Bangladesh Geology

Bangladesh Geology The country occupies major part of the bengal delta, one of the largest in the world. The ganges-brahmaputra delta basin or the bengal basin includes part of the Indian state of West Bengal in the west and Tripura in the east. geological evolution of Bangladesh is basically related to the uplift of the Himalayan Mountains and outbuilding of deltaic landmass by major river systems originating in the uplifted himalayas. This geology is mostly characterised by the rapid subsidence and filling of a basin in which a huge thickness of deltaic sediments were deposited as a mega-delta outbuilt and progressed towards the south. The delta building is still continuing into the present bay of bengal and a broad fluvial front of the Ganges-Brahmaputra-Meghna river system gradually follows it from behind. Only the eastern part of Bangladesh has been uplifted into hilly landform incorporating itself into the frontal belt of the indo-burman range lying to the east. All the above has been the result of the Indian plate colliding with the Asian plate as explained by the universally accepted theory of plate tectonics.

The geology of Bangladesh may be discussed under the following headings: (i) tectonic framework, (ii) stratigraphy, and (iii) Economic geology.

Tectonic framework this refers to the basic structural frame on which Bangladesh stands. Bangladesh is divided into two major tectonic units: (i) Stable Precambrian 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.

The stable Precambrian platform occupies rajshahi, bogra, rangpur and dinajpur areas and is characterised by limited to moderate thickness of sedimentary rocks above a Precambrian igneous and metamorphic basement. This unit is geologically stable in relative terms and has not been affected by fold movement. Some fault bounded graben basins occur within the Precambrian Basement. These basins contain coal bearing rock units of the Permian Period (286 to 245 million years ago), the oldest sedimentary rock found in Bangladesh. The Precambrian platform is divided into a northern Rangpur Saddle with a very shallow Precambrian basement (130 to 1,000m) and a southern Bogra Shelf with a basement at moderate depth (1 to 6 km). Sedimentary layers in the Bogra shelf dip very gently towards the southeast until it reaches the hinge zone when the dip suddenly increases to 15 to 20 degrees and the sedimentary units plunge to a great depth into the deep geosynclinal basin to the southeast.

The 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 areas of greater dhaka, faridpur, noakhali, sylhet, comilla and chittagong and the Bay of Bengal. 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 geosynclinal basin is subdivided into two parts, ie fold belt in the east and a foredeep to the west. The fold belt is characterised by folding of the sedimentary layers into a series of anticlines (upward folds) and synclines (downward fold). The anticlines form the hills and the synclines form valleys as seen in the topography of the eastern Chittagong-Comilla-Sylhet regions. The intensity of the folding is greater towards the east, causing higher topographic elevation in the eastern chittagong hill tracts. As the intensity of folds decreases towards the west, the fold belts unit merges with the foredeep unit, which is characterised by only mild or no folding. So the sedimentary layers are mostly horizontal to sub-horizontal and are free from major tectonic deformation in the foredeep area covering the central part of the basin and this is expressed as river to delta plain topography of the land.

The hinge zone is a 25-km wide northeast-southwest zone that separates the Precambrian platform in the northwest from the geosynclinal basin to the southeast. There is no surface expression of this unit but it is marked by the sudden increase of dip in subsurface sedimentary layers as shown strongly by the seismic marker at the top of the Sylhet Limestone unit of Eocene age. This is why it is also known as the Eocene hinge zone.

Stratigraphy Stratigraphic subdivision of the rock sequences in Bangladesh follows the broad tectonic divisions. In the Precambrian platform, a thin to limited sedimentary sequence of Permian to Recent age lies on a crystalline basement conforming with the relative tectonic stability of the area. In the geosynclinal basin, on the other hand, a huge clastic sequence of mostly Tertiary age testifies to high tectonic mobility. The stratigraphy of Bangladesh is discussed under two headings corresponding to the above two divisions.

Stratigraphy of Precambrian platform 'in northwestern Bangladesh, Precambrian igneous and metamorphic rocks form the base of all sedimentary rock units. The Precambrian basement is composed mainly of granite, granodiorite and gneiss. The basement occurs at a shallowest depth of 130m below the surface in the Rangpur area and dips towards southeast with increasing sedimentary cover ranging in age from Permian to Recent.

Permian the oldest sedimentary unit in Bangladesh is the Gondwana group of Permian age, resting unconformably on the Precambrian crystalline basement. The Gondwana group is composed of hard sandstone with some coal and shale layers. The Group is about 1000m thick and is found in fault bounded graben basins.

Jurassic-Cretaceous Above the Gondwana group of sediments lies a sequence of volcanic basaltic rock layers of Jurassic age called Rajmahal Trap formation, named after the rajmahal hills in adjacent India where the unit is exposed on the surface. The unit is about 500m thick and found in the drill holes in the Rajshahi-Bogra area. The Rajmahal trap is overlain by the Shibganj Trapwash formation, a relatively thin cover of the weathered product of volcanic rocks consisting of red ferruginous sandstone and mudstone. It is Cretaceous in age.

Early Tertiary The next upward sequence of rocks is named the Jaintia group, which belongs to the Palaeocene and Eocene age and were formed under marine conditions. The Jaintia group is divided into three units, from bottom upward, the Tura formation, Sylhet Limestone formation and Kopili Shale. The Tura formation is composed mainly of whitish sandstone with occasional thin coal beds near the top. Overlying these lies the Sylhet Limestone formation, nummulitic fossiliferous limestone units of middle Eocene age with average thickness of 250m. The Sylhet Limestone formation is the most extensively developed unit in the subsurface of northwestern Bangladesh and is a marker horizon in the seismic section. The overlying Kopili formation is composed of dark grey to black fossiliferous shale with a few limestone beds. The unit has a thickness of 40 to 90m and marks the end of open marine conditions of deposition.

Late Tertiary The Barail group, 150 to 200m thick and consisting of sandstone, shale and siltstone with occasional carbonaceous layers represents the deposition under deltaic conditions in Oligocene times. This is overlain by the Surma group (undifferentiated and often called the Jamalganj formation), about 400m thick, and consisting of deltaic sandstone, shale and siltstone. Both the Barail group and the Surma group are far less in thickness in the stable platform compared to what these units have in the geosynclinal basin part.

The Surma Group is overlain by the Dupi Tila formation of the Pliocene-Pleistocene age, composed of medium to coarse-grained sandstone, often pebbly and having a thickness of about 270m. This unit represents river plain deposit following the delta filling of the basin. The overlying Dihing formation is pebbly sand to occasional gravel deposits of river origin, having a thickness of about 150m. All the above are covered by about 100m of soft sandy, silty and clayey sediments of Bengal Alluvium of Recent age.

Stratigraphy of the Geosynclinal basin in contrast to the platform part in northwestern Bangladesh, the stratigraphy of the geosynclinal basin to the southeast is characterised by an enormous thickness of sedimentary succession (upto about 20 km), mostly Tertiary in age, recording rapid subsidence and sedimentation. Rocks older than Oligocene have not been encountered in the drill holes or in the outcrop in the geosyncline basin because pre-Oligocene rocks are very deeply buried.

Late Tertiary The Oligocene is represented by the Barail group, named after the Barail Range in nearby India where the unit has its type locality. The Barail group is composed of alternating sandstone, shale, siltstone and occasional carbonaceous rich layers. In neighbouring assam, about 3,000m of Barail sediments are recorded in the outcrop and the unit is divided into three formations. In Bangladesh most of the Barail group is very deeply buried, although about 350m of the upper part of the Barail is exposed on a small strip of land in jaintiapur, on the northern Sylhet border with India. Top parts of the Barail sediments have been encountered in deep subsurface (more than 4,000m deep) in Atgram and Fenchuganj wells in Sylhet area.

The Surma group of the Miocene-Pliocene age overlies the Barail group with an unconformity. The Surma group has a thickness of about 3,500m to 4,500m and is composed of monotonous alternating sandstone, shale, siltstone and some conglomerate. It is traditionally believed to be deposited in deltaic to shallow marine environments. However recent field studies suggest that the basal parts of the Upper Bhuban formation are formed in a deep marine turbidite environment in some areas in the Chittagong Hill Tracts. The Surma group is divided into two formations, a lower more sandy Bhuban formation and an upper more argillaceous (clayey) Bokabil formation. Both the Bokabil and Bhuban formations show extensive lateral facies change and vertical variations in sand to shale ratio within themselves. This has rendered correlation of the rock units across the basin difficult. The Surma sediments are poor in diagnostic fossil and therefore their age designation is also difficult. The Surma group is probably the most important stratigraphic unit in Bangladesh because it is represented by great thickness in all the wells drilled in the geosynclinal basin and it forms the backbone of the eastern hilly areas of Bangladesh including those of Sylhet and Chittagong hills where it is extensively exposed.

The Surma group is overlain by the sand dominating Tipam group of the Pliocene-Pleistocene age. The Group is subdivided, from bottom upward, into the Tipam Sandstone formation, Girujan Clay formation and Dupi Tila formation. The Tipam Sandstone formation, about 1,200m to 2,500m thick, is dominantly composed of medium grained sandstone frequently cross bedded, with little shale and this indicates deposition under a river environment. The overlying Girujan Clay formation is a shale unit with a thickness of 100 to 1,000m and was formed under a lake environment. The Dupi Tila Sandstone formation is 2,000 to 3,000m thick and is composed of medium to coarse loosely compacted cross-bedded sandstone, occasionally pebbly and this indicates deposition in a river environment. The above is covered with about 100m of sandy, silty and clayey sediment of Bengal Alluvium of Recent age.

The stratigraphic subdivision and nomenclature follow the one established by the geologist P Evans in 1932 for the rock sequence in the adjacent Indian State of Assam to the northeast. Given the complexities of facies and their interrelations in a deltaic to fluvial regime, the present correlation of Evan's units over the entire basin in Bangladesh appears to be too generalised. Many geologists believe, for example, that the lithological criteria for the Bhuban and Bokabil formations are not so explicit that these units can be extended over the entire basin and their uses should be restricted if not abandoned. The concept of Diachronism should be incorporated for the deltaic upper Tertiary basinal sedimentary units, which are not isochronous but do cut time lines and are therefore diachronic in nature. A stratigraphic committee of Bangladesh has been formed to formulate a revised classification in the light of the above.

Economic geology occurrence of economic deposits of petroleum and mineral resources has been controlled by the geological parameters specially by the tectonic-structural setting and stratigraphy. For example, the large bituminous coal deposits are restricted to northwestern Bangladesh because of the occurrence of Permian rocks in graben basins within the Precambrian platform. On the other hand the large natural gas reserves are located in eastern Bangladesh because folded structures are available to trap the gas in thick Mio-Pliocene sandstone reservoirs there. Bangladesh has so far discovered significant amounts of economic mineral resources including natural gas, coal and peat, limestone, crystalline hard rocks, heavy minerals, kaolinite and glass sand. Each of these resources is associated with specific geological settings.

All the 22 gas fields discovered so far in Bangladesh are located in the eastern half of the country. The gas occurs at depths ranging from about 1,000m to 3,500m below the surface. The occurrence of natural gas depends, among other factors, on the existence of suitable traps. The eastern part of Bangladesh falls in the fold belt division and the anticlinal folds form an excellent structural trap for gas accumulation. The sandstone in the Miocene-Pliocene Surma group rocks have served as excellent reservoir rocks capped by interbedded shale forming seals. The natural gas originating from source rocks below has conveniently migrated upward to be accumulated in the structural traps in the Mio-Pliocene reservoir sandstone.

Major bituminous coal deposits are located in Rangpur and Dinajpur districts because of the occurrence of Permian sediments in the fault bounded graben basin above the Precambrian basement. Permian sediments in most parts of the world are rich in coal because of the favourable coal forming environment during the Permian geologic time, ie thick vegetation and swampy conditions and coals are conveniently preserved where there are graben basins, a setting found in the northwestern part of Bangladesh.

Limestone is formed in a shallow calm open marine condition. During the middle Eocene time northwestern Bangladesh was covered with such a shallow open marine condition and the Sylhet Limestone formation was formed during that time. Limestone is thus confined to areas where the Sylhet limestone formation occurs. This formation is found in the subsurface in the Bogra-Rajshahi area and found on the surface in the Lalghat-Bhangerhat areas of northern Sylhet.

Rich deposits of glass sand are found in many areas, mainly in the eastern part of the country. The occurrence of glass sand depends on the process of concentrating quartz in sand by washing away impurities. The glass sand deposits are found as pockets or lens of quartz rich sand within the Dupi Tila formation, itself a sand-dominating unit. The quartz rich sand pockets were formed by the second cycle erosion and deposition of pre-existing Tipam and older sandstone hills. The location of glass sand deposits is controlled mainly by the occurrence of Dupi Tila Sandstone formation.

The Precambrian basement, all over the world, is formed of hard crystalline igneous and metamorphic rocks. Occurrence of granitic igneous and metamorphic hard rocks in the shallow subsurface in the Rangpur and Dinajpur areas is due to tectonic stability because of which the Precambrian basement rock has not subsided too deeply but has remained very near to the surface. [Badrul Imam]

See map in tectonic framework.

Bibliography FH Khan, Geology of Bangladesh, University Press Limited, Dhaka, 1991; Haroun Er Rashid, Geography of Bangladesh, University Press Limited, Dhaka, 1991; Klaus-Ulrich Reimann, Geology of Bangladesh, GebrFCder Borntraeger, Berlin-Stuttgart, 1993; Badrul Imam, Bangladesher Khaniz Sampad in Bangla (Mineral Resources of Bangladesh), Bangla Academy, Dhaka, 1996.