Preliminary Study On Sedimentary Geology Of Gyachala Formation Of Gyangzi,Southern Tibet

Studies of Paleogene allow to interpret closure of the eastern Tethys and the age of initiate of a collision between India and Eurasia.Gyangzi area of southern Tibet is located in the northern subzone of Tethys-Himalaya, It belongs to the northern Indian continent. It is located in the southern Indus-Yarlung Zangbo Suture(IYZS), north of South Tibetan Detachment (STD) and High-Himalaya and west of Dingri and Gangba of Eastern Tethys. Gyachala Formation of Gyangzi area is the latest marine sequence, which conformably overlies Cretaceous Zongzhuo Formation. It is 880 met- res thick and mainly consists of interbeded sandstone, shale and fine-grained conglomerate. Many sandstones occur as lenses. Also few limestone and silicalite occur in lensoid form too. Sandstones are comprised by quartz, feldspar and debris. Quartz from in an average 65%, feldspars 12% and 23% are debris. The latter are mostly calcalkaline composition, represented by andesite, basaltic andesite and acidic liparite. Gyachala Formation is rich in Dinophyceae and spore-pollen assemblages, which indicate that the age of it Paleogene to Eocene.Field study allowed to separate the formation into four reverse graded cycles that range from fine grain at the base to a coarse grain at the top (shale-siltstone-fine sandstone-medium sandstone and fine conglomerate), and to many sublevel cyclic sequence. There occur Bouma sequences and recognized can be A—D, A—B, B —D, B—C and B —E cycles; slumpes, slipping structure. Also were observed flute casts and groove casts. The section is tectonically highly deformed, so any detial sedimentary observations are extremly difficult to make due to a poor exposure of the strata. It is concludecd that the Gyachala Formation is a deep-water turbidite fan deposit. The presence of spores and pollens suggested that sediment deposition of Gyachala Formation was occurring not far from the land. The plot of sediment composition into triangular diagrams of Dickinson indicate that the provenance of Gyachala Formation sediments was the recycled orogen , mixed orogen and subduction complex. The magmatite zone of IYZS also provided sedimentary material for the Gyachala Formation.