| 论晚期共和古湖时代、演化过程及其与地壳运动和黄河发育的关系 |
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| 关键词:Gonghe Basin late Gonghe Paleolake ESR dating Pliocene evolution of the paleolake the Yellow River development Qiabuqia (Gonghe) Movement |
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| A Discussion on the Age and Evolution Process of the Late Gonghe Paleolake and Its Relations with the Crustal Movement and the Development of the Yellow River |
| Gonghe Basin, as one of the numerous basins in northeastern Tibetan Plateau, is a kind of medium-scale tectonic basin, consisting of three parts: endorheic basin—the Chaka Salt Lake in the northwest, nontypical endorheic basin represented by small salt water lake such as the Dalian Lake in the middle, and external basin incised by the Yellow River and its tributaries in the southeast. There are two series of contraposed stratigraphic units: Gonghe Formation in the upper part and Qugou Formation in the lower part, which are filled in the Gonghe Basin. The thicknesses of these two sets of strata are mainly between 200 and 300 m, with the thickest reaching 500~600 m. The facies types are mainly coarse clastic deposits from the foothills around the basin and lacustrine fine-grained deposits in the center. The latter strata have redder color, stronger diagenesis, and wider distribution ranges, and experienced more tectonic activities. The paleolake, reconstructed by these two strata, is named Gonghe Paleolake. The early Gonghe Paleolake was reconstructed by the Qugou Formation, and the late one, the Gonghe Formation. The late Gonghe Paleolake, which is the focus of this paper, experienced the vegetation change process of stappe, forest-steppe, stappe, forest-steppe, stappe, forest-steppe and present steppe, indicating that the change from warm temperate zone to temperate zone, experiencing several times of climatic changes of cold-warm and dry-humid, and the general trend being aridification. During the maximum stage, the paleolake reached its maximum height, being (3 160±10) m a.s.l.. The Gonghe Paleolake was a deep freshwater lake, with the area of nearly 7 000 km2 and the depth of more than 300 m, where the Yellow River flowed in from the Gamayangqu Village, Xinghai Countay, and flowed out from east of the Gahai, through the ancient Duolong River, and hence the Yellow River was a river flowing through Duolonggou and Qunaihai Village to Guide Paleolake. During the period of the strongest development of paleolake, the Yellow River flowed into the Gonghe Basin and Gonghe Paleolake nearby the Gamayangqu, around which a large, northward extending, and irregularly shaped-fan delta system was formed The alluvial fan-delta system consists of two sets of gravel beds: the upper and the lower, with each bed having thickness of more than 200 m. When the lake level rose and exceeded the bedrock watershed of Gonghe and Guide basins in the south part of Waligang Mountain, a new channel was formed in the Longyang Gorge. With the basins changing from sinking-depositing to rising-eroding, the ancient Duolong River flowing around Waliguan Mount was abandoned. Meanwhile, the Gonghe Paleolake began to die gradually. The dying process consisted of three stages. During the first stage, the 21th and 20th terraces of the Yellow River were formed and the surface of the paleolake was less than 3 108 m a.s.l., the Chaka Salt Lake in the northwest began to separate from the main Gonghe Paleolake. During the second stage, the 16th terrace was formed, the surface of the Gonghe Paleolake was between 2 850 m and 2 865 m a.s.l., and most of the paleolake in the middle area was drained because of leakage. When the height of the riverbed of the Yellow River was 2 650~2 700 m a.s.l. or earlier because of the eroding action by the Yellow River, the paleolake area in the southeast had been probably already emptied. There were only some small salt water lakes such as Dalianhai (dried up), Genggahai and Yingdehai, and the Chaka Salt Lake developed to the stage of NaCl extraction. According to the quartz thermal-active ESR data, the ages of the above-mentioned strata and geological events are as fallows: the Gonghe Formation of lacustrine facies began to form before (4.31±0.40) Ma and continued to about (2.58±0.20) Ma/(2.54±0.20) Ma; The upper gravels of the fan-delta of the Yellow River near the Gamayangqu were formed between (3.15±0.30) Ma and (3.07±0.30) Ma, and the gravels in the delta front were formed at about (2.91±0.25) Ma, while the age of the synchronous proluvial gravels from the mountain front in the north is (2.97±0.27) Ma, which indicates that the late Gonghe Paleolake developed to the highest level in the late Pliocene between (3.07±0.30) Ma and (2.91±0.15) Ma. The ages of the 21th, 19th, 18th, 17th, 16th terraces of the Yellow River are (2.47±0.30) Ma, (2.36±0.20) Ma/(2.32±0.30) Ma, (2.31±0.20) Ma, (2.23±0.20) Ma, (2.12±0.15) Ma, respectively. When the late Gonghe Paleolake reached its maximum extension between (3.07±0.30) Ma and (2.91±0.15) Ma, the nature of regional crustal movement changed. The move state of the basins changed from the differential uplift and subsidence to the whole uplift, and this included Gonghe Basin, adjacent basins, and even most of the basins in the first and second ladder of topography of China, except Yinchuan–Hetao Basins and Fenwei Basin. This movement is named “Qiabuqia (Gonghe) Movement” by XU Shu-ying and some others. However, this name has been changed into “Gonghe Movement” and used as the continuation of Qingzang (Qinghai–Tibet) movement by LI Ji-jun and some others, indicating that the Yellow River eroded to the Gonghe Basin and the source area through incision of the Longyang Gorge around 100 or 150 thousand years ago. Due to the fact that the ancient Duolong River connected Gonghe and Guide Paleolakes and the formation of the Longyang Gorge was caused by “the bends of the cut-off” instead of headward erosion, the authors suggest retaining the name and basic meaning of the “Qiabuqia (Gonghe) movement”. As to the debate about the time of the movement, it can be solved by further research. |
| ZHAO Xi-tao,YANG Yan,JIA Li-yun,HU Dao-gong,LI Wei-dong,ZHANG Yao-ling,LIN Xu.2021.A Discussion on the Age and Evolution Process of the Late Gonghe Paleolake and Its Relations with the Crustal Movement and the Development of the Yellow River[J].Acta Geoscientica Sinica,42(4):451-471. |
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