五台山北麓南峪口段晚第四纪洪积作用与影响因素分析

LATE QUATERNARY ALLUVIAL PROCESSES IN THE NORTH PIEDMONT OF WUTAI MOUNTAIN AND THE INFLUENCING FACTORS

山前洪积作用受控于构造活动、气候变化与基准面变化等因素,区分各因素的贡献是河流地貌学的研究热点。山西地堑系五台山北麓地区,山前断裂活动强烈,晚第四纪垂直滑动速率不小于1mm/a;另外,研究区为半干旱地区,第四纪黄土地层中黄土与古土壤颜色反差大,反映该地区第四纪气候波动显著,为开展研究热点提供了理想的条件。本研究通过野外调查与探槽建立了五台山北麓晚第四纪洪积台地与全新世洪积扇的地层层序,并通过碳同位素测年对沉积地层进行精细断代,发现距今3.5万年以来山前洪积作用具有强弱交替的特征,较强的洪积作用发生在距今32ka至29ka之间、距今7.5ka至4.7ka之间和距今1ka以来,在山前形成了粗碎屑堆积,沉积速率高;其他时间段为弱洪积期,在山前形成了细粒堆积,沉积速率低,地层土壤化程度高。与中国北方高分辨率晚第四纪气候记录的对比表明,这3个较强的洪积期在时间上对应于气候从温暖期到寒冷期的快速转变阶段或温暖气候鼎盛期后的剧烈波动阶段,其共同特点是气候波动剧烈,波动幅度远大于其他时期。根据前人对五台山北麓断裂全新世古地震研究,2次地震在时间上对应强洪积期,1次地震在时间上对应弱洪积期。尽管地震可能导致崩塌、滑坡等地质灾害,增加山区河床碎屑物源,有助于山前洪积作用,但根据目前有限的古地震研究资料尚看不出大地震对山前洪积作用的显著影响。初步认为,断层活动为山前洪积作用提供了空间与河床坡降,气候变化则是控制山前洪积作用强弱交替的主导因素。进入气候波动最剧烈的时期,地表植被退化严重,物理风化强烈,在暴雨作用下易发山洪,有利于山区的侵蚀作用和山前的洪积作用。该项研究深化了对半干旱地区山前洪积作用影响因素的认识。

The alluvial processes in the piedmont may be controlled by tectonics, climate fluctuations and base level of erosion ,etc. To distinguish the contributions of each factors is a hot field in fluvial geomorphology. The north piedmont of Wutai Mountain in Shanxi graben is an ideal area for this kind of study. The piedmont fault is very active with a slip rate of no less than 1 mm/a in Late Quaternary. This semiarid region experienced significant climatic fluctuations in Quaternary time which is indicated by striking contrast in colors between loess and paleosoils in loess strata. The stratigraphic sequence of alluvial fans in Late Pleistocene and Holocene is established through field surveying and trenching. Ages of the strata have been well constrained by the method of radiocarbon dating. It is discovered that the alluvial processes in front of the mountain were strong and weak alternately. The strong periods were from 32kaB. P. to 29kaB. P., from 7.5kaB. P. to 4. 7kaB. P., and since 1.0kaB. P., when coarse-grained sediments deposited and the sediment rate is relatively high. The other times are weak periods when fine-grained sediments deposited and the sediment rate is much lower. The three strong periods correspond either with the transform stages from warm to cool climates, or with the intense fluctuation stage after the maximum of warm climate. These three periods share a common feature of intense fluctuations of weather whose amplitudes are larger than other periods. According to former studies on Holocene paleoearthquakes about Wutai Mountain piedmont fault, two events happened in strong periods of alluvial processes, one event in weak period. Strong earthquakes may lead to landslides and rock-falls in mountain area, which increment the clastic provenance and contribute to alluvial processes ,but the limited information of paleoearthquakes cannot support a salient effect of big earthquakes on alluvial processes in front of mountain. As our preliminary view, fault slips provided the space for deposition and slope of river bed, and climatic changes controlled the alternations of strong or weak alluvial processes. In the strongest fluctuations stage of climates, the vegetation degraded, physical weathering strengthened and storms easily led to flood, which contributed to erosions in mountain area and diluvium in front of mountain. This research deepen our understanding on the influencing factors of alluvial processes in semiarid area.