Basement a complex usually of igneous and metamorphic rocks that is overlain unconformably by sedimentary strata. Generally the igneous and metamorphic rocks form the basement of all geological group-formations and as a consequence they are referred to as basement complex. In many regions the basement is of Precambrian age (the period of time from the consolidation of the earth's crust to the base of the Cambrian ie 570 million years ago), but it may be much younger also.
In Bangladesh the sedimentary rocks lying over the basement range in thickness from 128m in the Rangpur Saddle area to 21 km in the Basin area. The rocks of the Gondwana, Rajmahal, Tura, Jaintia and the Surma Groups, the Dupi Tila and the Madhupur Clay Formations overlie the basement whose thickness varies from 0.12 km to 3 km in the pre-cambrian indian platform region and 3 km to 21 km in the basin part. In the Indian Platform part basement rocks encountered at varying depths in the boreholes include granite, granodiorite, diorite, gabbro, gneiss, schist, amphibolite, etc. These rocks may contain precious metallic minerals. In many places of the Platform region the rocks of the Surma Group directly overlie the basement. In most parts of the Rangpur Saddle area the sedimentary rocks of the dupi tila formation unconformably overlies the basement. The top of the basement at the Rangpur Saddle exhibits a good degree of flatness, but it generally slopes in all directions. The basement has suffered from intense faulting. [Eunuse Akon]