: Paleoclimatology: oncoid

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oncoid

An oncoid is a rounded nodule, usually calcareous, ranging in size from mm to cm, composed of a core and thin laminar layers concentrically surrounding the core. The core may be composed of a grain of limestone or other rock, or of shell fragments. Unlike ooids, laminar layers are irregular, may overlap and are not spherical. Laminar layers are the product of the adhesion of very fine grains (encrustation) on a mucilaginous surface of an algal or microbial mat covering the core. They can also be caused by the precipitation of calcium carbonate due to the use of CO2 by bacteria or algae during photosynthesis. This mode of formation is similar to that of stromatolites. Scientists currently believe that it is mainly cyanobacteria that are involved in oncoid formation, but other bacteria, algae, diatoms, bryozoans and crust-forming foraminifera may also be involved. Oncoids need currents and movement to turn over regularly and grow. They can therefore be found in lakes, rivers, karsts and in the marine realm affected by wave action, most often at water depths of a few meters, but also up to 50 m deep. Rock with more than 50% oncoids is called oncolite. A rock containing 10 to 49% oncoids is called oncoidal limestone. (Adapted and translated from: https://fr.wikipedia.org/wiki/Onco%C3%AFde)

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