REDEFINITION OF THE SEDIMENTARY TIME OF THE SALAWUSU FORMATION IN THE HONGSIBU BASIN, NINGXIA AND ITS SIGNIFICANCE

Based on the detailed field profile observation and stratigraphic correlation works, we re-divide the Late Pleistocene sedimentary cycle in the Hongsibu basin, Ningxia. What's more, we also redefine the sedimentary frame of the Pleistocene in the Hongsibu basin combining with the accurate time limit from the OSL or ¹⁴C dating results. The lacustrine deposit of the former Salawusu formation can be subdivided into the lower part which corresponds with the traditional Salawusu formation, and the upper part which corresponds with the Shuidonggou formation according to the regional stratigraphic comparison. Overall, the sedimentary process of these two parts covered two periods in which large paleo-lakes developed at the north-eastern margin of the Tibet Plateau during the Late Pleistocene. Besides that, these two periods in which large paleo-lakes developed corresponded to the marine oxygen isotope stage 3 (MIS 3) and stage 5 (MIS 5). During these two stages, the paleoclimate around the world was warm and wet, which means the evolution of the paleoclimate at the north-eastern margin of the Tibet Plateau took concerted action with the whole world. Moreover, there also exits a regional unconformity between these two sets of lacustrine deposits which means there developed a significant tectonic uplift during the sedimentation of these two large paleo-lakes. The tectonic uplift might have close relationship with the uplift of the Tibet Plateau during the Late Pleistocene. Therefore, the sedimentary time re-definition of the Salawusu formation is of great significance to the study of the formation and evolution of the Pleistocene large paleo-lakes, transition of the paleo-environment, and the tectonic uplift of the Tibet Plateau during the Late Pleistocene.