Geo-environmental characteristics of weathering deterioration of red sandstone relics: a case study in Tongtianyan Grottoes, Southern China

It is reasonable to study the destruction and preservation of stone relics from a geo-environmental point of view. The Tongtianyan Grottoes, known as “the First Grottoes in Southern China”, are located in the city of Ganzhou, Jiangxi province, China, and are the largest cave temples in eastern China and southern China. The grottoes were carved into sandstone palisades during the period from the late Tang Dynasty (755–763 AD) to the Republic of China (1912–1949 AD). In China, the Danxia landform is a special landform characterized by red sandstone cliffs. Different from the grottoes located in arid climate regions, the Tongtianyan Grottoes have the lowest latitude (25°45′N) among all the grottoes in China, and the stone relics are under a perennially warm and humid environment. The monuments have suffered from various forms of weathering deterioration, which can be summarized as five types: exfoliation, bio-weathering deterioration, crack, erosion, and loss. Detailed investigations have been conducted to analyze the mechanisms of the weathering deterioration of the stone relics based on different aspects of geo-environmental characteristics. The weathering sandstone samples were analyzed using X-ray diffraction and X-ray fluorescence, and the chemical weathering indices (LOI, WPI, and WI) were calculated. We found that there is a boundary zone at a depth of 10 ± 2.5 mm from the surface to the inner rock based on the curves of the chemical weathering indices, and the rock outside of the boundary zone has much stronger weathering than that inside.