兰州“930505”特大尘暴沉积物特征研究

本文首次从沉积学角度对兰州现代特大尘暴沉积物进行了粒度组成、矿物成分和化学成分的分析研究，并同兰州黄土进行了比较。结果表明，沉积物属粉质重亚粘土，分选差，平均粒径仅为０．０１６ｍｍ。矿物成分以石英、长石、云母为主，富含碳酸盐和粘土矿物。与马兰黄土相比，尘暴沉积物粒度偏细；石膏和白云石含量较高；常量元素中镁含量较高，而硅、钠含量较低。微量元素镍、铜、锌、铅、锶含量较高，可能受物源区或降尘区的矿山、工业及交通污染等因素影响所致。总之，特大尘暴指示为大气动力条件很不稳定的快速气下堆积；物质主要源自河西走廊东部干旱地区。现代尘暴事件是地质历史时期黄土沉积的继续，而黄土堆积中心已由马兰期的陇西盆地退向北西。

On the Wind-blown Deposits From a Heavy Dustfall Numbered "930505" in Lanzhou, North-Central China

For the first time and from the sedimentologic view, this paper deals with the grainsize distribution, mineral and chemical composition of the deposits from a huge dustfall that took place in the Lanzhou area on May 5, 1993. Studies show that the deposits were badly sorted(SD=1.65)by wind, and might be defined as a silty clayey (Md= 0.0016mm ) according to the grainsize distribution. By X-ray diffraction analysis, the deposits are composed of quartz, feldspar, mica and some other minerals, e. g. carbonate and clay minerals in small quantity. Compered with the Lanzhou loess, the modern dustfall deposits are smaller in grainsize, and contain more gypsum (2.7%) and dolomite (3.65%). Mg as a macroelement is higher than that in the loess, while Si and Na are lower. Studies also reveal that the microelements of Ni, Cu, Zn, Ph and Sr are comparatively higher in the deposits, and this seems to be caused by the pollution materials in the air from mines, modern industries and automobiles when the air-born dusts are transported.Furthermore. the dePOsitS of the heary dust fall indicate an extremely unstable subaerial environment. The dust (or loess) materials were derived mainly from the arid area of the eastern Hexi Corridor, Gansu Province. The dePOSition centre must have shifted from the Lanzhou area during the last glaciation (Malan Epoch) to its northwest at present (postglaciation).