Status of Divisions of the International Geologic Scale


Ogg, J.G. 2004 0615: Status of Divisions of the International . 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 Section and Point (GSSP) or Global Standard Strati- graphic (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 divisions of some systems, such as the or , are similar to tradi- tional usage in European . However, in order to utilize global correlation hori- zons, the international stage divisions of other systems, such as the or Per- mian, have required assembling new stage nomenclatures or hybrids of different regional stages. A reference table by the International Commission on itemizes the current or potential GSSP and GSSA de®nitions of all international geolo- gic time units. & , , GSSP, , , .

James G. Ogg, [[email protected]], Department of 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 , and tion of a standard global stratigraphic time scale. Each stage units, but their boundaries were often demarked formal unit of the (542 Ma to by hiatuses. These facies shifts were commonly the ) and latest ( System/ local manifestation of a global environmental change, Period) will be de®ned by a Global Stratotype Section which was also accompanied by or extinc- and Point (GSSP) at its base. The and tion of biologic , 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 (, ammo- initiation of a geochronologic interval (geologic time noids, graptolites, planktonic foraminifers, certain units of `period', `', etc.) and its corresponding types of , 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 (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 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- (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- As of March 2004, approximately two-thirds of the GSSP de®ning the base of the Upper 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 GSSP is signi®cantly younger than the major nomenclature for the chronostratigraphic units are mass and geochemical anomalies that were provided in Tables 1 and 2, and are schematically formerly grouped as the `/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 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 of only a few GSSPs (e.g. base- different regional scales. Indeed, this is the heart of Triassic, base-) or their primary correlation compiling a global Earth history ± how to relate the horizons (e.g. base-, 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 EHI 7(2004) 37 LETHAIA iiin fteItrainlGooi ieScale Time Geologic International the of Divisions

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 . 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) Ogg G. James 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 episodes) that was initiated at about 2.6Ma, therefore encompasses the , and uppermost . It is not a formal chronostratigraphic unit. Holocene Series base Holocene 11.5 ka 0.00 Carbon-14 dating exactly 10,000 Carbon-14 Informal working calibration (= 11.5 ka years BP) at the de®nition end of the 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 under de®nition before ®nal glacial episode of the Netherlands Pleistocene (Eemian type area) base Middle Pleistocene 0.781 0.00 Astronomical cycles in Brunhes-Matuyama magnetic reversal Informal working subseries sediments de®nition base Pleistocene Series 1.8060.00 Astronomical cycles in Just above top of magnetic polarity Top of sapropel layer Rati®ed 1985 Episodes 8 (2), p.116± sediments C2n (Olduvai) and the `e', Vrica section, 120, 1985 extinction level of calcareous Calabria, nannofossil brouweri (base Zone CN13). Above are lowest occurrence of calcareous nannofossil medium Gephyrocapsa spp. and extinction level of planktonic foraminifer extremus. Pliocene Series base Stage 2.588 0.00 Astronomical cycles in Isotopic stage 103, base of magnetic Midpoint of sapropelic Rati®ed 1996Episodes 21 (2), p.82± sediments polarity chronozone C2r Nicola Bed (ªA5º), 87, 1998 (Matuyama). Above are extinction Monte San Nicola, levels of calcareous nannofossil Gela, , Italy Discoaster pentaradiatus and D. surculus (base Zone CN12c). base Stage 3.600 0.00 Astronomical cycles in Base of magnetic polarity chronozone Base of beige layer of Rati®ed 1997 Episodes 21 (2), p.88± sediments C2An (Gauss); extinction levels of carbonate cycle 77, 93, 1998 planktonic foraminifers Punta Piccola, Sicily, margaritae (base Zone PL3) and Italy Pulleniatina primalis. base Stage, base 5.332 0.00 Astronomical cycles in Top of magnetic polarity chronozone Base of Trubi Fm (base Rati®ed 2000 Episodes 23 (3), p.179± Pliocene Series sediments C3r, 100 kyr before Thvera normal- of carbonate cycle 187, 2000 polarity subchronozone (C3n.4n). 1), Eraclea Minoa, Calcareous nannofossils ± near Sicily, Italy extinction level of Triquetrorhabdulus rugosus (base Zone CN10b) and the lowest occurrence of Ceratolithus acutus. (2004) 37 LETHAIA Series base Stage 7.2460.00 Astronomical cycles in Astrochronology age of 7.246Ma; Base of red layer of Rati®ed 2000 Episodes 23 (3), p.172± sediments middle of magnetic polarity carbonate cycle 15, 178, 2000 chronozone C3Br.1r; lowest regular Oued Akrech, Rabat, occurrence of the Globorotalia Morocco conomiozea planktonic foraminifer group. EHI 7(2004) 37 LETHAIA

base Stage 11.608 0.00 Astronomical cycles in Last Common Occurrences of the Midpoint of sapropel Rati®ed 2003 Episodes article in sediments calcareous nannofossil Discoaster 76, Monte dei Corvi preparation kugleri and the planktonic beach section, foraminifer Globigerinoides Ancona, Italy subquadratus. Associated with the short normal-polarity subchron C5r.2n. base Stage 13.65 0.00 Astronomical cycles in Near lowest occurrence of nannofossil GSSP anticipated in sediments Sphenolithus heteromorphus, and 2004 within magnetic polarity chronozone C5ABr. base Stage 15.97 0.0 Calibrated magnetic Near ®rst occurrence of planktonic GSSP anticipated in anomaly scale foraminifer Praeorbulina glomerosa 2004 and top of magnetic polarity chronozone C5Cn.1n base Stage 20.43 0.0 Calibrated magnetic Near lowest occurrence of planktonic Guide is anomaly scale foraminifer Globigerinoides undecided altiaperturus or near top of magnetic polarity chronozone C6An base Stage, 23.03 0.0 Astronomical cycles in Base of magnetic polarity chronozone 35 m from top of Rati®ed 1996Episodes 20 (1), p.23± base Miocene Series, sediments C6Cn.2n; lowest occurrence of Lemme-Carrosio 28, 1997 base planktonic foraminifer section, Carrosio Neogene System Paragloborotalia kugleri; near village, north of extinction of calcareous nannofossil Genoa, Italy Reticulofenestra bisecta (base Zone NN1).

Paleogene System Series base Stage 28.4 0.1 Calibrated magnetic Planktonic foraminifer, extinction of Probably in Umbria- GSSP anticipated in anomaly scale relative to Chiloguembelina (base Zone P21b) Marche region of 2004 base-Miocene and C24n. Italy

Arbitrary 100 kyr Scale Time Geologic International the of Divisions uncertainty assigned. base Stage, base 33.9 0.1 Calibrated magnetic Planktonic foraminifer, extinction of Base of marl bed at Rati®ed 1992 Episodes 16(3), p.379± Oligocene Series anomaly scale relative to 19 m above base of 382, 1993 base-Miocene and C24n. Massignano quarry, Ancona, Italy Series base Stage 37.2 0.1 Calibrated magnetic Near lowest occurrence of calcareous Probably in Umbria- anomaly scale relative to nannofossil Chiasmolithus Marche region of base-Miocene and C24n. oamaruensis (base Zone NP18) Italy base Stage 40.4 0.2 Calibrated magnetic Near extinction of calcareous anomaly scale relative to nannofossil Reticulofenestra reticulata base-Miocene and C24n. base Stage 48.60.2 Calibrated magnetic Planktonic foraminifer, lowest Leading candidate is GSSP anticipated in anomaly scale relative to occurrence of Hantkenina Fortuna section, 2004 base-Miocene and C24n. Murcia province, Betic Cordilleras, Spain base Stage, base 55.8 0.2 Astronomical cycles in Base of negative carbon-isotope Dababiya section near Rati®ed 2003 Micropaleontology v.49 Eocene Series sediments scaled from excursion Luxor, Egypt (Suppl. 1), 2003. base- Episodes article in preparation Paleocene Series base Stage 58.7 0.2 Astronomical cycles in Magnetic polarity chronozone, base of Leading candidate is Guide event is sediments scaled from C26n, is a temporary assignment Zumaya section, undecided base Paleocene, using northern Spain base of magnetic polarity chronozone C26n. Arbitrary 0.1 (2 precession cycles, plus the base-

radiometric) uncertainty 187 assigned to all estimates.

Continues. Table 1. continued. 188

Global Boundary Stratotype Sections and Points (GSSPs)

Status in March 2004; see ICS website (www.stratigraphy.org) for updates Ogg G. James EON, Era, System, Age (Ma) Est. GSSP and Series, Stage GTS2004 Æ myr Derivation of Age Principal correlative events location Status Publication

base Stage 61.7 0.2 Astronomical cycles in Boundary task group is considering a Leading candidate is Guide event is sediments scaled from higher level ± base of calcareous Zumaya section, undecided base Paleocene, using nannofossil zone NP5 ± which would northern Spain magnetic polarity be 1 myr younger. chronozone placement of C27n.9 base Stage, base 65.5 0.3 Ar-Ar and U-Pb age Iridium geochemical anomaly. Base of boundary clay, Rati®ed 1991 Paleogene System, base agreement Associated with a major extinction El Kef, Tunisia (but Cenozoic horizon (foraminifers, calcareous deterioration may nannofossils, , etc.); require assigning a replacement section) Mesozoic Era Cretaceous System Most substages of Cretaceous also have recommended GSSP criteria Upper base Stage 70.60.6 Estimated placement relative Mean of 12 biostratigraphic criteria of 115.2 m level in Grande Rati®ed 2001 Episodes 24 (4), p.229± to Ar-Ar calibrated Sr- equal importance. Closely above is CarrieÁre quarry, 238, 2001; Odin curve lowest occurrence of ammonite Tercis-les-Bains, (ed.) IUGS Spec. Pachydiscus neubergicus. proxy Landes province, SW Publ. Series, v.36, is lowest occurrence of belemnite Elsevier, 910pp. Belemnella lanceolata. base Stage 83.5 0.7 Spline ®t of Ar-Ar ages and , extinction of Leading candidates are ammonite zones. testudinarius in southern England and in base Stage 85.8 0.7 Spline ®t of Ar-Ar ages and Inoceramid bivalve, lowest occurrence of Leading candidates are ammonite zones. undulatoplicatus in Spain, England and Texas base Stage 89.3 1.0 Spline ®t of Ar-Ar ages and Inoceramid bivalve, lowest occurrence of Base of Bed MK47, GSSP anticipated in ammonite zones. Cremnoceramus rotundatus (sensu Salzgitter-Salder 2004 TroÈger non Fiege) Quarry, SW of Hannover, Lower Saxony, northern base Turonian Stage 93.5 0.8 Spline ®t of Ar-Ar ages and Ammonite, lowest occurrence of Base of Bed 86, Rock Rati®ed 2003 Episodes article in ammonite zones. Watinoceras devonense Canyon Anticline, preparation east of Pueblo, Colorado, west- central USA base Cenomanian Stage 99.60.9 Spline ®t of Ar-Ar ages and Planktonic foraminifer, lowest 36m below top of Rati®ed 2002 Episodes 27 (1), p.21±32, ammonite zones, plus occurrence of Rotalipora Marnes Bleues 2004 monitor standard globotruncanoides Formation, Mont correction. Then cycle Risou, Rosans, stratigraphy to place Haute-Alpes, SE foraminifer datum France relative to ammonite zonation. Lower base Stage 112.0 1.0 Estimated placement relative Calcareous nannofossil, lowest Guide event is to bases of Cenomanian occurrence of Praediscosphaera undecided and , with large columnata (= P. cretacea of some uncertainty due to lack earlier studies), is one potential

of GSSP criteria. Ar-Ar marker. (2004) 37 LETHAIA age of 114.6 ‡/À 0.7 Ma from Parahoplites nut®eldensis below. base Aptian Stage 125.0 1.0 Base of M0r, as recomputed Magnetic polarity chronozone, base of Leading candidate is from Ar-Ar age from M0r Gorgo a Cerbara, MIT guyot Piobbico, Umbria- Marche, central Italy base Stage 130.0 1.5 Paci®c spreading model for Ammonite, lowest occurrence of Leading candidate is magnetic anomaly ages Spitidiscus hugii ± Spitidiscus RõÂo Argos near (variable rate), using vandeckii group Caravaca, Murcia placement at M5n.8. province, Spain EHI 7(2004) 37 LETHAIA

base Stage 136.4 2.0 Paci®c spreading model for Ammonite, lowest occurrence of Leading candidate is La magnetic anomaly ages Acanthodiscus (especially A. radiatus) Charce village, (variable rate), using DroÃme province, placement at base M11n. southeast France base Stage 140.2 3.0 Paci®c spreading model for , lowest occurrence of Leading candidate is magnetic anomaly ages darderi (base of near Montbrun-les- (variable rate), using Calpionellid Zone E); followed by the Bains, DroÃme placement at M14r.3 lowest occurrence of ammonite province, southeast (base T. pertransiens). ªThurmannicerasº pertransiens France base Stage, 145.5 4.0 Paci®c spreading model for Maybe near lowest occurrence of Guide event is base Cretaceous System magnetic anomaly ages ammonite jacobi undecided (variable rate), assigning to base of Berriasella jacobi zone (M19n.2n.55) Jurassic System Upper base Stage 150.8 4.0 Paci®c spreading model for Near base of Hybonoticeras hybonotum Guide event is magnetic anomaly ages ammonite zone and lowest undecided (variable rate), assigning occurrence of genus, and to base M22An the base of magnetic polarity chronozone M22An base Stage 155.7 4.0 Paci®c spreading model for Ammonite, near base of Pictonia baylei Leading candidates are GSSP anticipated in magnetic anomaly ages ammonite zone of Boreal realm In Scotland, SE 2004 (variable rate), age is France and Poland assigned as base M26r.2, using the Boreal ammonite de®nition base Stage 161.2 4.0 Paci®c spreading model for Ammonite, Brightia thuouxensis Horizon Leading candidates are GSSP anticipated in magnetic anomaly ages at base of the Cardioceras in SE France and 2004 (variable rate), assigning scarburgense Subzone southern England to base M36An (Quenstedtoceras mariae Zone) Middle base Stage 164.7 4.0 Equal subzones scale Bajo- Ammonite, lowest occurrence of the Leading candidate is GSSP anticipated in

Bath-Callov genus (Kosmoceratidae) Pfef®ngen, Swabian 2004 Scale Time Geologic International the of Divisions (de®nes base of Macrocephalites Alb, SW Germany herveyi Zone in sub-Boreal province of Great Britain to southwest Germany) base Stage 167.7 3.5 Equal subzones scale Bajo- Ammonite, lowest occurrence of Bath-Callov (G.) convergens (de®nes base of zigzag Zone) base Stage 171.63.0 Equal subzones scale Bajo- Ammonite, lowest occurrence of the Base of Bed AB11, Rati®ed 1996Episodes 20 (1), p.16± Bath-Callov genus Hyperlioceras (de®nes base of 77.8 m above base of 22, 1997 the Hyperlioceras discites Zone) Murtinheira section, Cabo Mondego, western Portugal base Stage 175.62.0 Duration of Aalenian- Ammonite, lowest occurrence of base of Bed FZ107, Rati®ed 2000 Episodes 24 (3), p.166± from cycle Leioceras genus Fuentelsalz, central 175, 2001 stratigraphy Spain Lower base Toarcian Stage 183.0 1.5 Duration of Aalenian- Ammonite, near lowest occurrence of a Guide event is Toarcian from cycle diversi®ed Eodactylites ammonite undecided stratigraphy fauna; correlates with the NW European Paltus horizon. base Stage 189.61.5 Cycle-scaled linear Sr trend Ammonite, lowest occurrences of Wine Haven section, GSSP anticipated in Bifericeras donovani and of genera Robin Hood's Bay, 2003 Apoderoceras and Gleviceras. , England, UK base Stage 196.5 1.0 Cycle-scaled linear Sr trend Ammonite, lowest occurrence of 0.9 m above base of Bed Rati®ed 2000 Episodes 25 (1), p.22± arietitid genera and 145, East 26, 2002 Metophioceras Quantoxhead, , West , SW England, UK base Stage, 199.60.6 U-Pb age just below Near lowest occurrence of smooth Guide event is

base Jurassic System proposed GSSP for base- Psiloceras planorbis ammonite group undecided 189 Jurassic

Continues. Table 1. continued. 190

Global Boundary Stratotype Sections and Points (GSSPs)

Status in March 2004; see ICS website (www.stratigraphy.org) for updates Ogg G. James EON, Era, System, Age (Ma) Est. GSSP and Series, Stage GTS2004 Æ myr Derivation of Age Principal correlative events location Status Publication

Triassic System Upper base Stage 203.61.5 Magnetostratigraphic Near lowest occurrence of ammonite Key sections in Austria, Guide event is correlation to cycle- Cochlocera, conodonts spp. British Columbia undecided scaled Newark magnetic and mosheri, and (), and polarity pattern radiolarian Proparvicingula Turkey moniliformis. base Stage 216.5 2.0 Magnetostratigraphic Base of Klamathites macrolobatus or Leading candidates are Guide event is correlation to cycle- Stikinoceras kerri ammonoid zones in British Columbia undecided scaled Newark magnetic and the communisti (Canada), Sicily polarity pattern or M. primitius zones. (Italy), and possibly Slovakia, Turkey (Antalya Taurus) and Oman. base Stage 228.0 2.0 Magnetostratigraphic Near ®rst occurrence of the ammonoids Candidate section at Guide event is correlation to cycle- Daxatina or Trachyceras, and of the Prati di Stuores, undecided scaled Newark magnetic conodont Metapolygnathus Dolomites, northern polarity pattern polygnathiformis Italy. Important reference sections in Spiti () and New Pass, Nevada (USA). Middle base Stage 237.0 2.0 U-Pb array by Mundil et al. Alternate levels are near base of Reitzi, Leading candidates are Guide event is on levels near Nevadites Secedensis, or Curionii ammonite Bagolino (Italy) and undecided (= Secedensis) ammonite zone; near ®rst occurrence of the Felsoons (Hungary). zone in Dolomites, plus conodont genus Budurovignathus. Important reference placement relative to sections in the Humboldt Range, correlations to cycle- Nevada (USA). scaled Newark magnetic polarity pattern base Stage 245.0 1.5 Proportional subzonal Ammonite, near lowest occurrences of Candidate section GSSP anticipated in scaling genera Japonites, Paradanubites, and probable at Desli 2004 Paracrochordiceras; and of the Caira, Dobrogea, conodont Chiosella timorensis Romania; signi®cant sections in Guizhou Province (). Lower base Stage 249.7 0.7 Composite standard from Near lowest occurrence of Candidate sections in Guide event is conodonts scaled to Hedenstroemia or Meekoceras (Russia) and undecided base-Anisian and base- gracilitatis ammonites, and of the probably Chaohu, Triassic conodont waageni. Anhui Province, China. Important sections also in Spiti. base Stage, base 251.0 0.4 U-Pb ages bracket GSSP Conodont, lowest occurrence of Base of Bed 27c, Rati®ed 2001 Episodes 24 (2), p.102± Triassic System, base (Bowring et al., 1998) parvus; termination of Meishan, , 114, 2001 Mesozoic major negative carbon-isotope China excursion. About 1 myr after peak of Late Permian .

Paleozoic Era (2004) 37 LETHAIA Permian System Permian- time scale is derived from calibrating a master composite section to selected radiometric ages Series base Stage 253.8 0.7 º Conodont, near lowest occurrence of Leading candidates are conodont wangi in China base Stage 260.4 0.7 º Conodont, near lowest occurrence of Candidate section is Rati®cation pending conodont Clarkina postbitteri Tieqiao rail-bridge (Feb'04) section, Laibin Syncline, Province, China EHI 7(2004) 37 LETHAIA

Guadalupian Series base Stage 265.8 0.7 º Conodont, lowest occurrence of 4.5 m above base of Rati®ed 2001 Episodes article in postserrata Pinery preparation Member, Nipple Hill, SE , Texas, USA base Stage 268.0 0.7 º Conodont, lowest occurrence of 7.6m above base of Rati®ed 2001 Episodes article in Jinogondolella aserrata Getaway Ledge preparation , Guadalupe Pass, SE Guadalupe Mountains, Texas, USA base Stage 270.60.7 Conodont, lowest occurrence of 42.7 m above base of Rati®ed 2001 Episodes article in Jinogondolella nanginkensis Cutoff Formation, preparation Stratotype Canyon, southern Guadalupe Mountains, Texas, USA Series base Stage 275.60.7 Conodont, near lowest occurrence of Leading candidates are conodont Neostreptognathus pnevi-N. in southern Ural exculptus Mtns. base Stage 284.4 0.7 º Conodont, lowest occurrence of Leading candidates are conodont whitei in southern Ural Mtns. base Stage 294.60.8 º Conodont, near lowest occurrence of Leading candidate is at conodont Sweetognathus merrelli Kondurovsky, Orenburg Province, Russia. base Stage, base 299.0 0.8 º Conodont, lowest occurrence of 27 m above base of Bed Rati®ed 1996Episodes 21 (1), p.11± Permian System isolatus within the 19, Aidaralash Creek, 18, 1998 S. ªwabaunsensisº conodont AktoÈbe, southern chronocline. 6m higher is lowest , fusilinid foraminifer northern Sphaeroschwagerina vulgaris aktiubensis. Scale Time Geologic International the of Divisions

Carboniferous System Subsystem Series classi®cation approvedin 2004 Upper base Stage 303.9 0.9 º Near lowest occurrences of the Guide event is fusulinids Daixina, Jigulites and undecided Rugosofusulina, or lowest occurrence of simulator (s.str.). Close to lowest occurrence of ammonoid Shumardites. base Stage, 306.5 1.0 º Near base of Obsoletes obsoletes and Guide event is base Upper Protriticites pseudomontiparus undecided Pennsylvanian Series fusulinid zone, or lowest occurrence of conodont Streptognathodus subexcelsus or ammonoid Parashumardites Middle base Moscovian Stage, 311.7 1.1 º Near lowest occurrences of conodonts Guide event is base Middle donetzianus and/or undecided Pennsylvanian Series Idiognathoides postsulcatus, and of fusulinid Aliutovella aliutovica. Lower base Stage, 318.1 1.3 º Conodont, lowest occurrence of 82.9 m above top of GSSP rati®ed 1996. Episodes 22 (4), p.272± base Pennsylvanian Declinognathodus nodiliferus s.l. Battleship Wash Subsystem rank 283, 1999 Subsystem Fm., Arrow Canyon, of southern Nevada, and USA Pennsylvanian names rati®ed 2000. Mississippian Subsystem Series classi®cation approvedin 2004 Upper

base , base 326.4 1.6 º Near lowest occurrence of conodont, Guide event is 191 Upper Mississippian ziegleri undecided Series Continues. Table 1. continued. 192

Global Boundary Stratotype Sections and Points (GSSPs)

Status in March 2004; see ICS website (www.stratigraphy.org) for updates Ogg G. James EON, Era, System, Age (Ma) Est. GSSP and Series, Stage GTS2004 Æ myr Derivation of Age Principal correlative events location Status Publication

Middle base Visean, base Middle 345.3 2.1 º Foraminifer, lineage Eoparastaffella Leading candidate is Mississippian Series simplex morphotype 1/morphotype 2 Pengchong, south China Lower base , base 359.2 2.5 º Conodont, above lowest occurrence of Base of Bed 89, La Rati®ed 1990 Episodes 14 (4), p.331± Mississippian sulcata Serre, Montagne 336, 1991 Subsystem, Noir, CabrieÁres, base Carboniferous southern France System time scale is a Devonian System statistical ®t of a composite biostratigraphic zonation (based on Figure 8 of Williams et al., 2000) to selected radiometric ages Upper base Stage 374.5 2.6 º Just above major extinction horizon base of Bed 32a, upper Rati®ed 1993 Episodes 16(4), p.433± (Upper Kellwasser Event), including Coumiac quarry, 441, 1993 conodonts Ancyrodella and Cessenon, Montagne and of Noir, southern Gephuroceratidae and Beloceratidae France base Stage 385.3 2.6 º Conodont, lowest occurrence of Base of Bed 42a', Col Rati®ed 1986Episodes 10 (2), p.97± Ancyrodella rotundiloba (de®nes base du Puech de la 101, 1987 of Lower asymmetricus Suque section, St. conodont Zone) Nazaire-de-Ladarez, SE Montagne Noir, southern France Middle base Stage 391.8 2.7 º Conodont, lowest occurrence of Base of Bed 123, Jebel Rati®ed 1994 Episodes 18 (3), p.107± Polygnathus hemiansatus, near base Mech Irdane ridge, 115, 1995 of Maenioceras Stufe Ta®lalt, Morocco base Stage 397.5 2.7 º Conodont, lowest occurrence of Base unit WP30, trench Rati®ed 1985 Episodes 8 (2), p.104± Polygnathus costatus partitus; major at Wetteldorf 109, 1985 faunal turnover Richtschnitt, SchoÈnecken- Wetteldorf, Hills, western Germany Lower base Stage 407.0 2.8 º Conodont, lowest occurrence of Base of Bed 9/5, Rati®ed 1995 Episodes 20 (4), p.235± Polygnathus kitabicus (= Po. Zinzil'ban Gorge, SE 240, 1997 dehiscens) of Samarkand, Uzbekistan base Stage 411.2 2.8 º Conodont, lowest occurrence of Base of Bed 12, Velka Rati®ed 1989 Episodes 12 (2), p.109± Eognathodus sulcatus Chuchle quarry, 113, 1989 southwest part of Prague city, Czech Republic base Stage, 416.0 2.8 base-Devonian from scale in Graptolite, lowest occurrence of Within Bed 20, Klonk, Rati®ed 1972 Martinsson (ed.), The

base Devonian System Cooper (this volume), uniformis Barrandian area, -Devonian (2004) 37 LETHAIA which is 1 myr younger southwest of Prague, Boundary, IUGS than Tucker et al (1998) Czech Republic Series A, no.5, 349 estimate. pp., 1977

Silurian System Silurian and Ordovician Holland and Bassett time scales are from (eds), A Global calibrating a CONOP Standard for the composite graptolite Silurian System, Nat. zonation to selecte Mus. , Geol. radiometric ages Series No.10, Cardiff, 325 pp., 1989 EHI 7(2004) 37 LETHAIA

Pridoli Series base PrõÂdolõÂ Series (not 418.7 2.7 º Graptolite, lowest occurrence of Within Bed 96, PozaÂry Rati®ed 1984 Episodes 8 (2), p.101± subdivided in stages) Monograptus parultimus section near Reporje, 103, 1985 Barrandian area, Prague, Czech Republic Series 2.6 base Stage 421.3 2.6 º Imprecise. May be near base of Base of lithologic unit Rati®ed 1980 Lethaia 14, p.168, 1981; Saetograptus leintwardinensis C, Sunnyhill Quarry, Episodes 5 (3), p.21± graptolite zone. Ludlow, Shropshire, 23, 1982 southwest England, UK base Stage 422.9 2.5 º Imprecise. Just below base of local Base of lithologic unit Rati®ed 1980 Lethaia 14, p.168, 1981; Leptobrachion longhopense F, Pitch Coppice Episodes 5 (3), p.21± range zone. May be near base of quarry, Ludlow, 23, 1982 Neodiversograptus nilssoni graptolite Shropshire, zone. southwest England, UK Wenlock Series base Stage 426.2 2.4 º Graptolite, lowest occurrence of Graptolite Rati®ed 1980 Lethaia 14, p.168, 1981; Cyrtograptus lundgreni (de®nes base intersection in Episodes 5 (3), p.21± of C. lundgreni graptolite zone) stream section in 23, 1982 Whitwell Coppice, Homer, Shropshire, southwest England, UK base Stage 428.2 2.3 º Imprecise. Between the base of acritarch Base of lithologic unit Rati®ed 1980 Lethaia 14, p.168, 1981; biozone 5 and extinction of G, Hughley Brook, Episodes 5 (3), p.21± conodont Apedale, Shropshire, 23, 1982 amorphognathoides. May be near base southwest England, of Cyrtograptus centrifugus graptolite UK zone. Llandovery Series iiin fteItrainlGooi ieScale Time Geologic International the of Divisions base Stage 436.0 1.9 º , just above extinction of Locality 162 in transect Rati®ed 1984 Episodes 8 (2), p.101± Eocoelia intermedia and below lowest d, Cefn Cerig road, 103, 1985 succeeding species Eocoelia curtisi. Llandovery area, Near base of Monograptus south-central Wales, turriculatus graptolite zone. UK base Stage 439.0 1.8 º Graptolite, lowest occurrence of Base of locality 72 in Rati®ed 1984 Episodes 8 (2), p.101± Monograptus austerus sequens transect h, Trefawr 103, 1985 (de®nes base of Monograptus forestry road, north triangulatus graptolite zone) of Cwm-coed-Aeron Farm, Llandovery area, south-central Wales, UK base Stage, 443.7 1.5 º Graptolites, lowest occurrences of 1.6m above base of Rati®ed 1984 Episodes 8 (2), p.98± base Silurian System Parakidograptus acuminatus and Birkhill Fm., 100, 1985 Akidograptus ascensus Dob's Linn, Moffat, Scotland, UK

Ordovician System Upper base Stage 445.61.5 º Potentially at base of the Candidate section is Normalograptus extraordinarius-N. Wangjiawan, China ojsuensis graptolite biozone base of sixth stage (not 455.8 1.6 º Potentially near ®rst appearance of the Candidate sections ae yet named) graptolite Diplacanthograptus Black Knob Ridge caudatus (Oklahoma, USA) and Hartfell Spa (S. Scotland, UK) base of ®fth stage (not yet 460.9 1.6 º Graptolite, lowest occurrence of 1.4 m below Rati®ed 2002 Episodes 23 (2), p.102± named) Nemagraptus gracilis in E14a 109, 2000 (proposal; outcrop, FaÊgelsaÊng, formal GSSP Scane, southern publication in preparation). 193

Continues. Table 1. continued. 194

Global Boundary Stratotype Sections and Points (GSSPs)

Status in March 2004; see ICS website (www.stratigraphy.org) for updates Ogg G. James EON, Era, System, Age (Ma) Est. GSSP and Series, Stage GTS2004 Æ myr Derivation of Age Principal correlative events location Status Publication

Middle base Stage 468.1 1.6 º Graptolite, lowest occurrence of Base of Bed AEP184, Rati®ed 1997 Episodes 20 (3), p.158± Undulograptus austrodentatus 22 m below top of 166, 1997 Ningkuo Fm., Huangnitang, Changshan, Zhejiang province, southeast China base of third stage (not 471.8 1.6 º Conodont, potentially lowest occurrence Candidate sections at yet named) of aranda or of Niquivil () triangularis and Huanghuachang (China) Lower base of stage (not 478.61.7 º Graptolite, lowest occurrence of Just above E bed, Rati®ed 2002 Episodes article in yet named) Tetragraptus approximatus Diabasbrottet preparation quarry, VaÈstergoÈtland, southern Sweden base of 488.3 1.7 º Conodont, lowest occurrence of Within Bed 23 at the Rati®ed 2000 Episodes 24 (1), p.19± Stage, base Ordovician ¯uctivagus; just above 101.8 m level, Green 28, 2001 System base of lindstromi Point, western conodont Zone. Just below lowest Newfoundland, occurrence of planktonic graptolites. Canada Currently dated around 489 Ma. Potential GSSP correlation levels include Overview of Cambrian System Cordylodus proavus, Glyptagnostus potential reticulatus, Ptychagnostus punctuosus, subdivisions in Acidusus atavus, and Oryctocephalus Episodes 23 (3), indicus. p. 188-195, 2000. Upper (ªFurongianº) Series upper stage(s) in Potential GSSP levels in upper Cambrian are basedon trilobites andconodonts base Stage, base 501.0 2.0 Radiometric ages near , lowest occurrence of agnostoid 369.06 m above base of Rati®ed 2003 Episodes article in Furongian Series primary marker level. Glyptagnostus reticulatus. Coincides Huaqiao Fm, Paibi preparation Estimated age and with base of large positive carbon- section, NW uncertainty only. isotope excursion. province, south China Middle 513.0 2.0 Radiometric ages near Potential GSSP levels in Middle primary marker level. Cambrian are basedmainly on Estimated age and trilobites uncertainty only. Lower Potential GSSP levels in Lower Cambrian are basedon archaeocyatha, small shelly , andto a lesser extent, trilobites base Cambrian System, 542.0 1.0 U-Pb age from Oman , lowest occurrence of 2.4 m above base of Rati®ed 1992 Episodes 17 (1&2), p.3± base Paleozoic, base coinciding with the (Phycodes) pedum. Near Member 2 of Chapel 8, 1994. PHANEROZOIC negative carbon base of negative carbon-isotope Island Fm., Fortune excursion. excursion. Head, Burin Peninsula, southeast Newfoundland, Canada

PROTEROZOIC Pre-Cambrian eras andsystems below (2004) 37 LETHAIA Ediacaran are de®ned by absolute ages, rather than stratigraphic points. Era base Ediacaran System 630 Age as suggested by Termination of Marinoan (or Varanger) Base of the Marinoan Age-de®nition Ediacaran Subcomm: glaciation, and distinctive C-13 cap carbonate (650 Ma) for bracketed by radiometric change. (Nuccaleena ªNeoproterozoic ages of 600 and 635 Ma Formation), IIIº rati®ed immediately above 1990, and the Elatina replaced by diamictite in the Ediacaran GSSP Enorama Creek in Feb 2004 section, Flinders Ranges, South . LETHAIA 37 (2004) Divisions of the International Geologic Time Scale 195

each period/system are primarily extracted from detailed discussions in the corresponding chapters of A Geologic Time Scale 2004. 15(2), p.122±123, 1992. 15(2), p.122±123, 1992. 15(2), p.122±123, 1992. 15(2), p.122±123, 1992. 140, 1991 140, 1991 140, 1991 140, 1991 140, 1991 140, 1991 140, 1991 140, 1991 Informally in Episodes Informally in Episodes Informally in Episodes Informally in Episodes

Archean and Proterozoic Eons of the `Precambrian'

Subdivisions and correlation of the Phanerozoic Eon 1996, but not submitted to ICS 1996, but not submitted to ICS 1996, but not submitted to ICS 1996, but not submitted to ICS are traditionally based on the evolution of metazoan Subcomm. decision ± the fossil record. In contrast, the Precambrian generally lacks well-documented precise global corre- lation horizons, and correlation of regional Precam- brian generally utilizes comparison of radiometric ages. Therefore, the current Precambrian time scale is divided into the chronometric subdivi- sions, in which time boundaries (GSSAs) have been selected to demarcate the estimated ages of principal cycles of sedimentation, mountain building and volcanism. These GSSA de®nitions were subsequently formalized in Plumb (1991). The `Precambrian' is not a formal stratigraphic and simply refers to all rocks that formed prior to the Cambrian Period. The Precambrian is formally divided into an older Archean Eon and a younger Proterozoic Eon. The informal Archean divisions of Eo-, Paleo-, Meso- and Neo-Archean eras were approved by the Precambrian subcommission in 1996, but were never formally submitted to and rati®ed by ICS/IUGS. No base was de®ned to the Archean, but the term `' (after Hades, hence forever unknown) has been suggested to span the time from the moon-forming catastrophic impact to the end of heavy bombardment. The Archean would begin with the preserved rock record. The Proterozoic Eon begins at 2500 Ma. The Paleo-, Meso- and Neo-Proterozoic eras are poetically divided Base is not de®ned De®ned chronometrically Base = 3600 Ma Subcomm. decision De®ned chronometrically Base = 3200 Ma Subcomm. decision De®ned chronometricallyDe®ned chronometrically Base = 850 Ma Base = 1000 MaDe®ned chronometrically Base = 2800 Ma Rati®ed 1990 Episodes 14 (2), p.139± Subcomm. decision De®ned chronometrically Base = 1600 Ma Rati®ed 1990 Episodes 14 (2), p.139± De®ned chronometrically Base = 1400 Ma Rati®ed 1990 Episodes 14 (2), p.139± De®ned chronometrically Base = 1200 Ma Rati®ed 1990 Episodes 14 (2), p.139± De®ned chronometricallyDe®ned chronometricallyDe®ned chronometrically Base = 2050 Ma Base = 2300 Ma Base = 2500 Ma Rati®ed 1990 Rati®ed 1990 Rati®ed 1990 Episodes 14 (2), p.139± Episodes 14 (2), p.139± Episodes 14 (2), p.139± De®ned chronometrically Base = 1800 Mainto periods named Rati®ed 1990 after Episodestheir 14 (2), p.139± typical tectonic, sedimentary or environmental characteristics (Table 2). The ®nal Neoproterozoic period, which had been temporarily called `Neoproterozoic III', was formally named as the Ediacaran period/system in early 2004. The Ediacaran period begins at the termination of `' glacial episodes of the 850 3600 3200 2800 1000 1600 1400 1200 2050 2300 2500 1800 Period, and is de®ned by a GSSP in the Flinders Range of Australia. Potentially, some of the current schematic divisions of the Precambrian based on absolute ages (GSSAs) may be suitable for high-resolution GSSP de®nitions and global correlation levels. Some possibilities for `natural' Precambrian subdivisions that utilize impor- tant correlative geologic events are presented by Era Era Era Cryogenian System Era System System System System System System System System Bleeker (2004, this issue). ARCHEAN Era Era 196 ae .Ogg G. James

Table 2. Origin of nomenclature for periods of the Proterozoic Eon. [This ®gure is modi®ed from a table in the Precambrian chapter of Geologic Time Scale 2004].

PERIOD NAME base DERIVATION GEOLOGICAL PROCESS EDIACARAN GSSP, 630 Ma Ediacara = from Australian Aboriginal ªEvolution of ®rst megascopic marine term for place near water animalsº Oldest record of megascopic . GSSP in Australia coincides with termination of glaciations and a pronounced carbon-isotope excursion

CRYOGENIAN À850 Ma Cryos = ice; Genesis = birth ªGlobal glaciationº Glacial deposits, which typify the late Proterozoic, are most abundant during this interval

TONIAN À1000 Ma Tonas = stretch Further major platform cover expansion (e.g., Upper , Russia.; Qingbaikou, China; basins of Northwest ), following ®nal cratonization of polymetamorphic mobile belts, below.

STENIAN À1200 Ma Stenos = narrow ªNarrow belts of intense metamorphism & deformationº Narrow polymetamorphic belts, characteristic of the mid-Proterozoic, separated the abundant platforms and were orogenically active at about this time (e.g., Grenville, Central Australia)

ECTASIAN À1400 Ma Ectsis = extension ªContinuedexpansion of platform coversº Platforms continue to be prominent components of most shields

CALYMMIAN À1600 Ma Calymma = cover ª`Platform coversº Characterised by expansion of existing platform covers, or by new platforms on recently cratonized (e.g. Riphean of Russia)

STATHERIAN À1800 Ma Statheros = stable, ®rm ªStabilisation of ; Cratonizationº This period is characterized on most by either new platforms (e.g. North China, North Australia) or ®nal cratonization of fold belts (e.g. , )

OROSIRIAN À2050 Ma Orosira = mountain range ªGlobal orogenic periodº The interval between about 1900 Ma and 1850 Ma was an episode of on virtually all continents;

RHYACIAN À2300 Ma Rhyax = stream of ªInjection of layeredcomplexesº The Bushveld Complex (and similar layered intrusions) is an outstanding event of this time; the age of the Bushveld seems unlikely to change dramatically

SIDERIAN À2500 Ma Sideros = ªBanded iron formationsº The earliest Proterozoic is widely recognised for an abundance of BIF, which peaked just after the Archaean-Proterozoic boundary EHI 7(2004) 37 LETHAIA LETHAIA 37 (2004) Divisions of the International Geologic Time Scale 197

Paleozoic era classi®cation and correlation. The well established, but independent, regional subdivisions in Europe, Russia, The Cambrian system/period has undergone radical North America, and elsewhere are dif®cult to corre- changes in its de®nition and subdivisions during the late. For the international standard series and stages, decade, but it will require another few years until the Carboniferous subcommission decided to aban- an international set of stages are pinned by GSSPs. don the western Europe system of ± Once regarded as beginning with the earliest preserved ±, and adopt a Russian-based shells of , the base of the Cambrian is now nomenclature for the Carboniferous stages within de®ned at a GSSP associated with the ®rst occurrence each series. The potential GSSP for each stage will be of the activity of complex metazoans in the form of coincident with widespread conodont or fusulinid widespread characteristic feeding traces (ichnofossil appearance. Currently, only the major division of the Tricophycus pedum). This level is approximately Carboniferous into the Mississippian and Pennsylva- coincident with a brief but pronounced negative nian `sub-systems/sub-periods' has been de®ned by a excursion in carbon isotopes. The Cambrian interval GSSP. has numerous regional subdivisions based on facies The Permian has a tripartite division ± the lower patterns and endemic fauna, but most of these are not Cisuralian series/epoch will have four GSSPs in Russia conducive for global usage. However, in the Middle and Kazakhstan, the middle series/epoch and the Late Cambrian, global biostratigraphic mar- has placed three stage GSSPs in Texas, and the upper kers, such as those based on pandemic and pelagic Lopingian series has assigned two stage GSSPs in agnostoid trilobites, are present (Geyer and Shergold China. All GSSPs are coincident with conodont 2000), and the global subdivision of these appearances. epochs of the Cambrian will have GSSPs coinciding with a subset of these levels. At present, only the base of the upper Cambrian `Furongian' epoch/series (base Mesozoic era of Paibian stage) is de®ned by a GSSP. Existing regional suites of series and stages for the Most of the traditional stages of the Triassic (Anisian, Ordovician were also found to be unsatisfactory for Ladinian, Carnian, Norian, Rhaetian) were named global division. The Ordovician subcommission has from ammonoid-rich successions of the Northern identi®ed a set of widespread biostratigraphic datums Calcareous Alps of Austria. However, the stratigraphy using graptolites and conodonts and then used these of these Austrian tectonic slices proved unsuitable for events for assigning GSSPs for global subdivision. The establishing formal boundary stratotypes, or even resulting international stages are being named accord- deducing the sequential order of the stages (Tozer ing to their approximate correspondence to the extent 1984). The general lack of unambiguous historical of regional stages (e.g. Tremadocian and Hirnantian precedents for placement of Triassic stage boundaries, stages of British regional succession, Darriwilian of coupled with dif®culties in achieving consensus on Australian succession). global correlation horizons, have retarded establish- The Silurian subdivisions were mainly ®xed in 1980 ment of formal GSSPs. The base of the Triassic (and and 1984 by assigning GSSPs to a combination of Mesozoic) at the GSSP in China coincides with the graptolite datums and classical lithology-based stages recovery from the end-Permian mass extinctions as (and nomenclature) within southwest England and indicated by the near-global occurrence of a conodont Wales (Holland & Bassett 1989). However, some of and the termination of a major negative carbon- these levels have either lacked a precise correlation isotope excursion. These basal-Triassic events occur horizon or proved challenging to achieve a global signi®cantly after the peak of Late Permian extinc- correlation (e.g. bases of Wenlock and Ludlow series). tions. The Triassic stages will retain the Alpine Therefore, the Silurian subcommission is reviewing nomenclature, but await decisions on the primary some of these GSSP assignments. correlation horizons using a combination of conodont Devonian, Carboniferous and Permian subdivisions and ammonoid criteria. have mainly utilized conodonts for global correlation Ammonites have provided a high-resolution corre- horizons associated with rati®ed or potential GSSPs. lation and subdivision of Jurassic strata throughout The Devonian stages are fully delimited by GSSPs the globe (e.g. Arkell 1956). The bases of nearly all distributed in Czech Republic, France, Germany, Jurassic stages and substages are traditionally assigned Uzbekistan and Morocco. to the base of ammonite zones in marginal-marine Even through the Carboniferous was one of the ®rst sections in western Europe (e.g. Oppel 1856±1858), established geological periods; it is one of the most and the current or potential GSSPs have retained this complicated and confusing in terms of stratigraphic close tie to European ammonite markers. The identi- 198 James G. Ogg LETHAIA 37 (2004)

®cation of GSSPs within the upper Jurassic has been base of the Middle Pleistocene tentatively assigned as hindered by a pronounced latitudinal segregation into the base of a magnetic reversal, and the base of the Tethyan, Boreal and other faunal realms, which have Upper Pleistocene to coincide with the inter-glacial frustrated global correlation for over a century. climate interval at about 126ka. The base of the This faunal provincialism, especially for ammonites, Holocene epoch/series will probably be chronometri- continued through the , thereby cally de®ned by a GSSA (not GSSP) at the termination complicating decisions on GSSPs for the base-Cretac- of the Younger Dryas cold interval at 10,000 `C-14 eous (Berriasian) and the other ®ve Lower Cretaceous years' or 11,500 calendar years before `Present', where stages. As a result, a calpionellid () marker `Present' is ®xed as 1950 AD. is suggested for base-Valanginian, and magnetic reversal horizons have been suggested as primary correlation criteria for GSSPs for base-Cretaceous Conclusion (Chron M19n) and base-Aptian (Chron M0r). GSSPs for the have included a diverse set of It was not until the establishment of the International palaeontological groups, including planktonic fora- Commission on Stratigraphy (ICS) by the Interna- minifers, pelagic , inoceramid bivalves, tional Union of Geological Sciences (IUGS) that the ammonites and . goal of establishing an international chronostrati- graphic scale had a means of ful®lment. The GSSP concept of de®ning the base of each successive unit at a Cenozoic era level that coincided with potential markers for global correlation has required the concerted efforts of The division of the Cenozoic into the Paleogene and paleontologists, geochemists, paleomagnetists, cycle Neogene periods/systems abandons the formal use of stratigraphers and other specialists, combined with `' and `', which were residual international consensus. The goal of the resulting concepts from when all Earth history consisted of stratigraphic divisions of the International Geologic only four major divisions. However, the `Quaternary' Time Scale is a global language for Earth history. The is retained as a valuable climate-stratigraphic term for international standard developed by ICS is formally the past 2.6myr that was dominated by major glacial published in conjunction with the International oscillations, and therefore encompasses the Holocene, Geological Congress every four years; but current Pleistocene and latest Pliocene epochs. updated tables and charts (PDF format) are freely The epoch/series divisions of the Paleogene period/ available from the ICS website, www.stratigraphy.org. system are de®ned by GSSPs associated with an The complete suite of GSSP-de®ned units will be in iridium anomaly caused by the end-Cretaceous impact place by 2008. (base-Paleocene), a brief negative carbon-isotope excursion (base-Eocene), and the extinction of a Acknowledgements. ± Portions of this compilation was condensed planktonic foraminifer (base-Oligocene). Stages from information provided by the subcommissions of the Inter- within each of these epochs/series have not yet been national Commission on Stratigraphy and by the authors of forthcoming chapters of the Geologic Time Scale 2004 (Gradstein, formally de®ned by GSSPs, nor have primary correla- Ogg, Smith et al. 2004). Drafts of this article were reviewed by tion criteria been selected. However, these GSSPs will Felix Gradstein and Gabi Ogg. This material is based upon work potentially utilize calcareous nannofossil and/or supported by the National Science Foundation under Grant No. 0313524. planktonic foraminifer datums and magnetic rever- sals. Each GSSP within the Neogene period/system has been calibrated to multiple correlation horizons References (microfossil, magnetic, stable isotope or astronomical Arkell, W.J. 1956: Jurassic Geology of the World, 806pp. Oliver & cycle). In addition, the GSSPs have been chosen to Boyd Ltd, Edinburgh. allow direct calibration to the astronomical `Milanko- Bleeker, W. 2004: Towards a `natural' time scale for the Precambrian ± a proposal. Lethaia 37, 219±222. vitch' cyclicity, hence directly to absolute age. The base Gradstein, F.M. & Ogg, J.G. 2004: Geologic Time Scale 2004 ± why, of the Pleistocene epoch/series through the base of the how, and where next!. Lethaia 37, 175±181. upper Miocene have these calibrated GSSPs. The Gradstein, F.M., Ogg, J.G., Smith, A.G., Agterberg, F.P., Bleeker, W., Cooper, R.A., Davydov, V., Gibbard, P., Hinnov, L.A., Pleistocene has a high-resolution -isotope House, M.R. (²),, Lourens, L., Luterbacher, H-P., McArthur, J., `stage' nomenclature; therefore, to avoid confusion, a Melchin, M.J., Robb, L.J., Shergold, J., Villeneuve, M., Wardlaw, new set of chronostratigraphic `stages' de®ned by B.R., Ali, J., Brinkhuis, H., Hilgen, F.J., Hooker, J., Howarth, R.J., Knoll, A.H., Laskar, J., Monechi, S., Powell, J., Plumb, K.A., GSSPs will be avoided. Instead, the Pleistocene will be Raf®, I., RoÈhl, U., San®lippo, A., Schmitz, B., Shackleton, N.J., subdivided into `sub-series/sub-epoch' units, with the Shields, G.A., Strauss, H., Van Dam, J., Veizer, J., van LETHAIA 37 (2004) Divisions of the International Geologic Time Scale 199

Kolfschoten, Th. & Wilson, D. 2004 in press: A Geologic Time forschende Jahreshefte Hefte 12±14, iv ‡ 857 pp. Ebner & Seubert, Scale 2004. 500 pp. Cambridge University Press, Cambridge. Stuttgart ( publishedas a series of 3 volumes through 1858) . Geyer, G. & Shergold, J. H. 2000: The quest for internationally Plumb, K.A. 1991: New Precambrian time scale. Episodes 14, 139± recognised divisions of Cambrian time. Episodes 23, 188±195. 140. Tozer, E.T. 1984: The Trias and its Ammonites: The Evolution of a Holland, C.H. & Bassett, M.G. (eds) 1989: A Global Standard for the Time Scale. Geological Survey of Canada, Miscellaneous Report, Silurian System. National Museum of Wales, Geological Series 9, 35, 171 pp. 325 pp. Walsh, S.L., Gradstein, F.M. & Ogg, J.G. 2004: History, philosophy, Oppel, C.A. 1856: Die Juraformation Englands, Frankreichs und and application of the Global Stratotype Section and Point des SuÈdwestlichen Deutschlands. WuÈrttemberger Natur- (GSSP). Lethaia 37, 199±217.