冰楔假型是反映古气候环境的重要标志,其蕴涵的气候地层信息对地貌演化过程具有重要的指示意义。通过对黄土高原北缘中部环江T1阶地上新发现的冰楔假型群特征的研究和光释光(OSL)测年,结果显示这些发育在冲积砂砾石中的冰楔假型的充填砂体的年代为(30.01 3.31) ka B. P.,其上部的河流相粉砂层的年代为(28.64 3.13) ka B. P.。冰楔假型的年代结果和发育地层揭示出晚更新世末次冰期的MIS 3阶段出现的一次寒冷波动事件,并根据冰楔假型的形成环境推测当时的年平均地温比现今要低17.7 ℃。该冰楔假型的发现也为末次冰期阶段性冻土南界的位置和变迁提供了证据。另外,据冰楔与阶地冲积物的关系推测,在冰楔形成的冰冻期河流阶地的堆积缓慢甚至停滞,在(28.64 3.13) ka B. P.之后的气候温暖期河流快速下切、T1阶地形成。这为河流阶地成因机制的探讨提供了依据,反映了气候波动对阶地发育的控制。
英文摘要:
The ice-wedge casts were considered to be an important indicator on paleoenvironment. The climatostratigraphic information in them could implicate the geomorphic evolution. In this paper,the ice-wedge casts were described in detail,which were found in the lower terrace of Huanjiang river in the northern margin of Loess Plateau. Through the field investigation and optically stimulated luminescence (OSL) dating,the age of sandy gravel deposited in the wedges was about (30.01 3.31) ka B. P., the fluvial silty sand layer on the upper of the ice wedges was deposited about (28.64 3.13) ka B. P.. This finding shows that a cold event occurred in the phase of Marine Oxygen Isotope Stage 3 (MIS 3) in the Last Glaciation. Also,the average ground temperature then was estimated to be 17.7 ℃ lower than the temperature nowadays. Besides,the discovery of the ice-wedge casts provided evidence for the phasic variation of the permafrost boundary during the Last Glaciation. In addition,the climatostratigraphic position of ice-wedge casts in the river terrace indicated that the fluvial aggradation occurred slowly and even ended during the freezing period. When the climate was getting warmer in (28.64 3.13) ka B. P., the river started to undercut and the terrace was forming. This provided a basis for the formation mechanism of river terrace and showed the development of terrace was controlled by the changing climate.