Carlsbad Caverns (a.k.a. Carlsbad Cavern) in southeastern New Mexico, USA, is one of the world's most spectacular caves. The speleothem is abundant, large-scale, and magnificent. It has one of the largest cave rooms anywhere on Earth (the Big Room - 14 acres in size & 370' maximum height). Unlike many tourist-accessible caves, much of Carlsbad Caverns is self-guided. On top of all that, the geologic origin of Carlsbad Caverns (& nearby Lechuguilla Cave) is unlike that of almost any other cave.
Carlsbad Caverns is developed along the crest of Carlsbad Ridge. This ridge is part of the prominent Guadalupe Escarpment - a long, NE to SW-trending mountain range extending from southeastern New Mexico to western Texas.
Guadalupe Escarpment (as seen from highway between Carlsbad Caverns National Park and Guadalupe Mountains National Park), consisting of Upper Permian reefal and perireefal limestones.
Guadalupe Ridge (as seen from Carlsbad Ridge; looking ~NE), consisting of backreef dolostones of the Yates Formation (upper Guadalupian Series, ~mid-Upper Permian).
The natural entrance to Carlsbad Caverns was discovered in the late 1800s. Smoke was seen rising from the ridge at sunset. Close inspection revealed that the smoke was millions of bats flying from the mouth of a cave. The site was first called Bat Cave.
Carlsbad Caverns - natural entrance (looking ~ESE). The rocks surrounding the cave mouth are Permian backreef dolostones of the Tansill Formation (upper Guadalupian Series, mid-Upper Permian). Cave swallows are commonly seen here in the daytime. Near sunset, the swallows disappear in apparent anticipation of the bat flight. The most common species (see below) is the relatively small Mexican freetail bat, Tadarida brasiliensis mexicana (Saussure, 1860). >8 million bats occupied the cave in early days, but the population is now down to an estimated 250,000-300,000 individuals.
Mexican freetail bat - Tadarida brasiliensis mexicana (Saussure, 1860). NPS public display photo.
Guano deposit, near Bottomless Pit, southern end of the Big Room, Carlsbad Caverns. Thick guano deposits were found in the the Bat Cave portion of Carlsbad Caverns (east of & about 160' below the natural entrance). Because guano deposits are rich in phosphorus, they were mined from here in the early 1900s for use as fertilizer. Ancient lithified guano deposits are a variety of the uncommon sedimentary rock phosphorite.
Most caves on Earth (e.g., Mammoth Cave and Ohio Caverns) are formed by downward percolation of slightly acidic water. Dissolution of bedrock (usually limestone) is done by weak carbonic acid (H2CO3), formed by mixing of atmospheric CO2 gas with water.
In contrast, Carlsbad Caverns & nearby Lechuguilla Cave were formed by upward movement of sulfuric acid-rich waters. The sulfuric acid (H2SO4) likely formed by oxidation of H2S gas derived from regional petroleum fields. Alternatively, the sulfur component in the sulfuric acid may be derived from Upper Permian Castile Formation gyprocks in the adjacent Delaware Basin.
So, instead of "up-down" dissolution by weak carbonic acid, Carlsbad and Lechuguilla originated by "down-up" dissolution by strong sulfuric acid - quite unusual.
Sculpted limestone cave wall & spongework - before interesting cave features such as stalactites and stalagmites can form, the cave's empty spaces have to be dissolved out from the limestone bedrock. Some of Carlsbad Cavernsí cave walls are unadorned with speleothem, and often show an irregularly sculpted pattern formed by differential dissolution of acid-rich waters. Note also the many small pits in the wall (especially on the right, lower right, and bottom-center areas of photo). The abundance of small dissolution pits is called spongework. All limestone dissolution by acids is called karstification. Stratigraphically, the limestone walls of the main portions of Carlsbad Caverns (i.e., the Big Room, King's Palace area, Lower Cave) are reef core limestones of the Capitan Limestone (Upper Permian).
Info. on these pages mostly synthesized from:
Hill & Forti (1997) - Cave Minerals of the World, Second Edition.
Kiver & Harris (1999) - Geology of U.S. Parklands, Fifth Edition.
Harris et al. (2004) - Geology of National Parks, Sixth Edition.