Abstract:

The foreland basins of central-western China are mostly characterized by intracontinental-subduction, so the application of sequence stratigraphy to these basins is a new research field. Sedimentary facies analysis and sequence stratigraphic division of five measured outcrop sections and one well log arecarried out in detail. The patterns of sedimentary facies transformation, the types of sequence boundaries, sequence characteristics and the related base level changes are described as well. The result suggests that the Upper Triassic Xujiahe Formation of the northern Sichuan foreland basin be divided into four thirdorder sequences (TS1, TS2, TS3 and TS4). The thickness of Xujiahe Formation is about 400-600 m in Dazhu-Kaijiang basin where the four sequences are relatively complete. In contrast, the Xujiahe Formation tends to thin westward to less than 100 m in Wanyuan area where only one sequence could be recognized. Sequence stratigraphic framework is constructed based on the above sequence stratigraphic division of the six sections. TS1-TS2 is dominated by meandering river and delta depositional system, which implied stable subsidence and an underfilled basin. In contrast, TS3-TS4 is characterized by alluvial fan and braided river depositional system, which showed strong subsidence and an overfilled basin. The spatial and temporal distribution of sedimentary facies and stacking patterns were restrained and controlled by tectonics of northern Qinling orogenic belt.

Key words: continental sequence stratigraphy, Upper Triassic, Xujiahe Formation, northern Sichuan foreland basin, Qinling orogenic belt, stratigraphy