File:Oncolitic limestone (Flagstaff Limestone, Upper Paleocene; Fayette Wash, San Pitch Mountains, Utah, USA) 1.jpg

English: Oncolitic limestone from the Tertiary of Utah, USA.

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 sample is from a Paleocene-aged lake deposit in central Utah.

Stratigraphy: Flagstaff Limestone, Upper Paleocene

Locality: Fayette Wash, just northeast of Fayette Springs, northeast of the town of Fayette, western side of the near-southern end of the Gunnison Plateau / San Pitch Mountains, southwestern Sanpete County, central Utah, USA