ABSTRACT

The Assam-Arakan fold-thrust belt of NE India runs from the Patkai hills at its easternmost end through Naga Hills and Manipur in the southwest to Mizoram in the south. The Patkai Hills in the fold belt presents a complete Tertiary stratigraphic succession from Eocene Disang to Pliocene Dihing Group. The outcropping Lower Tertiary Disang and Barail groups occur as thrust slices in which the rocks of the Disang Group are more argillaceous whereas they become more arenaceous upward in the stratigraphic horizons of the Barail Group. The upper part of the Disang Group is comprised of alternating succession of sandstone and shale in which the sand: shale ratio varies from 1:7 to 1:0.26. The Barail Group is dominantly arenaceous with minor intervening shales in which the sand: shale ratio varies from 1:0.07 to 1:1.2. The sandstones are fine to coarse grained, light grey in colour, well indurated and cross-bedded. The sandstones of the Disang and Barail groups are “Lithic-Arenites” and “Lithic Graywackes” types in which the matrix content varies between 8.5 and 26%. The quartz overgrowth, authigenic mica, secondary chert and sericitization of feldspar are present in the sandstones. Although minor the feldspar dissolution produces secondary porosity in the sandstones. Silica, clay and iron oxides are the cement types present in the sandstones. Besides floating grains, the framework elements show point, long, concavo-convex and sutured grain contacts.

Key words: petrography, sandstone, Patkai Hills, NE India.