Discovery of abundant pyroclasts in the Namunagarh Grit, South Andaman: evidence for arc volcanism and active subduction during the Palaeogene in the Andaman area

The facies characteristics indicate that the coarse-grained facies was emplaced as debris flows while the finer-grained facies was transported by turbidity currents. Preservation of intact glassy constituents and their abundance, imply direct sedimentation, or very rapid re-sedimentation, with limited traction and limited time for alteration during transport. These pyroclast-rich sandstones were laid down in a submarine forearc environment on an accretionary complex. The petrology of the juvenile clasts indicates the subaerial or shallow subaqueous explosive eruption of crystal-rich vesicular magma from andesitic arc volcanoes located on an arc massif, inferred to have existed on the western margin of the Burma-Malaya continent during Eocene-Oligocene times. This evidence of arc volcanism indicates active subduction, accretion and emplacement of ophiolite and the associated sediments from the Andaman-Java trench. The pyroclast-rich sandstone represents a distinct and hitherto unrecorded unit in the Tertiary stratigraphy of the Andaman Islands.