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
JurassicgastropodBactroptyxis 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.
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.[4]
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.[5][6][7] 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.[8]
^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), vol. 81, Innsbruck, pp. 229–253