revised correlation of silurian provincial series of north ... · advancement at the end of the...

Revised correlation of Silurian Provincial Series of NorthAmerica with global and regional chronostratigraphic unitsand d13Ccarb chemostratigraphy

BRADLEY D. CRAMER, CARLTON E. BRETT, MICHAEL J. MELCHIN, PEEP MANNIK, MARK A. KLEFF-

NER, PATRICK I. MCLAUGHLIN, DAVID K. LOYDELL, AXEL MUNNECKE, LENNART JEPPSSON, CARLO

CORRADINI, FRANK R. BRUNTON AND MATTHEW R. SALTZMAN

Cramer, B.D., Brett, C.E., Melchin, M.J., Mannik, P., Kleffner, M.A., McLaughlin, P.I.,Loydell, D.K., Munnecke, A., Jeppsson, L., Corradini, C., Brunton, F.R. & Saltzman,M.R. 2011: Revised correlation of Silurian Provincial Series of North America with globaland regional chronostratigraphic units and d13Ccarb chemostratigraphy. Lethaia, Vol. 44,pp. 185–202.

Recent revisions to the biostratigraphic and chronostratigraphic assignment of stratafrom the type area of the Niagaran Provincial Series (a regional chronostratigraphic unit)have demonstrated the need to revise the chronostratigraphic correlation of the SilurianSystem of North America. Recently, the working group to restudy the base of the Wen-lock Series has developed an extremely high-resolution global chronostratigraphy for theTelychian and Sheinwoodian stages by integrating graptolite and conodont biostratigra-phy with carbonate carbon isotope (d13Ccarb) chemostratigraphy. This improved globalchronostratigraphy has required such significant chronostratigraphic revisions to theNorth American succession that much of the Silurian System in North America is cur-rently in a state of flux and needs further refinement. This report serves as an update ofthe progress on recalibrating the global chronostratigraphic correlation of North Ameri-can Provincial Series and Stage boundaries in their type area. The revised North Ameri-can classification is correlated with global series and stages as well as regionalclassifications used in the United Kingdom, the East Baltic, Australia, China, the Barran-dian, and Altaj. Twenty-four potential stage slices, based primarily on graptolite andconodont zones and correlated to the global series and stages, are illustrated alongside anew composite d13Ccarb curve for the Silurian. Conodont, graptolite, isotope, New York,Ontario, series, Silurian, stage.

Bradley D. Cramer [[email protected]], Division of Earth History, School of Earth Sci-ences, The Ohio State University, 125 S. Oval Mall, Columbus, Ohio 43210, USA, now Kan-sas Geological Survey, Department of Geology, University of Kansas, 1930 Constant Avenue,Lawrence, Kansas 66047, USA; Carlton E. Brett [[email protected]], Department ofGeology, University of Cincinnati, 2624 Clifton Avenue, Cincinnati, Ohio 45221, USA;Michael J. Melchin [[email protected]], Department of Earth Sciences, St. Francis XavierUniversity, 1 West Street, Antigonish, Nova Scotia B2G 2W5, Canada; Peep Mannik [[email protected]], Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, Tallinn19086, Estonia; Mark A. Kleffner [[email protected]], Division of Earth His-tory, School of Earth Sciences, The Ohio State University at Lima, 4240 Campus Drive,Lima, Ohio 45804, USA; Patrick I. McLaughlin [[email protected]], Wisconsin Geo-logical and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705,USA; David K. Loydell [[email protected]], School of Earth and Environmental Sci-ences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth P01 3QL,UK; Axel Munnecke [[email protected]], GeoZentrum Nordbayern,Fachgruppe Palaoumwelt, Universitat Erlangen, Loewenichstraße 28, Erlangen D-91054,Germany; Lennart Jeppsson [[email protected]], Department of Geology, GeoBio-sphere Science Centre, Lund University, Solvegatan 12, Lund SE-223-62, Sweden; CarloCorradini [[email protected]], Dipartimento di Scienze della Terra, Universita di Cagliari,via Trentino 51, Cagliari I-09127, Italy; Frank R. Brunton [[email protected]], Sed-imentary Geoscience Section, Ontario Geological Survey, Ministry of Northern Development,Mines and Forestry, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, Canada; MatthewR. Saltzman [[email protected]], Division of Earth History, School of Earth Sciences,The Ohio State University, 125 S. Oval Mall, Columbus, Ohio 43210, USA; manuscriptreceived on 9 ⁄ 2 ⁄ 2010; manuscript accepted on 31 ⁄ 5 ⁄ 2010.

The Silurian System was the first to have a globallyapplicable classification of series and stages when thepresent definitions of the four Silurian series

(Llandovery, Wenlock, Ludlow, and Pridoli) andseven stages (Rhuddanian, Aeronian, Telychian,Sheinwoodian, Homerian, Gorstian, and Ludfordian)

DOI 10.1111/j.1502-3931.2010.00234.x � 2010 The Authors, Journal compilation � 2010 The Lethaia Foundation

were formalized by the International Subcommissionon Silurian Stratigraphy (ISSS) more than 25 yearsago (Holland 1984; Bassett 1985). Stable chronostrati-graphic nomenclature and the comparatively cosmo-politan nature of Silurian marine fauna have led mostregional stratigraphic bodies to abandon regionalchronostratigraphic terms in favour of the global Silu-rian series and stages ratified by the ISSS. NorthAmerica is among the last examples where regionalchronostratigraphic terms (particularly at the serieslevel) are still in use (Fig. 1).

As early as 1970, it was becoming clear that corre-lation within North America accomplished usingNorth American Provincial Series and Stages (e.g.Alexandrian, Niagaran, and Cayugan series) had beenproblematical and calls for the adoption of the globalseries and stages for the Silurian of North America hadbegun (Berry & Boucot 1970). Decades of misuse ofstratigraphic terms throughout North American Silu-rian literature, and the inconsistency between Silurianchronostratigraphic terms used by the United StatesGeological Survey (USGS), the American Associationof Petroleum Geologists (AAPG), the GeologicalSurvey of Canada (GSC), and the Ontario GeologicalSurvey (OGS), have adversely impacted the utility andmeaning of these regional chronostratigraphic terms.The purpose of this report is to show how commonly

used regional terms from North America correlatewith each other and to show how recent findingschange their placement within the global chronostrati-graphic scheme. In addition, we present an improvedcomposite d13Ccarb curve for the Silurian. Finally, fol-lowing the lead of the International Subcommissionon Ordovician Stratigraphy (Bergstrom et al. 2009),we informally introduce potential subdivisions of theglobal Silurian stages that are biochemostratigraphical-ly defined, referred to as stage slices. It is important tonote that this report does not constitute ISSS supportfor or official revision of any regional stratigraphicscheme. The authors are in agreement with Berry &Boucot (1970), Norford (1997), and many others thatthe global series and stages should be used for theNorth American Silurian succession.

Global correlation of Silurianchronostratigraphic units

Silurian biostratigraphy underwent a period of majoradvancement at the end of the last century (late1990s) with the publication of a generalized graptolitezonation (Koren’ et al. 1996; Loydell 1998) for theentire Silurian as well as major revisions to Silurianconodont taxonomy and biostratigraphy during a

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Fig. 1. Chronostratigraphic chart showing proposed correlation between the revised North American classification and global andregional series and stages presently in use from regions of major Silurian outcrop. The GSSP boxes show the location of the Global BoundaryStratotype Section and Point for the global Silurian series and stages.

186 Cramer et al. LETHAIA 44 (2011)

span of just a few years (Jeppsson 1997; Mannik1998). The current Silurian timescale (Ogg et al.2008) assumes a correlation between stage (and there-fore series and systemic) boundaries and graptolitezones and as a result graptolite-bearing successionscan be correlated directly to the global chronostrati-graphic classification. Conodont-bearing successionshave traditionally been more difficult to correlate pre-cisely with the global scheme, but significant improve-ments to Silurian conodont biostratigraphic zonationshave begun to alleviate this problem (e.g. Jeppsson1997, 2005; Mannik 1998, 2007; Jeppsson & Aldridge2000; Jeppsson & Calner 2003; Jeppsson et al. 2006;Corriga & Corradini 2009). Major advancements incorrelation of conodont and graptolite zones (e.g.Loydell et al. 1998, 2003) and their integration withcarbonate carbon isotope d13Ccarb chemostratigraphyas a chronostratigraphic tool (Kaljo et al. 2003, 2007;Munnecke et al. 2003; Cramer et al. 2006a,b; Kaljo &Martma 2006; Melchin & Holmden 2006; Jeppssonet al. 2007) have made it possible to correlate someintervals of the Silurian globally and with precision farfiner than the stage level (Cramer et al. 2010). Adetailed discussion of global chronostratigraphic cor-relation at the stage level (and finer) is beyond thescope of this report and below we include a few briefnotes as updates on Silurian chronostratigraphic issuesfrom selected regions worldwide.

United Kingdom

Seven of the eight Silurian Global Boundary Strato-type Section and Point (GSSP) ‘golden spikes’ arelocated in the United Kingdom, which means thatBritish Silurian chronostratigraphy is necessarily cou-pled to the global chronostratigraphic classification(the exception is the base of the Pridoli Series). Arecent compilation produced for the British Geologi-cal Conservation Review Series (Aldridge et al. 2000)combined with the seminal work of Holland & Bassett(1989) provide an excellent overview of British Silu-rian stratigraphy and the reader is referred to theseworks for more detailed information. The results oftwo working groups of the ISSS to restudy Silurianchronostratigraphic boundaries have recently revisedor suggested revision to the biostratigraphic correla-tion of the base of the Silurian System and the base ofthe Wenlock Series respectively, and their findings aresummarized briefly here.

The new biostratigraphic definition of the base ofthe Silurian System proposed by Melchin & Williams(2000), as coincident with the first appearance of thegraptolite Akidograptus ascensus and the base of the A.ascensus graptolite Zone, 1.6 metres above the base ofthe Birkhill Shale at the GSSP, was accepted by the

ISSS in 2006 and ratified by the International Unionof Geological Sciences (IUGS) in 2007 (see Rong et al.2008). The GSSP for the base of the SheinwoodianStage and Wenlock Series was originally stated tocoincide with the first appearance of the graptoliteCyrtograptus centrifugus (Bassett et al. 1975) and thebase of the C. centrifugus graptolite Zone. Based uponinformation not yet available when the base WenlockGSSP was ratified (Martinsson et al. 1981), it is nowclear that the position of the GSSP is significantlyabove the first appearance of C. centrifugus (e.g.Mabillard & Aldridge 1985; Loydell et al. 2003;Mullins & Aldridge 2004; Cramer et al. 2010). There-fore, the ISSS was faced with a decision, either: (1) tomove the GSSP to a location known to be coincidentwith the base of the C. centrifugus graptolite Zone; or(2) retain the current GSSP and redefine the biostrati-graphic correlation of this position.

In a report to the ISSS, the working group to rest-udy the base of the Wenlock Series suggested that theGSSP for the base of the Wenlock Series be retained,but that the biostratigraphic correlation, as coincidentwith the base of the C. centrifugus graptolite Zone,needed to be revised (Loydell 2008). There are twopotential new levels (one graptolite and one cono-dont) for the base of the Wenlock Series: (1) correlat-ing to a position at or slightly above the base of the C.murchisoni graptolite Zone, marked by the firstappearance of C. murchisoni (a full zone higher thanthe original position); or, (2) close to the IrevikenEvent Datum 2, the base of the Upper Pseudooneoto-dus bicornis conodont Zone, marked by the lastappearance of the conodonts Ozarkodina polinclinatapolinclinata, Apsidognathus ruginosus, and A. walmsleyi(see Jeppsson 1997). Recent integration of detailedglobal graptolite and conodont biostratigraphy withhigh-resolution d13Ccarb chemostratigraphy surround-ing the base of the Wenlock Series has documentedthe global utility of these definitions and suggestedthat these two positions (base of the C. murchisoniZone and Ireviken Event Datum 2) are likely sepa-rated by less than 100 kyr (Cramer et al. 2010).Therefore, following Loydell (2008), Cramer et al.(2010) considered the base of the C. murchisoni grap-tolite Zone to correlate with the base of the WenlockSeries.

The most recent iteration of the Geological TimeScale (Ogg et al. 2008) also recognized that the GSSPcorrelates with this biostratigraphic position (theirtable 6.1), but in their correlation chart (their fig. 6.4)showed the base of the Wenlock at the base of a com-bined C. centrifugus–C. murchisoni graptolite Zone.An official position regarding revisions to the base ofthe Wenlock Series has yet to be reached by the ISSS,and although the de facto correlation of the GSSP is

LETHAIA 44 (2011) Silurian chronostratigraphic correlation 187

currently at a level that approximates the base of theC. murchisoni graptolite Zone, that is subject tochange should the ISSS choose in the future to select anew GSSP that coincides with the originally intendedbiostratigraphic level (i.e. the base of the C. centrifugusgraptolite Zone).

Silurian units smaller than the stage level have beenformally described for the Homerian Stage of the Wen-lock Series from the Welsh Borderlands (Bassett et al.1975). The two Homerian divisions, Gleedon and Whit-well, although not officially recognized by the ISSS, arestill occasionally found in print (e.g. Jeppsson et al.2006).Thefour-stagedivisionoftheLudlowSeries(Elto-nian, Bringewoodian, Leintwardian, and Whitcliffian)defined by Holland et al. (1963) was replaced by the late1970s with the present two stage (Gorstian and Ludfor-dian) division. Because the type localities for all six termsare located in the United Kingdom, the older terms wereabsorbed by the present global classification. Theseregional terms have been largely abandoned and are indisuse, as is the term, Downton, which had been pro-posed as the fourth Silurian Series (Bassett et al. 1982)andforatimewascommon(sometimesasDowntonian)in UK Silurian literature. Following the suggestions ofthe ISSS, the term Pridoli has been used in the UnitedKingdomformorethan20 years.

North America

The Silurian succession of North America has beendivided into a complex array of chronostratigraphicunits and schemes over the past 150 years. Varying useof classical terms such as Alexandrian, Niagaran, andCayugan series, combined with ‘official’ use of theunofficial terms Lower, Middle, and Upper Silurian,have crippled international correlation of North Amer-ican Silurian units. Here we have included the relevantportion of the current AAPG–COSUNA (Correlationof the Stratigraphic Units of North America) chart(Shaver et al. 1985), the recently revised Silurian termsin use by the OGS (Brunton 2009), and the Silurianterms utilized by the USGS (Brett et al. 1995) for com-parison in Figure 2. A revised correlation of the USGSclassification scheme is presented here (far right panelin Fig. 2) and represents the North American columnshown in Figure 1. Terms such as Lockport were for-mally introduced as lithostratigraphic terms (LockportFormation or Lockport Group, see Brett et al. 1995)but have been misused for decades as stage level termsthroughout North America (e.g. Lockportian inFig. 2). To compare these terms and their use with glo-bal and regional classifications, we follow the common(incorrect) treatment of these terms as being chrono-stratigraphic for this report. Again, we point out thatthese regional series and stage names should be

abandoned in favour of the global classification. TheGeological Survey of Canada has demonstrated thatthe global Silurian series and stages can be recognizedthroughout the Canadian succession (Norford 1997)and use of regional terms within Canada is now largelyrestricted to the Ontario region. Work is currentlyunderway to identify properly the global Silurian seriesand stage boundaries within Ontario and the GreatLakes region (Bancroft 2008; Brunton 2008, 2009;Brunton et al. 2009) and the United States as well(Saltzman 2002; Bergstrom et al. 2006; Cramer et al.2006a; b; McLaughlin et al. 2008; Barrick et al. 2009;Kleffner et al. 2009).

The classic term Alexandrian Series has beenapplied traditionally to an interval spanning from thebase of the Silurian System to somewhere in the mid-dle of the Llandovery Series and remains pervasive inNorth American literature (e.g. AAPG-COSUNA).However, revisions to the North American timescaleand recent biochemostratigraphic data have combinedto make the Alexandrian Series a meaningless term.The original type area of the Alexandrian is located inthe American mid-continent (SW Illinois and SE Mis-souri) and covers an interval from the base of theGirardeau Limestone to the top of the BrassfieldLimestone. Bergstrom et al. (2006) demonstrated thatthe base of the Girardeau Limestone correlates to aposition within the lower part of the global Hirnan-tian Stage, which by necessity would force the base ofthe Alexandrian Series down into the Ordovician. Thetop of the Alexandrian Series contains strata (Brass-field Limestone) that are undoubtedly of Silurian age(Berry & Boucot 1970) and likely belong to the Aero-nian Stage. Therefore in present global terms, the clas-sical Alexandrian Series would cover an interval fromthe Hirnantian Stage to the Aeronian Stage and as aseries would cross a systemic boundary. This wasavoided in the present definition of the North Ameri-can Ordovician System by extending the term Cin-cinnatian Series to the top of the Ordovician(Bergstrom et al. 2009). Similarly, Brett et al. (1995)placed the base of the previously overlying NiagaranSeries at the base of the Silurian System, and thus ren-dered the term Alexandrian Series obsolete.

The Niagaran Provincial Series, as defined by Brettet al. (1995) from the Niagara region of western NewYork State was thought to encompass all of the Llan-dovery Series, all of the Wenlock Series, and most ofthe Ludlow Series. The Niagaran Series is equivalentto the antequated Lower and Middle series tradition-ally used by the OGS (Johnson et al. 1992 – but aban-doned by Brunton 2009) in that both systems ofnomenclature include all Silurian strata below the Sal-ina Group. Brett et al. (1995) include three groupswithin the Niagaran Series (Medina, Clinton, and


Lockport groups). Brett et al. (1995) and Brunton(2009) provide a detailed correlation between theOGS classification and the USGS classification of thisinterval, and we restrict ourselves here to discussionand refinement of the USGS terms unless otherwiseindicated. At present only the base of the LockportGroup has been precisely correlated with the globalchronostratigraphic classification and global correla-tions of the remaining units (Medina, Clinton, Salina,etc.) require further refinement. Major researchprojects are currently underway to constrain moreprecisely the remaining North American regionalboundaries and a few notes of update are includedhere.

The base of the Clinton Group was placed histori-cally at the base of the Thorold Sandstone in NewYork (Gillette 1947) and continued to be placed at thislevel by the Ontario Geological Survey (Johnson et al.

1992), such that the OGS Cataract Group + ThoroldSandstone = USGS Medina Group (Fig. 2). However,the base of the Clinton Group was elevated to the baseof the Neahga Shale by the USGS (Brett et al. 1995)because the latter unit was found to be separated fromthe Thorold Formation by a regionally angular uncon-formity in western New York and Ontario, whereasthe Thorold Formation is underlain, and locally over-lain, conformably with red mudstones and sandstonestypical of the Medina Group. A similar position(above the Thorold Formation) has been adopted bythe OGS (Brunton 2009). Here, we restrict our use ofthe term ‘Clinton Group’ to the USGS (Brett et al.1995) sense as placing the base of the Clinton Groupat the base of the Neahga Shale and including theThorold Sandstone within the underlying MedinaGroup. In westernmost New York and adjacentOntario the Clinton Group includes in stratigraphic

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Fig. 2. Chronostratigraphic chart showing the relationship between the North American Silurian chronostratigraphic terms in use by theAAPG, OGS, and USGS (three central columns) with global series and stages (far left column) as well as the revised correlation of the USGSterms with the global series and stages (far right column). We acknowledge that the term ‘Group’ (a lithostratigraphic term) is out of placeon a ‘Chronostratigraphic Chart’. Lithostratigraphic terms referring to rock units at formation and group level historically have been misap-plied as chronostratigraphic terms throughout North America for decades (e.g. Lockport Formation or Group vs. Lockportian Stage in theAAPG column). Because these terms are embedded with a chronostratigraphic context in North American literature, they are shown here(incorrectly) in their chronostratigraphic context (see text for detailed discussion). The AAPG column is from Shaver et al. (1985), the Llan-dovery–Wenlock portion of the OGS column is from Brunton (2009), and the Ludlow–Pridoli portion of the OGS column is modified fromJohnson et al. (1992) using Brunton (2009) and Norford (1997), and the USGS column is after Brett et al. (1995). For Ontario, Johnson et al.(1992) used the general terms ‘Lower’ and ‘Upper’ instead of Provincial Series names such as ‘Cayugan’, but these general terms were recentlyabandoned by the OGS (Brunton 2009). Brunton (2009) did not address strata above the Guelph Formation, but the replacement of generalterms (Lower and Upper) by Provincial Series terms would therefore require the assignment of all strata above the base of the Salina Groupto the Cayugan Series. The revised correlation of the USGS terms (far right column) was included in Figure 1. AAPG-COSUNA – AmericanAssociation of Petroleum Geologists-Correlation of Stratigraphic Units of North America. OGS – Ontario Geological Survey. USGS – UnitedStates Geological Survey.


order the Neahga, Reynales, Merritton (upper part ofthe Fossil Hill Formation in Ontario, e.g. Brintnellet al. 2009; Brunton 2009), Williamson, Rockway,Irondequoit, Rochester, and Decew formations. Incentral New York the Clinton is much more completeand includes in ascending order the Reynales or equiv-alent Bear Creek Shale, Lower and Upper Sodus, Wal-cott, Sauquoit, Williamson-Willowvale, Rockway,Irondequoit, Rochester or equivalent Herkimer, andDecew formations.

The base of the Wenlock Series is likely locatedwithin, or at the top of, the Rockway Formation as thebase of the Irondequoit Formation clearly coincideswith the lower part of the Sheinwoodian Stage(Cramer et al. 2006a, 2010), although the preciseposition has yet to be determined. The lowermostWilliamson Formation, at the type section in Roches-ter, New York, contains the graptolite Stimulograptusclintonensis as well as the conodont Pterospathodusamorphognathoides angulatus clearly indicative of aposition no higher than the middle part of theTelychian Stage (Loydell et al. 2007). According toKleffner in McLaughlin et al. (2008, p. 124), the con-odonts from the Merritton Formation, referred to asbelonging to the Neospathognathodus celloni conodontZone by Rexroad (1970, though the genus is no longerin use and zone redefined), likely belong to the Pteros-pathodus eopennatus Super Zone of Mannik (2007).The conodont fauna and the position of the Merrittonbelow the Williamson indicate that the Merrittoncorrelates with the lower part of the Telychian Stage.

The base of the Merritton overlies a major unconfor-mity at which underlying Reynales and Neahga forma-tions are truncated westward into Ontario. Thisdisconformity appears to be coextensive with a regio-nal, angular unconformity at which the Sauquoit, Wol-cott, Upper Sodus, Lower Sodus, and Reynales aresuccessively removed from east to west commencing incentral New York. In westernmost New York–south-western Ontario this unconformity separates the Nea-hga and Reynales formations from the rest of theClinton Group. The assignment of these lower Clintonunits to the Aeronian Stage by Brett et al. (1995) is sup-ported by the presence of the conodont Pranognathustenuis throughout the Reynales Formation and upperpart of the Neagha Formation in New York (Kleffner2004). Rhuddanian–Aeronian conodont biozonationsrequire significant improvement and for the moment itis unclear where the base of the Telychian should beplaced in North American carbonate successions if it ispreserved at all. It is clear, however, that the baseof the Telychian must lie somewhere between the topof the Reynales and the base of the Merritton. The baseof the Clinton Group, therefore, correlates with a posi-tion no higher than the Aeronian Stage.

The term ‘Lockport’ was first introduced by Hall(1839) at the rank of formation. Rickard (1975) raisedthe term to group status and this assignment wasmaintained by Brett et al. (1995) for the USGS. Theterm Lockport has continued to be occasionally usedat the formational level in Ontario (e.g. Johnson et al.1992), but Brunton (2009) has abandoned this prac-tice. Based upon conodont biostratigraphy andd13Ccarb chemostratigraphy from the type area of theNiagaran Series, Cramer et al. (2006a, 2010) dem-onstrated that the base of the Gasport Formation andtherefore the base of the Lockport Group coincideswith the middle part of the Sheinwoodian Stage(Fig. 2). The upper limit of the Lockport Group how-ever remains a source of confusion, both chronostrati-graphically and nomenclaturally. The USGS includesall strata between the base of the Gasport Formationand the base of the Salina Group within the LockportGroup (Brett et al. 1995). The OGS had traditionallyvaried from this viewpoint and removed the upper-most units (Guelph and Eramosa formations) fromthe Lockport Group (or Lockport Formation withGasport, Goat Island, and Eramosa members, Johnsonet al. 1992) and left them without a group designa-tion. Brunton (2008, 2009) demonstrated that theUSGS sense of a Lockport Group (as one that containsthe Gasport, Goat Island, Eramosa, and Guelph for-mations) can be extended throughout southernOntario, which illustrates that the Ontario terms‘Albemarle Group’ and Amabel Formation are redun-dant and should be abandoned. Here, we use the term‘Lockport Group’ in the recently uniform OGS–USGSsense as including all strata between the base of theGasport Formation and the base of the Salina Group.Conodonts indicative of the late Sheinwoodian (K.ortus ortus) have been found in strata belonging to theupper part of the Lockport Group (within the EramosaFormation, Bancroft 2008). The base of the HomerianStage can be correlated at the formation level through-out the mid-continent (e.g. Cramer et al. 2006b; Bar-rick et al. 2009), although its placement in theAppalachian Basin remains less clear. The base of theHomerian Stage however, likely correlates with a posi-tion in the lower part of the Guelph Formation.

The term ‘Salina’ was introduced by Dana (1863) asthe Salina Period to refer to the time represented bythe Guelph and overlying limestones and salts of thethen so-called Saliferous Epoch of New York State.After the labrythine evolution of the term in NewYork, it has come to be used at the group level (SalinaGroup) and has become virtually synonymous withthe Cayugan Provincial Series as well. The term ismore broadly known from what is called the SalinaGroup in the Michigan Basin that contains over 700metres of chiefly alternating salts and limestones,


however the term is defined by its use in New YorkState. As currently recognized by the USGS and theOntario Geological Survey, the Guelph Formation isincluded within the Lockport Group as its uppermostformation (Brett et al. 1995; Brunton 2009), whichplaces the Niagaran–Cayugan series contact at theLockport-Salina group contact.

The position of the base of the Ludlow Series withrespect to the base of the Cayugan Provincial Series iscurrently unknown. The next positive correlation inthe northern Appalachian Basin comes from therecovery of the Ludfordian conodont Polygnathoidessiluricus from the Vernon Formation in New York(Miller et al. 1988). Therefore the interval from thebase of the Homerian Stage to the base of the Ludfor-dian Stage is likely contained within the Guelph toVernon (upper Lockport–lower Salina) succession,and significant further research is required to refinethe chronostratigraphic classification of this intervalof the North American sequence. As a result we havetentatively placed the base of the regional CayuganSeries as coinciding with the base of the global LudlowSeries until further research can be conducted.

The use and meaning of group designations for theuppermost part of the Cayugan Series have beenhighly variable and it remains among the least chrono-stratigraphically understood intervals of the SilurianSystem in North America. The Cayugan Seriesincludes strata assigned to the Salina Group, BertieGroup (or Bertie Formation), and Bass Islands Group(or Bass Islands Formation). These strata appear to liewithin the upper Ludlow to Pridoli, although bio-stratigraphic control is very poor. The HelderbergGroup was previously considered to be entirely ofDevonian age but it now appears that the lower for-mations (e.g. Manlius and Coeymans) contain stratathat cross the base of the Devonian System (e.g. Kleff-ner et al. 2009). Based on conodont and d13Ccarb data,the base of the Devonian has begun to be refined inthe Appalachian Basin (e.g. Saltzman 2002) but recentfindings have highlighted the continuing uncertaintyof the position of this boundary in North America(Kleffner et al. 2009).

Eastern Baltic

A variety of stratigraphic classifications of the SilurianSystem of the eastern Baltic (particularly Estonia andLatvia) were in place by the 1970s (e.g. Aaloe et al.1976) and regional stages have been erected through-out the region. Referred to as ‘gorizont’ in Russianliterature (e.g. Adavere Regional Stage = AdaverskijGorizont [in Russian]), this term directly translates to‘horizon’, as in ‘sky line’. However, because in Eng-lish the term ‘stratigraphic horizon’ is a geological

term particularly noted for its thinness (e.g. Salvador1994) ‘gorizont’ is better translated as ‘regional stage’for geological use. Bassett et al. (1989) consideredthese terms to be regional stages and presented theircorrelation with global series and stages and graduallyit has become common for East Baltic studies toinclude global series and stages in addition to regionalstages in publication (e.g. Mannik 2007). The currentEast Baltic classification of a three-stage Llandovery,three-stage Wenlock, two-stage Ludlow, and two-stage Pridoli has changed little from what was pre-sented by Bassett et al. (1989). The correlation shownin Figure 1 for the East Baltic is modified slightlyfrom Nestor (1997) and a few notes are included here.

Traditionally, the base of the Telychian Stage hasbeen considered to lie in the lower part of the AdavereRegional Stage and to be coincident with the boundarybetween the Rumba and Velise formations (e.g. Nestor1997). However, a stratigraphic gap at this positionthroughout much of the East Baltic has obscured theprecise correlation between global and regional units inthis interval (Loydell et al. 1998; Poldvere 2003). Simi-larly, the Jaani Stage has been considered to be coinci-dent with the lower part of the Sheinwoodian Stage(the lower stage of the Wenlock Series), althoughincreasing evidence has begun to indicate that the baseof the Jaani Stage may be slightly below the base of theWenlock Series (Mannik 2007; Cramer et al. 2010).The base of the Jaagarahu Stage is within the globalSheinwoodian Stage (e.g. Kaljo & Martma 2006) andcorrelates with a position above the base of the Lock-port in North America. The base of the RootsikulaStage is within the global Homerian Stage (Loydellet al. 1998) and the base of the Paadla Stage is taken tobe coincident with the base of the Ludlow Series. Thebase of the Kuresaare Stage is clearly within the Ludfor-dian Stage and the Pridoli Series is divided into the Ka-ugatuma and Ohesaare regional stages. Refinements ofthese regional stages within the global classification areongoing, but whereas North American revisions arerequiring boundary positions to be moved by tens ofmetres, recent refinements to East Baltic boundarypositions are requiring only minor changes of a fewmetres at most (centimetres in most cases). With theexception (perhaps) of the United Kingdom, the EastBaltic is the chronostratigraphically best constrainedmajor Silurian region in the world.

Australia

At the time of publication of Holland and Bassett(1989 – A Global Standard for the Silurian System),the bio- and chronostratigraphic correlation of theSilurian System in Australia was known only in a gen-eral way, which prompted Jell & Talent (1989) in the


same volume to comment that the Australian Siluriansuccession was likely to be of limited global impor-tance. The work of Young & Laurie (1996) began theprocess of standardizing the Australian Silurianwithin the global classification. Later, Talent et al.(2003) presented a significantly different view fromthat of Jell & Talent (1989) and demonstrated thatthe Australian Silurian succession contains areas withmore than a kilometre of fossiliferous Silurian marinesedimentary rocks that can be easily tied to the globalscheme and can serve as an important source of datafrom outside of the well-known regions of Baltica,Laurentia, and Avalonia. A recent review by Strusz(2007) documented in detail the application of globalseries and stage boundaries to the Australian sequenceand no regional chronostratigraphic terms are cur-rently in use. Jeppsson et al. (2007) demonstratedhigh-resolution global correlation much finer thanthe stage level for the Ludlow portion of the Austra-lian sequence and it is likely that such resolution ispossible for the rest of the Silurian succession as well(Talent et al. 2003; Strusz 2007). Future research fromthis region may prove to be of considerable globalimportance.

China

A tripartite division of the Silurian System of Chinainto Lower, Middle, and Upper Series (Yin 1949)had been erected by the middle of the last centuryand Mu (1962) proposed the names Lungmachi, Loj-oping, and Hanchiatien for the Lower, Middle, andUpper Series respectively. Following the work of Geet al. (1979), the Lower Series was considered to beroughly equivalent to the Llandovery Series, the Mid-dle Series roughly equivalent to the Wenlock Series,and the Upper Series encompassed both the Ludlowand the Pridoli Series. Following the establishment ofa standard global Silurian chronostratigraphic classifi-cation, Mu et al. (1989) suggested that the Chineseregional terms should be abandoned in favour of theglobal classification. This decision followed Mu et al.(1986) and was supported by Rong & Chen (2003)who demonstrated unequivocally that the global Silu-rian classification can be readily identified within theChinese succession and regional stratigraphic termsare now largely in disuse. There is however, one his-torically significant feature of the Silurian System inChina that for some years after its ratification, thebase of the Silurian System continued to be placed atthe base of the Normalograptus persculptus graptoliteZone, which is the base of the uppermost zone of theOrdovician System in the global classification (seeeditorial note in Mu et al. 1989, p. 205). This practicewas formally abandoned by Chen et al. (1995).

Barrandian

Over the past 100 years, the only major addition tothe division of the Silurian System has been the addi-tion of the Pridoli Series as the fourth and final Silu-rian series (Bassett 1985). The Pridoli Series is uniquein the Phanerozoic as it is the only series recognizedby the IUGS that has not been divided into stages.The GSSP for the base of the Pridoli Series is alsounique in that it is the only Silurian GSSP located out-side of the United Kingdom. The GSSP is located inthe Pozary Section, near Prague, in the Czech Repub-lic (Krız 1989) and was placed to coincide with thefirst appearance of the graptolite Monograptus parulti-mus and the base of the M. parultimus graptoliteZone. Following Berdan et al. (1969), a four-fold divi-sion of the Barrandian Silurian System, essentiallyequivalent to the current global classification, wasadopted by Chlupac (1972) and by Krız (1975), andsince then, regional chronostratigraphic terms havebeen in disuse. Krız (1990) demonstrated that the glo-bal Silurian series and stages can be easily identified inthe highly graptolitic Barrandian succession. The lackof good conodont control for the Pridoli Series in thetype region as well as the United Kingdom howeverhas hindered global correlation of Pridoli strata in car-bonate sequences and significant improvement in ourunderstanding of latest Silurian conodont biostratigra-phy and palaeobiology is still needed.

Altaj

Silurian strata in the modern Altaj were deposited onthe tectonically passive south-western shelf of theSiberian Paleocontinent (Yolkin et al. 2003). Follow-ing the work of Kul’kov (1967), Yolkin & Zheltonog-ova (1974), Sennikov (1976), and several others, theSilurian succession in this region was divided intoseven formations that were shown to be allostrati-graphic units (Yolkin & Zheltonogova 1974). Recentstudies have demonstrated that these units are nearlycoeval across the Altaj region and are now consideredto be regional stages (gorizonts; e.g. Sennikov et al.2008). The Altaj column in Figure 1, and the correla-tions briefly discussed below are modified from Yolkinet al. (2003) and Sennikov et al. (2008) unless other-wise stated.

The base of the Syrovaty Formation (Stage) and thebase of the Gromotukha Series correlate to a positionwithin the uppermost Aeronian Stage, likely near thebase of the Stimulograptus sedgwickii graptolite Zone.It is unclear if this is equivalent to the base of the Ada-vere Stage of the East Baltic, but is likely above thebase of the Clinton Group in North America. Basedon conodont data of Moskalenko (1970), the base of


the Polaty Formation is no lower than the middle partof the Telychian Stage. The base of the ChesnokovkaFormation and the Tigerek Series has been correlatedwith the uppermost part of the Oktavites spiralis grap-tolite Zone in the upper part of the Telychian Stage.Above the lower part of the Chesnokovka Formationgraptolites do not occur in this region, and thereforethe positions of the remaining regional boundarieswithin the global scheme are less precise. Kul’kov(1967) demonstrated that the Chagyrka Formationlikely belongs to the upper part of the Wenlock Series.The base of the Kuimov may be coincident with thebase of the Ludlow Series and the base of the ChernyjAnui is correlated tentatively with the base of the Prid-oli Series.

Other regions

We have discussed only some regions with extensiveSilurian exposures although the mainland Europeansections in, for instance, Belgium, Sardinia, Germany,and Austria, have been of significant historical impor-tance as well. Their absence here, along with manyother regions, is simply the result of limited space inthis article and the fact that most of these otherregions now employ the global standard chronostrati-graphic classification. The reader is referred to thesummary of Krız et al. (2003) for data from classicalEuropean regions. In the same volume (Landing &Johnson 2003) good summaries of Silurian strata frommany other regions not discussed in this article areprovided as well. Another useful overview of the Silu-rian stratigraphy of Europe was provided by Vernierset al. (2008). It should be noted here that the excellentSilurian exposures on the Swedish Island of Gotlandspan an interval from the uppermost part of the Tely-chian Stage to the upper part of the Ludfordian Stageand serve as the type area for Wenlock and Ludlowconodont biostratigraphy and d13Ccarb chemostratig-raphy. It has been primarily through comparison withGotland that global correlation of Wenlock and Lud-low carbonate successions has begun to improve.Samtleben et al. (1996, 2000), Calner et al. (2004),Eriksson & Calner (2005), and Jeppsson et al. (2006)provide a summary overview of the lithostratigraphy,biostratigraphy, and chemostratigraphy of Gotland.

A new Silurian d13Ccarb

chemostratigraphic curve

Starting in the late 1980s to early 1990s, it was becom-ing apparent that the stable carbon isotopic ratio ofSilurian seas was highly variable and several discrete

features in the d13Ccarb curve began to be documented(Popp et al. 1986; Veizer et al. 1986; Corfield et al.1992; Wadleigh & Veizer 1992; Talent et al. 1993).The pioneering data produced from the Swedishisland of Gotland and the East Baltic region duringthe middle to late 1990s (Samtleben et al. 1996;Wenzel & Joachimski 1996; Bickert et al. 1997; Kaljoet al. 1997, 1998) demonstrated three positive d13Ccarb

excursions within the Wenlock to Ludlow interval,and these excursions have since been documentedworldwide (Saltzman 2001; Porebska et al. 2004;Cramer & Saltzman 2005; Noble et al. 2005; Crameret al. 2006a; b; Jeppsson et al. 2007; Kaljo et al. 2007),even in non-tropical settings (Lehnert et al. 2007).A fourth Silurian positive d13Ccarb excursion was alsofound to be associated with the Silurian–Devonianboundary interval in the type area of the Barrandian(Czech Republic) by Hladikova et al. (1997). Sincethen, this feature has been recognized in other parts ofthe world as well (Saltzman 2002; Buggisch & Mann2004), yet the d13Ccarb record of the Pridoli Seriesremains less well documented globally than that ofthe Wenlock and Ludlow Series. The most significantpositive d13Ccarb excursions appear to occur within theWenlock, Ludlow, and Pridoli Series however impor-tant isotopic features have been recognized within theLlandovery Series as well. Positive d13Ccarb excursionsduring the early Aeronian, late Aeronian, and earlyTelychian (each approximately +2.0& change) havebeen documented (Kaljo & Martma 2000; Kaljo et al.2003; Poldvere 2003; Melchin & Holmden 2006;Munnecke & Mannik 2009), but the Llandovery is theleast known portion of the Silurian d13Ccarb curve onthe global scale. At present, only the early Aeronianand early Telychian d13Ccarb excursions have been doc-umented from more than one paleobasin, althoughthe late Aeronian excursion has been documented ind13Corg records from multiple paleobasins.

Previous versions of a composite d13Ccarb curve forthe Silurian (e.g. Azmy et al. 1998) suffered from poorbiostratigraphic control and low stratigraphic resolu-tion, which complicated the use of Silurian d13Ccarb

chemostratigraphy as a chronostratigraphic tool (Kaljo& Martma 2006). An improved d13Ccarb curve for theentire Silurian is included here that allows direct corre-lation between d13Ccarb chemostratigraphy and theSilurian graptolite and conodont biozonations. It isbeyond the scope of this report to discuss the Siluriand13Ccarb record in detail, but there are a few pointsworth mentioning here.

The middle Ludfordian (Lau) d13Ccarb excursionappears to be the highest magnitude positive d13Ccarb

excursion in the post-Cambrian Phanerozoic. Middleto late Ludfordian d13Ccarb values typically reach


+8.0& Vienna PeeDee Belemnite (VPDB) (Samtlebenet al. 1996, 2000; Martma et al. 2005; Jeppsson et al.2007) and values in excess of +10.0& VPDB havebeen documented (Wigforss–Lange 1999; Jeppssonet al. 2007). By the middle Ludfordian it had beenmore than 150 million years since d13Ccarb values hadbeen at such an elevated level and this d13Ccarb excur-sion appears to be unique among those of the Silurian.The Silurian d13Ccarb curve indicates relative instabilityin the Silurian global carbon cycle, and the presence ofat least seven discrete positive d13Ccarb excursions in<30 million years demonstrates that the SilurianPeriod was among the climatically least stable intervalsof Earth history.

Stage slices

In regions that are well constrained within the globalchronostratigraphic classification, intercontinental cor-relation far finer than the stage level is possible. Asnoted above, the terms Gleedon and Whitwell wereintroduced from the Homerian of the United King-dom (Bassett et al. 1975), and the use of allostrati-graphic units finer than the stage level in correlations ofSilurian strata within North America is common prac-tice (e.g. Brett et al. 1998; McLaughlin et al. 2008),whilst biozones (especially graptolite, conodont, andchitinozoan) are routinely used in intercontinentalcorrelation. Frequently in stratigraphic studies, it isnecessary or useful to apply units between the stagelevel and the biozone level. In particular, Silurian car-bonate sequences that have as yet yielded little or noconodont biostratigraphic information often can becorrelated only at a level coarser than the biozone,and providing a level of correlation between the stagelevel and biozone level would be useful. Here weinformally introduce the ‘stage slice’ for the Silurianas such a unit.

The term ‘stage slice’, as used in the OrdovicianSystem (Bergstrom et al. 2009), refers to biochemo-stratigraphically defined units most closely resemblingthe concept of assemblage zones (Salvador 1994). Atpresent there is no accepted term for a stratigraphicunit that is defined on the basis of more than onetaxon and ⁄ or more than one faunal group togetherwith the chemostratigraphic content of a given set ofstrata, but that is effectively what stage slices are.Clearly, further discussion about the rank, utility, defi-nition, and scope of stage slices is warranted, and theirintroduction to the Silurian here is informal.

In this article, the levels defined as boundaries ofthe stage slices are selected based on their chrono-stratigraphic utility. That is, they are levels that can bereliably identified and correlated on several differentpaleocontinents. Following Bergstrom et al. (2009) wehave used a two letter-number designation for eachstage slice where the two letters refer to the globalstage (global series in the case of the Pridoli) and thenumber refers to the relative position within the stage.With the exception of Sh3, the base of each stage sliceis defined by the base of a graptolite or conodont zone(Fig. 3). The definition of each stage slice and theirrelationship to the generalized Silurian d13Ccarb curveis provided here. The graptolite biozones referred tobelow are slightly modified and refined from Melchinet al. (2004) and Sadler et al. (2009), and the cono-dont biozones are a composite of several differentbiozonations combined here for global (as opposed toregional) utility. The first or last appearances (FADsand LADs) used to define the base of each Silurianconodont biozone are shown in Figure 3, and the totalbiozonation follows, in stratigraphic order: Mannik(2007); Jeppsson (1997); Jeppsson & Calner (2003);Jeppsson & Aldridge (2000); Jeppsson et al. (2006);Corradini & Serpagli (1999); and Corriga & Corradini(2009).

Fig. 3. Silurian graptolite and conodont biostratigraphic zonations, shown in relation to global stages and a generalized d13Ccarb curve for theSilurian System. With the exception of Sh3, the base of each stage slice is coincident with the base of either a graptolite or conodont zone.The generalized d13Ccarb curve is based on a composite from the following sources: Rhuddanian-Aeronian: Poldvere (2003); UppermostAeronian: Melchin & Holmden (2006); Telychian: Kaljo et al. (2003), and Cramer et al. (2010); onset of the Ireviken Excursion: Mun-necke et al. (2003); Sheinwoodian: Cramer et al. (2006a) and Cramer & Saltzman (2005); Homerian: Cramer et al. (2006b); Ludlow: Samtle-ben et al. (2000) and Jeppsson et al. (2007); Pridoli: Saltzman (2002). The graptolite zonation is slightly modified and refined from Sadleret al. (2009) and the FADs and LADs shown to the right of the conodont zonation refer to the definition of the conodont zones utilized inthis report. Graptolite abbreviations: N. – Neodiversograptus; A. – Akidograptus; P. – Parakidograptus; C. – Cystograptus; M. – Monograptus;Co. – Coronograptus; D. – Demirastrites; Pr. – Pribylograptus; L. – Lituigraptus; S. – Stimulograptus; St. – Streptograptus; Sp. – Spirograptus;Mcl. – Monoclimacis; Cy. – Cyrtograptus; G. – Gothograptus; Pri. – Pristiograptus; Col. – Colonograptus; Lo. – Lobograptus; Sa. – Saetograptus;Po. – Polonograptus. CONODONT ABBREVIATIONS: D. – Distomodus; Asp. – Aspelundia; Pr. – Pranognathus; Pt. – Pterospathodus; am. – amorpho-gnathoides; procerus – Pterospathodus pennatus procerus; K. – Kockelella; ranuli. – ranuliformis; O. – Ozarkodina; s. – sagitta; v. – variabilis; A. –Ancoradella; Po. – Polygnathoides; Ou. – Oulodus; w. – woschmidti; S.Z. – SuperZone; I.Z. – Interval Zone; s.l. – sensu lato. A slash betweennames in the graptolite zonation indicates a single ‘combined’ zone (e.g. Co. cyphus ⁄ M. revolutus). Dashed lines in the graptolite columndenote uncertainty in the placement of that boundary with respect to Silurian series and stage boundaries and the relative duration of thezone. Dashed lines in the conodont column denote uncertainty in the placement of that boundary with respect to the graptolite biozonation.The base of the Wenlock Series is shown with a solid line (base C. murchisoni – Ireviken Event Datum 2) representing the approximate corre-lation of the current GSSP, and a dashed line (base C. centrifugus) representing the originally intended correlation of the base of the WenlockSeries. See text for discussion of this boundary.


LAD K. crassa

FAD A. ploeckensisFAD P. siluricus

LAD P. siluricus

FAD O. crispa

LAD O. crispa

FAD Ou. e. detortus

FAD O. sagitta rhenanaFAD K. walliseriFAD K. ortus ortusFAD O. sagitta sagitta

FAD O. bohemica longa

FAD K. ortus absidata

FAD K. crassa

LAD Pt. am. amorph.LAD Pt. penn. procerus

FAD Pt. am. amorph.

FAD Pt. am. lithuanicusFAD Pt. am. lennarti

FAD Pt. am. angulatus

FAD Pt. eopennatus

FAD Asp. expansa

FAD Pr. tenuis

LAD Pr. tenuis

A.ascensus

P.acuminatus

C.vesiculosus

Co. cyphus /

D.kentucky-

ensis

D. tri-angulatus /

M. argenteus /L. convolutus

S.sedgwickii

Sp. guerichi

Sp. turriculatus

S. crispus

Mcl. grieston.

Mcl.crenulata

O.spiralis

Cy.lapworthi

Cy. insectus

Cy. centrifugus

Pr. tenuis

D. stauro-gnathoides

Pt. eopennatus

S.Z.

Pt. am.angulatus

Pt. am.lithuanicus

Pt. am. lennarti

Pt. am.amorpho-gnathoides

procerus S.Z.K. ranuli. S.Z.O. s. rhenana

K. walliseriK. ortus ortus

O. s. sagitta

M. riccarton.

Cy. rigidus /

Cy.lundgreni

Pri. parvus /G. nassa

Col. praedeubeliCol. deubeli

Col. ludensis

N. nilssoni /

O.bohemica

longa

K. ortusabsidata

Lo.scanicus

K. crassa

K. v. vari-abilis I.Z.

Po. siluricus

O. snajdri I.Z.

O. crispa

Sa. leintward. /Sa. linearis

Ne. kozlowskii /

M. formosus

O. eostein-hornensis

s.l. I.Z.

Ou.elegansdetortus

I. w.woschmidti

M.uniformis

M. parultimus

M. branikensis /M. bouceki

N.persculptus

A.ordovicicus

grapt-olite

cono-dont

Lla

ndov

ery

Wen

lock

Lud

low

ORDOVICIAN

Rhu

ddan

ian

Aer

onia

nTe

lych

ian

Shei

n.H

omer

.G

or.

Lud

.

Ep-och Age

Cy. murchisoni

A. ploeckensis

Po. podoliensis

M. transgrediens

/ M. perneri

M. lochkovensisM. ultimus

Rh1

Rh2

Rh3

Ae1

Ae2

Ae3

Te1

Te2

Te3

Te4

Te5

Sh1

Sh2

Sh3

Ho1

Ho2

Ho3

Go1

Go2

Lu1

Lu2

Lu3

Pr1

Pr2

s.s.

DEVONIANPr

idol

i415

420

425

430

435

440

445

Ma

422.9

426.2

428.2

436.0

439.0

443.7

418.7

416.0

D. pectinatus

Asp.expansa

M. revolutus

Pr. leptotheca

M.belophorous

Lo. progenitor

Bohemograptus

421.3

Mulde

Lau

Klonk

Valgu

Late Aeronian

Early Aeronian

Ireviken

+9 +10–2 –1 0 +1 +2 +4+3 +5 +6 +7 +8


Definitions of potential Silurian stage slices

Rh1. – Base of the Silurian (first appearance of thegraptolite Akidograptus ascensus) to the base of theCystograptus vesiculosus graptolite Zone. This includesthe Parakidograptus acuminatus graptolite Zone and iswithin the Distomodus kentuckyensis conodont Zone,roughly covering the lower half of the RhuddanianStage. The end of the Hirnantian d13Ccarb excursion iswell within the Hirnantian and baseline values persistthroughout this stage slice.

Rh2. – Base of the Cystograptus vesiculosus graptoliteZone to the base of the Coronograptus cyphus ⁄ Monog-raptus revolutus graptolite Zone. This includes theCystograptus vesiculosus graptolite Zone and is withinthe Distomodus kentuckyensis conodont Zone, roughlycovering the middle of the Rhuddanian Stage. A dis-tinct negative excursion near the top of this stage sliceis known from the eastern Baltic but the global pres-ence and precise chronostratigraphic position of thisexcursion has not been demonstrated. This excursionmay occur either within the C. vesiculosus graptoliteZone or the lower part of the Co. cyphus ⁄ M. revolutusgraptolite Zone and may correspond to one of twoweak negative excursions present in this interval inArctic Canada (Melchin & Holmden 2006) but it hasyet to be studied in detail elsewhere.

Rh3. – Base of the Co. cyphus ⁄ M. revolutus graptoliteZone to the base of the Demirastrites triangulatus ⁄ D.pectinatus graptolite Zone. This includes the C. cy-phus ⁄ M. revolutus graptolite Zone and most of thestage slice is within the upper part of the Distomoduskentuckyensis conodont Zone. The lowermost part ofthe Aspelundia expansa conodont Zone is included inthe upper part of this stage slice that represents theupper part of the Rhuddanian Stage. The base of theRaikkula Stage in the East Baltic probably lies lowwithin this stage slice.

Ae1. – Base of the Demirastrites triangulatus ⁄ D. pec-tinatus graptolite Zone to the base of the Monograptusargenteus ⁄ Pribylograptus leptotheca graptolite Zone.This includes the D. triangulatus ⁄ D. pectinatus grap-tolite Zone and is within the Asp. expansa conodontZone roughly corresponding to the lower third of theAeronian Stage. There is a positive d13Ccarb excursionrecorded in strata from this interval in the East Baltic(upper part of the D. triangulatus graptolite Zonein the Ruhnu core; Poldvere 2003), which was alsoobserved in lower Aeronian strata from ArcticCanada by Melchin & Holmden (2006), but this hasyet to be recognized elsewhere. It is restricted to thisstage slice.

Ae2. – Base of the Monograptus argenteus ⁄ Pribylo-graptus leptotheca graptolite Zone to the base of the Sti-mulograptus sedgwickii graptolite Zone. This includesthe M. argenteus ⁄ Pr. leptotheca and Lituigraptus convo-lutus graptolite zones as well as the upper part of theAsp. expansa and lower part of the Pranognathus tenuisconodont zones of the middle part of the AeronianStage. Generally, non-excursion or ‘baseline’ d13Ccarb

values have been recovered from strata within this stageslice. The base of the Clinton Group in North Americais likely located within this stage slice.

Ae3. – Base of the Stimulograptus sedgwickii grapto-lite Zone to the base of the Spirograptus guerichi grap-tolite Zone. This includes the St. sedgwickii graptoliteZone, the top of the Pr. tenuis and the lower part ofthe Distomodus staurognathoides conodont zones andrepresents the uppermost part of the Aeronian Stage.A positive d13Ccarb excursion has been recorded inAe3 strata from the Canadian Arctic and appears tobe restricted to this stage slice. This excursion has yetto be reproduced elsewhere in d13Ccarb records, butoccurs in d13Corg records from Scotland (Melchin &Holmden 2006) and Bohemia (Storch & Fryda 2009).A protracted interval of low d13Ccarb values has beenrecovered from the uppermost part of this stage slice(and the lowermost part of Te1) from Arctic Canadathat appears to follow immediately the positived13Ccarb excursion. The base of the Syrovaty Forma-tion and the Gromotukha Series in Altaj coincide withthe base of this stage slice. A similar position (orwithin Ae3) is likely for the base of the Adavere Stagein the East Baltic.

Te1. – Base of the Spirograptus guerichi graptoliteZone to the base of the Pterospathodus eopennatusconodont Superzone and the Pterospathodus eopenna-tus ssp. n. 1 conodont Zone. This includes all of theSp. guerichi and almost all of the Sp. turriculatus grap-tolite zones, is within the upper part of the D. stauro-gnathoides conodont Zone, and represents thelowermost part of the Telychian Stage.

Te2. – Base of the Pterospathodus eopennatus cono-dont Superzone and the Pt. eopennatus ssp. n. 1 cono-dont Zone to the base of the Monoclimacis crenulatagraptolite Zone. This includes the uppermost part ofthe Spirograptus turriculatus, and all of the Strepto-graptus crispus, Streptograptus sartorius, and Mcl. grie-stoniensis graptolite zones, as well as all of thePterospathodus eopennatus ssp. n. 1 conodont Zoneand most of the Pt. eopennatus ssp. n. 2 conodontZone (most of the Pt. eopennatus conodont Superz-one). A positive d13Ccarb excursion (the Valgu excur-sion) has been known from Te2 strata in the eastern


Baltic for more than a decade and this excursion hasrecently been identified in North America (Munnecke& Mannik 2009). This excursion appears to berestricted to this stage slice. The base of the PolatyFormation in Altaj may correlate with a positionwithin this stage slice (Sennikov et al. 2008, p. 13record graptolites of the Mcl. griestoniensis Zone).

Te3. – Base of the Monoclimacis crenulata graptoliteZone to the base of the Pterospathodus amorphognatho-ides amorphognathoides conodont Zone. This includesthe uppermost part of the Pterospathodus eopennatusssp. n. 2 conodont Zone, all of the Pt. amorphognatho-ides angulatus, Pt. am. lennarti, and Pt. am. lithuanicusconodont zones, as well as all of the Mcl. crenulata andOktavites spiralis graptolite zones. Stage slice Te3 coversroughly the middle third of the Telychian Stage. Thebase of the Tigerek Series and Chesnokovka Formationin Altaj are probably within this stage slice.

Te4. – Base of the Pterospathodus amorphognathoidesamorphognathoides conodont Zone to the base of theCyrtograptus insectus graptolite Zone. This includesthe lower part of the Pterospathodus amorphognatho-ides amorphognathoides conodont Zone and probablyall of the Cyrtograptus lapworthi graptolite Zone corre-sponding to an interval within the upper part of theTelychian Stage.

Te5. – Base of the Cyrtograptus insectus graptoliteZone to the base of the Cyrtograptus murchisoni grapto-lite Zone. This includes the Cyrtograptus insectus and C.centrifugus graptolite zones as well as most of the upperpart of the Pterospathodus amorphognathoides amorpho-gnathoides conodont Zone of the uppermost TelychianStage. The base of the Jaani Stage in the East Baltic likelycorrelates with a level within this stage slice.

Sh1. – Base of the Cyrtograptus murchisoni graptoliteZone to the base of the Upper Kockelella ranuliformisconodont Zone within the K. ranuliformis conodontSuperzone. At present, the base of the C. murchisonigraptolite Zone is tentatively considered to correlatewith the base of the Wenlock Series (see Loydell 2008;Cramer et al. 2010), and the base of this stage slicecorrelates with the base of the Sheinwoodian Stageand Wenlock Series as defined by the current GSSP(see discussion above). This stage slice includes all ofthe Cyrtograptus murchisoni, Monograptus firmus, andlower part of the M. riccartonensis graptolite zones.This stage slice contains the uppermost part of thePterospathodus amorphognathoides amorphognathoidesconodont Zone and the entirety of the Ireviken Event(Datum 1-8), which includes the Lower and UpperPseudooneotodus bicornis (Datum 1-2 and Datum 2-3

respectively), Lower and Upper Pterospathodus penna-tus procerus (Datum 3-4 and Datum 4-6 respectively),and Lower Kockelella ranuliformis (Datum 6-8) cono-dont zones. The onset of the well-known early Shein-woodian (Ireviken) positive d13Ccarb excursion lieswithin the lower part of this stage slice and d13Ccarb

values continue to rise throughout the rest of Sh1. Atpresent, the base of the Cyrtograptus murchisoni grap-tolite Zone is tentatively considered to correspond tothe base of the Wenlock Series (see Loydell 2008;Cramer et al. 2010), and the base of this stage slicecorrelates with the base of the Sheinwoodian Stageand the Wenlock Series.

Sh2. – Base of the Upper K. ranuliformis conodontZone (within the K. ranuliformis conodont Superzone)to the end of the early Sheinwoodian (Ireviken)d13Ccarb excursion. This includes the remainder of theMonograptus riccartonensis graptolite Zone and thelower part of the Cyrtograptus rigidus ⁄ Monograptusbelophorus graptolite Zone as well as all of the upper K.ranuliformis, Ozarkodina sagitta rhenana and lowerpart of the Kockelella walliseri conodont zones. d13Ccarb

values first reach their maximum near the base of thisstage slice and return to a level consistently at or belowpre-excursion values by the top of Sh2. The base of theLockport Group in North America correlates with aposition in the middle of Sh2 and may be equivalent tothe base of the Jaagarahu Stage in the East Baltic.

Sh3. – End of the early Sheinwoodian (Ireviken)d13Ccarb excursion to the base of the Cyrtograptuslundgreni graptolite Zone. This includes the remainderof the Cyrtograptus rigidus ⁄ Monograptus belophorusgraptolite Zone, the Cyrtograptus perneri graptoliteZone, as well as the upper part of the K. walliseri, allof the K. ortus ortus, and the lowermost part of theOzarkodina sagitta sagitta conodont zones. This stageslice coincides with the upper part of the Sheinwoo-dian Stage above the Ireviken d13Ccarb excursion.

Ho1. – Base of the Cyrtograptus lundgreni graptoliteZone to the base of the Pristiograptus parvus ⁄ Gotho-graptus nassa graptolite Zone. This includes most ofthe Ozarkodina sagitta sagitta and the lower part ofthe Oz. bohemica longa conodont zones. This stageslice is equivalent to the Cy. lundgreni graptolite Zoneand approximates to the Gleedon Chronozone ofBassett et al. (1975) (see discussion in Zalasiewicz &Williams 1999). The Mulde Event (conodonts) andthe ‘big crisis’ (graptolites) begin near, and at the toprespectively, of this stage slice.

Ho2. – Base of the Pristiograptus parvus ⁄ Gothograptusnassa graptolite Zone to the base of the Colonograptus


ludensis graptolite Zone. This includes the Pri. Par-vus ⁄ G. nassa, Col. praedeubeli, and Col. deubeli grapto-lite zones as well as the remainder of the Ozarkodinabohemica longa, and lower part of the Kockelella ortusabsidata conodont zones. The first peak of the Homeri-an (Mulde) d13Ccarb excursion appears to be restrictedto Ho2. The base of the Rootsikula Stage in the easternBaltic lies within Ho2.

Ho3. – Base of the Colonograptus ludensis graptoliteZone to the base of the Neodiversograptus nilssoni ⁄Lobograptus progenitor graptolite Zone. This includesthe upper part of the Kockelella ortus absidata cono-dont Zone to the base of the Kockelella crassa cono-dont Zone and is equivalent to the Col. ludensisgraptolite Zone. Ho2 and Ho3 combined approximateto the Whitwell Chronozone of Bassett et al. (1975).This stage slice represents the uppermost part of theHomerian Stage and includes the second peak of theHomerian (Mulde) d13Ccarb excursion.

Go1. – Base of the Neodiversograptus nilssoni ⁄ Lobo-graptus progenitor graptolite Zone to the base of theLobograptus scanicus graptolite Zone. This includes allof the Kockelella crassa conodont Zone and lowermostpart of the Kockelella variabilis variabilis conodontInterval Zone. This stage slice is equivalent to the N.nilssoni ⁄ L. progenitor graptolite Zone covering thelower part of the Gorstian Stage. The base of Go1 iscoincident with the base of the global Gorstian Stageand Ludlow Series, with the base of the regionalPaadla Stage in the East Baltic, and potentially withthe base of the Salina Group and Cayugan Series inNorth America.

Go2. – Base of the Lobograptus scanicus graptoliteZone to the base of the Saetograptus leintwardinen-sis ⁄ Saetograptus linearis graptolite Zone. This includesthe remainder of the Kockelella variabilis variabilisconodont Interval Zone of the upper part of the Gors-tian Stage. Go2 is equivalent to the Lobograptus scani-cus graptolite Zone and contains the biotic eventtermed the Linde Event (Jeppsson 1993).

Lu1. – Base of the Saetograptus leintwardinensis ⁄Saetograptus linearis graptolite Zone to the base of theOzarkodina snajdri conodont Interval Zone. Thisincludes all of the Sa. leintwardinensis ⁄ Sa. linearis andthe lower part of the Bohemograptus graptolite zones aswell as all of the Ancoradella ploeckensis and Polygna-thoides siluricus conodont zones. The position of thebase of the A. ploeckensis conodont Zone with respectto the base of the Sa. leintwardinensis ⁄ Sa. linearis grap-tolite Zone, and the position of the base of either ofthese biozones with respect to the base Ludfordian

GSSP remains uncertain and these three positions aretentatively correlated at the same level here. It mayhowever be that the base of the A. ploeckensis conodontZone is below the base of the Sa. leintwardinensis ⁄ Sa.linearis graptolite Zone, which says nothing of theircorrelation to the GSSP. A low amplitude (but strati-graphically consistent) positive d13Ccarb excursion asso-ciated with the underlying Linde Event has been foundon Gotland from Gorstian–Ludfordian boundarystrata, but this feature remains to be reproduced fromother regions. The onset of the much higher amplitudemiddle Ludfordian (Lau) d13Ccarb excursion begins inthe uppermost part of this stage slice.

Lu2. – Base of the Ozarkodina snajdri conodontInterval Zone to the base of the Ozarkodina crispaconodont Zone. This includes the upper part of theBohemograptus, all of the Neodiversograptus kozlow-skii ⁄ Polonograptus podoliensis, and lowermost part ofthe Monograptus formosus graptolite zones. This stageslice is equivalent to the Oz. snajdri conodont IntervalZone spanning the middle part of the LudfordianStage. The middle Ludfordian (Lau) d13Ccarb excur-sion is largely restricted to this stage slice whered13Ccarb values rise over most of the lower part of thisstage slice, peak values are restricted to the middle partof Lu2, and d13Ccarb values decrease through the upperpart of this stage slice.

Lu3. – Base of the Ozarkodina crispa conodont Zoneto the base of the Monograptus parultimus graptoliteZone. This includes the remainder of the M. formosusgraptolite Zone and is equivalent to the O. crispaconodont Zone coinciding with the upper part of theLudfordian Stage. Primarily, this stage slice containsLudfordian strata above the middle Ludfordian (Lau)d13Ccarb excursion. The base of the Kuressaare Stage inthe East Baltic region may be coincident with the baseof Lu3 or lies somewhat below it.

Pr1. – Base of the Monograptus parultimus graptoliteZone to the base of the Monograptus bouceki graptoliteZone. This includes all of the M. parultimus, M. ulti-mus, and M. branikensis ⁄ M. lochkovensis graptolitezones as well as the majority of the Ozarkodinaeosteinhornensis sensu lato conodont Interval Zone andrepresents roughly the lower half of the Pridoli Series.The base of the regional Kaugatuma Stage in the EastBaltic is considered to be coincident with the base ofthe global Pridoli Series. The top of the Salina Groupin North America is likely within this stage slice.

Pr2. – Base of the Monograptus bouceki graptoliteZone to the base of the Monograptus uniformis grapto-lite Zone. This includes all of the M. bouceki and M.


transgrediens ⁄ M. perneri graptolite zones, the remain-der of the Ozarkodina eosteinhornensis sensu lato cono-dont Interval Zone, all of the Oulodus elegans detortusconodont Zone, and likely the lowest part of the Icrio-dus woschmidti woschmidti conodont Zone. The onsetand most of the rising limb of the Silurian–Devonianboundary (Klonk) d13Ccarb excursion is within Pr2strata. This stage slice represents the uppermost partof the Pridoli Series and the base of the Ohesaare Stagein the East Baltic probably lies within this stage slice.

Conclusions

The chronostratigraphic correlation of the SilurianSystem of North America is currently undergoingmajor revision. As a result, the global correlation ofmuch of the Silurian strata of North America remainsuncertain by more than a stage in most cases, andmore than a series in others. This report serves as anupdate on changes to the global correlation of Silurianchronostratigraphic terms in use within North Amer-ica as well as a framework for improved global chro-nostratigraphic correlation. The new composited13Ccarb curve for the Silurian is directly tied to thebiochemostratigraphically defined stage slices intro-duced here, which facilitates the use of d13Ccarb chem-ostratigraphy as a chronostratigraphic tool in wellstudied and biostratigraphically well-constrainedintervals such as the Wenlock. With the new Siluriand13Ccarb curve and the recent improvements in Silu-rian conodont biozonations there remains little justifi-cation for retaining regional chronostratigraphicclassifications of the Silurian System. As more databecome available, refinements to this curve will berequired and the definitions of the stage slices willremain similarly open to revision.

Acknowledgements. – The authors thank the Lethaia editors,Dimitri Kaljo, and an anonymous reviewer whose detailed andinsightful comments made this a stronger manuscript. This work isa contribution to the IGCP 503 project and was partially funded byNSF grants # EAR 0518511 to CEB and W. Huff as well as # EAR0517929 to MAK and J. Barrick; NSERC (Canada) Discovery Grantto MJM; Estonian Science Foundation grants #7138 and #7640 toPM; NERC Small Grant (Ref. GR9 ⁄ 1129) to DKL; DFG grantMu2352 ⁄ 1 to AM; and grants from the Swedish Natural ScienceResearch Council, its successor the Swedish Research Council, andthe Geological Survey of Sweden to LJ.

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