五台山北麓东段晚第四纪洪积扇形成年代与成因分析

A research of Late Quaternary alluvial and diluvial geomorphic units along the northeast section of the northern piedmont fault of Wutai Mountain

洪积扇与河流阶地的发育受控于构造活动、气候变化与基准面变化等因素。识别和理解各控制因素在山前地貌-沉积体系中所起的作用是过程地貌学研究的一个重要方面。对山西地堑系五台山北麓地区山前地貌-沉积体系的研究结果表明,该区三万年以来经历了三期快速堆积过程,在山前表现为多期洪积扇堆积,在山区表现为河流阶地堆积;较早两期洪积扇历经多次断层活动,位于下降盘一侧的主扇体部分被埋藏,上升盘一侧仅残余扇体后缘部分,沿山前形成洪积台地,其对应上游河段的第一、二级阶地;通过碳同位素测年获得了这三个堆积期的精确年代:最老一期发生在30kaBP左右,中间一期发生在6kaBP左右,最新一期开始于1.5ka并持续至今。与冰芯记录的晚第四纪高精度气候旋回对比表明这三期堆积均发生在气候由暖湿突然变干冷的时段。这一时段五台山北麓断裂活动强烈,构造条件不利于山区河段的堆积作用,因此推断气候变化是控制三期快速堆积的主导因素。从暖湿到干冷的气候变化,导致植被的退化与物理风化的加强,为河流提供了丰富的碎屑物源。

The northern piedmont fault of Wutai Mountain is a normal fault which controls the geomorphic units along the fault and the sedimentation process in Fan-Dai basin;the field investigation reveals that,there are three periods of rapid sedimentation during the late Quaternary,which typically consists of three different river terraces and fans.The first period went on quickly,which probably began and finished at the time of 30ka B.P;the second one finished in 6ka B.P,whereas the nearest sedimentation time dated back to 1.5ka B.P,and it lasted till hundreds years ago.In this paper,we compare the three rapid sedimentation periods with the climate records cast in loess in northern China and ice-cores in northern margin of Tibet Plateau,namely the Gulia and Dunde ice core.We also take into account the lake surface’s fluctuation in the past thousands years,the paleoclimate cycle recorded in plant’s spore and the sediments records in Deep Ocean.The result shows that all the three sedimentation periods share the same character that they happened at the time when the climate changed drastically,mainly from a relatively wet,warm climate to harsh and dry weather.Considering background the tectonic backgrand at that time,we concluded that the climate events in the past 30ka may control the sedimentation along the fault,or at least the northeast part of the fault.On the contrary,the slip rate of the fault demonstrates that the fault underwent increasing activities,which is not beneficial for a sedimentation period.