AMCS ACTiViTiES NEWSLETTER NUMBER 32

Mexico. By Marcus O. Gary, John M. formed. the cenotes of Sistema in Mexico is predominant in the Sharp, Jr., todd Halihan, and Juan Zacatón are perhaps the deepest un- late-Pleistocene, as recent as 10,000 Alonso ramirez Fernandez. derwater shafts. the period of their years b.p. The identification of these Defining the timing of karstifica- formation had been constrained to fossils may provide accurate timing tion in a specific geologic setting is only within the last 1.5 million years, of the formation of this impressive often a challenging task. isotopy, when volcanic activity dominated karst system. sediment deposition, paleontology, the nearby landscape. the sinkholes Source: Paper presented April 2007. geologic super-position, and geomor- are hypogenic—formed by volcanic http://gsa.confex.com/gsa/2007SC/ phology are among the tools used gasses. Following the opening of finalprogram/abstract_120214.htm; to provide clues for understanding many sinkholes in Sistema Zacatón, a Geol. Soc. America, Abs. with Programs when a particular karst system secondary phase of travertine depo- (South-Central Sec. Mtg.), v. 39, no. sition sealed expansive water-filled 3, p. 28. voids beneath a 2-to-5-meter-thick crust of rock, as demonstrated by Abstract: Volcanogenic Karstifica- electrical resistivity data and cave tion: implications of this Hypogene diving. in June 2006 while conduct- Process. By Marcus O. Gary and John ing geophysical surveys of some of M. Sharp, Jr. the sealed cenotes, vertebrate fossils Hypogenic karst forms from water were found in travertine, previously in which the aggressiveness has been mapped as Cretaceous limestones, produced at depth, independent of adjacent to the sinkhole Poza Seca. acid sources at or near the surface. These fossils have been identified numerous geologic conditions fa- as belonging to the genus Mam- cilitate a setting for hypogenic karst muthus by the molar and tusks that processes to evolve, including the were located. the significance of interaction of igneous rocks and this discovery relates to the lithol- groundwater in carbonate rocks. ogy of rocks that pre-date sinkhole Hydrothermal, deep-seated karst is development to the surface and documented, but the mechanisms are subsequent collapse. the fossils not always applied in geologic evalu- are lithified in dense travertine de- ations. volcanic activity provides posited by hydrothermal springs, conditions that can rapidly dissolve and the sinkholes and dry caves in large voids deep below the earth’s the system have formed within this surface. Volcanogenic karstification travertine, thus major dissolution relies on four components to initi- and collapse episodes at Sistema ate and develop deep, subsurface 3-D models of Cenote Zacatón Zacatón must have occurred coinci- voids: 1) thick carbonate strata, 2) (above), Cenote Caracol (below left), dentally or following this travertine preferential groundwater flow paths and Cenote la Pilita, tamaulipas. ian Reichert-Watts. formation. Mammuthus distribution (fractures), 3) volcanic activity that

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Cenote Zacatón, Cenote Caracol, Cenote La Pilita Rancho La Azufrosa, Mpo. Aldama Tamaulipas AMCS Activities Newsletter 32, p. 34, 2009