Abstract

The Guadalupe Mountains of New Mexico and west Texas are a relict erosional feature on the southeastern flank of the Rio Grande Rift, frequently characterized as a northeast-tilted fault block. The mountains are cut by canyons that increase in frequency and topographic relief from east to west, and whose orientations match cave passage orientations throughout the Guadalupes (DuChene and Martinez, 2000). Erosion and mass wasting have exposed more than 300 known caves ranging from systems like Lechuguilla Cave (>161 km) and Carlsbad Caverns (>49 km) in the east, to caves with less than 10 m of passage in the west. Many are hypogenic caves formed by dissolution of carbonate by sulfuric acid (Hill, 1987).

Hypogenic caves in the Guadalupe Mountains formed predominantly below the water table where oxygenated meteoric water mixed with hydrogen sulfide-bearing water within porous and permeable facies of the Capitan Reef Complex (Cunningham and Takahashi, 1992). Mixing of these waters along deep, pressurized, upwardly-curving flow paths formed sulfuric acid that dissolved large passages and galleries at various levels, connected by enlarged near-vertical joints with hundreds of feet of vertical relief. Where joints intersected the surface, flowing springs may have existed (Palmer and Palmer, 2000). Minimum ages for hypogenic caves in the Guadalupe Mountains range from 12.3 Ma to 3.9 Ma from west to east (Polyak, et. al, 1998).

Virgin Cave (11.3 Ma) in Big Canyon 3 mi from the western escarpment of the mountains has an entrance elevation of 6,600 ft, a depth of 722 ft, and is 2,000 ft above the base of the escarpment west of Guadalupe Peak. When this cave was formed, the water table was at least at the level of the cave entrance. This indicates that that the structural and topographic relief between the Guadalupe Mountains and the Salt Basin Graben had not yet formed and the recharge area for the Capitan aquifer extended considerably farther to the west.

Studies of the Rio Grande Rift show that broad, continental scale arching began 35 Ma and was followed by strong tectonic events ~17 Ma and 7.4 - 4 Ma (Eaton, 1986). We suggest that conditions suitable for the development of hypogenic caves existed 35 -17 Ma, and that uplift ~17 Ma enhanced the process by water-washing of oil and gas traps in Permian (Guadalupian) beds (Lindsay, 1998)(Figure 1). Hydrocarbons and hydrogen sulfide were transported into the Capitan Reef Complex to mixing zones where caves were formed. Cave formation migrated east as downfaulting of the Salt Basin graben from 7.4 - 4 Ma, accompanied by erosion in the Delaware basin and Guadalupe Mountains, caused gradual lowering and eastward migration of the water table.