17.6 Chemical Sedimentary Rocks—Carbonates and Evaporites
Limestones, dolomites, and, to a lesser extent, evaporites like gypsum and halite, are widespread over the surface of the Earth. These rocks have an extremely wide range of hydrogeologic characteristics and are responsible for both some of the most productive aquifers in the world as well as extensive low permeability units with very low water quantity and quality.
Limestones can have a wide range of primary porosities and permeabilities. Limestones created from lime muds usually have low permeability and low porosity, while limestones created by the accumulation of invertebrate shells or in reef environments have high primary porosity and permeability. Since calcite—one of the primary carbonate minerals—is fairly soluble, these rocks can easily become well‑cemented, and some limestones have very little open pore space.
The high solubility of limestones means that they are also subject to extensive karstification. Karstification refers to the formation of caves and other voids associated with dissolution of soluble minerals. Karst development usually starts in fractures and tends to occur at the water table where [latex]{CO}_{2}[/latex] outgasses from the system and groundwater becomes acidic by mixing with the [latex]{CO}_{2}[/latex] to form carbonic acid. The acidic groundwater flows through fractures, small pores, and other voids, dissolving the soluble carbonate minerals and enlarging the interconnected pores to create open spaces with very high permeabilities. The resultant cavities may be filled with reddish silt, re‑precipitated calcite, or they may remain open for fluid flow. Extensive karstification results in extremely transmissive and productive aquifers as discussed in Introduction to Karst Aquifers↗ by Kuniansky and others (2022) and Karst: Environment and Management of Aquifers↗ by Stevanović and others (2024), both are available as free downloads from The Groundwater Project. Some well‑known and extensively studied karst aquifers include the Floridian aquifer and The Edwards Aquifer↗ as presented by Sharp and Green (2022); their book is available as a free download from The Groundwater Project. These aquifers respond quickly to recharge from storms, and flow rates are very high. They are also extremely susceptible to contamination.
Evaporite units rarely form aquifers. They tend to have low primary porosity and are so ductile that fractures don’t stay open for long. Some aquifers occur in gypsum—for example, the Blaine aquifer of Texas and southwest Oklahoma, US—but these aquifers typically yield low‑quality water and are limited in extent.