Provenance and Depositional Environment of Eastern Himalayan Gondwana Rocks of Siang District, Arunachal Pradesh

The Gondwana sequence of West Siang district, Arunachal Pradesh may be subdivided into three units namely diamictite, black shale and sandstone-shale heterolithic facies in ascending order. In the sandstone-shale alternating unit, the sand facies gradually thickens upwards, in which the lenses and pockets of coal are local in extent. With low quartz to feldspar plus rock fragment ratio (<8), the sandstones are lni~ieralogicallyi mmature. Matrix dominantly consists of clay and silt-sized quartz which accounts for 11% on average. The chief authigenic constituents found are muscovite, sericite, quartz, and iron oxide, Fine to medium grained, poorly to moderately sorted sandstones exhibit moderately intense compaction. The sediments under humid climatic conditions were derived from the uplifted cratonic block comprising dominantly igneous and metamorphic rocks, and subordinately from the veneer of sedimentary rocks. Sediments were introduced in the basin under cold glacio-marine conditions and progressively passed through tranquil deep marine euxinic environment to lower shore face shelf environment.