Sedimentary facies and depositional evolution of Member 3 of Upper Triassic Xujiahe Formation in Yuanba area of Northeast Sichuan,China

The sedimentary environments of Member 3of Upper Triassic Xujiahe Formation(T3x3)are mainly related with the Longmen Mountain episodic orogeny in the Yuanba area of Northeast Sichuan,China.Through the comprehensive research of the field outcrops,3Dseismic data and drilling data,the authors believes that T3x3 in the Yuanba area is mainly composed of the braided river delta facies and lacustrine facies.4subfacies including delta plain,delta front,prodelta and shallow lake can be identified in these facies.These subfacies can be further divided into nine microfacies including distributary channel,river course,underwater distributary channel,estuary dam,sheet sand,far sand dam,underwater distributary bay,underwater distributary channel,prodelta-shallow lake mud.In the relative weak period of the Longmen Mountain activities,the distant fine metamorphic rock source of the Songpan-Ganzi fold belt mix with the proximal fine calcium chip source of the Longmen Mountains,and the distant fine braided river delta form in the study area,for example,the third bottom member of Xujiahe Formation.In the relative strong period of the Longmen Mountain activities,the proximal coarse grain calcium chip source of the Longmen Mountains mix with the distant fine metamorphic rock source of the Songpan-Ganzi fold belt,and the proximal coarse grain braided river delta form in the study area,for example,the third middle-top member of Xujiahe Formation.