(unnumbered FMNH specimen, Field Museum of Natural History, Chicago, Illinois, USA)

Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.

Limestone is a common 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 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.

Stratigraphy: attributed to the Gros Ventre Formation, Upper Cambrian