CLASSIFICATION OF TECTONIC SEQUENCE AND BASIN EVOLUTION OF THE UPPER TRIASSIC IN THE SICHUAN BASIN

The Upper Triassic in the Sichuan Basin is a key to understand the evolutionary history of this basin. Using bore hole, outcrop, and seismostratigraphic data, the authors studied the tectonic sequence of the Upper Triassic in the Sichuan Basin. Four sequence boundaries were recognized, including 1) the tectonic unconformity between the Upper Triassic and Middle or Lower Triassic, 2) the lithostratigraphic boundary between the second member of the Xujiahe Formation and the Xiaotangzi Formation, 3) the tectonic unconformity between the third and fourth members of the Xujiahe Formation, 4) the Jurassic-Triassic tectonic unconformity.Therefore, the Upper Triassic is divided into three tectonic sequences, which can be correlated across the entire basin. Each sequence is divided into two system tracts using the largest lake flooding surface, and these represent the basin-expansion system tract and basin-reduction system tract. On the basis of tectonic sequence data,the evolution history of the Sichuan Basin is discussed. The Late Triassic in the Sichuan Basin are divided into three stages, which are the marginal foreland basin evolution stage (TS1), the western Sichuan Foreland Basin formation stage (TS2), and the western Sichuan Foreland Basin development stage (TS3). Because of influence by the An’xian Movement in TS2, the Longmenshan Thrust Belt was uplifted and became a mountain, which changed the Sichuan Basin into a terrestrial sedimentary environment. The most important factor controlling the basin evolution of the Late Triassic in the Sichuan Basin is tectonic activity.