A stratigraphic unit is defined as a volume of rock that has an identifiable origin and relative age range, characterized by distinctive and dominant features such as petrographic, lithologic, or paleontologic attributes that make the unit easily mappable and recognizable.
These units are distinct from one another based on shared characteristics and must be mappable, though the contacts between units don’t always have to be sharply defined.
Stratigraphic units are classified based on different criteria including lithology (lithostratigraphic units), fossil content (biostratigraphic units), or other properties, and are organized hierarchically into ranks such as bed, member, formation, group, and supergroup.
The naming of these units typically involves a geographic reference combined with a term indicating their rank or lithology. Formal definitions of stratigraphic units often involve identifying type sections or localities that serve as benchmarks for characterizing and distinguishing these units in the field.
