Tectonic evolution and sequence filling patterns in the western Sichuan foreland-like basin in the Upper Triassic Xujiahean period

Abstract：The tectonic activity in the Longmen Mountains exhibited notable phased changes in the Upper Triassic Xujiahean period. The deposition stages of the second and fourth members of the Xujiahe Formation coincided with the stages of thrusting in the Longmen Mountains. The depocenter and subsidence center of the western Sichuan foreland-like basin are located in the foredeep near the basin-margin thrust belt, and the reservoirs on both sides of the basin are well developed and their deposits are very asymmetric. As the downcutting erosion increased with enhancing tectonic activities in the Longmen Mountains orogen, the lowstand systems tract on the side of the basin-margin thrust is better developed, while the highstand systems tract is preserved incompletely because of later subaerial exposure and erosion. On the other hand, the side of the forebulge slope was marked by steady slow uplift and weak downcutting erosion, and so the lowstand systems tract is absent or not well developed and the highstand systems tract is better developed due to weaker erosion in the late stage. The deposition stages of the third and fifth members of the Xujiahe Formation coincided with the repose stage of thrusting in the Longmen Mountains. The depocenter and subsidence center of the western Sichuan foreland-like basin moved toward the forebulge；the deposits on both sides of the basin are less asymmetric, which are characterized by the development of regional caps and source rocks. The sequence filling patterns of the two stages are similar to those of the thrusting stages. However, their depositional sequences, especially the highstand systems tract, in the front of the orogen are completely preserved because of weaker tectonic erosion; whereas in the forebulge the sequences are relatively incomplete mainly owing to lack of supply of sediments and thus only the sediments near the maximum flooding surface are preserved.