Folk classification
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Thin-section of an oosparite with calcitic ooids and sparry calcite cement; Carmel Formation, Middle Jurassic, of southern Utah, USA; the largest ooid is 1.2 mm in diameter.

The Folk classification, devised by R. L. Folk in 1959, is a technical descriptive classification of calcareous sedimentary rocks. It 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.

Applying Folk Classification

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.

Common Prefixes

oo-

Used to denote the presence of ooids

bio-

Used to describe biogenetic remnants - for example, shells, echinoiderm ossicles or other tests

pel-

Describes the presence of peloids (fossilised droppings). 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.

References

• Folk, R.L., 1959, Practical petrographic classification of limestones: American Association of Petroleum Geologists Bulletin, v. 43, p. 1-38.

Bioclasts in an Ordovician limestone; T = trilobite; E = echinoderm. Scale bar is 2.0 mm.

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