Geology of Mammoth Cave National Park
Mammoth Cave National Park is a designated UNESCO World Heritage Site as well as an International Biosphere Reserve located in central Kentucky. The park was established on July 1, 1941 and encompasses 52,830 acres of wilderness.
Mammoth Cave National Park is known for housing the world’s longest cave system, with over 405 miles (651 km) mapped. The vast cavern system was formed by water slowly dissolving Mississippian-aged carbonate rocks, creating sinkholes, tunnels, and underground rivers. This national park is also host to a diverse group of animal and plant species due to the microclimates created by varying light and temperature conditions.
Geologic History
Mammoth Cave National Park has one the most famous karst topographies in the entire world. Karst terrain is created by dissolution of soluble rocks like limestone or dolomite and is characterized by the presence of cave systems, sinkholes, springs, and disappearing streams (surface water that suddenly flows underground). The park is located within the Central Kentucky Karst, a limestone belt that extends from southern Indiana through Kentucky into Tennessee. It is part of the Chester Upland and the Pennyroyal Plateau and is dissected by the Green River, which controls the cave development. The sedimentary rocks that form the park were produced from sediments that were deposited when this area was submerged by an ancient ocean 330 million years ago as great inland seas or as near-shore deposits. The erosional force of the Green River has helped to carve the limestone into the unique and extraordinary karst topography of this region over millions of years.
Stratigraphy
Stratigraphy is the study of rock layers and their relationship to one another, helping researchers understand how the environment has changed over time. Rock layers are split into large sediment packages called formations, grouped based on common characteristics. Formations are sometimes split into smaller members, narrowed down to a more specific group of characteristics.
The four rock formations, from oldest to youngest, found in Mammoth Cave are the St. Louis Limestone, the Ste. Genevieve Formation, the Girkin Formation, and the Big Clifty Formation. The largest caves are formed in the first three limestone units, while the Big Clifty, predominately shale and sandstone, forms a caprock of more chemically resistant rock.
The St. Louis Limestone is the oldest unit exposed, the first unit deposited that we can still see from the surface and cave exposures. This unit was deposited during the Mississippian about 330 million years ago and is composed of fine to medium-grained limestone, dolomite, sandstone, siltstone, and greenish-gray shale. These sediments are typically found in marine environments. The fossilized remains of corals, bryozoans, brachiopods, shark teeth, gastropods, and crinoids have been found in this area and support that this part of what is now Kentucky was an ancient sea. Other important characteristics of this formation are horizontal beds and flat nodules of chert (a specific type of hard, very fine-grained sedimentary rock composed of silica) that protrude from the cave walls because they weather more slowly than the rest of the limestone and gypsum (a soft, hydrated calcium sulfate mineral).
The Ste. Genevieve Formation overlies the St. Louis Limestone and consists of interbedded limestone and dolomite, no gypsum is found in this unit. The Ste. Genevieve Formation contains similar fossils as in the St. Louis Formation.
The Girkin Formation lies above the Ste. Genevieve Formation and is predominantly made up of fine-to coarse-grained crystalline limestone. When these rocks were deposited, the area was still under water. An interesting feature found in this formation is the presence of oolites (small concentric spheres of calcium carbonate that form in shallow marine waters). This formation is separated by local interbedding of shale and sandstone, forming a lower oolitic portion and an upper fossiliferous portion. This upper fossiliferous section include the remains of many marine organisms such as corals, brachiopods, crinoids, and echinoids.
The Big Clifty Formation, the uppermost strata, is the oldest of the cap rocks. These brown and gray sedimentary rocks are mostly sandstone, siltstone, and fissile shale (shale that splits into thin sheets). These resistant cap rocks are critical to cave preservation. If too much water entered the cave system, the passages and caverns we marvel at today may have eroded away.
Cave Formation
The largest of the caves are found in the St. Louis Limestone, Ste. Genevieve, and Girkin Formations. Groundwater began interacting with the Girkin Limestone about 10 million years ago and began to carve the caves. The upper levels of the cave system were fully formed by 3.2 million years ago, based on radiometric dating of quartz pebbles. Water is a powerful force that can carve through rock, but it works very slowly.
To form this massive cave system, water percolated through cracks and pores in the cap rocks, where it reached the soluble limestone (calcium carbonate). Rainwater and groundwater are slightly acidic (naturally containing carbonic acid through the reaction of water and carbon dioxide), therefore chemically dissolving the rock over very long periods of time, also physically weathering the rock with the erosional power of water. The large size of this cave system is attributed to the amount of time it has been forming and to the size of the Green River drainage basin. At various points in the geologic past, parts of the cave system have filled with sediment that accumulated from the Green River, in some cases filling up previous passageways.
Speleothems
Mammoth Cave has many subterranean cave formations called speleothems that form from the precipitation of minerals that were once suspended in the groundwater percolating through the limestone. Stalactites, stalagmites, evaporites, helictites, gypsum formations, and travertine dams are just a few examples.
Evaporites are deposits of minerals that remain after water has evaporated. These often form in areas with a constant drip or slow flow, or areas with intermittent water. For example, evaporite dams can often form along the edges of pools, where the water ebbs periodically and dissolved minerals can precipitate out and accumulate.