155 / 2019-06-01 18:19:08

The palaeogeography and episodic sedimentation response to the Early-Middle Permian Dongwu Movement in the western Sichuan Basin, China

Western Sichuan Basin, Permian, Chihsia Formation, Maokou Formation, Dongwu Movement, palaeogeography, Extensional Movement, Overlap

The precise understanding of the distribution of carbonate reservoirs was significantly hampered by the intense magmatic activity and complicated depositional processes during the Permian (the latest Hercynian orogeny) in the Yangtze area. Multiple tectonic activities in the Early-Middle Permian were revealed by the boreholes of Shuangtan 9 and Yongshen 1 in the western Sichuan Basin in recent years. Based on the detailed analysis of outcrops, cores, seismic interpretation and logging data, the evolution of paleogeography during the Early-Middle Permian in western Sichuan Basin was reconstructed. Four episodes of the Dongwu movement was identified according to the characteristics of the strata filling, facies variation and the occurrence of magmatic rocks. The palaeogeography of the western Sichuan Basin generally increased towards the southeast and northeast, resulting in the gradually overlap to the Beichuan-Kuangshanliang and Hannan paleo-continents in the early stage of transgression during the Liangshan-Chihsia period. After that, the basement faults were reactivated, forming a secondary slope belt in the lower part of intraplatform along the Zhongba-Shuangyushi area during the first episode. This secondary slope belt began to control the occurrence of grain-shoal in the western Sichuan platform Marginal zone. The second episode extensional movement during the end of Chihsia period induced not only a large-scale temporal exposure of the shallow shoals in the platform Marginal zone of Western Sichuan Basin and Hannan subaqueous paleo-uplift, but also the accelerating subsidence in the Guangyuan-Wangcang area, resulting in the overlap of lower part of Maokou Formation. Afterwards, the third episode of Dongwu movement induced the volcanic eruption and persistent subsidence along the Chengdu-Ya’an and Guangyuan-Wangcang area, respectively. This tectonic activity ended the development of carbonate platform and switched to under compensated deposit, forming the rudiment of the Kaijiang-Liangping trough. The Emei taphrogeny, which resulted in the extensive eruption of balsalt in the southwestern Sichuan Basin and subsequent overall exposure in the Western Sichuan Basin, reached climax in the fourth episode after the deposition of Maokou Formation.