HYPOGENE ORIGIN FOR CAVES IN THE IBERIAN RANGE, SPAIN

Caves in mountainous settings are typically epigene in origin. Characteristics of these types of caves include distinct connection to surface drainage systems, fluvial sediment deposits, branching morphologies, and steep passage gradients. We investigated numerous caves in the Iberian Chain, Zaragoza Province (Spain), a deeply dissected region of folded and faulted Mesozoic rocks. Land elevations range from 750 to 2300 m, and the climate is semi-arid. Known caves are numerous, but relatively short in length. All appear relict with respect to groundwater flow as a genetic agent. From our analysis we conclude that the caves are not of epigene origin. Evidence supporting this includes: entrances that are not integrated with the modern surface drainage system, dead-end passages, and absence of fluvial sediments. Based upon these characteristics, the caves are most suitably classified as hypogene. However, the source of the cave-forming water and the acidifying agent is uncertain. The absence of reaction products (i.e. secondary gypsum) suggests that hydrogen sulfide did not participate. Further, there are not apparent sources for reduced sulfur in the local stratigraphy. Significant post sub-aqueous modification by condensation corrosion has obscured many details of the original cave morphology; 1 meter or more of wall rock and speleothems has been removed by this process in some caves (e.g. Cueva Honda). Evidence of active corrosion is also seen in parts of Cueva del Muerto. Nevertheless, numerous factors support the interpretation that the caves are of hypogene origin, and subsequent rock and fluid studies should clarify the process.