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Geochronology is the branch of stratigraphy aimed at determining and dating the time sequence of events in the Earth's history.[1]

Geochronology deals thus with geologic time, in opposition to chronostratigraphy which rather deals with rock bodies. This distinction has historical reasons, and has been challenged recently[2][3] (see also chronostratigraphy). The branch of geochronology that deals with the quantitative measurement of geologic time is called geochronometry.

Geochronologic units

A geochronologic unit is a interval of geologic time. Any geochronologic unit has a chronostratigraphic counterpart, which denotes the body of all rocks formed during the corresponding geochronologic unit. The geologic time is organized in a hierarchy of geochronologic units, i.e., relative time intervals:

Chronostratigraphic units Geochronologic units

When a geochronologic unit inherits its name from a higher-rank unit, it is preceded by Early, Mid or Late. For example, the Triassic system is subdivided into three epochs: the Early Triassic, the mid Triassic, and the late Triassic ('mid-' and 'late Triassic' are not capitalized because there is not formal definition yet for these epochs).

Definition of geochronologic units

Geochronologic units are automatically defined along the corresponding chronostratigraphic units when their GSSP is established. The fundamental unit of geochronology is the age, corresponding to the stage in chronostratigraphy.

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References and notes

  1. Salvador A.E., 1994 - International Stratigraphic Guide - A guide to stratigraphic classification, Terminology and procedure. Geological Society of America. ISBN 0-8137-7401-2
  2. Gradstein F.M., Ogg J.G. and Smith A.G., 2004, A Geologic Time Scale 2004, Cambridge University Press, Cambridge, UK.
  3. Zalasiewicz J., Smith A., Brenchley P., Evans J., Knox R., Riley N., Gale A., Gregory F.J., Rushton A., Gibbard P., Hesselbo S., Marshall J., Oates M., Rawson P., Trewin N., 2004 – Simplifying the stratigraphy of time. Geology, v. 32, pp.1-4.