Physical Features

Physiography of Bangladesh

Bangladesh belong to South Asia and lies between 20034´ and 26038´ N, and 88001´ and 92041´ E. The area of the country is 147,570 square km with more than 700 km long coastlines.
About 80% of the land are flat, intersected by numerous rivers and their distributaries. The land area has a general slope of 1-20 from north to south.
Physiographically Bangladesh can be divided into 7 divisions.
Each of these divisions has distinguished characteristic of its own.

a) Hilly regions (in the eastern and northern frontier) i) Chittagong Hill Tracts, ii) Hill Ranges of northeastern Sylhet, iii) Hills along the narrow northern strip of Sylhet and Mymensingh

b) Pleistocene uplands i) Barind Tract in the north western part, ii) Madhupur Tract in the central part, iv) Lalmai Hill

c) Tippera Surface

d) Tista Fan (in the northern part)

e) Flood plains i) Ganges flood plain, ii) Atrai flood plain, iii) Brahmaputra-Jamuna flood plain, iv) Old Brahmaputra flood plain, v) Meghna flood plain

f) Deltaic plain of the Ganges-Brahmaputra-Meghna Delta Complex i) Inactive delta, ii) Active delta, iii) Tidal delta

g) Sylhet Depression and Inland marshes (Scattered all over Bangladesh)

The hilly areas occupy about 18% of the country that encompasses Chittagong, Cox’s Bazar, Three Hill Tract Districts, Sylhet and adjacent areas. The Pleistocene uplands cover about 10% of the country. The floodplains of the Ganges, the Atria, Brahmaputra-Jamuna, the Old Brahmaputra, the Meghna rivers cover about 40% of the country including the Tista Fan, Tippera Surface and Sylhet Depression. The delta complexes in the outhern part cover about 32% of the country. There are numerous depressions/ inland marshes scattered and distributed all over the country. Among these, the Sylhet Depression is a ectonically subsiding basin that is subsiding at a much faster ate than the Bengal Basin as a whole.

The Bengal delta is a combination of three deltas namely the Ganges delta, the old Brahmaputra-Meghna delta and the Ganges-Jamuna (the present Brahmaputra)-Meghna delta. In some recent literature the name, "Ganges- Brahmaputra-Meghna Delta Complex" has been used. Also, in the summer monsoon season when about 3 million cusecs of water passes through the delta, it behaves as a fluvial delta, whereas in the winter when the volume of water passing through the delta drops to 250,000 to 300,000 cusecs (with 2 billion tons of sediments) it behaves as a tide dominated delta. These unusual features make this delta one the most complex deltas in the world. Holocene or Holocene sediments from a few hundred to thousands of meters cover the Flood plains and the Delta.

Tectonic Set-up of the Country

Tectonically Bangladesh is divided broadly into the following divisions.

Indian Platform and Shelf

a) Dinajpur slope (Himalayan Fore Deep); b) Rangpur Saddle, c)
Bogra Slope Hinge Zone (Eocene slope break) Bengal Fore Deep
a) Folded flank, b) Basinal area; i) Sylhet Trough, ii) Chandpur-
Barisal High, iii) Patuakhali Depression (Hatia Trough)

Map showing the Bengal basin with tectonic elements. Crustal section lines along X-X and Y-Y ref Figure 2.8 (BOGMC, 1986)

Geologic evolution of the Bengal Basin starting from Upper Paleozoic time is directly related with the break up of eastern Gondwanaland and collision of the Indian plate with the Asian plate, it can be divided into four major stages: I. Permo-carboniferous pre-breakup stage, II. Early Cretaceous Rift stage, III. Late Cretaceous-Eocene Plate or drift stage, and IV. Oligocene-Holocene Orogenic stage. The sedimentary cover of the basin with a maximum thickness of 20 km includes three major lithostratigraphic units separated by three major unconformities. The western part of Bangladesh is the platform shelf, whereas the eastern part of the country is represented by the folded belt. The central part representing the most subsided part of the basin comprises two major depressions at the north (Sylhet Trough) and south (Patuakhali Depression). The transition zone from the shelf to basin is represented by the hinge zone-a Eocene shelf/slope break.

Rapid subsidence of the foredeep of the Bengal Basin was compensated by the influx of huge amounts of detritus originating from the nearby sources of the basin. Shallow water conditions and in deltaic environment persisted. In addition to the western and Northern foreland shelves, which were source areas earlier, the rising chains of the Himalayas and the Indo- Burman Ranges were increasingly subjected to erosion and supplied much of the sediments since the Mid-Miocene in the basinal area (Shamsuddin
and Abdullah, 1997).

Stratigraphy

Bangladesh contain thick sediment (up to 20 km in the southern part) sequences of Permian to Holocene. The sediment thickness is shallowest in northern Bangladesh (114 m). Major part of the sediment is deposited by the Ganges-Brahmaputra-Meghna river systems during Miocene to Holocene time.

Generalized stratigraphic sequence of Bangladesh.
Age	Formation / Group	Lithologic description	Comment
Holocene	Alluvium	Sand, silt and clay: Grey; layered inter layered with peat and decomposed vegetal matters. ~~unconformity~~	Top and Middle & Lower aquifers
Pleistocene	Madhupur Clay	Red clay with patches of sandstone: Deeply oxidized, variegated colors, Contains ferruginous nodules and laterite	Stable Paleosol
Pio- Pleistocene	Dihing	~~unconformity~~ Sandstone: Variegated colors with pebbles and laterites.
	Dupi Tila Formation	Sandstone with clay beds: Grey, oxidized to variegated colors, coarse, pebbles and petrified woods present ~~unconformity~~	Deep aquifers
Miocene	Tipam Sandstone	Top: Girujan Clay: Grey Bottom: Tipam Sandstone: Grey, Medium to coarse with lignite bands. ~~unconformity~~	Very deep aquifers
	Bokabil Bhuban	Sandstone and shale ~~unconformity~~ Sandstone and shale	Gas producing zone
Oligocene	Barail ~~~~~~~~~~~ Kopili ~~~~~~~~~~~ Sylhet Limestone	Sandstone ~~unconformity~~ Shale with sandstone and fossiliferous beds ~~unconformity~~ Lime stone with sandstone beds	Limestone deposits
Paleocene	Tura Sandstone	~~unconformity~~ Sandstone	Oldest exposed rock of BD
Cretaceous	Rajmahal Trap	Volcanic trap with sandstone and shale
Permian	Gondwana	Sandstone with shale and coal beds ~~unconformity~~	Coal deposits
Pre- Cambrian	Basement	Granite, Granodiorite, Gneiss and Schist	Igneous & metamorphic rock

In Bangladesh the oldest sediment is the Permian Gondwana rock that lies over the Pre-Cambrian Basement complex. Over the Gondwana rock successively lies the Cretaceous Rajmal Trap, Paleocene Tura Sandstone, Eocene Sylhet limestone and Kopili, Oligocene Barail, Miocene Bhuban, Bokabil and Tipam Sandstone, Pliocene Dihing and Dupi Tila Sandstone, Pleistocene Madhupur Clay and Holocene Alluvium sediments. This generalized sequence is not common in all parts of the country and in some places many formations are missing due to depositional, non-depositional, and post depositional erosion. The
stratigraphic sequence is also variable between the Fore Deep area and the Shelf areas.
In Bangladesh, the Quaternary (including Plio-Pleistocene) sediments are the most important for the purpose of groundwater extraction. Hence, these are described in some detail.

Shekor'71 Bangladeshlive@yahoo.com