Study On The Characteristics And Sedimentary Evolution Of The Mesozoic In Chiawa Area,Southern Tibet

The Chiawa area in southern Tibet is located between Yarlung Zanbo Suture Belt(YS)and Southern Tibet Detachment System(STDS).It belongs to the Middle-East of the Himalayan Tethys Orogenic Belt,Himalayan Block.It comprises neo-Tethys ocean basin filling formations from the Upper Triassic through Lower cretaceous,which is influenced by the collision of the Indian Plate and the Eurasian continent.Combined methods of stratigraphy,sedimentology and chronology have been applied to study the Mesozoic in Chiawa area in order to real the geological settings,distribution,lithology,sequence and related aspects of the Mesozoic in the study area.Based on the regional data,the filling characteristics and sedimentary facies of the Mesozoic in Chiawa area were analyzed,and the geological evolutionary history was discussed.Primary results of this paper are including:The Ridang Formation of the Lower Jurass:ic is generally characterized by argillaceous siltstone,thin-bedded siliceous rock,shales with few thin-bedded feldspar quartz sandstone,thin-bedded limestone,and medium-thick bedded basalt,which belongs to basin facies deposit.The Lower—Middle Jurassic Lure Formation is composed of grey and dark grey medium-thick micrite,bioclastic micrite,thin micaceous siltstone,mud-shales,mica-bearing quartz argillaceous biological oriatic microcrystalline dolomite;the Lure Formation is featured by horizontal stratification representing a mixed depositional facies of shallow sea shelf and platform.The Zhela Formation of the Middle Jurassic is mainly consists of argillaceous siltstone,silty mudstone intercalated sandstone,limestone,basalt,pyroclastic rock and siliceous rock;the sandstones possess horizontal beddings and cross-beddings,the volcanic rocks are basalt and anglaite;the sedimentary environment is interpreted as terrigenous clastic shallow sea and volcanic spillages.The lithology of Weimei Formation of Upper Jurassic consists of coarse-grained thick layer quartz sandstone,gravel-bearing quartz sandstone,argillaceous silty sandstone and limestone;the sandstone generally shows upward thinning,and the bedding reason shows the transition of oblique bedding,parallel bedding and horizontal bedding from bottom to top,with delta-littoral shallow sea deposition as the main sedimentation.The particle grains in the Sangxiu Formation of the Upper Jurassic and the Lower cretaceous is generally characterized by upward fining,which was formed at the junction of the continental shelf and the sea;it is mainly composed of basalt,volcanic breccia,yinganite,rhyolite and other volcanic rocks.According to the litho-faciesanalyses,the sedimentary environment of the Mesozoic in Chiawa area is concerned a continental shelf rift basin.Since the Late Triassic through to the Early Cretaceous,two major expansion periods could be recognised,i.e.shelf rift I period,during the Late Triassic to Middle Jurassic;shelf rift ? period,during the Late Jurassic to the Early Cretaceous.