KARST: FORM-PROCESS-FUNCTION – AN EXPANSION OF THE CONCEPT

Karst was first defined as a landscape form – a landscape that develops on soluble limestone and dolomite. That landscape typically contains sinkholes, thin soils or bare rock, very rough topography, disrupted surface drainage, caves, large springs, etc. The classic description of the process creating karst is dissolution of the carbonate by the reaction with dilute carbonic acid from soil water. This process (with coupled mechanical erosion) creates multiple sets of porosity and permeability in the underlying karst aquifers. The most transmissive set self-organizes into mass transfer conduits which function to transport the dissolved solids out of the landscape, to erode the landscape. This description has been expanded to cover analogous forms/processes/functions on Earth and can be further expanded to extraterrestrial objects.

Terrestrially karst is well documented to form in rocks other than carbonates: in salt, gypsum, quartz sandstones, etc. The “solution” process, in the carbonate case, is chemically an acid/base neutralization not simple solution. A better description of the karst process is “congruent weathering”; processes in which all of the products are in solution, i.e. mobilized. In terrestrial karst the mobilizing fluid is water. That water ranges in composition from rain water to concentrated brines and from oxidizing to highly reducing. In extraterrestrial cases, fluids other than water can be the transporting agent. On Mars, concentrated brines that are stable at surface temperatures and pressures may form karst rimstone dams. On Titan, liquid methane is the fluid that may be transporting dissolved hydrocarbon solids to form karst. Rosetta’s recent discovery of sinkholes on Comet 67P raises illustrates that, in low gravity environments, gases can effectively function as the mobilizing fluid and transport solid dust particles out of the comet.

A suggested, more generalized karst definition (paraphrasing and expanding Klimchouk and Ford’s 2002 definition) is: “Karst is an integrated mass-transfer system in solids with a permeability structure dominated by conduits eroded into the solids by chemical and physical processes (often with coupled mechanical corrosion) that mobilize the solid and which is self-organized to facilitate the erosion and transport of the solids by fluid flow.”