Dunham classification

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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 1964, and refined by Embry and Klovan in 1971[1] to include sediments that were organically bound during deposition.

History[edit]

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[edit]

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.
  • Rudstone is coarse limestones supported by grains larger than 2 mm.
  • Grainstone lacks mud and is grain supported.
  • 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 acted as baffles during deposition, reducing the local depositional energy. They will contain traces of baffling organism and smaller grains than would be expected from the paleocurrent strength.
  • Bindstone is produced where organisms (such as algae) encrust the elements during deposition and bind them together.
  • Framestone is a solid calcareous or siliceous framework which is maintained by an organism such as a coral or sponge.
  • Crystalline carbonate does not have recognisable depositional structures.

Summary[edit]

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[edit]

  1. ^ Embry, A.F. III and Klovan, J.S. 1971. A Late Devonian reef tract on northeastern Banks Island, N.W.T. Bulletin of Canadian Petroleum Geology vol. 4, p. 730-781.