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Appendix 2-A

Geologic Time Scale Divisions of Geologic Time— Major Chronostratigraphic and Geochronologic Units

Introduction.—Effective communication in the geosciences rank of the terms “Tertiary” and “.” Although some requires consistent uses of stratigraphic nomenclature, especially stratigraphers have suggested that these terms be abandoned, divisions of geologic time. A is composed the issue remains unresolved. If the terms are retained, there of standard stratigraphic divisions based on rock sequences and will need to be agreement on the status of the Quaternary as a calibrated in (Harland and others, 1982). Over the years, the /period or subsystem/subperiod. Another controversial development of new dating methods and refinement of previous issue is the position of the base of the Quaternary; is it at the base ones have stimulated revisions to geologic time scales. of the or within the upper ? These positions Since the mid-1990s, geologists from the U.S. Geological have age estimates of 1.8 Ma and 2.6 Ma, respectively (see box Survey (USGS), State geological surveys, academia, and other for age terms). Until a decision is made on the subdivisions of the organizations have sought a consistent time scale to be used , the Divisions of Geologic Time will follow the general in communicating ages of geologic units in the United States. structure of the time scale in STA7 (Hansen, 1991) in accepting Many international debates have occurred over names and the use of the terms “Tertiary” and “Quaternary” and the equiva- boundaries of units, and various time scales have been used by lence of the bases of the Quaternary and Pleistocene. The map the geoscience community. symbols “T” (Tertiary) and “Q” (Quaternary) have been used on New time scale.—Since the publication by the USGS of the geologic maps for more than a century and are widely used today. 7th edition of “Suggestions to Authors” (STA7; Hansen, 1991), .—For many years, the term “Precambrian” no other time scale has been officially endorsed by the USGS. was used for the division of time older than the . For consistency purposes, the USGS Geologic Names Committee For consistency with the time scale in STA7 (Hansen, 1991), the (GNC; see box for members) and the Association of American term “Precambrian” is considered to be informal and without State Geologists (AASG) developed Divisions of Geologic Time specific stratigraphic rank (although it is capitalized). (fig. 1). The Divisions of Geologic Time is based on the time Map colors.—Geologic maps utilize color schemes based on scale in STA7 (Hansen, 1991, p. 59) and updates it with the unit standards that are related to the time scale. Two different schemes names and boundary age estimates ratified by the International are used, one by the Commission for the Geologic Map of the Commission on Stratigraphy (ICS). Scientists should note that World (CGMW) and another by the USGS. Colors typically other published time scales may be used, provided that these are shown on USGS geologic maps have been used in a standard specified and referenced (for example, Palmer, 1983; Harland and fashion since the late 1800s and recently have been published in others, 1990; Haq and Eysinga, 1998; Gradstein and others, 2004). the digital cartographic standard for geologic map symbolization Advances in stratigraphy and require that any time (Federal Geographic Data Committee, Geologic Data Subcom- scale be periodically updated. Therefore, the Divisions of Geo- mittee, 2006). The GNC decided in 2006 that the USGS colors logic Time is dynamic and will be modified as needed to include should be used for large-scale and regional geologic maps of the accepted changes of unit names and boundary age estimates. United States. For international maps or small-scale maps (for The Divisions of Geologic Time shows the major chrono- instance, 1:5 million) of the United States or North America, the stratigraphic (position) and geochronologic (time) units; that is, GNC recommends the use of the international colors. Specifica- eonothem/eon to series/ divisions. Workers should refer to tions for the USGS colors are in Federal Geographic Data Com- the ICS time scale (Ogg, 2004) for /age terms. Most systems mittee, Geologic Data Subcommittee (2006), and those for the of the and are subdivided into series utiliz- CGMW colors are in Gradstein and others (2004). ing the terms “Lower,” “Middle,” and “Upper.” The geochrono- Acknowledgments.—This Fact Sheet benefited from logic counterpart terms for subdivisions of periods are “Early,” thoughtful reviews by John Repetski (USGS) and Donald E. “Middle,” and “Late.” The international geoscience community Owen (Lamar University, Beaumont, Tex., and North American is applying names to these subdivisions based on stratigraphic Commission on Stratigraphic Nomenclature). sections at specific localities worldwide. All series/epochs of the and have been named. Although the usage of By U.S. Geological Survey Geologic Names Committee 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 has been named Members of the Geologic Names Committee of the “” in the ICS time scale. However, the GNC will not U.S. Geological Survey, 2006 recognize this name and include it in the Divisions of Geologic Randall C. Orndorff (chair), Nancy Stamm (recording secretary), Time until all series/epochs of the Cambrian are named. Steven Craigg, Terry D’Erchia, Lucy Edwards, David Fullerton, Cenozoic.—There has been much controversy related to Bonnie Murchey, Leslie Ruppert, David Soller (all of the USGS), subdivisions of the Cenozoic, particularly regarding retention or and Berry (Nick) Tew, Jr. (State Geologist of Alabama).

U.S. Department of the Interior Fact Sheet 2007–3015 U.S. Geological Survey March 2007 E-mail: [email protected] Reston, VA 20192 908 NationalCenter, 12201Sunrise Valley Drive Randall C.Orndorff, U.S.GeologicalSurvey For more information, please contact: Palmer, A.R., comp.,1983, The DecadeofNorth Ameri- Ogg, James,comp.,2004,Overview ofglobalboundary North American CommissiononStratigraphicNomencla- Harland, W.B., Cox, A.V., Llewellyn, P.G., Picton, Harland, W.B., Armstrong, R.L.Cox, A.V., Craig,L.E., Haq, B.U.,andEysinga, F.W.B., van, eds.,1998,Geologi - Hansen, W.R., ed., 1991, Suggestions to authors of the Gradstein, Felix,Ogg,James,andSmith Alan, eds.,2004, Federal GeographicDataCommittee,Geologic References Cited years; andGaforgiga-annum, 10 for kilo-annum,10 of Units)prefixes coupledwith“a”forannum:ka abbreviations foragesinSI(InternationalSystem on StratigraphicNomenclature,2005)recommends Stratigraphic Code”(North American Commission present (before A.D. 1950). The “North American geologic event maybeexpressed inyearsbefore Age Terms ued for2m.y. For example, depositionbegan at85Maandcontin- time shouldbeexpressed inmillionsofyears(m.y.). ogy, v. 11, no.9,p.503–504. can Geology[DNAG] 1983geologictimescale:Geol- http://www.stratigraphy.org/gssp.htm. Commission onStratigraphy, available onlineat stratotype sectionsandpoints(GSSPs):International code2.html.) http://ngmdb.usgs.gov/Info/NACSN/Code2/ v. 89,no.11,p.1547–1591.(Alsoavailable onlineat can Association ofPetroleumGeologistsBulletin, ture, 2005,North American stratigraphiccode: Ameri- University Press,131p. geologic timescale:Cambridge,U.K.,Cambridge C.A.G., Smith A.G., and Walters, R.W., 1982, A Press, 263p. scale, 1989:Cambridge,U.K.,CambridgeUniversity Smith, A.G., andSmith,D.G.,1990, A geologictime cal timetable(5thed.): Amsterdam, Elsevier, 1sheet. http://www.nwrc.usgs.gov/lib/lib_sta.ht 289 p. (Also available online at edition [STA7]: Reston, Va., U.S. Geological Survey, reports of the United States Geological Survey, seventh bridge University Press,589p.,1pl. A geologictimescale2004:Cambridge,U.K.,Cam- at http://ngmdb.usgs.gov/fgdc_gds/. FGDC–STD–013–2006, 290p.,2pls.,available online Geographic DataCommitteeDocumentNumber standard forgeologicmapsymbolization:Federal Subcommittee, 2006,FGDCdigitalcartographic The ageofastratigraphicunitorthetime 3 years;Maformega-annum, 10 v

9 years.Durationof m .) 6

Phanerozoic EONOTHEM / EON

Paleozoic (\) Mesozoic (]) Cenozoic ([) ERATHEM /

Carboniferous (C) Tertiary (T) Cambrian Silurian Permian Quaternary SYSTEM,SUBSYSTEM / (?) (O) (S) (D) (P) (>) (J) (K) (Q) PERIOD,SUBPERIOD (M) ( ) () (N) Lower /Early Lower /Early Lower /Early Lower /Early Lower /Early Lower /Early Lower /Early Lower /Early Upper /Late Upper /Late Upper /Late Upper /Late Upper /Late Upper /Late Upper /Late Upper / Llandovery Pliocene Wenlock EPOCH SERIES / Ludlow Middle Middle Middle Middle Middle Middle Middle Pridoli 4. ±1.0 542.0 ±2.0 513.0 ±2.0 501.0 ±1.7 488.3 ±1.6 471.8 ±1.6 460.9 ±1.5 443.7 ±2.3 428.2 ±2.5 422.9 ±2.7 418.7 ±2.8 416.0 ±2.7 397.5 ±2.6 385.3 ±2.5 359.2 ±2.1 345.3 ±1.6 326.4 ±1.3 318.1 311.7 ±1.1 ±1.0 306.5 ±0.8 299.0 ±0.7 270.6 ±0.7 260.4 ±0.4 251.0 ±1.5 245.0 ±2.0 228.0 ±0.6 199.6 ±2.0 175.6 ±4.0 161.2 ±4.0 145.5 ±0.9 99.6 ±0.3 65.5 ±0.2 55.8 ±0.1 33.9 ±0.05 23.03 ±0.005 5.332 ±0.005 1.806 11,477 ±85yr

Age estimates of boundaries in mega-annum (Ma) unless otherwise noted symbols are in parentheses. Stratigraphy (Ogg, 2004). Map the International Commission on boundary age estimates from reflects ratified unit names and and geochronologic units. It shows major chronostratigraphic Committee, 2006. The chart logical Survey Geologic Names Time Figure 1. approved by the U.S. Geo (") (A) () EONOTHEM / EON

Eoarchean (X) (Y) (Z) ERATHEM / ERA Divisions of Geologic PERIOD SYSTEM / Ectasian

~4000 3600 3200 2800 2500 2300 2050 1800 1600 1400 1200 1000 850 630 Age estimates of boundaries in mega-annum (Ma)

- unless otherwise noted