Sedimentary sequence and paleogeographic characteristics of the Upper Triassic Xujiahe Formation in eastern Sichuan basin

Abstract: Based on the data of field sections, thin sections and well logging, combined with the characteristics of petrology, palaeontology and previous research results, the authors hold that the 1st member of Xujiahe Formation in Dianjiang area of eastern Sichuan basin was developed in the barrier coast sedimentary system, and the 2nd to 6th members of Xujiahe formation were developed in the lacustrine?braided river delta sedimentary system. Based on the recognition of sequence boundaries and flooding surface, the authors divided the Xujiahe Formation into 3 tectonic sequence cycles (Sup?LSC1—Sup?LSC3), which represent the “remnant bay basin”, “lower Xujiahe basin” and “upper Xujiahe basin” formation periods respectively and can be traced through the whole basin. 6 long?term sequence cycles (LSC1—LSC6) which correspond to the 1st to the 6th members of the Xujiahe Formation respectively were also established. At the same time, using the sequence boundaries of long?term cycle and flooding surface as the marks of isochronous stratigraphic correlation, the authors established the isochronous stratigraphic framework from central Sichuan uplift to southeast Sichuan depression zone. On such a basis, the authors chose each long?term sequence cycle as an isochronous stratigraphic unit, and drew the maps of sequence?based lithofacies and paleogeography of the Xujiahe Formation in the study area. The maps indicate the paleogeography and distribution characteristics of sedimentary facies.