GREATER HIMALAYAN GNEISS PRESERVES SEDIMENTARY STRUCTURES – PHENOMENON OR FANTASY?

Sedimentary relicts, including trace fossils, conglomerate with oriented pebbles, and laminated, channeled dolostone, within the gneisses of Greater Himalaya Sequence demonstrate the possibility of recognizable sedimentary structures surviving metamorphism in upper amphibolite facies with partial melting. Found in favorable exposures, these relicts could provide important information about the depositional history of the protolith and aid in paleogeographic mapping.

The type Takhtsang Gneiss, within granulite facies, consists of alternating millimeter bands rich in quartz and biotite, interpreted as silt and clay interlaminations, respectively, in the protolith. This suggests possible tidal deposits or varves. This banding drapes around isolated “lenses” of contrasting and varied compositions, interpreted as dropstones. This combination suggests deposition in a glacial lake. Published comparisons of Takhtsang lithologies indicate a minimum dimension of 315 km for the postulated lake. These examples demonstrate the potential utility of relict sedimentary features in reconstructing geologic history, even in high-grade metamorphic terrains.