New Mexico Bureau of Mines & Mineral Resources Bulletin 117 Geology of Carlsbad Cavern and other caves in the Guadalupe Mountains, New Mexico and Texas

The most important conclusions of the speleogenesis part of this study are:

(1) Three solution episodes occurred in the Guadalupe Mountains. Solution Stage I caves are an early-stage paleokarst development of probable tectonic and/or solutional origin; these are small fissure caves formed at the contact of the reef-backreef facies, probably during the Late Permian. Solution Stage II caves are enlargements of primary pores and joints in the reef; these are spongework caves phreatically formed in one or more episodes between the Late Permian and the Tertiary. Solution Stage III caves are large passages that represent the final episode of cave development in the Guadalupe Mountains; they formed in the late Pliocene-Pleistocene during uplift and tilting of the Guadalupe Mountains.

(2) Solution Stage III caves dissolved primarily by sulfuric acid. The sulfuric acid was derived from hydrogen-sulfide gas that originated in the oil and gas fields of the Delaware Basin and entered the reef via joints or possibly along the Bell Canyon Formation. Massive gypsum and native sulfur in the caves have σ³⁴S values as low as -25.6 and -20, respectively. These values correspond to σ³⁴S values of H₂S gas and sulfur in the Gypsum Plain, which are known to have been generated by hydrocarbon-related reactions.

(3) The progressive eastward migration of the halite dissolution margin in the Gypsum Plain may have been a factor controlling Solution Stage III cave development in the Guadalupe Mountains. Where halite beds remained intact in the basin, they acted as impermeable barriers preventing hydrogen sulfide from rising to the surface in the basin; instead, the gas rose into the Capitan reef to dissolve out the caves there.

(4) The systematic excavation and integration of Solution Stage III cave passages in the Guadalupe Mountains and the evolution of their unique three-dimensional form are the result of deep phreatic (bathyphreatic) and water-table conditions combined with a sulfuric-acid speleogenesis. Bathyphreatic conditions were responsible for the strong vertical development of Guadalupe caves; water-table conditions were responsible for the horizontal development of caves along certain levels.

(5) The general sequence of deposits in Guadalupe caves as seen in Carlsbad Cavern is: breccia, montmorillonite, spar, calcified siltstone-cave rafts, cobble gravel, endellite, silt and sand, chert, gypsum, breakdown, speleothems, sulfur, bat guano, and animal bones.

(6) The breccia fills Solution Stage I cavities and is encased in a mudstone or spar matrix which, based on carbon- and oxygen-isotope data, is believed to be Late Permian in age. As Solution Stage I caves enlarged along the reef-back reef margin due to tectonic and/or solutional processes, breccia clasts were either sheared in place or gravitated down into Solution Stage I voids.

(7) Montmorillonite clay fills Solution Stage II sponge work caves and is probably an autochthonous residue derived from that solution episode. The montmorillonite reconstituted from limestone residue in a basic (high pH), high-bicarbonate, slow-flow environment. A potassium-argon date of 188 ± 7 my (Early Jurassic) was obtained for the clay; while highly speculative, this date probably indicates that the montmorillonite clay predates the Solution Stage III episode.

(8) U-series dates of >350,000 ybp, an ESR date of 879,000 ± 124,000 ybp, and carbon-oxygen data on the spar suggest that the large Spar III crystals in Guadalupe caves formed in the shallow phreatic zone during the Solution Stage III episode.

(9) The calcified siltstone-cave rafts are interpreted to be the result of early fluctuation phases of water-table development that antedated the final solutional enlargement of Solution Stage III passages. U-series dates, ESR dates, and carbon-oxygen compositions suggest that the rafts of the siltstone-raft sequence formed on top of the water table at the same time as Spar III crystals were being formed below the water table; this occurred during the present erosion cycle, early in the Solution Stage III episode of cave development.

(10) The cobble gravel exposed in Carlsbad Cavern is an allochthonous deposit composed of backreef clasts and is possibly Ogallala (or Gatuña) material. It is a heterogeneous, poorly sorted, crudely stratified, matrix-supported deposit that is interpreted as a debris flow.

(11) The endellite and chert were derived from the reaction of sulfuric acid on montmorillonite during the Solution Stage III episode.

(12) The fine-grained sediment is an autochthonous residue derived from dissolution of the large, Solution Stage III cave passages. In Lower Cave, Carlsbad Cavern, the silt is >730,000 yrs old as determined by paleomagnetic dating.

(13) The massive gypsum blocks and rinds are late-stage, lagoonal-type deposits resulting from hydrodynamic stagnation during the water-table stage of cavern development. Textures and distribution of the gypsum attest to its precipitation, replacement, recrystallization, solidification, and compaction.

(14) The greatest amount of breakdown fell soon after the lowering of the water table. An exception to this rule is Iceberg Rock, Carlsbad Cavern, which fell later, between about 180,000 and 500,000 ybp.

(15) Speleothem deposition has occurred in Guadalupe caves since the caves became air-filled. The great mass of travertine material dates from more humid, pluvial stages of the Pleistocene. Many of the speleothems have become highly corroded due to high levels of CO₂ in the caves.

(16) Native sulfur in the caves is the result of oxidation of H₂S gas in a subaerial environment.

(17) Bat guano and animal bones in the caves attest to the fact that bats and other animals entered the caves of the Guadalupe Mountains as soon as the passages became connected to the surface. The entrance of Carlsbad Cavern may have been open as long ago as 112,000 ybp, as indicated by dates on Nothrotheriops bones in Lower Devil's Den.

(18) The caves of the Guadalupe Mountains are late Pliocene-Pleistocene in age. Dating results suggest that the Big Room level of Carlsbad Cavern is about 800,000 yrs old and the Bat Cave level is about 1.2 my old. Other caves higher in the Guadalupe Mountains may be as old as 3 my.

(19) The caves of the Guadalupe Mountains may be a manifestation of the evolution of intracratonic basins, as are hydrocarbons and Mississippi Valley-type ore deposits.