北京西山古山洪堆积——马兰砾石形成环境及年代

在北京西山永定河支流清水河流域构成马兰台、燕家台、梁家台和青龙涧小台等台地的砾石堆积物可称之为马兰砾石.其岩性、岩相、地形和地貌等方面都不同程度地显示出山洪泥石流沉积的特征.释光和14C测年结果表明,马兰砾石形成于约30kaB.P.至约13kaB.P.,覆盖在马兰砾石之上的全新世复合古土壤发育于约8kaB.P.至约2.5kaB.P..马兰砾石上部及上覆古土壤的植物硅酸体组合反映,马兰砾石堆积时期研究区可能发育着以藜科、蒿属等草本植物为主的干旱-半干旱草原植被;而全新世古土壤发育时期出现了森林或草原-森林植被.马兰台、燕家台、梁家台和青龙涧小台等台地应是末次冰期盛冰阶冰缘气候环境中暴雨山洪事件的稀性泥石流产物.

DATES AND ENVIRONMENTS OF THE MALAN GRAVEL FORMATION AS THE TORRENT DEBRIS DEPOSITS ALONG QINGSHUI RIVER IN WESTERN HILLS, BEIJING

There are a series of the tablelands of marked relief, such as Malan tableland, Yanjia tableland, Liangjia tableland and Qinglongjianxiao tableland, in the reach of Qingshui River. These tablelands, located at tributary mouths joined Qingshui River, have been mainly composed of the gravel accumulates which are referred as the Malan gravel formation in this paper. According to the statistic data of the lithographic properties, deposition structures and topographic relief, the Malan gravel formation can be recognized as the rapid deposits transported by sub-viscous debris flow in high energy circumstance when torrent events occurred in the tributaries of Qingshui River. IRSL(Infrared stimulated luminescence) and GLSL(Green light stimulated luminescence) dating of five samples collected from the remain of loess-like sand sediments underling the Malan gravel, thin loess-like sand interbeds near bottom, middle and top of the Malan gravel formation respectively, were undertaken by the fine grain multiple aliquot added dose and regenerative method. On the basis of IRSL and GLSL dates of the five samples, as will as 14 C ages of three samples taken from the Holocene polygenetic soils overlying the Malan gravel formation, the beginning age of the Malan gravel accumulation is estimated as about 30 ka B.P., and the end age as 13 ka B.P. to 11 ka B.P. The Holocene polygenetic soils were developed during a period from about 8 ka B.P. to about 2.5 ka B.P.The phytoliths assembles of nineteen samples from the upper part of the Malan gravel formation and the Holocene paleosols at two sections in the Malan tableland reveal that the arid and semi-arid steppe vegetation with the dominant components of Artemisia and Chenopodiaceae could be developed in the periglacial environment during the Malan gravel deposition. There are several kinds of periglacial phonomena, such as a periglacial involution, congelifold, ice wedges, and so on, presented in the late period of Late Pleitocene in the reach of Qingshui River 8,9] . However, the relative warm-moist forest-steppe or grass-forest vegetation took place in the epochs as the Holocene polygenetic soils formed.