Folk classification

Last updated July 01, 2023

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.

Folk's sandstone (clastic) classification

Folk's philosophy is that the name of a rock must convey as much information as possible without being a complete description. For this, he proposed five important properties of sandstones to use as defining characteristics. These five properties are: grain size, chemically precipitated cements, textural maturity, miscellaneous transported constituents, and clan designation. Folk's fivefold name must be in the following format:

(Grain size): (chemically precipitated cements) (textural maturity) (miscellaneous transported constituents) (clan designation)

However, Folk stated that cements and miscellaneous transported constituents are optional categories as they are not always observed. The other three properties should always be mentioned.

The following are examples of rock names using Folk's fivefold name:

Coarse sandstone: calcitic submature micaceous subarkose

Fine sandstone: supermature quartzarenite

Sandy granule conglomerate: calcitic submature calclithite

Very fine sandstone: chert-cemented submature quartzose phyllarenite

Clayey very fine sandstone: immature fossiliferous plagioclase arkose

Clan designation

QFR diagram for Folk's sandstone classification Folk-sandstone.classification.png — QFR diagram for Folk's sandstone classification

As others before him, Folk proposed a classification for sandstones based on the relative abundances of quartz (Q), feldspars (F), and rock fragments (R). These are the main poles of the classification diagram.

To define the clan name one must normalize the sum of abundances of quartz, feldspars and rock fragments to 100%. This means that other constituents that don't fit in these categories are disregarded. After this, the relative percentages of quartz, feldspars and rock fragments are used to plot the appropriate point on a QFR triangle and obtain the clan designation.

There are some exceptions when summing the abundances. Due to the difficulty in distinguishing quartz from metaquartzite rock fragments, metaquartzite is always plotted on the Q pole of the QFR diagram along with quartz. Granites and other phaneritic igneous rock fragments are plotted in the F pole of the diagram.

If the abundances of quartz, feldspars and rock fragments indicate that the rock is an arkose, a subarkose or a lithic arkose, one must then normalize the abundance of feldspars to 100% and attempt to identify the relative abundances of K-feldspars to plagioclase in the sample. If there is more plagioclase than there is K-feldspar, the rock is either a plagioclase arkose, a plagioclase subarkose or a lithic plagioclase arkose, respectively. If there is more K-feldspar than there is plagioclase, or if it is too difficult to make a distinction between the feldspars, the name stays as arkose, subarkose or lithic arkose, respectively.

If the abundances of quartz, feldspars and rock fragments indicate that the rock is a litharenite, a sublitharenite or a feldspathic litharenite, one must then normalize the abundance of rock fragments to 100% and attempt to identify the relative abundances volcanic rock fragments (VRFs), metamorphic rock fragments (MRFs) and sedimentary rock fragments (SRFs). If the relative abundances cannot be identified, then the clan name is simply obtained from the QFR triangle. If the relative abundances can be obtained, one must plot the appropriate point in the VRF-MRF-SRF triangle to obtain the clan name. If the point plots in the sedarenite field, one must then normalize all the sedimentary rock fragments to 100% and attempt to find the relative abundances of carbonate rock fragments (CRFs), chert fragments and sandstone fragments (Ss) and shale fragments (Sh). Using this information one must plot the point in the CRF-chert-Ss, Sh triangle and find the appropriate clan name. If the relative abundances of different sedimentary rock fragments cannot be obtained, then the rock is called a sedarenite, subsedarenite or feldspathic sedarenite, respectively.

The name must be as specific as possible and one must try to avoid using broad terms like litharenite or sedarenite if the necessary information is available.

Miscellaneous transported constituents

Miscellaneous transported constituents are any grains that do not fall into the categories described by the QFR diagram. These usually include heavy minerals or fossil fragments. These constituents provide a signature for the observed formation and will help correlating between various samples.

Textural maturity

Textural maturity is a property that relates to the amount of mechanical energy input on transported sediments through the abrasive power of currents and tides. It is observed in certain characteristics such as rounding and sorting of the grains. Folk states that as more mechanical energy is applied to transported sediment, this sediment will pass through the following four stages sequentially:

Immature stage: The sediment contains more than 5% clay and sand grains are poorly sorted and angular.

Submature stage: The sediment contains less than 5% clay and sand grains are poorly sorted and subangular to subrounded.

Mature stage: The sediment contains little to no clay and sand grains are well sorted but not well rounded.

Supermature stage: Sediment contains no clay and sand grains are well sorted and well rounded.

Cements

Cements are authigenic minerals precipitated in the pores of clastic rocks. The composition and texture of these cements depends on the chemistry of the water in the pore, the surrounding mineralogy, and the temperature and pressure conditions during cementation.

Grain size name

Grain size refers to the diameter of the largest possible inscribed circle in a grain. In Folk's classification scheme, one uses the Wentworth scale to find the appropriate grain size name.

Folk's carbonate classification

Folk's carbonate rock classification details the relative proportions of allochems in the rock and the type of matrix. The classification scheme covers most common carbonate rocks, however the more inclusive although less precise alternative, Dunham classification, may be preferred in some instances. Folk classification consists of one or two prefixes followed by a suffix.^[1]

A recent study of carbonate classification within academia and industry by Lokier and Al Junaibi (2016)^[2] has highlighted a strong decline in the use of the Folk Classification with 89% of classifications employing some form of the Dunham classification system.

Suffixes

The suffix -sparite is used if the rock has a crystalline matrix, and -micrite if it contains a micritic, or mud-based, matrix.

Prefixes

The prefix describing the main (non-matrix) component of the rock immediately precedes the suffix, a second prefix describing a second important component may be tagged on to the front of this.

oo- used to denote the presence of ooids
bio- used to describe biogenetic remnants - for example, shells, echinoderm ossicles or other tests
pel- describes the presence of peloids (fossilised fecal pellets). May also be used to describe any pellet under 2 mm in diameter (as peloids are often hard to distinguish from intraclasts after diagenesis)
intra- denotes the presence of intraclasts, for example quartz grains or carbonate clasts eroded from the surrounding rocks.

Example

A rock consisting mainly of ooids with some shelly fragments, with a crystalline matrix, would be termed a biooosparite.

Components Gallery

Thin-section of a biooosparite with calcitic ooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah, USA.
Biooosparite with calcitic ooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah, USA.
Thin section of a biopelsparite showing a geopetal structure consisting of peloid sediment and sparry calcite cement in a recrystallized bivalve shell; Bird Spring Formation (Carboniferous) of southern Nevada, USA.
Bioclasts in an Ordovician biosparite of southern Ohio, USA.;
T = trilobite;
E = echinoderm
Broken surface of Brassfield Formation (Early Silurian, Ohio) showing coarse sparry cement.

Thin Section Gallery

Photomicrograph (PPL) of lithoclasts and skeletal material in an intrasparite.
Photomicrograph (PPL) of lithoclasts and skeletal material in an intramicrite. Note that some of the dark micrite has started to recrystallize in the left half of the image
Photomicrograph (PPL) of an oosparite.
Photomicrograph (PPL) of an oomicrite from the Silurian Keel Formation, Oklahoma
Photomicrograph (PPL) of a biosparite
Photomicrograph (PPL) of a biomicrite from the Mississippian Lodgepole Formation, SW Montana
Photomicrograph (PPL) of a pelsparite.
Photomicrograph (PPL) of a possible pelmicrite. Note that the odds of being able to distinguish peloids from the surrounding micrite are slim.
Photomicrograph (PPL) of a stromatoporoid boundstone.
Photomicrograph (PPL) of a lime mudstone.
Photomicrograph (PPL) of a dismicrite
Photomicrograph (PPL) of a recrystallized (and dolomitized) limestone

Related Research Articles

<span class="mw-page-title-main">Limestone</span> Sedimentary rocks made of calcium carbonate

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.

Feldspar is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars. Feldspars make up about 60% of the Earth's crust, and 41% of the Earth's continental crust by weight.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and subsequent cementation of material

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.

<span class="mw-page-title-main">Lithology</span> Description of its physical characteristics of a rock unit

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.

Ooids are small, spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas. After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. Oolites usually consist of calcium carbonate; these belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad in the Czech Republic.

<span class="mw-page-title-main">Greywacke</span> Hard, dark sandstone with poorly sorted angular grains in a compact, clay-fine matrix

Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. The term "greywacke" can be confusing, since it can refer to either the immature aspect of the rock or its fine-grained (clay) component.

<span class="mw-page-title-main">Hornfels</span>

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

Arkose or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose.

<span class="mw-page-title-main">Mudrock</span> Type of sedimentary rock

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.

<span class="mw-page-title-main">Allochem</span>

Allochem is a term introduced by Folk to describe the recognisable "grains" in carbonate rocks. Any fragment from around 0.5 mm upwards in size may be considered an allochem. Examples would include ooids, peloids, oncolites, pellets, fossil or pre-existing carbonate fragments. Fragments are still termed allochems if they have undergone chemical transformations – for example if an aragonite shell were to dissolve and be later replaced by calcite, the replacement would still be deemed an allochem.

<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

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.

In optical mineralogy and petrography, a thin section is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat. It is then mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method uses the Michel-Lévy interference colour chart to determine thickness, typically using quartz as the thickness gauge because it is one of the most abundant minerals.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

<span class="mw-page-title-main">Cementation (geology)</span> Process of chemical precipitation bonding sedimentary grains

Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms "bridges" between original sediment grains, thereby binding them together. In this way, sand becomes sandstone, and gravel becomes conglomerate or breccia. Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz, and silica phases like cristobalite, iron oxides, and clay minerals; other mineral cements also occur.

<span class="mw-page-title-main">Gazzi-Dickinson method</span> Point-counting technique used in geology

The Gazzi-Dickinson method is a point-counting technique used in geology to statistically measure the components of a sedimentary rock, chiefly sandstone. The main focus part of the technique is counting all sand-sized components as separate grains, regardless of what they are connected to. Gazzi-Dickinson point counting is used in the creation of ternary diagrams, such as QFL diagrams.

<span class="mw-page-title-main">Lithic sandstone</span>

Lithic sandstones, or lithic arenites, or litharenites, are sandstones with a significant (>5%) component of lithic fragments, though quartz and feldspar are usually present as well, along with some clayey matrix. Lithic sandstones can have a speckled or gray color, and are usually associated with one specific type of lithic fragment.

<span class="mw-page-title-main">Pellets (petrology)</span> Form of carbonate sedimentary structure

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.

The Catoctin Formation is a geologic formation that expands through Virginia, Maryland, and Pennsylvania. It dates back to the Precambrian and is closely associated with the Harpers Formation, Weverton Formation, and the Loudoun Formation. The Catoctin Formation lies over the a granite basement rock and below the Chilhowee Group making it only exposed on the outer parts of the Blue Ridge. The Catoctin Formation contains metabasalt, metarhyolite, and porphyritic rocks, columnar jointing, low-dipping primary joints, amygdules, sedimentary dikes, and flow breccias. Evidence for past volcanic activity includes columnar basalts and greenstone dikes.

References

Folk, R.L. (1980) Petrology of Sedimentary Rocks (pdf), Austin (Texas), Hemphill

↑ Folk, R.L. (1959). "Practical Petrographic Classification of Limestones". AAPG Bulletin. 43 (1): 1–38. doi:10.1306/0BDA5C36-16BD-11D7-8645000102C1865D.
↑ Lokier, Stephen W.; Al Junaibi, Mariam (2016). "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.

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[Folk_1959-1] Folk, R.L. (1959). "Practical Petrographic Classification of Limestones". AAPG Bulletin. 43 (1): 1–38. doi:10.1306/0BDA5C36-16BD-11D7-8645000102C1865D.

[2] Lokier, Stephen W.; Al Junaibi, Mariam (2016). "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.

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