Gypsum flowers & crust on limestone walls of a giant canyon passage in Great Onyx Cave.

Great Onyx Cave is located in the northern part of Flint Ridge in Mammoth Cave National Park, Kentucky, USA. It has 8 miles worth of mapped passages. Geologically, Great Onyx Cave is part of the Mammoth Cave System, but it has become erosively separated from it (although an air flow connection with the Mammoth Cave System has been identified). Great Onyx Cave is the downstream continuation of the Salt Cave section of the system.

The walls of Great Onyx Cave are limestones of the Paoli Member, shales of the Bethel Member, and limestones of the Beaver Bend Member of the Girkin Formation (lower Upper Mississippian). The travertine speleothem-rich areas of Great Onyx Cave are wet and occur where a cap of overlying Big Clifty Sandstone is absent. The dry portions of the cave are below an intact Big Clifty Sandstone "caprock", and include the giant canyon passage areas and the gypsum speleothem areas.

The main cave passage of Great Onyx Cave is called Edwards Avenue, developed at Level B in the Mammoth Cave System. Level B passages formed about 2 to 4 million years ago during the Pliocene. Edwards Avenue is a giant canyon passage. Canyon passages are tall and narrow, and form in the vadose zone (above the water table). Giant canyon passages (a.k.a. rectangular passages) are much larger and can form in either the vadose or phreatic zone (at or below the water table). They are usually more than half-way filled with sediments and breakdown. Famous examples of giant canyon passages in Mammoth Cave park include Main Cave, Audubon Avenue, Broadway Avenue, and Sandstone Avenue.

This cave is sometimes accessible to the general public by guided lantern tours during boreal summer months. This photo was taken during a field trip in June 2011 as part of a cave geology course at Mammoth Cave park.

"Speleothem" is the technical term for "cave formations", and refers to all secondary mineral deposits in caves. The most common speleothem-forming mineral is calcite (CaCO3 - calcium carbonate), the same mineral in limestone, which is the host rock for almost all caves on Earth. Speleothem composed of calcium carbonate is given the compositional rock name travertine. Some of Great Onyx Cave's travertine has a minor aragonite component (see Siegel, 1965).

Travertine speleothem in Great Onyx Cave only occurs in the wet areas, where there is no overlying "caprock" of Big Clifty Sandstone (it has been erosionally removed). In such areas, groundwater readily enters the cave from the ceiling and walls.

Gypsum speleothem (see above photo) in Great Onyx Cave only occurs in the dry areas, where there is an intact "caprock" of Big Clifty Sandstone. Everything visible in the above photo is gypsum that covers a cave passage wall of Paoli Member limestones. The thin, elongated structures projecting from the ceiling are gypsum helictites. The surrounding material is a gypsum crust. Helictites consist of twisted to curved to irregularly-formed, elongated speleothem that project from cave ceilings or walls

Gypsum speleothem in Great Onyx Cave forms by small amounts of water seeping from bedrock walls and evaporating, leaving behind hydrous calcium sulfate crystals. The chemical constituents of the gypsum come from oxidation of minor disseminated pyrite in the surrounding limestone bedrock - pyrite is iron sulfide (FeS2 - "fool's gold). Most or all of the gypsum speleothem in Mammoth Cave park appears to have this origin, regardless of morphology - crusts, helictites, snowballs, flowers, hair, etc. (see Metzger et al., 2015).

References cited:

Metzger, J.G., D.A. Fike, G.R. Osburn, C.J. Guo & A.N. Aadison. 2015. The source of gypsum in Mammoth Cave, Kentucky. Geology 43: 187-190.