Final Disappearing Process Of Ancient Lake During Quaternary In Nihewan Basin

Nihewan Basin is a Cenozoic rift basin and it was occupied by a thick succession of lacustrine and fluvial sediments. Due to its advantaged geological characteristics, such as continuous sedimentation, rich of fossil and relics of ancient human beings, and so on, it plays an important role in Quaternary geological research in China. Previous studies during the past nearly one hundred years, suggested that Nihewan Basin was occupied by lake for a long time during Quaternary. The palaeolake was named ancient Nihewan lake. One of the important study contents about the Nihewan Basin is the evolution of ancient Nihewan lake, about which researchers have done a lot and obtained fruitful achivements. However, the extinction process of ancient Nihewan lake hasn’t been studied deeply, because of limitations of the dating methods, which brought many difficulties on the study of stratum comparison of middle-late Pleistocene, palaeogeography and palaeoenvironment, as well as Archaeology and palaeoanthropology. Therefore, this study is important and necessary.In this study, seven typical middle-late Pleistocene deposit sections were sampled throughout the Nihewan basin from east to west, including Hougou, Xiaodukou, Hutouliang, Jing’erwa, Beiliang, Houjiayao and Liyigou. These deposit sections were firstly well dated, measured for multiple climate proxies, and compared about lithology among each other. Thus, timing framework during middle-late Pleistocene of the study area was established, and palaeoclimate and palaeoenvironment evolution were uncovered. At last, periods, processes and causes about the extinction of ancient Nihewan lake were discussed.The main advance and innovation results of this study are as follows:(1) Timing framework of the extinction process of the ancient Nihewan lake was established. Since～266 ka, the ancient Nihewan lake started to shrink and it was extinct at-30 ka. However, it should be noted that influenced by the palaeogeomorphological place, the extinction time of ancient Nihwan lake were different in different palces of the basin. At the Datianwa-Haojiatai area in southeast of Nihewan basin, the lake started to extinct～266 ka on large scale, and the residual lake continued till about 110ka. In the east part of Huashaoying area of northeast basin, the lake became extinct around 30ka. Around Dongjingji area in the eastern part of Nihewan basin, the lake became extinct around 190ka. In the north of Liyigou area of northwest Nihewan basin, the lake became extinct around 1 lOka. Around Shanghuicun-Dongmulian area in the southwest of Nihewan basin, and Houjiayao-Qiangjiaying area in the northwest of Nihewan basin, lakes became extinct around 80ka. In the Hutouliang and Jing’erwa area where near the center of basin, lakes became extinct around 53ka and 30ka respectively.(2) Extinction pattern of the ancient Nihewan lake was established. Under certain climatic and tectonic situations, the uniform big ancient Nihewan lake began to shrink and gradually broke up into several residual lake with different sizes. Those residual lakes were effected by climate changes, and hence shrank and disappeared.(3)There were three process patterns about the finally died out of ancient Nihewan lake. Pattern I:lots of loess deposited into lake, and caused the extinction of the lake, which can be seen in the vicinity of Huashaoying, as well as Liyigou-Houjiayao in Dongjingji area. Pattern II:strong evaporation led to a salt lake, and finally caused the extinction of the lake, which can be seen in the vicinity of Datianwa-Haojiatai and Jing’ erwa area. Pattern III: alluvial fan extended into lake and quickly filled up, finally caused the extinction of the lake, which can be seen around Qiangjiaying in Dongjingji area.(4) The role that climatic and tectonic activities played in the extinction of the ancient Nihewan lake was discussed. Ancient Nihewan lake in different places died out concurrently with the relatively dry period between 280-30ka. Tectonic activities changed the landscape in Nihewan Basin, and ancient Nihewan lake finally extincted under the drier climate. The extinction processes of the Nihewan lake had close relationship with the tectonic activities, as well as raletivcly dry climate conditions.