Sedimentary characteristics of upper Triassic Xujiahe formation in Guanba area of southern Chongqing

By taking the outcrop of the upper Triassic Xujiahe formation in Guanba area of southern Chongqing as the research object,the petrological and sedimentological analyses were carried out.The results show that the 2~(nd),4~(th) and 6~(th) member of the upper Triassic Xujiahe formation are mainly composed of grey-white medium to medium-coarse feldspathic lithic sandstones,with medium-well sorting,sub sharp-sub circle roundness pore cementation,large-scale inclined beddings and erosional basal surfaces.The 3~(rd) and 5~(th) member of the upper Triassic Xujiahe formation are mainly composed of grey and grey-yellow mud shale with horizontal bedding.In vertical direction,the'dual structure'is very developed in the Xujiahe formation,characterized by the thickness of the bottom layer being much larger than the top layer.In horizontal direction,sandstones are divided by erosional basal surfaces to lens-shaped and wedge-shaped sand body,which form large sand bodies by continuous superimposition.The comprehensive study shows that the 2~(nd),4~(th) and 6~(th) members of Xujiahe formation in the study area are braided delta front sedimentation,and the 3~(rd) and 5~(th) members of Xujiahe formation are braided river delta sedimentation.