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 geology of the Guadalupe Mountain region has been described by Lloyd (1929), King (1942, 1948), Newell et al. (1953), Hayes (1957, 1964), Hayes and Koogle (1958), Kelley (1971, 1972), and many others. In this study, the regional geology is discussed only insofar as it applies to cave-related questions.

Permian rocks in the region can be divided from southeast to northwest into three facies: the Delaware Basin, reef, and Northwest Shelf (Figs. 4, 5). The rock units of interest in the basin are the Bell Canyon Formation of the Guadalupian Series and the Castile, Salado, and Rustler Formations of the Ochoan Series (Table 1). In the basin-margin reef area the formation of interest is the Capitan Limestone, which is subdivided into a massive (reef) member and a breccia (forefeef) member. These two members are transitional with each other both laterally and vertically. The breccia member of the Capitan grades southeastward into the Bell Canyon Formation of the Delaware Basin. In the backreef Northwest Shelf area, the units of interest are the Queen, Seven Rivers, Yates, and Tansill Formations of the Artesia Group. The upper three units are laterally equivalent to the Capitan Limestone.

FIGURE 4—Stratigraphic units of the Northwest Shelf, reef, and Delaware Basin during Permian time. After Bachman (1980).

FIGURE 5—Geologic map of the Guadalupe Mountains showing physiographic features, bedrock units, and cave locations. After King (1948) and Moore (1960a). (click on image for a PDF version)

TABLE 1—Major stratigraphic divisions, southeastern New Mexico (after Bachman, 1980).

Tertiary deposits in the area include the Ogallala and Gatuña Formations, alluvial gravels of probable Pliocene and Pleistocene age, respectively (Bachman, 1976).

Summary of geologic history

A lowland that existed in southeastern New Mexico during the Precambrian and Cambrian became inundated by shallow seas in the Late Cambrian through Mississippian. The region first became tectonically active in the Precambrian, and during the Pennsylvanian compressional forces caused movement along a northwest-trending thrust fault. Renewed tectonic activity in the Early Permian resulted in a rapid sinking of the Delaware Basin, an elongate, bowl-shaped depression around which grew an extensive organic reef known as the Capitan Limestone. As this reef grew upward, reef debris slumped basinward, and, as new generations of reef organisms progressively colonized the debris pile, the reef grew over its own debris and outward, toward the basin. In shallow lagoons behind the reef, sandstone, limestone, and dolomite beds of the Artesia Group were deposited contemporaneously with the reef limestone; in the basin itself, thin petroliferous limestone and sandstone layers of the Bell Canyon Formation and thick evaporite sequences of the Castile Formation accumulated.

At the close of Permian time, roughly 230 my ago, the entire region was tilted and uplifted above sea level, as indicated by an angular unconformity between Late Permian beds and overlying terrestrial beds of the Triassic Dockum Group. The rock record for most of the rest of the Mesozoic is absent, possibly because the Guadalupe Mountain region was above sea level throughout the Jurassic and most of the Cretaceous. The sea again spread over at least part of the area late in the Cretaceous as confirmed by small patches of fossiliferous Cretaceous rock found on the surface and collapsed into the subsurface of the Gypsum Plain. In the Late Cretaceous or early Tertiary, the area was affected by another broad uplift which gently folded bedrock and tilted it slightly to the northeast.

By the middle Tertiary, or roughly 10 my ago, the area was reduced to an erosion surface of low relief. The presence of siliceous gravel, dendritic stream patterns, and entrenched meanders in the Guadalupe uplands are evidence that the area was at or near base level during that time.

The main uplift and tilting of the Guadalupe Mountains occurred in the Pliocene to Pleistocene (1-3 my ago). Concurrent with uplift, oil and gas accumulations in the Bell Canyon Formation of the Delaware Basin migrated updip toward the reef. Hydrologic connection between the Capitan Limestone aquifer and the Pecos River was probably established during this time, and, since then, movement of ground water in the reef aquifer has been controlled by changes in the regional base level of the Pecos River.

Stratigraphy

Permian deposits

Backreef—In Permian time the Capitan reef acted as a discontinuous barrier behind which the light-colored, thin-bedded limestones, sandstones, and dolomites of the back-reef Northwest Shelf facies accumulated. These beds were deposited contemporaneously with the reef itself and dipped toward the reef at angles of a few degrees. The oldest major cave-bearing rocks of the shelf facies are the Queen and Seven Rivers Formations, units composed of dolomite beds which thicken conspicuously as they approach the limestone reef. Overlying the Seven Rivers Formation are the Yates and Tansill Formations which act as caprock protecting the more soluble Seven Rivers from dissolution and truncation.

The Yates Formation is an especially silty and sandy unit containing limonite pseudomorphs after pyrite disseminated in the upper two-thirds of the formation. The Yates Formation is too silty to be cave-bearing except near its contact with the reef or with the Seven Rivers Formation; such caves as Spider Cave and the McKittrick Hill caves are developed at these localities. In Spider Cave, an apparent dip of 2°S has been measured on Yates bedding planes (R. Bridgemon, written comm. 1980). Hill (1972) reported a porosity of 0.5% and a permeability of 0.02 millidarcies parallel to bedding for a silty limestone believed to be part of the Yates Formation in the Main Corridor of Carlsbad Cavern near Devil's Spring.

The Tansill Formation is a thin to medium-bedded, white to tan, microgranular to fine-grained dolomite which Tyrrell (1964) believed to be equivalent in age to the Lamar Member of the Bell Canyon Formation in the Delaware Basin. No younger beds of Permian age overlie the Tansill Formation within the study area, but the Salado Formation overlies it in the subsurface east of the Pecos River. In the vicinity of the entrance to Carlsbad Cavern, the Tansill Formation strikes N77°E and dips about 3-4°SE (Sanchez, 1964). Hill (1972) reported that the Tansill Formation near the cave entrance has a porosity of 2.2% and a permeability of 0.51 millidarcies parallel to bedding and 0.01 millidarcies perpendicular to bedding.

Reef—The Capitan Limestone includes a massive reef-core facies and a breccia (talus) forereef facies. These two facies grade laterally and vertically into each other and together reach thicknesses of 450-600 m. The reef-core facies is poorly bedded to massive and weathers to form steep cliffs in the canyons of the Guadalupe Mountains. The forereef facies is a well-cemented breccia which weathers to ragged slopes; it consists of thick, crudely bedded layers of limestone that have depositional dips of 20-35° (Hayes, 1957; Ward et al., 1986).

The reef-core facies of the Capitan Limestone is a fine-grained, light-gray to cream-colored, vuggy limestone. Fossils are mostly algae and sponges and, in general, are poorly preserved. The Capitan reef limestone is a porous unit which serves as the aquifer for much of the Guadalupe Mountain area. Hill (1972) reported a porosity of 1.5% and a permeability of 14.0 millidarcies for the Capitan Limestone exposed near Iceberg Rock, Carlsbad Cavern. Primary cavities of the reef core are up to several centimeters in diameter, and secondary enlargements can range from pore-size holes to cavern-size chambers (Palmer, 1975). Most of the caves in the Guadalupe Mountains are developed in the reef-core facies.

The forereef facies is a light-buff to pink, dolomitic brecciated limestone. Brachiopods (Fig. 6), bryozoans, crinoids, foraminifers, and sponges are plentiful in the forereef and are commonly well preserved. Breccia fragments in the forereef facies range from sand-sized particles up to boulder-sized clasts. The forereef facies is not nearly as cave-bearing as the reef core; cave passages usually terminate near where forereef beds are encountered.

FIGURE 6—Spiriferid brachiopod in a passage near Mabel's Room, Carlsbad Cavern. The delicate spiralia, which provided support for internal organs, are unusually well preserved in this specimen. Photo Arthur Palmer.

The Capitan Limestone has been interpreted by most investigators to be a barrier reef (Lloyd, 1929; King, 1942; Adams and Frenzel, 1950; Newell et al., 1953; Hayes, 1964), but others have considered it either a linear organic bank (Achauer, 1969) or a line of organic mounds separated by lagoons connecting the shelf and basin provinces (Motts, 1957; Dunham, 1972; Cronoble, 1974). Achauer divided the Capitan Limestone into three distinct lithologic facies: an organic-skeletal limestone which constitutes the massive reef core, an organic-skeletal dolomite which constitutes the reef talus, and a dolomite breccia, a local facies found at or near the contact of the reef limestone and backreef shelf rocks.

Basin—While an immense reef structure was building upward around the margins of the Delaware Basin, shaly, organic limestones and clean, fine-grained sandstones of the Bell Canyon Formation were contemporaneously being deposited within the basin. Then, as the Delaware Basin became closed off and sea water was restricted from entering it, thick evaporite sequences of the Castile, Salado and Rustler Formations were deposited subsequent to reef growth.

The Bell Canyon Formation is composed of five limestone members and five sandstone members. The Lamar Member is the principal oil- and gas-bearing rock in the basin. It is an easily recognizable, black, shaly, petroliferous limestone of low permeability and porosity, widely used by oil geologists as a subsurface marker. The Ramsey sandstone member of the Bell Canyon Formation consists of very clean, well-sorted, fine-grained quartz-sand beds (Grauten, 1965; Watson, 1979); it averages about 26% in porosity and 50 millidarcies in permeability (Table 12). Near the Capitan reef margin the Bell Canyon Formation is characterized by gentle dips (less than 1°), but where it interfingers into the reef, beds dip upward at angles of 10-30° (King, 1942; Adams and Frenzel, 1950).

Overlying the Bell Canyon is the Castile Formation, a thick, deep-water, evaporite sequence composed of several hundred meters of intricately layered anhydrite, gypsum, halite, and subordinate limestone. Layering within the Castile is cyclic, with calcite and anhydrite alternating every few millimeters and with each calcite-anhydrite couplet representing an annual varve or periodic accumulation of material. Based on varve counts, Dean and Anderson (1978) estimated that the rate of gypsum precipitation within the Delaware Basin was approximately 1.9 mm per year. Cycles having periods of 1,000 to 3,000 years are persistent features of the Castile Formation; these cycles have been correlated with great precision over the entire basin for distances up to 113 km (Anderson et al., 1972).

Three halite members occur within the Castile Formation. The lowermost member, Halite I, is the thickest and most extensive. The halite members have become significantly eroded, the top ones having experienced more erosion from meteoric water than the bottom one. All three members extended beyond their present limit into the western part of the basin earlier in the Tertiary, but since then the halite-solution margin has moved progressively eastward and dissolution of the salt has formed blanket-solution breccia zones where the halite used to exist (Anderson et al., 1972).

The eroded surface of the Castile Formation south of Carlsbad, New Mexico, forms what is known as the Gypsum Plain (Fig. 7). It is a karst terrain containing sinkholes, troughs, collapse breccia, sinking streams, and gypsum caves (Lee, 1924b; Weberneck, 1952; Kirkland and Evans, 1976; Gutierrez, 1981; Smith, 1981; Sares, 1984). Unusual features of the Gypsum Plain are the "castiles"—circular, haystack-like, limestone buttes rising a few to 30 m above the level of the plain (Adams, 1944). These steep-sided buttes were created by the replacement of anhydrite and gypsum by calcite, a process which preserved even the most minutely varved and microfolded textures in the evaporite rock (Anderson and Kirkland, 1966). The castile buttes are located at or near the contact of the Castile Formation with the underlying Bell Canyon Formation (Fig. 84) and are the sites of hydrogen-sulfide degassing and sulfur mineralization.

FIGURE 7—The Gypsum Plain as viewed from Big Canyon, Guadalupe Mountains. Note blade-like projections of the unbedded Capitan Limestone and the horizontal bedding planes of the Seven Rivers Formation. Photo Alan Hill.

Tertiary deposits

Tertiary deposits of the Ogallala and Gatuña Formations overlie Permian rocks because Triassic, Jurassic, and Cretaceous sediments were never deposited or have been mostly (Cretaceous) removed by erosion. Gravel of the Ogallala Formation consists primarily of worn cobbles of limestone and pebbles of quartzite, chert, jasper, and basalt (Bretz and Horberg, 1949b). Horberg (1949) reported quartzose gravel on the flat-topped summits of the Guadalupe Mountains and believed they indicated a former peneplain at that level. Horberg thought these gravels were Ogallala in age, but G. Bachman (pers. comm. 1986) favors a Gatuña identity for this gravel. Remnants of this gravel veneer the summit ridge of Carlsbad Cavern 0.4-1.0 km east of the cave entrance (Fig. 8). This gravel consists of: (1) very well-rounded quartzite pebbles of assorted colors (white, tan, gray, pink, black); (2) subrounded pieces of black flint; (3) small subrounded pieces of red jasper; and (4) rare pieces of rounded, scoriaceous lava (Fig. 9). No rounded limestone cobbles coexist with the siliceous gravel on the Carlsbad Cavern Ridge; this is probably due to preferential erosion of the limestone over time with respect to the more resistant siliceous pebbles.

FIGURE 8—Location of Ogallala (or Gatuña) gravel remnants on the surface near Carlsbad Cavern (dotted area). (click on image for PDF versio)

FIGURE 9—Ogallala (or Gatuña) gravel remnants on Carlsbad Cavern Ridge about 0.5 km east of the Natural Entrance. Photo Ronal Kerbo.

Stratigraphic controls on cavern formation

Four main zones of preferential cave solution are controlled by stratigraphy (Jagnow, 1979; Fig. 10):

(1) Below the Yates transition into the massive Capitan Formation. The three largest caves in Carlsbad Caverns National Park—Carlsbad Cavern, Ogle Cave, and New Cave—occupy this stratigraphic position. The relatively impermeable, fine-grained siltstone of the Yates Formation collects oxygenated ground water from the Northwest Shelf area and discharges it directly into the massive member of the Capitan Limestone.

(2) At the contact between the massive and breccia facies of the Capitan Limestone. The contact between the massive and breccia facies of the Capitan Limestone is another stratigraphic location for cavern development because it is here that the bedding planes of the breccia facies interrupt the movement of ground water in the vertical joints of the massive facies. Caves which have formed at this contact are Musk Ox, Helen's, Wen, Vanishing River, Pink Fink Owl-cove, and Frank's. Carlsbad Cavern, Ogle Cave, and New Cave also occupy this stratigraphic position, in addition to being below the Yates-Capitan transition.

(3) At the transition of the Capitan Limestone with other members of the Artesia Group. Oxygenated ground water moves downdip along the Tansill, Seven Rivers, and Queen Formations until it meets with the Capitan Limestone. Caves formed at the junction of these units are Three Fingers, Sentinel, Big Door, Lechuguilla, Pink Dragon, Pink Palette, Pink Panther, Wind (Hicks), Madonna, Goat, Virgin, and Damn.

(4) Beneath the Yates contact with the Seven Rivers. Where the Yates caprock is broken by major joints, oxygenated ground water descending from the surface has access to the underlying Seven Rivers Formation. Caves formed at or near this contact are Cottonwood, Queen of the Guadalupes, Hidden, Hell Below, Cave Tree, Decorated, McCollhum's Pit, Black, Dry, Endless, McKittrick, Sand, and Little Sand.

FIGURE 10—Four zones of preferential solution (shaded areas): (1) below the Yates transition into the massive Capitan Limestone, (2) at the contact between the massive and breccia members of the Capitan Limestone, (3) at the transition between the Artesia Group members and the Capitan Limestone, and (4) immediately beneath the Yates Formation in the Seven Rivers. Arrows indicate movement of ground water along impermeable siltstone in the Yates Formation. After Jagnow (1979).

Structure

Regional structure

Detailed accounts of the structural geology of the Guadalupe Mountains have been given by King (1948), Hayes (1964), Kelley (1971), and DuChene (1971, 1978). The Guadalupe Mountains are structurally simple compared to many other parts of New Mexico. Faults are spaced far apart, fracture frequency is low, and folds are very gentle (a few degrees). The Guadalupe Mountains were uplifted and tectonically folded and faulted in the Tertiary along with the entire western United States, starting about 10 my ago and continuing even today (Reilinger et al., 1979). The eastward to northeastward regional dip of the Guadalupe Mountains from Guadalupe Peak to Carlsbad is about 1°

Faults—The Guadalupe Mountains are faulted along their western flank where northwest-trending, arcuate, high-angle faults have stratigraphic displacements on the order of 600-1,200 m (Fig. 11). As the Guadalupe Mountains uplifted along their southwest margin, beds were tilted to the northeast. Kelley (1971) proposed that two strike-slip faults parallel the reef escarpment, the Carlsbad fault in the north near Carlsbad, and the Barrera fault which occurs south of the Carlsbad fault but north of Double Canyon. Hayes and Bachman (1979) questioned the presence of both of these faults.

FIGURE 11—Structural map of the Guadalupe Mountains region of southeastern New Mexico and western Texas: (1) Guadalupe Ridge anticline, (2) Walnut syncline, (3) Reef anticline, (4) McKittrick Hill anticline, (5) Huapache monocline, and (6) linear scarps in the Gypsum Plain. After DuChene (1978) and Jagnow (1979). (click on image for a PDF version)

The Castile Formation in the Delaware Basin is marked on its western part by sets of linear fault scarps trending N75°E to N80°E, or approximately parallel to the direction of regional dip (Figs. 2, 11). Olive (1957) called these scarps "solution-subsidence troughs," and described them as being 1-15 km long, up to 1.6 km wide, and less than 6 m deep. The troughs do not extend into the underlying Bell Canyon Formation, but have formed as subsidence blocks from the solution of the Castile evaporites (Davis and Kirkland, 1970). They are restricted to halite-free areas of the Castile Formation (i.e. west of the halite limit; Fig. 85). A second group of faults in the Gypsum Plain have been determined by sulfur test drilling. Smith (1978a) called these faults "graben-boundary faults" and described them as being nearly vertical, trending about N65°E and having displacements on the order of 7-25 m that affect both the upper Bell Canyon Formation and lower Castile Formation (Fig. 84). Sulfur deposits in the castile masses are localized along graben-boundary faults.

Folds—Crossing regional dip and paralleling the reef escarpment is a fold belt approximately 8 km wide, known as the Guadalupe Ridge folds (Fig. 11). Where the reef escarpment turns sharply from northeast to northwest just south of Carlsbad, the Guadalupe Ridge folds also veer in this same direction. A series of folds in the northern part of the belt is called the Carlsbad fold complex; the McKittrick Hill caves are developed along these anticlinal folds. Another main structural feature in the area is the Huapache monocline, a flexure 0.8-4 km wide which dips 3-5° to the northeast. This monocline becomes obscured both at its northwestern and southeastern terminations and also where it intersects the reef escarpment. According to recent sulfur-exploration maps, the monocline in the basin intersects the reef somewhere between Rattlesnake Canyon and Carlsbad Cavern (Fig. 12). The structural relief of the monocline is 90-300 m in the Guadalupe Mountains and on the order of 90-200 m in the Delaware Basin (Hayes, 1964; Kelley, 1971). The Huapache monocline is a surface representation of the draping of Permian and younger sediments over a thrust fault or series of faults in the pre-Permian basement (Powers et al., 1978).

FIGURE 12—Location of the Huapache monocline in the Delaware Basin, contoured (in meters) on top of the Bell Canyon Formation in the subsurface. Courtesy of Leonard Minerals. (click on image for a PDF version)

Small-scale folding has occurred in some backreef shelf beds, especially in the Tansill Formation. These folds, called "tepee structures," have been mildly deformed into chevronlike patterns resembling inverted V's. Tepee structures are up to 9 m high and wide and deformed beds are joined together so as to form steep-crested buckles.

Joints—The Capitan Limestone displays a conspicuous system of nearly vertical joints. Major joints trend either parallel or perpendicular to the reef escarpment; less frequently, joints follow the strike of bedding. Where joints have become case-hardened by mineralizing solutions, prominent blade-like projections stand out in relief along limestone cliffs (Fig. 7). Jagnow (1979) reported a strong correlation between cave locations and mineralized joints, and speculated that solutions mineralizing the rock may have also been responsible for carving out the caves. Within the Delaware Basin, joint sets with northeasterly and northwesterly strike are recognizable (Anderson, 1981).

Sandstone dikes—Sandstone dikes fill joint sets at or near the reef-backreef contact between the Capitan Limestone and the Seven Rivers, Yates, and Tansill Formations (Newell et al., 1953). Hayes (1957, 1964) and Dunham (1972) classified the sandstone dikes in the Guadalupe Mountains into two types based on textural, mineralogical, and size differences: (1) tens of centimeters thick, irregularly branching dikes composed of homogeneous, fine-grained sand cemented in a quartz or quartz-calcite matrix, and (2) several meters wide dikes, sometimes nearly vertical, with a trend generally parallel to the reef escarpment, and composed of coarse sand to gravel cemented with dolomite. Age of the dikes is questionable. Adams and Frenzel (1950) thought that type (1) sand was Yates debris intruded into fractures during reef growth in the Permian. These appear to have filled original connecting voids in the reef limestone (Newell et al., 1953). Horberg (1949) believed type 2 dikes to be of Ogallala age, and King (1948) believed that they might be Early Cretaceous. Jagnow (1979) identified no less than 15 dikes transecting the walls of Ogle Cave and proposed that type (1) dikes were of Permian age and type (2) dikes of Pliocene age.

Structural controls on cavern formation

Five structural controls influence cavern formation; one of them is of regional importance, the others are only of local importance.

(1) Joint control. Joints are the primary structural control on cavern development and the major trunk passages of the caves in the Guadalupe Mountains are, almost without exception, aligned along joints which either parallel the reef or are perpendicular to it. Jagnow (1979) found that east of Rattlesnake Canyon the major joints along which caves are developed trend in a northeastern direction parallel to the reef front, whereas west of Rattlesnake Canyon the cave-bearing joints trend in a northwestern direction perpendicular to the reef front, a change which Jagnow ascribed to the structural influence of the Huapache monocline.

(2) Huapache monocline. The draped, folded strata of the Huapache monocline act as avenues for solutional enhancement. This control may be one factor responsible for the development of caves in Slaughter Canyon which are located near the monocline in the reef.

(3) Sandstone dikes. Sandstone dikes are zones of structural weakness along which water can move. This structural influence seems to be especially pronounced in the case of Ogle Cave.

(4) Anticlinal folds. The McKittrick Hill caves—Endless, Dry, Sand, Little Sand, and McKittrick—are all formed along the flanks of the McKittrick Hill anticline. Flexed rocks provide avenues for ground-water movement.

(5) Tepee structures. Minor cave development has occurred along tepee structures, local avenues for ground-water movement. Small caves developed along the axial fractures of tepee structures are Tepee Cave and Jurnigans Cave #2.

Hydrology

Regional hydrology

The hydrology of the Carlsbad region has been described by Motts (1957), Bjorklund and Motts (1959), and Hiss (1974, 1975, 1980). The permeable Capitan Limestone acts as the aquifer for the region, and the entire upland surface of the Guadalupe Mountains is the recharge area (about 2,000 km²) Water percolating through the shelf rocks of the Artesia Group drains downdip into the Capitan Limestone aquifer and then moves slowly through the reef to its point of discharge at Carlsbad Springs. In the vicinity of Carlsbad, the Capitan reef plunges beneath the surface and comes in contact with the alluvial fill of the Pecos River valley. The water of the Capitan Limestone aquifer at Carlsbad is of good quality and has a low dissolved-salt content (Table 2).

TABLE 2—Water analyses (in ppm) for the Capitan Limestone aquifer: (a) Dark Canyon water supply area, City of Carlsbad; (b) White City; (c) potassium content of water, Dark Canyon water supply area, City of Carlsbad. Data supplied by J. Wright, State Engineer Office, Roswell, and J. Smith, Environmental Improvement Division, Carlsbad.

Both water-table and artesian conditions exist in the Capitan reef aquifer, but unconfined conditions prevail. Only where the aquifer is confined by overlying alluvial deposits, as in the vicinity of Carlsbad, do continuous artesian conditions persist, and then the hydrologic head is only a few meters at most (Bjorklund and Motts, 1959). Hiss (1975) estimated that the Capitan aquifer has an average gradient of about 0.2-0.4 m/km (1-2 ft/mi), but, based on limited well data, it may be as high as 0.85 m/km (4.5 ft/mi) (Sheet 1). Calculations using Darcy's Law and a permeability of 14.0 millidarcies for the Capitan reef limestone (Hill, 1972) show that velocity of flow through primary pores should be approximately 5-20 cm/yr. Secondary conduit flow in the Capitan Limestone aquifer, as determined from the flow rate at Carlsbad Springs, is on the order of 0.4 m³/sec (13-15 cfs) (Pecos River Joint Investigation, 1942).

The hydrologic system of the Gypsum Plain operates in dependently of the hydrologic system of the Capitan reef aquifer. The impermeable anhydrite of the Castile Formation forms an effective seal inhibiting water movement between the evaporite basin and the limestone reef. In an important recent study, Sares (1984) showed that the potentiometric (piezometric) level of the alluvial-evaporite aquifer in the Gypsum Plain differs by nearly 107 m from the Capitan aquifer in the reef (Fig. 13). He also showed that upland erosional terraces in the Black Canyon-Chosa Draw area of the Gypsum Plain do not correlate with the major horizontal cave levels in Carlsbad Cavern, a fact that implies that these two aquifers have operated independently in the past as well.

FIGURE 13—Comparison of major cave levels and cave passages in Carlsbad Cavern with terrace levels in the Gypsum Plain. Note that the potentiometric surface in each aquifer is not at the same level. After Sares (1984). (click on image for a PDF version)

The same hydrologic conditions that exist today in the Carlsbad region have probably operated in a similar manner during the last half-million years or so since the time when the Ancestral Pecos River flowed along the western margin of the Delaware Basin (Bachman, 1984). As the Guadalupe Mountains uplifted and the Gypsum Plain lowered by solution subsidence, the resurgence point for the Capitan aquifer moved from southwest to northeast along the reef until it reached its present location at Carlsbad Springs (Fig. 86).

Hydrologic controls on cavern formation

Caves develop in three hydrologic regimes: the vadose zone, the water-table zone, and the phreatic zone. Vadose caves enlarge predominantly by free-surface streams eroding downward or laterally; water-table caves form along or at shallow depth beneath a potentiometric surface that is of greater extent than the cave; and phreatic caves form under total, permanent water fill where water is under pressure. Deep phreatic caves, where water is under high hydrostatic pressure, are termed "bathyphreatic." A given cave may be wholly vadose, phreatic, or water-table, but more usually it is a combination of two or all of these types.

The caves of the Guadalupe Mountains are a combination of deep phreatic (bathyphreatic) and water table. Water-table conditions have been responsible for the horizontal development of caves along certain levels, and bathyphreatic conditions have been responsible for the strong vertical development of these caves. No vadose cave enlargement has occurred in the Guadalupe Mountains, with the exception of minor modification of cave passages by intermittent, local-invasion, vadose drainage. Typical vadose features such as predominant scalloping, incised meanders, or dome pits are rare or absent in Guadalupe caves.

Bathyphreatic flow in the Guadalupe Mountains was caused by the tectonic uplifting of the mountains and by the low fracture frequency of the reef limestones. If the frequency of fissures penetrable by ground water is low, then recharge water must take a deep course through the bedrock if there is no efficient alternative at lesser depths (Ford and Ewers, 1978; Fig. 14). If substantial uplift occurs in a region, such as it has in the Guadalupe Mountains, then hydraulic gradients can become steep and there is a movement of water from upland recharge areas to springs at lower elevations. Where reservoir capacity is low and the minimum flow path to the spring is long, the potentiometric surface remains high in the rock and is steeply inclined.

FIGURE 14—(A) Model of bathyphreatic flow in the Guadalupe Mountains. Since the massive Capitan reef has a low fissure frequency and a relatively low permeability, water takes a deep course through the limestone. The water ascends under high hydrostatic pressure along joints, bedding planes, and at the contact of the reef with forereef and backreef facies, and eventually exits at a spring down-gradient from the recharge area. Since the reservoir capacity of the limestone is low and the minimum flow path to the spring is long, the potentiometric surface remains high in the rock and is steeply inclined. The future course of integrated passages in the Guadalupe Mountains is highly influenced by the development of these early bathyphreatic flow routes, even though, initially, these routes may be inaccessibly small. (B) Caves with a mixture of phreatic and water-table components. As the caves enlarge and water discharges primarily along developing trunk passages, the potentiometric surface "lowers" into sections of these passages which become ideal water-table caves. Spring shafts or joint chimneys constitute phreatic loops between horizontal levels. Spring positions shift down and northeastward as base level lowers. After Ford and Ewers (1978).

The caves of the Guadalupe Mountains show substantial horizontal development where major levels cut across bedding and base-level control is traceable between different caves and cave passages (Table 3). After phreatic cave systems have evolved for a substantial length of time, they are subject to modifications which change their character to a system increasingly more concordant with the water table. This is because as fissures enlarge after the onset of karstification and the volume of a cave-trunk system increases, the potentiometric surface slowly "lowers" into the trunk passage (Ford and Ewers, 1978; Fig. 14). The end result are caves, like those in the Guadalupe Mountains, which possess a mixture of phreatic and water-table components.

TABLE 3—Major cave levels in Guadalupe Mountains.