Limestone Classification

Classifying limestones is fundamentally about describing what the rock is made of and in what proportions — a task that turns out to carry direct environmental significance. Most carbonate rocks are essentially monomineralic, so mineralogy plays only a small role in classification; the principal parameters are the types of carbonate grains or allochems present, the grain/micrite ratio, and the grain-packing fabric (grain-supported vs. mud-supported). ^[1] A grain-supported fabric is one in which grains are in contact, creating an intact grain framework in which voids may or may not be filled with mud, whereas in a mud-supported fabric most grains do not touch and appear to float in carbonate mud. ^[1] This distinction matters because grain support implies high-energy agitated deposition while mud support implies quiet water where fine sediment was not winnowed away.

Folk’s Classification (1959/1962)

Folk’s classification is based on the relative abundance of three major constituents: carbonate grains (allochems), microcrystalline carbonate mud (micrite), and sparry calcite cement. ^[1] Classification proceeds in two steps: first determining the ratio of total allochems vs. micrite plus sparry calcite cement, then subdividing on the basis of which type of allochem dominates and whether micrite or spar is more abundant. ^[1]

This approach produces a bipartite name combining the dominant allochem type with the matrix type. The prefix codes the allochem: intra- for intraclasts, oo- for ooids, bio- for skeletal grains, pel- for pellets. The suffix codes the matrix: -sparite means sparry calcite dominates (micrite absent or minor); -micrite means carbonate mud dominates. ^[1] For example, a biomicrite tells you immediately that skeletal grains are present in a mud-dominated matrix — quiet-water conditions. A biosparite tells you that skeletal grains are cemented with spar and that mud has been winnowed away — high-energy conditions.

Additional textural maturity descriptors (packed, sparse, poorly washed, sorted, rounded) can be prepended to provide information on grain sorting and packing. ^[1] Folk’s classification can also be applied to dolomite rocks where allochems survive as ghosts after dolomitization. ^[1]

Dunham’s Classification (1962) and the Embry-Klovan Extension

Dunham’s classification is based solely on depositional texture and considers two aspects: grain packing relative to micrite abundance, and whether grains were organically bound at the time of deposition. ^[1] Unlike Folk’s system, it does not distinguish between different types of allochems — the grain identity is irrelevant to the name.

Dunham’s original scheme produces five terms for rocks whose depositional texture is recognizable:

• Mudstone — mud-supported, less than 10% grains. ^[1]
• Wackestone — mud-supported, more than 10% grains. ^[1]
• Packstone — grain-supported, contains mud. ^[1]
• Grainstone — grain-supported, lacks mud. ^[1]
• Boundstone — components were organically bound during deposition (reefs, stromatolites). ^[1]

An important nuance of Dunham’s system is that the boundary between grain-supported and mud-supported is not a fixed grain/micrite ratio, because grain support also depends on grain shape — platy or elongate grains such as bivalve shells may form a grain-supported fabric at much lower abundances than spherical particles such as ooids. ^[1]

Embry and Klovan (1972) extended Dunham’s scheme with two additional names — floatstone and rudstone — to better accommodate gravel-size (>2 mm) carbonate grains, and divided Dunham’s boundstone into three types — framestone, bindstone, and bafflestone — based on the presumed biological binding mechanism. ^[1]

Combined Use

Because Dunham’s classification ignores grain identity and Folk’s ignores the grain/mud packing relationship, the two systems are complementary rather than competing, and using both together gives more complete information. ^[1] For instance, a rock described as a packed oomicrite in Folk’s terminology would be called an oomicrite packstone using both systems together. ^[1]

Informal Terms

Several informal names are in common use for carbonate rocks.

A coquina is a mechanically sorted and abraded, poorly consolidated carbonate sediment consisting predominantly of fossil debris; coquinite is its consolidated equivalent. ^[1]

Chalk is a soft, earthy, fine-textured limestone composed mainly of the calcite tests of floating microorganisms such as foraminifers. ^[1]

Marl is an old and rather imprecise term for an earthy, loosely consolidated mixture of siliciclastic clay and calcium carbonate. ^[1]

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