Timing of Disappearance of the ancient Fen River in the Yuncheng Basin in the southern margin of the North China Craton: Implications from sedimentary and paleoclimatic records

The withdrawal of the Ancient Fen River from the Yuncheng Basin and its flow into the Yellow River from the present-day Linfen Basin was an important geological event in the Quaternary of the Yuncheng Basin on the southern margin of the North China Craton. There are still many controversies about whether the time frame for the withdrawal of the Ancient Fen River from the Yuncheng Basin occurred in the Middle Pleistocene or the Late Pleistocene， and whether the main geological factors were the result of tectonics or climatic processes. In this paper， we use optical luminescence (OSL) dating， detrital zircon chronology， heavy mineral assemblage analysis， pollen assemblage analysis and other experimental methods to answer these controversial questions. The Late Pleistocene stratigraphic sequence in the Yuncheng Basin consists of eolian facies loess in the upper part and fluvial facies sand layers in the lower part. An unconformity， characterized by distinct lithological differences above and below， clearly indicates the timing of the ancient Fen River’s disappearance. The detrital zircon U-Pb age sequence and heavy mineral assemblages of the Late Pleistocene fluvial facies sands from the ancient Fen River closely resemble those of the present-day Fen River. The unconformity between the overlying eolian loess and the underlying fluvial sands formed between 75.01 ± 7.98 ka B.P. and 67.57 ± 7.88 ka B.P.， about 7.0 ka B.P.， indicating that the ancient Fen River disappeared in the Yuncheng Basin in middle Late Pleistocene. Regional tectonic uplift led to channel realignment of the ancient Fen River， while cold and arid paleoclimatic conditions further intensified the drying of its paleochannels. This study provides new evidence to constrain the disappearance time of the ancient Fen River in the Yuncheng Basin and identifies the main geological controlling factors.