52. Sistema Zacatón: Volcanically Controlled Hypogenic Karst, Tamaulipas, Mexico

Sistema Zacatón includes the second deepest underwater cave in the world. It is hypothesized to have formed by volcanogenic karstification, a process that relies on four components to initiate and develop deep subsurface voids: a carbonate matrix, a system of preferential flow paths (e.g., fractures), volcanic activity that increases groundwater acidity, and groundwater flux through the system. Results are compiled into a multiphase speleogenetic model, most phases of which are of Late Pleistocene age. Surface rocks consist of carbonate travertine with Pleistocene mammoth fossils found within the rock matrix. The rocks are interpreted as a hydrothermal travertine terrace formed as nearby volcanic activity peaked, and thus representing the end member of a carbonate mass transfer system originating deep in the subsurface. The modern karst system includes a dynamic array of deep, phreatic sinkholes (also called cenotes) propagated upward through the travertine and exposing hydrothermal water supersaturated with carbon dioxide to the atmosphere. In some cenotes, seals of a second stage of travertine formed as CO₂ degassed, capping the sinkholes with hydrologic barriers. Volcanogenic karstification worldwide is not limited to Sistema Zacatón, although the localized nature, coupled with the extreme degree of karstification, makes it an ideal modern analog for classifying certain other karst systems as volcanogenic.