Oncolite

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]