Photo by W. W. Little

The primary function of carbonate rocks is, through weathering and erosion, to provide the cement found within clastic rocks.

Cement

Photo by W. W. Little

Carbonates are rocks composed of at least 50% calcite and/or dolomite and are produced primarily through biological (limestone) and diagenetic (dolostone) processes.

Carbonates

As with clastics, there are many schemes for classifying carbonate rocks. The two most commonly used are referred to as “Dunham” and “Folk,” named for the individuals who devised them. Because of the difficulty of distinguishing between matrix and cement in carbonate rocks, most current classifications are based on thin-section analyses.

Classification of Carbonate Rocks

The Dunham classification is similar to that of clastic rocks, in that it clearly shows the role of energy in sediment accumulation. The main distinction between rock types is based upon the relative abundance of allochems (framework grains) and matrix. Cement is treated as open pore space.

Dunham Classification

Some restrict mudstone through grainstone to carbonate rocks that contain “framework” grains of sand size and smaller. Wackestones with particles greater than sand size are sometimes referred to as floatstones. Packstones and grainstones with grains larger than sand size can be called rudstones.

Variation on the Dunham Classification

Mudstones are composed of at least 90% carbonate mud. In hand-sample, it is often extremely difficult to distinguish between mud and cloudy calcite spar cement.

Mudstone

Carbonate mud originates through four processes: algal disaggregation, mechanical and biological erosion, and direct precipitation from sea water.

Origin of Carbonate Mud

Calcareous Algae

Halimeda and Penicillus are the two most common forms of calcareous algae.

Penicillus Halimeda Carbonate Mud

Slide modified from Gahn, 2006

Wackstones are mud-supported carbonate rocks in which allochems make up more than 10% of the rock volume but do not come into regular contact with one another. They can appear to “float” in the matrix.

Wackstone

Fossiliferous wackstoneCrinoidal wackstone

Packstone

Packstones are closed-framework, grain-supported carbonate rocks.

Crinoidal packstone Bivalve/ehinoidal packstone

Grainstone

Grainstones are open framework, grain-supported carbonate rocks.

Gastropodal grainstoneOolitic grainstone

An allochem is a carbonate particle that has been transported a short distance and deposited as sediment.

Common Allochem Types

Intraclasts are intraformational lithic fragments derived through the erosion of previously deposited carbonate sediment.

Intraclasts

Ooids are spherical “coated” carbonate grains consisting of concentric laminations and are typically less than 1 mm in diameter. Coated grains larger than a few millimeters are pisoids.

Ooids

Boundstones are carbonate rocks in which components have been bound together through organic processes.

Boundstone

Bioclasts are the skeletal remains of organisms that lived contemporaneously with depositional process that formed the deposit that contains them.

Bioclasts (fossils)

Pelloids are composed of micro- to cryptocrystalline carbonate mud, typically in elongate spheroidal shapes. Pelloids of fecal origin are referred to as pellets.

Pelloids

Embry and Klovan (1971) expanded Dunham’s classification to emphasize the importance of grain size in packstones and grainstones and types of binding organisms in boundstones.

Expanded Dunham Classification

Floatstone

Floatstones are packstones in which at least 10% of the grains are gravel size.

RudistFloatstone

Rudstone

Rudstones are grainstones in which at least 10% of the grains are gravel size.

RudistRudstone

Bafflestones are boundstones in which the binding organism acted as a baffle (flow barrier).

Bafflestone

Algalbafflestone

Bindstones are boundstones formed by encrusting organisms.

Bindstone

Modern stromatolitesStromatolitic dolobindstone

Bindstones are boundstones formed by organisms that produce a rigid framework.

Framestone

Modern reefCoralline framestone

Photo by F. J. Gahn

Crystalline Carbonates

Crystalline carbonate rocks are those that do not retain their depositional texture due to diagenetic alteration. A crystalline texture is unusual in limestones but common in dolostones.

The Folk classification is more complex than that of Dunham and incorporates relative abundances of allochems, matrix, and cement. Allochem type also plays part in this classification.

Folk Classification

Micrite is equivalent to mudstone in the Dunham classification. As the relative abundance of allochems increase, the type and abundance of the allochem is used to modify the term micrite.

Micrite

Packed biomicriteMicrite

Sparite

Pelsparite

Biopelsparite

Biosparite

Sparite is equivalent to open space in the Dunham classification. As the relative abundance of allochems increase, the type and abundance of the allochem is used to modify the term sparite.

Orthochemical carbonate rocks are divided into two categories: microcrystalline (equivalent to Dunham’s crystalline category) and autochthonous reef rocks/biolithite (equivalent to Dunham’s boundstone).

Orthochemical Carbonate Rocks

Biolithites are formed by organisms that bind sediment as part of their growth process.

Biolithite/Reefrock

Dunham vs. Folk

Pores are cavities in the rock and can be related to depositional, diagenetic, and tectonic processes.

Porosity

Primary intergranular porosity consists of depositional open space between allochems that tends to decrease with burial through compaction and cementation.

Intergranular Porosity

Primary intragranular porosity consists of depositional open space within allochems that tends to decrease with burial through compaction and cementation.

Intragranular (sheltered) Porosity

Secondary intercrystalline porosity consists of open space between authigenic.

Intercrystalline Porosity

Secondary moldic porosity consists of open space within allochems produced through dissolution.

Moldic Porosity

Secondary dissolution porosity consists of open space within or between allochems formed by the removal of primary and secondary material.

Dissolution (vug) Porosity

Dolomites form mostly through diagenetic processes that replace Ca with Mg.

Dolomite