Regional Geological Setting
Bangladesh lies between 20o 34′ and 26o 38′ north latitudes and 88o 01′ and 92o 41′ east longitudes and as a consequence falls in the north eastern South Asia. The Indian States of West Bengal, Meghalaya and Tripura border Bangladesh in the west, north, and east and Burma forms the southern part of the eastern frontier. The Bay of Bengal limits the land area in the south. Bangladesh having about 80% of its area as flat fertile alluvial land is rightly considered as an agricultural country. But the presence of abundant coal, hard rock, and limestone within mineable depths, large reserve of gas, peat, white ceramic clay, and glass sand can assist Bangladesh in gaining recognizing as an industrial country. Physically Bangladesh may be classified in to four distinct regions each having distinguished characteristis of its own.
I. The Eastern and Northern Frontier Hilly Regions comprising the extensive eastern hilly regions, hills of Lalmai and north-eastern Sylhet district and a narrow strips of a series of low hill ranges and isolated circular and elongated hillocks separated by recent alluvium along the northern frontier of districts of Sylhet and Mymensingh.
II. The great Table Land is trisect by the river systems of Meghna and Jamuna giving rise to three large blocks of high lands that exhibit smooth rolling topography.
III. Flood plains of the Ganges, the Brahmaputra and the Meghna River systems cover approximately 40% of Bangladesh.
IV. The Delta at present appears to be a combination of three deltas, namely, the Ganges delta, the Old Brahmaputra-Meghna delta, and the Ganges-Jamuna delta.
Structure and Tectonics
The major tectonic elements of Bengal Basin are the Pre-Cambrian Indian Platform and the Bengal Foredeep. The Indian Platform is divided into four parts including Rangpur Saddle, Northern Slope of Rangpur Saddle, Southern Slope of Rangpur Saddle and a Hinge Zone [Bakhtine, 1965].
The Bengal Foredeep filled with thick strata of Neogene is divided into two parts naming the Western Platform Flank and Eastern Folded Flank. The Western Platform Flank represents an area of deep subsidence, having simple structure of platform type, which sharply differs from eastern folded part of the Flank.
The Eastern Folded Flank is characterized by folding of intermediate or transitional type. By structural peculiarities of folds, it can be sub-divided into the following three tectonic zones;
A. The western zone:
The quietest zone of box like structures, which indicates that this folding is not yet accomplish. This zone includes Dakhin Nhila, Inani, Sitakind Maheshkhali, St. Martin’s, Jaldi, and Walataung.
B. The middle zone:
More disturbed, predominantly asymmetrical and includes Matamuhuri, Bandarban, Gilasari, Siatpahar, and Kasalong, etc.
C. The eastern zone:
This zone includes narrow, ridge like elongated and tightly folded structures like Mowdak, Borcal, Uttar Chatra and Shisok etc.
The present study area, Chittagong city lies in the western marginal part Chittagong Hill Tracts within the folded part (Eastern Folded Flank) of the Bengal Foredeep of the Bengal Basin. The development of Bengal Foredeep is directly related to the development of Himalayan Mountains in the north and the Arakan Yoma Hill Range in the east due to the north and northeastern collision of Indian plate with Eurasian Plate and Burmese Plate respectively. Folds of Chittagong and Chittagong Hill Tracts are situated near the eastern edge of the Indian plate. An attempt has been made to interpret the mechanism for the formation of the folded belt of Chittagong and Chittagong Hill Tracts in the light of plate tectonic theory.
Figure 2.4: Major structural elements of the Bengal basin and its adjacent areas (modified after Bakhtine, 1966; Guha, 1978; Alam, 1990; Reimann, 1993 and Gunguly, 1997).
According to Curray and Moore (1974), the present relative plate motion between the Indian and Eurasian plates is apparently in a north-east south-west direction at a rate of convergence 5 to 6 cm per year. This movement is evidenced by the seismic activities in the plate boundaries and is confirmed by the studies of paleomagnetism of rocks and oceanic magnetic anomalies (McKenzie and Sclater, 1971). The Indian plate being subducted and has been underthrusting the Burmese plate in the east and the Tibet plate in the north and finally is being consumed beneath the Benioff zone. The under thrusting of the Indian plate has also been supported by the study of local mechanism solution for some earthquakes in the north eastern India and Burma (Rastogi et al., 1973). The movement of the Arakan subplate (bounded by the Ninety East Ridge and covering the eastern and the southeastern part of Bangladesh) has been suggested to has resulted the formation of folds of the eastern flank of the Bengal Basin (Faruquee, 1975).
The regional setting of South- Eastern Folded Belt has been described in various literature- Bakhtine, Raju , Guha , Hossain (1985) Alam, Khan, Shamsuddin and Abdullah, Sikder , Murphy and stuff of BOGMC etc.
All works conclude that the Chittagong and Chittagong Hill Tracts, the Geological name of southeastern folded belt is considered to be the youngest structural subdivision of the western flank of the Indo-Burma Ranges. The rocks of these north-south trending hills of this belt are ranging in age from Lower Miocene to Recent. The folded belt is separated from Arakan-Yoma Anticlinorium by the NNW-SSE striking Kaladan Fault [Rajib, 2002].
Folds of Chittagong and Chittagong Hill Tracts are the western extension of the Arakan Yoma geanticlinorium, which are known in Bangladesh as folded flank of Bengal Foredeep of Bengal Basin. The surface relief of the area is represented by north-south stretched hills of sedimentary cover. The axes of folds run along NNW-SSE directions, which are parallel to the trend of the Arakan Yoma geanticlinorium. The structure is disrupted and complicated by the presence of numerous faults. It is generally observed that the intensity of folding increases towards the east. The amplitudes of the anticlines range from several hundred meters to more than 3500m. Most of the anticlines are asymmetrical and plunging in nature. The axial planes are mostly inclined to the east and the plunge of anticlines ranges from 20 to 150.
From the above discussion, it is observed that the relative movement of the Indian and Burmese plate has developed the main east west compression in the region. The Burmese plate being overridden the Indian plate has been serving as the main pushing agent and as a result the magnitude of force is higher in the east than the west. The analysis shows that the intensity of deformation and folding is higher in the east than the west, which is very much conformable with the field evidences. The tectonic forces thus generated from the east caused intensive deformation in the east become progressively weak towards the west to raise the formation of relatively broad and box-shaped folds in the western zone.
The folded region of Chittagong and Chittagong Hill Tracts is characterized by asymmetrical box-shaped folds associated with reversed type of faults. Such structures can not be satisfactorily explained to have formed by horizontal compression alone. Similar structures have been studied extensively by Millar (1971) in the Tripura–Surma valley of India.
From his studies it has been found that such structure is developed due to vertical crustal movement. In the present study it is believed that the differential vertical movement of the curstal rock might have taken place due to the relative movement of the Indian and Burmese plate. When two plates came finally closer to each other, the basinal material may have bounced up to form the present structure. The upward movement of material in the region has also been confirmed by the abnormal gravity anomaly and elevation relationship (Verma & Mushapadhya, 1976). From the field study it has been observed that adjacent to the high angle reverse fault like Sitakund (Hossain & Akhter, 1983), the sedimentary rocks have been on the up thrown block due to gravity. This bending or draping effect can be responsible for the formation of asymmetrical anticlines.