Lithological and environmental controls on large tafoni along conglomerate cliffs in subtropic humid Danxiashan UNESCO Global Geopark

Tafoni are a type of cavernous weathering features widespread in different climate zones around the world, but their origin has not been fully understood. Little attention was paid on tafoni in conglomerates. Danxiashan UNESCO Global Geopark, located within the subtropic humid region of South China, has a landscape of large tafoni that occur on cliffs of red conglomerates. Both the Dinosaur Rock and Pagoda Peak of the geopark were selected to investigate the formation mechanisms of tafoni in conglomerates. During the field investigation, the size, rock hardness, and the external and internal meteorology of tafoni were measured and tested. The texture and composition of rock samples were observed, and the ion chromatography experiments were performed as well. Results show that tafoni have well ellipsoidal openings. Clasts of the conglomerates are largely sub-rounded and poorly sorted, and the cement is dominantly composed of calcite and iron oxide. The hardness of the lips is almost equal to that of the columns, and both are slightly higher than the backwalls. Compared with the external intense environmental changes, the interior of the tafoni is characterized by smaller temperature and relative humidity ranges and windless condition. Therefore, the porous and permeable conglomerates provide favorable lithology for tafoni development, and the micro-environment within the caverns is conducive to improving water utilization efficiency and enhancing salt weathering. The salts mainly consist of sulfates and nitrates, which might be derived from frequent raining, water evaporation and red beds as well. Consequently, tafoni are the result of the synergistic effects by multiple factors, rather than a single mechanism.