Jurassic Stratigraphic Characteristics And Sedimentary Environment Analysis In South Qiangtang Region Of Tibet

South Qiangtang area in northern Tibet, is a Jurassic marine sedimentary stratawas developed completely and outcropped good well, which, according to this studybelong to the sedimentary environment shallow continental shelf sedimentaryenvironment, more sensitive to changes in sea~level fluctuations,and therefore is thestudy of the Jurassic sea level changes in the history of the best depositional system.The well~studied Jurassic strata in Qiangtang Basin is mainly in NorthQiangtang.There are two section which completely developed Jurassic stratas:(1)TheYanshiping section is the representation in eastern region in which the lower Jurassichave been missing and the Quemocuo group is the lowest level of the Jurassic;(2)TheNadigangri section that havenot significant differences with South Qiangtang area andthe lower Jurassic is composed of volcaniclastics is standard in area of the northShuanghu area,and havenot find black shale formation that as Quse formation andSewa formation. According to Jilin Institute of Geological Survey1:250,000regionalsurvey, Longeni—Angdaer area which lie in South Qiangtang Basin have fully exposedJurassic strara,such as Uper Triassic-Lower Jurassic Suobucha formation, LowerJurassic Quse Formation, middle Jurassic Sewa Formation, Shaqiaomu Formation,Buqu Formation,Xiali Formation,Upper Jurassic Suowa Formation, this area is bestoptimize region for build the stratigraphic sequence of the southern Qiangtang area.The depositional environment of the SEWA formation and QUSE formation havecontinuity,that fully demonstrated by the Toarcian Ammonoid’s finding in top of theQUSE formation,it is from the study of the five formation,such as theRugumiqiong,Dongbula, Migaiertuoba,Ganbeixiama,GanbeiXiamanan that lie in thenear of the Shuanghu region. The siliceous rock that output in a stripe or nodular lie inbottom of the Buqu formation comparison with Saqiaomu formation is a clasticconverted to carbonates interface. Ammonite that was first find in Xiali formation thatconformitied sedimentation over the Buqu formation is deepwater plankton,that indicate the deep-water facies. There have a transgression submerged interface betweenBuqu and Xiali formation.Suowa formation have a conformity sedimentation withXiali formation,in wich the typical signs is a grainstone that containing coral fossil andmetacrinus.These grainstone have a tendency that shallowing along with deepthbecame decrease,and have a transgression interface between this two formation. Theseevidence prove the continuous superpositied sedimentation of the Quseformation,Sewa formation,Shaqiaomu formation,Buqu formation and Xialiformation.it is as not as Wang Jian who saied this four formation is the fingers crosssuperimposed sedimentation. The South Qiangtang area Jurassic stratigraphic’ssequence is the lower Jurassic Quse formation, middle Jurassic Sewaformation,Shaqiaomu formation,Buqu formation,Xiali formation,upper JurassicSuowa formation from down to up.it is same with the data of1:25regional survey byGeological Survey of Jilin.During the deposition of the Jurassic sedimentary the South Qiangtang Basin wasmainly shallow continental shelf sedimentary,in which the type of the sedimentaryenvironment is mainly the carbonates and clastics Mixed deposition platformenvironment,the overall background of the ancient geography is North shallow andSouth deep,the particle size is North coarse and South fine.The shallow continentalshelf sedimentary is bounded SeWa-ZiGeTangCuo,and the southern region thatdeposition a gray~black shale,argillaceous limestone deep~water sediments is thecenter of the main sedimentary of the basin area,and the Northern region that depositionthe deposition Shallow marine carbonate rocks and sandy mudstone formedShelf~Shore facies was deposition of shallow-water platform.Hallam,Haq et al studied the global trends of the Jurassic sea level is the risingtrends, which in accordance with trends of the Jurassic sea level in southern Tibet by thestudy of LixiaoChi,but the trends of the Jurassic sea level in South Qiangtang area onthe contrary, in which the deposition becomes shallow gradually, it is because Yanshanmovement made the Qiangtang plot hit and Weld with the Gandise-Nyainqêntanglhaplots that Rapid northwarding in began Middle Jurassic to the early Cretaceous,andcaused Bangong~Nu Trough of the southern boundary of the basin reduction, off,forming a massive regression, the sea retreated from east to west, so that the vastmajority of the Qiangtang area exposed surface, ended the marine sedimentary history.