Long-timescale variation in bulk and clay mineral composition of Indian continental margin sediments in the Bay of Bengal, Arabian Sea, and Andaman Sea

Abstract

This study documents X-ray diffraction results from bulk powders and oriented clay-size aggregates using samples from sites drilled and cored during the Indian National Gas Hydrate Expedition 01 (NGHP01). These sites are located in the Krishna–Godavari Basin, Mahanadi Basin, and Andaman accretionary wedge of the Bay of Bengal, and the Kerala-Konkan Basin of the Arabian Sea. Calcite is more abundant at the pelagic sites of the Andaman Sea and Kerala-Konkan Basin, which is consistent with previous studies of biological productivity and dilution by lithogenous influx. Hemipelagic sediments in the Krishna–Godavari Basin and Mahanadi Basin are comprised primarily of smectite-rich and illite-rich clay mineral assemblages, respectively. We attribute those contrasts to differences in detrital sources between the Deccan basalts (smectite sources) and Precambrian rocks of the Eastern Ghats Belt; those sources remained consistent over the entire history of sedimentation (0–9 Ma). Higher quartz content in the Mahanadi Basin and higher feldspar content at the Krishna–Godavari Basin reinforce these interpretations of detrital provenance. Smectite is the most abundant clay mineral in the Andaman Sea sediments likely due to weathering of volcanic sources along the Sunda Arc. Strata from the Kerala-Konkan Basin show a shift at 23 Ma from a smectite–kaolinite clay mineral assemblage to an increasingly illite-rich assemblage. We also see steady decreases in kaolinite and increases in chlorite and quartz over the 30-Myr record, which indicates increasing influences of material derived from physical weathering. The higher abundance of fully hydrated smectite in the Krishna–Godavari Basin may play a minor role in gas hydrate formation by sustaining higher permeabilities at any given value of mudstone porosity or void ratio.