Tectonic Framework of Bengal Basin

1. The Bengal basin is divided
generally into two major tectonic
units:
1) Stable platform in the west-
northwest.
2) Geosynclinal basin in the
east-northeast.
The third unit, a narrow northeast
– southwest trending zone,
known as Hinge zone which
separates the two units. The
hinge zone is also known as
paleo-continental slope.

2.  The Bengal Basin became a remnant
ocean basin (Graham et al., 1975;
Ingersoll et al., 1995) (Fig.) at the
beginning of Miocene because of the
continuing oblique subduction of India
beneath Burma (West Burma Block).
 The Bengal Basin, as a remnant ocean
basin, has three distinct geo-tectonic
provinces: (1) Passive to extensional
cratonic margin in the west, the Stable
Shelf Province; (2) the Central Deep Basin
Province or the remnant basin; and (3) the
subduction-related orogen in the east, the
Chittagong-Tripura Fold Belt (CTFB)
Province.
 These geo-tectonic provinces have
been related to a regional plate
tectonic scenario. Each of these
provinces has its own distinctive
tectonic and stratigraphic framework
and history of basin-fill.

4. The stable platform refers to the
stable shelf of Bengal basin,
which is divided into three major
zones:
a.Northern slope of the Rangpur
saddle
b. Rangpur saddle (Dinajpur
Block)
c. Southern slope of the
Rangpur saddle or western
foreland shelf or Bengal shelf or
Bogra shelf
Rangpur Saddle represents Indian Platform and connects the
Indian Shield with the shillong massif and the Mikir Hills. Shillong
Massif is a large thrust block of the Indian Shield. In Rangpur
Saddle the basement is the most uplifted and is covered with thin
sedimentary deposits.

5. It is continental crust overlain by the
sediments of Cretaceous to Recent
times.
- in isolated basin on the stable
shelf, there are permo
carboniferous sediments with
considerable amount of
Gondwana Coal.
- The thickness of the upper part of
the shelf is 130m and lower part
of the shelf is 5000m.

6. The basement is shallowest in the
Rangpur saddle (Dinajpur Block),
from there the basement plunges
greatly towards the south-
southeast up to hinge zone, which
is known as southern slope of
Rangpur saddle (Dinajpur block) or
the western foreland shelf.
The plunge of the basement in the
northwest towards the sub-
Himalayan Foredeep is rather
sharp and known as northern slope
of Rangpur saddle.

7. Rangpur saddle: This structural
feature is the part of north eastern
prolongation of the Indian shield and
separates the sub-Himalayan
Foredeep in the north from the
Bengal basin, particularly from the
latter’s stable shelf in the south.
The Rangpur saddle connects the
main shield area with the Shillong
Massif and Mikir Hills to the east.
The results of gravimetry, seismic
and aeromagnetic surveys as well
as those of drilling reveal that the
Rangpur saddle is overlain merely
by Recent to sub-Recent deposits.
However, there are also indications
of N-S aligned graben structures
with Gondwana fill.

8. The sedimentary cover on the Rangpur saddle is
about 130m at the shallowest part in Maddhapara
and is represented by Dupitila sandstone and
Madhupur clay of Plio-Pleistocene age, while the
thickness gradually increases to 700m both in north
and in south direction.
The tectonic boundary of the Rangpur saddle with
the northern and southern slopes has been marked
at 700m contour line on the basement. Here the
sedimentary deposit form monoclinal beds with 10-20
dip. The width of Rangpur saddle is approximately
100km. It is connected with the Shillong Massif by a
series of sub-meridional (N-S trending) trending
faults.

9. According to Ameen et al (2007) the rock of the Dinajpur
Block consist of a suite of tonalite and granodioritic
rocks, various deformed to granitic gneiss and intruded
by younger monzogranite. A tonalite obtained in this
block at a depth of 227.48m in drill hole Bh-2 at
Maddhapara records an age of 1722±6 Ma.
Paleoproterozoic rocks with similar magnetic ages are
unknown in the adjacent Indian Blocks of the
Chhotanagpur Plateau (IS) or Shillong Massif. This lack
of comparable ages may be due to the paucity of precise
radiometric ages from the Indian Terrains or more likely,
because there are real age differences, with the buried
rock at Maddhapara. Representing a separate and
discrete micro-continental fragment (Dinajpur Block) that
was trapped by the northward migration of India during
Gondwana dispersal.

10. NORTHERN SLOPE OF
THE RANGPUR SADDLE
AND SUB HIMALAYAN
FOREDEEP:
The basement of the northern
slope of the Rangpur saddle
(Dinajpur block) occasionally
designated as Dinajpur slope,
dips down to a depth of about
4000m and is covered mainly by
the Siwaliks of Middle Miocene to
Pleistocene age.
The plunge of the basement is
about 30-40towards the sub-
Himalayan Foredeep. There are
indicates of wedges of Paleogene
rocks in the northernmost sector
of this area as well as Gondwana
graben features in N-S alignment.

11. However a well drilled on the north-westernmost tip at
Salbanhat in 1988 for Oil and Gas exploration to probe a
carbonate reef but ended in Basement with depth of
about 2518m penetrating the Miocene formations lastly
encountering Eocene Limestone.
Further plunges are more pronounced. Unlike the
southern slope, the northern slope is rather narrow as
the sub-Himalayan Foredeep. The nature of junction
between the northern slope or the Dinajpur Slope and
the Sub-Himalayan Foredeep is not known. The beds
are of simple monoclinal dipping.

12. SOUTHERN SLOPE OF THE RANGPUR
SADDLE OR THE WESTERN FORELAND
SHELF:
The western Foreland shelf is build up of the
gently southeastward dipping Archean
Basement complex and of overlying deposits
of the Gondwana system, the Mesozoic
Rajmahal volcanics, Late Cretaceous-
Paleocene deposits and of Neogene
progradational delta sequence.
The width of the stable shelf between paleo-
continental slope (Hinge zone) and Rangpur
Saddle (Dinajpur block) ranges from 60km to
about 130km. Seismic contours on the top of
Eocene (Sylhet) Limestone show regional
dip of 10-30, besides revealing a number of
NE-SW trending faults. There is no closed
anticlinal structures found in this tectonic
zone.

13. The structural development
of the stable shelf, which also
has been designated as the
Bogra shelf in the literature,
began in the Carboniferous-
Permian times, when graben
and half graben features with
terrestrial fluviatile basin fill
were formed. The next major
structural feature deformation
took place at the turn from
Jurassic to Cretaceous, when
the east Gondwana broke
away from the rest of
Gondwanaland. This event is
evidenced by the Rajmahal
Lava flow.

14. PALEO-CONTINENTAL
SLOPE OR HINGE
ZONE:
The paleo-continental slope
also known as hinge zone is
one of the major structural
units of regional importance
within the Bengal Basin. Since
it has no surface outcrop, it is
known merely by evidence of
gravimetric, seismic and
aeromagnetic surveys. This
zone marks the structural as
well as the depositional
transition zone between the
Foredeep in the southeast and
stable shelf in the northeast.

16. This Hinge zone has been often
interpreted as an Eocene hinge
line or Hinge zone, which was
active as Neogene hinge line,
stretches from the Calcutta area
of west Bengal in the
northeastern direction across
the Mymenshing area as far as
the Shillong Plateau, where it is
intersected by the E-W trending
Dauki Fault zone.

17. The hinge zone is characterized by
sharp change in dip of the basement
rocks associated with deep-seated
displacements in faults and is
reflected on the gravity and magnetic
anomalies. The Eocene Limestone in
this zone dips at an angle of 150-300
in southeastern direction as
compared to 20-30 in the shelf zone.
The seismic interpretation shows that
to the depth of the Sylhet Limestone
a strong seismic reflector increases
from 4000 to 9000m within a narrow
zone of 25km.

18. -During the subsurface interpretation of
the south eastern part of the west Bengal
in India, a zone of flexure on the top of the
sylhet Limestone was recognized, which is
the extension of the hinge zone in India. In
the area of surma basin, the paleo-
continental slope is apparently burried
beneath the thick pile of Neogene strate.
-Hinge zone is connected with Bengal
Foredeep by deep basement faults that
probably started with the breakup of
Gondwanaland. Since then they have
been repeatedly reactivated.
-In the northeast of Bangladesh the hinge
zone turns to the east and seems to be
connected with the Dauki fault, probably
by a series of E-W trending faults.

19. 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 basin is subdivided into two
parts
a. Foredeep in the west/Central
Deep Basin
b. Mobile fold belt in the
east/Subduction related
orogeny/Chittagong Tripura
Fold Belt (CTFB)

20. Bengal Foredeep/Central Deep Basin
The Bengal Foredeep is a region of great
subsidence of the earth’s crust between
the Indian Platform al Shelf in the west
and Indo-Burman Ranges in the east.
It is about 200 km wide in the north-east
and broadens southwards to more than
500 km.
It is characterized by huge thickness of
sedimentary rocks mostly sandstone and
shale of Tertiary age due to the result of
tectonic mobility or instability of the
areas causing rapid subsidence and
sedimentation in relatively short span of
geological time (Imam, 2005).

21. The maximum thickness of
sedimentary pile at the
deepest part of the Bengal
Foredeep exceeds 20 km
(Hiller, 1988).
The Bay of Bengal serving as
present depositional area for
the progradational delta
sequences forms the
continuation of the
southwards structurally open
Bengal Foredeep (Sengupta,
1966; Curray and Moore,
1971, 1974).

22. The Deep Basin has been sub-
divided into the following tectonic
domains1,12,35: (
1) Sylhet Trough;
(2) Tangail–Tripura High (also
known as Madhupur High);
(3) Faridpur Trough;
(4) Barisal Chandpur High;
(5) Hatia Trough (Figure 3).
Although most of the Bengal Basin
is slowly subsiding, subsidence in
the trough zones is more rapid.
The Sylhet Trough in the
Southeastern Sub-basin is the
deepest part of the Bengal Basin,
which is marked by a zone
negative gravity anomaly (–84 mgl)

23. THE FARIDPUR TROUGH
-occurring south of Tangail-Tripura
high
-It is bounded by hinge zone in the
west and Barisal-Chandpur high in
the east. The width of the zone
varies from 75km to 125km. The
Fardpur trough is characterize by a
general gravity low of N-E trend
indicating a general subsidence of
the Basement. The thickness of
sedimentary cover is rather
enormous here. In Khulna and
Bagerhat area two small amplitude
structures are known to exist.
-Munshiganj and Kamta structures
occupy the northeastern part of the
trough.

24. THE HATIA TROUGH
It covers part of Noakhali and
vast areas of the Bay of Bengal
and represents the deepest
trough within the Bengal
Foredeep, which opens
southward.
-The axis of Bengal Foredeep
runs through the apex of Hatiya
trough. The trough is sharply
asymmetric with gentle western
flank of the Bengal Foredeep.
-Hatiya trough is probably
connected with a series of step
like faults in the basement.
Hatia Trough extends south of the
deltaic area into the Bay of Bengal, where
the basin edge is confined near the 200 m
bathymetric contour

25. -The sedimentary sequence here is
over 20km thick. The Paleogene
sediments probably lie at a great
depth, whereas the later Neogene
sediments have considerable
thickness in the trough.
-The eastern part of the trough
consists of predominantly argillaceous
sediments while the percentage of
arenaceous sediments is more in the
structures of eastern part.
Begumganj, Sandwip, Hatiya, Sangu
etc. are the prominent structures in the
zone.
-The offshore Islands of Rabnabad,
Donmanik, Shahbazpur, Char-Jabbar
represents anticlinal fold.

26. The southern part of the Bengal
Foredeep is subdivided into two
sub-basins, the Faridpur Trough
and The Hatia Trough, separated
from each other by the Barisal-
Chandpur High (Alam, 1972;
Dasgupta, 1977; and Guha,
1978).
Banerji (1984) has assumed the
existence of an “East-Bengal
Ridge”, a paleo-high stretching
from the Barisal-Chandpur high in
north-eastern direction as far as
Upper Assam.

27. The width of the zone is about
60km. The ridge was interpreted
to turn south of the Barisal-
Chandpur high in N-S direction,
merging the Ninetyeast Ridge.
But, there is no proof of the
existence of such a major
structure dividing the foredeep
into sub-basins.
Thus, the Barisal-Chandpur high
can be interpreted as a gravity
and magnetic anomaly caused
by a magmatic body at great
depth.
Patharghata, Muladi, Chandpur,
Lakshmipur, Munshiganj, Kamta
and Daudkandi are the main
anticlinal structure of this zone.

28. Murphy and Staff
BOGMC (1988) and
BOGMC
(1997) show the area
between the Hinge
Zone and
Barisal–Chandpur
Gravity High to be
attenuated or
thinned continental
crust, so that the
continent–ocean
crust boundary lies
along the Barisal –
Chandpur
Gravity High (Fig. 2a
and b).
-A Continent–Ocean Crust Basalt
(COB) line has been recognized
between the shelf-break in the west and
the Barisal–Chandpur Gravity High
(BCGH) in the east

30. TRIPURA-MADHUPUR THRESHOLD:
The trough of Bengal Foredeep narrows
northeastward between Tripura uplift on
the eastern side and the Madhupur high
on its western side. Both are connected
by the Tripura-Madhupur threshold.
The Tripura-Madhupur, being aligned
NW-SE, forms the south western limit of
the Surma basin, which is the
northernmost sub-basin of the Bengal
Foredeep.
The area represents an undulated
topography, slightly elevated from the
adjacent active floodplain. At Madhupur,
tectonic disturbance can easily be
observed where the Dupitila sandstone
is exposed under the Madhupur
formation.

31. -The Tangail–Tripura High
(Madhupur High) is a NW–
SE trending band of relatively
uplifted basement.
-This is approximately a100
km wide zone recognized by
the analysis of gravity and
aeromagnetic data

32. The northernmost sub-basin of the
Bengal Foredeep is known as the
Surma Basin, which is often
referred to as Sylhet Trough.
It is situated south of the Shillong
Massif and corresponds to the vast
low lands of surma valley with
numerous swamps (Haors) where
absolute elevation marked even
below the sea level.
It is a sub basin of the Bengal
Foredeep and is characterized by a
very pronounced, vast, closed
negative gravity anomaly upto
84mgal.

34. -Shillong Massif forms the
northern boundary of the Sylhet
trough while the Dauki fault
seprates the trough from the
massif.
-The trough is bounded on the
east and south east by the sub-
meridional trending folded belt
of the Assam-Burmese Ranges.

35. Johnson & Alam (1991) proposed
that the Sylhet Trough evolved from
a passive continental margin (Pre-
Oligocene), through a foreland
basin linked to the Indo-Burman
Ranges (Oligocene and Miocene) to
a foreland basin to the south-
directed thrusting of the Shillong
Plateau (Pliocene to Holocene).
All the structural features forming
trough in the north, east and south
(Fig,2) are probably the results of
tensional or compressional
movements in post-Miocene times
(Reimann, 1993).

36. The anticlinal folds of Habiganj,
Rashidpur, Bibina, Maulavi Bazar,
Kathalkandi, Fenchuganj,
Hararganj, Patharia, Beani bazar
and Kailas tila which occupy the
southern rim of sylhet trough have
sub-meridional trended in contrast
to sub-latitudianl trending Chhatak,
Jalalabad, Sylhet, Dupitila, and
Jaintia Structures.
These two structural trends form a
syntaxialpattern at the northeastern
rim of the Sylhet trough.
The Neogene sediments have
excellent development in the sylhet
trough while the Paleogene lie at
greater depth.

38. DAUKI FAULT ZONE:
The northern Border of the Surma
Basin is marked by the Dauki fault.
The fault system is of regional
importance described as “Dauki
Tear Fault” by Evans (1964) is the
result of vertical torsions caused by
subduction complexes along the
southern and northern edges of the
part of the Indian Shield, also
known as Shillong Foreland Shield.
Additionally, the central portion like,
the present day Shillong Plateau
and the Mikir Hills experienced
increasing buoyancy at the same
time.
The Dauki fault is a major fault along the southern
boundary of the Shillong Plateau that may be a source
of destructive seismic hazards for the adjoining areas,
including northeastern Bangladesh.[ The Dauki Fault lies in the Dauki town of Shillong
Plateau.

39. -The Dauki Fault, which is inferred to
go through the southern margin of the
Shillong Plateau, is an E–W trending
reverse one inclined towards the north.
-The Dauki fault has a vertical uplift to
the north, causing the Shillong-Mikir
Hills Plateaus as an uplifted region with
northward tilting. These faults are
similar to upthrusts.
-The Dauki Fault was believed to be
active during the Late Quaternary time
by the geomorphic features of the
Shillong Plateau, the gravity anomaly
data, and uplifted Tertiary and
Quaternary deposits on the southern
foothills of the Shillong Plateau.
The Dauki fault is a major fault
along the southern boundary of
the Shillong Plateau that may
be a source of destructive
seismic hazards for the
adjoining areas, including
northeastern Bangladesh.

40. The ~300 km long Dauki Fault
demarcates the elevated
Shillong Plateau, part of the
Indian Shield to the north and
the deep basin to the south.

41. The great Assam earthquake of 12th
June, 1897 was responsible for the
formation of major tectonic disturbance
in Shillong, Meghalaya.
The release of these combined torsional
forces took place along the Dauki Fault.
The fault zone has divided the former
Northern Foreland shelf into two
sections. The northern part was uplifted
with the rising Shillong Massif, the
southern part has been down faulted
and concealed beneath a thick clastic
sequence of Plio-Pleistocene age. The
maximum throw of the fault system is as
much as 18km in the eastern half
whereas the throw decreases
significantly towards the west.
According to Evans (1964) the amount
of actual topographic displacements on
both sides is about 13000m.

42. The Eocene Flysch sequences
of the Indo-Burman ranges
constitute the eastern limit of
this tectonic province. However
based mainly on the structural
configuration of the anticline, a
sub-division of this zone into
two belts have been suggested
1. Mizo fold belt in the east
2. The Tripura-Chittagong fold
belt in the west
The mentioned belts are
separated by the NNW-SSE
striking Kaladan fault.

43. A line drawn from the western slope of the
St. Martin’s Island in NNW direction to the
western shore of Sondwip island marks
approximately the western limit of the fold
belt in the Bay of Bengal.
Onshore, the line continues as far as
Daudkandi, follows the Meghna River
upwards, turns in the Ashuganj area to N-
E and finally at the foot hills of the Shillong
Massif NE of Sylhet.
-The fold belt extends within Bangladesh
for 450km (N-S) and about 150km wide
covering an area of 350002km of onshore
area.

44. The sediments outcropping in the fold
belt range age wise from Lower
Miocene to Recent. Data reveal that
thickness of Miocene strata increases
eastwards, from 4000m in Tripura to
as much as 7000m in Mizoram. The
same trend prevails in the Chittagong
Hill tracts.
In response to the eastward directed
subduction of Indian plate, the
Molasse sediments of this zone were
folded into a series of elongated,
doubly plunging, asymmetrical folds
arranged en-echelon.
The alignment of the folds follows a
NNW-SSE in the eastern portion of the
surma basin. A slight convexity to the
west characterizes the anticlinal
structures in the Tripura and parts
Mizoram.
Mizoram, state of India. It is located
in the northeastern part of the
country and is bounded by
Myanmar (Burma) to the east and
south and Bangladesh to the west
and by the states of Tripura to the
northwest, Assam to the north, and
Manipur to the northeast.

45. Paleogene sediments lie at
a greater depth and have
not been encountered any
well so far. The folds belts
are divided into two zones
according to structural
features and intensity of
tectonic forces:
1.The western Zone
(Neogene sediment)
2. The Eastern Zone
(Paleogene sediments)

46. THE WESTERN ZONE:
Tectonically the western zone consists
of relatively simple anticlinal structure.
A large number of structures are
included in this zone like Beani bazar,
Kailas tila, Fenchugang, Kathalkandi,
Maulavi bazar, Rashidpur, Habigang,
Titash, Saldanadi (Rokhia),
Lalmai,Trichna, Agartola, Lambusara,
Gojalia, Feni, Semutaung, Halda,
Lambaghona, Maheshkhali, Patiya,
Inani, St. Martin’s etc.
Besides the moderately light folded
anticlines like Bachia, Langtrai,
Barmura, Aharamura, Dhumbura,
Tulamura, Changotaung, Sardengal,
Gobramura, Sitapahar, Bandarban,
Matamuhuri, Olahtaung, DakhinNhila
etc. and Mayu, Pingna and Sin in
Myanmar.

49. The width of the zone varies from
100km in the north 60km in the south.
The length of the fold varies from 20-
70km whereas the width varies from 8-
12km.
Among the above structures, Titas and
Sonagazi are covered by Recent Alluvial
deposits.
Dupitila sediments are reported from
Beani bazar, Kailas tila, Fenchuganj,
Rashidpur, Habiganj, Maulavi bazar and
Lalmai structures.
In Saldanadi, Semutaung, Jaldi and
Maheshkhali structures Tipam
sandstone is reported from the core of
the folds.

50. Bokabil sediments are
exposed in Kathalkandi,
Patiya, Lambaghona and
Inani structures. Bhuban
sediments are exposed in
Changotaung, Sitakund,
Sitapahar, Bandarban,
Olahtaung, DakhinNhila
and Matamuhuri.
Beani bazar, Kailas tila,
Fenchugang, Kathalkandi,
Maulavi bazar, Rashidpur,
Habigang, Titas,
Bakhrabad, Lalmai,
Sonagazi are simple closed
structures with wide flat and
gentle flanks.

51. In structures like Rokhia, Semutaung,
Jaldi, Patiya and Inani are relatively steep
with box like cross section.
The width of the synclines is more than
anticlines and ranges from 15-20km.
The width of the synclines decreases
towards the east. The amplitude of the fold
varies from 200m to 3500m or more in the
crest.
The dip varies from 30-100 and in the
flanks they increase from 300-600 and in
structures go up to 800.

52. THE EASTERN ZONE:
A number of likely structures of
Chittagong Hill Tracts, Mizoram
(India), Tripura (India) and
southeastern part of the Sylhet
and Assam (India) constitute the
eastern zone of the fold belt.
Patharia, Haraganj, Chargola,
Kanchanpur, Machlithum,
Sarkhan, Langai, Badrapur,
Chatrachura, Masmipur, Rengte,
Bhuban,Bhairabi, Sentet,
Zanlawan, Zabwak, Maul vawm,
Jampai, Bhuchari, Shishak,
Kasalang, Barkal, Utanchatra,
Belasari, Gailasari, Mowdok,
Langsen, Thuaphuli, Maurawap,
Phalphang and Kaletwa.

54. The zone is characterized by
narrow ridge like faulted
structures tightly folded cores
and steep flanks.
Linearly elongated folds lie at
en-echelon to each other. The
width of the structures varies
from 10-12km towards east
within Bangladesh.
The Bhuban sediments are
exposed in the core of
Habiganj, Patharia, Kasalong,
Jamalpur, Shishok, Barkal,
Utanchatra, Gailasari and
Mawdok structures.

57. This deeper basin passes east into the onshore
Chittagong Hill Tracts (CHT) which is deformed
into a series of
N– S trending folds and east dipping thrusts.
The Neogene strata of the Chittagong Hill Tracts
and their easterly continuation in Burma are
separated from the Cretaceous-Palaeogene Indo-
Burman Ranges to the east by the Kaladan Fault
(Sikder & Alam, 2003).

58. Kaladan Fault marked the eastern boundary of the
Mizoram-Tripura-Chittagong folded belt and bears
the important clue to the tectonic evolution of the
whole folded area. The fault trends north-east-south-
west along the Kaladan river of Arakan coast. It
extends from latitude 20'30'N to 22'20'N and covers
a distance of almost 270 km. This long fault is
traceable and identified on satellite images. On the
basis of faulting nature and pattern it is termed as a'
transformed fault.
The Sagaing Fault is a major fault in Burma, a
continental transform fault between the Indian plate
and Sunda Plate that connects spreading centers in
the Andaman Sea and the continental convergence
zone along the Himalayan front.

59. Name of the Structure Trend
Kaladan fault NNW-SSE
Pataposagiri NE-SW
Western Foreland
Shelf
S-E
Continental slope
(hinge zone)
S-E
Dauki fault E-W
Surma basin E-W
Shillong Plateau E-W
CHT NNW-SSE
DIRECTIONS: