【摘要】 在黄河源区两岸的第二级阶地砂砾层和基座中发现了两种不同形态的冰楔假型群。一种是发育在阶地砂砾石层的冰楔假型,其特点宽而浅,底部边界呈圆滑锅状,深约0.5~0.9m,宽0.8~1.4m;另一种是发育在第二级阶地基座的基岩中,以窄深倒三角状为特点,其底部尖锐,深约0.7~2.0m,宽为0.3~1.0m。前者形成于全新世中期(5.69±0.43kaB.P.,5.43±0.41kaB.P.),后者形成于末次冰期的冰消期(13.49±1.43kaB.P.)。另外,还在洪积的砂砾石层中发现了规模较大的冻融褶皱(宽3~4m),其时代晚于39.83±3.84kaB.P.,也是末次冰期的产物。根据冰楔假型的对比研究,在全新世的中期(约5.5kaB.P.前后)和冰消期,黄河源区的降温幅度达6~7℃。尤其值得注意的是全新世中期的冰楔假型形成,表明了大暖期气温的不稳定性。更多还原

【Abstract】 The source area of Yellow River is located at Northeastern Tibetan Plateau,and is a high elevation region with the annual mean temperature of-3.89℃ and annual precipitation of 322mm.In this area,permafrost is widely but isolatedly developed.The permafrost layers are about 6.5～65.5m in thickness,while active layers are about 0.8～1.5m.Our field work has confirmed that frost-heaved mound,stone polygon,debris slope,and freeze-thaw terrace are common in this area,but active and in-active ice wedges are absent.Ice-wedge casts and freeze-thaw folds,however,have been discovered in this area.Two types of ice-wedge casts were recognized.One type was found in sandy gravel beds of the second terrace of Yellow River,which is characterized by higher ratio of breadth to depth,and is 0.8～1.4m in breadth and 0.5～0.9m in depth.The lower border of the ice-wedge cast is round curve on section.The other type was discovered in bedrocks of the second terrace and has lower ratio of breadth to depth,and is 0.3～1.0m in breadth and 0.7～2.0m in depth.Its lower border is sharp.Based on the TL dating,the former was formed at the Middle Holocene(5.69±0.43kaB.P.and 5.43±0.41kaB.P.),and the latter was formed during the deglaciation period,about 13.49±1.43kaB.P.,which is correlated to the oldest Dryas in age. In additiona,the freeze-thaw folds were discovered from the late Late Pleistocene proluvium,and is 39.83±3.84kaB.P.in age.These discoveries infer that the temperature during the formation of the ice-wedge casts and the thawing-freezing folds might have dropped remarkably to 6～7℃ and that there might exist a sharp temperature-drop event in the Megathermal.更多还原