Oncolite

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.

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

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]

Oncolites may have either a porostromate or spongiostromate texture. Most oncolites are spongiostromate, having no recognisable cellular texture or microstructure. Porostromate oncolites are mostly unknown during the Precambrian; since the Eocene they have mostly been confined to freshwater environments. [7]

References
  1. Corsetti, F.A.; Awramik, S.M.; Pierce, D. (2003-04-15). "A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA". Proceedings of the National Academy of Sciences. 100 (8): 4399–4404. Bibcode:2003PNAS..100.4399C. doi:10.1073/pnas.0730560100. PMC 153566. PMID 12682298. Retrieved 2007-06-28.
  2. Gutschick, R.C.; Perry, T.G. (1959-11-01). "Sappington (Kinderhookian) sponges and their environment [Montana]". Journal of Paleontology. 33 (6): 977–985. Retrieved 2007-06-28.
  3. Riding, Robert. 1991. Calcareous Algae and Stromatolites, pp. 32. Springer-Verlag Press.
  4. D. Hägele (2006), Morphogenese, Wachstum und Ökologie der modernen Süßwasser-Onkoide der Alz (Online-Version; pdf-Datei; 8,6 MB) (in German), Dissertation an der Fakultät für Geowissenschaften, Ludwig-Maximilians-Universität München
  5. E. Rott (1994), "Der Algenaufwuchs in der Oberen Alz (Oberbayern)" (Online-Version; pdf-Datei; 7,6 MB), Berichte des Naturwissenschaftlich-medizinischen Vereins in Innsbruck (in German), Innsbruck, 81, pp. 229–253
  6. N.N. (2008), "Stop 7: Tufa oncoids of the river Moosach, Molasse Basin" (Online-Version; pdf-Datei; 8,4 MB), Geobiology of Stromatholites - Internationales Kalkowsky-Symposium (in German), Göttingen, pp. 193–197
  7. Monty, C. L. (1981). Monty, Claude (ed.). "Spongiostromate vs. Porostromate Stromatolites and Oncolites". Phanerozoic Stromatolites. Berlin, Heidelberg: Springer: 1–4. doi:10.1007/978-3-642-67913-1_1. ISBN 978-3-642-67913-1.


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