陕北黄土覆盖型丹霞地貌特征及成因

Characteristics and Genesis of Loess-Covered Danxia Landscape in Northern Shaanxi Province of China

丹霞地貌在中国东南湿润地区研究程度较高,但西北干旱-半干旱地区的研究资料较少。西北丹霞地貌在形态特征和形成过程等方面独具特色,对其研究有利于提高对丹霞地貌共性特征的认识,具有重要理论意义。陕北甘泉大峡谷地处黄土高原中部,是黄土覆盖型丹霞地貌的典型代表,具有明显波状起伏的峡谷特征,对该地区的研究有利于更好地认识黄土高原丹霞地貌的演化过程。本文采用野外考察、施密特锤岩石原位硬度测量、样品薄片偏光显微镜和扫描电子显微镜观察及地理信息系统(ArcGIS)中面积高程积分等多种方法,探究陕北地区丹霞地貌的造景岩石特征、演化阶段及其控制因素。研究结果表明:(1)研究区的造景岩石为洛河组砂岩,发育大型风成交错层理,孔渗性较好,长石、岩屑及钙质胶结物含量较高,岩石抗风化能力较弱,在流水侵蚀和盐风化作用下形成峡谷密布的丹霞地貌。(2)甘泉大峡谷与桦树沟的面积高程积分值分别为0.51和0.49,指示该地区的地貌演化阶段为壮年期,流水侵蚀为地貌演化的主要控制因素。(3)研究区丹霞地貌于新近纪晚期之前可能已经形成,随后第四纪气候变干,风成黄土堆积在丹霞山峰之上和峡谷之间,流水溯源侵蚀使峡谷进一步加深变长,形成了独具特色的“黄土覆盖型丹霞地貌”。本研究可为加深对西北地区丹霞地貌形成过程的理解提供新材料。

Danxia landscape in southeastern China has been a scientific concern,but in northwestern China it received little attention from scientists.The northwestern Danxia landscapes are unique in both morphological appearances and geologic genesis as compared with those in the southeast;thus research on them in the northwest Danxia would lead to some important geomorphologic advances on the formation of Danxia landscape.The Ganquan Grand Canyon,located in the middle of the Loess Plateau of China,exemplifies a representative of loess-covered Danxia landscape with undulated canyons in northwestern China.Research on this canyon is conducive to better understanding the evolution process of Danxia landscape in the Loess Plateau.In this study,a variety of research approaches,such as field survey,in-situ rock hardness measurement by using a Schmidt hammer,sample observation under a polarizing microscope and scanning electron microscope,Hypsometric Integral(HI)(a tool developed in ArcGIS software to quantitatively describe the geomorphic evolution stage of drainage basin)were collectively used to explore the bedrock characteristics,evolution stages and controlling factors of Danxia landscape in northern Shaanxi.It found some interesting results.(1)The bedrock of the Ganquan Grand Canyon is composed predominantly of eolian sandstones of the Luohe Formation,with large-scale cross-beddings,high porosity and permeability,high contents of feldspar particles,lithic fragments and carbonate cements,and hence less resistant to weathering.Influenced dominantly by water erosion and salt weathering,the Danxia landscapes walled by dense valleys were formed.(2)The HI values of the Ganquan Grand Canyon and the Huashugou gully are 0.51 and 0.49,respectively,suggesting that they are in the mature stage and the river erosion acted as the primary controlling factor of landscape evolution.(3)The Danxia landscape in the study area had been possibly formed by the late Neogene period,followed by a drying Quaternary climate,when the loess covered on the top of peaks of Danxia mountains and valleys;and then headward erosion by flowing water resulted in the deepening and lengthening of valleys,and the formation of the distinct loess-covered Danxia landscape.The research can provide new materials for deepening the understanding of the formation process of Danxia landscape in northwestern China.