Science News

Bangladesh was once under the ocean

K. S. Jayaraman

doi:10.1038/nindia.2019.131 Published online 27 September 2019

Researchers have found the presence of an ocean floor beneath Bangladesh and are calling it a "unique case on planet Earth”.

Bangladesh is not underlain by the rocks usually found beneath global continents but by oceanic rocks, they report1. Such a geologic phenomenon, they say, has been observed for the first time.

“Most of the present territory of Bangladesh was under the ocean at least until about 23 million years ago," says marine geophysicist Kolluru Sree Krishna, also the corresponding author of the study at the University of Hyderabad (UoHd). The finding is an outcome of research carried out over the years to understand the tectonic evolution of the Bay of Bengal (BoB).

Other researchers in the study include M. Ismaiel of the university, K. Srinivas of the National Institute of Oceanography Re-gional Centre in Visakhapatnam, and D. Saha of Oil and Natural Gas Corporation.

The breaking away of Greater India from the Gondwana supercontinent 132 million years ago (Ma) was followed by the opening of a new oceanic domain – called the proto BoB – along India's eastern margin. The present location of Kolkata, the report says, was once on the verge of the continental margin neighbouring the proto BoB. Krishna and his team showed that oceanic rocks were accumulated on the seafloor and spread from Kolkata to the Rajmahal–Sylhet line, close to the Shillong Plateau.

As BoB continued to evolve, these primitive oceanic rocks were completely buried under large sediment volume carried by the Ganges and Brahmaputra river systems from the Himalayas. The sediments built an alluvial cover of more than 18 km thickness in the Bengal Basin to Kolkata's east, which is a major part of present day Bangladesh.

The researchers say the Bay of Bengal, therefore, is unique among global ocean basins for its thick accumulated sediments that completely carpet the oceanic rocks. Earth scientists have been curious to know what kind of crust lies beneath the thick sediments in Bangladesh.

"Using different geological and geophysical tools it has now been shown that oceanic- and not continental-type of crust lies beneath the alluvial cover over most parts of Bangladesh," says the report. A petroleum company drilled more than 70 wells in the Bangladesh region but could not recover basement rocks as the sediments were about 18 km thick.

"Extension of oceanic crust well within the land mass is a rare feature," the report concludes. "The Bengal Basin region constitutes a good example."


References

1. Krishna, K. S. et al. Ancient ocean floor hidden beneath Bangladesh. Curr. Sci. (2019) Article