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

Purpose

The origin of Carlsbad Cavern and other caves in the Guadalupe Mountains has been one of the great unsolved mysteries of speleogenesis. Geomorphically, Guadalupe caves bear little resemblance to other great cave systems of the world. Rooms are huge, yet passages are not long and terminate abruptly. The caves seem unrelated to surface topography or to ground-water-flow routes. Especially enigmatic are the large deposits of gypsum and the colorful waxy clay in the caves.

For over 30 years the prevailing theory has been that Guadalupe caves formed similarly to other caves; that is, by carbonic-acid dissolution at the water table. Within the past 10 years three new theories of origin have been proposed, all of which differ significantly from one another and from the earlier theory. All of these four theories are based mainly on field observations, despite the pertinence of a number of analytical techniques to speleogenesis problems.

This paper is a summary of a lengthy investigation which has utilized observational, stratigraphic, geochemical, dating, and isotopic techniques. The purpose of Part I is to describe cave deposits and discuss speleogenesis events from Permian time to the present.

Previous work

The first geologist to study any Guadalupe cave was Lee (1924a, 1925a, b). His articles were primarily of a popular nature, and it was not until 1949 that Bretz published his now classic Carlsbad Caverns and other caves of the Guadalupe Block, New Mexico. In that paper, Bretz invoked a concept of Davis (1930) to explain cave levels and spongework as phreatic features formed in the zone of saturation. The only exceptions to Bretz's phreatic rule for Guadalupe cave development was a late-stage, gypsum-depositing episode and a brief, silt- and cobble-carrying, vadose-stream episode in Lower Cave of Carlsbad Cavern. Bretz thought that the massive gypsum deposits in Carlsbad Cavern were a consequence of local pooling and that the source of the gypsum was the Castile Formation which lies close to the reef escarpment. The 1.5 m thick gypsum in McKittrick Cave was interpreted by him as a secondary flowstone rather than as correlative in character and origin to the gypsum deposits in Carlsbad Cavern.

Bretz's (1949) interpretation of regional geology hinged strongly on the observations he made in Carlsbad Cavern; together with Horberg (1949b), he postulated three stages of reef modification: (1) a pre-Ogallala exhumation of the reef, during which time the caves in the Guadalupe Mountains formed; (2) filling of the Pecos valley with 400 m of Ogallala alluvium; and (3) a Pleistocene exhumation of the reef, which is still continuing today. Inasmuch as Guadalupe caves are truncated by present-day erosion of the reef and hence are older than the escarpment, and inasmuch as the supposed vadose "stream" in Lower Cave had to exit and descend to a lower base level, Bretz concluded that the caves predated the present erosion cycle. Gale (1957) and Thomas (1971) echoed Bretz's theory of two stages of reef exhumation, and other investigators following Bretz's lead attributed the massive gypsum in the caves to a late-stage back-up of water (Good, 1957; Sanchez, 1964; Bullington, 1968). Black (1954) and Gale (1957) reiterated Bretz's view that the source of the cave gypsum was the Castile Formation in the Delaware Basin.

Nearly 30 years elapsed before the speleogenesis model of Bretz was challenged. Queen (1973) and Queen et al. (1977a, b) introduced an entirely new theory: a speleogenesis model in which gypsum replaced limestone where gypsum-saturated brines in the Delaware Basin mixed with fresh meteoric water in the reef aquifer. When the gypsum was later dissolved by vadose water, cave passages were left void except for a few remnant gypsum deposits on cave floors and walls. Queen et al. (1977a, b) cited replacement textures in the gypsum deposits of Cottonwood Cave as evidence supporting their theory. Queen believed, as had Bretz, that the caves in the Guadalupe Mountains formed in pre-Ogallala time and that there have been at least two exhumations of the reef.

A few years after Queen proposed his startling new theory, Jagnow (1977, 1979) came out with an entirely different one. Jagnow essentially followed Bretz's model of phreatic cave development, but he invoked the notion that sulfuric acid was partly responsible for the dissolution of Guadalupe caves. According to Jagnow, pyrite from the Yates Formation was the source of the sulfuric acid. Acidic solutions moved along bedding planes in the Yates until they reached vertical joints and descended to the Seven Rivers or Capitan Formations where they dissolved out the caves. The reaction between sulfuric acid and limestone produced gypsum as a by-product.

The fourth model of speleogenesis for Guadalupe caves has been advocated by Davis (1979a, b, 1980), whose model is a modification of a theory of speleogenesis first proposed by Egemeier for the caves of the Big Horn Basin, Wyoming. Egemeier (1971, 1973, 1981) termed his theory of speleogenesis "replacement-solution." According to this theory, sulfide-bearing water ascends via thermal springs to base level, where hydrogen sulfide reacts with oxygen in the cave air to form sulfuric acid. The acid attacks the limestone, which is then directly replaced by a thin crust of gypsum. Davis' modification of Egemeier's theory stressed cavern dissolution along upwelling limbs of deeply curving flow paths, and the pits underlying large cave rooms were regarded by him as input points for ascending water. The source of the sulfuric acid necessary for "replacement-solution" was ascribed by Davis to the oil and gas fields of the Delaware Basin and his claim was supported by the finding of Hinds and Cunningham (1970, p. 7) that "hydrogen sulfide is commonly found in formation waters throughout most of Eddy Co."