Karst hydrogeology and speleogenesis of Sistema Zactón, Tamaulipas, Mexico
Understanding geologic mechanisms that form karst is of global interest. An estimated 25% of the world's population obtains water from karst aquifers and numerous major petroleum reserves are found in paleokarst reservoirs, so characterization and classification of specific types of karst is essential for resource management. Sistema Zacatón, which includes the second deepest underwater cave in the world, is hypothesized to have formed from volcanogenic karstification, defined as a process that relies on four components to initiate and develop deep, subsurface voids: a carbonate matrix, preferential groundwater flowpaths (fractures), volcanic activity that increases groundwater acidity, and groundwater flux through the system. Phases of karstification creating this modern hydrogeological environment are defined using numerous methods: field mapping, 3-D imaging of surface and aqueous environments, geophysical investigations, physical and chemical hydrogeologic characterization, and microbial analysis. Interpretation of the results yields a multi-phased speleogenetic model of the karst, with most phases occurring in the late Pleistocene. The surface rocks are carbonate travertine with Pleistocene mammoth fossils found within the rock matrix, and are interpreted as a hydrothermal travertine terrace formed as nearby volcanic activity peaked, thus representing the end member of a carbonate mass transfer system originating deep in the subsurface. The modern karst system includes a dynamic set of deep, phreatic sinkholes, also called cenotes, which propagated up through the travertine, eventually exposing hydrothermal water supersaturated with carbon dioxide to the atmosphere. In some cases these cenotes have precipitated seals of a second stage of travertine as CO₂ degassed, capping the sinkhole with a hydrologic barrier of travertine. Evidence of these barriers is observed in aqueous physical and geochemical characteristics of the cenotes, as some have high hydrologic gradients and contrasting geochemistry to those of neighboring cenotes. Investigations of electrical resistivity geophysics and underwater sonar mapping support the hypothesis of the barriers and define the morphology in intermediate and final phases of sinkhole sealing. Volcanogenic karstification is not limited to Sistema Zacatón, although the localized nature coupled with rapid and extreme degrees of karstification makes it an ideal modern analogue for classifying other karst systems as volcanogenic.