美国西部丹霞地貌特征及演化过程研究——以犹他州Zion国家公园为例

Geomorphic Features and Evolution Process of Danxia Landforms in Western United States:A Case Study at Zion National Park,Utah

丹霞地貌是一种重要的风景地貌,研究其地貌特征和演化过程可以为保护与利用这一地质遗迹提供科学支持。中生代红层是我国丹霞地貌发育的物质基础,在美国西部的科罗拉多高原亦有大量分布。为增进国内同行对美国西部红层与丹霞地貌的了解,以犹他州Zion国家公园为例,通过实地考察,岩样理化性质测试,并结合GIS和DEM分析,对其地貌特征和演化过程进行探讨。结果表明:区域构造方面,Zion公园红层沉积于大型的弧后盆地——西部内陆盆地,盆地抬升时形成的节理群对丹霞地貌发育具有控制作用;红层岩性方面,Zion公园发育丹霞地貌的红层以风沙沉积为主,岩性为细砂—粉砂岩,但不同段的风成砂岩在胶结特性和岩石强度上存在较大差异;地貌特征方面,Zion公园整体为高原—深切峡谷景观,其中高程为1800~2000 m的部分占比最大,公园南部比北部切割侵蚀程度更高,形成的单体地貌以块状山崖—沟谷组合为主,同时还因地下水的基部渗流侵蚀和风蚀作用在崖壁上发育有各种洞穴景观;演化过程方面,Zion公园丹霞地貌发育经历了构造盆地形成—红层堆积—地壳隆升—外力塑造四个阶段,三叠纪末期形成的西部内陆盆地为红层堆积提供了相对稳定的沉积环境,侏罗纪期间干旱的气候环境下形成了大规模的风沙沉积,白垩纪末期科罗拉多高原的抬升及形成的节理裂隙为外力侵蚀提供了条件。其中,强烈的流水下切是该地区丹霞地貌发育的主导外营力,并一直持续至今。

Danxia landforms is an important type of landscapes and research on their geomorphic features and evolutional processes can provide scientific support for the protection and utilization of this geoheritage.Mesozoic red beds are the material basis for the development of Danxia landforms in China,and they are also widely distributed on the Colorado Plateau of the Western United States.In order to enhance understanding on red beds and Danxia landforms in the Western United States,this study took Zion National Park in Utah as a case to discuss the geomorphic features and evolution process of Danxia landforms by field investigation and analysis of physical and chemical properties of rock samples,and analysis by the application of GIS and DEM.In terms of regional geological setting,the depositional environment for red beds deposited in Zion was a large back-arc basin—the Western Interior Basin,the development of Danxia landforms was controlled by closely-spaced joints developed when the basin was uplifted.In terms of lithologic features,red beds for the development of Danxia landforms in Zion are dominated by aeolian deposition,which are featured by fine-grained sandstones and siltstones,while coarse-grained clastic deposits such as conglomerates and sandy conglomerates are scarce in this area.In addition,the cementation characteristics and rock strength of these aeolian sandstones are varied in different rock members.In terms of geomorphic features,the overall landscape in Zion is a combination of plateau and deeply-incised valleys,with the elevation of 1800~2000 m accounts for the largest proportion,and the southern part has more down-cutting and erosion than the northern part.As for single Danxia landforms,they include sheer cliff walls,valleys,and various caves developed by sapping process of ground water and wind erosion.In terms of geomorphic evolution,the evolution processes of Danxia landforms in Zion could be divided into four stages:formation of structural basin—deposition of red beds—tectonic uplift—erosion of external forces.The Western Interior Basin formed in the late Triassic provided a relatively stable depositional environment for the deposition of red beds.Then,during the Jurassic period,a large scale of aeolian deposits was formed in arid climate.Finally,the uplift of the Colorado Plateau in late Cretaceous and the formation of joints provided conditions for erosion of external forces.Among them,the strong downcutting process was the dominant external force for the formation of Danxia landforms in this area,and it continued to this day.