Surma Basin

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Surma Basin occupying the northeastern part of the country is the prime gas producing province of Bangladesh. The River surma, the right bank tributary of the Barak originating from Manipur and Mizoram of India, passes along Sylhet town and joins the kushiyara west of Baniachong, the combined flow of which is the mighty meghna river.

The Surma Series of Miocene consisting of Bhuban and Boka Bil Formations takes the name from this river and has excellent development in Surma Basin. Surma Valley and Sylhet Trough also conform more or less with the Surma Basin. Shillong Massif and Barail Range form the northern boundary while Tripura Fold Belt bounds it on the south. On the east it is bounded by Barail-Manipur ridge and on the west the basin gradually rises to Hinge Zone finally passing into the Bengal Foredeep in the southwest. Surma Basin has an area of about 1,00,000 sq km occupying the northeastern part of bengal basin that subsided mainly between Oligocene and Pliocene times. Dauki Fault of regional extent passing all along the northern part of Surma Valley south of Shillong Massif is the most important tectonic element, which drove down the Archean basement to about 18 km in the Surma valley. Structurally Surma Basin has been formed due to simultaneous interaction of the two major tectonic elements- the rising Shillong Massif in the north and westward moving Indo-Burman mobile fold belt on Burmese Plate.

In the border belt of greater Sylhet district, there are outcrops of Palaeocene Tura Sandstone's (Takerghat), Eocene Limestone (Bagali Bazar Lamakata, Charagaon, Lalghat, Bhangarghat and Jafflong), Upper Eocene Kopili Shale (Dauki) and Oligocene Barail sediments (Jenam/ Renji) has limited development in Jafflong-Tamabil-Jaintiapur area. In the sub-latitudinal trending foothills east of Jaintiapur Mio- Plio-Pleistocene sequence representing Bhuban, Boka Bil, Tipam Sandstones, Girujan Clay, Dupi Tila (Type area) and Dihing are exposed dipping southwards occupying the northern flank of the Surma Basin.

Among the other structures of the Surma Basin Bhuban sediments are exposed in Patharia and Harargaj, Boka Bil in KathalKandi, Tipam in Kailas Tila, Dupi Tila deposits cover Beanibazar (Mama Bhagna), Fenchuganj, Maulvibazar, Rashidpur and Habiganj anticlines. Bibiana is covered with Recent and Sub-Recent Holocene deposits. [DK Guha]