Folk classification
Encyclopedia
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.
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:
(Q), feldspars (F), and rock fragment
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
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.
or fossil fragments. These constituents provide a signature for the observed formation and will help correlating between various samples.
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:
circle in a grain. In Folk's classification scheme, one uses the Wentworth scale to find the appropriate grain size name.
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.
Used to describe biogenetic remnants - for example, shells, echinoderm ossicles
or other tests
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
)
Denotes the presence of intraclasts
, for example quartz grains or carbonate clasts eroded from the surrounding rocks.
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:: (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
As others before him, Folk proposed a classification for sandstones based on the relative abundances of quartzQuartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
(Q), feldspars (F), and rock fragment
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 , or finer-grained materials...
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
Alkali feldspar
The alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....
to plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
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 mineralsHeavy mineral sands ore deposits
Heavy mineral sands are a class of ore deposit which is an important source of zirconium, titanium, thorium, tungsten, rare earth elements, the industrial minerals diamond, sapphire, garnet, and occasionally precious metals or gemstones....
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 roundingRoundness
*Roundness — sharpness of handwriting patterns*Roundness — measure of sharpness of a particle's corners*Roundness — roundness of clastic particles...
and sorting
Sorting
Sorting is any process of arranging items in some sequence and/or in different sets, and accordingly, it has two common, yet distinct meanings:# ordering: arranging items of the same kind, class, nature, etc...
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 inscribedIncircle and excircles of a triangle
In geometry, the incircle or inscribed circle of a triangle is the largest circle contained in the triangle; it touches the three sides...
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 allochemsAllochems
Allochem is a term introduced by Folk to describe the recognisable 'grains' in carbonate rocks. Any fragment from around ½mm upwards in size may be considered an allochem. Examples would include ooids, peloids, oncolites, pellets, fossil or pre-existing carbonate fragments...
in the rock and the type of matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
. The classification scheme covers most common carbonate rocks, however the more inclusive although less precise alternative, Dunham classification
Dunham classification
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 sediments that were organically bound during deposition.-History:...
, may be preferred in some instances. Folk classification consists of one or two prefixes followed by a suffix.
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.bio-
Used to describe biogenetic remnants - for example, shells, echinoderm ossicles
Ossicles
The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss...
or other tests
pel-
Describes the presence of peloids
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. Another type of peloid is pseudo-oolith....
(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
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
)
intra-
Denotes the presence of intraclasts
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 whole, disaggregated and broken skeletal debris...
, for example quartz grains or carbonate clasts eroded from the surrounding rocks.