Definition of oncoid
In North American usage, an oncoid is a coated grain of algal (but not red algal) or microbial origin that is coarser than 2 mm in diameter; a spheroidal form of microbial stromatolite showing a series of concentric (often irregular or scalloped) laminations. These unattached stromatolites are produced by mechanical turning or rolling, exposing new surfaces to microbial/algal growth.
Common European usage is less genetic, and in that usage a microbial/algal origin is not a prerequisite. Almost identical structures can form in calcrete layers in calcareous and saline soils, possibly by inorganic pedogenic origins.
An oncolite is a rock composed of oncoids; the term, however, is often used improperly as a synonym for “oncoid”.
Oncoids, ooids and other coated grains require conditions suitable for inorganic or microbial precipitation and for biological encrustation of grains. They also require repeated rotation of grains to allow the formation of concentric coatings. Thus, the best environments for ooid formation are tidal deltas and bars, or beaches (marine or lacustrine) where surficial grains are kept in daily motion. Because reefs or bioherms “compete” with ooids in high-energy settings, biologically stressed areas (with abnormal salinities or temperatures) can favor ooid formation by inhibiting organism growth and enhancing rates of carbonate precipitation. Because of their rounded shape ooids are easily reworked into adjacent environments (especially aeolianites).
Coated grains (superficial ooids, pisoids and oncoids) can be formed in soils and caves (vadoids; cave pearls), in relatively deep-water, current-scoured platform areas (rhodoids), in shelf areas prone to periodic storm action, in partially protected lagoons, and in a wide variety of other settings.
Ref: Scholle, PA, Ulmer-Scholle, DS, (2003) A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis. American Association of Petroleum Geologists, AAPG MEMOIR, 77, DOI: https://doi.org/10.1306/M77973, ISBN electronic: 9781629810041
Common European usage is less genetic, and in that usage a microbial/algal origin is not a prerequisite. Almost identical structures can form in calcrete layers in calcareous and saline soils, possibly by inorganic pedogenic origins.
An oncolite is a rock composed of oncoids; the term, however, is often used improperly as a synonym for “oncoid”.
Oncoids, ooids and other coated grains require conditions suitable for inorganic or microbial precipitation and for biological encrustation of grains. They also require repeated rotation of grains to allow the formation of concentric coatings. Thus, the best environments for ooid formation are tidal deltas and bars, or beaches (marine or lacustrine) where surficial grains are kept in daily motion. Because reefs or bioherms “compete” with ooids in high-energy settings, biologically stressed areas (with abnormal salinities or temperatures) can favor ooid formation by inhibiting organism growth and enhancing rates of carbonate precipitation. Because of their rounded shape ooids are easily reworked into adjacent environments (especially aeolianites).
Coated grains (superficial ooids, pisoids and oncoids) can be formed in soils and caves (vadoids; cave pearls), in relatively deep-water, current-scoured platform areas (rhodoids), in shelf areas prone to periodic storm action, in partially protected lagoons, and in a wide variety of other settings.
Ref: Scholle, PA, Ulmer-Scholle, DS, (2003) A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis. American Association of Petroleum Geologists, AAPG MEMOIR, 77, DOI: https://doi.org/10.1306/M77973, ISBN electronic: 9781629810041
