甘肃兰州黄河北岸疏松砂岩成因

Genesis of the Unconsolidated Sandstones in the North Side of the Yellow River in Lanzhou,Gansu

广泛分布于甘肃兰州黄河北岸红城以东至皋兰以西一带的桔红色疏松砂岩体几乎全由石英颗粒组成,对其研究不仅具有较高的经济价值,而且对于了解中国西北地区干旱环境演变具有极其重要的意义。长期以来,该层砂岩的成因问题一直备受中外地质学家的关注。为此,针对该层疏松砂岩,通过野外沉积构造描述、室内颗粒分析试验及扫描电镜分析等方法,分别从沉积构造、粒度及石英颗粒表面形态特征等方面对其成因进行分析。研究结果表明,兰州黄河北岸水阜乡至曹家湾一带疏松砂岩水平层理沉积特征明显,砂岩粒度累积曲线具有典型水成特性,石英颗粒表面可见比较发育的贝壳状断口及因溶蚀作用而导致的鳞片状剥落等水下沉积特征;朱家井至红城一带疏松砂岩中风成板状斜层理沉积特征明显,其粒度累积曲线具有典型风成特性,石英颗粒表面可见比较发育的麻面、蝶形坑等风成特征。该层砂岩并非单纯的风成或水成,其成因具区域性。

The orange-red unconsolidated sandstone which consists almost entirely of quartz is widely distributed from the east of Hongcheng to the west of Gaolan on the north side of the Yellow River in Lanzhou,Gansu Province.The research on unconsolidated sandstone not only has high economic value but also an extremely significance for the drought condition study in northwest China.For a long time,the genesis of this sandstone has attracted wide attention and dispute at home and abroad.By field observation of the sedimentary structure,indoor particle size analysis test and SEM scanning,the genesis of this sandstone were discussed in aspects of the sedimentary characteristics,the grain-size characteristics and the surface characteristics of the quartz in this sandstone.The unconsolidated sandstone widely distributed from Shuifu to Caojiawan has the following characteristics of underwater sedimentary： the horizontal bedding is evident in the sedimentary structures;the change of cumulative grain-size distribution curves consists with the neptunian theory;the conchoidal fracture and scaly peeling caused by erosion on the quartz surface are developed.The unconsolidated sandstone distributed from Zhujiajing to Hongcheng has the following aeolian features： the tabular oblique bedding is evident in the sedimentary structures;the change of cumulative grain-size distribution curves meets the aeolian theory;the surface of the quartz in the unconsolidated sandstone possesses pockmarked face,butterfly pit,and so forth.These results indicate that the genesis of this sandstone is very complicated and is regionally different;it cannot be simply analyzed by aeolian theory or neptunian theory.