Wackestone

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Fragmented bioclastic wackestone Wackestone PPL.jpg
Fragmented bioclastic wackestone
A Wackestone in thin section (width of image is 10 mm) DunhamWackestone.jpg
A Wackestone in thin section (width of image is 10 mm)

Under the Dunham classification (Dunham, 1962 [1] ) 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 (<63 μm) component supports a fabric comprising 10% or more very fine-sand grade (63 μm) or larger grains but where less than 10% of the rock is formed of grains larger than sand grade (>2 mm). [2]

Dunham classification classification system for carbonate sedimentary rocks

The Dunham classification system for carbonate sedimentary rocks was originally devised by Robert J. Dunham in 1962, and subsequently modified by Embry and Klovan in 1971 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% of workers currently adopting this system over the alternative Folk classification scheme

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolomite, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In fact, in old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.

Carbonate rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3) and dolomite, also known as dolostone, which is composed of the mineral dolomite (CaMg(CO3)2).

Contents

The identification of wackestone

Schematic wackestone as seen in thin section under the petrographic microscope. Type of carbonaceous rock according to the depositional texture: Alloctonous carbonates - Original components not bound at the deposition time. Less than 10% of components larger than sand size (> 2 mm) Contains carbonate mud (micrite, silt/clay size <63 mm) Fabric supported by carbonate mud (micrite, <63 mm) 10% or more composed of 63 mm or greater grains Legend: Dotted background: micritic matrix. Curved blue particles: bioclasts (indeterminate fossils, e.g. bivalve fragments). Blue cones: bioclasts (e.g. fossils of gastropods) Black spheroids: bioclasts (pellets). Wackestone.svg
Schematic wackestone as seen in thin section under the petrographic microscope. Type of carbonaceous rock according to the depositional texture: Alloctonous carbonates – Original components not bound at the deposition time. Less than 10% of components larger than sand size (> 2 mm) Contains carbonate mud (micrite, silt/clay size <63 μm) Fabric supported by carbonate mud (micrite, <63 μm) 10% or more composed of 63 μm or greater grains Legend: Dotted background: micritic matrix. Curved blue particles: bioclasts (indeterminate fossils, e.g. bivalve fragments). Blue cones: bioclasts (e.g. fossils of gastropods) Black spheroids: bioclasts (pellets).

A study of the adoption and use of carbonate classification systems by Lokier and Al Junaibi (2016) [2] highlighted that the most common problem encountered when describing a wackestone is to incorrectly estimate the volume of 'grains' in the sample – in consequence, misidentifying wackestone as mudstone or vice versa. The original Dunham classification (1962) [1] defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971) [3] to a grain size of less than or equal to 30 μm. Wright (1992) [4] proposed a further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt (62 μm).

Mudstone Fine grained sedimentary rock whose original constituents were clays or muds

Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.

Related Research Articles

Mudrock Class of fine grained siliciclastic sedimentary rocks

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 0.0625 mm and are too small to study readily in the field. At first sight the rock types look quite similar; however, there are important differences in composition and nomenclature. There has been a great deal of disagreement involving the classification of mudrocks. There are a few important hurdles to classification, including:

  1. Mudrocks are the least understood, and one of the most understudied sedimentary rocks to date
  2. It is difficult to study mudrock constituents, due to their diminutive size and susceptibility to weathering on outcrops
  3. And most importantly, there is more than one classification scheme accepted by scientists

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.

Clastic rock type of sedimentary rock

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 with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

Calcarenite A type of limestone that is composed predominantly of sand-size grains

Calcarenite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) sand-size, carbonate grains. The grains consist of sand-size grains of either corals, shells, ooids, intraclasts, pellets, fragments of older limestones and dolomites, other carbonate grains, or some combination of these. Calcarenite is the carbonate equivalent of a sandstone. The term calcarenite was originally proposed in 1903 by Grabau as a part of his calcilutite, calcarenite and calcirudite carbonate classification system based upon the size of the detrital grains composing a limestone. Calcarenites can accumulate in a wide variety of marine and nonmarine environments. They can consist of grains of carbonate that have accumulated either as coastal sand dunes (eolianites), beaches, offshore bars and shoals, turbidites, or other depositional settings.

Grainstone Type of limestone

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.

Packstone class of limestone in the Dunham classification system

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'.

Micrite limestone constituent formed of calcareous particles formed by the recrystallization of lime mud

Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to four μm formed by the recrystallization of lime mud.

Floatstone

Floatstone is a type of carbonate rock.

Calcisiltite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) silt-size carbonate grains. These grains consist either of the silt-size particles of ooids, fragments of fossil shells, fragments of older limestones and dolomites, intraclasts, pellets, other carbonate grains, or some combination of these. Calcisiltite is the carbonate equivalent of a siltstone. Calcisiltites can accumulate in a wide variety of coastal, lacustrine, and marine environments. It is typically the product of abrasion and bioerosion.

Calcilutite is a type of limestone that is composed of predominantly, more than 50 percent, of either clay-size or both silt-size and clay-size detrital (transported) carbonate grains. These grains consist either of fossil fragments, ooids, intraclasts, pellets, other grains, or some combination of them. The term calcilutite was originally proposed in 1903 by Grabau as a part of his calcilutite, calcarenite and calcirudite classification system based upon the size of the detrital grains composing a limestone. In the original classification of limestone according to the dominant grain-size, calcisiltites were not named and are classified as calcilutite. In this classification, which the majority of geologists follow, a calcilutite consists of both silt- and clay-size, less than 0.062 mm in diameter, grains. It is the carbonate equivalent of a mudstone. Calcilutites can accumulate in a wide variety of marine and lacustrine environments.

Pellets (petrology)

Pellets are small spherical to ovoid or rod-shaped grains that are common component of many limestones. They are typically 0.03 to 0.3 mm long and composed of carbonate mud (micrite). Their most common size is 0.04 to 0.08 mm. Pellets typically lack any internal structure and are remarkably uniform in size and shape in any single limestone sample. They consist either of aggregated carbonate mud, precipitated calcium carbonate, or a mixture of both. They either are or were composed either of aragonite, calcite, or a mixture of both. Also, pellets composed of either glauconite or phosphorite are common in marine sedimentary rocks. Pellets occur in Precambrian through Phanerozoic strata. They are an important component mainly in Phanerozoic strata. The consensus among sedimentologists and petrographers is that pellets are the fecal products of invertebrate organisms because of their constant size, shape, and extra-high content of organic matter.

Rudstone

Rudstone is a type of carbonate rock.

Bafflestone

Bafflestone is a type of carbonate rock.

A Boundstone is a special type of carbonate rock in the Dunham classification

A Bindstone is a special type of carbonate rock in the Dunham classification. The term Bindstone 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.

References

  1. 1 2 Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. American Association of Petroleum Geologists, Tulsa, Oklahoma, pp. 108–121.
  2. 1 2 Lokier, Stephen W.; Al Junaibi, Mariam (2016-12-01). "The petrographic description of carbonate facies: are we all speaking the same language?". Sedimentology. 63 (7): 1843–1885. doi:10.1111/sed.12293. ISSN   1365-3091.
  3. Embry, A.F. and Klovan, J.E., 1971. A Late Devonian reef tract on Northeastern Banks Island, NWT. Bulletin of Canadian Petroleum Geology, 19(4)), 730–781.
  4. Wright, V.P., 1992. A revised classification of limestones. Sedimentary Geology, 76(3–4), 177–185.