Thrombolite

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.

The clots are on the scale of millimetres to centimetres and may be interspersed with sand, mud or sparry carbonate.

[3] Thrombolites can be distinguished from microbialites or stromatolites by their massive size, which is characterized by macroscopic clotted fabric.

[2] 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.