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Divisions of geologic time—Major chronostratigraphic and geochronologic units

August 27, 2018


Effective communication in the geosciences requires a consistent nomenclature for stratigraphic units and, especially, for divisions of geologic time. A geologic time scale is composed of standard stratigraphic divisions based on rock sequences and is calibrated in years.

Geologists from the U.S. Geological Survey (USGS), State geological surveys, academia, and other organizations require a consistent time scale to be used in communicating ages of geo­logic units in the United States. Many international debates have occurred over names and boundaries of units, and various time scales have been used by the geoscience community.

For consistent usage of time terms, the USGS Geologic Names Committee and the Association of American State Geologists devel­oped the Divisions of Geologic Time; the 2018 update in this fact sheet contains the unit names and boundary age estimates ratified by the International Commission on Stratigraphy in 2018. Scientists may use other published time scales, provided that these are specified and referenced.

Publication Year 2018
Title Divisions of geologic time—Major chronostratigraphic and geochronologic units
DOI 10.3133/fs20183054
Publication Type Report
Publication Subtype USGS Numbered Series
Series Title Fact Sheet
Series Number 2018-3054
Index ID fs20183054
Record Source USGS Publications Warehouse
USGS Organization Eastern Geology and Paleoclimate Science Center