Sedimentation and Tectonics of Lower Cenozoic Sequences from Southeast of Shillong Plateau, India: Provenance History of the Assam-Bengal System, Eastern Himalayas

Abstract

The present study area is located along the southeastern part of the Shillong Plateau in the eastern and southeastern part of the Meghalaya State and in the lower (southern) part of Assam State, northeast India. A continuous Eocene to Miocene sequence is exposed along the Jowai-Badarpur Road (NH-44). The study area is at the junction between Assam and Bengal basins. Detrital history from this area provides data pertaining to uplift and erosional history of the hinterland areas (i.e., Himalayas, Indo-Burma Ranges and Shillong Plateau). Lower Cenozoic units from the study area bear signatures of the both orogenic belts and stable craton. Lower Eocene Tura and Sylhet sandstones are quartzose (Q98F2L0), which points to the Indian Craton as a potential source area. Sandstones from the late Eocene Kopili Formation are quartzolithic (Q73F4L23). Younger Oligocene and Miocene sandstones are also quartzolithic, rich in unstable grains, suggesting derivation from adjacent orogenic belts. Eocene sandstones are rich in opaque and ultra-stable heavy minerals (e.g., zircon, rutile, and tourmaline). Concentrations of unstable heavy minerals (e.g., amphibole, chrome-spinel, and serpentine) increase in Oligocene and Miocene strata. Progressively lower maturity indices of heavy mineral assemblages in up-section suggest that the orogenic fronts are getting closer. Mineral chemistry of detrital tourmaline grains suggests that derivation from metapelites with a subordinate Li-poor granitoid source. The majority of the detrital tourmalines belong to the schorl-dravite series. Low Al, low Fe, and high Mg-bearing chrome spinels suggest derivation from Alpine-type peridotites, likely from the Indo-Burma Ranges (Naga ophiolites) to the east of the study area. Detrital garnet chemistry suggests a broad P-T range for source rock terranes. Mudrock geochemical data suggest that the majority of the sediments were derived from recycled orogenic sources. Based on the chemical index of alteration, the intensity of weathering in the source area was moderate to high. Future research project in the area should explain the uplift history of source terranes using garnet and tourmaline chemistry and detrital geochronology.