美国Zion国家公园红层地貌发育研究

A Study of the Development of Red Bed Landforms in Zion National Park, the United States

随着中国红层和丹霞地貌研究逐步走向国际,国内学者走出国门去认识和了解国外的红层及其地貌发育显得尤为必要。Zion国家公园是美国西部红层的典型代表,在干旱气候环境下发育了高原-峡谷景观。本文通过野外考察和岩样分析,从地质构造、岩性特征和外动力等方面来探讨Zion公园的红层地貌发育机制。研究表明:Zion公园为科罗拉多高原西部边缘的断块山地,公园内褶皱很少,断层稀疏,但节理密集,控制了峡谷群的发育。公园内地貌发育的主体地层为红层,主要形成于侏罗纪,其沉积环境复杂多变,兼具滨/浅海、河流、湖泊和沙漠等。岩性特征上,Zion公园的红层以细砂岩和粉砂岩为主。其中,形成崖壁的主要为风成相石英砂岩,但各段岩石的胶结特性不同,导致其岩石强度和颜色有较大差异。此外,还发现Zion公园红层的岩石强度和山体坡度并不呈正相关关系,它还与岩性是否均一有关。细粒均一的风成相砂岩和河流相Springdale段砂岩发育成陡崖,其余含有软岩夹层的岩层只能形成缓坡。外动力方面,流水下切是主导因素,同时流水侧蚀和地下水的基蚀作用等对Zion公园的地貌发育也有重要影响。

As the research on red beds and Danxia landforms in China is becoming increasingly known by geoseientists outside China, it is quite necessary for Chinese researchers to know and understand red beds and their geomorphic development in other countries. As a representative of red beds distributed in the western United States, Zion National Park is characterized by a geomorphie combination of plateau and canyons in arid climate. Based on field investigation and analysis of rock samples, this paper looks at the developmental mechanism of landforms in Zion National Park from the aspects of geological setting, lithological features, and exogenic forces. In terms of general geologic structure, Zion is a fault block formed on the western margin of the Colorado Plateau. There are few folds and faults, but joints are exceptionally well developed in Zion, they are responsible for the orientation of canyon network. The dominant rock strata exposed in Zion belong to red beds, which were mainly deposited in the Jurassic with a wide range of environments, including near shore/shallow sea, streams, lakes, and deserts. In terms of lithological features, most red beds in Zion are fine-grained sandstones and siltstones. Among them, the spectacular cliffs in Zion are made of eolian quartz sandstones. However, due to cement differences of each rock member, there are remarkable diversities in rock strength and colors. Moreover, this study also found that rock hardness values do not positively correlate with mountain slopes in Zion. The uniformity of a certain rock layer can also affect mountain slopes. Therefore, being composed of uniform sandstones, the aeolian rock formations and Springdale Member can form steep cliffs, while other rock layers, which contain soft interlayers, can only form gentle slopes. Finally, in terms of exogenic forces, fluvial incision plays a dominant role in the formation of cliffs and canyons in Zion, followed by lateral erosion and sapping process.