Late Pleistocene stratigraphic sequence and geologic significance of the Kaolao Tableland in the Yuncheng Basin

The ancient Fen River diversion was a crucial earth's surface transformation in the Yuncheng Basin during the Cenozoic. The time frame for the diversion of the ancient Fen River is still characterized by two views: the middle Pleistocene and the late Pleistocene, which has yet to be finalized. This study investigated the late Pleistocene sedimentary sequence of the Kaolao Tableland in the Yuncheng Basin, and the critical time frame of the sedimentary sequence transition was determined based on optically stimulated luminescence (OSL) dating results. The causes of the late Pleistocene sedimentary sequence of the Kaolao Tableland and the geological factors that controlled the sequence were analyzed using detrital zircon U–Pb isotope dating. It is concluded that the late Pleistocene sedimentary sequence of the Kaolao Tableland in the Yuncheng Basin is characterized by a two-layer structure, with fluvial sands in the lower part and eolian loess in the upper part. Based on the OSL dating results, the formation time of the boundary between these two parts is between ~76–63 ka B.P. Comparative analysis of detrital zircon age sequences indicates that the early Pleistocene fluvial sands in the Kaolao Tableland and sediments in the ancient Fen River have similar age sequence characteristics. Therefore, it can be deduced that the regional tectonic uplift of the northeastern Emei Terrace in the middle of the late Pleistocene resulted in the diversion and exit of the ancient Fen River from the Yuncheng Basin and the sedimentary facies began to change from fluvial to eolian. The tectonic uplift in the middle of the late Pleistocene extensively developed around the Ordos Basin, and that indicates a significant tectonic uplift of the Tibet Plateau during this time, whose remote effect might be the major cause for the exit of the ancient Fen River from the Yuncheng Basin. This research provides new sedimentary evidence for the time frame of the ancient Fen River diversion in the Yuncheng Basin