Most modern works (e.g. Nichols 2009; Allaby & Allaby 1999) seem to restrict the definition of ‘concretion’ specifically to formations in sedimentary rocks. I have recently come across a taxonomy of concretions in Crimes (1966), which includes concretions ‘formed as a result of metamorphism’. He gives little detail about formation processes but writes that ‘[m]etamorphic concretions have also received much attention and have been reviewed briefly by Barth and Ramberg.’ (I don’t have access to the cited references.) It seems plausible to me that the concretionary processes that operate in sedimentary rock would also be possible at very low grades of metamorphism, provided that some pore space remains. However, Crimes’ concretions are apparently formed as a result of metamorphism.
Do such metamorphic concretions have a place in currently accepted taxonomies of sedimentary features, or have they been reclassified as a different phenomenon altogether?
- Allaby, A. & Allaby, A., 1999. A dictionary of earth sciences, 2nd ed. Oxford University Press.
- Barth, T. F. W., 1952. Theoretical Petrology. New York.
- Crimes, T. P., 1966. The relative age of some concretions in Cambrian sediments of St. Tudwal's Peninsula, North Wales. Geological Journal, 5(1), 33-42.
- Nichols, G., 2009. Sedimentology and Stratigraphy, 2nd ed. Wiley-Blackwell.
- Ramberg, H., 1952. The origin of metamorphic and metasomatic rocks; a treatise on crystallisation and replacement in the earth’s crust. Chicago.