File:Oncolitic limestone (Wirrapowie Limestone, Lower Cambrian; Mernmerna railroad bridge dry creek cut, South Australia) 3.jpg

English: Oncolitic limestone in the Cambrian of South Australia.

Limestone is a biogenic sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. Many geologically young limestones are composed of aragonite (also CaCO3). Numerous varieties of limestone exist (e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, rubblestone, coralstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc.). Most limestones represent deposition in ancient warm, shallow ocean environments.

Oncolitic limestones have oncolites - macroscopic, concentrically layered, ~irregularly spheroidal masses of variable size. 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 are often called “algal balls”. Referring to cyanobacteria as “algae” is a common error - they are frequently called “blue-green algae”. Cyanobacteria are photosynthetic, as algae are, but they are not algae.

This oncolitic limestone is in a South Australian Lower Cambrian unit called the Wirrapowie Limestone. It consists of a fossiliferous limestone succession having archaeocyathans, trilobites, stromatolites, shell hash horizons, limestone conglomerates/intraclastic limestone breccias, and oncolitic limestones.

Stratigraphy: Wirrapowie Limestone, Tommotian Stage to lower Atdabanian Stage, lower Lower Cambrian

Locality of section: dry creek cut upstream from an abandoned railroad bridge at Mernmerna, South Australia (31° 34’ 25.30" South latitude, 138° 24’ 44.77" East longitude)