High-resolution sequence stratigraphy and the lithofacies-paleogeographic characteristics of Upper Triassic Xujiahe Formation in Sichuan Basin,China

Under the guidance of the high-resolution sequence stratigraphic theory,the Upper Triassic Xujiahe Formation in Sichuan Basin was divided into 2 super-long-term,5 long-term,18 medium-term and 10 short-term base-level cyclical sequences.Based on this and taking long-term asending and descending half-cycle as a unit,this paper analzes the lithofacies-paleogeographic characteristics and the evolution of the Xujiahe Formation at different periods. The result is that:(1) the Xujiahe deposition in Sichuan Basin has been gradually advanced from the peripheral mountain system to intrabasins and from the central Sichuan uplift to the centre of the basin,which mainly consists of developed(alluvial fan) braided river-braided delta-shallow lake deposits,resulting in shallow lake areas inclining to the southwest in ∩ shape,with a scatter and sparse distribution of small type shallow lake sandbars;(2) the distribution of the sedimentary facies belts is strictly controlled by the structure,especially on the basin margin;(3) the late Triassic Xujiahe stage is dominated by the thrusting in the Longmen structural belt,and the Micang-Daba activity is relatively weak.