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 sequence of events in Guadalupe caves is based on the stratigraphic relationships of cave deposits, on the dating results, and on the carbon-oxygen-isotope paleoenvironmental data. The proposed sequence of deposits and events (numbered 1 through 19) is presented in graphic form in Fig. 83 and in tabular form in Table 29.

FIGURE 83—Sequence of geologic events in Carlsbad Cavern. (click on image for a PDF version)

TABLE 29—Sequence of geologic events in Carlsbad Cavern.

Because the development of Guadalupe caves was both multi-level and multi-sequential, each numbered deposit/event represents the optimum time for the occurrence of that deposit/event. Some events overlap in time (e.g. Solution Stage III cave development and the formation of the endellite) and others occurred continuously over an extended period of time (e.g. speleothems have formed since the caves became air-filled till the present). Or, a deposit could have formed at a lower level (e.g. spar), while at the same time another deposit could have formed at a higher level (e.g. the cave rafts of the siltstone-raft sequence).

Permian deposits and events

Deposition of bedrock

The relatively pure limestone of the reef facies and the less pure dolomites of the backreef and forereef facies were laid down and diagenetically altered in Guadalupian time, a compositional arrangement which would later confine cave development mainly to the residue-free, calcitic reef core. The forereef inclined away from the reef at approximately 20-30° a depositional slope which may have later defined the slope of some Guadalupe cave passages (e.g. the Main Corridor, Guadalupe Room, Mystery Room, and Lake of the Clouds Passage, Carlsbad Cavern, all of which descend at angles of 20-30°). The backreef facies dipped gently (a few degrees) towards the reef, and this slope was later accentuated by post-Permian uplift and was responsible for directing cave-forming ground water into the limestone-reef core.

In Guadalupian time, the reef was probably emergent during periodic drops in sea level. The fresh-water, carbon-oxygen signature of Spar I occurring with marine-cement intergrowths is thought to represent successive episodes of a single diagenetic event during this time (Given and Lohmann, 1986; Fig. 73).

Solution Stage I

The earliest caves are small, often fissure-like cavities filled with breccia, They are located exclusively at or near the contact of reef-backreef sediments (Fig. 30), suggesting formation in response to a lithologic zone of instability between the Capitan reef core and the backreef shelf members of the Artesia Group, perhaps when the shelf facies lithified, compacted, and pulled away from the reef core,

Rarely, individual clasts of mudstone and limestone can be seen exposed in the caves, not as cavity fillings but incorporated into the limestone itself as if they had settled in place before, or contemporaneously with, the solidification of the bedrock. For this reason nearby Solution Stage I cavities filled with similar breccia clasts are believed to date from a very early age—probably from the Late Permian, very soon after the limestone itself had deposited and solidified. Epeirogenic uplift later in the Permian caused the entire region to tilt and rise slightly above sea level; this uplift caused the first flushing of the Capitan Limestone. Fissures which developed along the zone of weakness between reef and backreef sediments thus became the first avenues for water movement and solutional enlargement, and were to influence the position of later cave development from the Permian to the present.

Origin of breccia

Solution Stage I cavities are filled with breccia cemented in a mudstone or calcite-spar matrix. The breccia fragments are angular and many of them appear to have been sheared in place (Fig. 67), suggesting origin at, or close to, the fissures they occupy. The fine-grained mudstone matrix also seems to have been derived locally, as it is composed of the same, although finer-grained, material as the larger breccia clasts (Queen, 1981). The breccia strongly resembles the dolomite breccia facies of Achauer (1969), a local facies found at or near the contact between reef and backreef beds (compare Pl. 1A with Achauer's fig. 10). Achauer, and also Queen (1981), ascribed the fissure-filling breccia clasts to tectonic movement or to solutional collapse just subsequent to the deposition of the limestone. Tectonic activity associated with the uplift and tilting of the reef in the Late Permian may have brecciated the limestone along fissure zones at the shelf-reef margin. As the fissures enlarged by either tectonic or solutional processes, the breccia clasts gravitated downward and filled the voids. The Type I sandstone dikes of probable Permian age may have also formed in response to this tectonic, fissure-filling event.

In Virgin Cave, in the Spar Room of the Secondary Stream Passage, Carlsbad Cavern, and in some breccia exposures in the Guadalupe Room, Carlsbad Cavern, a crystalline spar matrix (Spar II) fills the space between breccia clasts. According to Given and Lohmann (1986), Spar II represents a fresh-water deposit related to a presumed Ochoan (or younger) regional meteoric or shallow-phreatic burial system (Fig. 73). This fresh-water, shallow-burial origin fits with Achauer's and Queen's models of tectonic activity and uplift in the Late Permian. It also fits with the temperature of deposition for Spar II (e.g. 55.4°C), as calculated from the fluid-inclusion data.

Permian-Tertiary deposits and events

Solution Stage II

The second episode of cave development involved the enlargement of primary pores and joints in the massive Capitan Limestone into a three-dimensional maze of spongework. Solution Stage II caves are small, randomly orientated, and many of them are partly filled with either large spar crystals or colorful montmorillonite-endellite clay.

In the earliest stages of development of a cave system, there is a complex, three-dimensional array of pores and unsolutioned joints of minimal cross-sectional area in the rock. The pores and joints are not necessarily integrated, so that flow under these "pre-cave" conditions is diffuse, with phreatic water under pressure creating a spongework array of passages. As solution continues over time, this array expands, and there is a progressive integration of cave passages and enlargement of small portions of the array to create a cave system that eventually becomes continuous from input to output (Ford and Ewers, 1978). Solution Stage II caves represent such a development in the Guadalupe Mountains. They dissolved under conditions of complete waterfill and are phreatic passages created by slow-flow, diffuse circulation of aquifer water.

Solution Stage II caves in the Guadalupe Mountains may have had an origin similar to those now forming in the San Andres Limestone aquifer of the Roswell, New Mexico, area. The San Andres Formation, a cavernous limestone full of holes and fissures, was described by Fiedler and Nye (1933) as possessing two types of solutional cavities: (1) solution channels along joints, bedding planes, and fractures, and (2) enlargement of primary pores so as to produce a honeycombed, "worm-eaten" network of interconnected passages. In the Roswell Basin, spongework cavities have been found down to a depth of 297 m below the surface (Lee, 1924b).

Solution Stage II caves were most likely dissolved exclusively by carbonic acid, as is typical of most cave systems, and not by sulfuric acid as has been proposed for the large (Solution Stage III) cave passages in the Guadalupe Mountains. Firstly, Solution Stage II caves are not large, as might be expected from rapid dissolution by sulfuric acid. Secondly, gypsum deposits are rarely associated with sponge work except where these cavities have been noticeably enlarged by Solution Stage III acids into boneyard. And thirdly, Solution Stage II spongework is sometimes filled with grayish-green montmorillonite clay, a mineral derived from limestone residue which characteristically forms under a high pH (basic) rather than low pH (acidic) conditions.

The age of Solution Stage II caves is debatable. They predate the Solution Stage III episode because: (1) they and the deposits filling them (montmorillonite and spar) are truncated by the large caves; (2) the spar filling Solution Stage II caves has been etched, pitted, and dissolved with respect to its position relative to the large passages; and (3) the spar at the Big Room level dates from 879,000±124,000 yrs, which means that the Solution Stage II cavities (which the spar fills) must be older. There is evidence to suggest that at least some Solution Stage II cavities may be Late Permian in age. A few dogtooth spar crystals filling Solution Stage II cavities (e.g. sample 1, Fig. 73) display carbon-oxygen-isotope signatures characteristic of Spar II. Since Spar II is believed to be Ochoan (or younger) in age (Given and Lohmann, 1986), the Solution Stage II cavities which the spar fills may be as old as Late Permian.

Solution Stage II spongework cavities may have dissolved in one episode, or, more likely, they dissolved in a number of minor episodes during limestone mesogenesis. Bachman (1980) proposed that pre-Cenozoic times of non-deposition in the basin (mainly in the Jurassic) must have been times of extensive erosion and subsurface dissolution through circulation of meteoric water; the 188 my Jurassic K-Ar date on the montmorillonite clay (which fills a Solution Stage II cavity) supports this idea. According to K. Given (pers. comm. 1986), spar material in the Guadalupe Mountains can display luminescent Spar II centers, but be surrounded by non-luminescent overgrowths of later (Spar III?) material of different isotopic composition. Thus, it appears that Solution Stage II cavities, and the spar filling those cavities, may have a history of continued development extending from the Late Permian to the Tertiary.

Origin of montmorillonite clay

Montmorillonite clay fills spongework (Solution Stage II) cavities. In Carlsbad Cavern this clay is found most abundantly in the lower sections, such as in Lower Cave and the Scenic Rooms. Bjorklund and Motts (1959) reported deep caverns filled with red silt in test wells of the Carlsbad region and speculated that this silt had percolated down from above through open conduits in the limestone. White and White (1968) theorized that horizontal movement of silt through small openings in the limestone is minor because flow velocities are too low to exceed the threshold of transport. Lower paths of water movement are especially prone to low-velocity flow and, therefore, deeper paths tend to fill with silt, a pattern further enhanced by normal gravity settling.

Montmorillonite is a common weathering product of limestone, forming very slowly from limestone residue by the mixing of dilute solutions or colloidal suspensions of aluminosilicates at pH's in the range of 8-9 (Krauskopf, 1967). Four conditions seem to favor the formation of montmorillonite: (1) a basic (high pH) environment, (2) very slow-flow rates, (3) a long period of time for reconstitution, and (4) a relatively high HCO₃ concentration (>100 ppm) (Berner, 1971). Under slow-flow conditions, a reaction of cations with Al(OH)₃ and silica can take place, resulting in the formation of montmorillonite.

The potassium-argon date of 188 ± 7 my for the montmorillonite clay is highly speculative; nevertheless, it still indicates that the clay has a high radiogenic-argon content and suggests that the deposit is very old. Montmorillonite is a mineral which takes a long time to reconstitute from limestone residue. This residue had to have changed to the mineral montmorillonite before the Solution Stage III event, in order for the montmorillonite to change to the mineral endellite during the Solution Stage III event. The montmorillonite could not have formed during the Solution Stage III sulfuric-acid event because of the high pH (basic), carbonate-rich environment required for its formation.

All of these factors suggest that the montmorillonite pre-dates the Solution Stage III episode of cavern dissolution. After the interconnected spongework (Solution Stage II) passages had formed, silt residue released from the dissolution of limestone filtered down through available path ways. The residual silt partially filled interconnected spongework pockets and then remained in a basic bicarbonate environment for a long time, slowly reconstituting into montmorillonite clay.

Pliocene-Pleistocene deposits and events

Solution Stage III

Past theories of speleogenesis have rarely related the caves of the Guadalupe Mountains to regional geology. Davis (1979a) was the first to suggest the possibility of a connection between Guadalupe caves and the oil and gas fields of the Delaware Basin. This study carries the possibility one step further: it attempts to identify the connection and proposes that sulfuric acid derived from oil and gas was responsible for dissolving out the large (Solution Stage III) cave passages.

Sulfur-isotope values of the cave gypsum and sulfur are the primary evidence on which this sulfuric-acid model of speleogenesis is based. Only biologically-produced reduction and oxidation reactions such as have occurred in the basin could have produced the large isotopic fractionations that characterize the cave deposits. Other evidence in the caves for a sulfuric-acid origin is the endellite, a type of clay that is known to form only in an acidic environment.

Relationship of Solution Stage III caves to oil and gas fields in Delaware Basin—The Delaware Basin of southeastern New Mexico is one of the largest oil and gas producing areas in the world. Characteristic of the natural gas of the region is a high content of hydrogen sulfide and carbon dioxide. Lambert (1978) reported as much as 55% carbon dioxide and 28% hydrogen sulfide from boreholes in the Castile Formation.

When the Guadalupe Mountains uplifted during the Pliocene-Pleistocene, the oil-producing Bell Canyon Formation in the basin was tilted a few degrees to the northeast. A primary consequence of this tilting was the initiation of oil and gas migration updip through interconnecting permeable zones. Graben-boundary faults and joints at or near the base of the Castile Formation allowed hydrocarbons to ascend from the underlying Bell Canyon Formation into overlying Castile anhydrite beds, where they reacted to form hydrogen sulfide, carbon dioxide, and the limestone of the castile buttes (Fig. 84). The following reactions are believed to have produced the hydrogen-sulfide and carbon-dioxide gas in the basin:

CaSO₄ + CH₄ = H₂S + CaCO₃ + H₂O (4)

2H⁺ + SO₄^2- + CH₄ = H₂S + CO₂ + 2H₂O (5)

FIGURE 84—Location of the castile buttes underground and in relationship to the Bell Canyon Formation, Gypsum Plain. After Smith (1978a).

These simple reactions should be regarded only as symbolic of the many biochemically related reactions that could have taken place in the basin with respect to oil and gas. Hydrogen-sulfide gas and sulfur mineralization originating in the castile buttes of the basin are significantly enriched in ³²S (Fig. 72); such a pronounced isotopic fractionation of sulfur is known to be caused in nature only by the sulfur bacteria.

Sulfuric acid formed where oxygenated meteoric water mixed with the hydrogen-sulfide gas of the basin (Kirkland and Evans, 1976). The acid immediately reacted with the calcium carbonate of the castiles and dissolved out tubular shafts in the buttes (see the two locations marked "cave" in Fig. 85). Kirkland and Evans (1976), and also Smith (1978a), reported a cylindrical, vertical cavern about 3-4.5 m in diameter and about 30 m deep in one castile butte, which is lined with a 10 cm thick layer of crystalline sulfur and has hydrogen-sulfide gas still issuing from it. The crystalline sulfur formed in cavernous parts of the buttes wherever hydrogen sulfide mixed with oxygen.

FIGURE 85—Location of Carlsbad Cavern in relation to oil, sulfur, and the castile buttes (circular black dots) of the Gypsum Plain. The straight line is an extension of the trend of the Big Room (N 15° W). Dashed and solid line is the west margin of Halite I beds. Caves in the Gypsum Plain are tubular and have degassing H2S and native sulfur. (click on image for a PDF version

Hydrogen-sulfide and carbon-dioxide gas rose to the surface in the basin only where the rock was not capped by impermeable halite or clay beds. Halite is among the least permeable of any rock type, plastic flow of the salt closing up any fractures which might develop in the rock (Swenson, 1974). Sulfur exploration companies find H₂S gas below intact halite beds in the basin, but no native sulfur; they find sulfur only along the western erosion edge of Halite I beds where hydrogen sulfide has been oxidized (Fig. 85). Native sulfur is not found far west of the Halite I erosion edge because where ground water has had access to the subsurface over a long period of time, the sulfur has oxidized to sulfuric acid and sulfate; all that is left of past sulfur zones are blanket-solution breccias with textures that suggest the former presence of sulfur.

It is proposed that the progressive eastward migration of the halite dissolution margin over time could have been a factor controlling the position of cave development in the adjacent reef (Fig. 86). Where H₂S gas moved updip to the western erosion edge of the halite beds, it was oxidized to native sulfur. But where it was trapped below the halite, it sought other avenues of escape out of the system. Northwest-trending joints could have been avenues of gas ascent for such caves as Cottonwood and Hell Below in Black and Gunsight Canyons as demonstrated by the northwest clustering of the caves in these canyons (Fig. 5). Extrapolated into the basin, the N15°W trend of the Big Room, Carlsbad Cavern, passes near a dense concentration of exposed castile buttes (some with sulfuric-acid-derived caves), oil deposits, sulfur deposits, and the western limit of the Halite I beds (Fig. 85). Another avenue along which gas could have travelled from the basin into the reef was along interfingerings of the Bell Canyon Formation (Fig. 87). This scenario may especially apply to the Big Room and New Mexico Room of Carlsbad Cavern, where the Bell Canyon Formation is believed to crop out in the cave. One other possible control for cave development in the case of Carlsbad Cavern is the Huapache monocline. The monocline in the basin is located between Rattlesnake Canyon and Carlsbad Cavern (Fig. 12); the basal part of this fold may have been a structural influence on siphoning gas into the reef in the vicinity of Carlsbad Cavern.

FIGURE 86—Model of Halite I unit in the Castile Formation of the Gypsum Plain in relation to the development of caves in the Guadalupe Mountains. (A) Rivers meandered across a low-lying erosion surface in the Miocene-Pliocene. (B) In the late Pliocene-Pleistocene the Guadalupe Mountains area uplifted and tilted. Oil and gas moved updip in the basin and reacted with the Castile anhydrite to form H2S, CO2. and the castile masses underground. Impermeable halite beds trapped the gas in the subsurface so that it either escaped at the western edge of the halite beds or ascended into the reef along joints or interfingerings of the Bell Canyon Formation. As the halite beds in the basin were eroded from west to east, and as spring positions continually shifted from west to east, caves in the Guadalupe Mountains also developed from west to east. As soon as the western limit of the halite beds moved east, past a particular cave location, development of that cave ceased because gas could no longer ascend into the reef at that point. (C) Native sulfur is today found along the western limit of Halite I beds, but never east of this limit below halite beds, or far west of this limit in solution-breccia deposits. H2S gas is found in the subsurface where halite beds are still intact.

FIGURE 87—Model of gas ascension from the basin into the reef along the Bell Canyon Formation. Natural gas migrated updip from the oil fields to the east and encountered anhydrite at the base of the Castile Formation. Reactions between the gas and the anhydrite produced hydrogen sulfide, carbon dioxide, and the castile limestone masses. The hydrogen sulfide and carbon dioxide continued updip along the Bell Canyon Formation into the Capitan reef, where they mixed with oxygenated ground water moving downdip along backreef beds. The hydrogen sulfide and oxygen combined to form sulfuric acid which dissolved out the large cave passages in the Guadalupe Mountains. (click on image for a PDF version)

It is not at all clear what factors caused the gas to move from basin to reef. Present-day ground-water flow in the alluvial-evaporite aquifer of the basin is approximately perpendicular to the direction of proposed gas flow into the reef (Hiss, 1980). According to Sares (1984), the potentiometric level of the alluvial-evaporite aquifer is nearly 107 m higher than the potentiometric level of the adjacent Capitan reef aquifer so that very little movement of water, if any, occurs today between basin and reef in the vicinity of the Guadalupe Mountains. The migration of fluids from intracratonic basins into surrounding carbonate rocks is a problem which, in general, is poorly understood. Ore deposits are often found along the margins of basins (as Guadalupe caves are found near the margin of the Delaware Basin; Fig. 5), but the source of the ore, and how the metals together with reduced sulfur (as H₂S) move from basins into surrounding carbonate reef rocks, is still very much a matter of debate (Anderson and Macqueen, 1982). However, it is known that such movement does occur, both during basin formation and long afterwards, in response to gradients established by hydrologic, compaction, thermal, relief, or deformation factors.

Episodes of tectonic movement and faulting may have triggered the release of subsurface gas from solution so that it could begin to migrate from basin to reef. Once released, the gas could have ascended buoyantly into the reef in response to a pressure gradient between the two areas, perhaps by diffusion as microbubbles, as suggested by Ash and Wilson (1985a, b). Local gas migration under a "cap" (the halite beds in the basin) could have been facilitated by avenues which ascended at high angles into the reef (e.g. the Bell Canyon Formation(?), Bottomless Pit, Carlsbad Cavern, dips 20-50°). Water flow may not have been required for the migration of the gas as long as the reservoir rock was water-wet (McAuliffe, 1979).

Sulfuric-acid dissolution of Solution Stage III caves—By whatever avenue and whatever means hydrogen-sulfide gas ascended into the reef, it would have remained in the reduced state until it combined with oxygenated water in the reef, Oxygenated water could have entered the caves as meteoric ground water moving downdip along backreef beds and into the reef (Jagnow, 1979), or it could have entered by diffuse seepage through an overlying land surface deficient in impermeable caprock (Palmer, 1975). Either way, where ascending hydrogen sulfide met with oxygenated water, sulfuric acid was produced.

Two reactions, which may have been aided by bacteria, describe the oxidation of hydrogen sulfide to sulfuric acid:

2H₂S_(aq) + O_2(aq) = 2S_(s) + 2H₂O₍₁₎ (6)

2H₂O_(l) + 2S_(s) + 3O_2(aq) = 2HSO₄^-_(aq) + 2H⁺_(aq) (7)

The sulfuric acid produced by reaction (7) immediately reacted with the limestone bedrock and carved out the large cave voids:

2H⁺ + SO₄^2- + CaCO_3(s) + 2H₂O_(l) =
CaSO₄.H₂O(s) + H₂O_(l) + CO_2(g) (8)

The carbon dioxide liberated in this reaction, and also carbon dioxide which may have migrated up from the basin along with the hydrogen sulfide, would not have been subject to a chemical reaction which rapidly removed it from the system, and so it would have been more likely to degas at the surface of the water table than the hydrogen sulfide. Any carbonic acid that might have been produced from the CO₂ would have dissolved only a very small amount of limestone compared to that dissolved by the sulfuric acid, since the ionization constant of sulfuric acid is 10^-1.9 whereas that of carbonic acid is only 10^-6.35 (Krauskopf, 1967).

From equations (6), (7), and (8) the volume of hydrogen-sulfide gas that produced a cave void the size of the Big Room, Carlsbad Cavern, can be calculated. The volume of the Big Room, as determined by digitizing a map of it on a computer, is roughly 1,343,000 m³. Assuming that the density of limestone is 2.3 g/cm³, then 3 x 10¹² g of limestone must have been removed in order to create the Big Room void, and 6.7 x 10¹¹ liters of hydrogen-sulfide gas were needed to dissolve so much limestone. This is about 10 times less gas than the amount of natural gas removed from Eddy County in 1978 (Arnold et al., 1980). Thus, the amount of gas that it took to dissolve the caves of the Guadalupe Mountains is small considering the total amount of gas which has probably been generated in the Delaware Basin over time, and it is not quantitatively unreasonable to assume that the gas came from this source.

Sulfuric-acid dissolution and bathyphreatic cave development—The systematic excavation and integration of cave passages in the Guadalupe Mountains and the evolution of their unique three-dimensional form are the result of bathyphreatic and water-table conditions combined with a sulfuric-acid speleogenesis. Imposed upon the three-dimensional matrix of spongework passages dissolved earlier (i.e. Solution Stage II caves) is a complex of large passageways that descend vertically at steep angles, major horizontal levels containing large rooms, fissures or tubular pits connecting the levels, and boneyard passages underlying and surrounding the large rooms. With the exception of the spongework, all of these types of passages developed during the Solution Stage III episode of sulfuric-acid dissolution. The ideas put forth in the following discussion of bathyphreatic Solution Stage III passages are based on the work of Ford and Ewers (1978), Davis (1979a, 1980), this study, and conversations with D. C. Ford.

The basic pattern of bathyphreatic flow in the Guadalupe Mountains was probably set up before cave development was pronounced (Fig. 14). Flow was guided by joints, bedding planes, joint intercepts, and zones of weakness such as at the reef-backreef and reef-forereef contacts. Cave development was primarily north-south or east-west because water followed major joints that trended either parallel or perpendicular to the reef. High-velocity flow presumably existed when joints were still relatively tight, but with enlargement of the cave passages the velocity decreased. Progressively more water ensued along enlarged paths with the least resistance to flow, developing trunk passages which carried most or all of the water. Water in these large conduits discharged at major springs, the outlet positions of these springs having been controlled by step-wise lowering of the water table (Fig. 14).

The first spring outlet for Carlsbad Cavern may have been the Natural Entrance (Fig. 88), or what was then the entrance before the Carlsbad Cavern Ridge eroded down to its present position. The Natural Entrance displays a phreatic form to the very top of the entrance lip, a form that was most likely produced by lifting water. As the water table dropped, the Natural Entrance paleospring was abandoned; this shift brought Bat Cave into play as an eastward drain for water and the second natural entrance into play as the next discharge spring. The flow along Bat Cave at that time (as determined by scallop length) was 3-12 m³/sec (as compared to a 0.4 m³/sec flow rate for Carlsbad Springs today). Basal water-injection points for both the Natural Entrance spring and the Bat Cave spring probably encompassed practically all of the then-formed cave below the level of Bat Cave, and it is possible that bathyphreatic water could have excavated cave passage all the way from the Bat Cave level down to the Lower Cave level at that time. The Main Corridor was the main line of ascent for lifting water coming up from the Mystery Room, Guadalupe Room, Big Room, and Lower Cave. Water ascending from the Guadalupe Room encountered the impermeable lower units of the Yates Formation and so was forced to diffuse toward the Main Corridor along the level of the New Section. Bottomless Pit in the Big Room was not only a probable water-injection point but also a probable injection point for hydrogen-sulfide gas ascending into the reef along the Bell Canyon Formation(?) (Fig. 87) and/or along a major north-south trending joint (Fig. 85). Hence this area was a region of intense mixing and dissolving that eventually produced an exceptionally large void, the Big Room. Another possible water- and gas-injection point was Lower Cave (along the Cable Slot and Nicholson's Pit); where acidic water ascended at the Jumping Off Place and mixed with water ascending from the Bottomless Pit, it created the large dome in the Big Room ceiling known as the Top of the Cross. The ascending, bathyphreatic water probably never exited via the Top of the Cross (recent balloon exploration of that area shows that the Top of the Cross may not continue upward); rather, the water probably moved northeast along the Big Room, ascended Appetite Hill at an angle toward the Main Corridor, and then lifted vertically up the Main Corridor.

FIGURE 88—Proposed route of bathyphreatic flow and successive spring outlets in Carlsbad Cavern. Straight arrows represent main-flow routes and wavy arrows represent diffuse-flow routes. (click on image for a PDF version)

As erosion of the reef and basinal rocks continued, spring positions shifted eastward and downward, as did new basal injection points for lifting water. The next spring outlet for water in Carlsbad Cavern may have been somewhere on the New Section level or, later on, in the Lake of the Clouds-Bell Cord Room section of the cave. The unlikely position and configuration of the Lake of the Clouds Passage, Bell Cord Room, and Bifrost Passage with respect to the rest of the cave at that level (see Sheet 2) suggest that perhaps these passages were the avenues along which water ascended at the time when the water table was lowering to the Big Room level. This spring outlet could have been either in the high dome of the Bell Cord Room or in a passage not yet discovered or one covered by collapse. At that time water no longer proceeded up the Main Corridor, but instead flowed from the Big Room along Left Hand Tunnel and toward the Bell Cord Room spring outlet (Fig. 88).

Sulfuric-acid dissolution and water-table development—The greatest amount of sulfuric-acid dissolution probably took place at the water table, which was a mixing zone for oxygenated vadose water descending from the surface and hydrogen-sulfide gas ascending from the basin. Acidic water dissolved out the major horizontal rooms and passages, transformed the montmorillonite clay into endellite and, as a by-product of the acidic reactions, autochthonous silt released from the limestone settled to the floor and gypsum precipitated on top of the silt (Fig. 89).

FIGURE 89—Model of hydrogen-sulfide reaction with dissolved oxygen near the water table. Hydrogen sulfide and carbon dioxide from the basin ascended into the reef along injection points and reacted with oxygen in the zone of oxygenation to form sulfuric acid. Sulfuric acid was neutralized by the limestone away from the injection points and, therefore, horizontal rooms end abruptly. The sulfuric-acid reaction did not occur below the zone of oxygenation and hence vertical passages die with depth below large, horizontal rooms. With successive lowering of base level, new horizontal levels became connected with older horizontal levels by spring shafts and joint chimneys.

As water-table conditions prevailed over bathyphreatic conditions (Fig. 14), the velocity of flow decreased. This change explains the sequence of deposits in Lower Cave, where cobbles (fast-flow environment) are overlain by finely laminated silt (slow-flow environment). The position of gypsum on top of silt is the result of continuous dissolution of limestone (source of silt) but precipitation of the gypsum only when the water reached saturation. Left Hand Tunnel became the lateral connection between the Big Room and the Bell Cord Room spring outlet, and water flowing slowly in that direction could have caused the diffuse pattern of that passage. Slow-flowing water may have also dissolved the diffuse pattern of passages in the Polar Regions, Big Room, and many boneyard passages (such as those at the base of the New Mexico Room and Appetite Hill; Wilson and Ash, 1984b). Boneyard may represent the contemporary enlargement of Solution II spongework caves in rock surrounding enlarging passages, especially in slack current places.

According to the proposed model of Solution Stage III sulfuric-acid speleogenesis. vertical tubes, fissures, and pits are interpreted as formed along injection points for gas and/or bathyphreatic water, and horizontal levels are interpreted as formed at the water table where dissolved oxygen was most concentrated (Fig. 89). Cave walls of large rooms end abruptly in the horizontal plane because, away from gas-injection points, the acid was neutralized by bedrock. Since the amount of dissolved oxygen decreased with depth below the water table, fissure passages and pits terminate vertically and do not possess bottom drains. Degassing of carbon dioxide at the water table produced atmospheric corrosion of cave ceilings and walls, and the large cave passages grew further by sloping upward.

Origin of spar

U-series and ESR dates, carbon-oxygen-isotope data, and nucleation crystal kinetics suggest that Spar III crystals in Guadalupe caves formed during the present erosion cycle, under saturated conditions, and in the shallow phreatic zone.

The growth of large spar crystals implies that: (1) water flow in the zone of crystallization was extremely slow and crystals thus were free to nucleate and develop; (2) water in the zone of crystallization was barely saturated with calcite; and (3) the right saturation conditions were maintained over a sufficiently long period for the spar to grow large. Under supersaturated conditions the nucleation of crystals is so rapid that most of the excess dissolved mass is precipitated as nuclei, with little material left over for large crystal growth. Cave rafts or cloud linings, such as in the Lake of the Clouds Passage, Carlsbad Cavern, are examples of this type of precipitate which consists of extremely small crystallites of calcite. Under barely saturated conditions nucleation is slow and only a negligible proportion of excess mass is used to form crystal nuclei. In this case, when the nucleation rate is considerably less than the growth rate, surface-reaction-controlled crystallization predominates and crystals can grow large and at the expense of smaller crystals around them (Berner, 1971). Thus, the growth of large spar crystals implies a very delicate balance between depositional and nondepositional equilibrium, at a point where saturation is barely maintained. Zoned spar crystals, such as have been found in the Secondary Stream Passage and Guadalupe Room of Carlsbad Cavern, also demonstrate that equilibrium may have been very delicately balanced between deposition and nondeposition.

Thrailkill (1965b) thought that water movement in a karst aquifer is concentrated in the upper 100 m or so of the phreatic zone, and mainly in the uppermost 10 m, very near the surface of the water table. It is this oxygenated water that is generally undersaturated with calcite and dissolves most of the limestone along a horizontal zone at the surface of the water table. Below this zone the aquifer is much slower-flowing and water can become saturated with calcite. Far below the water table, deep in the phreatic zone, greater pressure causes an increase in the solubility of calcite and, therefore, the water is again undersaturated with calcite. Thus, theoretically at least, there is between the water table and the bathyphreatic zone a middle, shallow-phreatic zone which is barely saturated with calcite and is the ideal location for the growth of large spar crystals.

The concept of a saturated, shallow-phreatic zone as the site of spar formation concurs with the carbon-oxygen data on Spar III crystals (Fig. 73), which were dated at >350,000 yrs by the U-series method and at 879,000 ± 124,000 yrs by the ESR method. The carbon depletion of Spar III crystals indicates origin from ¹³C-poor organic soil and little interaction with carbonate host rock. This implies a shallow-phreatic origin for the spar. According to this interpretation the spar formed after fast-flow bathyphreatic cave development, during slow-flow water-table development, but before the large horizontal rooms became highly integrated by sulfuric-acid dissolution at the water table. Thus, spar and spar linings, such as can be seen in the Secondary Stream Passage and Mystery Room, have been dissolved and etched by sulfuric-acid solutions (Pl. 9A, Fig. 90).

FIGURE 90—Proposed sequence of events producing the calcified siltstone-cave-raft sequence. (A) At the beginning of Solution Stage III dissolution, acidic water enlarged Solution Stage II cavities and silt residue settled to the floor. Cave rafts formed at the surface of the water table where CO2 was degassing, and the rafts sank to the floor on top of the silt, (B) Acidic water at and below the water table and corrosive air above the water table dissolved away the bedrock so that, over time, early Solution Stage III caves became large, interconnected voids. Passages sloped upward by corrosion, and the siltstone-rafts became truncated concordant with the wall. (C) The water table lowered from the passage and breakdown blocks (containing exposures of siltstone-rafts) fell to the floor. Sites (1), (2), and (3) correlate in each of the three views, and the dashed lines in (A) correspond to the final cave passage outline in (C).

Origin of calcified siltstone-cave rafts

Whereas spar precipitated in the saturated, shallow-phreatic zone just below the water table, the Type I cave rafts of the siltstone-raft sequence formed on the surface of the water table where carbon dioxide was degassing and supersaturated conditions were achieved. The almost identical carbon-oxygen composition of Type I rafts and Spar III crystals (Fig. 73) suggests a genetic relationship between the two; that the rafts were forming on the water-table surface during the same time the spar was forming just below the water table (Fig. 90). This model fits the sequence of events as seen in the Guadalupe Room, where the calcified siltstone-cave raft sequence directly overlies spar crystals (Fig. 68); it is also not in discord with the dating results on the spar and siltstone-rafts, assuming that the <350,000 yrs siltstone-raft dates (Table 24, samples 12, 13, 14) are young ages due to post-depositional calcite cementation of the rafts.

Considering these factors and also the occurrence of the calcified siltstone-cave rafts, these deposits are thought to be the result of early fluctuation phases of water-table development that antedated the final enlargement of Solution Stage III cave passages. The siltstone-rafts occur in the backreef-reef transition zone (Fig. 40) and the siltstone probably represents insoluble residue released from silty backreef beds upon bedrock dissolution. The siltstone-raft deposits always rest on upward-facing surfaces (Fig. 68), signifying that these silts settled out of suspension once they were released from the limestone. The siltstone and rafts almost always occur together, with the rafts directly overlying the siltstone and with both deposits being highly calcified; also, the space above the siltstone-raft sequence is usually air-filled (or covered with subaerially formed travertine). These facts imply that the cave rafts deposited very soon after the siltstone and that the siltstone-rafts date from the present erosion cycle rather than an earlier one, since otherwise the rafts would be covered with silt or spar. This is confirmed by the dates on the rafts.

The fact that the calcified siltstone-cave rafts are truncated by large cave passages may be explained if one assigns the siltstone-rafts to an early episode within the Solution Stage III event (Fig. 90). As with the spar, the siltstone-raft deposits became corroded and truncated as small, isolated passage segments became integrated into large cave passages by sulfuric acid dissolution or by the sloping upward of passages by atmospheric corrosion. Only a small percentage of the siltstone-rafts remains of those originally deposited—those exposed in walls and ceilings and those in pieces of breakdown that fell from the walls and ceilings after the water table had permanently receded from the cave passages (Fig. 90).

Not all Type I rafts are truncated by the large cave passages: some are still located in small solutional rooms. Examples of this are the well-indurated rafts (overlain by sulfur crystals) which occur on the floor and ledges of a small room off the Christmas Tree Room and which have a carbon-oxygen signature characteristic of Type I rafts (Fig. 73, sample 8). It is believed that these rafts may have formed in an area of less acidity, where solutions quickly lost their aggressiveness for dissolving cave wall and thus became saturated with calcite. Solutional cavities with this kind of Type I cave raft represent an early development of Solution Stage III cave which never became fully integrated into large rooms (i.e. the cavities remained in the stage of development shown in A (2) of Fig. 90).

Origin of cobble gravel

Bretz (1949) proposed that the cobbles and silt in Lower Cave and Secondary Stream Passage, Carlsbad Cavern, were the result of a late-stage, short-lived vadose stream which traversed down the Main Corridor, into the Secondary Stream Passage, and then down into Lower Cave, finally exiting somewhere in the Pecos River valley. Gale (1957) further elaborated on Bretz' idea by stating that an earlier period of vadose-stream activity produced the terraced silt banks in Lower Cave, and then a younger vadose stream carved out a channel in the silt and deposited locally derived limestone cobbles along its course. The thesis of a vadose-stream origin for the cobbles was also accepted by Moore (1960a); however, Good (1957) thought the cobbles were reef material rounded in place. Neither theory is supported by the evidence.

(1) The cobbles are backreef material and hence originated outside the cave. This is supported by lithologic studies done by M. Queen (pers. comm. 1984) and by size analyses of the cobbles (Table 9). The cobbles have an average long-axis dimension twice as great as the short-axis dimension, indicating that they are bedded, backreef material rather than nonbedded, reef limestone. The chert pieces found in the Lower Cave trench and in the gravel of the Main Corridor, and the quartzite pebble found by M. Queen in the Secondary Stream Passage (Table 8) may be remnants of the Ogallala (or Gatuña) Formation.

(2) The cobble gravel is a heterogeneous, poorly sorted, non-stratified to crudely stratified deposit. Individual cobbles do not touch each other as is typical of water-laid gravel, but rather are matrix-supported. These facts argue strongly against a stream or flood-water genesis.

(3) The cobbles in Lower Cave were not deposited in a downcut trench after the silt as stated by Bretz (1949) and Gale (1957); they underlie the silt (Pl. 1B. Fig. 34). If the contact between the well-indurated cobbles with tan matrix and the fine-grained, homogeneous, orange silt overlying the cobbles represents a true unconformity, then the cobbles and silt in Lower Cave were emplaced at different times and may be genetically unrelated to each other.

(4) Cobbles are encased in a gypsum block in Upper Devil's Den. If indeed a vadose stream had deposited the cobbles, then why did it not dissolve away the gypsum?

(5) Cobbles in the lower end of Lower Cave near Nicholson's Pit mysteriously disappear near the pit and do not fill the pit, which suggests that the cobbles may have been emplaced before the pit existed. Cobbles in the upper end of Lower Cave at the Green Clay Room could not have possibly been carried through the crawl-sized passages there by a stream. Flow velocities on the order of 1 m/sec would have been required to carry cobbles so large (up to 19.5 cm in diameter) along a streambed (White and White, 1968). On the other hand, flow velocities of only about 0.3 cm/sec would be required to transport the silt which overlies the cobbles (mean particle size 0.04 mm; see Hjulstrom's diagram, Krumbein and Sloss, 1963).

(6) The cobbles are not deposits of a late-stage stream; they underlie >730,000 yrs old, paleomagnetically reversed silt, which in turn underlies subaerial dripstone dated at 600,000 ± 200,000 yrs. The cobbles underlie silt which in turn underlies travertine; the cobbles never abut up against travertine material. This implies that the cobbles were not emplaced by a free-flowing stream in the vadose zone, but rather were deposited in the phreatic zone; otherwise travertine and cobbles would be adjacent and contemporaneous with each other.

Because of the above objections to a stream-deposited or in-place origin of the cobbles, the theses of Bretz (1949) and Good (1957) are rejected and a new one is proposed. The cobble gravel in Carlsbad Cavern originated similarly to a debris flow—a muddy mixture of water and fine particles that supports and transports abundant coarser material (Friedman and Sanders, 1978). The cobbles originated in backreef beds, were carried and rounded by a surface stream, were dumped into then-forming bathyphreatic cave passages early in the Solution Stage III episode, and then gravitated down into the lower levels of the cave along the Main Corridor. In limestone terrains rocks are honeycombed with voids and surface material may find its way into subterraneous cavities on a large scale (Lee, 1924b).

Much of the evidence points to a subaqueous-debris-flow origin. The cobbles are backreef, possibly Ogallala or Gatuña material. Limestone cobbles, siliceous pieces, and pieces of very old spar (see the ²³⁴U/²³⁸U ratio of sample 11, Table 24) were carried and rounded by a surface stream and then dumped into pre-existing passages. Before entering the cave, the cobbles were weathered, some of them developing moonmilk-like or hollow centers such as described by Bretz and Horberg (1949a) for the leached caliche deposits of the Ogallala Formation in the Pecos River valley. Moonmilk-centered cobbles lie directly adjacent to crystalline cobbles in the "cobblestone" floor of Nooges Realm, Lower Cave, and were also found in the lower levels of the trench (Table 9); both trends would be expected if the cobbles had been weathered prior to entering the cave and then were randomly dumped into place within the cave.

Debris flows are poorly sorted, chaotic mixtures which generally lack internal stratification. Large clasts such as cobbles are easily transported in debris flows because they are supported in part by buoyancy forces due to the high density of the mud in which they are submerged (Blatt et al., 1972). The largest clasts end up on top of the deposit (Gloppen and Steel, 1981) and sometimes display a weak upcurrent imbrication (Allen, 1982). The size data on the cobbles (Table 9) and the "upstream" cobble imbrication measured at Nooges Realm show that both conditions exist for the Lower Cave cobble gravels. Most important, a matrix-supported internal structure, where the cobbles are separated by a finer matrix material, is diagnostic of debris flows.

A debris-flow origin satisfies all the statistical data on the cobbles and explains the distribution of the gravels in the Main Corridor and Secondary Stream Passage (Fig. 31), but it is much less satisfactory in explaining the horizontal distribution of the cobbles along Lower Cave where the passage slope is low (1-1.5°). No really good explanation can be offered for this distribution, but one possibility is suggested. If the cobbles entered the cave early in the Solution Stage III episode when bathyphreatic conditions still prevailed in Lower Cave, then perhaps they were moved along Lower Cave by rapid, sub-water-table flow. Later, when Lower Cave was subject to a slow-flow water-table development, the silt settled out of suspension on top of the cobbles and protected the gravel from further dissolution by Solution Stage III acids.

The relative time of cobble-gravel emplacement is not known with certainty. A debris flow could cut across cave levels and thus antedate events going on at one level, be contemporaneous with events at another level, or postdate events at still another level. The fact that the cobble gravel is interbedded with silt in the Secondary Stream Passage suggests that at this level of the cave the cobbles were slumping into passages contemporaneous with passage enlargement. The cobbles in the gypsum of Upper Devil's Den may have slumped into the gypsum while the gypsum was consolidating, but they had probably entered the Main Corridor long before then.

The proposed scenario for the emplacement of the cobble gravel in Carlsbad Cavern is as follows: The presence of siliceous gravels, dendritic stream patterns, and entrenched meanders on the Guadalupe upland surface indicates that during Ogallala time the Guadalupe Mountains were low and near base level (Motts, 1957). With uplift, sharp meanders such as the Serpentine Bends of Dark Canyon (Fig. 5) were incised into the formerly low-lying plain. Solution Stage II cavities and then-forming bathyphreatic Solution Stage III passages were exposed by incised-valley erosion so that rounded, backreef, stream cobbles were dumped into the voids, progressively working their way down into the lower levels of the cave (Fig. 91). In the higher levels of Carlsbad Cavern such as Bat Cave, the Main Corridor, and Secondary Stream Passage, gravel infilling may have been contemporaneous with Solution Stage III dissolution, and the result was cobbles crudely stratified with silt. According to this model, the cobbles were brought into the cave as a subaqueous debris flow, and the interbedded and overlying orange silt originated as residue from the dissolving limestone. However, on the Lower Cave level the cobbles arrived before substantial water-table development began, and so a sharp unconformity exists between the cobbles and silt at that location.

FIGURE 91—Proposed model for emplacement of cobble gravel. As the reef uplifted in the late Pliocene-Pleistocene, rivers that once meandered across a low-lying erosion surface downcut into forming Solution Stage III bathyphreatic caves, and Ogallala (or Gatuña) gravel was dumped into these voids.

Downcutting streams filled other early caves exposed high along valley walls, such as Pink Fink Owlcove, with gravel. Type 2 dikes of coarse sand to gravel may also date from this episode of cavity filling. The Ogallala (or Gatuña) siliceous gravel remnants still present on the ridge tops of the Guadalupe Mountains, including the ridge east of the Carlsbad Cavern entrance (Fig. 8), may be part of the same cobble gravel that fills the cave. On the Carlsbad Cavern Ridge, most (99%) of the original mass of this gravel (the limestone cobbles) has been eroded away, and the more resistant remnants (1%) of this gravel (the siliceous pebbles) still remain in place.

Origin of endellite

The mineral endellite is another important indicator that the large cave passages of the Guadalupe Mountains had a sulfuric-acid genesis. Endellite is a kaolin mineral and species of this group are highly susceptible to crystallographic change in an acidic environment (DeKimpe et al., 1964).

Cox (1875) and Callaghan (1948) described porcellaneous endellite in Gardner Mine Ridge, Indiana, as bluish, gray, brownish, or snowy-white nodules which occur in a dark mahogany clay; the nodules are dense, waxy, translucent, have prominent conchoidal fracture, and contaminating material (such as iron oxide or manganese), expelled from the structure of the clay, stains the surface of the endellite. The Indiana clay is a residual soil layer derived from limestone; sulfuric acid derived from pyrite in overlying sandstone beds turned the residue to endellite (Ross and Kerr, 1934; Callaghan, 1948).

Keller et al. (1966) described an occurrence of endellite from Stanford, Kentucky, and postulated the same, sulfuric acid origin. The Stanford endellite is white to light gray, granular to porcellaneous, and forms as lenses and stringers in weathered limestone residue. The lenses are about 10 cm wide and follow pre-existing fractures in the residue. At the zone of endellite formation, pH was 3.0-3.7; pH at the source (pyrite in overlying shale beds) was 1.0.

A third reported location of endellite is at Les Eyzies, France, where kaolin-halloysite (endellite) is trapped in pockets as much as 50 m deep within the pyritic limestone of the Dordogne and Vezere Valleys (Brindley and Coiner, 1956). The kaolinitic sediments were stirred and washed and then altered to endellite by sulfuric acid solutions derived from the pyrite.

In Guadalupe caves, endellite occurs with montmorillonite in Solution Stage II spongework cavities that have been cut across by Solution Stage III passages, or it is associated with overlying silt (Fig. 70). Montmorillonite is a clay mineral which can transform to palygorskite (attapulgite) under drying conditions and to endellite under acidic conditions. Berner (1971) stated that montmorillonite weathers to kaolinite (endellite is a type of kaolinite) in an acidic environment, in which case silica (chert) is liberated in the reaction. When Solution Stage III acids corroded limestone honeycombed with Solution Stage II spongework, the montmorillonite clay in the spongework was exposed and some of it transformed to endellite—especially along cracks or seams in the clay or at the contact of the montmorillonite and limestone.

Origin of sand and silt

Clastic sediments in caves are classified as autochthonous if they derive from inside a cave, or as allochthonous if they are brought from outside. The cobble gravel in Carlsbad Cavern is believed to be allochthonous. The fine-grained sediment in Guadalupe caves, however, is believed to be autochthonous, i.e. it is not a stream deposit but a residue released from the limestone at the time of Solution Stage III dissolution. Rare exceptions to this non-stream mode of deposition are: (1) Vanishing River Cave, which is located in the bottom of a canyon wash and so receives flowing water during the time of heavy rain; (2) the entrance of Dry Cave, where flowing water has channeled down several centimeters into the sediment of Fool's Hole (Lindsley and Lindsley, 1978); and (3) the connection between Bat Cave and New Section, Carlsbad Cavern, where a small, abandoned stream bed is entrenched in the floor of the upper part of the connection area (D. Davis, pers. comm. 1984).

Gardner (1935, p. 1269) was the first to propose that the silt in Carlsbad Cavern derived from a stream: "in its lowermost level the channel, now dry, of a stream which was quite active throughout a long period until recent geologic time." Bretz (1949) also thought a stream had traversed through Lower Cave, first depositing the silt and then the cobbles in a downcut trench in the silt. This study contends that the silt is autochthonous residue, based on the following evidence:

(1) Floor sediment is almost always traceable to nearby silty-sandy limestone; the sediment has not been transported more than tens of meters from its source.

(2) Insoluble residue in the bedrock matches nearby floor sediment in color, grain size, and type of impurity. For example, the orange sand on the floor of Sand Passage matches the sand of the Yates Formation which can be seen interfingering along the north wall of the passage. The cream-colored silt on the floor at Bottomless Pit matches the cream-colored insoluble residue in the limestone at that spot (compare Table 10 with Table 27).

(3) Silt underlies floor speleothems such as stalagmites, but it has not covered them or piled up around them as would be expected if the silt was stream-laid.

(4) Sediment terrace banks are level across cave passages. This can be seen in Left Hand Tunnel and also in Lower Cave where the sediment banks are almost level from the upper end to the lower end of the passage (Table 11). The level terraces suggest that the sediment was deposited by flood or aquifer water, not by stream water.

(5) No signs of active streams exist in silt-laden areas; there are no scallops or incised meanders to suggest fast flow under vadose conditions. Laminations in the silt (Pl. 2A) attest to a quiet mode of deposition such as might be produced by flooding or by slowly flowing aquifer water. Also, the fact that excellent paleomagnetic data have been obtained for the silt in Lower Cave suggests that the silt settled out of non-turbulent water where iron mineral grains could align to the Earth's magnetic field.

(6) The silt in Lower Cave was not brought in by a late-stage stream as suggested by past researchers (e.g. Bretz, 1949). The silt is older than 730,000 yrs as determined by paleomagnetic dating.

(7) The laboratory experiment duplicated on a small scale the autochthonous process by which the silt in Guadalupe caves originated. As the limestone was dissolved by acid, the liberated silt residue settled out of suspension and to the cave floor.

Some objections can be raised to an autochthonous origin of the silt.

(1) The up to 7 m high silt banks in Lower Cave cannot be directly traced to a correspondingly high percentage of silt in the limestone (Table 27). So where did the silt in Lower Cave come from?

(2) If the silt deposits in both the Big Room and Lower Cave are a product of Solution Stage III dissolution, then why does the Big Room silt contain color bands, chert, and is of limited extent, whereas the Lower Cave silt occurs in high banks and is orange colored throughout?

(3) If the silt in Lower Cave and Secondary Stream Passage is autochthonous, then why is it associated with the supposedly allochthonous cobble gravel?

Question (2) partly answers question (1). Consider the amount of residue that had to be liberated by the dissolution of the Big Room. Most of it did not accumulate on the floor of the Big Room, so where did it go? As discussed previously, deeper-flow paths tend to fill with silt because of normal gravity settling; therefore, lower cave passages should fill with silt liberated from the dissolution of upper cave passages so long as the two passages are connected. This is the explanation offered for the Lower Cave silt: It is in soluble residue derived not only from the dissolution of Lower Cave and Talcum Passage, but also, in part, from the dissolution of the connected Big Room. To support this hypothesis, a digitized map of the Big Room, Talcum Passage, and Lower Cave was made, and the volume of residue that should have been liberated from the dissolution of these three rooms was calculated and compared with the actual amount of silt on the floor of Lower Cave. The amount of insoluble residue in the limestone was calculated by averaging the values in Table 27 (excluding the New Mexico Room values), and the height of the silt banks in Lower Cave was calculated using the values in Table 11. The estimated average height of the silt banks in Lower Cave is 1,8 m and, based on the total volume of the three rooms and the average insoluble-residue content in the limestone, the theoretical height that the banks should be is 1.7 m. These calculations do not take into account the amount of silt on the floor of the Big Room, a quantity difficult to estimate since the Big Room is covered with travertine and gypsum in many places. Exposed patches of silt in the Big Room do not nearly match the amount of silt present in Lower Cave, however, and probably would not affect these calculations by more than 10%.

Objection (2) may be countered by invoking a higher overall acidity for the dissolution of the Big Room than for the dissolution of Lower Cave. If the cream-colored sediment at Bottomless Pit and the quartz-sand half-cones of the Big Room really represent interfingerings of the Bell Canyon Formation, as suggested by this study, then these could have been avenues along which hydrogen sulfide entered the Big Room (Fig. 87). In addition, the N15°W joint of the Big Room may have been another avenue along which gas ascended from the basin into the reef (Fig. 85). More hydrogen-sulfide gas entering the system would produce more acid, which in turn would dissolve more limestone, produce more gypsum on the cave floor, liberate more endellite and silica from a montmorillonite transformation, and cause more colloidal migration of silica and iron to form chert and color-banding in the silt. This is exactly what is seen in the Big Room. The Big Room is much larger than Lower Cave and there are more gypsum blocks on the floor. A higher percentage of the Big Room clay is endellite, whereas Lower Cave it is predominantly montmorillonite. The silt of the Big Room is color-banded due to its iron-oxide content and there is chert interlayered with the silt.

Objection (3) is not difficult to counter in the case of Lower Cave where the possible unconformity suggests the lack of a genetic relationship between the cobbles and silt. Cobbles do not fill Nicholson's Pit because they predate the pit; the cobbles descended into Lower Cave in the bathyphreatic stage of cave development, and, subsequently, Nicholson's Pit and the Cable Slot became injection points for gas. The cobbles were not dissolved by Solution Stage III acids because they were protected by silt released from the dissolution of the Big Room before the water table has descended to the Lower Cave level.

The interbedding of silt and cobbles in the Secondary Stream Passage is not as easy to explain, but the association of silt and cobbles where the passage emerges into the Lunch Room may be a clue. Here the cobbles and pebbles are mantled by layers of laminated orange silt, as if the silt had filtered out of suspension around the upper parts of exposed cobbles. If periodic slumping of the cobbles occurred contemporaneously with the dissolution of this part of the cave, it might account for the interbedding of the two deposits at this level. As Solution Stage III acids enlarged the cave, the cobble gravel slumped into these voids. These cobbles were immediately covered with silt residue and thus were protected from further acid corrosion. A 1.5 m thick layer of silt once covered the cobble gravel in the Secondary Stream Passage. This layer may have been Solution Stage III residue deposited after slumping had stopped; it has since been mostly eroded away.

Silt banks, such as those in Lower Cave (Fig. 37), were probably entrenched by a fluctuating water table in the zone of flooding. V-type entrenchment by flood-zone water is readily apparent in such caves as Flint-Mammoth, Kentucky, where periodic flooding of passages by the back-up of the Green River has cut steep-sided banks in mud, but it is much less apparent in Guadalupe caves where the present-day water table is not intersected by explorable cave passages. In the forereef sections of Left Hand Tunnel, Carlsbad Cavern, where canyons exist in the passage floor, the silt banks slope at an angle concordant with the canyon, as do the slumped layers in the silt. In Lower Cave, Carlsbad Cavern, silt banks were downcut into V's as far as the cobble-gravel horizon, but apparently the entrenching water did not have enough energy to breach this more competent horizon. Entrenchment of the silt in Lower Cave postdated the precipitation of the gypsum, as shown by the sequence of events in the Nooges Realm region where the silt deposited first and the gypsum second, and then both deposits were downcut concordant with each other by the V-shaped channel (Fig. 69).

There is evidence for one or more fluctuations of the water table around base level. At least one back-up of water stirred up the silt sufficiently in Lower Cave in order to produce the <30 cm high layer of silt on top of the gypsum blocks in Nooges Realm. This back-up happened subsequent to a subaerial episode, as evidenced by the silt exposed along the sides of drip tubes in the gypsum blocks (Fig. 69). Corrosion rillenkarren in Lower Cave which are overlain by a layer of silt (M. Queen, pers. comm. 1983), and mudcracks in the area between the Green Clay Room and Naturalist Room which possibly show more than one episode of submergence and wet-dry cracking, also suggest that a back up of water took place some time after the establishment of subaerial conditions at the Lower Cave level.

Origin of chert

Because of its close association with endellite and montmorillonite (Fig. 70), the chert in the Big Room of Carlsbad Cavern is believed to have been derived from the reaction of acidic solutions on montmorillonite clay with the production of endellite and silica (chert). Silica is a hardened colloidal gel, which may explain the position of chert in the silt deposits of the Big Room, its association with color-banded silt, and its rhythmically banded nature. Colloids carry an electric charge, the colloidal particles migrating to a reversely charged location where they coagulate as their charge is neutralized. Ferric oxide is practically insoluble except when it acts as a colloid, and then it becomes easily transportable. Therefore, the association of chert with color-banded silt may not be a coincidence. Both the ferric oxide and silica may have migrated through the silt as colloids as soon as subaerial conditions prevailed in the cave but before the silt dried out (Fig. 92). As the silica-rich water was neutralized by excess carbon dioxide near the top of the silt, the solubility of amorphous silica decreased because of the conversion of H₃SiO₄ to H₄SiO₄; supersaturation resulted and silica was precipitated (Berner, 1971). The same type of chemical migration may have been responsible for the color-banding of the silt. As the colloidal iron moved toward the top of the silt bank, it became more highly oxidized and in turn produced colors characteristic of increasingly higher oxidation states (greens to browns to reds and oranges). Cyclic banding in the chert may also relate to colloidal behavior. The rhythmic layers in the chert may have been produced by a liesegang-ring type of phenomena whereby gel-like layers are deposited in alternating porous-compact bands. Richardson (1919) related liesegang-ring gel behavior to the rhythmic or periodic precipitation of silica solutions diffusing through a porous medium, wherein a banded precipitate is separated by constant intervals and is sometimes replaced by a granular zone.

FIGURE 92—Diagrammatic presentation of montmorillonite-endellite clay, color-banded silt, and chert deposits of the Big Room, Carlsbad Cavern. Silica and ferric hydrates acted as colloidal "sols" which moved to positively or negatively charged sites to be neutralized. Colloidal behavior produced the color banding of the silt and the rhythmic banding of the chert.

The movement of silica upward to the top of silt may have occurred in response to alternating subaerial and subaqueous conditions during the time when water was fluctuating around base level in the Big Room. If water was slightly above the level of silt, silica would have moved toward the top of silt in response to water acidity; if water was slightly below the top of silt, the silt would have dried out, cracked, and then silica could have deposited within polygonal mudcracks in the silt (as it has in the Texas Trail occurrence, Table 16).

Origin of gypsum blocks and rinds

The sulfur-isotope data are essential to a proper interpretation of the gypsum blocks and rinds in Guadalupe caves. The gypsum of the Castile Formation in the Delaware Basin averages σS³⁴ = +10.3, while the gypsum in Guadalupe caves averages σS³⁴ = -15.1 (Table 23). These differences in isotopic fractionation show that the cave gypsum could not have been derived from the Castile Formation according to the local pooling model of Bretz (1949) or the mixing model of Queen et al. (1977a), nor is it likely that it has been derived according to Jagnow's pyrite model of speleogenesis. Instead, the sulfur-isotope data indicate that the cave gypsum is the end result of a series of biological oxidation-reduction reactions related to the oil and gas fields of the Delaware Basin.

There are three theories of how the gypsum has been deposited: (1) by precipitation from a saturated solution; (2) by a replacement-solution mechanism (Egemeier, 1973); and (3) by an in-situ replacement of limestone by gypsum (Queen et al., 1977a). All three mechanisms have probably occurred in Guadalupe caves, but mechanism (1) is believed to have been the predominant one for the following reasons:

(1) Gypsum always overlies silt and sand in Guadalupe caves. If the gypsum formed as a direct replacement of limestone, either according to Queen's mixing model or Egemeier's replacement-solution model, then it should contain the same amount of insoluble residue as the limestone. Comparison of Table 15 with Table 27 shows this not to be the case. The bedrock at Bottomless Pit, Big Room, has an insoluble-residue content of 11.7%, whereas a gypsum block very near this limestone contains only 0.01% of insoluble residue.

(2) While the sulfuric-acid experiment on limestone was only a model not simulating the geologic parameters of time and space, it nevertheless showed that the cave gypsum could have formed according to a precipitation mechanism. The laboratory-precipitated gypsum mimicked the cave gypsum in that it overlay silt and was residue-free except for tiny pieces of chert. The gypsum precipitated out onto the top of a small glass beaker (Fig. 81), a situation that could not possibly have been produced by a replacement mechanism.

(3) Most of the gypsum in Guadalupe caves does not seem to contain replacement textures as noted by Queen et al. (1977a) in the gypsum of the upper Gypsum Passage, Cottonwood Cave. Rather the laminations and microfolding in the gypsum blocks are reminiscent of the varved and microfolded texture of the Castile Formation in the Gypsum Plain. The gypsum in Guadalupe caves also does not resemble the thin crustal replacement gypsum in Wyoming caves as described by Egemeier (1981). With a few exceptions, in Guadalupe caves the gypsum forms as floor blocks and thick rinds rather than as thin crusts.

(4) Sulfate content of limestone from the drill core in the Big Room bedrock was 0.1% (maximum) at the surface of the wallrock and decreased steadily until it stabilized at about 0.008% (Fig. 80). If, as Queen's model of replacement suggests, the limestone was replaced by gypsum brine, then one would expect sulfate values to be high in the wallrock—at least in some sort of a gradational zone which contains stringers of gypsum or reaction rims such as occur around the partially replaced dolomite inclusion in the upper Gypsum Passage of Cottonwood Cave (Fig. 52). Rather, this sulfate trend (like a similar nitrate trend in bedrock at the Natural Entrance) probably reflects the concentration of solutes by seeping ground water at the cave-wall—air evaporation interface.

(5) Paired specimens of gypsum and limestone collected from Guadalupe caves showed no systematic correlation in amount of cations as did Egemeier's replacement-solution gypsum crusts in the Big Horn Basin caves (Fig. 79).

Gypsum precipitation from saturated solution—According to the precipitation model proposed here, the gypsum in Guadalupe caves deposited in much the same manner as did the gypsum of the Castile Formation in the Gypsum Plain. Laminations in the cave gypsum raise the question of whether or not these features are true varves (annual deposits). If the 0.5-1.0 mm translucent-opaque laminations in the gypsum blocks are indeed related to an annual cycle, as are the varves in the anhydrite of the Gypsum Plain, then the 6 m high gypsum blocks in the Big Room of Carlsbad Cavern represent roughly 8,000 years of accumulation. Seasonal variations in the volume or rate of water flow might have triggered the precipitation of the laminated gypsum; or, such triggering might also have been accomplished by seasonal variations in evaporation, temperature, carbon-dioxide loss, or calcium-ion concentration from dripping, speleothemic water.

Assuming a completely closed system, the amount of gypsum that theoretically could have precipitated out of solution according to equation (8) would have exceeded the volume of the limestone excavated, The molar volume of calcite is 36.8 cm³/gmole and that of gypsum is 74.2 cm³/gmole; i.e. the solid volume approximately doubles when limestone is converted to gypsum. This explains such areas as the Middle Maze of Endless Cave where the gypsum completely filled the passage before being compacted or partially dissolved (Figs. 44, 45), but it does not explain passages which contain no gypsum, or gypsum which fills 10-30% of the passage height. In the Big Room, the 4-6 m high gypsum blocks represent only about 7% of the gypsum that theoretically could have precipitated. This means that 90-95% of the gypsum must have been lost to flow within the aquifer or other processes such as vadose drippage.

According to the precipitation model proposed here, the gypsum blocks and rinds in Guadalupe caves represent a late-stage, lagoonal-type deposit resulting from hydrodynamic stagnation during the water-table stage of cavern development. Ponding of stagnant water was especially pronounced in the Big Room of Carlsbad Cavern. After water became diverted to the Bell Cord Room spring outlet (Fig. 88), the Big Room became a de-coupled and stagnating entity so that gypsum could precipitate out of solution and onto the floor as thick, massive deposits.

The purity of the gypsum blocks suggests that the gypsum was not a direct, immediate product of wallrock dissolution, but precipitated subsequent to passage dissolution. If numerous cycles of bedrock dissolution and gypsum precipitation occurred, then silt should alternate in layers within the gypsum or at least be mixed with it. Evidently, as with the laboratory experiment, the silt fraction settled out to the cave floor first and the gypsum precipitated out later in a single, fairly continuous "snow fall" during a phase of intense brine concentration. The purity of the gypsum also implies that the dissolution of Guadalupe caves may have been very rapid (in the geologic sense) and continuous, otherwise silt should be interbedded with gypsum.

Gypsum replacement of limestone—Replacement textures in the gypsum, especially those in the rinds along the walls of the upper Gypsum Passage, Cottonwood Cave, verify that at least some of the gypsum formed by a replacement mechanism. But how much of the gypsum had a replacement origin (as opposed to a direct precipitation origin) and how much of it fits Queen's replacement model (versus Egemeier's model) is still a matter of debate.

In the replacement model proposed by Queen et al. (1977a), sulfate ions directly replace carbonate ions in solution. The only way replacement can thermodynamically occur is if the sulfate ion is much more concentrated than the carbonate ion (i.e. SO₄^2- >5.4 x 10³ CO₃^2-; A. Palmer, pers. comm. 1986), a situation which may possibly be the case for Capitan aquifer water considering that in Table 2 HCO₃ exceeds SO₄^2- by a factor of 2 or 3 and the solubility of HCO₃ is about 8 x 10³ >CO₃^2-. Thus, under conditions approaching saturation or supersaturation with gypsum, where the SO^4-2 ion is much greater than it is in Capitan aquifer water, Queen's mechanism of replacement may be a viable one.

The replacement envisioned by Egemeier takes place in the zone of aeration, where sulfuric acid reacts with limestone and produces a thin crust of gypsum on the wall. This type of replacement is thermodynamically more likely than that proposed by Queen et al. (1977a), but Queen's model is actually more in accord with the type of replacement seen in some Guadalupe caves. The reaction rims around dolomite inclusions (Fig. 52) and the thick replacement rinds in the upper Gypsum Passage of Cottonwood Cave do not suggest a subaerial mechanism of replacement, but rather a replacement of carbonate ions by sulfate ions in a concentrated brine solution. On the other hand, the thin (<1 cm) crusts overlying limestone at Bottomless Pit, Big Room, Carlsbad Cavern, and those in Rim City and Windy City, Lechuguilla Cave, seem to morphologically conform to Egemeier's type of replacement. The low sulfate content in the bedrock core (Fig. 80) also favors either a purely precipitation origin or Egemeier's method of replacement where limestone alters to gypsum only on the outermost surface of the bedrock.

Solidification of gypsum—Fluctuations of the water table just before its final abandonment of a cave passage alternately caused the gypsum to dissolve, consolidate, dry out, harden, compact, and then partially dissolve again. These processes probably began as soon as the gypsum first started to precipitate, and may extend even to the present day in the case of dissolution by condensation water; however, they occurred primarily while the water table was fluctuating around base level.

Laminations in the gypsum formed during its precipitation, while microfolding, slumping, angular unconformities, brecciation, slickensides, inclusions, overgrowth crusts, recrystallization, and flow features attest to the gypsum's mode of solidification, Egemeier (1981) compared the consistency of wet gypsum in the Big Horn Basin, Wyoming, caves to that of mud. This was also probably true of the wet gypsum in Guadalupe caves; such features as the gypsum cascade in the Insane Rain Drain Trench Pit of Dry Cave or flow features in the Talcum Passage gypsum (Fig. 53) further suggest that the gypsum may have had an almost plastic consistency while still wet.

Possibly the first consequence of drying was the movement of interstitial water to the surface of the gypsum blocks to form overgrowth crusts. In some cases, where the crusts are vertically oriented, as in the Balcony Room of Dry Cave and the Talcum Passage of Carlsbad Cavern, hardened overgrowth crusts slid down over the still plastic interiors of the blocks (Fig. 43). Slickensides were gouged where overgrowth crusts slid down against underlying, more solidified gypsum (Pl. 3B). Precipitation of new gypsum over previously deposited, partly hardened, slumped layers resulted in angular unconformities. And, where hardened upper layers broke and sunk down at various angles into the less consolidated, lower layers of gypsum, a breccia texture was produced (Fig. 50). If pieces of bedrock fell or slumped off into the gypsum while it was still solidifying, the limestone became partially or totally replaced by gypsum depending on the viscous state of the gypsum and the amount of ionic exchange that occurred (Figs. 51, 52). Continued seepage of water to the outside of the gypsum block caused partial recrystallization of its interior mass, resulting in partial or total obliteration of original textures.

As the gypsum blocks completely dried out, they compacted. Undulations in the tops of the gypsum blocks in the Middle Maze of Endless Cave match those in the ceiling limestone (Fig. 45); at such localities the compaction factor seems to be about one-fourth of the original mass. Microfolding of the laminations may have been produced from the compaction, shrinking, and compression of the gypsum while it consolidated.

Dissolution of gypsum—The presence of gypsum blocks and rinds in some caves and cave passages and their absence in others is one of the most puzzling aspects of the gypsum deposits in Guadalupe caves. As discussed above, most of the gypsum (90-95%) that theoretically could have precipitated according to equation (8) must have been carried out of the caves in solution; otherwise all of the cave passages would be completely filled with gypsum. Four mechanisms appear to have caused the dissolution of the gypsum:

(1) Aquifer water—Most of the solute gypsum that did precipitate from saturated solutions probably redissolved and was removed by fresh aquifer water almost as soon as it formed. In this connection it is interesting to note that Bat Cave and the Main Corridor of Carlsbad Cavern, the Entrance Hall of Cottonwood Cave, the Main Corridor of Hell Below Cave, and the Expressway Passages of Endless and Dry Caves all are nearly devoid of gypsum. This distribution may possibly be explained as due to aquifer water moving more rapidly and less diffusely through these large trunk passages than through constricted passages such as the Gyp Joint of Hell Below Cave, the Gypsum Passage of Cottonwood Cave, the Middle Maze of Endless Cave, or the Polar Regions of Carlsbad Cavern. Also noteworthy is the trend seen in Carlsbad Cavern where very little gypsum exists in passages that trend northeast, parallel to the reef escarpment (e.g. Bat Cave, Main Corridor, and Left Hand Tunnel), whereas in the Big Room (a passage which trends perpendicular to the escarpment) deposits of gypsum are plentiful (Sheet 2). Flow of aquifer water in the northeast direction, toward outlet springs, may have been responsible for preferential gypsum dissolution in the passages which parallel the reef.

(2) Flood-zone water—Normal fluctuations of the water table in the zone of flooding may account for some gypsum dissolution, especially for archways of gypsum (Fig. 49), pillars of gypsum (Fig. 44), carved channels in gypsum (Fig. 47), commode holes in gypsum (Fig. 57), and scallops in gypsum (Fig. 59). Also, the entrenchment of gypsum where it overlies silt was probably caused by a fluctuating water table (Fig. 69).

Flood-zone water would have been only slightly under-saturated with gypsum and, therefore, only gypsum exposed to turbulence-where water rose up through small holes in the limestone or at the intersections of passageways (Pl. 5A)—would have dissolved. This may have been the same aquifer water out of which the gypsum had previously precipitated; only during flooding was the water undersaturated enough to dissolve gypsum.

(3) Vadose drippage—This is the most obvious mechanism of gypsum dissolution, but it probably accounts for only a small fraction of the total reduction of the original solute mass. Drip water has severely eroded many gypsum blocks into sharp-spiked crags, crevices, and tubes (Fig. 55). Some of the gypsum has been almost or entirely eroded away by this mechanism, as shown by remnant blocks, molds, and casts. On a small scale, dissolution patterns of vadose dripping can be seen in such places as the Gyp Joint, Hell Below Cave, where gypsum remains in alcoves or under archways protected from dripping along joints (Fig. 49). On a large scale, the effects of vadose drippage can be seen in such places as the Big Room, Carlsbad Cavern, where the gypsum abruptly begins and ends at the forereef-reef contact. In the forereef, talus beds tilt at an angle of about 30° away from the reef core so that meteoric vadose water is diverted downdip along these avenues. Hence water does not drip into the cave in most areas of the forereef and the gypsum blocks have been preserved there. In the reef core, however, dripping water readily enters the cave along ceiling joints, forming stalactites and stalagmites and also dissolving away the floor gypsum. Queen (1981) related the trend of gypsum preservation in the Big Room to joint position. At the Jumping Off Place the prominent joint is in the center of the passage and so the gypsum blocks are preserved against the wall; near the Lunchroom, in an area which has no prominent joints marking the Big Room axis, gypsum is found in the center of the room.

(4) Condensation water—Condensation water may have been responsible for streamlining the surfaces of gypsum blocks and for scouring the insides of commode holes. Streamlining appears to be most pronounced in areas where air is moving between different cave levels. Examples of this may be observed in the gypsum blocks in the Talcum Passage and at Bottomless Pit, Carlsbad Cavern, where the blocks at the edge of a pit or precipice are streamlined flush with the void (Figs. 38, 41). In the Talcum Passage and in the upper Gypsum Passage, Cottonwood Cave, the under sides of gypsum blocks are partially dissolved where air blows from a lower passage, and in Endless Cave gypsum blocks have dissolved concordant with the limestone walls at a place where air emerges from the Lower Maze into the Mud Crack Room. In the case of commode holes in gypsum, condensation water has smoothed and scoured the inside of the holes and has built up speleothem rims along the sides of the holes (Fig. 58).

Breakdown fall

The greatest amount of breakdown in a cave falls at the time of, or soon after, the lowering of the water table. According to Sweeting (1973), collapse is most likely when water that is forced through passages under considerable hydrostatic pressure loses that pressure and starts flowing freely. Bogli (1980) further stated that the change from a water-filled state to a drying state causes breakdown collapse.

Most of the breakdown in Guadalupe caves seems to coincide with a short interval between water-table lowering and breakdown falling. The breakdown overlies gypsum, but very little or no travertine has deposited between the gypsum and breakdown events. Iceberg Rock in the Main Corridor of Carlsbad Cavern is a notable exception to this rule; it is a piece of breakdown that fell some time after the water table had lowered in the Main Corridor. Tilted dripstone on the bottom of Iceberg Rock has a U-series date of >350,000 ybp and an ESR date of 513,000 ybp (Table 24, samples 28, 29). The Georgia Giant stalagmite, which grew on top of Iceberg Rock subsequent to its fall, has been U-series dated at 65,000-180,000 ybp (Fig. 74). Hence, it is known that Iceberg Rock fell some time between about 500,000 and 180,000 ybp—probably closer to 500,000 ybp, considering that the dripstone may have been actively growing before the fall.

Speleothem deposition

The various types of speleothems began to decorate the caves of the Guadalupe Mountains as soon as the passages became air-filled. The great mass of speleothemic material dates from humid stages earlier in the Pleistocene when the climate in the Guadalupe Mountains was much wetter than it is today (Fig. 77, Table 24). Notable exceptions to present speleothem inactivity are Crystal Dome, the largest actively forming speleothem in Carlsbad Cavern; the Chocolate Drop, New Mexico Room, Carlsbad Cavern; Temple of the Cave God, Three Fingers Cave (Pl. 16A); and numerous deposits in Virgin Cave, especially in the Cavernacle area. Helictites, soda straws, cave pearls, cave rafts, popcorn, and anthodites are examples of smaller, presently growing speleothems.

Dates on travertine material indicate that the Texas Toothpick stalagmite in the Lower Cave level of Carlsbad Cavern began forming about 600,000 ± 200,000 yrs ago (Table 24). Since speleothems in the upper levels of a cave can be older than those in the lower levels, the travertine in passages such as Bat Cave, Carlsbad Cavern, can significantly exceed this age. In other, higher-elevation Guadalupe caves such as Cottonwood and Virgin, the speleothems can be even older than those in Carlsbad Cavern (Fig. 86).

Subaerial speleothem growth has continued uninterrupted to the present day except where it has been subject to a growth hiatus (Fig. 77), condensation-corrosion, or where possible earthquake tremors have broken speleothems. Davis (1980) speculated that some of the stalactites in the Temple of the Sun, Big Room, were broken by earthquake tremors. In Colonel Boles alcove, Nooges Realm, Lower Cave, two columns are cracked and both are offset about 1 cm in the same direction. This displacement may be explained by earthquake tremors or, more likely, by subsidence and slumping of the silt banks beneath the columns. In the Mystery Room, Carlsbad Cavern, and in Deep and Ogle Caves, massive columns detached along the ceilings and then toppled to the floor, fracturing into naturally cross-sectioned pieces like giant columns in a Roman temple (Fig. 131). Likewise, in the Main Corridor of Carlsbad Cavern, broken pieces of massive stalagmites can be seen along the trail. The fracturing and toppling of these speleothems may have been caused by earthquake shocks or, alternatively, it may simply be the result of speleothem old age.

Origin of sulfur

The native sulfur in Guadalupe caves is believed to have formed as a direct sublimation product of hydrogen-sulfide gas which entered the caves via injection points (i.e. along the Bell Canyon Formation or along joints). Evidence for a subaerial interpretation is:

(1) Sulfur crystals coat the undersides of tilted Bell Canyon(?) bedrock in the New Mexico Room and tilted forereef beds in the Christmas Tree Room, occurrences which suggest that hydrogen-sulfide gas ascended up-dip along bedding planes until it reached the air-filled caves where it reacted with atmospheric oxygen to form native sulfur.

(2) The Big Room sulfur occurs over a drip tube, a subaerially formed dissolution feature.

(3) Sulfur crystals fill the crevices between masses of stacked cave rafts in the Christmas Tree Room; this suggests gas infiltration into the crevices.

(4) Sulfur directly overlies gypsum flowers and crusts in the New Mexico Room. If the sulfur had been in aqueous form, it would have dissolved away the gypsum speleothems.

(5) The Cottonwood Cave sulfur is enclosed in massive gypsum, as if H₂S had been pumped up through the drained floor and the decanted gypsum.

The direct oxidation of H₂S cannot occur in the gas phase unless a combustion source is present, but it can form on wet surfaces, especially with the assistance of sulfur bacteria:

2H₂S_(l) + O_2(g) = 2S_(s) + 2H₂O

The sulfur could have deposited any time after the caves became air-filled, but the sulfur overlying speleothems suggests that it may be a relatively young deposit. In the Christmas Tree Room, the sulfur crystals overlie a yellowish-brown, iron-rich crust on limestone fins; this occurrence may be explained if one postulates more than one episode of H₂S gas infiltration and sulfur deposition. Older sulfur may have oxidized in the moist cave environment to sulfuric acid which dissolved the limestone fins. It may have also reacted with iron in the bedrock to form pyrite which then oxidized to the limonite crusts. If this speculation is correct, it may indicate the episodic release of gas under pressure in the basin and "pulses" of gas injection into the caves of the reef.

Condensation-corrosion

Condensation-corrosion (also called "gas weathering") is the process by which water contained in the air and charged with a high level of carbon dioxide condenses out on bedrock or speleothem surfaces and corrodes them. Three atmospheric conditions are needed in a cave before condensation-corrosion can occur: a high CO₂ level in the air, a high amount of moisture (humidity) in the air, and a temperature gradient between the air in different passages. The temperature gradient drives warm, moisture-laden air to areas of lower temperature and humidity; the dew point of the air is reached, and water condenses on speleothems and bedrock and corrodes them.

The carbon dioxide needed for condensation-corrosion in Guadalupe caves could have derived from meteoric water or, according to the theory of speleogenesis proposed in this study, some carbon dioxide may have migrated up from the basin along with hydrogen-sulfide gas. Carbon dioxide, degassing at the surface of the water table, formed calcite rafts at the water surface and corroded bedrock and speleothems above the water surface (Fig. 89). Both Type I and Type II cave rafts are believed to have originated in this manner. Type I rafts, which are usually associated with calcified siltstone, formed near the beginning of Solution Stage III as depicted in Fig. 90; Type II rafts formed near the end of Solution Stage III probably from static backwater associated with late-stage climatic fluctuations.

Condensation-corrosion as modification of geomorphic and speleothemic forms—Characteristic geomorphic and speleothemic forms are caused by the process of condensation-corrosion. Air scallops develop in areas of pronounced corrosive air flow; they modify phreatic solution pockets and can cut across both bedrock and speleothems. Rillenkarren and spitzkarren are caused by acidic water which condenses on cave ceilings. When the water drips to the floor, it drills corrosion furrows in floor bedrock, breakdown, and flowstone (Figs. 16, 27, 29). Corrosion channels form when CO₂-rich air moves along a cave ceiling. "Punk rock" forms where corrosive air attacks and weathers the bedrock.

Condensation-corrosion can also modify speleothem surfaces or produce distinct speleothem types. Rims are a type of speleothem produced by this process, and the dull-white speleothems in Spider Cave and in the Lake of the Clouds area, Carlsbad Cavern (Pls. 10A, B, Fig. 64), have been highly corroded by this process. Condensation-corrosion may be an ongoing process in some passages, e.g. at Taffy Hill in the Main Corridor, Carlsbad Cavern. Dense fogs sometimes accumulate there, especially in the fall (McLean, 1976), and the condensation water may be responsible for the corrosion of drapery speleothems. Thrailkill (1965b, figs. 28, 31) showed that the water on Taffy Hill flowstone goes from slightly calcite-supersaturated to slightly calcite-undersaturated down-flow, as might be expected if condensation water was diluting flowstone-depositing water. The process of condensation-corrosion may be exemplified by the Lake of the Clouds-Bell Cord Room area, Carlsbad Cavern (Figs. 93, 94):

(1) Carbon dioxide degassed from the water table when the water table was at the level of the Lake of the Clouds Passage. The degassing caused cave rafts to precipitate at the water surface and cave clouds to coat rock projections beneath the water surface (Fig. 94). Water dripping from the ceiling sank the floating rafts and they accumulated at drip points to form the giant cones of the Balcony.

FIGURE 93—Corrosion features in the Lake of the Clouds Passage, Bell Cord Room, and Left Hand Tunnel, Carlsbad Cavern. The corroded sides of speleothems face in the direction of the Lake of the Clouds. Passage width is exaggerated and some side passages are not shown. Arrows indicate direction of air flow causing corrosion. Compare with Fig. 94. (click on image for a PDF version)

FIGURE 94—Vertical view of Lake of Clouds Passage, Bell Cord Room, and Bifrost Room, Carlsbad Cavern, showing corrosion features. Darkened parts of stalagmites are the corroded sides. Arrows show direction of ascending air flow. Compare with Fig. 93.

(2) The solubility of carbon dioxide in water increases as the temperature decreases. If the air at the Lake of the Clouds was warmer than the air in upper connected passages (as is the case today, Fig. 19), then this temperature gradient would have forced the air to rise from the Lake toward higher cave passages and to collide with bedrock and speleothems equilibrated at a lower temperature. The carbon-dioxide-charged water thus condensed on speleothems or bedrock and corroded them in a direction facing the Lake of the Clouds. Air flow translocated the condensed water (now saturated with calcium carbonate) around to the edges of the corroded areas, and degassing of excess carbon dioxide caused the deposition of rims along the perimeter of the corroded speleothems or bedrock (Fig. 63).

(3) As the corrosive, moisture-laden air continued to rise toward the Bell Cord Room, it moved along the ceiling, carving out a corrosion channel and corroding speleothems on the ceiling, but not those on the floor (Fig. 94). Ceiling speleothems became so corroded by the aggressive moisture that, in places, they completely disintegrated down into the joints along which they had formed. Where air flow directly impacted bedrock, the carbonate content of the rock was dissolved away, leaving insoluble residue of soft, friable, dark-brown "punk rock." The residue then flaked off onto the floor and formed mounds beneath the corroded punk rock.

(4) In the Bell Cord Room, aggressive water condensed on the ceiling and dripped down onto the floor bedrock and flowstone, corroding out drip points (Fig. 28) and rillenkarren (Fig. 27). This "acid rain" also dripped onto the apices of stalagmites and cave cones, drilling out their hollow centers or volcano-like shapes (Fig. 94).

The process of condensation-corrosion has been documented from caves of other regions as well. In Guisti Cave, Italy, Forti and Utili (1984) found cave clouds, cones, rafts, folia, and corrosion "furrows" (rillenkarren) in an actively forming part of the cave. In Castellana Cave, Italy, P. Forti (pers. comm. 1985) found "corrosion domes" in a dead-end passage of the tourist section of the cave. The air in that passage had 2.5% CO₂, whereas in the non-tourist parts of the cave it had 0.05% CO₂. The corrosion domes in Castellana Cave are believed to have formed within the last 20 years or so, since the cave became commercialized. Sweeting (1973, pgs. 79, 81) described rillenkarren as being "most perfectly developed where water is most highly charged with CO₂. . . . Rillenkarren are formed rapidly, possibly in the space of a few months or years." It is quite conceivable that the corrosion features seen in Carlsbad Cavern and other caves in the Guadalupe Mountains were also produced in a short period of time.

Condensation-corrosion and popcorn line—Condensation-corrosion is believed to have been responsible for the popcorn line of the Big Room-Left Hand Tunnel-Green Lake areas of Carlsbad Cavern. This is supported by the CO₂ measurements and the dating results of this study. U-series and ESR dates on the popcorn of the popcorn line varied from 33,000 ybp to >350,000 ybp (Table 24, samples 23, 24, 25, 26). Thus, the popcorn formed over an extended length of time rather than in one "waterline" episode, as suggested by Jagnow (1979). The dates on the popcorn by the Lion's Tail, Big Room, and on the travertine beneath the popcorn (Table 24, sample 27) show a gradual change from a moist, travertine-depositing environment to a more evaporative, popcorn-depositing environment.

Carbon-dioxide levels along Left Hand Tunnel are consistently higher near the ceiling than they are near the floor (Table 5). (Measurements obtained by the second bridge are an exception to this rule; these were made one-third of the way to the ceiling and one-third of the way to the floor because the actual ceiling and floor were impossible to reach.) Thus, condensation-corrosion should be expected to take place preferentially near the ceiling; this is corroborated by the corrosion on the upper walls and ceilings of the passage, above the popcorn line.

The present-day carbon-dioxide levels (up to 1,000 ppm; Table 5) are probably not nearly as high as in the past, when the water was much higher in the reef and CO₂ was degassing from the surface of the water table. The process of condensation-corrosion has probably been in effect from the beginning of the subaerial stage until the present, but it appears not to be nearly as active today. This is evident from a number of wet and dry "post-corrosion" speleothems in the Lake of the Clouds area that are uncorroded (Pl. 11A). Flowstone in the Bell Cord Room, which displays rillenkarren corrosion (Fig. 27), has been dated at about 150,000 yrs (Table 24, samples 18, 19); the main corrosion event in this area thus took place after that time. One small area of condensation-corrosion in progress can be seen near the second bridge in Left Hand Tunnel, where there is a sudden change from 89% to 100% humidity (Table 5). Here, droplets of water are condensing on the eastward (humid) side of a limestone pendant, and wet, flat-bottomed (tray) popcorn is depositing on the westward (drier) side of the pendant, below the popcorn line.

The popcorn line in Carlsbad Cavern can be related to the process of condensation-corrosion and also to patterns of air flow and density. The Green Lake-Lion's Tail-Crystal Springs Dome areas of popcorn all line up (Fig. 62), as if dry air from the entrance has moved into the cave along this route. Side passages do not experience such direct-line air flow, and so the popcorn line "falls off" into side passages; or, where passages make a turn (such as near the Temple of the Sun), the popcorn line jags abruptly downward. Cold, dense air sinks to the floor, whereas warm, less dense, humid, corrosive air rises to the ceiling. Based on the fact that the popcorn line is approximately at the same elevation everywhere in the cave (Table 19), the air that was responsible for it must have been highly stratified, with cold, dry air from the entrance settling to the floor in passages at the Big Room level, and with warm, moist air reaching the ceiling as it flowed out of the cave. The sharp est stratification in temperature, humidity, and CO₂ content appears to have been at the wall "notch," which corresponds to the maximum diameter of the passage.

Bat guano

Bats are known from the fossil record back to Eocene time (Romer, 1966), and so they could have inhabited Guadalupe caves since the passages became open to the surface. Bat guano in Guadalupe caves has been carbon-dated by other studies, but only minimum dates have been achieved for the deposits using this method (Table 24). The bat guano in New Cave has been carbon-dated at >17,800 ybp in one analysis, and from >28,150 (22 cm below a flowstone caprock) to >32,500 ybp (2.2 m below the caprock) in another analysis. A guano sample from an upper level in a miner's trench, Ogle Cave, has been dated at 4,150-7,300 ybp (D. DesMarais, pers. comm. 1983).

Animal bones

From the dates on animal bones (Table 21) it appears that Carlsbad Cavern has been open to the surface longer than other Guadalupe caves. This is perhaps due to the fact that the Natural Entrance is at the top of a ridge, whereas entrances to other Guadalupe caves are deeper in canyons which have been more recently dissected by erosion. The 111,900 ybp date on the sloth bones of Lower Devil's Den, Carlsbad Cavern (Table 24), suggests that the Natural Entrance was open during, and possibly ever since, that time. (Type II rafts on the Balcony of the Lake of the Clouds Passage, dated at 50,000 yrs, have a carbon-oxygen signature which also suggests substantial air flow and evaporation by this time; see Fig. 98, plot of Type II versus Type I rafts.) The dates on animal bones in other Guadalupe caves confirm that erosion of the reef had exposed many cave entrances and pits into which animals wandered or fell by at least 35,000 yrs ago.