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Folk limestone classification


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A widely used classification of carbonates, based on the type of particles and the nature and proportion of the matrix and/or cement present. In his original classification Robert L. Folk defined three main components to limestones. These are allochems, comprising various grains and particles; micrite (microcrystalline calcite mud matrix); and sparite (sparry calcite cement). The main allochems are bioclasts (‘bio-’, see below), pellets (‘pel-’), intraclasts (‘intra-’), and ooids (‘oo-’). In defining a limestone by the Folk classification the rock is named according to the nature of the material filling the spaces between the particles (i.e. micrite matrix or sparite cement), prefixed by an abbreviation to denote the main allochems present: bio- for bioclasts, pel- for pellets, oo- for ooids, and intra- for intraclasts. For example, a limestone comprising pellets in a mud matrix is called a ‘pelmicrite’, and a limestone comprising shell fragments with a sparry calcite cement is a ‘biosparite’. It is possible to combine the prefixes of several allochems where appropriate, e.g. ‘oobiosparite’. Limestones which lack allochems, consisting only of micrite are termed ‘micrites’. Dismicrite is used for micrites with fenestral cavities (see fenestrae). Organically bound limestones, e.g. reef rocks, or stromatolites, are termed biolithites. Folk subsequently modified his classification to include various carbonate textures. Under his textural scheme a limestone with varying proportions of bioclasts, mud matrix and cement would be classified as follows: micrite with less than 1% shell fragments = micrite; micrite with 1–10% shell fragments = fossiliferous micrite; micrite with 10–50% shell fragments = sparse biomicrite; micrite with over 50% shell fragments = packed biomicrite. Limestone with micrite and sparry calcite together with shell fragments = poorly washed biosparite; poorly sorted shell fragments with sparry calcite cement = unsorted biosparite; well sorted shell fragments with sparry calcite = sorted biosparite; rounded and abraded shell fragments with sparry calcite cement = rounded biosparite. Similar textural terms apply for other allochems with micrite and/or sparite. See biomicrite; intramicrite; intrasparite; pelsparite; oosparite; and oomicrite.

Subjects: Earth Sciences and Geography.


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