and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. Supergroup A supergroup is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. Biostratigraphic units A sequence of fossil-bearing sedimentary rocks can be subdivided on the basis of the occurrence of particular fossil taxa. A unit defined in this way is known as a biostratigraphic unit, generally shortened to biozone. The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones. An assemblage zone is a stratigraphic interval characterised by an assemblage of three or more coexisting fossil taxa that distinguish it from surrounding strata. A range zone is a stratigraphic interval that represents the occurrence range of a specific fossil taxon, based on the localities where it has been recognised. An abundance zone is a stratigraphic interval in which the abundance of a particular taxon (or group of taxa) is significantly greater than seen in neighbouring parts of the succession. An interval zone is a stratigraphic interval whose top and base are defined by horizons that mark the first or last occurrence of two different taxa. A lineage zone is a stratigraphic interval that contains fossils that represent parts of the evolutionary lineage of a particular fossil group. This is a special case of a range zone. See also Chronostratigraphy Glaciology Magnetostratigraphy Sedimentology Sequence stratigraphy Stratigraphy References External links International Commission on Stratigraphy Stratigraphic Guide Stratigraphy Geology terminology