Sedimentary Characteristics And The Proto-basin Analysis Of The Western Sichuan Basin In Late Triassic

The Western Sichuan basin is located at the western margin of the Yangtze block. It’s separated from the central Sichuan by the Longquan Shan and the western bounder is Beichuan-Yingxiu fault in the Longmen Shan. Before the middle Triassic, the Western Sichuan basin as the passive continental margin of the Yangtze block deposited huge thick marine carbonate rocks. But from the late Triassic, accompanying with the Indosinian movement, this basin transformed from a marine basin to a continental sedimentary basin and deposited the Maantang and Xujiahe Formation. From the recently NE strike of the Longmen Shan, Western Sichuan basin and Longquan Shan, a mating relationship may exist between the mountain and basin. So, for a long time, the Western Sichuan basin has been considered as a foreland basin which was under control of the Longmen Shan thrust belt and the Xujiahe Formation(T3x) as represented was the sedimentary rocks in the foreland basin. However, northwest most outcrop of the T3 x was either cut by the Beichuan-Yingxiu fault in the southwest segment, or far away-from northeast striking Longmen Shan thrust belt. There are also some exploratory wells drilled T3 x at the hanging wall of the Beichuan-Yingxiu fault in the hinter land of the Longmen Shan thrust belt, which indicated that the western margin of the Western Sichuan basin maybe much more far than the recent location. So reconstructing the proto-basin of the T3 x is the key to understand the tectonic framework in late Triassic and the evolution of Western Sichuan basin. We analyzing the outcrops and oil wells,obtain the lithic and distribution features of the T3 x. Test the detrital zircons in order to analyze the sedimentary age of the T3 x was 222 Ma, and the main source rocks was from the Western Qinling. With the margin facies movement from N to NW of the basin of the basin, we construct a model of the basin’s tectonic evolution and considered that it was mainly controlled by the south direction thrusting of the south margin of the Western Qinling.