3.2 Karst Occurrence where Soluble and Less Soluble Units Occur Together

Carbonate and evaporite rocks are sedimentary rocks that accumulate from shallow tidal flats to deep-water basins in coastal or marine environments. Because carbonates are a product of precipitation of limestone or biological activity rather than being a sediment that is transported from elsewhere, some carbonates can form far from a coastline such as a reef or atoll. Carbonates can form by calcite precipitation from inland waters into rocks called tufa. Often in coastal environments near large rivers where large quantities of sand and gravel are available, karst aquifer systems form within layered sequences of sandstones, mudstones, shales, and carbonates. Many major karst aquifer systems are composed of carbonate and sandstone or are adjacent to other aquifers composed of unconsolidated sediments. Where carbonate and sandstones are interbedded, the carbonate rocks generally yield more water than the sandstones.

If sandstones, shales, or less soluble and less permeable rock overlie the carbonates, they can form caprocks that protect the limestone beneath from dissolution and serve as an area of slow, diffuse recharge. The Mammoth Cave system is within a Mississippian age carbonate and sandstone aquifer. The sandstone caprock overlies the cavern system within the Mammoth Cave Plateau, which overlooks the dissolved limestone plain called the Pennyroyal Plain (Figure 10).

Illustrations showing the Mammoth Cave Plateau

Figure 10  a) Karst aquifers commonly occur between strata of other types of sedimentary rocks such as sandstone and shale as shown in this cross-section from the Mammoth Cave Kentucky region. Large volumes of water move rapidly from sinkholes and swallow holes through a well-developed network of solution cavities in the St. Louis and Ste. Genevieve Limestones to discharge at springs or to the Green River. The openings were formed by dissolution of the limestones as water moved along bedding planes and fractures. The sandstone that remained intact protected the upper formations forming the Mammoth Cave Plateau. The Chattanooga Shale forms a confining unit below the Warsaw Limestone and Fort Payne Formation (Miller, 1999). b) East looking view of the Pennyroyal Plain from overlook on Mammoth Cave Plateau at Mammoth Cave National Park with Rick Toomey discussing hydrogeology of the area. Photograph by Kuniansky (2008b).

In some parts of the world sediments of gravel, sand, silt, and clay have been deposited on top of older karstified limestones and fill in the conduits. This occurs frequently in the confined parts of the Floridan aquifer system in peninsular Florida where in some cases the sediment plugs up all the older solution features and lakes form (Figure 11a), or the process is ongoing with sediments gradually filling in the solution features. In both cases, recharge occurs slowly as in a traditional porous media aquifer (Figure 11b and c). The dangerous, rapid form of sinkhole collapse makes the news when there is a sudden cover-collapse sinkhole where the sediment falls into a large underground cavern within a period of hours or over several days (Figure 11d). In Canada and northern areas of the central United States, glaciers deposited sediment over many areas of limestones. When both the inlets and outlets of the conduits are clogged with sediment, the flow system reverts to behaving like a fractured-rock or granular flow system because there is no longer rapid inflow to and outflow from interconnected conduits. Sinkhole lakes in Canada and the northern United States are similar to the circular sinkhole lakes of Florida, but were filled in by glacial unconsolidated sediments rather than alluvial and coastal unconsolidated sediments.

Cross section showing karst terrain covered with unconsolidated sediments

Figure 11  In some areas, historically exposed karst terrain has been covered with unconsolidated sediments that fill in the dissolution features: a) sediment infilling over a completely clogged sinkhole; b) sediment slowly settling into solution openings forming a gradual depression in land surface and plugging the karst, but not enough to create a lake; c) a dry depression where sediments have completely plugged the solution openings at an elevation where rainwater moves slowly into the subsurface; and d) a sudden cover-collapse sinkhole. All of these are typical types of sediment filled sinkholes in Florida. In Canada and the northern parts of the USA, unconsolidated sediments from glacial deposits overlie many older limestone systems and similar circular lake features occur. Modified from Rupert and Spencer (2004).


Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.