The Assam–Arakan Basin in the northeastern region of India (22°N–28°N: 90°E–96°E) is one of the largest sedimentary basins encompassing the states of Assam, part of Arunachal Pradesh, Meghalaya, Tripura, Nagaland, Manipur and Mizoram (Fig. 1). The exposed sedimentary sequence in the geosyncline ranges in age from Late Mesozoic to Cenozoic and the bulk of the succession comprises the Tertiary strata. The sediments are represented in two distinct shelf and basin lithofacies. The distinction is more pronounced in the Paleogene sequence than in the Neogene. The rocks of the shelf facies are exposed in the Garo Hills, Khasi–Jaintia Hills, part of North Cachar and Mikir Hills and also in the subsurface below the alluvium of Upper Assam. The basinal facies are developed in the Naga Hills, Manipur, Surma Valley of south Cachar, Tripura and Mizoram. Due to rarity of fossils, particularly microfossil markers in the region, lithostratigraphic classification formed the basis for correlation.