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New peculiar cave ceiling forms from Carlsbad Caverns (New Mexico, USA): The zenithal ceiling tube-holes Article in Geomorphology · April 2011 DOI: 10.1016/j.geomorph.2011.02.032

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Geomorphology 134 (2011) 43–48

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

New peculiar cave ceiling forms from Carlsbad Caverns (New Mexico, USA): The zenithal ceiling tube-holes Jose-Maria Calaforra a,⁎, Jo De Waele b a b

Water Resources and Environmental Geology, University of Almeria, Cañada de San Urbano s/n, 04120 Almeria, Spain Italian Institute of Speleology, University of Bologna, Via Zamboni 67-40127 Bologna, Italy

a r t i c l e

i n f o

Article history: Received 19 July 2010 Received in revised form 21 February 2011 Accepted 25 February 2011 Available online 23 March 2011 Keywords: Sulphuric acid cave Cave karren morphology Cave ceiling forms Speleogenesis

a b s t r a c t During a trip to the Hall of the White Giant, Carlsbad Caverns (NM, USA) cigar-shaped vertically upward developing holes were observed on the ceiling at different heights of the passages. They have a circular crosssection with diameters of 1 to some centimetres and taper out towards their upper end. Their walls are smooth and their bottom edges are sharp, while their length can reach several decimetres. Sometimes gypsum can be found inside. They often occur randomly distributed in groups and their development is not necessarily controlled by fractures or other bedrock structures. We name these peculiar karren-like cave microforms “zenithal ceiling tube-holes” because of their origin by H2S environment corrosion processes and their vertical (zenithal) upward growth in ceilings. A comparison is made between zenithal ceiling tube-holes and other karstic or non karstic similar forms such as bell holes, oxidation vents, snailholes, Korrosionskolke (mixture-solution hollows) or pockets, röhrenkarren, lightoriented photokarren, borings of (often marine) organisms and negative stalactites. Zenithal ceiling tube-holes are created by the corrosive effect of sulphuric acid. H2S(g) dissolves in water giving rise to widespread sulphuric acid corrosion. When H2S bubbles are trapped underneath overhanging surfaces or ceilings and water level rises steadily the corrosive effect is concentrated vertically upwards, drilling vertical holes that can also completely pass overhanging rock ledges. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Carlsbad Caverns is a world renowned show cave located in the Guadalupe Mountains (Chihuahua desert, New Mexico) and managed by the United States National Park Service. The cave is hosted in the Permian Capitan Reef limestones that border the Delaware basin (Fig. 1). It consists of elongated passages connecting wide rooms that extend to a depth of almost 300 m beneath the natural cave entrance, almost 200 m below the floor of nearby Walnut Canyon (Hill, 1987). As most caves of the Guadalupe mountains it has formed by rising hydrogen sulphide deriving from underlying oil and gas deposits, producing sulphuric acid by mixing with fresh water which corroded the limestone into unusually large chambers (Hill, 1990). This H2SO4 speleogenesis has produced gypsum that is present under various forms. Carlsbad Caverns has been widely studied by generations of cave scientists, and research has especially focalised on geology (Hill, 1995; Harwood and Kendall, 1999), speleothems (Hill and Forti, 1997; Melim et al., 2006), mineralogy (Polyak and Guven, 1996, 2000), water chemistry (Ingraham et al., 1990; Chapman et al., 1992), speleogenesis (Hill, 1995, 2000; Polyak et al., 1998), cave meteorology (Cheng et al., ⁎ Corresponding author. E-mail addresses: [email protected] (J.-M. Calaforra), [email protected] (J. De Waele). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.02.032

1997), cave fauna (Northup and Crawford, 1992; Geluso, 2008; Hristov et al., 2010) and, mostly in the last decennia, microbiology (Northup et al., 2000; Barton et al., 2007; Snider et al., 2009). This has resulted in a large number of publications in many scientific journals, making Carlsbad Caverns one of the best studied caves of the world. In the squeezing crawlways that connect the main Corridor to the Sand Passage and the further lying Hall of the White Giant, several cylindrical centimetre-sized vertical holes have been observed on the ceiling at different levels (Fig. 2). These holes, here named zenithal ceiling tube-holes, are the subject of this paper. After a review of similar vertical cave ceiling forms, the genesis of this new form is explained. 2. Morphology 2.1. Zenithal ceiling tube-holes Zenithal ceiling tube-holes are centimetre to decimetre long conical holes that perforate the ceilings of cave passages in a perfect vertical upward direction (Fig. 3A). The host rock is made out of limestone of the Yates Formation (Hill, 1987). Their cross-section is circular with diameter ranging between a centimetre up to 5 cm (most are 2–3 cm wide). They are cigar-shaped, tapering out towards their upper ends (Fig. 3B), and they sometimes completely perforate horizontal overhanging rock slabs (Fig. 3C). Their walls are smooth and their edges are

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Fig. 1. Location of Carlsbad Caverns and the Guadalupe Mountains (New Mexico, USA).

sharp. Their length can reach several decimetres. In some cases, at their upper ends, gypsum has been found inside (Fig. 3D). This mineral occurs as crusts or powdery masses at the apex of the holes. They often occur randomly distributed in groups and their development is not necessarily

controlled by fractures or other bedrock structures. They have an altimetric distribution on at least three levels that does not seem to be controlled only by cave structure (presence of horizontal ceilings or ledges) but might present old water level stands.

Fig. 2. Hall of the White Giant and location (asterisks) of the zenithal ceiling tube-holes (modified from Hill, 1987).

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Fig. 3. Zenithal ceiling tube-holes: A) collapsed rock, part of a roof, completely perforated with almost perfect centimetre-sized conical holes; B) a typical cluster of zenithal ceiling tube-holes with a diameter of 2–3 cm and a depth of a decimetre; C) zenithal ceiling tube-holes, some of which perforating the entire limestone slab that constitutes the local cave roof; and D) detail of the bottom of a zenithal ceiling tube-hole with encrustations of white gypsum.

2.2. Other convergent cave forms There are several forms that can be found on cave roofs or overhanging ledges that in some way resemble the zenithal ceiling tube-holes of Carlsbad Caverns. Fig. 4 shows typical cross-sections of these small-scale forms in comparison to the zenithal ceiling tube-holes. Small fracture-guided Korrosionskolke (mixture-solution hollows) or pockets (Bögli, 1964; Hedges, 1967; Dreybrodt and Franke, 1994; Lauritzen and Lundberg, 2000) are formed by mixing-solution at places where saturated water entered the phreatic cave environment through small joints or cracks (Fig. 4A). Their size often is metric and the fracture from where water entered the phreatic cave passage is always visible and often influences the plan shape of the forms, elongated along the fracture direction. Their diameter/depth ratio ranges between 3 and 0.1 and rarely reaches 0.05. These forms are very similar to thermal spherical niches or convection cupola formed above a thermal water table by condensation water (Szunyogh, 1990; Lauritzen and Lundberg, 2000) typical of thermal endogenic caves (Piccini et al., 2007) (Fig. 4B). Cupola, however, are much larger in size than zenithal ceiling tube-holes, and often show a relatively regular subspherical morphology (Osborne, 2004; De Waele et al., 2009b). Their shape is not influenced by fractures. The “negative stalactites” of vadose cave environments described by Lange (1965) grow vertically upwards along a crack in the roof (Fig. 4C). Seepage water, entering a cave atmosphere rich in carbon dioxide, gets aggressive and dissolves the carbonate rock along the crack from where it comes. This creates cigar-shaped vertical holes growing vertically on a cave ceiling. For their formation to be possible seeping water has to come out of an open crack, which is always visible. Oxidation vents, described from a single Sardinian cave, are subaqueous corrosion forms in close connection with bubble trails (Fig. 4D). They are due to oxidation of sulphides with related formation of sulphuric acid and reaction with hosting limestone or subaqueous

calcite speleothems. Carbon dioxide bubbles escape forming the oxidation vents and bubble trails. Oxidation vents grow upwards, perpendicular to the cave cloud surface, radially towards the centre of the spherical concretions (De Waele and Forti, 2006). Their long axis can be vertical, but often it is not. Bell holes are cigar-shaped vertical cylinders described from many tropical caves (Fig. 4E). They often have a polygonal distribution along primary phreatic cave ceilings, although their genesis is now believed to have occurred in aerial (vadose) conditions. Condensation corrosion can explain their formation, but there seems to be also a biological control on their development (e.g. bats). Also bell holes grow vertically and are not fracture-guided, but are much larger and wider than zenithal ceiling tube-holes (Tarhule-Lips and Ford, 1998; Lundberg and McFarlane, 2009). Light-oriented photokarren, also typical of tropical karst, have a similar morphology, but since they face towards the light they rarely are found on ceilings but normally cover large surfaces along the walls and floors close to cave entrances (Simms, 1990; De Waele et al., 2009a) (Fig. 4F). Mussel borings (e.g. Lithophaga lithophaga) also create cigar-shaped holes at or close to sea level and have often been used as sea level markers (Valentich-Scott and Dinesen, 2004; Antonioli et al., 2006; Devescovi and Ivesa, 2008). These borings, however, cover not only the submerged rock surfaces, especially walls and floors but also overhanging ledges and ceilings (Fig. 4G). Probably the most striking similar forms to zenithal ceiling tubeholes are the holes described by Stanton (1986) from caves in the Mendip Hills. This author believed that the small vertical cylindrical holes formed by activity of snails (they were coined “snailholes”) (Fig. 4H). Snailholes are very similar to the tubular lake shore karren (or röhrenkarren) described by Simms (2003) from Irish lakes formed by condensation-corrosion processes within air pockets trapped by changing water levels (Fig. 4I). Probably snailholes and röhrenkarren

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Fig. 4. Schematic representation of the different ceiling morphologies: A) Korrosionskolke (mixture-solution hollows) or pockets; B) thermal spherical niches or convection cupola; C) negative stalactites; D) oxidation vents; E) bell holes; F) light-oriented photokarren; G) Lithophaga lithophaga boremussel holes; H) “snailholes”; and I) tubular lake shore karren.

are identical forms, and, since their shape is almost identical to the Carlsbad zenithal ceiling tube-holes, a similar type of convergent genesis might be expected. 3. Discussion on their possible genesis Since zenithal ceiling tube-holes resemble very much the tubular lake shore karren (or röhrenkarren) described by Simms (2003) it seems obvious to start with his genetic hypothesis put forward for these forms. Röhrenkarren are found only within the zone of current, or former, seasonal flooding of lakes in Ireland. Their shape, an almost perfect cylinder that ends with a rounded apex, indicates that dissolution is most active at their upper ends and is almost inexistent along its lower walls. Biogenic dissolution is excluded since SEM observation of tube apexes did not evidence biogenic textures and etching features. They are believed to form by condensation corrosion, whereby the limestone is dissolved by water vapour condensing in air pockets trapped by rising lake level. That condensation occurs when air is trapped in pockets along a cave ceiling and compressed by rising of water level has been clearly demonstrated by modelling (Lismonde, 2000). This process leads to the development of cupola along seasonally epiphreatic inundated cave passages. Perhaps the most striking feature of the zenithal ceiling tube-holes is their extremely vertical shape development. Therefore, any hypothesis to explain their speleogenetic evolution should take into account this unique morphologic appearance. In our view, a mechanism that could explain such morphology would be the effect of an epiphreatic H2S corrosion under pressure. In short, it would be the effect of air bubbles trapped in small irregularities in a previous subhorizontal bedrock surface. In normal vadose conditions in a sulphuric acid cave environment, H2S is gradually released from the water in the cave atmosphere. When there is enough thermal contrast between the water bodies, the cave air and the rock of the cave walls and roof, water condenses

especially on the colder places (far from a warmer water body for example). H2S can dissolve into this water and oxidises forming sulphuric acid. This last reacts with the limestone host rock forming gypsum as a byproduct (Fig. 5A). This is how solution pockets form in sulphuric acid caves along the walls, the ceiling and on the rocky floor (Galdenzi and Maruoka, 2003). If air is trapped underneath a cave ceiling by a rising water level, corrosion would start at the top of the small pocket where H2S is accumulated and would then evolve upwards into the bedrock generating a “quasi perfect” vertical tube (Fig. 5B). Of course, entrapment is favoured on more or less horizontal surfaces (like the flat morphology of bedding planes). On vertical or inclined surfaces this entrapment does not occur and the corrosion only produces pockets. With a continuously rising water level (always keeping vadose conditions with trapped air pockets enriched in H2S) a hydrostatic pressure would be created between the two levels: the water level in the cave and the level that would remain inside the tube (Fig. 5C). The trapped bubbles would thus have a greater pressure than the air in vadose conditions of the passage. This overpressure is due to the hydrostatic height difference between water in the passage and water at the top of the zenithal ceiling tube. This greater pressure in the tube makes corrosion go vertically upward, leading to perfectly vertical cylinders. This could be the explanation that vertical progression in the development of these forms is dominant over generalised wall corrosion mechanisms. This process goes on as long as the tube continues to receive H2S and the water level remains above the top of the tube. The greater the difference between the water table and the tube apex the greater the power of vertical erosion (Fig. 5D). Some tubes maintain gypsum precisely at their top, indicating that acid corrosion was preferentially located at the top of the tube where the bubbles were trapped. Some of the tubes stop evolving when they go through the whole rock, thus degassing and losing pressure difference (Fig. 5E). After water table lowering the final appearance of the rock subjected by this

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Davis who transmitted part of his knowledge on Carlsbad Caverns. Pics of lake shore karren were kindly given to us by Michael Simms. Comments by an anonymous reviewer and Mario Parise have been very helpful. Finally thanks to the National Park Service for their encouragements of carrying out scientific research in Carlsbad Caverns National Park. This work has been partially supported by the project CGL-200601707-BTE (Ministry of Science and Innovation, Spain).

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Fig. 5. Schematic representation of the genesis of zenithal ceiling tube-holes (see text for explanation).

upwards piping process is like a series of tubes that hollows and/or crosses the bedrock with some gypsum crusts on their tops as remnants of the corrosion process. 4. Conclusions Zenithal ceiling tube-holes are unique corrosion features described for the first time from Carlsbad Caverns, New Mexico (USA). Their typical cigar-like shape, boring vertically upwards into almost horizontal carbonate surfaces, can be attributed to condensation corrosion processes in a H2S-rich environment. On the contrary to other convergent karstic forms their genesis is related not only to corrosion caused by CO2 or H2S acid effect but also by a peculiar situation in which this effect is magnified by that of localised overpressure in trapped air pockets of H2S-enriched air driven by the changing groundwater level in the passage. Their size might be an indication of the speed at which water level changed inside the cave passage, larger tubes testifying for a slower rise, while their altitudinal distribution reflects the change of water levels in the past. Acknowledgements Many thanks to Art and Peg Palmer for useful scientific discussion on these holes, Louise Hose for the logistics during our stay at Carlsbad, Dale Pate who guided us through the White Giant passages, and Donald G.

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