Carbonate rocks: components and classification

A limestone is made of grains (allochems), lime mud (micrite) and cement (spar). What the grains are and whether mud or grains hold the rock up give its name.

Components

Skeletal grains (bioclasts)	Calcareous shells and skeletal fragments; see Fossils in thin section.
Ooids	Coated grains, commonly ≤2 mm, built of concentric layers around a nucleus (a shell fragment, quartz grain or other particle). They form where waves keep grains moving, most commonly in shallow tropical seas such as the Bahamas and the Persian Gulf.
Oncoids	Coated grains formed where algae precipitate calcite and storms periodically roll the grain over.
Pisoids	Like ooids but larger than 2 mm; they grow in the vadose zone.
Peloids (pellets)	Silt- to sand-sized grains of lime mud, mostly invertebrate fecal pellets.
Intraclasts	Fragments of already-lithified carbonate reworked into younger sediment, like rip-up clasts.
Micrite	Clay-sized microcrystalline carbonate (lime mud): the carbonate equivalent of matrix.
Spar	Coarse, clear calcite cement filling pore spaces.

Folk classification

Folk names have two parts: the most abundant grain type (bio-, oo-, pel-, intra-) plus the material between grains (-sparite for spar cement, -micrite for lime mud). Examples: biosparite, biomicrite, oosparite, oomicrite, pelsparite, pelmicrite, intrasparite, intramicrite; dismicrite is mud with spar-filled patches. Most useful for thin sections.

Dunham classification

Based on depositional texture: is the rock supported by mud or by grains?

Mudstone	Mud-supported, under 10% grains.
Wackestone	Mud-supported, over 10% grains.
Packstone	Grain-supported, with mud between grains.
Grainstone	Grain-supported, little or no mud.
Boundstone	Components organically bound together during deposition.
Crystalline carbonate	Recrystallization has erased the depositional fabric.

Embry & Klovan additions (coarse grains and reefs)

Floatstone	Matrix-supported, with over 10% of grains larger than 2 mm floating in finer sediment.
Rudstone	The grains larger than 2 mm support the framework.
Bafflestone, bindstone, framestone	Subdivide boundstone by how organisms bound it: baffling sediment, encrusting and binding it, or building a rigid framework.

Photomicrographs

**Four main carbonate grains: bryozoans, ooids, peloids, lithoclasts (PPL).** Michael C. Rygel, CC BY-SA 4.0. Source

**Coated grains: ooids, pisoids and oncoids.** James St. John and Michael C. Rygel, CC BY-SA 4.0. Source

**Micrite (lime mud) examples.** Michael C. Rygel, CC BY-SA 4.0. Source

**Oolitic grainstone with spar cement (PPL).** Michael C. Rygel, CC BY-SA 4.0. Source

**Folk classification.** Page Quinton, CC BY-SA 4.0. Source

**Folk examples 1: intrasparite, intramicrite, oosparite, oomicrite, biosparite, biomicrite.** Michael C. Rygel, CC BY-SA 4.0. Source

**Folk examples 2: pelsparite, pelmicrite, boundstone, mudstone, dismicrite, recrystallized limestone.** Michael C. Rygel, CC BY-SA 4.0. Source

**Dunham classification.** Page Quinton, CC BY-SA 4.0. Source

**Dunham examples 1: lime mudstone, skeletal wackestone, skeletal packstone.** Michael C. Rygel, CC BY-SA 4.0. Source

**Dunham examples 2: grainstone, boundstone, dolomitized limestone.** Michael C. Rygel, CC BY-SA 4.0. Source

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Identify the fossils Limestone types

Sources

Rygel & Quinton, Sedimentary Geology 6.3: Carbonate Components and Classification · Wikipedia: Dunham classification · Wikipedia: Limestone · Wikipedia: Ooid