Karst Geoheritage of the Red Stone Forest in the Xiangxi UNESCO Global Geopark: Chromogenic Factors, Microgeomorphology and Dissolution Behaviour

Abstract

The Ordovician red karst forest is a core geoheritage site in the Xiangxi UNESCO Global Geopark and is of world-class value. However, the chromogenic factors and microgeomorphology of it have not been systematically studied, and the dissolution rate of the rocks remains unclear. The geopark contains the Sanbaidong profile, which completely traverses the bedrock of the red stone forest. We collected 355 samples from the strata within the profile for high-density geochemical analysis and found that the red stone forest contains more than one chromogenic factor. The TFe₂O₃ content in the lower boundary of red carbonate rocks increases significantly, controlling the occurrence of red beds. The proportion of Fe³⁺ controls the degree of red colouration. The proportions of Fe²⁺ and Cu²⁺ control the appearance of the greyish-green layer, and Mn²⁺ may control the darkness of the rocks. According to the study on the coupling between elements, TFe₂O₃ and Cu²⁺ are mainly supplied from continental sources, and Mn²⁺ is restricted by both marine and continental factors. The elemental distribution in layers obviously controls the concave and convex microgeomorphology. The clearest reason for this phenomenon is the CaO-SiO₂ proportions in adjacent layers, which are independent of the absolute values. According to an ideal model and comparisons with the Dalongdong area in Xiangxi and the Shilin UNESCO Global Geopark, it is calculated that the dissolution of the red stone forest may occur at a rate of 130.63 mg/a. The dissolution difference between the concave and convex surfaces is 1.3978 mg/a. This means that the compositional difference between the concave and convex surfaces may lead to an annual quality difference of approximately 1.07% between the layers. After tens of thousands of years, the microgeomorphology finally formed.