Tabla Geologica

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 (Harland and others, 1982). Over the years, thedevelopment 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 occurredover names and boundaries of units, and various time scales have been used by the geoscience community. New time scale.—Since the publication by the USGS of the 7th edition of “Suggestions to Authors” (STA7; Hansen, 1991), no other time scale has been officially endorsed by the USGS. For consistency purposes, the USGS Geologic Names Committee (GNC; see box for members) and the Association ofAmerican State Geologists (AASG) developed Divisions of Geologic Time (fig. 1). The Divisions of Geologic Time is based on the time scale in STA7 (Hansen, 1991, p. 59) and updates it with the unit names and boundary age estimates ratified by the International Commission on Stratigraphy (ICS). Scientists should note that other published time scales may be used, provided that these are specified andreferenced (for example, Palmer, 1983; Harland and others, 1990; Haq and Eysinga, 1998; Gradstein and others, 2004). Advances in stratigraphy and geochronology require that any time scale be periodically updated. Therefore, the Divisions of Geologic Time is dynamic and will be modified as needed to include accepted changes of unit names and boundary age estimates. The Divisions of Geologic Time showsthe major chronostratigraphic (position) and geochronologic (time) units; that is, eonothem/eon to series/epoch divisions. Workers should refer to the ICS time scale (Ogg, 2004) for stage/age terms. Most systems of the Paleozoic and Mesozoic are subdivided into series utilizing the terms “Lower,” “Middle,” and “Upper.” The geochronologic counterpart terms for subdivisions of periods are “Early,”“Middle,” and “Late.” The international geoscience community is applying names to these subdivisions based on stratigraphic sections at specific localities worldwide. All series/epochs of the Silurian and Permian have been named. Although the usage of these names is preferred, “lower/early,” “middle,” and “upper/ late” are still acceptable as informal units (lowercase) for these two systems/periods.Also the Upper Cambrian has been named “Furongian” in the ICS time scale. However, the GNC will not recognize this name and include it in the Divisions of Geologic Time until all series/epochs of the Cambrian are named. Cenozoic.—There has been much controversy related to subdivisions of the Cenozoic, particularly regarding retention or
U.S. Department of the Interior U.S. Geological Survey

rankof the terms “Tertiary” and “Quaternary.” Although some stratigraphers have suggested that these terms be abandoned, the issue remains unresolved. If the terms are retained, there will need to be agreement on the status of the Quaternary as a system/period or subsystem/subperiod. Another controversial issue is the position of the base of the Quaternary; is it at the base of the Pleistocene orwithin the upper Pliocene? These positions have age estimates of 1.8 Ma and 2.6 Ma, respectively (see box for age terms). Until a decision is made on the subdivisions of the Cenozoic, the Divisions of Geologic Time will follow the general structure of the time scale in STA7 (Hansen, 1991) in accepting the use of the terms “Tertiary” and “Quaternary” and the equivalence of the bases of the...