System (stratigraphy)

A system in stratigraphy is an idealized composite unit of the geologic record made up of a succession of rock layers that were laid down together within a certain corresponding geological time span, and are used in turn to date things to a certain corresponding geologic period. The system is thus a unit of the geologic record or rock column, pieced together using the Law of Superposition and mapped to its corresponding period&mdash; the associated continuous chronostratigraphical time unit, a relative metric that science committees have determined solid dating for as organized on the geologic time scale. A system is therefore a unit of chronostratigraphy, unrelated to lithostratigraphy, which subdivides rock layers on their lithology. Systems are subdivisions of erathems and are themselves divided into series, epochs and stages.

System is a term defining a unit of rocklayers formed in a certain time interval; it is in theory equivalent to the term period defining the interval of time itself, but unlike the system of time units, a system in many locations may be interrupted and incomplete as geologic forces alternately uplift or depress a region, bend the landscape and so expose a terrain feature once accumulating rock to weathering and vice versa. The overall rock record has been piecewise constructed throughout each physical system, series, et al. using superposition, and is treated in practice as one large continuous rock column, the whole matching the corresponding period. For this reason, the two words are sometimes confused in informal literature.

Systems in the geological timescale
The systems of the Phanerozoic eonothem were defined during the 19th century, beginning with the Cretaceous (by Belgian geologist Jean d'Omalius d'Halloy in the Paris Basin) and the Carboniferous (by British geologists William Conybeare and William Phillips) in 1822. The Paleozoic and Mesozoic erathems were divided into the currently used systems before the second half of the 19th century, except for a minor revision when the Ordovician system was added in 1879.

The Cenozoic has recently seen some revisions by the International Commission on Stratigraphy, it will most probably be divided into three systems (Paleogene, Neogene and Quarternary) while older (and currently still better known) names are now series (Paleocene, Eocene, Oligocene, Miocene and Pliocene) or abandoned (Tertiary).

Another recent development is the official division of the Proterozoic eonothem into systems, which was decided in 2004.