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Series are subdivisions ofrock layers based on the age of the rock and formally defined by international conventions of thegeological timescale. A series is therefore a sequence of strata defining achronostratigraphic unit. Series are subdivisions ofsystems and are themselves divided intostages.
Series is a term defining a unit of rock layers formed during a certain interval of time (a chronostratigraphic unit); it is equivalent (but not synonymous) to the termgeological epoch (seeepoch criteria) which defines the interval of time itself, although the two words are sometimes confused in informal literature.
| Segments of rock (strata) inchronostratigraphy | Time spans ingeochronology | Notes to geochronological units |
|---|---|---|
| Eonothem | Eon | 4 total, half a billion years or more |
| Erathem | Era | 10 defined, several hundred million years |
| System | Period | 22 defined, tens to ~one hundred million years |
| Series | Epoch | 38 defined, tens of millions of years |
| Stage | Age | 101 defined, millions of years |
| Chronozone | Chron | subdivision of an age, not used by the ICS timescale |
The geological timescale has allsystems in thePhanerozoiceonothem subdivided into series. Some of these have their own names; in other cases a system is simply divided into a Lower, Middle and Upper series, with official series being capitalized and unofficial designations (such as "middle Cretaceous") being left uncapitalized. TheCretaceous system is, for example, divided into the Upper Cretaceous and Lower Cretaceous Series, while theCarboniferous System is divided into thePennsylvanian andMississippian Series. As of 2008, theInternational Commission on Stratigraphy had not yet named all four series of theCambrian. Currently series are limited to the Phanerozoic, but the ICS has stated its intention of subdividing the three systems of theNeoproterozoic (Ediacaran,Cryogenian andTonian) into stages too.
Systems can include many lithostratigraphic units (for exampleformations,beds,members, etc.) of differing rock types that were being laid down in different environments at the same time. In the same way, a lithostratigraphic unit can include a number of systems or parts of them.