Paleozoic Era

2. • Bangladesh is divided into two major tectonic units: i)
Stable Pre-Cambrian Platform in the northwest, and ii)
Geosynclinal basin in the southeast. 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.
Stable Pre-Cambrian Platform in Bangladesh, refers to the
stable shelf of the bengal basin.
In Bangladesh, part of the Bengal Basin, the stable shelf can
be divided into three major zones. They are Dinajpur slope
(platform), Rangpur Saddle and Bogra slope. It is composed of
continental crust overlain by Cretaceous (144 to 66 million
years ago) to Recent sediment. 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. In Madhyapara area of Dinajpur the basement
is only 130m deep from the ground surface.

4. • Shield, Platform and Basin
• The Cambrian rocks form the basement of all geological
formations and as a consequence they are referred to as
Basement Complex or simply the Basement.
• The regions occupied by the Precambrian rocks are known as
Shields as they have virtually remained undisturbed and
unaffected by changes except for vertical movement.
• Literature on Plate Tectonics defines Shields as stable parts
of the Earth composed of Precambrian rocks with little or no
sedimentary cover. Metamorphic and plutonic rocks dominate
in the constitution of the rocks. Shield areas show very little
relief and tectonically remained stable for long periods of time.
• Platforms also include the stable parts of the earth’s crust
being composed of Precambrian Basement rocks which are
overlain by typically 1 to 3 km of relatively underformed
sediments.
• From the definitions, it appears that the depth of the
sediments on the Shield should be less than 1km.
• The Basin applies to the regions where the Precambrian
Basement lies 3km below msl.

5. • 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 [Silet]-Comilla
[Kumilla]-Chittagong areas.
A geosyncline generally forms along continental edges, and
is destroyed during periods of crustal deformation, which
often produces folded mountain ranges. Bengal
Geosyncline is one of the world's largest geosyncline that
includes the bengal basin and the bay of bengal.
The evolution of the Bengal Basin started in the Permo-
Carboniferous with the sedimentation in the faulted
Gondwana Basins. The break up of the Gondwanaland in
the Cretaceous Period and the marine transgression led to
the sedimentation in the Bengal Geosyncline.

7. For Bangladesh, the most important one is the Cenozoic
era, because most of the sedimentary layers as well as the
geological structures of the country were formed during
this era.
Bengal Basin, occupied mostly by Bangladesh, is the
result of plate collision between the Indian plate and the
Asian plate.
Before Cretaceous time (125 mya), the Indian plate
including part of Bangladesh (greater Rangpur and
Dinajpur area) was joined together with Antarctica, Africa,
Australia and South America, forming a super continent
called Gondwanaland.
The remaining part of Bangladesh did not exist at that
time. It was only after the rifting, northward drifting of the
Indian plate and the ultimate collision with the Asian plate
that the Himalayas were formed and the deltaic plain of
Bangladesh created.

8. • During the early Tertiary period (Mid-Palaeocene-
59mya) “soft collision” started between India and
Southeast Asia. Most of Bangladesh was under
open marine condition. The Tethys sea was
disappeared.
• During the late Tertiary period (44mya) (Middle
Eocene) “hard collision” started between India
and South Asia. The open sea retreated to the
south in response to the uplift of the himalayas in
the north and river systems built out a large
deltaic land that formed the backbone of the
present bengal delta.

10. About 22 Ma, Early Miocene, a time of major collision
between India and South Tibet in the north and India and
Burma in the east.
• In response to the eastwards directed subduction of the
Indian Plate, the molasses sediment (The term "molasse"
refers to the sandstones, shales and conglomerates formed
as terrestrial or shallow marine deposits in front of rising
mountain chains) of this zone were folded into a series of
elongated asymmetric folds arranged in en-echelon form.
The alignment of the folds follows NNW-SSE in the
Chittagong hill tracts.
During this time the direction of the Indian plate convergence
changed from north to north-east with increasing collision
with Southeast Asia.
• In the mean-time subduction of the Indian plate beneath the
Asian plate created a sutured zone of plate zone in the
northern Himalayas while in the east the Indo-Burman range
marks the zone of plate and the Asian plate continues and
this is evidenced from the release of stress in the form of
earthquakes in the vicinity of the plate boundary from time to
time

12. Since the Oligocene time (35 mya), plate collision continued
and major SEDIMENTS were shed by the rising Himalayas
as large river systems started filling up the proto-Bengal
basin to the south.
• From Miocene onward (from 25 mya), rapid rise in the
Himalayas accompanied by rapid SUBSIDENCE in the
basin resulted huge in deposition of a huge sedimentary pile
with the simultaneous development of a mega-delta. This
delta building continues at present as the Ganges-
Brahmaputra delta.
• Dauki Fault zone: The fault zone has divided the former
Northern Foreland shelf with the rising shillong Massif; the
southern has been down-faulted and concealed beneath a
thick clastic sequence of plio-pleistocene age.
• The Dauki fault has continued to be active as evedenced by
the continued sinking of the Surma Basin to its south

15. • Neotectonic Activity: “The study of geologically recent
motions of the Earth's crust, particularly those produced
by earthquakes,.
• …."recent tectonic movements occurred in the upper
part of Tertiary.
• Geological investigation, gravity, seismic, magnetic
data have indicated hat the late Pliocene-Pleistocene
Himalayan orogenic movement has continued to be
active upto the present time.
• Since the Bengal Basin is a part of the Himalayan
orogen, it is included in the intensive tectonic processes
of most recent and contemporary geological history.
• There are clear indications that Bengal Basin have
been uplifted and subsided as a result of neotectonic
activity beginning from the Pliocene age.

16. The Shillong Plateau in the northeast India gives an
excellent example of neotectonic activity. The plateau
occurs as an east-west trending horst block rising to an
elevation of nearly 2000m flanked by the surma basin to
its south which is known to have subsided 4-5 km during
the past 2-3 mys.
• The Dauki fault has continued to be active as evedenced
by the continued sinking of the Surma Basin to its south.
• The deltaic region of the Bengal basin is constantly
subsiding owing mainly to compaction of rcent sediments
and also to structural downwarping. Other areas of
subsidence in the basin include the Chalan Beel, the
Dhaka Depression, and the Faridpur Beel ( related to the
Faridpur trough).

17. • Regional structural feature:
• Zone of folded and faulted Neogene deposits on the
western side of the Indo-Burman ranges
• The present day Bengal foredeep, which is divided into
two sub basins with a sedimentary fill ranging from
Cretaceous to Recent in age, with a progading delta
sequence since middle Miocene.
• The continental slope as zone of facies transition
between the foreland shelf and deeper portion of the
basin in upper Cretaceous and Paleogene times.
• Western foreland shelf: consisting of Precambrian
crystalline rocks with remnants of Gondwana halfgraben
and an upper Creteaous to Paleogene shelf succession,
which is overlain by a Neogene prograding delta
sequence.
• E-W striking Dauki Fault zone: with block faulted shelf
and block faulted piedmont deposits of Plio-Pleistocene
age in the susang hills.

18. • Foreland basin systems:
• The wedge-top sits on top of the moving thrust
sheets and contains all the sediments charging from
the active tectonic thrust wedge.
• The foredeep is the thickest sedimentary zone and
thickens toward the orogen. Sediments are
deposited via distal fluvial, lacustrine, deltaic, and
marine depositional systems.
• The forebulge and backbulge are the thinnest and
most distal zones and are not always present.
When present, they are defined by regional
unconformities as well as aeolian and shallow-
marine deposits.