Geology of Bangladesh

It is the eastern two-thirds of the Ganges and Brahmaputra river delta plain stretching to the north from the Bay of Bengal.

[1] The downwarping of the basement rocks under central and southern Bangladesh result from the pressure of sediments that have been accumulating since the Cretaceous period, mostly a large quantity of carbonate.

As tectonic activity slowed somewhat, a river delta environment deposited of the other significant Cenozoic sedimentary units.

Renewed tectonism in the Pliocene formed the Dinajpur Shield into a graben, relative to the horst of the Shillong Plateau and Mikir Hills.

The Pliocene orogeny led into the Pleistocene ice age, resulting in regional extinction of many large mammals.

A final period of uplift raised the red clay table lad 30 to 100 feet, along with St. Martin's Island in the Pleistocene.

Drilling and mineral prospecting has revealed deeper rock units that record the geologic history of Bangladesh prior to the Cenozoic as far back as the Precambrian.

The Precambrian Basement Complex is composed of gneiss, granite, granodiorite and smaller sequences of schistose, amiphibolite and gabbro.

[8] The Sylhet limestone preserves additional rock units such as shale and sandstone, indicative of a shallow marine environment and contains 69 species of foraminifera, while the Upper Eocene Kopili formation contains pollen from mangrove swamps.

Ascending the stratigraphic column, a second unconformity separates Permian units from late Jurassic Rajmahal basalts and early Cretaceous Sibganj sedimentary rocks, with thicknesses between 131 and 546 meters.

An additional unconformity separates Eocene and Oligocene Jennum and Renji sandstone and shale sequences, which are 646 meters in thickness.