Photo by W. W. Littl The primary function of carbonate rocks is, through weathering and erosion, to provide the cement found within clastic rocks. Cement
Jun 14, 2015
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