蒋家沟泥石流堆积物的时代及成因

PRIMARY STUDY ON AGE AND GENESIS OF DEBRIS FLOW DEPOSITS IN JIANGJIA RAVINE, YUNNAN PROVINCE

据石英ESR和~14C测年结果,蒋家沟泥石流堆积可分为晚更新世形成的古泥石流堆积和近代泥石流堆积两类。古泥石流堆积再分为早、中、晚三期。扫描镜下观察蒋家沟古泥石流堆积中的石英砂表面机械结构特征和现代泥石流十分相同,例如两者的颗粒都呈尖锐棱角状,粒面上均发育很好的贝壳状断裂结构、解理面、凹坑、擦痕(或擦沟)等,这表明二者的形成环境是相同的。

The Jiangjia Ravine in the northeast of Yunnan Province is a typical area where debris flows are active in China. A comprehensive survey in this district shows that debris flows in modern times have been widespread and frequent,and clear evidence of debris flow activities from the remote past can also be found everywhere. Therefore,Jiangjia Ravine can be viewed as a natural museun of debris flow. In order to understand ages and genesis of debris flow deposits in the Jiangjia Ravine,the textures on the surface of quartz grains the debris flow deposits have been dated by application of the ESR technique and studied. According to the results obtained by ESR dating,the debris flow deposits in the Jiangjia Ravine can be classified into 2 typesiancient and new debris flow deposits. The ancient debris flow deposits can also be divided 3 subtypes:the early ancient debris flow deposit formed in 98. 3 Ka age: the middle one in 63. 1-56.1 Ka age; and the later one in 24. 0-20.1 Ka age. The three periods of debris flow deposits in Jiangjia Ravine are respectively consistent with the second interstadial of last interglacial (.100. 0-90. 0 Ka B. P. ) ;the second and the first interstadial of last glacial (i. e. , 62. 0-50. 0 Ka B. P. and 32. 0-21. 0 Ka B. P. ),showing that the three periods of debris flow deposits in the Jiangjia Ravine related closely to the three humid periods in Late Pleistocene.The results obtained by analysis under SEM show that the quartz grains of new debris flow deposits in Jiangjia Ravine are rather complicated in shape,their appraximate shapes which are observed are mainly sharp edgeangle, which may be formed during the period of transportation. There are cleavage planes,conchoidal fractures,pits,scratch and grooves,etc. on the surfaces of quartz grains. The shapes and surface microtextures of the quartz grains in ancient debris flow deposits are almostly similar to those of the new one. Therefore, the similarity between quartz grains of ancient and new debris flow deposits obviously shows that their genetic environment is alike.