Description of an emblematic geosite in Central Spain: A large sinkhole associated with paleo-landslide in gypsum karst

Emblematic geosites are those that introduce a new geological concept or strikingly display a well-known geological concept. In this context we introduce and describe a large sinkhole in Central Spain as an emblematic geosite due to the following reasons: (1) sinkholes associated with landslides are scarce; and (2) the described sinkhole has a large size providing a spectacular example of a sinkhole associated with a landslide in gypsum escarpments and with gypsum karst. Thus, although the proposed site is neither a unique example nor a new geomorphological feature, it is a particularly nice example that, by its rare development, deserves to be classed as an emblematic geosite. The studied geosite is characteristic of and represents the most typical features of sinkholes as a geo-hazard associated with paleo-landslides in gypsum karst. High steep scarps are common in gypsum terrains created by a combination of fractures and erosion caused by the lateral migration of rivers. Mass movements, including rotational landslides, are associated with these unstable scarps. These rotational landslides generate stepped slopes with a relatively rugged topography, creating flat benches and closed depressions that can accumulate runoff water from rainfall. Depending on their altitude with respect to the river floodplain, this can result in flood water or high ground water levels after intense rainfalls. This process can activate the rapid development of underground dissolution of gypsum, developing karst conduits, caves, and hollows, which can result in a self-accelerating process of rapid growth. As a consequence, the development of large underground karst voids (whether or not saturated) may increase the probability of subsidence and collapse creating sinkholes on the surface. These sinkholes are mostly small, but the large sinkhole observed in the Miocene gypsum scarps of the Tajuña River in the south of Madrid, near Titulcia and Chinchón (Madrid province) implies that the risk of a sinkhole hazard in these geological environments must be acknowledged and reconsidered. The sinkhole should be defined as an emblematic geosite that must be preserved, studied and explained.