Age, sedimentary environment and tectonic setting of two lacustrine sediments of the Late Pleistocene in Qingshuihe Basin, Ningxia

The sedimentology and chronology of the two Late Pleistocene lacustrine strata discovered in the middle of the basin were studied by means of drilling, grain size analysis and optically released luminescence dating in order to understand the sedimentary and tectonic evolution since the Late Pleistocene in Qingshuihe Basin, Ningxia. The results show that the lower part is the Salawusu Formation and the upper part is Shuidonggou Formation, which were formed at 76~63 ka and 25~11 ka, respectively. In addition, there is an obvious erosion surface between them. According to sedimentary evidence and grain size analysis results, the Salawusu Formation can be divided into four sedimentary stages from bottom to top, which constitute a complete set of lacustrine prograding and lacustrine recession sequence, representing a warm and humid climate environment. The Shuidonggou formation is a shallow lake formed in cold and dry environment. The comparative analysis of structure and environment shows that the formation and disappearance of the two ancient lakes in the late Pleistocene in Qingshuihe Basin indicate the transition of extensional-compression-extensional tectonic activity. The two extensional events were the main factors for the formation of Salawusu Lake and Shuidonggou Lake. The strong tectonic uplift event in the discontinuous period of the development of the ancient lake is the fundamental cause of the demise of the Salawusu Lake, and the abnormal cold in the late MIS 4 and MIS 2 of the last glacial period is also one of the reasons for the decline of the ancient lake. The evolution of the two ancient lakes in Qingshuihe Basin provides important evidence for the study of the formation and evolution of the ancient lakes, paleoclimate changes and uplift of the Tibetan Plateau during the late Pleistocene.