Carbonate Classification

Compare and contrast the carbonate classification schemes of Folk and Dunham.

Carbonate rocks are composed of allochems and orthochems. Allochems are the outside components, effectively grains that are the equivalent of clasts in sandstones. Orthochems are the inside components, the microcrystalline binding materials of matrix or cement. Common carbonate classification schemes distinguish between allochem and orthochem components, the type of matrix or cement, and describe the rock texture. The most popular classification systems are the Folk system, created by Robert Folk of the University of Texas, and the Dunham system, created by Robert J. Dunham of the American Association of Petroleum Geologists.

Folk Classification

Folk Classification System. Source: Imperial College Rock Library

Folk Classification System. Source: Imperial College Rock Library

The Folk classification scheme is based on pairing prefixes describing the allochem, and suffixes describing the orthochem. For prefixes, the allochems can be intraclasts (foreign fragments), ooids (coated, rounded calcium carbonate grains), bioclasts (skeletal remains), and peloids (fine-grained clasts without internal structure). The prefixes may be stacked in order of prevalence. For suffixes, the binding material is a sparite for micro-crystallized cement, micrite for fine grained matrix, and rudite for coarser grained matrix. This classification system focuses on objective descriptive as opposed to interpretation. For example, it does not differentiate between a cement and a recrystallized micrite: both are sparite.

Folk also applied the concept of textural maturity to carbonates by looking at grain-to-matrix ratios. Descriptors like sparse, packed, poorly washed, unsorted, sorted, and rounded are added to the front of the allochem-orthochem name. These descriptors add precision to the name, and give information about the energy level of the deposition site. A high energy site with strong currents would abrade more, rounding grains and washing away finer particles, resulting in increased sorting.

Dunham Classification

Dunham Classification System

Dunham Classification System. Source: Imperial College Rock Library

The Dunham classification scheme is primarily textural descriptions, with grain packing and grain-to-matrix ratio. While the allochem geometry is important, the allochem composition is ignored in this naming system. Describing grain packing includes observing the degree of grain support. A grain-supported limestone was probably porous at the time of formation, with primary sparry cement. A matrix-supported limestone with floating grains probably originally formed as micrite, which later recrystallized as secondary sparry cement.

Mudstone and wackestone are both mud-supported clastic rocks, where allochem grains are floating in a micrite matrix. Mudstone has fewer grains (<10% of total composition) and more silt, while wackestone has more grains (>10% of total composition) and more clay.

Packstone is a grain-supported clastic rock with some mud, where the allochem grains are in tangential content. It contains some mud. Grainstone is a grain-supported clastic rock with no mud, where allochem grains are in direct contact.

Embry and Klovan modified the Dunham scheme to distinguish limestones with large clasts, where at least 10% of the rocks are greater than 2 millimeters in diameter (>10% of components are >2mm).Floatstone is a large-clast matrix-supported rock, while rudstone is a large-clast grain-supported rock.

Boundstones are the only non-clastic limestones in this classification system, where the components are bound together during formation. The Embry and Klovan modification subdivides this by mechanism of binding: framestone for organisms that build rigid frameworks like reefs or sponges; bindstone for organisms that encrust and bind like algae and stromatolites; and bafflestone for organisms that trap sediment through parallel columns like vertical corals.

Finally, crystalline carbonates is the catch-all term for all limestones where the original depositional texture unrecognizable.

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