The Dunham classification system for carbonate sedimentary rocks was originally devised by Robert J. Dunham in 1962, [1] and subsequently modified by Embry and Klovan in 1971 [2] to include coarse-grained limestones and sediments that had been organically bound at the time of deposition. The modified Dunham Classification has subsequently become the most widely employed system for the classification of carbonate sedimentary rocks with 89% [3] of workers currently adopting this system over the alternative Folk [4] classification scheme
Robert J. Dunham published his classification system for limestone in 1962. [1] The original Dunham classification system was developed in order to provide convenient depositional-texture based class names that focus attention on the textural properties that are most significant for interpreting the depositional environment of the rocks.
The three criteria used to define the original Dunham classes were:
On the basis of these criteria, the following four classes were defined:
Recognising that these classes did not encompass all carbonate lithologies, Dunham defined two additional classes within his scheme:
Dunham specifically stated that, where appropriate, these six textural class names are intended to be combined with modifiers describing grains and mineralogy. The original classification can be summarized as follows:
Original Dunham classification (Dunham 1962) [1] | |||||
---|---|---|---|---|---|
Depositional texture recognizable | Depositional texture not recognizable | ||||
Original components not bound during deposition | Original components were bound during deposition | ||||
Contains mud | Lacks mud and is grain-supported | ||||
Mud-supported | Grain-supported | ||||
Less than 10% grains | More than 10% grains | ||||
Mudstone | Wackestone | Packstone | Grainstone | Boundstone | Crystalline Carbonate |
Following the publication of the original Dunham Classification System a number of modifications were proposed. The most widely adopted of these has been that of Embry and Klovan (1971) [2] who recognized that the Dunham classification scheme lacked detail when it came to the description of organically-bound and coarse-grained limestones.
Embry and Klovan proposed the subdivision of the Dunham 'boundstone' category on the basis of the means by which the sediment was organically-bound, thus yielding three new classes within the Dunham boundstone class:
Recognising that the identification of these structures is problematic at the limited scale of a petrographic thin section and typically requires examination of outcrop exposures or core, Embry and Klovan stated that where the mode of binding is not identifiable then the original Dunham classification term boundstone should be retained.
To address the issue of coarse-grained allochthonous limestones (lithologies where >10% of the components are >2 mm in diameter), Embry and Klovan proposed the introduction of two further new classes:
As with the original Dunham classification, modifiers should be employed to enhance the classification. Additionally, the class names should be employed as textural modifiers to describe the matrix. Embry and Klovan also redefined 'mud matrix' as material with a diameter of <30 μm.
Following the wide adoption of the Embry and Klovan (1971 [2] ) modifications, the Dunham Classification system is typically referred to as the 'modified Dunham Classification System' with both Dunham (1962 [1] ) and Embry and Klovan (1971 [2] ) being cited.
It can be summarized as follows:
Modified Dunham Classification (Dunham, 1962; [1] Embry and Klovan, 1971 [2] ) | |||||||||
---|---|---|---|---|---|---|---|---|---|
Allochthonous Limestones - No evidence that the original components were bound together at the time of deposition | Autochthonous Limestones - Original components were organically-bound during deposition | ||||||||
Less than 10% of the components are > 2 mm | Greater that 10% of the components are > 2 mm | ||||||||
Contains lime mud (<30 μm) | No lime mud | Bound by organisms that act as baffles | Bound by organisms that encrust and bind - the rock is supported by the matrix | Bound by organisms that build a rigid framework - the rock is supported by the fossil framework | |||||
Mud-supported | Grain-supported | Matrix-supported | Grain-supported by the >2 mm size fraction | ||||||
Less than 10% grains (>30 μm - 2 mm) | Greater than 10% grains (>30 μm - 2 mm) | ||||||||
Mudstone | Wackestone | Packstone | Grainstone | Floatstone | Rudstone | Bafflestone | Bindstone | Framestone |
. [3]
A revised classification was proposed by Wright (1992). [5] It adds some diagenetic patterns and can be summarized as follows:
Revised Dunham classification (Wright 1992) [5] | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
Depositional | Biological | Diagenetic | ||||||||
Matrix-supported (clay and silt grade) | Grain-supported | In situ organisms | Non-obliterative | Obliterative | ||||||
Less than 10% grains | More than 10% grains | With matrix | No Matrix | Encrusting binding organisms | Organisms acted to baffle | Rigid organisms dominant | Main component is cement | Many grain contact as microstylolithes | Most grain contacts are microstylolithes | Crystals larger 10 micrometers |
Calci-mudstone | Wackestone | Packstone | Grainstone | Boundstone | Bafflestone | Framestone | Cementstone | Condensed grainstone | Fitted grainstone | Sparstone |
Components larger 2 mm | Crystals smaller 10 micrometers | |||||||||
Floatstone | Rudstone | Microsparstone |
Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.
Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.
Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from shale by its lack of fissility.
Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 1⁄16 mm and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature.
The Folk classification, in geology, 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.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.
Under the Dunham classification system of limestones, a wackestone is defined as a mud-supported carbonate rock that contains greater than 10% grains. Most recently, this definition has been clarified as a carbonate-dominated rock in which the carbonate mud component supports a fabric comprising 10% or more very fine-sand grade or larger grains but where less than 10% of the rock is formed of grains larger than sand grade .
Under the Dunham classification system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as a carbonate-dominated rock that does not contain any carbonate mud and where less than 10% of the components are larger than 2 mm. The spaces between grains may be empty (pores) or filled by cement.
Under the Dunham classification system of limestones, a packstone is defined as a grain-supported carbonate rock that contains 1% or more mud-grade fraction. This definition has been clarified by Lokier and Al Junaibi (2016) as a carbonate-dominated lithology containing carbonate mud in a fabric supported by a sand grade grain-size fraction and where less than 10% of the volume consists of grains >2 mm.
Floatstone is a type of carbonate rock.
Rudstone is a type of carbonate rock.
Bafflestone is a type of carbonate rock.
A boundstone is a special type of carbonate rock in the Dunham classification
Bindstone is a special type of carbonate rock in the Dunham classification. The term did not appear in the original Dunham classification from 1962 and was introduced by Embry and Klovan 1971 in the modified Dunham classification.
A framestone is a special type of carbonate rock in the Dunham classification.