Nihewan Basin is a Cenozoic rift basin and it was occupied by a thick succession of lacustrine and fluvial sediments which were formed from the late Pliocene the Late Pleistocene. Due to its advantaged geological characteristics, such as continuous sedimentation, rich of fossil and relics of ancient human beings, etc. Nihewan Basin plays an important role in Quaternary geological research in China. Previous studies, over nearly one hundred years, suggested that during the Early and Middle Quarternary, Nihewan Basin was a great lake, which extent was from north to Yuxian Basin and west the region of Datong, Shuoxian County. 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, few of anylysis samples and lack of resolution, 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. Detailed strata description and samples collection were done on each typical section. These deposit sections were firstly well dated, measured for multiple climate proxies, and compared about lithology among each other. Dating method including OSL, Paleomagnetism and U-series, climate proxies including pollen analysis, grain-size, magnetic susceptibility, Chromaticity, chemical elements and so on. 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.

In late Middle Pleistocene, after ～266 ka, the Nihewan Lake shank widely. At the Datianwa–Haojiatai area in southeast of Nihewan Basin, the lake extinct and left residual lake in some places. Ages of stromatolites developed on the top of lacustrine sediments in Xiaodukou and Dongmulian were older than 200 ka and 220 ka respectively. The age of Aragonite deposition on the top of lacustrine sediments in Beiliang area is about 270 ka. Stromatolite mainly lived in the area of semi-salty lake and shore lake, chemical depositions like Aragonite formed in semi-salty lake with a shallow water. There appearance showed that those places where they lived/deposited were close to the lake shoreline.

In the late period of widely shank of Nihewan Lake, lake still developed in same places except Datianwa – Haojiatai area. And then, in Beiliang area around Dongjingji area in the eastern part of Nihewan Basin, the lake became extinct around 190 ka. In early late pleistocene, ancient Nihewan Lake had an expension in small-scale, which can be seen obviously in Hutouliang area. The strata of Hutouliang section showed a lake transgression sedimentary sequence. After this lake transgression, ancient Nihewan Lake began to extinct gradually. Around 110 ka, lake in the north of Liyigou area near and the residual lake at Datianwa–Haojiatai area extinct, and after that lake shank from Liuleng mountain front. Around Shanghuicun–Dongmulian area in the southwest of Nihewan Basin, and Houjiayao- Qiangjiaying area in the northwest of Nihewan Basin, lakes became extinct around 80 ka. Extinction of lake in Hutouliang area happened around 53 ka. In the Jing’erwa area where near the center of basin, as well as the east part of Huashaoying area, lakes became extinct around 30 ka.

(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, such as residual lake near Guanting at Datianwa–Haojiatai area. 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. In Huashaoying area, lacustrine strata were covered by Malan loess, and they were gradual transition deposition relationship without obviously boundary line. Around Liyigou–Houjiayao in Dongjingji area, loess content in the sediment top of the lacustrine sediments increased obviously. Those phenomenons showed that the final extinction of lake in those area had relation with the massive loess deposition, and under the gradually landfill of lake by loess, extinction happened. 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. In Datianwa–Haojiatai area, Beiliang area, as well as Jingerwa area, salt deposit such as Aragonite and Ankerite were widely distributed in the top of lacustrine and fluvial sediments, which were formed under a strong evaporation and salty lake environment. Those showed that the final extinction of lake in those area were accompanied with the evaporation and salty of lake water and deposition of chemical sediments. 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. Around Qiangjiaying in Dongjingji area, alluvial fan in the mountain front brought massive substances in lake, and finally caused the final extinction of lake.

(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 280～30 ka. 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 raletively dry climate conditions, but the later was the main factor.

Climate evolution of Nihewan Basin since 280 ka had a vary characteristic like warm and wet- cool and dry-dry warn-wet warm-warm and dry, which has good comparation with that of oxygen isotope phases of deep sea as well as loess-paleosoil deposition sequence. The climate change in Nihewan Basin is consistent with the global change, but had obvious regional features. Being a Cenozoic rift basin, neotectonics are active, which controlled the development of lakes in the basin. Tectonic activity in the eastern part of Nihewan Basin controlled the discharge of the lake in the basin. Ancient Nihewan Lake finally extinced after a serial process from wide lake in middle Pleistocene to widely shank in later Middle Pleistocene.

Around 380 ka and 330 ka, under drought climate, lake water concentrated, salt deposit like Ankerite separated out, which caused the shallow lake. Strong activities of Youfang Fault during 200～300 ka caused the uplift in Datianwa–Haojiatai area, and forbad the water supply of this area from northeast, which finally caused the widely disappearance of lake. Residential lake of this area such as that in Taiergou gradually shrank under drought climate around 110 ka, and finally extinct. The same thing happed on the lake near Liyigou.

In late Middle Pleistocene, uplift of Shabanliang caused the activity of fault along the Liuleng mountain, and tilt uplift by the fault activity maight be one of the reasons why the lake in Beiliang area disappeared. Around 190 ka, under the cold and dry climate, loess deposition increased, and lake in Beiliang area finally extincted. In early Late Pleistocene or later, pediment platform uplifted by the activity of fault along the Liuleng mountain, and under the drought climate, the lake shank and with drawed to the middle of the basin. In later period of late Pleistocene, climate changed cold and dry, lakes in basin shrank and extinct one after anothere. During 70～80 ka, lakes in Shanghuicun-Dongmulian area and Houjiayao–Qiangjiaying area extincted. Lake near Hutouliang disappeared around 53 ka, and lakes near Jingerwa and in Huashaoying area disappeared around 30 ka. At the end of Late Pleistocene, tectonic activity caused the lacustrine platform on the each side of sanggan river uplifted, which also prompted the undercut of sanggan river. Sanggan River was formed during 30～27 ka, and undercut two times during 27～13.7 ka and after 2.1 ka respectively, where after river valley and terrace like present were formed basiclly.

(5) For the first time found the late Middle Pleistocene chemical sediment in Nihewan Basin. In Quaternary lacustrine and fluvial sediments, several chemical sedimentary layers in the basin had been found previously and the chronologic results suggested that they belonged to the Late Pleistocene. A new chemical sedimentary layer was found in Beiliang area southwest of Yangyuan County by field investigation, which widely distributed in the top of lacustrine and fluvial sediments in western basin. In order to discuss the sedimentary processes and environmental history of the palaeolake Nihewan, this section was sampled in detail and analyzed by OSL dating, U-series dating, SEM, XRD, XRF, and diatom, ostracoda. Its U-series dating shows that the age of grayish white chemical sedimentary layer is about 270 ka, much older than that of the chemical sediments found before. Mineralogical data of this grayish white chemical layer reveal that it was mainly composed of Aragonite(71%) and Ankerite(12%). Ostracoda identification shows very low diversity (only two ostracod species) and very high abundance. These evidences suggest that the chemical sedimentary layer was formed under a relatively warmer and dryer climate. The palaeolake had shrank rapidly and become a salt lake during that stage, which led a large number of minerals like Aragonite to be precipitated. The disappearance of the palaeolake Nihewan was during the late middle Pleistocene at Beiliang area. This result would be of an important significance for reconstruction of paleoenvirenment in northern China during the late Middle Pleistocene.

Acknowledgements: This study was supported by National Natural Science Foundation of China (No. 40901001) and China Geological Survey (No. 1212011120147).

Final Disappearing Process of Ancient Lake during Quaternary in Nihewan Basin

JI Yun-ping, WANG Gui-ling*
Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Science, Shijiazhuang, Hebei 050061

Key words: Nihewan Basin; Middle–Late Pleistocene; Palaeoclimate; palaeoenvironment; ancient lake revolution; extinction pattern

本文由国家自然科学基金项目(编号∶ 40901001)和中国地质调查局地质调查项目(编号∶ 1212011120147)联合资助。

收稿日期∶ 2016-07-29; 改回日期∶ 2016-08-02。责任编辑∶ 闫立娟。

第一作者简介∶ 吉云平, 女, 1981年生。博士研究生, 助理研究员。主要从事第四纪地质与环境变化研究。E-mail∶ jiyp825@163.com。

*通讯作者∶ 王贵玲, 男, 1964年生。博士生导师, 研究员。主要从事地质工程研究。E-mail∶ guilingw@163.com。