Geology of Carlsbad Cavern and Other Caves in the Guadalupe Mountains, New Mexico and Texas

Hill, Carol

Sulfur-isotope data, whole-rock analyses, and pH-dependence of the mineral endellite support the hypothesis that the large cave passages in the Guadalupe Mountains were dissolved primarily by sulfuric rather than carbonic acid. Floor gypsum deposits up to 10 m high and native sulfur in the caves are significantly enriched in the light isotope of sulfur; σ34S values as low as -25.6 indicate that the cave sulfur and gypsum are the end products of biological oxidation and reduction reactions associated with the oil and gas fields of the Delaware Basin. As the Guadalupe Mountains uplifted and tilted to the northeast during the late Pliocene-Pleistocene, oil and gas moved updip in the basin and reacted with anhydrite at the base of the Castile Formation to form H2S, CO2, and the "castile" limestone masses. Where halite beds in the Castile Formation remained intact, they acted as impermeable barriers preventing the gas from rising to the surface in the basin. Instead, the hydrogen-sulfide gas rose into the Capitan reef along joints or the Bell Canyon Formation and there reacted with meteoric, oxygenated, ground water to form sulfuric acid. The sulfuric acid attacked the limestone and dissolved out the large caves of the Guadalupe Mountains.From oldest to youngest, the general sequence of deposits in Guadalupe caves is: breccia, mont morillonite, spar, calcified siltstone—cave rafts, cobble gravel, endellite, silt, chert, gypsum, breakdown, speleothems, sulfur, bat guano, and animal bones. The breccia fills fissures (Solution Stage I caves) truncated by the large cave passages and is believed to be a Late Permian deposit contemporaneous with sandstone-dike fillings in the Guadalupe Mountains. Montmorillonite fills small spongework cavities in the limestone (Solution Stage II caves) and has a speculative age of 188 ± 7 my (Late Jurassic), as determined by the potassium-argon method. It may be Solution Stage II residue which formed in a basic (pH = 8-9), bicarbonate-rich, slow-flowing aquatic environment some time between the Late Permian and the Tertiary.The large horizontal and vertical passages (Solution Stage III caves) formed in the late Pliocene Pleistocene when uplift and tilting of the Guadalupe Mountains caused H2S and CO2 gas to migrate from the basin into the reef. Uplift of the mountains also initiated a basic pattern of bathyphreatic flow whereby water entering the recharge area took a deep course through the reef bedrock to spring outlets controlled by the position of regional base level. Initial flow was guided by joints, bedding planes, joint intercepts, and facies contacts. Main trunk passages evolved along these routes, and, as discharge springs continually shifted to lower base-level positions, water-table conditions prevailed over bathyphreatic conditions and fast flow was replaced by slow flow.