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Formation of Kaptai Rangamati .pdf



Original filename: Formation-of-Kaptai-Rangamati.pdf
Author: Nazim Uddin Naeem

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The Major Formations of Kaptai,
Rangamati
Geological Group-Formation: A stratigraphic sequence that is presumed to divide
in whole or in part into formations in the future. A formation is a body of rock
identified by lithic characteristics and stratigraphic position; and is mappable at the
earth's surface or traceable in the subsurface.

In our field work at Kaptai, Rangamati we will find out the six major formations.
They are:
i)
ii)
iii)
iv)
v)
vi)

Alluvium Formation
Dupi Tila Formation
Girujan Clay Fromation
Tipam Sandstone Formation
Boka Bil Formation
Bhuban Formation.

The major characteristics of these six formations are described below:

i)

Alluivum Formation:
Location
Age
Thickness
Rock Type

Texture
Occurrence

:
:
:
:

The upper most part of stratigraphy.
Holocene
10 – 40 meters
It contains sand, clay, silt, peat, boulder, gravel sized
rock type. Corals and sea shells are also present. Mainly
this formation is a result of weathering and erosion.
Vegetation is found in this formation.
: i . Brown and Gray in color.
ii. Coarse to very coarse grained sand.
: Pockets and lenses of fravels and pebbles commonly
occur in the predmont alluvium of Pirajpur, Rangpur,
Sylhet and all along the western margin of eastern hilly
regions from Sylhet district to the southern frontier of
Chittagong district.

ii)

Dupi Tila Formation:
Location
Age
Group
Thickness
Rock Type

Texture

Occurrence

iii)

: Right under Alluvium Formations and it uncomfortably
overlies the Tipam group.
: Pliocene-Pleistocene
: Tipam Group
: 2500 meters
: Predominantly Sandstone with minor shale and clay
beds. Lower part is composed chiefly of massive
sandstone interbedded with subordinate claystone. The
upper part consists of claystone and siltstone with
Subordinate sandstone.
: i. The Sandstone is yellow to yellowish brown, medium
to coarse, cross-bedded and in places pebbly.
i i . In Chittagong, the Dupi Tila Formation consists
of yellowish-brown to brown, fine to medium grained
pebbly and cross-bedded sandstone with subordinate
claystone and siltstone.
: Well preserved leaf impressions in Dupi Tila sandstones
are also reported from our studied area, the Kaptai Lake
near Rangamati.

Girujan Clay Formation:
Location

Age
Group
Thickness
Rock Type

Texture
Occurrence

: The upper contact to the Girujan Clay Formation is also
conformable. In the case of absence of the Girujan Clay
Formation, the Tipam Sandstone Formation is
unconformably overlain by the Dupi Tila Formation.
: Pliocene-Pleistocene
: Tipam Group
: 1000 meters
: Mainly Shale. It is composed mainly of mottled clay
with subordinate mottled sandy clay, sandstone with
subordinate ferruginous sandstone. Fossil wood and
lignite are also present in the unit.
: Fine to medium coarse grained.
: Exposed in Upper Assam and the Surma Valley and
extends to Arakan coast.

iv)

v)

Tipam Sandstone Formation:
Location

:

Age
Group
Thickness
Rock Type

:
:
:
:

Texture

:

Occurrence

:

The lower part of Girujan Clay Formation. A part of
Tipam group.
Pliocene
Tipam Group
2500 meters.
Predominantly sandstone with minor shale. The Tipam
Sandstone Formation is a coarse-grained sandstone
sequence overlying the Boka Bil Formation. The contact
between these two units is unconformable in the
marginal portions of the basin as well as in the shelf
areas in the west. In the deeper sections of the basin the
contact is conformable.
The Tipam Sandstone Formation typically consists of
grey-brown to pale-grey, coarse-grained, cross-bedded,
massive sandstone.
i. Intercalations of grey shale, conglomerate horizons,
pebbles, wood fragments and petrified trunks also
occur.
ii. Outcropping Tipam Sandstone deposits occur
throughout the Frontal Fold Belt from the
southernmost portion of the Chittagong Hill Tracts,
through the western section of the Indian state of
Mizoram and across Tripura to the northeastern
section of the Surma Basin. The formation occurs
regularly on the flanks of the anticlinal trends in the
Frontal Folded Belt area, forming steep cliffs.

Boka Bil Formaiton:
Location

Age
Group
Thickness
Rock Type

: A transitional zone between the underlying dominantly
sandy Bhuban Formation and overlying Tipam
Sandstone Formation. Boka Bil Formation the upper
unit of the Surma Series.
: Milo-Pliocene
: Surma Group
: 1500 meters
: It is mainly composed of alternating bluish-grey to
yellowish-grey, well-bedded siltstone and shale. The

Texture

Occurrence

Economic
Importance

formation contains heavy minerals and is characterised
by the presence of zoisite, epidote, staurolite, kyanite
and hornblende.
: Light yellowish-brown to brown, fine to mediumgrained, bedded to massive, hard sandstone, locally
calcareous and conglomeratic.
: The formation is exposed in the hill ranges of greater
Sylhet, Chittagong district and the Chittagong Hill
Tracts.
: The Boka Bil Formation contains natural gas in
economic quantities.

vi) Bhuban Formation:
Location
: The lower unit of Surma Group.
Age
: Miocene
Group
: Surma Group
Thickness
: 3500 meters.
Rock Type
: Alternating Sandstone and Shale with minor
Conglomerate. The lower and upper members are made
up predominantly of sandstone, while the middle
member usually shows a higher percentage of shale.
Texture
: Light yellowish-brown to brown, fine to mediumgrained, bedded to massive, hard sandstone, locally
calcareous and conglomeratic.
Occurrence
: Outcropping rocks of the Bhuban Formation occur
throughout the Frontal Folded Belt and particularly in
the cores of anticlinal trends in the eastern part of the
chittagong hill tracts. Further outcrops are known from
three northern margins of the surma basin between the
Atgram area in the east and Jaintiapur in the west, where
the outcrops parallel the E-W trend of the Dauki Fault.
: The unit is a natural gas bearing horizon in Bangladesh.
Economic
Importance


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