A formation may stand alone or it may be linked with contiguous formations into a group. Definition of a group may be contentious. Some authors look for common lithological characteristics, e.g. the Lias Group is dominantly argillaceous but contains limestone- sandstone- and ironstone-dominated formations.
The current IUGS-sanctioned (2009) timescale for the Cenozoic Erathem/Era, in which the Quaternary and Pleistocene are coterminous with the base of the Gelasian Stage at 2.6 Ma. Stage names and boundary ages are from the (January 2008), with the Calabrian and ‘Ionian’ stages following
Cita et al. (2006, 2008) and the provisional ‘Tarentian’ Stage following Cita (2008 and references therein). Currently defined GSSPs are indicated by black arrows. The stratigraphical intervals are not scaled to geological time
Others focus on the genesis of the group and thus place less emphasis o. In the latter case, the bases of successive groups are commonly defined by basin-wide unconformities and their correlative conformities (see also sequence stratigraphy). A supergroup embraces contiguous groups and may also contain formations that have not been assigned to a particular group.
A member is a subdivision of a formation, but formations are not necessarily divided either wholly or partially into members. Members commonly occur in the marginal areas of a formation, representing, for example,