Gypsum speleogenesis: A hydrogeological classification of gypsum caves

This article reviews the state of the art of speleogenetic investigations in gypsum karsts from numerous studies carried out over the past 50 years in Spain. A classification of gypsum karsts is proposed based on the hydrogeological, tectonic and stratigraphic criteria that decisively control the evolution of gypsum karsts. In this respect, lithological aspects of Messinian and Triassic-Permian gypsum series in south-eastern Spain are considered, such as the alternation of rhythmic levels of marl and gypsum, as well as geodynamic aspects. The influence of the hydrogeological characteristics of evaporite aquifers on gypsum cave speleogenesis is discussed; this includes speleogenetic processes in confined, semi-confined or free aquifers controlled by regional and local base levels. Also, the importance of intense saline diapiric uplift is examined. To illustrate our classification, examples of gypsum caves developed in Spain are presented. Their similarities and differences with gypsum karsts in other regions (Italy, Ukraine, and USA) are discussed. A first general division addresses: (1) caves controlled by stratigraphic factors and (2) caves controlled by tectonic factors. Several typologies can be described, including (A) multilayer caves with confined hydrogeological origin, (B) confined hypophreatic caves with linear or maze configurations, (C) caves controlled by the variation or remanence of regional or local base-levels, and (D) caves controlled by the halokinetic evolution of salt/gypsum diapirs. The proposed classification is flexible and adaptable to each case, because different genetic mechanisms can coincide in time and space. Likewise, most considerations stated in this work about gypsum karst are valid for speleogenesis in other rock types.