Host Rocks

Karst most commonly forms in carbonate rocks. The common carbonate rocks are limestone and its metamorphic equivalent marble, which are made of CaCO₃ and dolomite, which is CaMg(CO₃)₂. Significant karst usually occurs in pure limestones (marbles) and dolomites (>70% limestone or dolomite). Dolomite dissolves much more slowly than calcite, and it may require more than 60% CaCO₃ or as much as 90% CaCO₃ in comparison to CaMg(CO₃)₂ to fully develop karst features.

But karst may form in any soluble rock type (eg.halite, gypsum).

First systematically studied in the northern Adriatic region of NW Yugoslavia & NE Italy. Besides this classic area, there are many well-known areas.

Florida - sink holes
Kentucky - Mammoth Caves
Puerto Rico - cockpit karst
South China - tower Karst

Karst Chemistry

carbonation: CO₂ + H₂O -> H₂CO₃ (carbonic acid)

dissolution of calcium carbonate, simplified: CaCO₃ + H₂CO₃ -> Ca²⁺ + 2HCO₃^-
calcium carbonate in acidic solution (carbonic acid) dissolves yielding calcium ions and bicarbonate ions in solution

note: these are reversible reactions
also, the more CO₂ in the air, or in the soil gas, the greater the solubility of CO₂ in water
and, the greater the dissolved CO₂ in water the greater the solubility of limestone (CaCO₃)

Mixing - caves may most rapidly form just below the water table where the mixing of saturated high CO₂ vadose water and saturated low CO₂ phreatic water yields undersaturation with respect to calcite.

vadose (soil water - water percolating down to the water table)
phreatic (water in the zone of saturation)

Biological Factors

plant decay (via bacterial & fungal respiration) releases CO₂ into the soil which increases CO₂ solubility in groundwater
plants produce organic acids
cyanobacteria form microenvironmnets causing surface pitting
guano forms phosphoric acid (calcium phosphate = rock phosphate)

Climate Factors

Deserts: Little karst in deserts because little water available and the water table is deep. Carbonates are resistant cliff-formers in arid climates.

Cold regions: Even though CO₂ is more soluble in cold water than in warm water, there is little bacterial decay of humus, Little or no biological CO₂ or organic acids, and slow reaction rate. So karst is rare or absent in cold/dry places and uncommon in cold humid places.

Midlatitude humid temperate climates: karst is effective
adequate rainfall, vegetation, infiltration

Humid tropics: Big Karst, rapid dissolution
hi temperature
hi rainfall
hi biological activity: organic acids, plant decay yielding CO₂

Structural Control

Karst is not important in limestone with high primary porosity + permeability (e.g., chalk, reef limestone). Thick bedded limestones with spaced fractures are best for karst development because groundwater flow (and dissolution) is focused.

Focussed groundwater flow will occur:

downward along vertical joints (may form solution shafts)
horizontally parallel to bedding plane parting (passageways)
galleries may form at the intersection of passageways and vertical shafts

 

Major Karst Features

Sinkholes: also called dolines (classic karst area), cenotes (Yucatan)
Sinkholes may form by:

a) gradual dissolution and subsidence from the top of a limestone unit
b) gradual subsidence of overlying sediments as they are washed down through a joint into a larger cave below
c) sudden collapse of a cavern roof

Sinkhole ponds and lakes form because either
bottom of sink below the local water table
drain clogged with clastic sediments and debris

Uvalas = compound sinkhole or doline

Sinking Streams: in well-developed karst, surface streams often swallowed up by sinkholes (swallow holes). As fluvial erosion continues in the remaining surface stream valley, a steep residual headwall remains above the swallow hole at the end of a blind valley.

Underground rivers may also rise to the surface, often downslope of a sinkhole that swallowed the river. They rise in a large volume spring or river rise at the head of a pocket valley.

Karst valleys (solution valleys) are the remains of former surface stream valleys whose streams have been diverted underground as karst developed. They may develop a series of sinkholes in the valley floor.

Landscape Transition from Fluvial to Karst

fluviokarst: a combination of surface and underground drainage; streams + sinkholes & cavern systems; usually where a mantle of clastic sediments (sands and clays) or clastic sedimentary rocks overlie limestone

holokarst: no surface drainage, all underground; where thick limestone unit lies at or near the surface with many sinkholes and caverns

Poljes &endash; tectonically controlled uvalas in folded / faulted area

Tropical Karst

Cockpits: intersecting sinkholes (Puerto Rico, Jamaica, Cuba)
They are typically large enough, perhaps 1 km across, to have drainage systems in their slopes that modify the sinholes into star shapes

Residual Karst (usually only in the tropcs)

tower karst (south China, southeast Asia)
steep-sided residual limestone hills separated by alluvial plain
steep sides may be reinforced by solution at base (in damp soil)
cone karst (Puerto Rico)
cone-shaped residual hills surrounded by sinkholes

Cave Systems

Perhaps caves mostly form in upper portion of the zone of saturation (phreatic zone) where vadose and phreatic water mixes, but significant dissolution also likely in the vadose zone. Changing water table levels over time will result in caves forming at different levels.

Cave Dripstones form by partial evaporation of water seeping into cave from above; calcite dissolved from overlying limestone precipitates on roof or floor as evaporation supersaturates the drop

stalactites from roof
helictites twisted from roof
stalagmites from floor

Clastic deposits may fill in a cave