CARLSBAD CAVERNS
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.
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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 Caverns’ 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).
Dripstone - Stalactites,
Stalagmites, Columns
Gypsum Blocks
& Gypsum Speleothem
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.