ONCOLITIC LIMESTONE
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)