Talk Title: Examples of Hypogenic (Sulfuric Acid) Speleogenesis from North America, Europe and Asia
Speaker: Harvey R. DuChene, Consulting Geologist
Publication: The Outcrop, November 2005, p. 4
Most caves are formed by processes related to the development of classic surface karst in limestone terrains. The term that describes these caves is “epigenic,” because they occur at or near the surface of the earth. Epigenic caves form from the action of weak acids derived from the atmosphere, soil, and oxidation of sulfides in the vadose zone.
A small number of caves form as a result of processes that occur within the earth’s crust. The term that describes these caves is “hypogenic.” Hypogenic caves comprise 5% – 10% of all caves and are formed by acids generated within the earth. The most common agents of hypogenic carbonate dissolution are sulfuric acid and carbonic acid. There are several known sources of hypogenic sulfur gases, including biogenic and thermal reduction of sulfate, magmatism, and volcanism. Hypogenic caves can be distinguished from more common epigenic caves on the basis of their morphology, mineralogy, hydrology and relationship to surface karst.
The Guadalupe Mountains of the southwestern United States contain hundreds of known caves, but classic surface karst features are rare or absent. There is no evidence of persistent, through-flowing underground streams, and caves lack significant detrital mud and clay fill. Cave passages and galleries tend to have large volume compared to length, and they are not part of a regional hydrologic system. Except for collapse and speleothem growth, there is little evidence of modification by vadose processes. In addition to common calcite and gypsum speleothems, Guadalupe caves contain minerals not common in caves, such as sulfur, sulfates, fluorite and uranyl-vanadates such as tyuyamunite and metatyuyamunite. Sulfur and gypsum are depleted in 34S, indicating metabolic modification by bacteria, and there are speleothemic and speleogenetic features that strongly suggest bacterial origin or influence.
Although there is abundant evidence supporting a hypogenic (sulfuric acid) origin for many caves in the Guadalupe mountains, speleogenetic processes have been dormant for at least 4 my. In the mid-1980s, active sulfur caves were recognized in Mexico, Romania, Italy, Turkmenistan, and the United States (Wyoming). These caves are morphologically and mineralogically similar to Guadalupe Mountain caves, and they contain abundant, sulfur-based microbial populations that support diverse ecosystems in environments without light. These caves provide modern analogies for the ancient caves of the Guadalupe mountains, and show that carbonate dissolution resulting in large cavernous porosity systems can develop independent of surface-related karst processes.