ABSTRACT
Surma basin that falls in Southern Assam (Barak valley) consists of a bowl shaped synclinal valley, elongated in the E-W direction while part of Surma basin that falls in Mizoram consist of N-S trending ridges in the hilly terrain. Geologically, sediments of the study area can be divided into, i) unconsolidated alluvial deposits of sub-recent to recent age, and ii) semi-consolidated deposits of the Surma Group-classified as Bhuban formation and Bokabil subgroup, as well as Tipam Group of Miocene -Pliocene age. The alluvial deposits in Barak valley consist of sand, silt and clay with gravel and occasional lignite beds while the semi -consolidated older sedimentary rocks are exposed in form of hillocks composed of shale, ferruginous sandstone, mottled clay, pebble bed and boulder beds. Bhuban and Barail group rocks of the area formed under marine conditions and the subduction of Indian plate under the Burmese plate resulted in the migrating accertionary prism complex (Dasgupta & Nandy 1995). This prism complex represents the remnant of ocean basin sediments. These formations together with the fossil assemblages present suggest a marine transgression indicating changes in the depositional environments throughout the stratigraphic sequence. The depositional environment changed from shallowmarine to near-shore and lagoonal i.e. coastal to fluvial (Kapesa, Lohha & Raju 2007). However, the younger Tipam formation has been deposited under freshwater condition with a thin seam of coal over the shaly Bhuban formation (Gaungly 1989). Generally, the exposed geosynclinal sediments of the Surma basin that contain Barail formation of the study area are rich in iron ore minerals such as pyrite, hematite etc. (Sinha & Sastri 1973).
