Palaeogene–Neogene sedimentary and tectonic evolution of the Yinchuan Basin, western North China Craton

The Yinchuan Basin is one of the Cenozoic rift basins around the Ordos Block in the North China Craton and is also the frontier zone of the north-eastern expansion of the Tibetan Plateau. Due to its special geotectonic position, the views on formation mechanisms and evolution process of the Yinchuan Basin are still controversial. Herein, we present a detailed study of the Palaeogene–Neogene sediment sequence in and around the basin coupled with activities of faults. Our results demonstrate that the Yinchuan Basin mainly underwent two evolutional stages during the Palaeogene–Neogene. At the first stage, from the Eocene to the late middle Miocene, the Yinchuan Basin was dominated by a NW–SE extension with deposits of the Sikouzi, Qingshuiying, and Zhangenbu Formations. The incipient Yinchuan Basin was developed on pre-existing basement faults and deposits of the Sikouzi Formation (E₂s). The Qingshuiying Formation (E₃q) consists mainly of lacustrine mudstones with unified eastward palaeocurrents in the basin and its periphery, indicating that a unitived lake existed in the studied area during the Oligocene. The alluvial fan conglomerates in the lower part of the Zhangenbu Formation (N₁z) developed on both sides of the Helan Mountains as a response to the initial uplift of the mountains. At the second stage, from the late Miocene to the Pliocene, the sedimentation of the Yinchuan Basin was dominantly controlled by a NE–SW compression that led to a brief uplift of the study area. After a short sedimentary discontinuity, the Ganhegou Formation (N_1-2g) was deposited in the basin. The evolutional features of the two stages of the basin are consistent with that of the regional tectonics. It is suggested that the tectonic evolution of the Yinchuan Basin in the early and late stages was mainly controlled by the Pacific plate’s subduction and the northeastward expansion of the Tibetan Plateau, respectively.