Karst, Sinkholes, and Salt Dissolution

Karst topography forms in areas where the underlying bedrock is composed of material that can be slowly dissolved by water. Examples of this type of sedimentary rock include carbonate rocks such as limestone, halite, gypsum, dolomite, and anhydrite. Carbonate rocks may develop karst and other dissolution features due to the effects of circulating groundwater that has been made slightly acidic through the presence of dissolved carbon dioxide (which creates carbonic acid that reacts with the rock, dissolving it). Sinkholes and caverns can form, creating potential hazards (i.e., the land surface could subside or collapse into the underground openings). This may principally occur in areas where cavities filled with water are emptied through groundwater withdrawal or other natural processes, resulting in the cavities being filled with air and reducing support for the overlying rock. Much of the South Central is underlain by karst and carbonates (see Figure 10.17).

Central and southern Missouri, including the Ozark Mountains and their extensions into northern Arkansas and northeastern Oklahoma, are famous for caves and karst in Ordovician and Carboniferous limestone and dolomite. Missouri is home to over 6000 limestone caverns, many of which are prominent tourist attractions (Figure 10.16). In Missouri, karst is also associated with exceptionally large springs such as Big Spring, Greer Spring, and Maramec Spring (Figure 10.17). Other karst formations are found in the Arbuckle Mountains of south central Oklahoma and the Limestone Hills in southwestern Oklahoma. Soluble gypsum and salt deposits near the surface in western Oklahoma and the Texas panhandle can also cause karst and dissolution problems.

Because karst terrain is very porous and fractures easily, groundwater pollution can be a serious problem. Contaminants that might otherwise be filtered through the sedimentary rock are quickly transported into aquifers by runoff. The hazards of pollution are increased by rampant industrial, agricultural, and residential development over karst features. This is occurring rapidly in northwest Arkansas and in Missouri, where 59% of the state sits atop thick layers of carbonate rock (Figure 10.17).

Figure 10.15: Meramec Caverns, a 7.4-kilometer (4.6-mile) limestone cave system near Stanton, Missouri, is the most visited cave in the state. Meramec Caverns was introduced as a tourist attraction in 1935; advertisements for the location involved one of the earliest uses of the bumper sticker.

Figure 10.16: Maramec Spring, located in the east-central Ozarks, has an average daily discharge of 360 million liters (100 million gallons) of water. The spring’s opening is underwater, at the base of the dolomite overhang.

Figure 10.17: Areas of karst in the continental US, associated with carbonate and evaporate rocks. NOTE: Clicking on this image will bring up a high-resolution version that may take a few minutes to load. This version will expand to the size of your browser window, so enlarge the window for maximum visibility.

The Coastal Plain of eastern Texas and Louisiana is dotted with many Jurassic subsurface salt domes that can collapse if salt is removed. For example, the Bayou Corne sinkhole in Assumption Parish, Louisiana, is a site where an underground salt dome collapsed in 2012 (Figure 10.18). Before its collapse, the Bayou Corne sinkhole was preceded by months of seismic activity and the release of methane bubbles. It originally spanned one hectare (2.5 acres) but has since grown to over 10 hectares (26 acres). It is still growing, swallowing surrounding cypress swamp and endangering the nearby community, from which many of the residents have been evacuated. Scientists believe the sinkhole was created by the salt dome cavern being excavated too close to the massive salt deposit’s outer face, making it incapable of maintaining pressure. A similar event occurred in Daisetta, Texas, a town also located on the edge of a major salt dome. In May 2008, a 330-meter-wide (1080-foot-wide) sinkhole caused by partial collapse of the dome swallowed a parking lot, construction equipment, and a small stand of trees over the course of a single day before filling with water (Figure 10.19). These types of situations present a growing hazard that will be studied by geoscientists for years to come.

Figure 10.18: Aerial view of Bayou Corne sinkhole in Assumption Parish, Louisiana.

Salt karst can also be a hazard through its association with pathways for fluids, such as the flow of natural gas, which is sometimes stored in salt caverns. For example, in 2001, the Yaggy storage field—consisting of dozens of caverns in the Permian-age salt deposits near Hutchinson, Kansas— leaked 4 million cubic meters (143 million cubic feet) of natural gas, leading to multiple large-scale explosions within and around the city. In this case, the leak was associated with drilling errors as well as with faults and fractures in rocks overlying the salt formation.

Figure 10.19: Aerial view of the Daisetta sinkhole, A) before and B) after the event. The sinkhole is now filled with water.