Oncolite
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Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by the growth of one or more species of microorganisms,[1] usually containing cyanobacteria.[2] Oncolites are very similar to stromatolites, but, instead of forming columns, they form approximately spherical structures.[3] The oncoids often form around a central nucleus, such as a shell fragment,[4] 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.[5] These structures rarely exceed 10 cm in diameter.
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.[6]
Distribution
The appearance of recent or near-recent freshwater oncoids has been documented in two rivers in Bavaria: the Alz, whose source is the Chiemsee, and the Moosach, near Freising.[7][8][9] Modern oncoids also grow in some springs in Atacama Desert in South America. In one particular case a system of oncoids have been observed interface between Salar de Antofalla and an adjacent wetland.[10]
References
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