Classification of Carbonates
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The document summarizes carbonate rock classification systems. It describes the Dunham and Folk classification schemes which are based on composition and textures. The Dunham scheme categorizes rocks as mudstone, wackestone, packstone, or grainstone depending on whether allochems or matrix is dominant. The Folk scheme additionally considers allochem types. Both aim to capture depositional environments but Dunham focuses on grain abundance while Folk considers all components. The document also discusses carbonate components, textures, porosity types and dolomitization.
Classification of Carbonates
- 1. 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
- 2. 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
- 3. 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
- 4. 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
- 5. 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
- 6. 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
- 7. Carbonate mud originates through four processes: algal disaggregation, mechanical and biological erosion, and direct precipitation from sea water. Origin of Carbonate Mud
- 8. Calcareous Algae Halimeda and Penicillus are the two most common forms of calcareous algae. Penicillus Halimeda Carbonate Mud Slide modified from Gahn, 2006
- 9. 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
- 10. Packstone Packstones are closed-framework, grain-supported carbonate rocks. Crinoidal packstone Bivalve/ehinoidal packstone
- 11. Grainstone Grainstones are open framework, grain-supported carbonate rocks. Gastropodal grainstoneOolitic grainstone
- 12. An allochem is a carbonate particle that has been transported a short distance and deposited as sediment. Common Allochem Types
- 13. Intraclasts are intraformational lithic fragments derived through the erosion of previously deposited carbonate sediment. Intraclasts
- 14. 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
- 15. Boundstones are carbonate rocks in which components have been bound together through organic processes. Boundstone
- 16. Bioclasts are the skeletal remains of organisms that lived contemporaneously with depositional process that formed the deposit that contains them. Bioclasts (fossils)
- 17. Pelloids are composed of micro- to cryptocrystalline carbonate mud, typically in elongate spheroidal shapes. Pelloids of fecal origin are referred to as pellets. Pelloids
- 18. 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
- 19. Floatstone Floatstones are packstones in which at least 10% of the grains are gravel size. Rudist Floatstone
- 20. Rudstone Rudstones are grainstones in which at least 10% of the grains are gravel size. Rudist Rudstone
- 21. Bafflestones are boundstones in which the binding organism acted as a baffle (flow barrier). Bafflestone Algal bafflestone
- 22. Bindstones are boundstones formed by encrusting organisms. Bindstone Modern stromatolitesStromatolitic dolobindstone
- 23. Bindstones are boundstones formed by organisms that produce a rigid framework. Framestone Modern reefCoralline framestone Photo by F. J. Gahn
- 24. 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.
- 25. 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
- 26. 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
- 27. 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.
- 28. 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
- 29. Biolithites are formed by organisms that bind sediment as part of their growth process. Biolithite/Reefrock
- 30. Dunham vs. Folk
- 31. Pores are cavities in the rock and can be related to depositional, diagenetic, and tectonic processes. Porosity
- 32. Primary intergranular porosity consists of depositional open space between allochems that tends to decrease with burial through compaction and cementation. Intergranular Porosity
- 33. Primary intragranular porosity consists of depositional open space within allochems that tends to decrease with burial through compaction and cementation. Intragranular (sheltered) Porosity
- 34. Secondary intercrystalline porosity consists of open space between authigenic. Intercrystalline Porosity
- 35. Secondary moldic porosity consists of open space within allochems produced through dissolution. Moldic Porosity
- 36. Secondary dissolution porosity consists of open space within or between allochems formed by the removal of primary and secondary material. Dissolution (vug) Porosity
- 37. Dolomites form mostly through diagenetic processes that replace Ca with Mg. Dolomite