Oncolitic limestones are readily identified by the presence of abundant oncolites.  Oncolites are macroscopic, concentrically layered, ~irregularly spheroidal masses of variable size.  The oncolites & surrounding matrix are composed of calcium carbonate (CaCO3), which bubbles in acid.


The oncolites are biogenic in origin.  They grew in successive layers by the active or passive precipitation of calcium carbonate by cyanobacterial films (oncolites can be considered as mobile varieties of stromatolites).  Oncolites are usually attributed to "algae", and these structures are often called "algal balls".  Referring to cyanobacteria as "algae" is a widespread error.  They are frequently called "blue-green algae".  Cyanobacteria are photosynthetic, as algae are, but they are not algae.



Oncolitic limestone (above & below; centimeter scale) from the Cambrian-aged Gros Ventre Formation at Beartooth Butte, Wyoming, USA (unnumbered FMNH specimen, Field Museum of Natural History, Chicago, Illinois, USA). 



Oncolitic limestone used as building stone in a stone walled structure at Port Wakefield, South Australia.



Oncolitic limestone (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA).



Oncolitic limestone (2.4 cm tall)



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