Dunham classification
Encyclopedia
The Dunham classification system for carbonate
sedimentary rocks was devised by Robert J. Dunham in 1964, and refined by Embry and Klovan in 1971 to include sediment
s that were organically bound during deposition
.
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 mat
s. 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 not the grains in the sample
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.
Carbonate minerals
Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3**Otavite CdCO3**Rhodochrosite MnCO3**Siderite FeCO3**Smithsonite ZnCO3...
sedimentary rocks was devised by Robert J. Dunham in 1964, and refined by Embry and Klovan in 1971 to include sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s that were organically bound during deposition
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...
.
History
Robert J. Dunham published his classification system for limestoneLimestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
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 mat
Algal mat
An algal mat is a layer of usually filamentous algae on marine or fresh water soft bottoms. It may be considered one of many types of microbial mats. Algae and cyanobacteria are ubiquitous, often forming within the water column and settling to the bottom. In shallow environments, they are often...
s. Unlike the Folk classification
Folk classification
The Folk classification is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas.-Folk's sandstone classification:...
scheme, Dunham deals with the original porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of the rock. The Dunham scheme is more useful for hand samples because it is based on texture not the grains in the sample
Dunham classes
The classification is a way of describing the composition of calcareousCalcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
rocks. For descriptions detailing the textural components of sediments and sedimentary rocks, the Folk classification
Folk classification
The Folk classification is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas.-Folk's sandstone classification:...
is generally preferred - both are equally valid methods of classification with different emphases.
- MudstoneMudstoneMudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
contains less than 10% grains (usually assessed by areaAreaArea is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...
in cut or thin sectionThin sectionIn optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
), supported by a limeLime (mineral)Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
mudMudMud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
. - WackestoneWackestoneWackestone is a matrix-supported carbonate rock that contains over 10% allochems in a carbonate mud matrix. This is part of the Dunham classification of carbonate rocks. In the other widely used classification due to Folk, an equivalent description would be, for example, an oopelmicrite, where the...
consists of more than 10% grains, supported by a lime mud. - Packstone contains lime mud, but is grain supported.
- Rudstone is coarse limestoneLimestoneLimestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s supported by grains larger than 2 mm. - GrainstoneGrainstoneUnder the Dunham classification, grainstones are grain-supported carbonate sedimentary rocks that contain no micrite. The spaces between grains are filled with sparry cement.-References:...
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 after deposition.
- Bafflestone develops where organismOrganismIn biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s have acted as baffleBaffleBaffle or baffles may refer to:* Baffle , a flow-directing vane or panel in some vessels such as shell and tube heat exchangers, chemical reactors, or static mixers...
s during deposition, reducing the local depositional energyEnergyIn physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
. They will contain traces of baffling organism and smaller grains that would be expected from the paleocurrentPaleocurrentA paleocurrent or paleocurrent indicator is a geological feature that helps one determine the direction of flowing water in the geologic past...
strength. - Bindstone is produce where organisms (such as algaeAlgaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
) encrust the elements during deposition and bind them together. - Framestone is a solidSolidSolid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
calcareous or siliceous frameworkFrameworkFramework may refer to:*Software framework, a reusable set of libraries or classes for a software system .**Application framework, a software framework used to implement the standard structure of an application for a specific operating system....
which is maintained by an organism such as a coralCoralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
or sponge. - 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 | ||||||