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Cavernous weathering is widely distributed in different climate zones worldwide. The dominant controlling factors and their formation processes have long been discussed in the geomorphological community; however, many controversies remain. In the danxia landscape, various forms of cavernous weathering develop on steep slopes. They are important elements of the landscape and provide shelter for the preservation of many precious historical cultures, yet there have been few studies investigating their origin. Danxiashan is representative of the Natural World Heritage site 'China Danxia', with widespread cavernous weathering features of various sizes and shapes, providing a good opportunity for investigating the origin of cavernous weathering in subtropical humid climates. Typical caverns on sandstone and conglomerate slopes at five sites in Danxiashan were selected and studied by field investigation, morphological measurement, meteorological monitoring, sample microscopic observation, and salt experiments. The results show the following. First, the diameters of the cavern openings vary from centimeters to meters, and the shape of the cavern openings is elliptical to elliptical. The tafoni on conglomerate slopes are generally large and tend to grow upward and inward, while the caverns on sandstone slopes are generally small and arranged in a honeycomb-like structure. Second, lithology is fundamental for the development of caverns. At the macroscopic scale, it determines their location and arrangement because the caverns generally align within the lamination of beddings or cross-beddings. Indeed, the morphological characteristics of caverns developed differently under varying lithologies. At the microscopic scale, the red beds consist of abundant soluble mineral components, such as feldspars and carbonate cements, which are chemically dissolved in the seepage of acid rainwater, the salt crystallization of which leads to the destruction of the rock texture. Collectively, they would have initiated the formation of caverns. Third, the favorable microclimate within the caverns is a key factor that is water and salt accumulation, and consequently, salt weathering and enlargement of the caverns over time. Notably, the microclimate within the Jinshiyan Cave has provided favorable conditions for algae colonization, which controlled the formation of the cell-and-wall structure of the regular honeycombs at the Longlingpian Rock. Finally, the dominant controlling factors were dissimilar between caverns and interplayed in different development stages of cavernous weathering, the entire progression of which was potentially regulated by a self-organized mechanism. However, the influencing factors and critical values of the positive and negative feedback mechanisms require further study. |
