Dunham classification

Grainstone in the Dunham Classification (Brassfield Formation near Fairborn, Ohio). Grains are crinoid fragments.

The Dunham classification system for carbonate sedimentary rocks was devised by Robert J. Dunham in 1962, and refined by Embry and Klovan in 1971 to include sediments that were organically bound during deposition.

History

Robert J. Dunham published his classification system for limestone in 1962. His scheme focuses on the depositional fabric of carbonate rocks. Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are essentially for rock families. His efforts deal with the question of whether or not the grains were originally in mutual contact; and therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk classification scheme, Dunham deals with the original porosity of the rock. The Dunham scheme is more useful for hand samples because it is based on texture and not the grains in the sample

Dunham classes

Grainstone rich in peloids and shell fragments; calcite cement; Upper Carboniferous of Nevada.

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

• 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.
• Packstone contains lime mud and is grain supported.
• Grainstone lacks mud and is grain supported.
• Rudstone is coarse limestones supported by grains larger than 2 mm.
• Floatstone contains over 10% >2mm grains but is matrix supported
• Boundstone describes sediment where the original components have been bound together during deposition.
• Bafflestone develops where organisms have hindered (acted as a baffle to) deposition, reducing the local depositional energy. These stones usually contain traces of the baffling organisms, and smaller grains than would be expected from the strength of the paleocurrent. (Type of Boundstone)
• Bindstone is produced where organisms (such as algae) encrust the elements during deposition and bind them together. (Type of Boundstone)
• Framestone is a solid calcareous or siliceous framework which is maintained by an organism such as a coral or sponge. (Type of Boundstone)
• Crystalline carbonate does not have recognisable depositional structures.

Summary

Dunham	Mudstone	Wackestone	Packstone	Grainstone	Boundstone	Crystalline
Frame	Less than 10% grains, more than 90% mud	More than 10% grains, less than 90% mud	Grain supported	Grain supported	Grain or mud supported	Crystal supported
Mud	Mud supported,	Mud supported	Less mud	No mud	With or without mud	No grains or mud
Deposition cementation	Original components not bound together during deposition	Original components not bound together during deposition	Original components not bound together during deposition	Original components not bound together during deposition	Bound during deposition	Depositional figures not recognizable
Thin section	Mudstone		Packstone	Grainstone		Crystalline

References

• Dunham, R.J. (1962). "Classification of carbonate rocks according to depositional texture". In Ham, W.E. Classification of carbonate rocks. American Association of Petroleum Geologists Memoir. 1. pp. 108–121.