Peloid

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Peloids (dark) surrounded by Sparitic cement (light) in the thin section of a limestone from the Nevada Carboniferous .

Peloid ( Greek  πέλος (pelos) = mud , εἶδος (eídos) = shape) is a term used to describe limestones in thin and polished sections. However, the definition is not completely uniform. It refers to round, oval to slightly elongated, micritic or cryptocrystalline grains in the range of 50 to 500 μm with or without internal structure (depending on the definition). Due to their size and structure, peloids can usually only be seen in thin sections under the microscope . Larger, completely micritic lime grains (> 0.5 mm) are counted as clasts .

Definition and naming

The term peloid goes back to ED McKee and RC Gutschick, who coined it in 1969. The terms pelettoide (depending on the author), mud peloid, bahamite, pseudooide and arrondide are synonyms for peloid. These are rounded, oval to slightly elongated, micritic or cryptocrystalline, calcareous components in the order of 50 to 500 μm with or without an internal structure. "Peloid" i. e. S. is basically a collective term for rock components whose origin or genesis is not certain, which is very often the case. More recent studies (e.g., Flügel, 2004) also include the so-called fecal pellets ( excrement pills from snails and crustaceans ) as peloids, as they also consist of a micritic matrix and are of the same size as the peloids. Fecal pellets often have a poorly developed internal structure and in many cases allow a genetic interpretation. However, they can also be structureless or have lost their internal structure secondarily.

The lack of internal structure can be primary, or an originally existing internal structure has been lost through recrystallization . Furthermore, slightly rounded fragments of biogens can be completely micritized by the drilling activity of microorganisms (drill fungi and algae), and thereby acquire the appearance of a peloid. Also ooliths can u. U. be completely micritized and become peloids. As a rule, however, the origin of such peloids from original ooids can be proven by other evidence. Another possibility for the formation of peloids are sediment crumbs ( algae peloids) that are formed with the help of cyanobacteria . Limestones, which consist mainly of peloids, are referred to as pelmikrite or pel sparite in the carbonate classification according to Robert L. Folk , depending on the matrix .

Similar components

The extensive lack of an internal structure is in contrast to the ooids, which are completely encrusted components with clear layers. Oncoids are also layered, but usually much larger and more irregular.

literature

  • Hans Füchtbauer and Detlev K. Richter: Carbonate rocks. In: Hans Füchtbauer (Ed.), Sediment-Petrology, Part 2, Sediments and Sedimentary Rocks. 4th edition, E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 1988, ISBN 3-510-65138-3 , p. 324 f.
  • Erik Flügel: Microfacies of Carbonate Rocks. Springer Verlag, Berlin / Heidelberg / New York 2004, ISBN 3-540-22016-X

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