Status of Divisions of the International Geologic Time Scale JAMES G. OGG Ogg, J.G. 2004 0615: Status of Divisions of the International Geologic Time Scale. Lethaia, Vol. 37, pp. 183±199. Oslo. ISSN 0024-1164. Each chronostratigraphic unit of the International Geologic Time Scale will be de®ned at its base by a Global Stratotype Section and Point (GSSP) or Global Standard Strati- graphic Age (GSSA). Nearly 50 GSSPs and 10 GSSAs have now been rati®ed. Ideally, the GSSP coincides with events having a global correlation potential. The international stage divisions of some systems, such as the Jurassic or Neogene, are similar to tradi- tional usage in European geology. However, in order to utilize global correlation hori- zons, the international stage divisions of other systems, such as the Ordovician or Per- mian, have required assembling new stage nomenclatures or hybrids of different regional stages. A reference table by the International Commission on Stratigraphy itemizes the current or potential GSSP and GSSA de®nitions of all international geolo- gic time units. & Cenozoic, chronostratigraphy, GSSP, Mesozoic, Paleozoic, Precambrian. James G. Ogg, [[email protected]], Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397, USA Standardization of the International universal language of a `Standard' Geologic Time Geologic Time Scale Scale. Prominent regional lithologic characteristics in A major role of the International Commission on marginal marine facies were utilized for assigning Stratigraphy (ICS) is the establishment and publica- many of the historic stratotypes for system, series and tion of a standard global stratigraphic time scale. Each stage units, but their boundaries were often demarked formal unit of the Phanerozoic Era (542 Ma to by hiatuses. These facies shifts were commonly the Present) and latest Proterozoic (Ediacaran System/ local manifestation of a global environmental change, Period) will be de®ned by a Global Stratotype Section which was also accompanied by evolution or extinc- and Point (GSSP) at its base. The Archean and tion of biologic species, sea-level excursions, and Proterozoic eons of the Precambrian interval are redistributions in the carbon cycle. The GSSP concept subdivided by absolute age (Global Standard Strati- of high-precision correlation requires that the stage graphic Age, or GSSA). Names, ranks and GSSPs of the boundaries be assigned within continuous successions, global scale proposed by its subcommissions are but very few of the historical stratotypes qualify in this approved by the International Commission on Strati- regard. Therefore, the GSSPs are commonly selected in graphy and rati®ed by the International Union of deeper marine facies in which evolutionary events of Geological Sciences (IUGS). Each GSSP de®nes the semi-cosmopolitan pelagic fauna (conodonts, ammo- initiation of a geochronologic interval (geologic time noids, graptolites, planktonic foraminifers, certain units of `period', `epoch', etc.) and its corresponding types of trilobites, etc.) and/or carbon-isotope excur- chronostratigraphic interval (time-rock units of `sys- sions serve as primary correlation markers. Stable tem', `series', `stage', etc.). Even though a GSSP is a isotope excursions and magnetic reversals are pre- point within the rock record at a speci®c locality, its ferred secondary correlation markers, because these placement generally coincides with primary correla- events are globally synchronous and can be utilized in tion criteria based on important faunal, magnetic, deep marine to terrestrial sections. cycle and/or isotopic events that can be globally The great majority of rati®ed or potential GSSPs are correlated. The origin, philosophy and implications in western Europe (Fig. 1). This distribution mostly of the GSSP concept are explored in a companion re¯ects that the stratigraphic studies developed in paper by Walsh et al. (2004, this issue). The ICS has western Europe, but is also due to tectonic processes the mandate to complete all GSSPs for global divisions that kept western Europe in low-latitude shallow-sea of Earth history by 2008. When all Phanerozoic units environments for much of the Phanerozoic eon and are de®ned by GSSPs, then geo-history will have a have subsequently exposed the richly fossiliferous DOI 10.1080/00241160410006492 # 2004 Taylor & Francis 184 James G. Ogg LETHAIA 37 (2004) Fig. 1. Geographic distribution of rati®ed (diamonds) and candidate (squares) GSSPs on a present-day (0 Ma) map (status in Jan., 2004; see Table 1). Most of the GSSPs are in western Europe, where the clustering has overlapped many additional GSSPs. [This ®gure is modi®ed from a compilation by Alan Smith for Geologic Time Scale 2004] sections that were the basis of the historical compila- and of geochemical cycling. This is one of the main tions of the chronostratigraphic scale. Many of these goals of the different subcommissions of the Interna- GSSP levels are partially consistent with historical tional Commission on Stratigraphy. usage of stage terminology (e.g. the base-Cenomanian As of March 2004, approximately two-thirds of the GSSP de®ning the base of the Upper Cretaceous has 90 stage-level Phanerozoic divisions of the interna- not departed signi®cantly from the beginning of the tional geologic scale are de®ned by GSSPs at their Cenomanian Stage as assigned by Alcide d'Orbigny in boundaries, and primary correlation criteria have been 1847. Other GSSP levels de®ning stages have necessi- decided for GSSPs of most other stages. An itemized tated a revision of the historical usage (e.g. the base- summary of these GSSP placements and associated Triassic GSSP is signi®cantly younger than the major nomenclature for the chronostratigraphic units are mass extinction and geochemical anomalies that were provided in Tables 1 and 2, and are schematically formerly grouped as the `Permian/Triassic boundary shown in Figure 2. The current listing, including a events'). standardized description, map and stratigraphic col- Even though the international geologic scale is umn for each GSSP, is maintained at the ICS website de®ned with global events (when possible), numerous (www.stratigraphy.org). The estimated absolute age regional scales of traditional usage are built from local (Ma) for each stage boundary is from A Geologic Time signi®cant changes in sediment, paleoenvironment or Scale 2004 (Gradstein, et al. in press) and is summarized fossil characteristics. For example, the `Cincinnatian' in a companion paper by Gradstein & Ogg (2004, this is an upper Ordovician unit that has been used for issue). In general, the age for each GSSP horizon is over a century in the central part of USA. One must be extrapolated after calibrating a primary scale (e.g. able to easily and con®dently convert from these biostratigraphic zonation, marine magnetic anomaly traditional regional scales to the international stan- pattern) to selected radiometric ages. There is direct dard as de®ned by GSSPs, and to correlation among radiometric dating of only a few GSSPs (e.g. base- different regional scales. Indeed, this is the heart of Triassic, base-Turonian) or their primary correlation compiling a global Earth history ± how to relate the horizons (e.g. base-Cambrian, base-Jurassic, Cretac- well-documented successions of events in each region eous/Palaeogene boundary). However, orbital-climate to the other regions of the planet. Only in this way can `Milankovitch' cycles enable direct age assignments to geoscientists understand the intricate workings of several of the Neogene GSSP levels. evolution, of environmental cause-effect, of tectonics These brief summaries of the GSSP placements for LETHAIA 37 (2004) Divisions of the International Geologic Time Scale Fig. 2. The International Stratigraphic Chart for the Precambrian and Phanerozoic with rati®ed GSSPs (status in March 2004). The colours are the standard suite of the Commission for the 185 Geologic Map of the World (UNESCO). Updated versions of this ®gure can be downloaded from the website (www.stratigraphy.org) of the International Commission on Stratigraphy of IUGS. Table 1. Status as of March 2004 of GSSP and GSSA de®nitions for lower boundaries of international geologic stages, periods and eras. The background shading is the standard colour scale of the 186 US Geologic Survey. Updates of this compilation can be obtained from the website (www.stratigraphy.org) of the International Commission on Stratigraphy of IUGS. Global Boundary Stratotype Sections and Points (GSSPs) James G. Ogg Status in March 2004; see ICS website (www.stratigraphy.org) for updates EON, Era, System, Age (Ma) Est. GSSP and Series, Stage GTS2004 Æ myr Derivation of Age Principal correlative events location Status Publication PHANEROZOIC Cenozoic Era Neogene System The ªQuaternaryº is traditionally considered to be the interval of oscillating climatic extremes (glacial and interglacial episodes) that was initiated at about 2.6Ma, therefore encompasses the Holocene, Pleistocene and uppermost Pliocene. It is not a formal chronostratigraphic unit. Holocene Series base Holocene 11.5 ka 0.00 Carbon-14 dating exactly 10,000 Carbon-14 years Informal working calibration (= 11.5 ka calendar years BP) at the de®nition end of the Younger Dryas cold spell Pleistocene Series base Upper Pleistocene 0.1260.00 Astronomical cycles in base of the Eemian interglacial stage Potentially, within Informal working subseries sediments (= base of marine isotope stage 5e) sediment core