甘肃新近系疏松砂岩成因

Genetic Analysis of the Neogene Unconsolidated Sandstone in Gansu,Northwest China

近年来重大地下工程中揭露一套结构疏松,自稳能力差,涌水、塌方等工程事故频发的岩层,即疏松砂岩。分析其成因及赋存环境对认识其特殊性质及应对相关工程问题都具有重要意义。本文结合甘肃新近系沉积环境,采用室内粒度分析试验、SEM及XRD试验,并与现代沙漠风成沙与河床水成沙对比,从其分布及产出特征、粒度特征、石英颗粒表面特征及胶结特征等方面对其成因进行分析。研究表明,这套疏松砂岩与相邻岩层产出相近,归属于甘肃新近系红层;其粒径相对集中,胶结物含量少、胶结差,孔隙度大;石英颗粒表面发育蝶形坑、麻面及硅质沉淀等典型特征。综合其各种特征,甘肃新近系疏松砂岩为新近纪干热气候环境下风沙活动的产物,且受后期赋存水环境作用的影响。

In recent years, a set of strata with loose structure and worse self-stabilization, which named unconsolidated sandstone, has been exposed by several large underground engineerings with frequent engineering accidents, such as gushing water, collapse and other problems. Analyzing its origin and occurrence environment is of great significance both in understanding the essence of its special properties and dealing with the relevant engineering issues. Combined with sedimentary environment of the Neogene in Gansu, the origin of the unconsolidated sandstone, which can be comparing with modern desert aeolian sand and beach sand, is studied by using grain size analysis, scanning electronic microscope （SEM） and X-ray diffraction （XRD） by which its distribution characteristics, occurrence and exposure characteristics, grain-size characteristics, quartz sand grain surface characteristics and cementation characteristics, would that the unconsolidated sandstone, which belongs to the parts be discussed in this paper. The analysis result shows of the Neogene red layers in Gansu province, has the same occurrence and exposure characteristics as adjacent strata. And this sandstone has well-sorted, large porosity and less clay minerals. Its quartz grain surface shows some butterfly shaped craters, surface pockmarks and siliceous sediments. All of those characteristics can be inferred that the unconsolidated sandstone in Gansu is the wind -sand activity product with dry and hot climate in late Neogene, and parts of the strata are influenced by water environment afterwards.