Pebble-Slates in Parts of Eastern Himalaya-Evidence for Pre-Gondwana Deformation in Himalayan Rocks

The pebble-slates in Sikkim and Bhutan are identical in character. They are conformably overlain by Gondwana rocks and have a gradational relationship with the underlying Buxa rocks in Sikkim. In Bhutan the main band overlies Buxa type lithology conformably but small conformable lenses of this rock are secn within the Buxas. It is suggested that they have been deposited over a span of time starting from the closing phases of Buxa deposition and continued till the base of the Gondwanas. Clasts of folded mica schist, folded carbon phyllite and veined dolomite indicate that they have been derived from a terrian exposing deformed and metamorphosed Buxa like lithology. Angularity of the clasts and presence of plenty of fresh feldspar fragments suggest very little transport of these sediments. The provenance belonged to the Himalayan domain and the deformation recorded in the clasts is pre-Gondwana tectonic element recorded in the provenance. This might be Hercynian or even pre-Hercynian.