Geology of Carlsbad Cavern and other caves in the Guadalupe Mountains, New Mexico and Texas
Authors: Hill Carol A. 
Published: March 2023

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; delta 34 S 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 hydrogensulfide 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, montmorillonite, 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.

Just below the descending water table, spar formed in the shallow phreatic zone where slow-flow, barely saturated conditions were responsible for the nucleation and growth of large crystals. Age of spar crystals collected at various levels in Carlsbad Cavern from the entrance down to the Mystery Room (about 200 m elevation difference) all exceed 350,000 yrs (the limit of the U-series dating technique), and spar at the Big Room level has been dated at 879,000 +/- 124,000 yrs by the Electron Spin Resonance (ESR) dating technique. While the spar was forming just below the water table, the rafts of the calcified siltstone-cave raft sequence were forming on the surface of the water table. Carbon-oxygen-isotope data and dates on the rafts confirm that these deposits formed early in the Solution Stage III episode and are not the product of a previous exhumation of the reef. The cobble gravel in Carlsbad Cavern may also be an early Solution Stage III deposit. The cobble gravel either underlies, or is interbedded with, paleomagnetically reversed (greater than 730,000 yrs) silt and is believed to be backreef, possibly Ogallala (or Gatuna) material which gravitated into bathyphreatic cave voids near the beginning of the uplift of the Guadalupe Mountains.

Sulfuric acid dissolved out the large Solution Stage III cave passages, and silt residue released during this dissolution settled directly to the floor. During this sulfuric-acid episode, montmorillonite altered to endellite and chert, and massive gypsum precipitated out on top of the silt during watertable conditions of hydrodynamic stagnation. Laminations, microfolding, slickensides, overgrowth crusts, limestone inclusions, insoluble residue, and recrystallization, replacement, and breccia textures in the gypsum attest to its method of precipitation and solidification. Commode holes, drip tubes, and streamlined surfaces in the gypsum represent dissolution features which formed after the gypsum had consolidated.

Breakdown, speleothems, sulfur, bat guano, and animal bones all deposited in the caves after they had become air-filled. Most of the breakdown fell a short time after the subsidence of the water table, one notable exception being Iceberg Rock, which fell between 180,000-513,000 ybp, some time after the water had subsided from the Main Corridor. Speleothems have been dated at approximately 500,000-600,000 ybp in Lower Cave and in the Main Corridor near Iceberg Rock, showing that these cave passages had become air-filled by that time. Speleothems in the upper, Bat Cave level of Carlsbad Cavern can be older than the speleothems in these lower passages, and speleothems in higher-altitude Guadalupe caves, such as Cottonwood and Virgin, can be older still. Many speleothems in Guadalupe caves are severely corroded due to high levels of CO2 and/or H2S which degassed at the surface of the water table. Degassing H2S, which ascended from the basin along with the CO2, oxidized to native sulfur under air-filled conditions and coated the undersides of bedrock and speleothems. Bat guano and animal bones attest to the fact that bats and animals entered the caves of the Guadalupe Mountains as soon as they had developed entrances. The entrance of Carlsbad Cavern may have been open as long ago as 112,000 ybp, as indicated by dates on sloth (Nothrotheriops) bones from the Lower Devil's Den area.

A variety of carbonate and sulfate speleothems formed in Guadalupe caves from dripping, flowing, seeping, pool, and condensation water. Evaporation and carbon-dioxide loss have been prime factors in the deposition of the magnesium-carbonate minerals hydromagnesite, huntite, and dolomite, and in the formation of speleothems such as moonmilk, popcorn, bell canopies, and tower coral. Most travertine material deposited during wet and humid glacial stages of the Pleistocene, about 350,000- 600,000 and 140,000-170,000 yrs ago. Then, at about 120,000-130,000 yrs ago and again during the last 10,000 yrs, most travertine ceased growing due to a shift from pluvial to arid climatic conditions. Sulfur-isotope data show that sulfate speleothems, where they do occur in Guadalupe caves, have probably derived from pyrite in the overburden.