TECTONIC SET UP OF BENGAL BASIN Tectonic Map of Bengal Basin DeVELOPMENT OF BENGAL BASIN TECTONIC ELEMENTS of Bengal Basin

1. TECTONICS OF BENGAL BASIN
Overview
TECTONIC SET UP OF BENGAL BASIN
 TECTONIC MAP OF BENGAL BASIN
 DEVELOPMENT OF BENGAL BASIN
 TECTONIC ELEMENTS OF BENGAL BASIN

2. The first tectonic map of the then East Pakistan was prepared in
1964. Concise characteristics of basic tectonic elements of the then
East Pakistan and its neighbouring areas were prepared in 1965 while
the major tectonic features of the then East Pakistan were compiled
in 1966. Tectonic map of Assam was published in 1964 and the
tectonic classification of Bangladesh was made in 1972.
On the other hand tectonic framework and oil and gas prospects of
Bangladesh were published in 1978. The tectonic map of Bangladesh-
past and present was introduced in 1983. It is essential to have a
clear conception about the tectonic framework of Bengal
Basin/Bangladesh in order to evaluate the prospect of MINERAL
RESOURCES including oil and NATURAL GAS.
TECTONICS OF BENGAL BASIN/Bangladesh

3. The Bengal basin occupies most of Bangladesh and West Bengal
subsurface. The extent of the basin is indirectly indicated by the hills
surrounding it. To the south the basin is open to the Bay of Bengal,
where it passes to the Bengal deep-sea Fan. It slopes south from its
head at latitude 200 north to 100 south.
The Bengal deep-sea fan is almost 3000km long and 1000km wide
with a thickness of more than 12 km of sediments. Sediments from
delta are funneled south through the “Swatch of No Ground” (submrine
canyon), to become spread out on the fan by turbidity currents.
Shield areas are exposed on sides of the Bengal basin with dis-
appearance below a blanket of alluvium within the basin. The
Precambrian shields are mainly constituted of gneisses and meta-
sediments and local granites. The Shillong Plateau and the
Precambrian shield of Bihar and West Bengal are separated from each
other by the Garo-Rajmahal Gap which is 200km wide and controls the
big bends of Ganges and Brahmaputra rivers.
The Rajmahal and Garo hilla are connected by a submerged basement
ridge (Rangpur saddle) at a depth of 200m.

4. Map showing major tectonic
elements in and around the
Bengal basin. Hinge zone
demarcates the deeper basin
from the Indian Platform area.
Right-lateral N–S faults (e.g.,
Kaladan fault) are in the east. The
Dauki fault separates the Sylhet
trough from the uplifted Shillong
Plateau at the north

5. Bangladesh is divided into two
major tectonic units:
i) Stable Pre-Cambrian
Platform/Shelf in the
northwest, and
ii) Geosynclinal basin in the
southeast.
iii) A third unit, a narrow
northeast-southwest
trending zone called the
hinge zone separates the
above two units almost
through the middle of the
country. This hinge zone
is currently known as
palaeo-continental slope.

6. Stable Pre-Cambrian Platform : Stable Pre-Cambrian Platform also refers to as
the stable shelf of the BENGAL BASIN. It is the part of the basin that lies on the west
and northwest of the line joining Calcutta and MYMENSINGH known as Calcutta-
Mymensingh gravity high, which represents the hinge zone of the basin or the
basin ward extension of the stable shelf. The stable shelf is divided into three
major zones : i) Dinajpur slope, ii) Rangpur Saddle and iii) Bogra slope.
The stable shelf is composed of continental crust overlain by Cretaceous (144 to 66
million years ago) to Recent sediments. However, in isolated basins on the stable
shelf, there is Permo-Carboniferous (360 million years to 245 million years ago)
sediments with considerable amount of COAL.
The thickness of sedimentary column on the stable shelf of Bengal Basin varies
from less then 200m to 8,000m.
A large part of the basin is covered by Sylhet limestone of Eocene age (58 million
years to 37 million years ago) and act as a marker horizon to define the basin ward
extent of the stable shelf.
In Bangladesh part coal, LIMESTONE and HARDROCK are considered to be major
mineral resources found on the shelf. However, there are evidences that precious
metal and base metal of Indian shield may also be present.
TECTONIC ELEMENTS

7. The tectonic framework of
Bangladesh can be divided into two
main elements: the Indian platform
comprising the north western part
and the Bengal foredeep representing
the thick sedimentary column forming
geosynclinal area in the south east.
These two elements are separated by
a flexure zone named as “Calcutta-
Mymenshing hinge zone”. Further
the Bengal foredeep is divided into
two parts the folded flank in the east
and platform flanks on the west
which is demarcated by “Barisal high”.
The platform flanks includes Sylhet
trough, Faridpur trough, Hatiya
trough and the Madhupur high
structures. The deepest part of Bengal
Basin is the Patuakhali trough (Figure
7).

8. Rangpur Saddle:
It represents Indian Platform and connects the Indian Shield with the SHILLONG
MASSIF and the Mikir Hills. Shillong Massif is a large thrust block of the Indian Shield.
In Rangpur Saddle the basement is the most uplifted and is covered with thin
sedimentary deposits. In Madhyapara area of DINAJPUR the basement is only 130m
deep from the ground surface and is overlain by Dupi Tila Sandstone and Madhupur
Clay of Plio-Pliestocene age.
Rangpur Saddle can be divided into 3 parts- Rangpur Saddle, Northern Slope of
Rangpur Saddle and Southern Slope of Rangpur Saddle. It is evident from seismic
data acquired out by OGDC (Oil and Gas Development Corporation) during 63-64 in
Rangpur-Dinajpur districts that both the northern and the southern slopes of
Rangpur Saddle are quite gentle. The basement plunges gently from Madhyapara
towards the southeast upto the Hinge Zone, which is known as the Southern Slope
of Rangpur Saddle. The tentative boundary of the Rangpur Saddle with the
northern and the southern slopes has been marked at approximately 700m contour
line on the basement. It separates the Bengal Foredeep and the Himalayan
Foredeep.
TECTONIC ELEMENTS

9. Dinajpur Slope: The Northern Slope of Rangpur Saddle also known as Dinajpur Slope,
occupies the north western part of Rangpur-Dinajpur districts and gently slopes towards
the Sub-Himalayan Foredeep with dips 3-4° which sharply increases in Tetulia area. The
only well drilled in 1988 located on the north-western most tip of Bangladesh at SALBANHAT
by Shell for oil and gas exploration to probe a carbonate reef ended in basement with
depth 2518m penetrating the Mio-Pliocene sequence without encountering Eocene
Limestone. The nature of junction between the Dinajpur Slope and the Sub-Himalayan
Foredeep is not clear.
Bogra Slope/Shelf: represents the southern slope of Rangpur Saddle which is a
regional monocline plunging towards southeast gently to Hinge Zone. This zone marks the
transition between the Rangpur Saddle and the Bengal Foredeep from depositional as
well as structural point of view. The width of Bogra Shelf varies from 60-125 km up to the
Hinge Zone and the thickness of the sedimentary sequence increases towards the
southeast. Stanvac Oil Company (SVOC) carried out aeromagnetic and seismic surveys in
the mid-fifties followed by two wells at Kuchma and BOGRA. Seismic contours on top of
Eocene Limestone (Bogra limestone) show regional dip of 2-3° besides revealing a
number of NE-SW trending faults of which BOGRA FAULT is the most prominent. The
attitude of Sylhet Limestone most possibly reflects the surface of the Archean Basement.
There is no closed anticlinal fold in this tectonic zone. The Bogra fault did not provide the
seal to the structural trends at Kuchma and Bogra for accumulation of commercial
HYDROCARBONs.
TECTONIC ELEMENTS

10. Calcutta-Mymensingh Gravity High: It reflects a tectonic element known as
Hinge Zone and more recently as palaeo-continental slope in the framework of
Bangladesh.
The Hinge Zone is a narrow strip of about 25 km wide complex flexure zone, which
separates the Bengal Foredeep from the shelf zone. It trends approximately N 30° E
along the Calcutta-Pabna-Mymensingh gravity high and extends upto the western tip
of Dauki fault. This zone is characterised by the sharp change in the dip of the
basement rocks associated with deep-seated displacements in faults and is reflected
on the gravity and magnetic anomalies. The Eocene limestone dips at about 200 in
this zone as compared to 2-30 in the shelf zone. The seismic interpretation shows that
the depth of the Sylhet Limestone - a strong seismic reflector - increases from 4000m
to 9000m within a narrow zone of 25-km.
Hinge Zone is connected with Bengal Foredeep by deep basement faults that
probably started with the break up of Gondwanaland. Since then they have been
repeatedly reactivated. In the northeast of Bangladesh the Hinge Zone turns to the
east and seems to be connected with the Dauki Fault, probably by a series of east-
west trending faults.
TECTONIC ELEMENTS

11. Geosynclinal Basin the geosynclinal basin in the southeast is characterised by the
huge thickness (maximum of about 20 km near the basin centre) of clastic sedimentary
rocks, mostly sandstone and shale of Tertiary age. It occupies the greater Dhaka-Faridpur-
Noakhali-Sylhet-Comilla-Chittagong areas. The huge thickness of sediments in the basin is a
result of tectonic mobility or instability of the areas causing rapid subsidence and
sedimentation in a relatively short span of geologic time. The geosynclinal basin is
subdivided into two parts i.e fold belt in the east and a foredeep to the west.
Bengal Foredeep occupies the vast area between Hinge Line and Arakan Yoma Folded
System and plays the most important role in the tectonic history of Bengal Basin.
Tectonically, Bengal Foredeep can be divided into two major regions- (a) Western Platform
Flank and (b) Eastern Folded Flank. The Western Platform flank is further subdivided into (a)
Faridpur Trough (b) Barisal-Chandpur High (c) Hatiya Trough (d) Madhupur High and (e)
Sylhet Trough.
Faridpur Trough situated adjacent to Hinge Zone is characterised by a general gravity
low with development of Neogene sequence. Sylhet Limestone is 6500 m deep in area
south of the confluence of the PADMA and the JAMUNA. Chalna and BAGERHAT are the two
notable structural highs of very low amplitude.

12. Hatiya Trough represents the deepest trough of Bengal Basin, which received highest
accumulation of clastic sediments. The axis of Bengal Foredeep runs through the apex of
Hatiya Trough. Shahbazpur (BHOLA), Kutubdia, Sangu and a large number offshore structures
are located here, of which Sangu is under production while Shahbazpur and Kutubdia are
awaiting development.
Madhupur High is represented by Pleistocene terrace of Madhupur Garh separates Faridpur
Trough from the Sylhet Trough (SURMA BASIN). Here, the basement is relatively uplifted as
evident from gravity and aeromagnetic data. Morphological studies reveal a pronounced
recent morphological upheaval east of Madhupur. Besides, Nagarpur dome, south of TANGAIL
and Nandina high showed that these are portions in the basement without deforming the
sedimentary sequence overlying these features.
Madhupur or Mymensingh High: the area represents an undulated TOPOGRAPHY slightly
elevated from the adjacent active FLOODPLAIN. At MADHUPUR, tectonic disturbances can easily
be observed where Dupi Tila sand is exposed under the Madhupur Formation.
Most of the authors believe that the MADHUPUR TRACT represents a tectonically uplifted
surface. A few also believe that the area was uplifted in very recent time and referred to the
EARTHQUAKE of 1762 as the Madhupur tract occurs along the axis of the belt of 'Volcano
action', extends in a north-western direction through CHITTAGONG and DHAKA. They
considered numerous low lakes in the Sylhet basin to be caused by SUBSIDENCE compensatory
to the elevation of the Madhupur tract. According to their belief, the uplift of Madhupur
tract resulted the diversion of the BRAHMAPUTRA River.

13. Barisal-Chandpur High is interpreted as a gravity and magnetic anomaly caused
by a magmatic body at great depth. This zone is located between Faridpur
trough and Hatiya trough of the Bengal Foredeep. The width of the zone is
about 60 km and apparently corresponds to an uplift of the sedimentary cover.
A number of gravity anomalies are spread over this zone. A paleo-high
stretching from Barisal-Chandpur High in the NE direction has been presumed
and the ridge was interpreted to turn south of Barisal-Chandpur High in north-
south direction merging with the Ninety east ridge. There is no definite
evidence for existence of such a ridge dividing the Bengal Foredeep into two
parts. PATHARGHATA, MULADI, CHANDPUR, LAKSHMIPUR [Laksmipur], MUNSHIGANJ,
Kamta and DAUDKANDI are the main anticlinal structures of this zone. Muladi
wells failed to yield positive result in respect of commercial accumulation of
HYDROCARBONs due to lack of northern closure. This zone has not been
sufficiently investigated by seismic surveys.

14. Sylhet Trough: Indian Platform bounds the trough from the west while it is
open in the southwest to the main part of Bengal Basin. It is an oval shaped
trough about 130 km long and 60 km wide. Sub-meridional trending anticlinal
folds of Chittagong-Tripura Folded Belt gradually plunge northward to the
Sylhet Trough. In cross-section the Sylhet Trough is sharply asymmetrical with
comparatively gentle southern and steep faulted northern slope.
Sylhet Trough is situated on the southern side of the Shillong Massif and
corresponds to the vast low lands of Surma Valley with numerous swamps
(haors) where absolute elevation marks even below the sea level. It is a sub-
basin of the Bengal Foredeep in the northeastern part of Bangladesh and is
characterised by a very pronounced, vast, closed negative gravity anomaly up
to 84 mgl (Milligal). Shillong Massif forms the northern boundary of Sylhet
Trough while the great Dauki Fault separates the trough from the Massif. The
Trough is bounded on the east and southeast by the sub-meridional trending
folded belt of Assam and Tripura as the frontal deformation zone of Indo-
Burman Ranges.

15. Dauki Fault with 5 km wide fault zone forms the contact between Shillong Massif
and Sylhet Trough. The evolution of Sylhet Trough includes (i) a passive
continental margin (Pre-Oligocene) to (ii) a foreland basin linked to the Indo-
Burman Ranges (Oligocene and Miolene) to (iii) a foreland basin linked to south-
directed over thrusting of Shillong Plateau (Pliocene-Holocene). The anticlinal
folds of Habiganj, Rashidpur, Bibiana, Maulvi Bazar, Katalkandi, Fenchuganj,
Harargaj, Patharia, Beani Bazar (Mama Bhagna) and Kailas Tila, which occupy the
southern rim of Sylhet Trough have sub-meridional trend in contrast to sub-
latitudinal trending Chhatak, Jalalabad, Sylhet, Dupi Tila and Jatinga structures.
These two structural trends form a syntaxial pattern at the northeastern tip of
Sylhet Trough. The Neogene sediments have excellent development in Sylhet
Trough while the Paleogenes are at greater depths.
Sylhet Trough is the most prospective petroliferous province of Bangladesh with
10 gas fields (Chhatak, Jalalabad, Sylhet, Kailas Tila, Beani Bazar, Fenchuganj,
Rashidpur, Maulvi Bazar, Bibiana and Habiganj) of which Jalalabad, Sylhet, Kailas
Tila, Rashidpur and Habiganj are producing now about 1000 million cubic feet per
day (MMCFD) for generation of power, manufacture of FERTILISER besides meeting
the industrial, commercial and domestic needs to a great extent thus contributing
immensely to the economic development of Bangladesh.

16. Folded Belt represents the most prominent tectonic element of Bengal Foredeep
with general sub-meridional trending hills parallel to the Arakan Yoma Folded
System. Folded belt extends within Bangladesh for 450 km (N-S) and about 150 km
wide covering an area of 35,000 sq km of on-shore area. A large number of narrow,
elongated N-S trending folds of the eastern part of Bangladesh (Sylhet and
Chittagong Divisions), Tripura, southern part of Assam, Mizoram and Myanmar
territory adjacent to S-E of the Chittagong Hill Tracts occupy the Folded Belt west
of the Arakan Yoma Folded System. The folds are characterised by ridge forming,
box-like in cross section, high amplitude with variable width and lie en-echelon
with the adjacent structures. The elevation of these elongated anticlinal folds in
Bangladesh ranges from 100 -1,000m. Some of the structures are faulted and
thrusted and the intensity of folding increases gradually from west to east.
Consequently, the structures of the eastern part are tightly folded, faulted and
thrusted with narrower synclines between them.

17. The Folded Belt is sub-divided into two tectonic zones- Western Zone and Eastern Zone
according to the intensity of folding and other structural features. Western Zone consists of
relatively simpler GEOLOGICAL STRUCTURES like Beanibazar (Mama Bhagna), Kailas Tila,
Fenchuganj, Kathalkandi, Maulvi Bazar, Rashidpur, Habiganj, Titas, Rokhia (Saldanadi),
Lalmai, Trichna, Agartala, Lambusara, Gojalia, Feni, Semutang, Halda, Lambaghona,
Maheshkhali, Patiya, Inani, St Martin besides the moderately tight folded anticlines like
Bachia, Langtrai, Barmura, Atharamura, Dhumbura, Tulamura, Changotang, Sardeng,
Gobamura, Sitapahar, Bandarban, Matamuhari, Olah Taung, Dakhin Nila etc and Mayu,
Pingna and Sin in Myanmar.
The Eastern Zone includes Patharia, Harargaj, Chargola, Kanchanpur, Machlithum, Sarkhan,
Langai, Badarpur, Chatrachura, Masimpur, Rengte, Bhuban, Bhairabi, Hatch Hak, Sentet,
Zanlawn, Zabwak, Maul Vawm, Jampai, Bhuachari, Shishak, Kasalang, Barkal, Utanchatra,
Belasari, Gilasari, Mowdok, Langsen, Thuamphui, Maurawap, Phalphang and Kaletwa.
(Dashed structures are in Bangladesh rest in Assam, Tripura and Mizoram of India).
The Western Zone is the most important and prospective oil and gas province of Bangladesh
with 12 fields from north to south eg Kailas Tila, Beanibazar, Fenchuganj, Maulvi Bazar,
Rashidpur, Bibiana, Habiganj, Titas, Bakhrabad, Rokhia, Feni and Semutang.
TECTONIC ELEMENTS Contd