| 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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| 引用本文：赵希涛,贾丽云,胡道功,李维东,林旭,张耀玲,杨艳.2020.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”[J].地球学报,41(4):453-468. |
| DOI：10.3975/cagsb.2020.070601 |
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| 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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| 中文摘要:共和盆地与贵德盆地位于黄河源头和上游的接合部位, 是青藏高原东北缘两个由巨厚的晚新生代沉积所充填的中型断陷盆地。而作为划分黄河源头与上游分界线并分隔共和与贵德两盆地的龙羊峡, 则为一长近40 km、最深逾700 m的基岩大峡谷。由于共和盆地大部由厚层的河湖相新近纪曲沟组与含早、中更新世哺乳动物化石的共和组及覆于其上的河流相砾石层所组成, 且后期形成的多级黄河阶地被14C、热释光(TL)、光释光(OSL)、电子自旋共振(ESR)等方法测得其年龄数据多为0.15～0.1 Ma以来, 因而人们多将黄河贯穿共和盆地、形成龙羊峡谷和黄河源头段, 归因于10到15万年前的地壳运动所引起的溯源侵蚀, 并命名为“共和运动”。 然而, 作者在龙羊峡段黄河之北, 发现了一条由厚逾200 m、主要由分选与磨圆良好、砾石成分复杂的大型河床相砾石层夹少量砂层与顶部河漫滩相砂层所组成的古河道, 自共和古湖东北端的尕海以东, 经多隆沟, 并横穿曲乃亥盆地, 流向贵德盆地。作者将这条古河道命名为古多隆河。其上段夹于青海南山与瓦里关山之间, 河谷狭窄, 只有2 km或稍宽。该古黄河砾石层与砂层或称曲乃亥组, 其顶面在曲乃亥地区高达海拔 3150 m, 已高出附近龙羊峡段黄河河面约768 m。而位于龙羊峡南岸、最高可达海拔3108 m的黄河阶地T21顶面, 只高出附近黄河河面不到718 m。显然, 后者是由于共和与贵德两盆地随地壳下沉而湖底与湖面上升至接近和达到其最高堆积位置时——共和古湖所能达到的最高湖面, 约为现今海拔(3160±10) m, 共和古湖的湖水在现今龙羊峡所在的最高基岩谷肩处向东溢出, 从而在瓦里关山南侧的低洼处所产生的一条新河道——龙羊峡段黄河所堆积。由于龙羊峡直接沟通共和与贵德两盆地, 其距离要比迂回瓦里关山之北的古多隆河近得多, 故按照地貌学的河流“截弯取直”原理及来自青海南山和拉脊山南麓洪积物的堆积, 其北的古多隆河必然被堰塞、淹没并最后废弃。随着青藏高原及其东北部地壳运动性质由总体上升但存在若干断陷盆地转为整体隆升仅极少数盆地继续断陷, 黄河水系因之下切, 龙羊峡的基岩峡谷及共和贵德两堆积盆地持续至今的切割由之形成, 完全不必用“共和运动”及其所引起的溯源侵蚀来加以解释。 根据曲乃亥剖面曲乃亥组古河道砾石层与砂层及各相关地层砂质样品的石英热活化ESR法测年结果, 该古河道发育于(3.79±0.34) Ma与(2.95±0.25) Ma之间的上新世中晚期。曲乃亥组古黄河砾石层下伏的河流相浅砖红色砂砾石互层, 其顶部样品的年龄为(4.49±0.38) Ma, 也应是早、中上新世古黄河——早期古多隆河所堆积。覆于古河道砾石层之上的巴卡台组洪积砾石层则堆积于(2.95±0.25) Ma与(2.63±0.23) Ma之间的上新世末期。位于龙羊峡南岸的最高黄河阶地T21, 其年龄为(2.47±0.22) Ma的早更新世初。二塔拉(T19)及铁盖乡剖面T18至T7(T6以下已被龙羊峡水库所淹没)的年龄为(2.36±0.20) Ma以来的第四纪, 表明龙羊峡的切割发生于整个第四纪时期, 与前人用其它方法所获得的测年结果明显不同。这些地层与地质事件因测年方法的不同所导致的年龄差异及与哺乳动物化石鉴定结果的矛盾, 值得深入探讨和今后进一步研究。 |
| 中文关键词:黄河 上新世 古河道 龙羊峡形成 “共和运动” |
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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” |
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| Abstract: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. |
| keywords:Yellow River Pliocene paleochannel formation of Longyang Gorge “Gonghe Movement” |
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