Provenance and depositional setting of the Disang Group exposed in the north-easternmost part of Assam-Arakan Basin, India: insights from petrography and clay mineralogy

The present study attempts to investigate the provenance, tectonic setting, paleoclimate, and diagenetic history of the Disang Group using an integrated approach of field investigation, petrography, and clay mineralogy. It has been observed that the Disang Group in the study area chiefly consists of shales with subordinate amounts of sandstone, siltstone, chert, and limestone. Dicotyledon fossil leaf impression in sandstone has also been observed during the study. Petrographic analysis of sandstones has shown that they are fine to medium-grained, poorly to moderately sorted, with sub-angular to sub-rounded grains, and classified mainly as wacke with minor arenite. They were derived mostly from low to middle and upper rank metamorphic source rocks in variable climatic conditions from arid to humid, and the source areas were of low to moderate relief. Tectonic discrimination diagrams have suggested craton interior and recycled orogen provenance. The occurrence of clay minerals like chlorite, illite, and smectite-montmorillonite has implied weathering in a cold, arid climate. The diagenetic processes involved were of early to late stages. The limestones are olistostromal deposits and are classified as micrite. The presence of thick shale beds and chert indicates a deep marine environment whereas rip-up clasts and fossil leaves are indicative of their deposition in a shallower part of the basin. Thus, there must have been a change in the environment from deep marine to shallow marine during the deposition of the Disang Group under study.