Research on Depositional System of Middle Jurassic Shaximiao Formation in Western Sichuan Foreland Basin

The Middle Jurassic Shaximiao formation distributed in the nappe front zone of Longmen Mountain and Micang-Daba Mountain is from several hundred to thousand meters thick. It consist of the conglomerate, felspar sandstone, felspar-quartz sandstone and siltstone.It includes alluvial fan, fluvial deposit, delta and lacustrine deposit. The alluvial fan is mainly developed in Lushan Lianghekou and Dujiangyan, the northwest of the study areas; The fluvial and delta deposits are mainly developed in Qionglai, Anxian, Mianyang and Zitong, the western-northern of the study areas; the lacustrine deposit is mainly developed in Renshou and Rongxian, the central and southern of the study areas. Sedimentary sources are mainly from the northwest of the basin; the sedimentary facies are alluvial fan facies-fluvial facies-delta facies-lacustrine facies from the northwest to the southeast. The vertical successions of the facies reflect the lake sediments development is a process with shrinkage-expansion-re-shrink-reexpansion.