陕西靖边统万城建城时期地表物质成因与自然环境

为查明距今约1600年的统万城建城时期的自然环境和该古城是否建在风沙沉积之上这一长期争议的问题,对6个钻孔剖面样品进行粒度和X-射线衍射等分析。结果表明:城墙内外地表之下均为湖泊沉积,沉积物特点与风沙沉积物显著不同,主要表现为以下五个方面:(1)沙层含有褐黄色针铁矿;(2)有多个深灰色沉积层;(3)沙层中粗砂含量显著大于风积沙层;(4)沙层中粒度成分的分选性比风积沙层明显差;(5)细粒斑点中Fe₂O₃与Al₂O₃含量比风沙沉积物高。本文获得的证据充分表明,在统万城建城前后,该区自然环境较好,普遍分布的是由湖积物组成的高平地,统万城建在湖积沙层之上。统万城建城时期基本没有发生沙漠化,沙漠化发生在建城之后的一定时期。

Causes of surface sediments and natural environment of Tongwan city in Jingbian county of Shaanxi province during its construction period

In order to figure out the natural environment during the construction period (around 1600 years ago) of Tongwan city and a long-standing controversy of whether the city is built on sand deposits, research is done on six profiles of sample bore holes by grain size analysis, X-Ray diffraction, etc. This study indicates that lacustrine deposits are identified under the surface of ground both inside and outside of the city wall, and their features are totally different from wind deposit. The differences are shown as follows: (1) There is yellowish-brown goethite in targeted sand layers. (2) Several dark gray deposited layers are identified. (3) The content of coarse sand in the targeted sand layers is much higher than that in wind deposits. (4) The grain content showed a mixture of coarse and fine sand with a higher degree of separation. (5) The contents of Fe₂O₃ and A1₂O₃ in fine-sand spots in lacustrine deposits are higher than those in sand deposits. Supported by concrete evidences collected, it is shown that the natural environment of the city was good during the construction period. And the basic landscape is plateau generated owing to the accumulation of lacustrine deposits, on which the ancient city was built. Desertification did not occur during the construction period, but did during a period after its construction.