The Tertiary rocks of Changkikong range constitute a part of belt of Schuppen - a most prominent morphotectonic unit of Naga Hills stretching along the western margin of Nagaland State. The Barail Group (Tikak Parbat Formation), Changki Formation and Tipam Group (Tipam Sandstone Formation) have been studied for their clay mineral content, ultrastructure of sand grain surfaces and pores as well as diagenetic changes. Presence of illite, chlorite, montmorillonite, and kaolinite in the sediments of the study area has been documented. A deep burial diagenetic environment has been envisaged for the Tertiary sediments of Changkikong range, Mokokchung district, Nagaland.