兰州现代特大尘暴沉积物粒度特征及其意义

Granulometric Characteristics and Significance of the Deposits from a Recent Extraordinary Heavy Duststorm in Lanzhou, Gansu, Northwest China

采用国内外较先进的ＳＫＣ－２０００光透式粒度分布测定仪对兰州１９９３年５月５日发生的特大尘暴沉积物进行组距为０．２５的粒度分析。结果表明，沉积物为均值６．４４４、众数５．００～５．２５、单峰极正偏的粉质重亚粘土，大致与黄土粒度旋回中古土壤的粒度构成相当；沉积物分选差（ＳＤ＝１．６５），属大气动力条件很不稳定的快速气下堆积；Ｋｄ值平均３．３７，较兰州马兰黄土高，且镜下发现石膏矿物晶形完好、含量高达２．７％，指示沉积物主要源于内陆干燥地带；沉积组分中大于０．０５ｍｍ的极细砂含量仅为０．０８％，较兰州马兰黄土低５６倍，表明特大沙尘暴进入兰州后已经演变为特大尘暴。黄土物质至今仍在源源不断地向本区输入，而黄土堆积中心已由马兰期（末次冰期）六盘山以西的陇西盆地退向北西。

This paper deals with the granulometric analysis of the deposits from a recent extraordinary heavy duststorm that happened in Lanzhou area on May 5, 1993. According to the granulometric analysis by the instruemt of SKC-2000, the deposits can be defined as a silty heavy clayey with a mean particle diameter of 6.444, and a sharp concerntration of 5.00￣5.25. The poor sorting degree (SD=1. 65) indicates an ustable air-current of the duststorm during its transportation process. The deposits originate from the inland arid area with one source only. This is also proved by a high Kd value (3. 3) and the high contents of calcium and gypsum in the sediments. The extraordinary duststorm in Lanzhou was formed after the heavy sand-dust storm in the adjacent areas. The deposition of the loessial materials tends to continue in Lanzhou area, but traces reveal that the deposition centre seems to have shifted from Lanzhou area during the Last Glacial (Malan Epoch), to its northwest at present (the Postglacial) .