Divisions of Geologic Time—Major Chronostratigraphic and Geochronologic Units
Introduction.—Effective communication in the geosciences requires
consistent uses of stratigraphic nomenclature, especially divisions of geologic
time. A geologic time scale is composed of standard stratigraphic divisions
based on rock sequences and calibrated in years.
Over the years, the development of new dating methods and 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 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 consistency purposes, the USGS Geologic Names Committee and the Association of American State Geologists developed Divisions of Geologic Time.
U.S. Geological Survey Geologic Names Committee, 2007, Divisions of geologic time—Major chronostratigraphic and geochronologic units: U.S. Geological Survey Fact Sheet 2007-3015, 2 p.
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