BASINWIDE CENOZOIC SEQUENCES OF ASSAM-BENGAL SYSTEM, EASTERN HIMALAYAS
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.