English: Oncolitic limestone in the Cretaceous of Texas, USA.

Oncolitic limestones are dominated by oncolite grains, which are macroscopic, concentrically layered, ~irregularly spheroidal masses of variable size. The oncolites and surrounding matrix are composed of calcium carbonate (CaCO3), which bubbles in acid.

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.

This outcrop is along the rim of the Odessa Impact Crater in western Texas. The impact structure formed about 64,000 years ago, during the Late Pleistocene. The impacting object was an iron meteorite, a 4.55 billion year old octahedrite from the Asteroid Belt (see: www.flickr.com/photos/jsjgeology/8284751939 ).

Stratigraphy: Fredericksburg Group, Lower Cretaceous

Locality: rim rock of the Odessa Impact Crater, ~9-10 miles southwest of the town of Odessa, southern Ector County, western Texas, USA