Oncolites are

sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different parti ...

composed of oncoids, which are layered structures formed by the growth of one or more species of

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s, usually containing

cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...

. Oncolites are very similar to

stromatolites Stromatolites ( ) or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria) ...

, but, instead of forming columns, they form approximately spherical structures. The oncoids often form around a central nucleus, such as a shell fragment, and a

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

structure is deposited by encrusting

microbes A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...

. Oncolites are indicators of warm waters in the

photic zone The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...

, but are also known in contemporary freshwater environments. These structures rarely exceed 10 cm in diameter. Oncolites may have either a

porostromate "Porostromata" is an antiquated form taxon that refers to fossil cyanobacteria. The term porostromate is also used as a descriptor of textures and microstructure of Stromatolite, stromatolites and Oncolite, oncolites which contain tubules or othe ...

spongiostromate "Spongiostromata" is an antiquated form taxon that refers primarily to fossil cyanobacteria. "Spongiostromate" is also used to describe stromatolites and oncolites that do not preserve clear tubules or other cellular microstructure. Pia (1927) e ...

texture. Most oncolites are spongiostromate, having no recognisable cellular texture or microstructure. Porostromate oncolites are mostly unknown during the

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

; since the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

they have mostly been confined to freshwater environments.

Distribution

The appearance of recent or near-recent freshwater oncoids has been documented in two rivers in

Bavaria Bavaria, officially the Free State of Bavaria, is a States of Germany, state in the southeast of Germany. With an area of , it is the list of German states by area, largest German state by land area, comprising approximately 1/5 of the total l ...

: the Alz, whose source is the

Chiemsee Chiemsee () is a freshwater lake in Bavaria, Germany, near Rosenheim. It is often called "the Bavarian Sea". The rivers Tiroler Achen and Prien (river), Prien flow into the lake from the south, and the river Alz flows out towards the north. The ...

, and the

Moosach Moosach is a municipality in the Upper Bavarian district of Ebersberg and a member of the ''Verwaltungsgemeinschaft'' (administrative community) of Glonn. Geography The community lies in a picturesque setting in an ice-age moraine landscape abo ...

, near

Freising Freising () is a university town in Bavaria, Germany, and the capital of the Freising (district), with a population of about 50,000. Location Freising is the oldest town between Regensburg and Bolzano, and is located on the Isar river in ...

. Modern oncoids also grow in some springs in

Atacama Desert The Atacama Desert () is a desert plateau located on the Pacific Ocean, Pacific coast of South America, in the north of Chile. Stretching over a strip of land west of the Andes Mountains, it covers an area of , which increases to if the barre ...

in South America. In one particular case a system of oncoids have been observed interface between

Salar de Antofalla Salar de Antofalla is a salt flat in the Andes of Catamarca Province in the Argentine Northwest. The salt flat contains systems of oncoids that grow in the interface between proper salt flat and an adjacent wetlands. The geology surrounding the ...

and an adjacent wetland.

References

Sedimentary rocks Trace fossils {{trace-fossil-stub