The Conglomerates of Middle Andaman and their Geologic Significance

Abstract

Fluvial conglomerates, separating two flysch sequences have been identified at several places in Middle Andaman island. These conglomerates are characterised by poorly sorted, polymictic, well-rounded to sub-rounded pebbles, and consist of black cherty quartzite, vein quartz, slaty phyllite, andesite-basalt, reddish jasper, limestone, serpentinite and tuffaceous rocks. The linear continuity of the conglomerate is broken and its thickness much affected by structural deformation.

The conglomerates appear to have been initially deposited as lags on steep slopes and later resedimented in deep waters of the basin. It is implied thereby that the Middle Andaman conglomerate represents unconformable unit separating two sequences of flysch beds, and marking a period of tectonic uplift. It is suggested that the uplift of the source area followed the deposition of the older flysch sequence and perhaps the basalt andesitic volcanism also. Such an uplift may be linked with the oft-suggested subduction of Indian Ocean plate along Indo-Burma-Andaman-Nicobar-Sunda arc boundary.