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The Assam Valley of northeast India is filled with more than 4 km of mainly terrigenous Tertiary sediments above Precambrian metamorphic and igneous basement. Near the Himalayan foothills in the north, where the total sedimentary thickness may exceed 7 km, upper Paleozoic-Mesozoic strata also are expected to be present. On the basis of gross lithologic characters, heavy-mineral assemblage, electric-log response, and elastic properties, the Tertiary succession has been divided into several rock-stratigraphic units. As far as possible, the stratigraphic nomenclature evolved for the exposed rocks in the adjoining hills has been retained. However, because of inadequate fossil control, the time-stratigraphic designation of system, series, and stage has been replaced by rock-st atigraphic terms--group, formation, and member.
The Tertiary sequence is divided into two supergroups--the Naga and Brahmaputra--separated by a major unconformity. The Naga Supergroup, containing shallow marine to paralic sediments of Paleogene age, comprises the Jaintia and Barail Groups. The Jaintia Group includes the Teok Formation, Sylhet Limestone, and Kopili Formation. The Barail Group includes the Naogaon Sandstone and Rudrasagar Formation. The Brahmaputra Supergroup, containing mainly fluvial and deltaic sediments of Neogene age, consists of the Surma, Tipam, and Moran Groups. The Surma Group includes the Geleki Sandstone. The Tipam Group includes the Lakwa Sandstone, Girujan Clay, and Nazira Sandstone. The Moran Group includes the Namsang and Dhekiajuli Formations.
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