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Discovery and geological significance of unconformity surface between Upper Triassic Ma'antang Formation and Xiaotangzi Formation in Hanwang of Mianzhu in Northwest Sichuan,China

ZHANG Biao1,WANG Zhong-jiao2,SHI Zhi-qiang1,CHENG Ming1, DUAN Xiong1,JIN Xin1,CUI Lei1 1.State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059,China; 2.College of Earth Science,Guilin University of Technology,Guilin 541004,China  
Based on the field geological investigation,a parallel unconformity surface between the Upper Triassic Ma'antang Formation and Xiaotangzi Formation is found in the Hanwang section of Mianzhu in Northwest Sichuan.The unconformity surface is characterized by the lithological mutation from the oolitic limestone of Ma'antang Formation to the calcareous lithic-quartzose sandstone of Xiaotangzi Formation.The weathering crust on the unconformity surface contains brown lenticular ferruginous and black carbonaceous layers.This irregular interface caused by leaching and erosive can be found on the top of the oolitic limestone in the uppermost layer of Ma'antang Formation.This interface is filled by mudstone or siltstone breccias.The unconformity surface is related to the Late Triassic tectonic activities and the global and regional sea level changes.It indicates that the Longmen Mountains partly uplifted from the Late Carnian to Early Norian.From this sedimentary section,it is concluded that the paleoclimate changed from arid to humid,then to arid,from Late Carnian to Early Norian and that this process is related to global climate changes under the background of megamonsoon system.
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