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

Compared to carbonate minerals, sulfate minerals are rare in Guadalupe caves. Cottonwood Cave is the only cave that contains abundant sulfate speleothems. Secondary sulfate minerals identified in Guadalupe caves are gypsum (CaSO₄ · 2H₂O), epsomite (MgSO₄ · 7H₂O), mirabilite (Na₂SO₄ · 10H₂O), and celestite (SrSO₄). All four occur in Carlsbad Cavern; gypsum and epsomite occur in Cottonwood Cave, and gypsum in Hell Below Cave, Wind (Hicks) Cave, and Spider Cave.

Deposition

The origin of sulfate minerals is not as well understood as that of carbonate minerals. Vadose seepage is the mechanism by which the sulfate ion enters the caves and evaporation is the cause of precipitation, but the ultimate source of the sulfate ion is still a matter of speculation. Three possible sources are: (1) the massive gypsum blocks and rinds in the caves, (2) evaporite beds of the Ochoan Series which might have once existed above the caves, and (3) pyrite in the overburden. Before sulfur-isotope data had been obtained on some of the sulfate speleothems, the first source was considered to be the most likely because of the spatial correlation of some of the secondary-gypsum mineralization relative to some of the gypsum blocks. Most of the gypsum and epsomite speleothems in the lower Gypsum Passage of Cottonwood Cave occur below a number of partially dissolved gypsum blocks residing in the upper Gypsum Passage. Gypsum stalagmites in Lower Cave, Carlsbad Cavern, are located below a number of gypsum blocks encountered on the climb up toward Mabel's Room and the Talcum Passage. In the New Mexico Room of Carlsbad Cavern, gypsum flowers and crust in the East Annex occur below gypsum blocks in the Balcony.

The isotope data do not support a genetic connection between the massive gypsum and the sulfate speleothems. The sulfate flowers and crusts in the East Annex of the New Mexico Room have σ³⁴S values of +9.2 and +8.7, respectively, while the gypsum blocks in the Balcony overlying the East Annex have a σ³⁴S value of -25.6 (Table 22). Similarly, the gypsum stalagmites in Lower Cave have a σ³⁴S value of +7.3, while the gypsum blocks on the climb up to Mabel's Room have a value of -17.8. This difference in isotopic composition shows that the two types of deposits are genetically unrelated.

The second possibility is that the sulfate mineralization in Guadalupe caves has derived from evaporite deposits that may have once existed in the overburden (i.e. the Salado and Rustler Formations; Fig. 4). Smith (1978b, fig. 3) showed the Salado Formation overlying the Yates and Tansill Formations in the vicinity of the McKittrick Hill caves; however, it is not known if the Salado and Rustler Formations ever extended over the Tansill Formation in the vicinity of Carlsbad Cavern. The Castile Formation in the basin has an average σ³⁴S value of +10.3 (Table 23), and sulfate beds within the overlying Salado and Rustler Formations may have similar isotopic compositions. This value correlates closely with the σ³⁴S values for the sulfate speleothems in Carlsbad Cavern, but not with the Cottonwood Cave sulfate speleothem (σ³⁴S=-0.8) (Table 22).

The most likely source of the sulfate ion for sulfate speleothems in Guadalupe caves is pyrite disseminated in back reef (mainly Yates Formation) beds. When pyrite weathers to limonite, it produces the SO₄^2- ion in solution which, when combined with calcium and magnesium derived from limestone and dolomite, forms gypsum and epsomite, respectively. Sulfate mineralization is most pronounced in the northwest parts of Carlsbad Cavern (i.e. along the north wall of the New Mexico Room, the northwest part of the Mystery Room, and near the Cable Slot in Lower Cave; Fig. 95), a relationship which suggests that the sulfate may have derived from Northwest Shelf beds. Cottonwood Cave contains more secondary gypsum and epsomite mineralization than any other Guadalupe cave, a fact that may relate to its stratigraphic position directly underlying the pyritic and dolomitic Yates Formation. The one isotope measurement done on a piece of secondary gypsum crust collected in Cottonwood Cave was -0.8; this compares with a σ³⁴S value of -2.5 for a piece of pyrite collected on Guadalupe Ridge near Cottonwood Cave (Table 22).

Stability

Sulfate minerals have a high solubility and for this reason are usually found in dry cave passages. Gypsum is stable when exposed to the air at ordinary cave temperatures and humidities, but the more soluble magnesium and sodium sulfates, epsomite and mirabilite, are unstable. Above a certain humidity, these two minerals absorb moisture from the air and dissolve. Even breathing on these minerals or bringing one's carbide lamp too close to them causes epsomite and mirabilite to melt. When the humidity of a cave rises in the summer months, as it does in Carlsbad Cavern (Fig. 21), epsomite and mirabilite will absorb moisture from the air, dissolve in that moisture, and remain dissolved within bedrock and silt. When the humidity lowers in the winter months, epsomite and mirabilite "sprout up" as efflorescences out of the silt, or deposit as dripstone. Due to their very soluble and deliquescent nature, epsomite and mirabilite also experience a very rapid growth and decline, disappearing or appearing in only a few weeks.

The growth of epsomite with respect to humidity was monitored from March 1985 to June 1985 in Lower Cave, Carlsbad Cavern, near the epsomite cotton (Fig. 95). Hydrothermograph measurements showed that the epsomite was in the crystallized state in March when the humidity and temperature were 87-88% and 12.2°C respectively, but was in the deliquescent (dissolved) state in June when the humidity and temperature were 88-89% and 12.2°C respectively. Apparently, 88% is the critical humidity at which the partial pressure of the water vapor in the air exceeds the vapor pressure of a saturated solution of epsomite at this temperature.

No similar hydrothermograph measurements were obtained for the mirabilite cotton in the Lunch Room of Carlsbad Cavern, but in April 1984 the mirabilite was observed to be in the process of disappearing, which may approximately correlate with the April 1969 humidity measurement of 87% made in the Lunch Room by McLean (1971) (Fig. 21).

Speleothems

Sulfate speleothems in Guadalupe caves vary in type depending on whether they have deposited from dripping water or seeping water, and whether they have formed on bedrock or sediment. Various types of sulfate speleothems observed in Guadalupe caves are: stalactites, stalagmites, helictites, popcorn, crusts, cotton, flowers, rope, hair, and selenite needles. The stalactites and stalagmites have deposited from dripping water, whereas the other types have deposited from seeping water. Sulfate solutions may seep out of cave walls to form crusts, flowers, or rope, or may seep up through cave sediment to form cotton or selenite needles. Sulfate speleothems are discussed in alphabetical order. As in the discussion of carbonate minerals, the classification system of Hill and Forti (1986) is followed.

Coralloids

In Lower Cave, Carlsbad Cavern, small (1 cm or less) nodules of white gypsum popcorn directly overlie more opaque knobs of carbonate popcorn. The gypsum covers the very tips of carbonate fronds and is more translucent than the calcite it overlies.

Gypsum popcorn is believed to deposit by evaporation from slowly moving films of water (instead of by carbon-dioxide loss). Carbonate popcorn is known to form partially by evaporation in Carlsbad Cavern, as can be seen by its position on the evaporation slope of Fig. 98, and this fact may also relate to gypsum-popcorn growth in the cave. If incoming solutions contain both carbonate and sulfate ions, then, upon evaporation, the sequence of deposition will be carbonate popcorn and sulfate popcorn as solutions evaporate to dryness.

Coral pipes

Gypsum coral pipes form in the same manner as calcite ones, except that soft gypsum takes the place of soft silt or guano in their genesis. Dripping water drills drip tubes into soft massive gypsum, and an outer, harder layer of more crystalline gypsum coats the sides of the tubes, protecting them from further erosion.

Good examples of gypsum coral pipes in Guadalupe caves are those near the Top of the Cross in the Big Room of Carlsbad Cavern. These are up to 1 m deep and centimeters in width, and consist of tabular gypsum crystals overlying a softer core of massive alabaster gypsum.

Crusts

Gypsum crusts line portions of wall in the lower Gypsum Passage, Cottonwood Cave (White, 1960), or they may occur as thin coatings over carbonate speleothems in parts of that passage (Hill, 1977a). The gypsum crust is massive granular and in some cases tabular, the tablets being up to 5 cm long in the area of the large gypsum stalactite known as the Chandelier. Gypsum crust can also be found with small gypsum flowers in Spider Cave; with selenite needles in the climb up to Mabel's Room and on the north wall of the Main Corridor, Carlsbad Cavern; and with gypsum stalactites in Dry Cave (Lindsley and Lindsley, 1978). A thin crust of gypsum also coats the surface of the large stalagmite known as the Christmas Tree in New Cave.

Celestite crusts have been found associated with gypsum crusts in Pickle Alley, off Left Hand Tunnel near the Lunch Room, Carlsbad Cavern. The crusts occur on the ceiling in two separate patches, one about 2 m² and the other about 0.5 m². The celestite crystals are tabular, grayish-blue, transparent, and up to 5 mm long.

Sulfate crusts deposit from solutions which seep uniformly out of bedrock or preferentially out along joints and seams in the bedrock. Uneven growth results in buckled or blistered crusts. "Tabular gypsum" refers to crusts where tabular crystals have grown to a macroscopic size.

Fibrous sulfates

Sulfate minerals often assume a fibrous habit that resembles cotton, hair, or rope. Fibrous sulfates grow as efflorescences on cave silt or bedrock. As solutions evaporate, crystals precipitate out as fibers which correspond to pore spaces within the silt or bedrock. Continued precipitation causes new growth to push old growth outward from the silt or away from the wall.

Hair—Cave hair is composed of thin, single-fiber strands of sulfate material. Gypsum hair has been observed in the Nailhead Spar Lining Room, Carlsbad Cavern, where it occurs as 5 cm long strands on ceilings and walls.

Cotton—Cave cotton consists of crystal fibers 1 cm or so long, which are intergrown and matted together. The cotton effloresces from cave dirt. As sulfate solutions evaporate at the surface of the dirt, crystals precipitate out as a fuzzy growth mat. Tufted cotton, composed of both gypsum and epsomite, can be seen in Cottonwood Cave (White, 1960; Hill, 1977a). The epsomite cotton fills wall recesses in the winter months, but the masses disappear in the summer months. A. Komensky (pers. comm. 1971) observed fibrous crystals of epsomite on the floor of the lower Gypsum Passage, Cottonwood Cave, which "blossomed" into a full growth of 25-35 cm in about a week's time. In Carlsbad Cavern "filamental hairy stuff" has been reported growing along the trail in the Amphitheatre region, the "stuff" being epsomite (report to the park from Edwin C. Alberts, Park Naturalist, 1945). Bitter-tasting epsomite also appears as cottony fibers on the floor of the Big Room and Lower Cave during the winter, most conspicuously in Lower Cave near the climb up to Mabel's Room (Hill, 1973b; Pl. 15A) and in the little crawlway at the top of the ladder going down into Lower Cave (N. Bullington, pers. comm. 1971). Salty-tasting, clear-white, fluffy mirabilite cotton up to 15 cm high forms during the winter months in the Lunch Room on the right side of the trail just before reaching the elevators (Hill et al., 1972, 1972b).

Rope—Cave rope is composed of bundles of parallel mineral fibers which hang down from ceilings or overhanging ledges and have the appearances of unspun rope flax. One gypsum rope 3-4 m long and 2.5 cm in diameter used to spiral down from a limestone ledge in the lower Gypsum Passage of Cottonwood Cave (J. Trout, pers. comm. 1982; Fig. 129a). This extremely rare speleothem was vandalized soon after the lower Gypsum Passage was discovered in 1960 (Fig. 129b), and since that time there has been no new growth.

FIGURE 129—Before (a) and after (b) pictures of gypsum rope and needles, lower Gypsum Passage, Cottonwood Cave. In (a) the gypsum rope is 3-4 m long and the selenite needles are up to 1.8 m long. Photo of the same location (b) after the rope and most of the needles were vandalized. Photos Jerry Trout.

Flowers, Cave

Cave flowers are a type of speleothem which have "petals" that radiate away from a common center, the petals being composed of branching and curving bundles of acicular crystals loosely bound together in a polycrystalline matrix. Cave flowers grow from the base, often transporting rock away from the wall with them as they grow. As sulfate solutions seep out from the wall, they crystallize at the air-wall boundary; subsequent crystallization pushes older flower growth out from the wall. If growth is somewhat more rapid on one side of the flower than on the other, a well-proportioned curve is the result.

When the Gypsum Passage of Cottonwood Cave was discovered in 1960, gypsum flowers were present all along the right wall of the passage. Since that time almost all of these flowers have been vandalized, with the exception of a few exquisite flowers up to 10 cm long remaining in a small room beneath floor breakdown (Hill, 1977a). A few epsomite flowers exist in the lower Gypsum Passage, just past the large gypsum stalactite known as the Chandelier, and also along the right-hand wall.

Carlsbad Cavern has gypsum flowers, the best of which occur in the Sand Pit area of the New Mexico Room (R. Kerbo, pers. comm. 1985). Also, a few small gypsum flowers occur in Lower Cave near the climb up to Mabel's Room, along the sides of the Bottomless Pit, Big Room, in the lower part of the Mystery Room, and in the Remarkable Crack, New Section. In Spider Cave the longest gypsum flower observed reaches 6 cm in length, and in the Gyp Joint, Hell Below Cave, gypsum flowers up to 15 cm long directly overlie gypsum rinds (Hill, 1983).

Helictites

One epsomite helictite 1.5 cm long and 0.5 cm in diameter has been observed on an epsomite soda straw in the Epsomite Room, lower Gypsum Passage, Cottonwood Cave. This is the only known occurrence of this speleothem type in a Guadalupe cave (Hill, 1976a, 1977a). Sulfate helictites are believed to form similarly to carbonate helictites, i.e. by hydrostatic pressure which forces solutions through a tiny capillary tube in the center of the helictite to its growth tip.

Needles, Selenite

Selenite needles are swallow-tail twins of gypsum which grow upward from cave silt, or, more rarely, outward from cave walls. Selenite needles grow from the base. Sulfate solutions move up through the silt to the base of the needle and new crystal growth pushes old growth up and out of the silt.

Stubby subhedral needles of selenite up to 2 cm long have been observed in the silt of Wind (Hicks) Cave (Hill, 1976c). and euhedral selenite needles up to 25 cm long occur in the area under the second bridge, Left Hand Tunnel, Carlsbad Cavern. The best selenite needles known to occur in any Guadalupe cave are in Cottonwood Cave (Fig. 129a). When the lower Gypsum Passage of Cottonwood Cave was discovered in 1960, wall needles up to 1.8 m long were found; free-hanging, they bent toward the floor, and upon touching the floor carved zig-zag paths in the silt (J. Trout, pers. comm. 1982). Since that time most of these needles have been vandalized (Fig. 129b).

Rims

A gypsum rim is analogous to a carbonate rim. It is a shell or projection of material smooth and scoured on the inside and rough on the outside, which surrounds an aperature in the cave wall or floor. Gypsum rims surround commode holes, a dissolution feature in massive gypsum, and these forms have been found in Carlsbad Cavern, Endless Cave, Cottonwood Cave, Hell Below Cave, and Spider Cave.

The best developed gypsum rim occurs in Endless Cave; this is "the Commode," a rim 1 m high and 6 cm thick at the base which surrounds a hole over 2 m deep in the floor (Fig. 58). Other well developed rims are the "lace" rims in Lechuguilla Cave. These are thin rims partially dissolved so as to display a lacy texture (Hill, 1986).

The origin of gypsum rims and carbonate rims is similar except that evaporation alone is responsible for precipitation of the gypsum. Moist air moving up through a commode hole in a gypsum block condenses out on the massive gypsum, partly dissolving and scouring the inside of the hole. Where this condensation water containing dissolved calcium sulfate meets with the dry air of a large room, evaporation causes the precipitation of a sulfate rim around the lip of the hole.

Stalactites

Gypsum stalactites can be observed in Cottonwood Cave, Carlsbad Cavern, and Dry Cave. These speleothems are faceted on their lower ends, the facets or blades extending downward and outward like claws. The largest gypsum stalactite known to occur in any Guadalupe cave is the Chandelier in Cottonwood Cave. When discovered in 1960, the Chandelier was 4-5 m long and extended all the way down to the passage floor, but since that time vandals have chipped away at the stalactite so that it is now only about 2-3 m long. Smaller gypsum stalactites occur in the Big Room of Carlsbad Cavern (along the trail near the Salt Flats) and in the Balcony of the New Mexico Room going toward the Rim Room. In the New Mexico Room, they have well-developed bladed ends, like the Chandelier. Small stalactitic gypsum adorns the bottom parts of gypsum blocks at the Jumping Off Place, Big Room, and Upper Devil's Den, Carlsbad Cavern.

Epsomite stalactites up to 0.8 m long and 20 cm in diameter occur in the Epsomite Room of the lower Gypsum Passage, Cottonwood Cave (Pl. 15B). These are smooth, colorless, and transparent, very similar in appearance to icicles (White, 1960; Russell, 1961; Frost, 1971). Epsomite stalactites, like their gypsum and carbonate counterparts, begin their growth as soda straws. Epsomite stalactites are very clear and transparent when first deposited, but they get more opaque when solutions cease to moisten their sides.

Stalagmites

The Chandelier stalactite with its many "claw" arms has its counterpart gypsum stalagmites, but these are comparatively small (only a few centimeters high) and have been partially covered over by silt kicked on them from the nearby path. Other, warty-looking gypsum stalagmites occur along the path just beyond the Chandelier, but these seem to be without counterpart stalactites (Hill, 1977a). The largest sulfate stalagmite in the lower Gypsum Passage is an epsomite one, 2.1 m tall and 0.9 m wide at its base. It is in part transparent and in part opaque, the transparent growth presumably having formed later than the opaque growth.

In Lower Cave, Carlsbad Cavern, near the climb up to Mabel's Room, is a cluster of four gypsum stalagmites 0.3-0.7 m tall that have drip tubes down their centers. The stalagmites have no counterpart stalactites, but directly above the stalagmites are gypsum crystals overlying carbonate popcorn on the ceiling. Gypsum stalagmites up to 1.3 m tall have also been observed in the Lower Guadalupe Room of Carlsbad Cavern (C. M. Seanor, pers. comm. 1984).