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

Compared to speleothems in many other caves, carbonate speleothems in Guadalupe caves are usually not brightly colored, but rather are delicately shaded in the pastel colors of cream, pink, and orange. There are exceptions to this rule, the most notable being the Temple of the Fire God in Three Fingers Cave (Pl. 16A). Sulfate speleothems are usually white or transparent, but selenite needles can be tinged a light tan due to included impurities.

The cause of coloration in speleothems is very complex and includes such factors as crystallinity, trace amounts of metal ions, complex organic chelates, surface contaminants, and even bacteria (Hill and Forti, 1986). Most of the orange-red-yellow-brown coloration of carbonate speleothems in Guadalupe caves is believed to be caused by iron oxide, but may also be caused by differences in crystallinity or incorporation of impurities into the travertine. Hill (1984c) speculated that the tan-to-brown travertine coloration seen everywhere in Virgin Cave (Pl. 12) may be caused by mud staining. Coloration of "fried egg" stalagmites in such caves as Pink Dragon is probably due to crystallinity factors.

Some coloration of Guadalupe speleothems may relate to impurities in the bedrock. In the Bell Cord-Bifrost Rooms, Carlsbad Cavern, speleothems such as cave cones and stalactites are peach to lemon yellow (Pl. 11A), a coloration that may be derived from the high silt content of the forereef limestone at this locality. Bright-red to purple stains on the limestone of the Secondary Stream Passage (near its junction with the Lunch Room) have been analyzed as iron oxide (memorandum to the park from Edwin C. Albert, Park Naturalist, 1945). Caldwell and Caldwell (1980) identified the filamentous iron bacteria Leptothrix from small pools in Left Hand Tunnel, Carlsbad Cavern; such bacteria may implement coloration by changing speleothem-depositing solutions from a ferrous-oxidation state to a ferric-oxidation state.

Gray, dark-brown, and black coloration of speleothems may result from manganese-oxide minerals, dirt, guano, or soot. A positive test for manganese has been obtained for black layers in the clouds and shelfstone of the Lake of the Clouds Passage, Carlsbad Cavern (Fig. 64). Black material in the shelfstone has been analyzed as 1.4% Mn and 6.4% Fe, and x-ray diffraction of the black material indicates the possible presence of kutnohorite, Ca(Mn,Mg,Fe²⁺)(CO₃)₂ (J. Husler, pers. comm. 1986).

Black Cave was named for the blackened nature of its speleothems and for the dark material which rests on the floor of the cave. Hill (1982b) proposed that the black of Black Cave is carbonaceous material originating from a surface fire and brought into the cave during a time of flooding. Davis (1982a) disagreed with Hill and proposed that the cause of the black material was airborne soot from a surface fire.

Suspended material settling out of the air has also been proposed as the cause of blackened speleothems in other Guadalupe caves. A dark coating on the massive stalagmites and columns at the cave entrance of Cottonwood Cave supposedly resulted from a goat-manure fire (Corcoran and Willis, 1964). The fire smoked for two weeks at the cave entrance and speleothems which were glistening white turned a dull gray from the airborne soot. In a side gallery known as the Black Dust Room off the Sand Passage, Carlsbad Cavern, speleothems and rocks are blackened on their upward-facing surfaces but not on their undersides; this black material has been attributed to guano dust which settled out of the air (Hill, 1982b). Other blackened areas in the side galleries of Sand Passage such as the black flowstone in Black River and Black Bat Bone Hollow may also be attributable to airborne guano dust or, alternatively, to black material which has been leached from bat guano in overlying Bat Cave (Pl. 16B). The blackened speleothems on the ceiling of the Lunch Room area are due to the smoke of kerosene lamps used by the early explorers and visitors to the cave.

The blackened speleothems in the front chambers of New Cave have alternately been explained as being due to manganese, bat guano, or soot from guano fires, but it now appears that the black may be guano dust that settled out of the air during the period of guano-mining activity in the 1920's-1940's. Many lines of evidence lead to this conclusion. The black material is not manganese as determined by chemical tests (less than 0.005% manganese), and x-ray-diffraction analyses show a carbon line. The black material cannot be soot from guano fires because the limestone ceiling is not blackened adjacent to blackened speleothems as would be expected from rising smoke. It is not bat guano staining as proposed by Baker (1960) because the black material occurs as a fine, dust-like layer only on the upper surfaces of speleothems, and not as pellets, streaks, or a slippery coating of dark material such as is found where bat guano covers speleothems. It is not black material leached from bat guano in above-lying chambers because gypsum blocks on the floor are blackened on their upfacing surfaces. The black material occurs only in guano-lined passages in the front part of the cave, whereas speleothems in the back of the cave (such as the Christmas Tree) are a pristine white. The blackening event was fairly recent as evidenced by non-black travertine that has grown over the blackened part of some speleothems. In one area in the front part of the cave, where the limestone wall has been gouged by a guano-mining cable, a white, fringed drapery 20 cm long and 1 cm wide partly covers the gouge, showing that non-blackened travertine has grown since the guano-mining era. It may be that a combination of guano dust, oil dust, and diesel fuel dust became suspended in the air during guano mining, and that this dust settled out on wet speleothem surfaces in the front part of the cave.