Divisions of geologic time—Major chronostratigraphic and geochronologic units

Randall C. Orndorff; Nancy R. Stamm; David R. Soller; Lucy E. Edwards; Julie A. Herrick; Leslie F. Ruppert; Janet L. Slate; Berry H. Tew

doi:10.3133/pp1879v1c

USGS · pp1879v1C

Divisions of geologic time—Major chronostratigraphic and geochronologic units

Randall C. Orndorff·Nancy R. Stamm·David R. Soller·Lucy E. Edwards·Julie A. Herrick·Leslie F. Ruppert·Janet L. Slate·Berry H. Tew

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Abstract

Introduction Effective communication in the geosciences requires consistent use of stratigraphic nomenclature, especially divisions of geologic time. A geologic time scale is composed of standard stratigraphic divisions that are based on rock sequences and is calibrated in years (Harland and others, 1982). Over the years, the development of new dating methods and the refinement of previous ones have stimulated revisions to geologic time scales. Since the mid-1990s, geologists from the U.S. Geological Survey (USGS), State geological surveys, academia, and other organizations have sought a consistent time scale that can be used in communicating ages of geologic units in the United States. Many international debates have occurred over the names and boundaries of units, and various time scales have been used by the geoscience community in years past (for example, U.S. Geological Survey Geologic Names Committee [USGS GNC], 2007, 2010, 2018). However, the numerical ages assigned to geochronologic units frequently change as the science of stratigraphy moves forward. Once ratified, changes in numerical ages are recorded by the International Commission on Stratigraphy (ICS) on their website ( https://stratigraphy.org/chart ). In addition, such advances in stratigraphy and changes in geochronology require that any time scale be periodically updated. Therefore, the time scale presented in this report (hereafter referred to as “Divisions of Geologic Time”) is dynamic, and it is modified as needed to include accepted changes of unit names and boundary age estimates (USGS GNC, 2007, 2010, 2018).

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Randall C. Orndorff, Nancy R. Stamm, David R. Soller, Lucy E. Edwards, Julie A. Herrick, Leslie F. Ruppert, Janet L. Slate, Berry H. Tew. 2023-07-07. Divisions of geologic time—Major chronostratigraphic and geochronologic units. https://doi.org/10.3133/pp1879v1c

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