Sedimentary characteristics of Permian-Triassic in Yangkang area, South Qilian Basin

The field geological profile measurement and the thin section identification of rocks were used, combined with stratigraphy, sedimentary petrology and petroliferous basin analyses, to study the rock assemblage types and sedimentary facies evolution of the Permian-Triassic in the Yangkang Uplift and its surrounding areas, the South Qilian Basin. The study area is mainly composed of three tectonic units:the Yangkang Uplift, the Halahu Depression and the Xiariha Depression. The Middle Permian to Upper Triassic strata were well developed and lithologically variable. The sediments include carbonate platform, shallow shelf, littoral, delta, fluvial and lacustrine facies. There are two distinct sedimentary cycles, corresponding to two large-scale transgressive and regressive deposits. The tectonic evolution of the study area is the key factor determining its sedimentary evolution. Since the Caledonian Movement, the Yangkang Uplift existed as a paleo-uplift, which controlled the whole Permian-Triassic sedimentary paleogeographic environment. The uplift has not received sediments since the Late Silurian, and only includes Permian-Triassic strata in the Halahu Depression and the Xiariha Depression.