Discussions on Carboniferous stratigraphy of Bachu uplift, Tarim Basin

Based on the comprehensive analysis of drilling data, the Carboniferous stratigraphy of the Bachu uplift in the Tarim Basin was discussed. Four new stratigraphic sequences were found, the original definitions of the Bachu and Kalashayi Formations were restored, and the Mazatage Formation was established. It was confirmed that the Bachu Movement took place at the end of the Early Carboniferous, and the movement was severe in the west and weak in the east of the Tarim Basin. In the east of the Bachu uplift, sandstones and mudstones lay on the upper mudstones with an angle unconformity. In the west of the uplift, the Xiaohaizi Formation lay on the middle mudstones or the bioclast limestones. From the west of the uplift to the Keping area, the Kangkelin Formation lay on the Sishichang Formation, and the Bachu and Mazatage Formations were eroded. Triticites belt and the strata of Xiaoyiaoan Stage developed in the Xiaohaizi Formation in the west of the Bachu uplift and disappeared in the east. A classification standard of lithologic and logging characteristics was founded between the Xiaohaizi Formation and the overlying Nanzha Formation of the Lower Permian. The S.suberectus-S. parvus-G. bella and Triticites belts developed in the Xiaohaizi Formation while the S. isolatus, S. whitei-N. pequopensis and Pseudoschwagerina belts developed in the Nanzha Formation. The discontinuity between the two formations gradually reduced from east to west in the Bachu uplift, and disappeared (with conformity) in the western edge of the uplift.