Most of Bangladesh sits on 18km thick sediments, researchers find

We all know that Bangladesh is the world's largest delta and it was created through millions of years of siltation. But did you know, how thick the sediment that created three-fourths of Bangladesh is? It's a staggering 18 kilometres!

In other words, most of Bangladesh is sitting on huge sediments piled on oceanic rocks—and not continental crust, like the rest of the countries of the world. Oceanic rocks are basalt—which is not more than 200 million years old, while continental rocks or granites are billion years old.

This is a rare or unique feature on the earth's crust not found in any other place, says a study by the University of Hyderabad (UoH), India.

It adds, most part of present Bangladesh was under ocean water some 23 million years ago.
This tectonic model is well-supported by the presence of granitic rocks in Maddhapara mine in north Bengal where 70 wells had been drilled, but the basement rocks could not be reached as the sediments reach up to 18km thickness.

Indian newspaper Hindustan Times adds that with many Indian petrochemical companies exploring natural gas or hydrocarbons owing to the thickness of sediments, the study findings will help look for fossil fuels from both ancient and younger sediment.

"Continental rocks cannot support 18km thick sediments because the continent cannot go down for sediment depositions, therefore it will lead to the formation of a mountain against the gravity. On the other hand, oceanic rocks will sink the moment sediments begin settling on them," K Srinivas, CSIR-NIO told Hindustan Times.

"Recent geophysical researches in Bangladesh and the BoB (Bay of Bengal) using different geological and geophysical proxis show that oceanic, not continental type of crust lies beneath the alluvial cover over most part of Bangladesh," said the study led by Professor KS Krishna, Centre for Earth, Ocean and Atmospheric Sciences (CEOAS), UoH.

The study was published in Current Science, peer-reviewed journal of Indian Academy of Sciences, on Wednesday.

According to the study, the rifting of eastern part of supercontinent Gondwanaland 132 million years ago led to the break-up of Greater India from Gondwana. With this, the proto Bay of Bengal was created. As the bay continued to evolve, rivers of the Indian peninsula and the Himalaya continuously carried terrigenous material into it.

"The sediments discharged principally by the Ganga and Brahmaputra rivers built an alluvial cover of more than 18 km thickness in the Bengal Basic to the east of Kolkata, which constitutes a major part of the region of present-day Bangladesh," the study co-authered by M Ismaiel, K Srinivas and D Saha.

"Beneath Bangladesh no rift-related structures representing the fracturing of continents are observable, implying that the line of continental break-up must be far to the north of the present-day Bangladesh coastline."

Absence of rifted continental crustal blocks on the Bangladesh coast and continuity of the continent-ocean boundary onshore along Rajmahal-Sylhet line suggest that the present Bangladesh region was under marine conditions at least until the beginning of Bengal fan sedimentation about 23 million years ago.

"There is also a general understanding that most continents are older than the oceans. So if Bangladesh is part of the Indian subcontinent, it should have granitic rocks. But we found the scenario here is different," said KS Krishna, lead author and professor, Centre for Earth, Ocean and Atmospheric Sciences (CEOAS), UoH.