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 particl ...

composed of oncoids, which are layered structures formed by

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

l growth. 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). Th ...

, 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 structure is deposited by encrusting microbes. Oncolites are indicators of warm waters in the

photic zone The photic zone, 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 pro ...

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

; since the

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

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 (german: Freistaat Bayern, link=no ), is a state in the south-east of Germany. With an area of , Bavaria is the largest German state by land area, comprising roughly a fifth 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 flow into the lake from the south, and the river Alz flows out towards the north. The Alz flows i ...

, and the Moosach, near

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

. Modern oncoids also grow in some springs in

Atacama Desert The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in th ...

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 th ...

and an adjacent wetland.

References

Sedimentary rocks Trace fossils {{trace-fossil-stub