Carlsbad Caverns: caves dissolved from below in a Permian reef

The caves of Carlsbad Caverns National Park in New Mexico were eaten out from below, by sulfuric acid rising from oil and gas fields deep underground — a process first described here. The rock they are cut into is itself a wonder: a barrier reef that grew in a Permian sea some 260 to 270 million years ago.

A reef from the Permian

The Guadalupe, Apache and Glass mountains are what is left of a vast Permian reef complex. Its main reef, the Capitan Reef, is exposed both on the surface and underground in the park, in the Upper Permian Capitan Limestone. It is up to 1,800 feet thick and runs for hundreds of miles.

It was not built like a modern coral reef. Its framework was sponges, algae and bryozoans. The reef complex, made of limestone and dolomite, also holds the Capitan Aquifer, a large freshwater aquifer that supplies the town of Carlsbad and some of its neighbours.

How the caves formed

Hydrogen sulfide from the oil and gas deposits beneath the park rose through fractures into the Capitan aquifer, where it made a mild sulfuric acid. That acid dissolved the huge cave systems of the park and the Guadalupe Mountains. Geologists call it hypogenic — formed from below — and the process was first described from these caves.

Two great caves

The reef and its caves made the park a World Heritage Site in 1995. All caves in national parks are protected by the Federal Cave Resources Protection Act of 1988.

Life in the dark

People came to the area about 11,000 years ago and used the caves for shelter and water. The entrance of Carlsbad Cavern was a sacred place, perhaps because of the evening flights of Brazilian free-tailed bats that pour out of it through the warm months. At least sixteen other bat species live in or pass through the park. The caves also shelter the largest and northernmost colony of cave swallows, mountain lions and ringtails, and many invertebrates found nowhere else. Hundreds of distinct strains of microbes have been cultured from the park's caves, some of them with possible medical uses.

Fossils

The reef preserves a rich marine fauna: fusulinids, brachiopods, scaphopods, cephalopods, crinoids, two species of trilobite, and the largest fossil sponge known from the Permian.

Much younger bones lie in the caves — 36 species of fossil vertebrates from Carlsbad Cavern, Musk Ox Cave, Lechuguilla Cave and Slaughter Canyon Cave. In 1975 James Goodbar found an 85-percent-complete skeleton of a Pleistocene shrub ox in Musk Ox Cave, where animals had fallen into a large sinkhole and died. Digs by Texas Tech University and the Smithsonian Institution turned up more: a horse (Equus), an even-toed hoofed animal, a ringtail (Bassariscus), a spotted skunk (Spilogale), a woodrat (Neotoma), a dire wolf and a bobcat-like cat. Park fossils are protected under the Paleontological Resources Preservation Act of 2009.

Carlsbad Caverns lies near the edge of the Great Plains province and shares its geologic history with country well beyond the park.

Maps and reports

The Park Service's Geologic Resources Inventory has published a geologic map and report for Carlsbad Caverns, free on the NPS DataStore.

Sources

Written from "NPS Geodiversity Atlas — Carlsbad Caverns National Park, New Mexico," National Park Service Geologic Resources Division, a work of the United States government in the public domain, which cites Santucci and others (2001, 2007) and Graham (2007).