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Modern thrombolites in Lake Clifton, Western Australia Lake Clifton SMC 2008.jpg
Modern thrombolites in Lake Clifton, Western Australia
Jurassic thrombolite formed around a tree trunk; Purbeck Formation, Isle of Portland, Dorset, England. PurbeckFormationPortland.jpg
Jurassic thrombolite formed around a tree trunk; Purbeck Formation, Isle of Portland, Dorset, England.
Fossil Thrombolites at Lake Walyungup Fossil Thrombolites at Lake Walyungup, April 2020 02.jpg
Fossil Thrombolites at Lake Walyungup

Thrombolites (from Ancient Greek θρόμβος thrómbos meaning "clot" and λῐ́θος líthos meaning "stone") are clotted accretionary structures formed in shallow water by the trapping, binding, and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria. [1]



Thrombolites have a clotted structure without the laminae of stromatolites. Each clot within a thrombolite mound is a separate cyanobacterial colony. The clots are on the scale of millimetres to centimetres and may be interspersed with sand, mud or sparry carbonate. [1] Clots that make up thrombolites are called thromboids to avoid confusion with other clotted textures. [2] The larger clots make up more than 40% of a thrombolite's volume and each clot has a complex internal structure of cells and rimmed lobes resulting primarily from calcification of the cyanobacterial colony. Very little sediment is found within the clots because the main growth method is calcification rather than sediment trapping. [1] There is active debate about the size of thromboids, with some seeing thromboids as a macrostructural feature (domical hemispheroid) and others viewing thromboids as a mesostructural feature (random polylobate and subspherical mesoclots). [2]


There are two main types of thrombolites: [3]

Calcified microbe thrombolites

This type of thrombolites contain clots that are dominantly composed of calcified microfossil components. These clots do not have a fixed form or size and can expand vertically. Furthermore, burrows and trilobite fragments can exist in these thrombolites.

Coarse agglutinated thrombolites

This type of thrombolites is composed of small openings that trap fine-grained sediments. They are also known "thrombolitic-stromatolites" due to their close relation with the same composition of stromatolites. Because they trap sediment, their formation is linked to the rise of algal-cyanobacterial mats. [3]

Differences from stromatolites

Thrombolites can be distinguished from microbialites or stromatolites by their massive size, which is characterized by macroscopic clotted fabric. Stromatolites are similar but consist of layered accretions. Thrombolites appear with random patterns that can be seen by the naked eye, while stromatolites has the texture of built up layers. [2]

Ancient fossil record

Calcified microbe thrombolites occur in sedimentary rocks from the shallow water ocean during the Neoproterozoic and early Palaeozoic. [3]


Thrombolites are rare on modern Earth, but exist in areas of groundwater discharge with high concentration of nutrients and organic ions, such as shallow seawater, freshwater, and saltwater lakes, and streams. [4] Thrombolites are now found in only a few places in the world, including:

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Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO3. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

<span class="mw-page-title-main">Stromatolite</span> Layered sedimentary structure formed by the growth of bacteria or algae

Stromatolites or stromatoliths are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota. These microorganisms produce adhesive compounds that cement sand and other rocky materials to form mineral "microbial mats". In turn, these mats build up layer by layer, growing gradually over time. A stromatolite may grow to a meter or more. Fossilized stromatolites provide important records of some of the most ancient life. Today the living forms are rare.

<span class="mw-page-title-main">Reef</span> A shoal of rock, coral or other sufficiently coherent material, lying beneath the surface of water

A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic (non-living) processes such as deposition of sand or wave erosion planing down rock outcrops. However, reefs such as the coral reefs of tropical waters are formed by biotic (living) processes, dominated by corals and coralline algae. Artificial reefs such as shipwrecks and other man-made underwater structures may occur intentionally or as the result of an accident, and are sometimes designed to increase the physical complexity of featureless sand bottoms to attract a more diverse range of organisms. Reefs are often quite near to the surface, but not all definitions require this.

<span class="mw-page-title-main">Phosphorite</span> Sedimentary rock containing large amounts of phosphate minerals

Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxide (P2O5). Marketed phosphate rock is enriched ("beneficiated") to at least 28%, often more than 30% P2O5. This occurs through washing, screening, de-liming, magnetic separation or flotation. By comparison, the average phosphorus content of sedimentary rocks is less than 0.2%. The phosphate is present as fluorapatite Ca5(PO4)3F typically in cryptocrystalline masses (grain sizes < 1 μm) referred to as collophane-sedimentary apatite deposits of uncertain origin. It is also present as hydroxyapatite Ca5(PO4)3OH or Ca10(PO4)6(OH)2, which is often dissolved from vertebrate bones and teeth, whereas fluorapatite can originate from hydrothermal veins. Other sources also include chemically dissolved phosphate minerals from igneous and metamorphic rocks. Phosphorite deposits often occur in extensive layers, which cumulatively cover tens of thousands of square kilometres of the Earth's crust.

<span class="mw-page-title-main">Algal mat</span> Microbial mat that forms on the surface of water or rocks

Algal mats are one of many types of microbial mat that forms on the surface of water or rocks. They are typically composed of blue-green cyanobacteria and sediments. Formation occurs when alternating layers of blue-green bacteria and sediments are deposited or grow in place, creating dark-laminated layers. Stromatolites are prime examples of algal mats. Algal mats played an important role in the Great Oxidation Event on Earth some 2.3 billion years ago. Algal mats can become a significant ecological problem, if the mats grow so expansive or thick as to disrupt the other underwater marine life by blocking the sunlight or producing toxic chemicals.

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<span class="mw-page-title-main">Oncolite</span> Trace fossil

Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, 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, but are also known in contemporary freshwater environments. These structures rarely exceed 10 cm in diameter.

<span class="mw-page-title-main">Microbial mat</span> Multi-layered sheet of microorganisms

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