Sequence stratigraphy of the second member of the Upper Triassic Xujiahe Formation in the middle part of western Sichuan depression

The second member of the Upper Triassic Xujiahe Formation is the regional gas horizons in the middle part of western Sichuan depression. The stratigraphic horizons have long been divided according to a lithostratigraphic boundary. The diachronism of this boundary may give rise to the differences in the examination of tectonic evolution. The findings based on sequence stratigraphy in this study have disclosed that the second member of the Upper Triassic Xujiahe Formation should be the delta deposits consisting of a complete medium-term cyclic succession, including the delta plain-delta front symmetrical cyclic sequence and deepening-upward delta plain asymmetrical cyclic sequence. The top boundary of the second member of the Upper Triassic Xujiahe Formation is interpreted as a third-order conformity.