Dunham classification

The Dunham classification system for carbonate sedimentary rocks was devised by Dunham in 1964, and refined by Embry and Klovan in 1971 to include sediments that were organically bound during deposition.

The classification is a way of describing the composition of calcareous rocks. For descriptions detailing the textural components of sediments and sedimentary rocks, the Folk classification is generally preferred - both are equally valid methods of classification with different emphases.

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Mudstone

Contains less than 10% grains (usually assessed by area in cut or thin section), supported by a lime mud.

Wackestone

Consists of more than 10% grains, supported by a lime mud.

Floatstone

Consists of more than 10% large grains (>2 mm), supported by a lime mud.

Packstone

Contains lime mud, but is grain supported.

Rudstone

Is supported by components larger than 2 mm.

Grainstone

Lacks mud, and is grain supported.

Boundstone

Describes sediment where the original components have been bound together after deposition.

Bafflestone

Organisms acted as baffles during deposition, reducing the local depositional energy. Will contain traces of baffling organism and smaller grains that would be expected from the palæocurrent strength.

Bindstone

Organisms (such as algæ) encrust the elements during deposition and bind them together.

Framestone

A solid calcareous or siliceous framework is maintained by an organism such as a coral or sponge.

Crystalline Carbonate

Does not have recognisable depositional structures.

References

• Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in Ham, W. E. ed., Classification of carbonate rocks: American Association of Petroleum Geologists Memoir 1, p. 108-121.