SEDIMENT COMPOSITION OF LOWER CENOZOIC SEQUENCES FROM SOUTHEAST OF SHILLONG PLATEAU, INDIA: PROVENANCE HISTORY OF THE ASSAM-BENGAL SYSTEM, EASTERN HIMALAYAS

A continuous Eocene to Miocene sequence is exposed along the Jowai-Badarpur Road section, southeast of the Shillong Plateau, NE India. This section is located at the junction between Assam and Bengal basins.

Lower Cenozoic units from the study area bear signatures of derivation from 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), which suggests a shift in sediment provenance from stable craton to orogenic belts. Younger Oligocene and Miocene sandstones are also quartzolithic and rich in unstable grains, suggesting derivation from adjacent orogenic belts.

Eocene sandstones are rich in opaque and ultra-stable heavy minerals. Concentrations of unstable heavy minerals increase in Oligocene and Miocene strata. Progressively lower maturity indices of heavy mineral assemblages in up-section suggest that the orogenic fronts were getting closer. Mineral chemistry of detrital tourmaline grains suggests that derivation from metapelites with a subordinate Li-poor granitoid source. 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 the source rock terranes.

Mudrock geochemical data suggest that the majority of 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.

This study suggests that the area started receiving orogenic sediments from the late Eocene, and the Shillong Plateau may have uplifted by the Miocene.