A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize it.
[1] A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit.
[5] Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres.
[4] A sequence of fossil-bearing sedimentary rocks can be subdivided on the basis of the occurrence of particular fossil taxa.
[6] The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones.