山东蒙山拦马墙全新世复合冰川堆积体的发现与测年——兼论冰川堆积的对称性及其成因

New Find and Dating of Holocene Composite Glacial Deposits in Lanmaqiang,Mengshan,Shandong Province:Concurrently Discussing the Symmetry of Glacial Deposits and Its Causes

近年来,山东蒙山拦马墙巨石垄的成因引起学界的关注,形成了冰川成因与泥石流成因两种观点。为进一步研究其冰川成因,在综合分析了众多冰川堆积体的研究资料之后,发现冰川堆积地貌普遍性特征是侧碛垄呈现出典型的“对称性”,这是由于冰川独特的堆积过程决定的,其与洪水、泥石流的扇形堆积之间存在着根本差别。基于此,对蒙山拦马墙一带巨石垄进行了更大范围的调查,发现拦马墙巨石垄并非以往认为的单侧存在,而是由左岸侧碛垄、中碛垄、右岸侧碛垄共同构成的对称性侧碛垄,是佛塔谷与兰溪谷交汇部位的两条冰川(复合冰川)共同作用的产物。经调查,拦马墙巨石垄上游兰溪谷内的3处侧碛垄也具有显著的对称性特征。对峡谷内多列冰碛垄进行宇生核素测年研究,获得堆积时间为距今(5.36±0.25)ka、(9.93±0.38)ka和(12.9±0.38)ka,分别对应于5.3 ka降温事件(清荣冰期)、C2降温事件和新仙女木事件,并与地衣测年结果具有较好的对应性。兰溪谷内由巨石堆积形成的对称性侧碛垄属于早全新世与中全新世降温事件的产物,并首次发现了新仙女木冷事件堆积的侧碛垄。该研究成果揭示了我国东部存在全新世冰川遗迹。

In recent years,the origin of the Lanmaqiang boulders ridges in Mengshan,Shandong Province has attracted the attention of academic circles,and two views of the cause of formation,glaciation and debris flow,have been formed.The research has integrated numerous research data of glacial deposits,it was found that these lateral moraines showed“symmetry”which was the most common feature of glaciation.This feature was determined by the unique deposition process of glaciers,as it was a fundamental difference in the topographic feature between it and the fan-shaped deposits formed by floods and debris flows.Therefore,a more extensive survey has been carried out on the boulders ridges in the study area based on this.It was found that the boulders ridges were not one-sided,as previously thought,but symmetrical moraines composed of left lateral moraines,middle moraines and right lateral moraines.These moraines were the products of two glaciers(composite glaciers)at the intersection of Fotagu Valley and Lanxi Valley.Additionally,the other three lateral moraines in Lanxi Valley upstream of Lanmaqiang boulder ridges also showed remarkable symmetry.The sedimentation time of several rows of moraines ridges in the valley was(5.36±0.25)ka,(9.93±0.38)ka and(12.9±0.38)ka obtained by Cosmogenic Nuclide Dating,The deposition time of these lateral moraines correspond to the 5.3 ka cooling event(Qingrong Glacial Age),C2 cooling event and Younger Dryas event,and had good correspondence with the results of lichenometry dating.Specifically,the symmetrical lateral moraines in Lanxi Valley formed in the Early Holocene and Middle Holocene cooling events,and the lateral moraines formed by the Younger Dryas cold event were first discovered.The research results reveal the existence of Holocene glacial remains in eastern China.