bioclast

Bioclast is a term used in geology and paleontology to refer to fragments of biological origin preserved in sedimentary rocks. A bioclast is any piece of formerly living material that survives weathering and burial, including calcareous shells and skeletons, tests of single-celled organisms, coral fragments, bryozoans, echinoderm spines, and other biogenic debris. Bioclasts can be macrofossils, such as large shell pieces, or microfossils, like foraminiferan tests or radiolarian siliceous remains.

Bioclasts are contrasted with lithoclasts, which are fragments of preexisting rocks derived from erosion of nonbiogenic material. They can be in place in primary sediments or reworked from older deposits. In carbonate settings, bioclasts are especially abundant and contribute to the texture and classification of rocks as packstones, wackestones, or grainstones, depending on the relative proportions of bioclasts, peloids, and matrix. In siliciclastic rocks, bioclasts may still be present as fossil fragments within sand or silt.

The presence and composition of bioclasts provide information on past ecosystems, depositional environments, and diagenetic history. Microfossil bioclasts are widely used in biostratigraphy and paleoenvironmental reconstruction, while the abundance of macrobioclasts can indicate proximity to a reef or other biotic carbonate factory. Bioclast studies are a key component of carbonate petrography and sedimentology.