Stage (geology)

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Stages are subdivisions of rock layers made on the age of the rock. A stage is therefore a chronostratigraphic unit, unrelated to lithostratigraphy, which devides rock layers on their lithology. Stages are subdivisions of series and are themselves divided into chronozones. As in 2008, the International Commission on Stratigraphy is nearly finished subdividing the Phanerozoic eonothem into internationally accepted stages. In many regions around the world local subdvisions are still used along the international ones, but they are supposed to be abandoned eventually.

Stage is a term defining a package of rocks formed in a certain interval in time; it is equivalent to the term age defining the interval of time itself, although the two words are often confused in informal literature.

[edit] Defining stages

Typically, a stage will be defined on fossil content (biostratigraphy) or paleomagnetic polarity in the rock. Usually one or more index fossils that are found worldwide, are common, easily recognized, and limited to a single, or at most a few, stages are used to define the stage's bottom. Thus, for example, in the (still used) local North American subdivision paleontologist finding fragments of the trilobite Olenellus would identify the beds as being from the Waucoban Stage whereas fragments of a later trilobite such as Elrathia would identify the stage as Albertan.

Stages were very important in the 19th and early 20th Century as they were the major tool available for dating rock beds until the development of seismology and radioactive dating in the second half of the 20th Century.

[edit] Stages and lithostratigraphy

Stages can include many lithostratigraphic units (for example formations, 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 stages or parts of them.

[edit] See also