The Bengal Basin is a

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

basin. The term Bengal basin is used to describe geological structure of

Bengal region Bengal ( ) is a historical geographical, ethnolinguistic and cultural term referring to a region in the eastern part of the Indian subcontinent at the apex of the Bay of Bengal. The region of Bengal proper is divided between the modern- ...

's sedimentary basin. It is spread across the

Indian states India is a federal union comprising 28 states and 8 union territories, for a total of 36 subnational entities. The states and union territories are further subdivided into 800 districts and smaller administrative divisions by the respe ...

West Bengal West Bengal (; Bengali language, Bengali: , , abbr. WB) is a States and union territories of India, state in the East India, eastern portion of India. It is situated along the Bay of Bengal, along with a population of over 91 million inhabi ...

and

Bangladesh Bangladesh, officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world and among the List of countries and dependencies by ...

. The Bengal basin in the eastern part of the

Indian subcontinent The Indian subcontinent is a physiographic region of Asia below the Himalayas which projects into the Indian Ocean between the Bay of Bengal to the east and the Arabian Sea to the west. It is now divided between Bangladesh, India, and Pakista ...

forms the world's largest fluvio-delta and shallow marine sedimentary basin. This shallow marine sedimentary basin coupled with the Bengal Fan beneath the

Bay of Bengal The Bay of Bengal is the northeastern part of the Indian Ocean. Geographically it is positioned between the Indian subcontinent and the Mainland Southeast Asia, Indochinese peninsula, located below the Bengal region. Many South Asian and Southe ...

to form the largest sedimentary dispersal system known in the modern world. The Bengal basin was initiated during the breakup of

Gondwanaland Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia, Zealandia, Arabia, and the ...

in the late

Mesozoic Era The Mesozoic Era is the era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Periods. It is characterized by the dominance of archosaurian reptiles such as the dinosaurs, and of gymnosperms s ...

. The basin consists of a thick Early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

sedimentary succession. Proximal deposition of a portion of orogenic sediments from the

Eastern Himalaya ] The Eastern Himalayas extend from eastern Nepal across Northeast India, Bhutan, the Tibet Autonomous Region to Yunnan in China and northern Myanmar. The climate of this region is influenced by the monsoon of South Asia from June to September. It ...

and Indo-Burman ranges deposited in the Bengal Basin. The thickest sediments in the basin occur in the coastal area of the

, where sediments are about 20 km thick. It is bounded by faults on all its sides except the southern side. The basin is bounded by the West Margin Fault and the Malda–Kishanganj Fault in the west, the Surma Margin Fault in the east and the Yamuna Fault and the Dauki fault in the northeast.

Origin

There are different theories about the origin of the Bengal Basin. However, four theories are the most accepted. The theories are: * The basin is the result of subduction of the

Indian plate The Indian plate (or India plate) is or was a minor tectonic plate straddling the equator in the Eastern Hemisphere. Originally a part of the ancient continent of Gondwana, the Indian plate broke away from the other fragments of Gondwana an ...

beneath the

Eurasian Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents dates back to antiq ...

and Burmese plates. * The Bengal basin is essentially a residual ocean basin and developed on the eastern continental margin of India. * The Bengal basin is a

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

with a succession of mainly deltaic sediments derived from the erosion of the Himalayan and Indo-Burmese ranges. * The Bengal basin developed into isolated graben controlled basins in the basement during the breakup of the

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

land.

Geologic

The Bengal Basin has a thin, gently sloping alluvial cover in the western and northern parts, which thickens basin-ward. Considerable variation in stratigraphy, sedimentary tectonics and basin evolution is observed in different parts of the basin. The entire basin is divided into three smaller sub-basins for structural variation: Northern sub-basin, Western sub-basin and South-eastern sub-basin.

Sub-basin

Northern sub-basin

The northern sub-basin mainly consists of the northern part of the Bengal basin. It lies between the 'saddle' of the Garo-Rajmahal gap (submerged ridge) in the south and the Himalayan Front, which is part of the Indian states of

and

. The shallowest part of the sub-basin is in the south, where Neogene sediments varies in thickness from 128 m to 1,160 m. The tectonically down-warped frontal belt of the Himalayas contains Neogene sediments. The southern edge of the submerged mountains along the Garo-Rajmahal is a fault zone, known as the east–west oriented Dauki fault.

Western sub-basin

The westernmost part of the Bengal basin forms the western sub-basin. It is more than 100 km wide in

Murshidabad Murshidabad (), is a town in the Indian States and territories of India, state of West Bengal. This town is the headquarters of Lalbag subdivision of Murshidabad district. It is located on the eastern bank of the Hooghly river, Bhagirathi Riv ...

district and relatively less wide in Medinipur area in the south. The stratigraphic succession of the western sub-basin shows virtually complete lithostratigraphic succession from

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

to Recent sediments. The western sub-basin portion belonging to West Bengal is characterized by gentle eastward slopes and numerous step faults with small displacements. The sediment thickness increases steadily from about 500 m near the western end to more than 6,500 m near

Kolkata Kolkata, also known as Calcutta ( its official name until 2001), is the capital and largest city of the Indian state of West Bengal. It lies on the eastern bank of the Hooghly River, west of the border with Bangladesh. It is the primary ...

. The presence of larite and laritic soil is seen in this part. The subsurface consists of layers of sandstone, red shale, grit and gravel. The Dhananjaypur Formation at the base of the Rajmahal trap is identified as the early stage of subsidence on the eastern side of the west margin fault. This sub-basin has deposits of rock from the

Rajmahal Rajmahal is a subdivisional town and a notified area in Rajmahal subdivision of the Sahebganj district in the Indian state of Jharkhand. It is situated at the banks of Ganges and was former capital of Bengal Subah under Mughal governor, Man S ...

and Jayanthi hills and alluvium from the

Bhagirathi The Bhāgīrathī (Pron: /ˌbʌgɪˈɹɑːθɪ/) is a turbulent Himalayan river in the Indian state of Uttarakhand, and one of the two headstreams of the Ganges, the major river of Northern India and the holy river of Hinduism. In the Hindu f ...

River. The sediments from the Rajmahal hills were deposited through the Bolpur and Ghatal Formations. The Bolpur Formation consists of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

, sandstone and trapwash and the Ghatal Formation consists of sandstone,

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

. Ranaghat and Malda Formation were formed by Bhagirathi river. Ranaghat Formation consists of

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

and shale, and

Sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

and

mudstones Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York ...

are present in the Malda Formation.

Southeastern sub-basin

The southeastern sub-basin is bounded on the west by the Eocene shelf-break and the easternmost edge of the CTFB. This sub-basin shows a huge sedimentary thickness of 10 km to 18 km. The southeastern sub-basin is filled with massive clastic sediments and the sediment thickness increases from northwest to southeast. The sub-basin is divided into 5 parts according to tectonic domains: Sylhet trough, Tangail-Tripura high (also known as Madhupur high), Faridpur trough, Barisal-Chandpur high and Hatia trough. The Hatia trough is the deepest trough in the Bengal Basin with the thickest clastic sediment deposits. In various parts of the Sylhet Trough, large amounts of sediment were deposited between the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

and

Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...

, forming Berail sediments layer 10 km to more than 17 km thick. A 1,500 m thick layer of

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

and

is observed in Barisal.

Mineral resources

The Bengal Basin has an abundance of natural gas. Natural gas is produced in Bangladesh. Also, large reserves of

crude oil Petroleum, also known as crude oil or simply oil, is a naturally occurring, yellowish-black liquid chemical mixture found in geological formations, consisting mainly of hydrocarbons. The term ''petroleum'' refers both to naturally occurring u ...

have been discovered in

. Crude oil is extracted from Ashoknagar oil field in West Bengal.

References

{{West Bengal Bengal basin Foreland basins