Understanding provenance, tectonic settings and diagenetic changes of Upper Disang (Middle to late Eocene) sandstones of Phek District, Nagaland (India): insights from petrography and heavy mineral assemblages

Abstract

The Upper Disang (Middle to Late Eocene) sandstones exposed in the Phek District of Nagaland were investigated using integrated petrographic and heavy mineral analysis to constrain their provenance, tectonic setting, and diagenetic evolution. The sandstones are light to dark grey in colour and are composed predominantly of very fine to fine grained sand fractions. Petrographic examination indicates that quartz is the dominant framework component, occurring mainly as angular to sub-angular grains with occasional sub-rounded forms. Both orthoclase and plagioclase feldspars are present, while lithic fragments constitute the next most abundant framework component and include sedimentary, metamorphic, and volcanic rock fragments. The heavy mineral assemblage is characterized by sub-angular to sub-rounded grains of zircon, tourmaline, rutile, kyanite, sillimanite, hypersthenes and opaque minerals. Detrital modal compositions plotted on Q–F–L diagrams indicate a dominant recycled orogen provenance, with contributions consistent with a passive margin tectonic setting. The combined framework grain composition and heavy mineral suite suggest a mixed provenance, reflecting sediment supply from multiple source terrains influenced by the complex tectonic framework of the region. Tectonic activity also played a significant role in drainage reorganization, facilitating sediment input from diverse directions and source areas. Diagenetic features preserved in the Upper Disang sandstones indicate predominance of shallow burial diagenesis, with localized signatures of early mesodiagenesis developed at burial depths of approximately 3–5 km and temperatures exceeding 150 °C. The integrated provenance and diagenetic signatures provide important insights into the tectono-sedimentary evolution of the Upper Disang Formation within the Indo–Myanmar orogenic system.