若尔盖盆地黄河第一湾河岸沉积地层序列及其成因研究

Formation of the sedimentary stratigraphy at the first bend of the Yellow River in the Zoige Basin

经过在若尔盖盆地进行全面野外考察,在黄河第一弯岸边发现了沉积序列清晰的、具有代表性的完整沉积地层剖面。通过沉积学和地层学特征和沉积相的观测分析,结合实验室粒度分析、数据处理和测年断代,其结果表明:该剖面底部蓝灰色古湖相沉积层反映出盆地内部在OSL年龄35 ka之前为较深的湖泊。其上覆盖的风沙与泥炭互层,反映出在该区域湖水消失之后,在30 ka之前古湖底出现泥炭沼泽,同时风沙作用盛行。而浊黄橙色古洪水沉积层(OFD1)则反映出在30~27 ka之间,曾经有来自于黄河源区流域的规模巨大的洪水进入若尔盖盆地,在古湖底泛滥并且将其携带的分选良好的细沙与粉沙质泥沙堆积下来。该剖面中部厚度达5~7 m的河漫滩-风沙层,沿着河岸追索,则可见其表现为高低起伏的古沙丘。这表明在末次冰期盛期和冰消期,气候干旱寒冷,黄河已经下切形成其河槽,其河漫滩沉积物被强劲的风力吹扬,形成连绵起伏的沙丘。该层之上所覆盖的浊黄橙色古洪水沉积层(OFD2),则反映出在全新世初期9.86~8.28 ka之间,来自于黄河源区流域的大洪水再次进入盆地,在黄河第一湾两岸的古湖底沉积了分选良好的细沙质与粉沙质的泥沙。到了全新世中期后,若尔盖盆地风沙作用依然盛行,黄河河槽深切,第一湾两岸接受沙尘暴沉积。在全新世中期的相对温暖湿润气候条件之下,沙尘暴沉积物被改造为亚高山草甸黑土类土壤,成熟度极高。到了全新世晚期以来,风沙作用与沙尘暴沉积过程持续,沙尘暴沉积物也被改造为亚高山草甸黑土层。本文研究对于深入理解黄河源区末次冰期以来环境变化与地表过程演变,以及晚更新世以来的气候水文变化规律具有重要的科学意义。

After a detailed field investigation in the Zoige Basin,a clear,representative and complete sedimentary stratigraphic profile was discovered on the bank of the first bend of the Yellow River.Through the observation and analysis of sedimentological and stratigraphic characteristics and sedimentary facies,combined with laboratory grain size analysis,data processing and dating.The results show that the blue-gray paleolacustrine sedimentary layer in the lower part of the profile reflects the deep closed lake in the basin before the OSL age of 35 ka.The alternate layer of sand and peat covered on it reflects that after the disappearance of the paleolake at 30 ka,peat swamps appeared at the bottom of the paleolake,and the effect of sandstorms prevailed at the same time.The turbid yellow-orange paleoflood sedimentary layer(OFD1)reflects that between 30~27 ka,largescale floods from the source area of the Yellow River entered the Zoige Basin,flooded at the bottom of the paleolake and carried good sorting.Fine sand and silt sand are accumulated.The floodplain-aeolian sand layer with a thickness of 5~7 m in the middle of the section can be seen as undulating paleo-sand dunes when traced along the river bank.This strongly indicates that during the last glacial peak and deglacial period,the climate was arid and cold,the Yellow River had been cut down to form its channel,and its floodplain sediments were blown up by the strong wind to form rolling sand dunes.The turbid yellow-orange paleo-flood sedimentary layer(OFD2)covered on this layer reflects that in the 9.86~8.28 ka of early Holocene,the flood from the source area of the Yellow River entered the basin again and deposited on the paleo lake bottom on both sides of the first bay of the Yellow River.Good sorting of fine sand and silty sand.After the mid Holocene,wind-sand action was still prevailing in the Zoige Basin,the Yellow River trough was deep cut,and both banks of the First Bay received sandstorm deposits.Under relatively warm and humid climatic conditions in the mid-Holocene,sand-dust storm sediments were transformed into subalpine meadow black soil at any time,with extremely high maturity.Since the late Holocene,the process of wind-sand and sand-dust storm deposition has continued,and the sand-dust storm sediments have also been transformed into subalpine meadow black soil layers.The research in this paper has important scientific significance for the in-depth understanding of environmental changes and the evolution of surface processes in the source region of the Yellow River and the climate and hydrological changes since the Late Pleistocene.