| 青海共和贵德两盆地间上新世黄河古河道的发现——兼论龙羊峡形成与“共和运动” |
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| 关键词:Yellow River Pliocene paleochannel formation of Longyang Gorge “Gonghe Movement” |
| 基金项目:中国地质调查局地质调查项目(编号: DD20190367);国家自然科学基金项目(编号: 41702194) |
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| The Discovery of the Pliocene Yellow River Paleochannel between Gonghe and Guide Basins, Qinghai Province: Concurrent Discussion on the Formation of the Longyang Gorge and the “Gonghe Movement” |
| Gonghe and Guide Basins are medium-scale fault basins, which are filled with extremely thick Cenozoic sedimentary sequence in northeastern Tibetan Plateau. Separating the headwater and upstream of Yellow River as well as Gonghe and Guide Basins, Longyang Gorge is a grand bedrock canyon with the length of about 40 km and the depth of 700 m. There are mainly Qugou Formation, Gonghe Formation and overlying fluvial gravel deposits filled in the Gonghe Basin, in which the Qugou Formation consists of thick Late Cenozoic fluvio-lacustrine deposits and the Gonghe Formation contains Early–Middle Pleistocene mammal fossils. The ages of multistage fluvial terraces of Yellow River in this region are since 0.15~0.1 Ma dated by 14C, TL, OSL, ESR and other dating methods. Therefore, some people attribute the formation of Yellow River and Longyang Gorge in this region to crustal movement in the late Middle Pleistocene or in the early stage of Late Pleistocene, which is named as “Gonghe Movement”. Nevertheless, the authors have found an ancient channel to the north of the Longyang Gorge, which flows from Gahai of north Gonghe County, through Duolonggou and Qunaihai Basin, to north Guide Basin. The ancient channel, or named ancient Duolong River, consists of large-scale gravel deposits with the thickness of more than 200 m with a small amount of overlying sand deposits of floodplain facies, in which gravels have good sorting, good rounding and complex lithology. The Upper Duolong River lies between the Qinghai Nanshan and the Waliguanshan, characterized by narrow valley of about two kilometer or more. The top surface of the ancient Yellow River gravel deposits (also named Qunaihai Formation) is at an altitude of about 3150 m a. s. l., with about 768 m height above the Yellow River bed. In addition, the 21th order Yellow River terrace south of Longyang Gorge is at an altitude of about 3108 m, with about 718 m height above the river bed, which is obviously a new channel formed in the Longyang Gorge after the Gonghe Paleolake was filled up and cut from the east. The new channel connects Gonghe and Guide Basins, whose distance is much shorter than the old one, which also accords with the principle of the geomorphology, i.e., “the bends of the cut-off”. What’s more, because of deposits from Qinghainanshan and Mt. Laji, the old channel in the north must be congested and then abandoned. With the integral uplift of the Tibetan Plateau, the Yellow River incised the bedrock and the new channel formed, which had nothing to do with “Gonghe Movement”. According to the quartz thermal-active ESR data of the ancient fluvial deposits of the Qunaihe Formation in the Qunaihe section, the paleochannel was developed during the middle and late Pliocene (3.79±0.34)~ (2.95±0.25) Ma. Underlying the Yellow River paleochannel was a set of light brownish red fluvial sand and gravel (date of its top sediment is about (4.49±0.38) Ma), which should also be sediments of the Early Pliocene ancient Yellow River, the early stage of the ancient Dulong River. Proluvial gravel deposits overlay the paleochannel, and the Bakatai Formation was accumulated during (2.95±0.25) Ma and (2.63±0.23) Ma in the latest Pliocene. The date ((2.47±0.22) Ma) of the highest Yellow River terrace (T21) south of Longyang Gorge is the early Pleistocene. The Yellow River Terraces (T19, T18 and T7,) have been all formed since (2.36±0.20) Ma (ages of terraces T1–T6 cannot be measured as they have been inundated by Longyangxia Reservoir). All these data show that the Longyang Gorge was eroded and incised during the whole Quaternary, which is obviously different from the dating results before. |
| ZHAO Xi-tao,JIA Li-yun,HU Dao-gong,LI Wei-dong,LIN Xu,ZHANG Yao-ling,YANG Yan.2020.The Discovery of the Pliocene Yellow River Paleochannel between Gonghe and Guide Basins, Qinghai Province: Concurrent Discussion on the Formation of the Longyang Gorge and the “Gonghe Movement”[J].Acta Geoscientica Sinica,41(4):453-468. |
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