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Discussion on the Stratigraphic Boundary between Middle Triassic and Upper Triassic

DENG Xiuqin1,2),LI Wenhou1),LIU Xinshe1,2),PANG Jinlian2),LIU Xin2)1) Geology Department of Northwest University,Xi'an,710069;2) Exploration and Development Research Institution of Changqing Oilfield Company,Xi'an,710018  
Considering the dispute over the Yangchang Formation classification,a comparative analysis and investigation was conducted.Sporo-pollen assemblage,petrologic characteristics and basin evolution all suggest the distinct differences between the Ch8-Ch10 oil-bearing formations and Ch1-Ch7 oil bearing formations.Sedimentary period of Ch8-Ch10 oil-bearing formations is characterized by even landform,well developed rivers,deltas and shallow lakes,and low sandstone maturity.Analysis on the samples from the Ch8-Ch10 strata shows that the sporo-pollen is dominated by pteridophyta spore,with high content of Punctatisporites one of the significant members of sporo-pollen living in middle Triassic up to 50.5%.However,the deep lake widened expeditiously at the beginning of the Ch7 depositional period.The quartz content in the Ch7 sandstone increases significantly,resulting in the corresponding change of rock types in the west and the southwest.Therefore,the sedimentary system in the west and southwest margins of the basin are evolved into alluvial fan and fan delta from braided channel and braided channel/delta system.The biotic community is characterized by the appearance of Duplexisporites-an important member of sporo-pollen assemblage in late Triassic.In addition,a suit of tuffs distributed at the bottom of Ch7 in the western,southwestern and central parts of the basin records geological events such as syn-depositional volcanism and magmatic activities,which are the important features of early Indo-Sinian movement.Consequently,the stratigraphic boundary between Ch7 and Ch8 oil bearing formations can be regarded as the boundary between mid-Triassic and upper Triassic as well as the representative of the first episode of the Indo-Sinian Movement.
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