Assam

Assam

Assam and Arakan basin, lies in the Northeast part of country, have spectacular stratigraphy of Cenozoic era. The Assam-Arakan sedimentary Basin is a shelf–slope–basinal system. The basin is famous for its hydrocarbon potential.

Evolution

• Evolution of the basin was occurred in two phases. First was formation of shelf-slope-basin system from cretaceous to oligocene. 

• During cretaceous, when Indian plate was moving towards north, numerous horst and grabens were formed followed by series of marine trangressions and regression. 

• Marine trangression was resulted in deposition of Sylhet limestone and regression deposition Kopili shales. Barail formation was deposited in marine regressing to deltaic environment.

• Second phase occurred when continental part of Indian plate collided with Eurasian plate which was resulted in formation of foredeep. There was upliftment and erosion all over the shelf and in a major part of the basinal area. The basin was then tilted southeast.

• In this foredeep was the site where Surma group deposited in marine to brackish condition. 

• Later, the sea was totally withdraw, and continental sedimentation deposited Tipam formation. 

Stratigraphy

Strati of Assam starts with Sylhet traps of Jurassic age which are lying on precambrian granitic terrains which are lying on shillong group.

Sylhet traps

These traps cover a small area on shillong plateau. Source was Kerguelen plume, same which caused Rajmahal traps. The area they cover is not easily accessible.  

Khasi group - Cretaceous

• It is named on Khasi hills, Meghalaya which are rich in sillimanite deposits. This group is mostly argelaceous.

• Mahadek formation is important in this group. Some authors suggested this formation is of KT boundary. This formation contains Uranium deposits.

Jantia group - Paleocene & Eocene

• Paleocene formation is Langpar. 

• Sylhet limestone have shales mix and are rich in plankton fossils. They represent marine transgression phase.

• Kopili formation is majorly of shale and sandstone. Sandstone is hydrocarbon reservoir and shale is cap rock.

Oligocene to Pleistocene  

• Strati of this range is important.

• Oligocene formation Barail contains coal seams and some oil traps. It is fossiliferous. 

• Surma group doesn't have coal and oil.

• Tipam formation contains wood fossils. Tipam sandstone is reservoir and girujan clay is cap rock. Famous Digboi oil field of assam has it reservoir in this formation.