The Saddle; and c) Bogra Slope (southern slope of

The study area is located in the Bengal Basin which
is one of the thickest sedimentary basins in the world and having maximum
thickness exceed 18 km. The Bengal Basin lies in the eastern side of the Indian
subcontinent and occupies most of Bangladesh and west Bengal of India and part
of the Bay of Bengal (Alam, 1989). The Bengal Basin is located in between
latitudes 20º and 26º 40′ N and longitudes 88º and 92º 41′ E. This basin is
bounded on the west, east and north by Indian shield, Arakan Yoma folded system
and Precambrian Shillong Massif successively and finally it plunges into the
Bay of Bengal. Tectonic elements of this area plays a significant role for both
the geological development of an area and In the case of evaluating the
economic resources of this country. That is why, A crystal cl ear concept about
the major tectonic zone of Bangladesh is essential. A series of articles worked
out concerning the tectonic framework of Bengal Basin and its adjoining areas
by several workers regarded in the course of Morgan and Mclnter, (1966).
According to Bakhtine (1966), the basic tectonic elements of Bangladesh, are:

a)     
Indian Platform-
a stable shelf with reduced thickness of sediment.

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b)     
Bengal Fore
deep-which is a deepest part of this basin.

c)     
ArakanYoma Mega
Anticlinorium’s, and 

d)    
Sub Himalayan
Fore deep

Bengal Basin has been Sub divided into several
tectonic zones using various nomenclatures on the basis of dominant tectonic
elements, analyzed the new aeromagnetic, gravity and seismic data. Which are
namely,

a)     
Indian
Platformal Shelf or Stable Shelf, 

b)     
Hinge zone,
and 

c)     
Bengal Fore deep

The Indian Platform which is the Stable Shelf  of Bangladesh is directly underlain  by Archaean crystalline rocks and constitutes
towards the southwestern part of the Platform and covers approximately
two-third of the Rajshahi Division that is  subdivided into three tectonic devision (Guha,
1978) which are: a) Dinajpur Slope (northern slope of the Rangpur Saddle), b)
Rangpur Saddle; and c) Bogra Slope (southern slope of the Rangpur Saddle).

In the recent year, on the basis of the nature of
Bouguer anomaly, the modeling of the residual anomaly, the recognizable
basement fault trends and bore hole data, Khan (1991); and Khan and Rahman
(1992) have proposed revised tectonic classification of the Indian Platformal
Shelf or Stable Shelf (Map 3.3) as:

a)     
The Northern
Slope of the Platform,

b)     
Stable Platform,

c)     
The
Nowabgonj-Gaibandha-Intra-cratonic High and

d)    
The Southern Slope
of the Platform.

The study area is Khalashpir coal bearing Gondwana
basin is located in the Rangpur saddle of the northwestern part of Bangladesh. Probably,
the Rangpur Saddle is the possible connection between Indian Platform and
Shillong Massif which is covered with thin sedimentary cover over the basement which
is around 128m at Madhyapara. The width of the Saddle is nearly 97 km, which
slopes both sides and towards the north and the south and formed an oval shaped
body (Zaher and Rahman, 1980). The width of the southern slope of the Rangpur
Saddle (Bogra Slope) is about 64 to 129 km wide and goes up to Hinge Zone. The
inclination of the basement is gentle towards Bogra, which next increased southeastward
direction. In this area Gondwana sediment were settled down in the faulted
troughs or subsiding basins in the Basement Complex (Zaher and Rahman, 1980).

The saddle area has suffered greatly from faulting
of different age, as it is observed from Geophysical survey. There are multiple
faulting along the Jamuna and Ganges rivers which down thrown the region and
made its a graben type structure corresponding to the horst structure of
Shillong Massif and Mikir Hills accordingly (Khan, 1991). Moreover the
aeromagnetic map shows that the frequency of faulting is low over the basement
of the stable platform. The faults which are observed within these tectonic
elements are actually belongs to the Gondwana period and formed intrabasinal
horsts and graben. This is the main characteristics of the Gondwana basins of
Peninsular India.

Networking of this fault ultimately made up as many
as five faults bounded Gondwana basins in the area (Uddin and Islam, 1992;
Islam, 1993). There are five Gondwana basins in Benagal basin (Map 3.4) and
approximately three billion tons of Permian Gondwana coal resources have been
estimated in these five basins (Farhaduzzaman et al., 2015).

The processes that led to the formation of the
Gondwana basin are rather complex. Its history commenced with the development
of the Precambrian and the global Permocarboninferous diastrophism which was
responsible for the breakup of the Gondwana land and the northward drift of the
Indian plate (Srikanta and Bhargara, 1983). According to Banerjii (1981), the
first phase of the formation of the basin was culminated in the development of
a set of faulted troughs or grabens in the crystalline basement. These basins
were simple depositional basins on the embryonic stage and were formed as
topographic depressions due to normal geomorphologic processes. Later on, these
basins acquired dynamic characters and formed sedimentary basins through down
breaking and subsidence of the basin floor along the pre-existing weak places.
The Gondwana sedimentation occurred in the Late Carboniferous – Early Permian
time in terrestrial fluvial to lacustrine environments. The subsidence kept
place with the sedimentation and the accumulation was accentuated by
differential mobility across the linear basin margin.

Khalashpir coal basin
is located within the southern slope of the Platform unit. The khalashpir is a
fault bounded half graben and oval-shaped basin. The eastern side of the basin
might be fault bounded, as evidenced from the Bouger gravity anomaly map.

The northeast-southwest trending fault may be the
northern limit of the basin. The basin might have formed by faulting in the
crystalline basement during Permo-Carboniferous time. Later on, these basins
were reshaped and basin marginal adjustments took place from time to time due
to the Himalayan upheavals. Subsequently the Tertiary and Quaternary sediments
were deposited over the crystalline basement within the half graben basin.

A number of small Gondwana basins (gravity low
areas) within the basement have been identified by regional gravity survey
(Islam et al., 1992). The basin is separated from the Dhighipara coal basin by the
Bhaduria high on the western side. Another small basin named Bhendabari is
located in the northern side of the basin. These basins may be formed by
faulting in the crystalline basement during Permo-carboniferous time (Banerjee,
1981).