Oncolite

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File:Oncolites.jpg
Oncolites from the upper Burgsvik beds (Silurian), with shell fragments as nuclei. The large flower is 2 cm in diameter.
File:Bactroptyxis trachaea 041811.jpg
Oncolites formed around the Middle Jurassic gastropod Bactroptyxis trachaea (Normandy, France).

Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but, instead of forming columns, they form approximately spherical structures.[1] The oncoids often form around a central nucleus, such as a shell fragment,[2] and a calcium carbonate structure is deposited by encrusting microbes. Oncolites are indicators of warm waters in the photic zone, but are also known in contemporary freshwater environments.[3] These structures rarely exceed 10 cm in diameter.

The appearance of recent/subrecent freshwater oncoids has been documented in two rivers in Bavaria: the Alz, whose source is the Chiemsee, and the Moosach, near Freising.[4][5][6]

References

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  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Riding, Robert. 1991. Calcareous Algae and Stromatolites, pp. 32. Springer-Verlag Press.
  4. ↑ D. Hägele (2006) (in German), [Online-Version; pdf-Datei; 8,6 MB Morphogenese, Wachstum und Ökologie der modernen Süßwasser-Onkoide der Alz], Dissertation an der Fakultät für Geowissenschaften, Ludwig-Maximilians-Universität München, Online-Version; pdf-Datei; 8,6 MB 
  5. ↑ E. Rott (1994), [Online-Version; pdf-Datei; 7,6 MB "Der Algenaufwuchs in der Oberen Alz (Oberbayern)"] (in German), Berichte des naturwissenschaftlich-medizinischen Vereins in Innsbruck (Innsbruck) 81: pp. 229–253, Online-Version; pdf-Datei; 7,6 MB 
  6. ↑ N.N. (2008), [Online-Version; pdf-Datei; 8,4 MB "Stop 7: Tufa oncoids of the river Moosach, Molasse Basin"] (in German), Geobiology of Stromatholites - Internationales Kalkowsky-Symposium (Göttingen): pp. 193–197, Online-Version; pdf-Datei; 8,4 MB 
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