The Folk classification, in
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...
, is a technical descriptive classification of
sedimentary rock
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 ...
s devised by
Robert L. Folk, an influential sedimentary
petrologist
Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...
and Professor Emeritus at the
University of Texas
The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,075 ...
.
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
![Folk-sandstone](https://upload.wikimedia.org/wikipedia/commons/1/16/Folk-sandstone.classification.png)
As others before him, Folk proposed a classification for sandstones based on the relative abundances of
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
(Q),
feldspars
Feldspars are 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) feldsp ...
(F), and
rock fragment
A rock fragment, in sedimentary geology, is a sand-sized particle or sand grain that is made up of multiple grains that are connected on the grain scale. These can include grains which are sand-sized themselves (a granitic rock fragment), or fi ...
s (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
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
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
Rounding means replacing a number with an approximate value that has a shorter, simpler, or more explicit representation. For example, replacing $ with $, the fraction 312/937 with 1/3, or the expression with .
Rounding is often done to ob ...
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
{{unreferenced, date=August 2012
An inscribed triangle of a circle
In geometry, an inscribed planar shape or solid is one that is enclosed by and "fits snugly" inside another geometric shape or solid. To say that "figure F is inscribed in figu ...
circle in a grain. In Folk's classification scheme, one uses the
Wentworth scale
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refer ...
to find the appropriate grain size name.
Folk's carbonate classification
Folk's carbonate rock classification details the relative proportions of
allochems
Allochem is a term introduced by FolkFolk, R. L. (1959) ''Practical petrographic classification of limestones.'' American Association of Petroleum Geologists Bulletin. 43, pp. 1–38. to describe the recognisable "grains" in carbonate rocks. Any fr ...
in the rock and the type of
matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** ''The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
. The classification scheme covers most common
carbonate rocks
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 rock (also known as doloston ...
, however the more inclusive although less precise alternative,
Dunham classification
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 organical ...
, may be preferred in some instances. Folk classification consists of one or two prefixes followed by a suffix.
A recent study of carbonate classification within academia and industry by Lokier and Al Junaibi (2016)
[{{Cite journal, last1=Lokier, first1=Stephen W., last2=Al Junaibi, first2=Mariam, date=2016, title=The petrographic description of carbonate facies: are we all speaking the same language?, journal=Sedimentology, language=en, volume=63, issue=7, pages=1843–1885, doi=10.1111/sed.12293, issn=1365-3091, doi-access=free] has highlighted a strong decline in the use of the Folk Classification with 89% of classifications employing some form of the
Dunham classification
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 organical ...
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
Ooids are small (commonly ≤2 mm in diameter), 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, m ...
*''bio-'' used to describe biogenetic remnants - for example, shells,
echinoderm ossicles or other tests
*''pel-'' describes the presence of
peloids
Peloids are allochems that are composed of micrite, irrespective of size, shape, or origin. The two primary types of peloids are pellets and intraclasts
Intraclasts are irregularly shaped grains that form by syndepositional erosion of partially ...
(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
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
)
*''intra-'' denotes the presence of
intraclasts
Intraclasts are irregularly shaped grains that form by syndepositional erosion of partially lithified sediment.
Gravel grade material is generally composed of whole disarticulated or broken skeletal fragments together with sand grade material of wh ...
, 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.
Gallery
Image:CarmelOoids.jpg, Thin-section of a biooosparite with calcitic ooids
Ooids are small (commonly ≤2 mm in diameter), 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, m ...
and sparry calcite cement; Carmel Formation
The Carmel Formation is a geologic formation in the San Rafael Group that is spread across the U.S. states of Wyoming, Utah, Colorado, north east Arizona and New Mexico. Part of the Colorado Plateau, this formation was laid down in the Middle J ...
, Middle Jurassic
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
, of southern Utah, USA.
Image:OoidSurface01.jpg, Biooosparite with calcitic ooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah, USA.
Image:GeopetalCarboniferousNV.jpg, Thin section
In optical mineralogy and petrography, a thin section (or petrographic 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 ...
of a biopelsparite showing a geopetal A way up structure'', ''way up criterion, or geopetal indicator is a characteristic relationship observed in a sedimentary or volcanic rock, or sequence of rocks, that makes it possible to determine whether they are the right way up (i.e. in the at ...
structure consisting of peloid
Peloid is mud, or clay used therapeutically, as part of balneotherapy, or therapeutic bathing. Peloids consist of humus and minerals formed over many years by geological and biological, chemical and physical processes.
Numerous peloids are av ...
sediment and sparry calcite cement in a recrystallized bivalve shell; Bird Spring Formation ( Carboniferous) of southern Nevada, USA.
Image:BioclastsBiosparite.jpg, Bioclasts in an Ordovician biosparite of southern Ohio, USA.;
T = trilobite;
E = echinoderm
Image:Biomicrite.jpg, Biomicrite
Image:Micrite.jpg, Micrite
Image:Spar cement.jpg, Spar
Image:Dismicrite.jpg, Dismicrite
Image:Intrasparite.jpg, Intrasparite
Image:BrassfieldLimestoneClose050412.jpg, Broken surface of Brassfield Formation
The Brassfield Formation, named by A.F. Foerste in 1906, is a limestone and dolomite formation exposed in Arkansas, Ohio, Kentucky, Indiana, Tennessee and West Virginia in the United States.Foerste, A.F. 1906. The Silurian, Devonian, and Irvine fo ...
( Early Silurian, Ohio) showing coarse sparry cement.
See also
*
Dunham classification
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 organical ...
*
Gazzi-Dickinson method
*
QFL Diagram
A QFL diagram or QFL triangle is a type of ternary diagram that shows compositional data from sandstones and modern sands, point counted using the Gazzi-Dickinson method. The abbreviations used are as follows:
* Q - quartz
* F - feldspar
* L - ...
, similar to the QFR diagram of the Folk classification system.
References
*Folk, R.L. (1980
''Petrology of Sedimentary Rocks''(pdf), Austin (Texas), Hemphill
Sedimentary rocks