PETROGRAPHIC EVIDENCE OF COMPOSITIONAL VARIATION IN GONDWANAN SISNE SANDSTONES OF NEPAL

The Gondwanan sequences are widely distributed in several intracratonic basins of peninsular India and also as discontinuous patches along the Himalayan orogenic belt. In Nepal Himalaya, these sequences are found in Surkhet, Dang, Palpa, Katari and Barahachhetra from west to east respectively. These Gondwanan sequences record pre-Himalayan orogenesis and segmentation of the Gondwanaland. The Tansen Group (2400 m) in western Nepal is thick and complete compared to the Katari (380 m) and the Barahachettra groups (350 m) located in eastern Nepal. Sandstone modal analysis data from the upper Carboniferous Sisne Formation reveal interesting detrital history of the Gondwanan sequences of Nepal.

The lowermost unit of the Tansen Group (Sisne Formation) in western Nepal consists mostly of Diamictites. Some interglacial sandstone beds are also present in this unit. Petrographic analyses of sandstone units from the Sisne Formation and its equivalent units from other two eastern sections show marked difference in composition. Framework grains in the Tansen section are texturally immature, consisting primarily of angular to subangular monocrystalline and polycrystalline quartz grains, plagioclase feldspars, chert and mudclasts. Quartz injections are observed in hand specimens and also in thin sections. Presence of abundant plagioclase feldspars suggests potential sediment source from rifted mafic margins during Gondwanaland segmentation (India from Australia). The Sisne-equivalent sandstones in eastern Nepal, however, contain medium-to coarse-grained sediments dominated by monocrystalline quartz grains. Some metamorphic lithic fragments are also found.

Sandstone composition and textural studies suggest that the Gondwanan Sisne Formation and its equivalents in eastern Nepal were developed in separate depositional basins and received detritus from different source terranes.