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

Stratigraphic problems

Past investigators have failed to decipher stratigraphic relationships between the different kinds of cave deposits and, consequently, the proper time sequence of speleogenesis events in Guadalupe caves has never been understood. There have been many reasons for this failure. For one, clean contacts between deposits are often difficult to find. For another, some deposits are hard to identify in the field; for example, decayed bat guano closely resembles silt. But perhaps the most fundamental reason is the basic misunderstanding of the mechanisms producing some of the cave deposits, particularly speleothem deposits such as popcorn coralloids. Previous investigators assumed that all popcorn forms under water because they saw nodular speleothems growing in cave pools. It is now known that, while some coralloids are subaqueous in origin, most coralloids form subaerially by a number of mechanisms: water seeping through cave walls, thin films of flowing water, or splash solutions. Thus, popcorn may underlie another deposit and look like it is older than that deposit, when in reality it may be younger. For example, thin crustal rinds of gypsum in Left Hand Tunnel, Carlsbad Cavern, overlie wall popcorn, and so look younger than the popcorn (Fig. 66); actually, the popcorn grew from water seeping through the wall behind the gypsum and pushed the earlier-formed gypsum out from the wall. In like manner, solutions can seep through bedrock and deposit popcorn and other carbonate material under gypsum blocks and breakdown or under and over silt. In Left Hand Tunnel, seeping water has deposited a moonmilk-like zone of carbonate mineralization at the contact between bedrock and floor silt (Sheet 9, column E); the moonmilk-like material underlies the silt, yet postdates it. Coralloidal crusts which overlie silt deposits in Left Hand Tunnel and Lower Cave are deposited by water seeping through the silt toward the silt-air evaporative surface (Figs. 34, 36).

FIGURE 66—A thin crustal rind of gypsum which has been pushed outward from the wall by seeping-water-type popcorn, Left Hand Tunnel, Carlsbad Cavern. The gypsum is older than the popcorn. Photo Alan Hill.

Another stratigraphic problem in Guadalupe caves concerns bat guano. Since guano falls from the ceiling in loose, uncompacted masses, it can easily gravitate below older features such as breakdown or gypsum blocks. For example, orange-colored bat guano has settled into the drip tubes of some of the gypsum blocks in the Big Room, Carlsbad Cavern (Pl. 4A). In the Talcum Passage, it has filtered completely through and under dissolved parts of the blocks so as to underlie portions of the gypsum. Another problem concerning bat guano is that it looks practically identical to silt when it decays. Both deposits are brownish orange and each may contain bat bones. The "bat guano under silt" described by Good (1957) as occurring in Bat Cave, Carlsbad Cavern, is a prime example of mistaken identity. M. Queen (pers. comm. 1984) analysed the "guano" material "under silt" and verified it to be more silt, a determination which is consistent with other observed stratigraphic relationships in the cave; that is, bat guano overlies silt, or in rare cases is mixed with silt, but it never underlies silt. The brick-red "guano" underlying gypsum blocks near the Salt Flats of the Big Room (Pl. 2B) is also silt, having an analyzed silica content of 57.55%. At least some of the material in New Cave thought to be pure bat guano is silt, as determined by analyses done by D. DesMarais (pers. comm. 1983). The only absolute way of distinguishing between bat guano and silt is to perform a chemical analysis on these deposits.

Stratigraphic sequence of deposits

The stratigraphic sequence of deposits in Guadalupe caves has been worked out mainly in Carlsbad Cavern where cave deposits of all types occur. This sequence was determined by crosscutting relationships and by stratigraphic superposition wherever clean contacts could be observed. Stratigraphic sections of cave deposits were correlated according to continuity and similarity of type; a number of representative sections are shown on Sheet 9. The sequence (ascending) of cave deposits in Carlsbad Cavern is as follows:

(1) Breccia—This is the earliest cave deposit. It is truncated by the large cave passages and thus predates all deposits lying within these passages. Breccia clasts are often embedded in a crystalline-spar matrix, and the breccia thus predates the spar event (events). The early age of the breccia relative to other deposits is shown most clearly by exposures in the wallrock and breakdown of the Guadalupe Room (Figs. 67, 68). The breccia predates the spar, the calcified siltstone-cave raft sequence, and all speleothemic material (Sheet 9, column A).

FIGURE 67—Sequence of deposits and events with respect to the breccia and spar, Guadalupe Room, Carlsbad Cavern: (1) formation of voids in the limestone; (2) shearing of breccia in place or emplacement of breccia into voids; (3) deposition of breccia mudstone on top of the breccia; (4) deposition of variegated siltstone on top of mudstone [before the siltstone and mudstone could harden, a large piece of breakdown moved down slightly (arrows) to produce deformation at (a), (b), and (c)], and deposition of siltstone layer (d) after breakdown had moved; (5) filling of remaining spaces between breccia clasts with spar. Photo Cyndi Mosch Seanor. (click on image for a PDF version)

(2) Montmorillonite-endellite—These clays are found compacted into spongework cavities within the limestone. Both the cavities and the clay have been crosscut by the large cave passages, and hence predate them. It should be mentioned that while the clay fillings themselves are unquestionably older than the large cave passages, the type of clay mineral making up the filling may have a later origin (i.e. the endellite is believed to have reconstituted from montmorillonite later on).

(3) Spar—Spar crystals often fill solution pockets which have been truncated by the large cave passages; these crystals have been pitted, etched, and dissolved according to their proximity to this truncation. The stratigraphic relationship of the spar to the montmorillonite clay is uncertain, but one occurrence of associated clay and spar suggest that the clay fillings may predate the spar fillings. In the Papoose Room, Carlsbad Cavern, next to the trail and right before the tunnel, are two solution pockets 0.3 m apart, one filled with grayish-green clay and the other lined with spar crystals. Assuming the two solution pockets date from the same episode, then, if the spar had preceded the clay, both the pockets should be crystal-lined because saturated solutions depositing the spar could not have varied over so short a distance. However, if the clay episode had preceded the spar episode, then it is conceivable that the clay could have filled only one of the pockets (assuming that one pocket was connected to other, residue-occupied cavities above and that the other pocket was isolated from above-lying cavities).

(4) Calcified siltstone-cave rafts—As observed in the Guadalupe Room and Lower Devil's Den, the calcified siltstone-cave rafts occur as cavity fillings in wallrock truncated by the large cave passages (Sheet 9, column B). In the Main Corridor, Guadalupe Room, and Lower Devil's Den the siltstone-rafts are included in pieces of floor breakdown, one of which shows the siltstone-raft sequence directly overlying (and postdating) spar (Fig. 68). In Lower Devil's Den, a siltstone-raft deposit has been found beneath a large gypsum block (Sheet 9, column C), but it is hard to tell whether this deposit is in place beneath the gypsum block or on a piece of breakdown; either way, the siltstone-rafts definitely predate the gypsum. Such stratigraphic exposures place the calcified siltstone-cave rafts after the spar, but before the gypsum and before the dissolution of at least some of the large cave passages.

FIGURE 68—Diagrammatic presentation of a breakdown block in the Guadalupe Room, Carlsbad Cavern, showing the spar, breccia, and calicified-siltstone-cave-raft sequence. The entire sequence is about 1.5 m thick. Breakdown is drawn in its originally upright position as determined by the horizontal orientation of 5a, 5b, and 5c. Sequence of events and deposits is: (1) formation of limestone; (2) possible emplacement of the breccia; (3) truncation of breccia; (4) deposition of spar with well-developed crystal faces at (a) and (b); (5) deposition of siltstone on upward-facing surfaces (5a, b, c); (6) deposition of cave rafts; and (7) subaerial crustal growth on top of the cave rafts. The relative time of emplacement of the breccia is speculative, but the breccia and its truncation definitely antedate the siltstone-raft sequence.

(5) Cobbles—The time of emplacement of the cobble gravel is uncertain both with respect to the dissolution of the major cave passages and to some of the other cave deposits such as the spar and the calcified siltstone-cave rafts. The cobbles may possibly postdate the spar in the upper levels of the cave and predate the spar in the lower levels—a speculation based on the well-indurated, sparry nature of the matrix material in the cobble gravel of Lower Cave versus the less-indurated matrix material in the Main Corridor and Secondary Stream Passage. The relative age of the siltstone-rafts as compared to the cobble gravel is also unclear, because the two deposits have never been found in direct superposition. However, in the far end of Secondary Stream Passage, where one starts the climb towards Lower Devil's Den, siltstone-rafts are exposed in the wall while cobbles are found nearby on the cave floor. From this setting it seems that the siltstone-rafts may either predate or be contemporaneous with the cobbles, at least in this level of the cave.

The relationship of the cobbles to the silt is more sure. Since the cobble gravel either underlies the silt or is interbedded with it, the gravel must predate or be contemporaneous with the silt. If the undulating contact between the cobble gravel and silt in Lower Cave represents a true unconformity (Fig. 34; Sheet 9, column F), then it cannot indicate a long hiatus, since in the Secondary Stream Passage and Main Corridor the silt and gravel are interbedded or mixed together. In Upper Devil's Den the cobbles are located under a pile of breakdown and hence predate a major breakdown event. At this locality, cobbles can also be seen included in the upper part of a gypsum block, a relationship that might be taken for an indication that the cobbles are contemporaneous with the deposition of the gypsum were it not for many other exposures in which the cobbles always underlie silt which underlies gypsum (Fig. 69). For this reason, it is strongly suspected that the cobbles included in the Upper Devil's Den gypsum reflect a late movement of the cobbles, such as slumping into the gypsum during the time of the gypsum's coalescence.

FIGURE 69—Sequence of events and deposits in Nooges Realm, Lower Cave, Carlsbad Cavern: (1) emplacement of cobble gravel; (2) formation of 3 m high silt bank; (3) deposition of 1 m high gypsum blocks; (4) drilling of drip tubes in gypsum by dripping water during first subaerial episode; (5) a back-up of water which stirred up the silt of silt bank and caused a thin (<30 cm) layer of silt to settle out on top of gypsum blocks and into drip tubes; (6) entrenchment of silt and gypsum to cobble-gravel horizon; (7) drying out of silt during second subaerial episode, formation of mud-cracks, and redrilling of drip tubes; (8) fall of breakdown block; (9) deposition of black flowstone on top of breakdown block.

(6) Sand and silt—Fine-grained clastic sediment in Guadalupe caves underlies the massive gypsum and thus predates it. There are numerous examples of this, e.g. at Bottomless Pit (Fig. 38; Sheet 9, column G), Nooges Realm (Fig. 69; Sheet 9, column F), and Left Hand Tunnel, Carlsbad Cavern (Sheet 9, column E); in the entrance area of Spider Cave; in the maze which connects the Entrance Hall with the Gypsum Passage, Cottonwood Cave; and in the Gypsum Room of Endless Cave. The only place where sediment has been observed to overlie gypsum is in Lower Cave, Carlsbad Cavern, where a thin (<30 cm) layer of silt covers a gypsum block which itself overlies a 3 m high silt bank (Fig. 69; Sheet 9, column F).

Silt overlies endellite and montmorillonite clay in the Big Room (Fig. 70; Sheet 9, column I), and it overlies the cobble gravel in Lower Cave. In the latter case, a possible unconformity exists between the silt and the underlying deposit. Silt always underlies flowstone and dripstone speleothems; for example, the stalagmite known as the Texas Toothpick directly overlies silt in Lower Cave. There is but one known exception to this rule: the 15 cm thick silt-breccia deposit of the Big Room overlies a portion of stalagmitic flowstone, which in turn overlies a 2 m high silt bank (Fig. 70).

FIGURE 70—Diagrammatic presentation of the deposits in the areas of the Texas Trail and Salt Flats, Big Room: (1) grayish-green montmorillonite; (2) white layer of endellite; (3) silt; (4) chert; (5) stalagmite; (6) silt-breccia. The montmorillonite was transformed to endellite under acidic conditions with the liberation of silica (chert). Silt and travertine were later reworked locally (possibly by pool or drip water) so as to form the silt-breccia which slumped over the travertine. The stalagmitic flowstone (5) has been U-series dated at 107,600 yrs (Table 24, sample 20).

(7) Chert—Chert lenses are interbedded with silt in the Big Room, usually along the top part of the deposit (Fig. 70; Sheet 9, column I). In the Salt Flats' exposure, chert interbedded with brick-red silt underlies gypsum blocks, which in turn underlie bedrock breakdown (Sheet 9, column H). The only place where this sequence is seemingly reversed (i.e. chert overlying gypsum) is where a large, out-of-place chert block (Pl. 5B) slid down on top of an in-place gypsum block, a late-stage movement which crushed part of the block and distorted the orientation of some of the block's drip tubes.

(8) Gypsum blocks and rinds—Gypsum blocks and rinds overlie clastic sediment (Fig. 38; Sheet 9, column F), or they may rest directly on bedrock limestone (Fig. 71; Sheet 9, column D). In some places in the Big Room, coralloidal crusts or flowstone can be seen beneath certain gypsum blocks, but these are speleothems formed from water which either seeped or flowed under the blocks. In the Talcum Passage, bat guano seemingly underlies certain sections of gypsum, but this guano has filtered down from above along drip tubes or cracks in the gypsum.

FIGURE 71—A gypsum block directly overlying bedrock limestone, Talcum Passage, Carlsbad Cavern. Photo Alan Hill.

(9) Breakdown—Most of the breakdown fell shortly after the gypsum had deposited, but prior to massive speleothem growth. Such a stratigraphic relationship is best exemplified in the Big Room where some of the gypsum blocks with large breakdown pieces overlying them have both tilted drip tubes and vertical drip tubes (Sheet 9, column G). The gypsum blocks were drilled by dripping water before as well as after the impact and tilting by the breakdown pieces. Since no carbonate speleothems occur between tilted gypsum blocks and overlying breakdown pieces, the time between gypsum block coalescence and breakdown falling was probably relatively short.

While most breakdown fell approximately at the time of subsidence of the water table, a few pieces are known to predate or postdate this time. Bretz (1949) reported silt deposits in Carlsbad Cavern which postdated collapse breakdown; perhaps Bretz was talking about the large breakdown pieces in the Stegosaurus Rock section of Lower Cave which were once completely covered with silt, but were later exhumed by an entrenchment of the silt. Iceberg Rock is a piece of breakdown which fell somewhat later in the cavern's history. The block has tilted dripstone deposits hanging along its bottom side, attesting to the fact that it fell some time after subaerial conditions became established.

(10) Speleothems—Speleothems have formed ever since the caves became air-filled. Some speleothems are known to have deposited prior to the native sulfur mineralization (i.e. the New Mexico and Christmas Tree Room sulfur), and some (those on the bottom of Iceberg Rock) grew prior to at least some major breakdown falls.

(11) Sulfur—In the Big Room of Carlsbad Cavern, sulfur occurs either mixed with gypsum or as a coating over it; thus, the sulfur either postdates, or is contemporaneous with, the gypsum at this site. In the New Mexico Room, sulfur coats speleothems (gypsum flowers and crust) and the undersides of bedrock, but it is not present on breakdown blocks adjacent to the sulfur-coated bedrock. For the sulfur at this locality to have deposited between the gypsum and breakdown events may mean that it formed very shortly after subaerial conditions were established in the cave. In the Christmas Tree Room, sulfur coats bedrock and speleothems (carbonate popcorn, rafts, and crinkle blisters), and, in this instance, it probably formed much later in the cave's history.

(12) Bat guano—Bat guano can be underlain, overlain, or interbedded with dripstone and flowstone travertine. Near the sign "decomposed bat guano" by Bottomless Pit in the Big Room, a thin layer of flowstone overlies decomposed bat guano; the guano rests on massive gypsum, which is underlain by silt, sand, and clay (Sanchez, 1964).

(13) Animal bones—All animal bones in Guadalupe caves have been found on top of cave sediment (e.g. the sloth bones in Carlsbad Cavern) or in talus piles near a cave's entrance (e.g. the diverse species in the entrance area of Dry Cave; Table 21). A few bones, such as those of the short-faced bear in Pink Panther Cave, or the bush ox in Musk Ox Cave, are covered with travertine and so antedate this late-stage speleothemic material (Fig. 65). The oldest animal bones known in any Guadalupe cave are the sloth bones found in Lower Devil's Den, Carlsbad Cavern; these have been dated at about 111,900 years (Table 24). The other bones found in Guadalupe caves do not exceed 33,590 years (Table 21).

(14) Mud—In the Mud Crack Room of Endless Cave, mud overlies a gypsum block and thus postdates the gypsum. Mud also locally covers travertine and animal bones. In all cases the mud is a late-stage material which has been washed into the caves, perhaps recording rare, catastrophic storms in the Guadalupe Mountains. The Mud Crack Room of Endless Cave and the passages of Little Sand Cave are approximately at the same elevation, with a downcutting valley between the two caves. Mud from valley erosion most likely entered both of these caves at the same time when the valley was at a higher level. In Vanishing River Cave, surface flood waters have been observed washing mud and debris into the cave. Mud infiltration into Guadalupe caves is a temporary phenomenon entirely unrelated to, and happening long after, speleogenesis events.