can be used for the construction
purpose. DupiTila is a good reservoir of ground water.
Girujan Clay:
It contains a good amount of pure clay, which may be used making potteries,
brick, handcrafts etc.
Gravels: The
studied area stores a great amount of gravels. Dauki
River, Rangapani River
and SonaTila gravel quarry accumulate a huge amount of boulders, cobbles,
pebbles etc. Which are very hard and compact. These gravels can be used for
constructing high buildings, heavy bridges, embankment, in the railway etc.
TABLE: Economic use of the rock units

REFERENCES
Aftab Alam Khan: Tectonics of the Bengal Basin, Journal of Himalayan Geology, volume2 (I),
1991,PP-91-101, printed in India.
Md. Hussain Monsur: An Introduction to the Quaternary Geology of Bangladesh, A complementary
Research of IGCP 347. Quaternary Stratigraphic Correlation of the Ganges-Brahmaputra
sediments (1995)
Reimann, R.V:
Geology of Bangladesh,
Berauder, Bonntaeger, Berlin, Studdgard, 1993
Kent C. Condie:
Plate tectonics & Crustal evolution, 2^nd edition, New Mexico. Institute of
Mining & Technology Soconno,
New Mexico.
Badrul Imam: Bangladesher
Khonij Sampad.
CHAPTER-4
STRATIGRAPHY
Stratigraphy means to study the
rocks of an area briefly and to arrange the rocks according to their time of
deposition on the earth surface. The studied area exposes a good sequence of
rocks of Bangladesh
from Eocene to Recent deposits. The studied area is in a large fault zone and
is a homocline. The thickness of rocks are enormous. The exposed rocks are
Jaintia Group, Barail Group, Surma Group, Tipam Group, and DupiTila Group from older
to younger.
The classification, nomenclature and
correlation of rock units are based on the ‘Stratigraphy of Assam’ proposed by
Evans (1932).
OUTCROP STUDIES
OUTCROP: Dauki
Location of outcrop: This out crop is in the NE part of Sylhet district having the GPS
reading 25˚10´53.0˝ and 92˚01´15.7˝
Description of outcrop: It is on the right side of Dauki
River near the Bangladesh-India border
only about 100 to 200 meter South from the Dauki town of India
where Dauki River
enters to Bangladesh.
It is situated in the footwall of Dauki fault and carries enough evidences of
that fault. The outcrop is an isolated hillocks, a detached block of Sylhet
limestone and a very important outcrop for Geological study. The exposure is
bush covered and weathered.
Structural attitude: The bedding attitude studied in this outcrop is 32˚S45˚E.There are
evidences of fault like presence of breccia, mylonite etc.
Lithological description: The rock is gray colored weathers to yellow, very hard and
compact, apparently nonporous, reacts with HCl and give effervescence that
indicates that the rock is composed of CaCO₃. The rock contains the
fragments of fossil shells in a matrix of crystalline calcite and clay
materials. The rock is thick bedded to massive. It contains fault breccias,
gauge, and milonites that are the evidence of large fault zone. Limestone is
composed of calcite fossil shells embedded in a matrix of calcite and clay
minerals. The fossils are mostly Nummulites, Discocyclina etc with some other
foraminifera.
Thickness: The
approximate thickness of the exposed rock is 30m
Depositional environment: The lithology and preservation of marine fossils indicate
that the rock was deposited in shallow marine water, continental shelf zone and
a clear water environment without any disturb. The area also could have been a
restricted arm of the sea. The deposition age was Eocene (Monsur 2004).
Diagnostic lithology and correlation: Gray to dark gray colored, very hard and compact,
non-porous, calcium carbonate, highly fossiliferous, crystalline limestone is
the diagnostic lithology of the rocks of outcrop 1. So it be correlated with
the Sylhet limestone of Eocene age (Evans 1932).
OUTCROP:2 Dauki
Location of outcrop: The outcrop is located in 25˚10´49.7˝ and 92˚01´16.4˝
Description of outcrop: This also in the bank of Dauki River,
about 300-m south from the Bangladesh-India border. It is a small exposure, not
good, covered by bush and vegetation. This is a slumping block and not in situ.
The outcrop is essentially composed of shale.
Structural attitude: The attitudes of beds observed here are:
23˚S60˚E
20˚S40˚E
Lithologic description: This formation is composed of predominantly shale with
subordinate impure limestone. Shale is dark gray colored, flat, hard and
fissile, composed of almost all clay particles, very thinly laminated.
Thickness:
This exposure covers about 15m area.
Depositional environment: Lithology of the rocks of this outcrop indicates that it was
deposited in a marine shelf environment with increased fresh water input during
the Eocene age. (Reimann 1983)
As the change of sediment supply from
no sediment to some clay sediment gradually took place, deposition of limestone
stopped and deposition of shale started.
Diagnostic lithology and correlation: Gray to dark colored, compact, fissile, laminated
shale is the diagnostic lithology of the rocks of this outcrop which is
correlated with the Kopili Formation of Assam (Evans 1932).
OUTCROP: 3 Barail
Location: The
outcrop is in the 25˚10´42.9 ˝ and 92˚01´39.9˝
Description of outcrop: The outcrop is in the south part of Dauki fault about 5 km
East to the Dauki
River. It is exposed in a
local stream named SonaTila chara. The stream trends almost N-S parallel to Dauki River
and 4km long. The rocks predominantly sandstone with some shale are very well
exposed along this stream. The stream cuts the rocks across the strike of the
fold. The stream was dried which helped to study the rocks conformity. The
exposure are almost continuous along this stream bed.
Structural attitude: The attitudes observed in different beds of Barail are given below:
17˚S23˚E
20˚S10˚E
33˚S17˚E
27˚S11˚E
Lithologic description: The outcrop is an exposure of sandstone beds overlain by
shale beds.
Sandstone is bright pink colored,
hard to very hard, fine grained, well sorted, fairly porous, thin to medium
bedded, bedding plane is sometimes wavy. Sandstone is composed of 80-90%
quartz, few micas and other dark colored minerals. The red color of rock
indicates the presence of hematite cement. In some places it shows laisigang
structure in sandstone beds.
Shale is bluish gray, weathers to
yellow colored, moderately hard, well laminated and not fissile. Shale is
composed of mainly clay particles.
Thickness:
Thickness of the outcrop is about 4km.
Depositional environment: During Oligocene, the sea moves southward from Shillong massif
due to the basinward differential movement in the area. Thus marine to
sub-marine deltaic transitional environment existed in the area and Barail
group sediments were deposited in that environment (Monsur 2004).
Diagnostic lithology and correlation:
The red to pink colored, fine grained, well sorted, bedded sandstone with
subordinate gray colored, non-fissile, laminated shale is the characteristics
of the rocks of this outcrop which can be correlated with that of Barail Group
of Assam
(Evans 1932). In particular it is correlated with Renji formation of Barail
Group.
OUTCROP: 4 Sripur
Location: The
outcrop is situated in the 25˚10´38.5˝ and 92˚04´21.2˝
Description of outcrop: This outcrop was studied in the left side of the Sylhet
–Jaflong highway in Sripur area. It is near the Nilima Restaurant. It is a
small exposure of Barail sandstone beside the road and is a good exposure.
Attitude of beds: The attitudes of beds are
14˚S19˚W
16˚S36˚W
Lithologic description: The exposure is totally composed of sandstone. Sandstone is
pink to brown colored, moderately hard, fine grained, very well sorted, fairly
sorted, thick bedded to massive and very quartz rich (about 95%) with few micas
(no biotite) insignificant dark colored minerals, small percentage of clay matrix.
The red color of sand indicates presence of hematite cement. The hematite
cementing occur the surrounding grains.
There was no shale bed in this
outcrop like the previous one of Barail exposure (outcrop 3).
Depositional environment: As described for outcrop 3.
Diagnostic lithology and correlation: As was describe for out crop 3.
OUTCROP: 5 the Nayagang
Outcrop location: The outcrop is located in 25˚08´23.9 ˝ and 92˚07´29.4˝
Description:
The outcrop is along the Nayagang
River bank about 2km
north from the Sripur Bazar. It is a good exposure of the top of Barail and the
lowest part of Bhuban with a distinct unconformity in the middle of these two
rocks. The unconformity bed is a laterite. This outcrops exhibits a good rock
sequence of both Barail and Bhuban rocks. The distinct and clear difference of
the characteristics between the rocks of Barail and Bhuban is best observed
here. So undoubtly this is a good exposure and a reference place for geological
study.
Structural Attitude: Attitudes of the Barail beds are 44˚S8˚E, 40˚S10˚E, 46˚S12˚E and that
for Bhuban are 44˚S12˚E, 46˚S4˚E.
Lithologic description: The top most part of Barail and the lowest part of Bhuban
rocks are well exposed here. There is an unconformity bed (laterite) between
two formations. So we get 3 types of rocks here.
The Barail formation covers 30-40m of
the exposed area. Barail rock is mostly sandstone, which is reddish to brown
colored, moderately hard, fine to very fine, fairly porous, thin to thickly
bedded. Sandstone is composed of almost 90% quartz with few muscovites and dark
colored minerals.
The unconformity surface is laterite
which is gray to black colored, ferruginous, pisolite, highly oxidized and
weathered that indicates it was exposed to air for a long period.
The lowest part of Bhuban rocks
covers about 50m area. It shows a good alternation of sandstone and shale.
Sandstone is yellowish brown colored,
moderately hard, medium grained, thinly bedded.
Shale is gray colored, hard, highly
laminated and composed of mostly clay particles.
Thickness:
Barail rocks has a thickness of 40 m.
Laterite covers 10m
Bhuban covers 50m
As a whole the exposure
covers 100m distance.
Depositional environment: All the rocks of this outcrop were deposited in the Bengal
Geosynclinal part. Barail rocks were deposited in a marine to deltaic
environment during the Oligocene.
Then a tectonic activity cropped out
the Barail rocks and was exposed to air for a certain period. These rocks then
in open air oxidized highly and of pisolite
structure formed laterite bed after the Oligocene age.
Bhuban rocks were deposited when the
above rock sequence again subsided long long after the Oligocene and probably
during the Early Miocene. Bhuban rocks were deposited in a transitional Marine
to Deltaic Environment.
Diagnostic lithology and correlation:
For the rocks below laterite: Reddish to brownish colored, fine grained, bedded, fairly
porous sandstone is the diagnostic property of the first observed rocks which
can be correlated with the Renji formation of “Bhuban Series” (Evans 1932).
For the rocks above the laterite: Alternations of yellow to gray colored, fine to medium
grained sandstone and gray colored, well laminated shale is the characteristics
of the rocks above the unconformity which can be correlated with the Bhuban
Formation of Assam (Evans 1932) of Early Miocene.
OUTCROP: 6 in Afifa Nagar
Location of outcrop: The outcrop is located at 25˚07´30.1˝ and 92˚11´27.3˝
Description of outcrop: This is an excellent outcrop of Bhuban rocks. It is on the
bank of Shari River about 50 m South from the
Bangladesh-India border in Afifa Nagar. This exposure is very very important
for the stratigraphy of Bangladesh
because the Bhuban rocks are best exposed here. The beds of the Bhuban rocks
are so much distinct here that they seem to be thick and large sheets of beds.
It can be a reference for sandstone, siltstone, shale and claystone.
Attitude: The
attitudes on the fresh surface are:
47˚S35˚W,
45˚S30˚W, 43˚S33˚W

44˚S28˚W, 45˚S23˚W, 44˚S31˚W
Thickness: It
covers about 80m distance.
Lithology: It
is repetitive sequence of very fine grained sandstone, siltstone, claystone and
shale.
Sandstone is
gray to dark gray, hard to very hard, fine grained and silty, very well sorted,
gradually thin to thick bedded. Sandstone is argillaceous, composed of
predominantly quartz with feldspars and micas, also few dark colored minerals.
The matrix of clay minerals is insignificant. Claystone is bluish gray, silty and dominantly nodulated. Shale is bluish gray colored,
moderately hard, silty and wavy laminated.
Depositional Environment: Sediments of this area were deposited under deltaic
estuarine during the Early Miocene. (Reimann 1983)
Diagnostic Lithology and Correlation: Repetitive sequence of gray, hard, silty, well
sorted, argillaceous sandstone; gray nodulated claystone and gray, silty,
laminated shale is the diagnostic lithology of the rocks of this outcrop.
According to the lithology the rocks
of this area cab be correlated with that of ‘Bhuban Formation’ of Assam.
(Evans 1932)
OUTCROP: 7 Hari River Bdr
Location: the
location of the outcrop is 25˚06´53.1˝ and 92˚11´7.6˝
Description of outcrop: The outcrop is along the bank of Hari River
and about 40m long. It is a good exposure of Bokabil Formation.
Attitude: The
attitude of bed is 43˚S21˚W
Lithology: The
exposed rocks are silty sandstone and laminated shale. Sandstone is gray
colored, moderately hard to breakable, porous, very fine, well sorted with some
clay matrix. Gary
color is probably for the argillaceous coating sandstone is composed of quartz,
high content of muscovite, and other dark colored minerals. It is silty in
nature. Shale is gray colored, moderately hard, wavy laminated and exposed of
clay and silt particles.
Thickness: The
outcrop covers about 50m area.
Depositional Environment: On the basis of lithology, it is determined that these rocks
were deposited in a Brakish, Subaerial condition. (Reimann 1983)
Diagnostic Lithology and Correlation: Gray colored, fine, well sorted, argillaceous
sandstone with interbedded gray colored, hard, laminated silty shale is the
diagnostic lithology of the studied rocks. It can be correlated with the ‘Bokabil
Formation’. (Evans 1932)
OUTCROP: 8 Shari
River Tipam sandstone
Location: The
outcrop is located in 25˚6´33.7˝ and 92˚10´56.4˝
Description of outcrop: The outcrop is by the bank of Shari River,
2km West from the Afifa Nagar Tea Garden. It is a small hillock that exposes
clearly the Tipam sandstone. The average coverage of the area is about 20km. It
is very good outcrop of Tipam sandstone. It is an excellent example of typical
Tipam sandstone.
Structural attitude: Attitude along the bedding plane of Tipam sandstone found DD-S26˚W and
AD66˚
Lithologic description: Sandstone is yellowish brown to reddish brown colored,
moderately hard to friable, medium to coarse grained, moderately sorted (for
fluvial environment) angular to subrounded grained, highly porous, very thick
bedded to massive. It is composed of mainly quartz, significant amount of mica
and dark colored minerals. The cementing material is ferruginous.
In some places strong cross bedding
in the sandstone had been observed.
Thickness: The
outcrop covers about 20m.
Depositional Environment: From the texture (medium to coarse grained, very poorly sorted)
and cross bedding structure the environment of deposition of Tipam sandstone
has been determined as a fluviatile environment during the Late Miocene.
Diagnostic Lithology and Correlation: Yellowish to reddish brown colored, medium to coarse
grained, very poorly sorted, cross bedded, massive sandstone is the diagnostic
lithology of the rock of this outcrop which can be correlated with the ‘Tipam Sandstone’ of Assam. (Evans 1932)
OUTCROP: 9 Hari st 2
Location of outcrop: The GPS location of this outcrop is 25˚05´52.0˝ and 92˚08´52.2˝
Description of outcrop: This is a good outcrop by the side of Shari River
near the Bargul after the Dupigaon. The exposure includes a distinct contact
between DupiTila and Girujan clay (underlying). The contact is sharp as seen in
the river bank.
Attitude of bed: The attitude of beds found here are 35˚N75˚E and the general dip is 35˚
Lithologic description: The rock exposed here is gray to bluish gray colored,
moderately hard, sticky, massive claystone. This claystone contains
ferrugeneous speck formed by the concentration of ferrugeneous materials.
Irregular cracks develop in the clay when dry. This claystone is very pure
without any silt or sand particles. In spite the clay being pure (no silt, no
sand) there is no lamination or fissility probably because the clay was
deposited very rapidly.
Thickness: The
exposure of Girujan clay is well developed along Shari River
about 250m.
Depositional Environment: The Girujan Clay Formation represents lacustrine, floodplain
and over bank deposits. The sedimentation took place under Subaerial condition.
(Ref. Reimann, R. U, Geology of Bangladesh, page-66)
Diagnostic lithology and correlation: Bluish gray colored, moderately hard, sticky,
massive, non-laminated pure claystone (without any sand and silt) is the
diagnostic lithology of the rock of this outcrop. On the basis of lithology it
is correlated with the ‘Girujan Formation’ of Assam. (Evans 1932)
OUTCROP: 10 Dupigaon
Outcrop location: The GPS reading of the outcrop is 25˚05´39.3˝ and 92˚07´41˝
Description of outcrop: It is a typical and ideal section of DupiTila, situated
about 3km east from the Sylhet-Jaflong highway road in a village named
Dupigaon. There are several small hillocks where DupiTila rocks are excellently
exposed. This is the only type section of Bangladesh. There is a right
lateral fault movement in this area. The beds are along the strike.
Structural attitude: It is a fresh outcrop and fresh beds exposure. The bush covering and
vegetation doesn’t disturb to take the attitude of beds. The attitudes on fresh
beds are given below:
55˚N67˚W
85˚N51˚W
Lithology: The
outcrop shows a good sequence of sandstone and silty claystone. Though it is a
very good exposure, so lithologic description is easy to describe.
Sandstone is pink to brown colored,
moderately hard, loose and friable, coarse to medium grained, poorly sorted,
contains some pebbles and highly porous. It is thick bedded to massive.
Sandstone is composed of quartz, significant amount of feldspar, mica and dark
colored minerals. The matrix is clay materials which is pink color that suggest
ferrugeneous coating. There are distinct iron bands in sandstone. The bands are
1-3cm thick. The iron bands developed along the bedding plane and fractures
that were formed by diagnostic process.
Claystone is light gray to yellow3ish
gray colored, moderately hard and friable, fine to very fine grained, fairly
porous and bedded to massive. It is composed of mostly clay particles with good
amount of silt particles that makes the claystone silty.
Thickness: The
total thickness of the outcrop in that area is about 2km.
Depositional Environment: From the above lithology, it was deposited under continental
fluviatile environment during Mio-Pliocene time.
Diagnostic Lithology and Correlation: Pink to brown colored, loose, friable, coarse
grained, containing pebble grains sandstone with subordinate claystone is the
characteristics of DupiTila Formation.
Evans (1932) named DupiTila Formation
after a hill halfway between Sylhet and Jaintiapur. This is the only type
section of Bangladesh.
As this outcrop is the type section of DupiTila it doesn’t need to be
correlated any more.
OUTCROP: 11 SonaTila
Location: This
outcrop is located on 25˚10´34.5˝ and 92˚01´31.7˝
Description of outcrop: The outcrop is studied beside the SonaTila Chara in a Boulder quarry. It is
about 3km East from the Dauki River and 4km South from the Dauki town of India. It is a good
exposure of SonaTila Gravel beds, that makes an unconformity between the
Alluvium and Barail Formation. It was also studied in the Jaintiapur area where
it makes an unconformity between Alluvium and Surma Group. It is also well
exposed in Mokambari, Muslin Nagar, Sripur hillocks. They cap all the hilltops
of Jaintiapur area.
Lithology: The
gravel beds consist of pebble, cobble, granule, sand, silt, and clay. Gravels
are off white to yellow colored, well rounded, smooth surface with high
sphericity and roundness. Gavels are composed of Quartzite, Granite, Pegmatites
etc. But gravel beds are composed of sediments of Barail and Surma Groups. The
shelly gravels are highly oxidized and weathered.
Depositional Environment: The Gravel beds were deposited as the rolling sediments or
bedload sediments on the errosional surface or on the paleoriver valleys. After
the deposition these gravel beds were uplifted and appears as small hillocks.
It was happened in the lower Pleistocene.
Diagnostic lithology and correlation: 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
(Monsur 1995).
It can be correlated with the Dihing
Formation of Assam by its lithology.
Alluvium
Alluvium is composed of
unconsolidated sand, silt, clay and gravels. It is loose and soft. Sediments
are of sedimentary origin. It covers the maximum area and almost covered by
vegetation and bush. Alluvium acts as a cap of the Barail and Surma and even
Sylhet limestone rocks making unconformity to each. It is highly weathered. The
age of the Alluvium is Recent or Subrecent.
TABLE: STRATIGRAPHIC SUCCESSION OF
JAINTIAPUR AREA

PALEOGEOGRAPHIC HISTORY
The Sylhet trough is a sub-basin of Bengal
basin in north-eastern Bangladesh,
contains a thick fill of Late Mesozoic and Cenozoic strata. The Sylhet trough
is bounded to the north by the Shillong Plateau, which is underlain by a
basement complex of Archean gneiss and minor green stone and Upper Proterozoic
granite. The paleogeography of the studied area is described below:
During the Middle Eocene time an
extensive marine transgression took place due to the conspicuous basinward
subsidence. The whole area was the site of deposition of shallow, clear water,
open marine. Sylhet limestone deposited at that time.
During Oligocene the sea moved
southwards from the Shillong massive due to the basinwide differential movement
of the area. Thus marine to deltaic trough transitional marine environment
prevailed. Barail group was deposited in this condition.
During the Early Miocene the
sandstone, shale and siltstone alternation in Surma group may have resulted
from fluctuating energy condition. Subsidence rate in Sylhet trough clearly
accelerated during the Miocene. The increased feldspar of Barail sandstone
indicates derivation from crystalline rocks of Eastern
Himalayas. Then a pelagic environment started the deposition of
Surma group.
During Early Pliocene the frontal
zone of Indo-Burman ranges deformed. The Sylhet trough continued to subsidence.
A fluvial environment took place in this time and an abrupt change in basin
geometry and tectonics happened.
In the Plio-Pleistocene a dramatic
subsidence was forced by south-directed overthrusting of the Shillong Plateau
over the Dauki fault. The formation of DupiTila indicates there was a
fluviatile environment. During the geological time the stream valleys represent
probably Lower Pleistocene time, when the gravels has deposited over the area.
After the deposition of gravel beds the area was tectonically affected and
hence the area was uplifted and took the shape of small hillocks. The gravels
are then highly oxidized and weathered may be in the Pleistocene age.
CHAPTER-5
LABORATORY ANALYSIS:
LABORATORY ANALYSIS OF SAMPLE:
It includes the following headings:
Grain size analysis
Thin section study
Grain size analysis: It is a procedure for sedimentary rock studies and by this a geologist
can get idea about the depositional mechanism of the sediments.
Grain size analysis dada:
Sample No. 1
Location: Hari
River
Weight sieved: 50gm

Sample No: 2
Location: Dupigaon
Weight sieved: 25gm

GRAPHIC REPRESENTATION:
The results of grain size analysis
that we got are represented in the graphical form by histogram and cumulative
curve for two samples.
HISTOGRAM:
A histogram has been drown in an
arithmetic paper taking weight percentage in the vertical scale and grain size
in horizontal scale. One histogram shows bimodal and another shows unimodal
distribution. In bimodal primary maxima is in between 0.2 5-0.125 mm and
secondary is in 0.63 to > 0.063 mm.
In unimodal the maxima is in between 0.25-0.125 mm
Bimodal distribution may be for following reasons:
Abnormal variation in deposition
history
or abundance of certain grain
size  in source material
Different modes of deposition
CUMULATIVE CURVE:
A cumulative curve has been drawn in
probability paper taking cumulative percent in vertical scale and grain size in
mm in horizontal scale. The figure gives an ‘S’ shaped curves from which
quartiles and percentiles have been determined. The method of Folk and Ward
(1957) has been used.
Graphic mean (Mz)=(Æ_ÆÆ)/3
Inclusive graphic standard deviation s_{ÆÆ)/4+(ÆÆ5)/6.4}
Inclusive graphic skewness Sk₁Æ_{Æ-2(Æ)}/2(ÆÆ)+{Æ5Æ-2(Æ)}/ 2(ÆÆ5)}
Graphic kurtosis, K_GÆ_Æ5ÆÆ)
VALUES OF STATISTICAL GRAIN SIZE PARAMETERS FOR THE TWO SAMPLES ARE GIVEN
BELOW IN TABULAR FORM:

RESULTS:
1.Tipam sandstone: Histogram of this sample shows it was deposited by unidirectional
current. Interpretation of cumulative curve shows the sediments deposited in
fluviatile environment.
2. DupiTila sandstone: The histogram of this sample shows that unidirectional
current deposited the sediment of this formation. The interpretation of
cumulative curve shows the rate of sediment deposition exceeds the dispersive
energy and the sediments are deposited in a high energy condition. Grain size
and sorting indicates that the sediments were deposited in fluviatile to
channel fill deposits.
THIN SECTION STUDY OF SYLHET LIMESTONE FORMATION:
Fig:
Observation under crossed polar (X₁₀)
Fossils:
Nummulites, Discocyclina, and shells of fossils.
Cement: The
cementing component is crystalline CaCO₃
Matrix: The
ground material is composed of clay matrix that gives the rock formation of gas
when react with HCl.
THIN SECTION STUDY OF SANDSTONE OF TIPAM SANDSTONE FORMATION:
Fig:
Observation under cross polarized light (X₁₀)
1. Framework grains:
Quartz: Both
monocrystalline and polycrystalline quartz have been found. Total amount is
50%.
Feldspar:
About 15%, mostly plagioclase feldspar is found.
Chert: About
10%
Mica: About 3%
Others: 5%
2. Matrix: The
matrix is mostly the silt and clay size particles. It is about 10%
3.Cement: The
cementing material is hematite. It is about 20%
4.Pore space:
Total area of pore space is 5%
Interpretation:
The presence if concavo-convex contact between two quartz grains indicates the
rock was undergone to a great depth in the region of high degree of compaction
and texturally matured.
THIN SECTION STUDY OF SANDSTONE OF BARAIL GROUP:
Fig:
Observation under cross polarized light (X₁₀)
1.
Framework grains:
Quartz: About
80% grains are quartz. Both mono and polycrystalline quartz grains are found.
Feldspar: Micas: About
5% grains are micas (mainly muscovite)
Lithic grain:
About 2%
Cement: There
is a significant amount of hematite cement. The total amount is 13%
Interpretation:
Both the presences of suture and concavo-convex contact in quartz grains
indicate the rock suffered high degree of compaction in a relatively high depth
and it is texturally matured.
THIN SECTION STUDY OF SILTSTONE OF BHUBAN FORMATION:
Fig:
Observation under crossed polars (X₁₀)
Framework grains:
Quartz: The
total amount of quartz grains is about 60%
Muscovite: The
lamella of muscovites is distinct. Amount of muscovite is 10%
Feldspar:
Tetragonal feldspars have been observed. It is about 1%
Biotite: The
presence of biotite is 3%
Chlorite: A
small amount of chlorite has been found which is about 2%
Matrix: The
amount of matrix is about 2%. The matrix is almost clay matrix.
Cement: The
cement is calcite cement. It is about 12%.