Unveiling the Presence of Limestone, Chert and Fossil Leaves in Disang Group Exposed in the North- easternmost Part of Assam-Arakan Basin, India: An Integrated Study on Lithology, Petrography, and Clay Mineralogy

Abstract

Disang Group of the north-easternmost part of Assam-Arakan Basin is predominantly made up of shales with sandstone and siltstone. The current study attempts to investigate its provenance, tectonic setting, paleoclimate, and diagenetic history using integrated approach of field investigation, petrography, and clay mineralogy. It has revealed for the first time that the Disang Group of the present study area also comprises fossil leaves, limestone, and chert in addition to the already reported shales, sandstone, and siltstone. 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 are classified as quartz arenite, quartz wacke, arkosic wacke, and lithic greywacke. The studied rocks were derived mostly from plutonic as well as metamorphic source rocks in a variable climatic condition from arid to tropical 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 presence of chert beds indicates deep marine and that of carbonates indicate shallow marine environment, whereas the fossil leaves are indicative of their deposition in a much shallower part of the basin. Thus, there must have been a change in environment from deep marine to shallow marine during the deposition of the Disang Group under study.