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Tectonic sequence framework and sedimentary basin evolution of upper triassic in the sichuan basin, China

Lin Liangbiao

The Late Triassic is an important geologic age for the evolution of the Sichuan Basin. Based on data from field outcrops, drilling, and seismic acquisition, a detailed study on the sequence boundaries, division, and characteristics of the Upper Triassic in the Sichuan Basin was conducted using tectonic sequence stratigraphy to establish a sequence stratigraphic framework. The research indicates that four sequence boundaries were distinguishable in the Upper Triassic: 1) regionally structural unconformity between the Upper Triassic and Middle and Lower Triassic; 2) the boundary between the second member of the Xujiahe Formation and the Xiaotangzi Formation; 3) the secondary structural unconformity between the third and fourth members of the Xujiahe Formation; and 4) regionally structural unconformity between the Triassic and Jurassic. Based on the occurrence of sequence boundaries, three tectonic sequences could be divided in the study area, each of which was bound by the maximum flooding surface and subdivided into basin extension (BE) and basin wither (BW) system tracts. The evolution of the Western Sichuan Foreland Basin was the main evolution in the Late Triassic, in which TS1 represents the evolution stage of the marginal foreland basin, TS2 represents the formation of the Western Sichuan Foreland Basin with the advent of the Longmen Mountain thrusting and napping body, and TS3 represents the development of the Western Sichuan Foreland Basin. In TS3, the Longmen Mountain was thrust and folded to form the mountain, which was affected by the An County Movement, such that the entire Sichuan Basin transferred into a continental depositional environment. This provided a large amount of carbonate fragments for western Sichuan Basin and became the principal provenance in this area. Tectonic movement is a major controlling factor of the Sichuan Basin evolution in the Late Triassic.