Chapter 32 Heavy Mineral Constraints on the Provenance of Cenozoic Sediments from the Foreland Basins of Assam and Bangladesh: Erosional History of the Eastern Himalayas and the Indo-Burman Ranges

Abstract

Abstract The Assam-Bengal Basin system, located near the eastern syntaxis of the Himalayas and the northern end of the Indo-Burman Ranges, has received synorogenic sediments of several kilometres thick from these orogenic belts. These deposits provide valuable information on tectonic events, palaeogeography, and evolution of the sedimentary basin. Studies of heavy minerals document temporal variations in detrital compositions reflecting changes in the hinterland. Heavy mineral weight percentages in the Palaeogene and Neogene samples from the Assam Basin vary from negligible to ∼1.5%, and from In the Bengal Basin , Oligocene heavy mineral weight percentages are low (0.2%), and most of the grains are opaque; non-opaque minerals are zircon, tourmaline, and rutile, suggesting intense weathering of the Oligocene sediments. Miocene and younger heavy minerals are much more diverse and include garnet, aluminosilicates, epidote group minerals, pyroxenes, chlorite, hornblende, tremolite-actinolite, micas, prehnite, pumpellyite, and opaques. In contrast to the Assam Basin, provenance-diagnostic minerals from the Cenozoic successions of the Bengal Basin show a distinctive pattern in their distribution, indicating gradual unroofing of the contributing orogenic belts. The presence of blue-green amphiboles in Mio-Pliocene strata from Pakistan, the Assam Basin, the Bengal Basin, and Bengal Fan signal orogen-wide unroofing of arc-type rocks. Modal analysis of framework components and heavy mineral analysis indicate that the sediments in both the Assam and Bengal Basins were derived from discrete sources during the Oligocene. Source areas were the incipient uplifted orogenic belts in the Himalayas for Assam, and the Indian craton for the Bengal Basin. Heavy mineral contents in Miocene and younger successions suggest that both the Bengal and Assam Basins received detritus from orogenic hinterlands, i.e., the Himalayas in the north and the Indo-Burman Ranges to the east. Overall, the Assam Basin appears to represent an earlier and more proximal repository of detritus, shed from the Himalayan convergence, whereas the Bengal Basin was a downstream and somewhat younger depocentre.