BASINWIDE CENOZOIC SEQUENCES OF ASSAM-BENGAL SYSTEM, EASTERN HIMALAYAS

The Assam and Bengal basins are two isolated but adjacent large depositional lobes south of the eastern Himalayas. Cenozoic sequences revealing early erosional remnants of the eastern Himalayas are preserved in these two basins. A continuous section exposing most of Cenozoic sequences are found along Jowai-Badarpur Road section of southeast Meghalaya and Assam State border in northeast India. The section is bounded in the north and west by the Shillong Massif, in the south by the Dauki fault and in the east by the Naga-Disang thrust. Interestingly, for a comparative study this section is located between not-so-continuous sequences in Margherita-Changlang area in the Schuppen belt at the northeast Assam, and the Sylhet trough toward southwest in the Bengal basin.

About 1 km Eocene sequence belonging to the Jaintia Group represents deltaic to shallow marine facies ranging from coaliferous clastics and fossiliferous limestones, to black shale. This sequence resembles more closely with the Eocene passive margin sequence of the Sylhet trough toward the southwest. The Eocene section in upper Assam (Margherita-Changlang area) is clastic (Disang Group), representing more of an active margin marine deposits. The Oligocene Barail Group is thick (4.65 km) and is composed of deep marine clastics. In contrast, the Oligocene units in both upper Assam (~5 km) and the Sylhet trough (~ 1.5 km) are of deltaic to marginal marine facies with thick coal seams present in upper Assam. Miocene Surma Group (~2.5 km) represents deep marine sequence in Jowai-Badarpur section. Although this sequence is as thick as in the Sylhet trough, but is thin in upper Assam sections. The Surma Group is overlain by upper Miocene to Pliocene Tipam Group (1+ km). Continental facies sedimentations prevailed during deposition of the Tipam Group in this section and the other two areas (upper Assam and Sylhet trough) because depocenters were shifting toward south as a result of encroaching mountain fronts close to these basins.

Overall, the Margherita-Changlang area appears to represent a more proximal and earlier repository of detritus shed from the Himalayan collision, while the Jowai-Badarpur Road section and the Sylhet trough of the Bengal basin serving as downstream and somewhat younger depocenters.