Revised correlation of Provincial Series of North America with global and regional chronostratigraphic units 13 and d Ccarb chemostratigraphy


Cramer, B.D., Brett, C.E., Melchin, M.J., Ma¨nnik, 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 global 13 and regional chronostratigraphic units and d Ccarb chemostratigraphy. Lethaia,Vol.44, pp. 185–202.

Recent revisions to the biostratigraphic and chronostratigraphic assignment of strata from the type area of the Niagaran Provincial Series (a regional chronostratigraphic unit) have demonstrated the need to revise the chronostratigraphic correlation of the Silurian of North America. Recently, the working group to restudy the base of the Wen- lock Series has developed an extremely high-resolution global for the and stages by integrating graptolite and biostratigra- 13 phy with carbonate carbon isotope (d Ccarb) chemostratigraphy. This improved global chronostratigraphy has required such significant chronostratigraphic revisions to the North 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 of the progress on recalibrating the global chronostratigraphic correlation of North Ameri- can Provincial Series and boundaries in their type area. The revised North Ameri- can classification is correlated with global series and stages as well as regional classifications used in the , the East Baltic, Australia, China, the Barran- dian, and Altaj. Twenty-four potential stage slices, based primarily on graptolite and conodont zones and correlated to the global series and stages, are illustrated alongside a 13 new composite d Ccarb curve for the Silurian. Conodont, graptolite, isotope, , , series, Silurian, stage.

Bradley D. Cramer [[email protected]], Division of 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 , University of Kansas, 1930 Constant Avenue, Lawrence, Kansas 66047, USA; Carlton E. Brett [[email protected]], Department of Geology, University of Cincinnati, 2624 Clifton Avenue, Cincinnati, Ohio 45221, USA; Michael J. Melchin [[email protected]], Department of Earth Sciences, St. Francis Xavier University, 1 West Street, Antigonish, Nova Scotia B2G 2W5, Canada; Peep Ma¨nnik [man- [email protected]], Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, ; Mark A. Kleffner [[email protected].ohio-state.edu], 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 Pala¨oumwelt, Universita¨t Erlangen, Loewenichstraße 28, Erlangen D-91054, Germany; Lennart Jeppsson [[email protected]], Department of Geology, GeoBio- sphere Science Centre, Lund University, So¨lvegatan 12, Lund SE-223-62, Sweden; Carlo Corradini [[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; Matthew R. Saltzman [[email protected]], Division of Earth History, School of Earth Sciences, The Ohio State University, 125 S. Oval Mall, Columbus, Ohio 43210, USA; manuscript received on 9 ⁄ 2 ⁄ 2010; manuscript accepted on 31 ⁄ 5 ⁄ 2010.

The Silurian System was the first to have a globally (Llandovery, Wenlock, , and Pridoli) and applicable classification of series and stages when the seven stages (, , Telychian, present definitions of the four Silurian series Sheinwoodian, , , and )

DOI 10.1111/j.1502-3931.2010.00234.x 2010 The Authors, Journal compilation 2010 The Lethaia Foundation 186 Cramer et al. LETHAIA 44 (2011) were formalized by the International Subcommission used regional terms from North America correlate on Silurian Stratigraphy (ISSS) more than 25 with each other and to show how recent findings ago (Holland 1984; Bassett 1985). Stable chronostrati- change their placement within the global chronostrati- graphic nomenclature and the comparatively cosmo- graphic scheme. In addition, we present an improved 13 politan nature of Silurian marine fauna have led most composite d Ccarb curve for the Silurian. Finally, fol- regional stratigraphic bodies to abandon regional lowing the lead of the International Subcommission chronostratigraphic terms in favour of the global Silu- on Stratigraphy (Bergstro¨m et al. 2009), rian series and stages ratified by the ISSS. North we informally introduce potential subdivisions of the America is among the last examples where regional global Silurian stages that are biochemostratigraphical- chronostratigraphic terms (particularly at the series ly defined, referred to as stage slices. It is important to level) are still in use (Fig. 1). note that this report does not constitute ISSS support As early as 1970, it was becoming clear that corre- for or official revision of any regional stratigraphic lation within North America accomplished using scheme. The authors are in agreement with Berry & North American Provincial Series and Stages (e.g. Boucot (1970), Norford (1997), and many others that Alexandrian, Niagaran, and Cayugan series) had been the global series and stages should be used for the problematical and calls for the adoption of the global North American Silurian succession. series and stages for the Silurian of North America had begun (Berry & Boucot 1970). Decades of misuse of stratigraphic terms throughout North American Silu- Global correlation of Silurian rian literature, and the inconsistency between Silurian chronostratigraphic units chronostratigraphic terms used by the Geological Survey (USGS), the American Association Silurian underwent a period of major of Petroleum Geologists (AAPG), the Geological advancement at the end of the last century (late Survey of Canada (GSC), and the Ontario Geological 1990s) with the publication of a generalized graptolite Survey (OGS), have adversely impacted the utility and zonation (Koren’ et al. 1996; Loydell 1998) for the meaning of these regional chronostratigraphic terms. entire Silurian as well as major revisions to Silurian The purpose of this report is to show how commonly conodont taxonomy and biostratigraphy during a











RHUDDANIAN GSSP Fig. 1. Chronostratigraphic chart showing proposed correlation between the revised North American classification and global and regional series and stages presently in use from regions of major Silurian outcrop. The GSSP boxes show the location of the Global Boundary Stratotype Section and Point for the global Silurian series and stages. LETHAIA 44 (2011) Silurian chronostratigraphic correlation 187 span of just a few years (Jeppsson 1997; Ma¨nnik ISSS in 2006 and ratified by the International Union 1998). The current Silurian timescale (Ogg et al. of Geological Sciences (IUGS) in 2007 (see Rong et al. 2008) assumes a correlation between stage (and there- 2008). The GSSP for the base of the Sheinwoodian fore series and systemic) boundaries and graptolite Stage and Wenlock Series was originally stated to zones and as a result graptolite-bearing successions coincide with the first appearance of the graptolite can be correlated directly to the global chronostrati- Cyrtograptus centrifugus (Bassett et al. 1975) and the graphic classification. Conodont-bearing successions base of the C. centrifugus graptolite Zone. Based upon have traditionally been more difficult to correlate pre- information not yet available when the base Wenlock cisely with the global scheme, but significant improve- GSSP was ratified (Martinsson et al. 1981), it is now ments to Silurian conodont biostratigraphic zonations clear that the position of the GSSP is significantly have begun to alleviate this problem (e.g. Jeppsson above the first appearance of C. centrifugus (e.g. 1997, 2005; Ma¨nnik 1998, 2007; Jeppsson & Aldridge Mabillard & Aldridge 1985; Loydell et al. 2003; 2000; Jeppsson & Calner 2003; Jeppsson et al. 2006; Mullins & Aldridge 2004; Cramer et al. 2010). There- Corriga & Corradini 2009). Major advancements in fore, the ISSS was faced with a decision, either: (1) to correlation of conodont and graptolite zones (e.g. move the GSSP to a location known to be coincident Loydell et al. 1998, 2003) and their integration with withthebaseoftheC. centrifugus graptolite Zone; or 13 carbonate carbon isotope d Ccarb chemostratigraphy (2) retain the current GSSP and redefine the biostrati- as a chronostratigraphic tool (Kaljo et al. 2003, 2007; graphic correlation of this position. Munnecke et al. 2003; Cramer et al. 2006a,b; Kaljo & In a report to the ISSS, the working group to rest- Martma 2006; Melchin & Holmden 2006; Jeppsson udy the base of the Wenlock Series suggested that the et al. 2007) have made it possible to correlate some GSSP for the base of the Wenlock Series be retained, intervals of the Silurian globally and with precision far but that the biostratigraphic correlation, as coincident finer than the stage level (Cramer et al. 2010). A withthebaseoftheC. centrifugus graptolite Zone, detailed discussion of global chronostratigraphic cor- needed to be revised (Loydell 2008). There are two relation at the stage level (and finer) is beyond the potential new levels (one graptolite and one cono- scope of this report and below we include a few brief dont)forthebaseoftheWenlockSeries:(1)correlat- notes as updates on Silurian chronostratigraphic issues ing to a position at or slightly above the base of the C. from selected regions worldwide. murchisoni graptolite Zone, marked by the first appearance of C. murchisoni (a full zone higher than United Kingdom the original position); or, (2) close to the Event Datum 2, the base of the Upper Pseudooneoto- Seven of the eight Silurian Global Boundary Strato- dus bicornis conodont Zone, marked by the last type Section and Point (GSSP) ‘golden spikes’ are appearance of the Ozarkodina polinclinata located in the United Kingdom, which means that polinclinata, Apsidognathus ruginosus,andA. walmsleyi British Silurian chronostratigraphy is necessarily cou- (see Jeppsson 1997). Recent integration of detailed pled to the global chronostratigraphic classification global graptolite and with 13 (the exception is the base of the Pridoli Series). A high-resolution d Ccarb chemostratigraphy surround- recent compilation produced for the British Geologi- ing the base of the Wenlock Series has documented cal Conservation Review Series (Aldridge et al. 2000) the global utility of these definitions and suggested combined with the seminal work of Holland & Bassett that these two positions (base of the C. murchisoni (1989) provide an excellent overview of British Silu- Zone and Datum 2) are likely sepa- rian stratigraphy and the reader is referred to these rated by less than 100 kyr (Cramer et al. 2010). works for more detailed information. The results of Therefore, following Loydell (2008), Cramer et al. two working groups of the ISSS to restudy Silurian (2010) considered the base of the C. murchisoni grap- chronostratigraphic boundaries have recently revised tolite Zone to correlate with the base of the Wenlock or suggested revision to the biostratigraphic correla- Series. tion of the base of the Silurian System and the base of The most recent iteration of the Geological Time the Wenlock Series respectively, and their findings are Scale (Ogg et al. 2008) also recognized that the GSSP summarized briefly here. correlates with this biostratigraphic position (their The new biostratigraphic definition of the base of table 6.1), but in their correlation chart (their fig. 6.4) the Silurian System proposed by Melchin & Williams showed the base of the Wenlock at the base of a com- (2000), as coincident with the first appearance of the bined C. centrifugus–C. murchisoni graptolite Zone. graptolite Akidograptus ascensus and the base of the A. An official position regarding revisions to the base of ascensus graptolite Zone, 1.6 metres above the base of the Wenlock Series has yet to be reached by the ISSS, the Birkhill at the GSSP, was accepted by the and although the de facto correlation of the GSSP is 188 Cramer et al. LETHAIA 44 (2011) currently at a level that approximates the base of the abandoned in favour of the global classification. The C. murchisoni graptolite Zone, that is subject to Geological Survey of Canada has demonstrated that change should the ISSS choose in the future to select a the global Silurian series and stages can be recognized new GSSP that coincides with the originally intended throughout the Canadian succession (Norford 1997) biostratigraphic level (i.e. the base of the C. centrifugus and use of regional terms within Canada is now largely graptolite Zone). restricted to the Ontario region. Work is currently Silurian units smaller than the stage level have been underway to identify properly the global Silurian series formally described for the Homerian Stage of the Wen- and stage boundaries within Ontario and the Great lock Series from the Welsh Borderlands (Bassett et al. Lakes region (Bancroft 2008; Brunton 2008, 2009; 1975). The twoHomerian divisions, Gleedon and Whit- Brunton et al. 2009) and the United States as well well, although not officially recognized by the ISSS, are (Saltzman 2002; Bergstro¨m et al. 2006; Cramer et al. still occasionally found in print (e.g. Jeppsson et al. 2006a; b; McLaughlin et al. 2008; Barrick et al. 2009; 2006).Thefour-stagedivisionoftheLudlowSeries(Elto- Kleffner et al. 2009). nian, Bringewoodian, Leintwardian, and Whitcliffian) The classic term Alexandrian Series has been defined by Holland et al. (1963) was replaced by the late applied traditionally to an interval spanning from the 1970s with the present two stage (Gorstian and Ludfor- base of the Silurian System to somewhere in the mid- dian)division.Becausethe typelocalitiesforallsixterms dle of the Llandovery Series and remains pervasive in are located in the United Kingdom, the older terms were North American literature (e.g. AAPG-COSUNA). absorbed by the present global classification. These However, revisions to the North American timescale regional terms have been largely abandoned and are in and recent biochemostratigraphic data have combined disuse, as is the term, Downton, which had been pro- to make the Alexandrian Series a meaningless term. posed as the fourth Silurian Series (Bassett et al. 1982) The original type area of the Alexandrian is located in andforatimewascommon(sometimesasDowntonian) the American mid-continent (SW and SE Mis- in UK Silurian literature. Following the suggestions of souri) and covers an interval from the base of the the ISSS, the term Pridoli has been used in the United Girardeau to the top of the Brassfield Kingdomformorethan20 years. Limestone. Bergstro¨m et al. (2006) demonstrated that the base of the Girardeau Limestone correlates to a North America position within the lower part of the global Hirnan- tian Stage, which by necessity would force the base of The Silurian succession of North America has been the Alexandrian Series down into the Ordovician. The divided into a complex array of chronostratigraphic top of the Alexandrian Series contains strata (Brass- units and schemes over the past 150 years. Varying use field Limestone) that are undoubtedly of Silurian of classical terms such as Alexandrian, Niagaran, and (Berry & Boucot 1970) and likely belong to the Aero- Cayugan series, combined with ‘official’ use of the nian Stage. Therefore in present global terms, the clas- unofficial terms Lower, Middle, and Upper Silurian, sical Alexandrian Series would cover an interval from have crippled international correlation of North Amer- the Stage to the Aeronian Stage and as a ican Silurian units. Here we have included the relevant series would cross a systemic boundary. This was portion of the current AAPG–COSUNA (Correlation avoided in the present definition of the North Ameri- of the Stratigraphic Units of North America) chart canOrdovicianSystembyextendingthetermCin- (Shaver et al. 1985), the recently revised Silurian terms cinnatian Series to the top of the Ordovician in use by the OGS (Brunton 2009), and the Silurian (Bergstro¨m et al. 2009). Similarly, Brett et al. (1995) terms utilized by the USGS (Brett et al. 1995) for com- placed the base of the previously overlying Niagaran parison in Figure 2. A revised correlation of the USGS Series at the base of the Silurian System, and thus ren- classification scheme is presented here (far right panel dered the term Alexandrian Series obsolete. in Fig. 2) and represents the North American column The Niagaran Provincial Series, as defined by Brett shown in Figure 1. Terms such as Lockport were for- et al. (1995) from the Niagara region of western New mally introduced as lithostratigraphic terms (Lockport York State was thought to encompass all of the Llan- Formation or Lockport Group, see Brett et al. 1995) dovery Series, all of the Wenlock Series, and most of but have been misused for decades as stage level terms the Ludlow Series. The Niagaran Series is equivalent throughout North America (e.g. Lockportian in to the antequated Lower and Middle series tradition- Fig. 2). To compare these terms and their use with glo- ally used by the OGS (Johnson et al. 1992 – but aban- bal and regional classifications, we follow the common doned by Brunton 2009) in that both systems of (incorrect) treatment of these terms as being chrono- nomenclature include all Silurian strata below the Sal- stratigraphic for this report. Again, we point out that ina Group. Brett et al. (1995) include three groups these regional series and stage names should be within the Niagaran Series (Medina, Clinton, and LETHAIA 44 (2011) Silurian chronostratigraphic correlation 189


STAGE GROUP GROUP GROUP STAGE SERIES SERIES SERIES SERIES SERIES SYSTEM (Murderian) (Bertie)/(Bass Islands) (Bertie)/(Bass Islands) (Bertie)/(Bass Islands)







(Tonawondan) NIAGARAN (Ontarian) CLINTON




Fig. 2. Chronostratigraphic chart showing the relationship between the North American Silurian chronostratigraphic terms in use by the AAPG, OGS, and USGS (three central columns) with global series and stages (far left column) as well as the revised correlation of the USGS terms with the global series and stages (far right column). We acknowledge that the term ‘Group’ (a lithostratigraphic term) is out of place on a ‘Chronostratigraphic Chart’. Lithostratigraphic terms referring to 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 the AAPG 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 from Johnson 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 recently abandoned by the OGS (Brunton 2009). Brunton (2009) did not address strata above the Guelph Formation, but the replacement of general terms (Lower and Upper) by Provincial Series terms would therefore require the assignment of all strata above the base of the Salina Group to the Cayugan Series. The revised correlation of the USGS terms (far right column) was included in Figure 1. AAPG-COSUNA – American Association of Petroleum Geologists-Correlation of Stratigraphic Units of North America. OGS – Ontario Geological Survey. USGS – United States Geological Survey.

Lockport groups). Brett et al. (1995) and Brunton 1992), such that the OGS Cataract Group + Thorold (2009) provide a detailed correlation between the Sandstone = USGS Medina Group (Fig. 2). However, OGS classification and the USGS classification of this the base of the was elevated to the base interval, and we restrict ourselves here to discussion of the Neahga Shale by the USGS (Brett et al. 1995) and refinement of the USGS terms unless otherwise because the latter unit was found to be separated from indicated. At present only the base of the Lockport the Thorold Formation by a regionally angular uncon- Group has been precisely correlated with the global formity in western New York and Ontario, whereas chronostratigraphic classification and global correla- the Thorold Formation is underlain, and locally over- tions of the remaining units (Medina, Clinton, Salina, lain, conformably with red mudstones and sandstones etc.) require further refinement. Major research typical of the Medina Group. A similar position projects are currently underway to constrain more (above the Thorold Formation) has been adopted by precisely the remaining North American regional the OGS (Brunton 2009). Here, we restrict our use of boundaries and a few notes of update are included the term ‘Clinton Group’ to the USGS (Brett et al. here. 1995) sense as placing the base of the Clinton Group The base of the Clinton Group was placed histori- at the base of the Neahga Shale and including the cally at the base of the Thorold Sandstone in New Thorold Sandstone within the underlying Medina York (Gillette 1947) and continued to be placed at this Group. In westernmost New York and adjacent levelbytheOntarioGeologicalSurvey(Johnsonet al. Ontario the Clinton Group includes in stratigraphic 190 Cramer et al. LETHAIA 44 (2011) order the Neahga, Reynales, Merritton (upper part of The term ‘Lockport’ was first introduced by Hall the Hill Formation in Ontario, e.g. Brintnell (1839) at the rank of formation. Rickard (1975) raised et al. 2009; Brunton 2009), Williamson, Rockway, the term to group status and this assignment was Irondequoit, Rochester, and Decew formations. In maintained by Brett et al. (1995) for the USGS. The central New York the Clinton is much more complete term Lockport has continued to be occasionally used and includes in ascending order the Reynales or equiv- at the formational level in Ontario (e.g. Johnson et al. alent Creek Shale, Lower and Upper Sodus, Wal- 1992), but Brunton (2009) has abandoned this prac- cott, Sauquoit, Williamson-Willowvale, Rockway, tice. Based upon conodont biostratigraphy and 13 Irondequoit, Rochester or equivalent Herkimer, and d Ccarb chemostratigraphy from the type area of the Decew formations. Niagaran Series, Cramer et al. (2006a, 2010) dem- ThebaseoftheWenlockSeriesislikelylocated onstrated that the base of the Gasport Formation and within, or at the top of, the Rockway Formation as the therefore the base of the Lockport Group coincides base of the Irondequoit Formation clearly coincides withthemiddlepartoftheSheinwoodianStage with the lower part of the Sheinwoodian Stage (Fig. 2). The upper limit of the Lockport Group how- (Cramer et al. 2006a, 2010), although the precise ever remains a source of confusion, both chronostrati- position has yet to be determined. The lowermost graphically and nomenclaturally. The USGS includes Williamson Formation, at the type section in Roches- all strata between the base of the Gasport Formation ter, New York, contains the graptolite Stimulograptus and the base of the Salina Group within the Lockport clintonensis as well as the conodont Group (Brett et al. 1995). The OGS had traditionally amorphognathoides angulatus clearly indicative of a varied from this viewpoint and removed the upper- position no higher than the middle part of the most units (Guelph and Eramosa formations) from Telychian Stage (Loydell et al. 2007). According to the Lockport Group (or Lockport Formation with Kleffner in McLaughlin et al. (2008, p. 124), the con- Gasport, Island, and Eramosa members, Johnson odonts from the Merritton Formation, referred to as et al. 1992) and left them without a group designa- belonging to the Neospathognathodus celloni conodont tion. Brunton (2008, 2009) demonstrated that the Zone by Rexroad (1970, though the genus is no longer USGS sense of a Lockport Group (as one that contains in use and zone redefined), likely belong to the Pteros- the Gasport, Goat Island, Eramosa, and Guelph for- pathodus eopennatus Super Zone of Ma¨nnik (2007). mations) can be extended throughout southern The conodont fauna and the position of the Merritton Ontario, which illustrates that the Ontario terms below the Williamson indicate that the Merritton ‘Albemarle Group’ and Amabel Formation are redun- correlates with the lower part of the Telychian Stage. dant and should be abandoned. Here, we use the term The base of the Merritton overlies a major unconfor- ‘Lockport Group’ in the recently uniform OGS–USGS mity at which underlying Reynales and Neahga forma- senseasincludingallstratabetweenthebaseofthe tions are truncated westward into Ontario. This Gasport Formation and the base of the Salina Group. disconformity appears to be coextensive with a regio- Conodonts indicative of the late Sheinwoodian (K. nal, angular at which the Sauquoit, Wol- ortus ortus) have been found in strata belonging to the cott, Upper Sodus, Lower Sodus, and Reynales are upper part of the Lockport Group (within the Eramosa successively removed from east to west commencing in Formation, Bancroft 2008). The base of the Homerian central New York. In westernmost New York–south- Stage can be correlated at the formation level through- western Ontario this unconformity separates the Nea- out the mid-continent (e.g. Cramer et al. 2006b; Bar- hga and Reynales formations from the rest of the rick et al. 2009), although its placement in the Clinton Group. The assignment of these lower Clinton Appalachian Basin remains less clear. The base of the units to the Aeronian Stage by Brett et al. (1995) is sup- Homerian Stage however, likely correlates with a posi- ported by the presence of the conodont Pranognathus tion in the lower part of the Guelph Formation. tenuis throughout the Reynales Formation and upper The term ‘Salina’ was introduced by Dana (1863) as part of the Neagha Formation in New York (Kleffner the Salina Period to refer to the time represented by 2004). Rhuddanian–Aeronian conodont biozonations the Guelph and overlying and salts of the require significant improvement and for the moment it then so-called Saliferous of New York State. is unclear where the base of the Telychian should be After the labrythine evolution of the term in New placed in North American carbonate successions if it is York, it has come to be used at the group level (Salina preserved at all. It is clear, however, that the base Group) and has become virtually synonymous with of the Telychian must lie somewhere between the top the Cayugan Provincial Series as well. The term is of the Reynales and the base of the Merritton. The base more broadly known from what is called the Salina of the Clinton Group, therefore, correlates with a posi- Group in the Basin that contains over 700 tion no higher than the Aeronian Stage. metres of chiefly alternating salts and limestones, LETHAIA 44 (2011) Silurian chronostratigraphic correlation 191 however the term is defined by its use in New York term particularly noted for its thinness (e.g. Salvador State. As currently recognized by the USGS and the 1994) ‘gorizont’ is better translated as ‘regional stage’ Ontario Geological Survey, the Guelph Formation is for geological use. Bassett et al. (1989) considered included within the Lockport Group as its uppermost these terms to be regional stages and presented their formation (Brett et al. 1995; Brunton 2009), which correlation with global series and stages and gradually places the Niagaran–Cayugan series contact at the it has become common for East Baltic studies to Lockport-Salina group contact. include global series and stages in addition to regional The position of the base of the Ludlow Series with stages in publication (e.g. Ma¨nnik 2007). The current respect to the base of the Cayugan Provincial Series is East Baltic classification of a three-stage Llandovery, currently unknown. The next positive correlation in three-stage Wenlock, two-stage Ludlow, and two- the northern Appalachian Basin comes from the stage Pridoli has changed little from what was pre- recovery of the Ludfordian conodont Polygnathoides sented by Bassett et al. (1989). The correlation shown siluricus from the Vernon Formation in New York in Figure 1 for the East Baltic is modified slightly (Miller et al. 1988). Therefore the interval from the from Nestor (1997) and a few notes are included here. base of the Homerian Stage to the base of the Ludfor- Traditionally, the base of the Telychian Stage has dian Stage is likely contained within the Guelph to been considered to lie in the lower part of the Adavere Vernon (upper Lockport–lower Salina) succession, Regional Stage and to be coincident with the boundary and significant further research is required to refine between the Rumba and Velise formations (e.g. Nestor the chronostratigraphic classification of this interval 1997). However, a stratigraphic gap at this position of the North American sequence. As a result we have throughout much of the East Baltic has obscured the tentatively placed the base of the regional Cayugan precise correlation between global and regional units in Series as coinciding with the base of the global Ludlow this interval (Loydell et al. 1998; Po˜ldvere 2003). Simi- Series until further research can be conducted. larly, the Jaani Stage has been considered to be coinci- The use and meaning of group designations for the dent with the lower part of the Sheinwoodian Stage uppermost part of the Cayugan Series have been (the lower stage of the Wenlock Series), although highly variable and it remains among the least chrono- increasing evidence has begun to indicate that the base stratigraphically understood intervals of the Silurian of the Jaani Stage may be slightly below the base of the System in North America. The Cayugan Series Wenlock Series (Ma¨nnik 2007; Cramer et al. 2010). includes strata assigned to the Salina Group, Bertie The base of the Jaagarahu Stage is within the global Group (or ), and Bass Islands Group Sheinwoodian Stage (e.g. Kaljo & Martma 2006) and (or Bass Islands Formation). These strata appear to lie correlates with a position abovethebaseoftheLock- within the upper Ludlow to Pridoli, although bio- port in North America. The base of the Rootsiku¨la stratigraphic control is very poor. The Helderberg Stage is within the global Homerian Stage (Loydell Group was previously considered to be entirely of et al. 1998) and the base of the Paadla Stage is taken to age but it now appears that the lower for- be coincident with the base of the Ludlow Series. The mations (e.g. Manlius and Coeymans) contain strata base of the Kuresaare Stage is clearly within the Ludfor- that cross the base of the Devonian System (e.g. Kleff- dian Stage and the Pridoli Series is divided into the Ka- 13 ner et al. 2009). Based on conodont and d Ccarb data, ugatuma and Ohesaare regional stages. Refinements of the base of the Devonian has begun to be refined in these regional stages within the global classification are the Appalachian Basin (e.g. Saltzman 2002) but recent ongoing, but whereas North American revisions are findings have highlighted the continuing uncertainty requiring boundary positions to be moved by tens of of the position of this boundary in North America metres, recent refinements to East Baltic boundary (Kleffner et al. 2009). positions are requiring only minor changes of a few metres at most (centimetres in most cases). With the Eastern Baltic exception (perhaps) of the United Kingdom, the East Baltic is the chronostratigraphically best constrained A variety of stratigraphic classifications of the Silurian major Silurian region in the world. System of the eastern Baltic (particularly Estonia and Latvia) were in place by the 1970s (e.g. Aaloe et al. Australia 1976) and regional stages have been erected through- out the region. Referred to as ‘gorizont’ in Russian At the time of publication of Holland and Bassett literature (e.g. Adavere Regional Stage = Adaverskij (1989 – A Global Standard for the Silurian System), Gorizont [in Russian]), this term directly translates to the bio- and chronostratigraphic correlation of the ‘horizon’, as in ‘sky line’. However, because in Eng- Silurian System in Australia was known only in a gen- lish the term ‘stratigraphic horizon’ is a geological eral way, which prompted Jell & Talent (1989) in the 192 Cramer et al. LETHAIA 44 (2011) same volume to comment that the Australian Silurian Barrandian succession was likely to be of limited global impor- tance. The work of Young & Laurie (1996) began the Over the past 100 years, the only major addition to process of standardizing the Australian Silurian the division of the Silurian System has been the addi- within the global classification. Later, Talent et al. tion of the Pridoli Series as the fourth and final Silu- (2003) presented a significantly different view from rian series (Bassett 1985). The Pridoli Series is unique that of Jell & Talent (1989) and demonstrated that in the as it is the only series recognized the Australian Silurian succession contains areas with by the IUGS that has not been divided into stages. more than a kilometre of fossiliferous Silurian marine TheGSSPforthebaseofthePridoliSeriesisalso sedimentary rocks that can be easily tied to the global unique in that it is the only Silurian GSSP located out- scheme and can serve as an important source of data side of the United Kingdom. The GSSP is located in from outside of the well-known regions of , the Poza´ry Section, near , in the Czech Repub- , and . A recent review by Strusz lic (Krˇı´zˇ 1989) and was placed to coincide with the (2007) documented in detail the application of global first appearance of the graptolite parulti- series and stage boundaries to the Australian sequence mus and the base of the M. parultimus graptolite and no regional chronostratigraphic terms are cur- Zone. Following Berdan et al. (1969), a four-fold divi- rently in use. Jeppsson et al. (2007) demonstrated sion of the Barrandian Silurian System, essentially high-resolution global correlation much finer than equivalent to the current global classification, was the stage level for the Ludlow portion of the Austra- adopted by Chlupa´cˇ (1972) and by Krˇı´zˇ (1975), and lian sequence and it is likely that such resolution is since then, regional chronostratigraphic terms have possible for the rest of the Silurian succession as well been in disuse. Krˇı´zˇ (1990) demonstrated that the glo- (Talent et al. 2003; Strusz 2007). Future research from bal Silurian series and stages can be easily identified in this region may prove to be of considerable global the highly graptolitic Barrandian succession. The lack importance. of good conodont control for the Pridoli Series in the type region as well as the United Kingdom however has hindered global correlation of Pridoli strata in car- China bonate sequences and significant improvement in our A tripartite division of the Silurian System of China understanding of latest Silurian conodont biostratigra- into Lower, Middle, and Upper Series (Yin 1949) phy and palaeobiology is still needed. had been erected by the middle of the last century and Mu (1962) proposed the names Lungmachi, Loj- Altaj oping, and Hanchiatien for the Lower, Middle, and Upper Series respectively. Following the work of Ge Silurian strata in the modern Altaj were deposited on et al. (1979), the Lower Series was considered to be the tectonically passive south-western shelf of the roughly equivalent to the Llandovery Series, the Mid- Siberian (Yolkin et al. 2003). Follow- dle Series roughly equivalent to the Wenlock Series, ing the work of Kul’kov (1967), Yolkin & Zheltonog- and the Upper Series encompassed both the Ludlow ova (1974), Sennikov (1976), and several others, the and the Pridoli Series. Following the establishment of Silurian succession in this region was divided into a standard global Silurian chronostratigraphic classifi- seven formations that were shown to be allostrati- cation, Mu et al. (1989) suggested that the Chinese graphic units (Yolkin & Zheltonogova 1974). Recent regional terms should be abandoned in favour of the studies have demonstrated that these units are nearly global classification. This decision followed Mu et al. coeval across the Altaj region and are now considered (1986) and was supported by Rong & Chen (2003) to be regional stages (gorizonts; e.g. Sennikov et al. who demonstrated unequivocally that the global Silu- 2008). The Altaj column in Figure 1, and the correla- rian classification can be readily identified within the tions briefly discussed below are modified from Yolkin Chinese succession and regional stratigraphic terms et al. (2003) and Sennikov et al. (2008) unless other- are now largely in disuse. There is however, one his- wise stated. torically significant feature of the Silurian System in The base of the Syrovaty Formation (Stage) and the China that for some years after its ratification, the base of the Gromotukha Series correlate to a position base of the Silurian System continued to be placed at within the uppermost Aeronian Stage, likely near the the base of the Normalograptus persculptus graptolite base of the Stimulograptus sedgwickii graptolite Zone. Zone,whichisthebaseoftheuppermostzoneofthe It is unclear if this is equivalent to the base of the Ada- Ordovician System in the global classification (see vere Stage of the East Baltic, but is likely above the editorial note in Mu et al. 1989, p. 205). This practice base of the Clinton Group in North America. Based was formally abandoned by Chen et al. (1995). on conodont data of Moskalenko (1970), the base of LETHAIA 44 (2011) Silurian chronostratigraphic correlation 193

13 the Polaty Formation is no lower than the middle part features in the d Ccarb curve began to be documented of the Telychian Stage. The base of the Chesnokovka (Popp et al. 1986; Veizer et al. 1986; Corfield et al. Formation and the Tigerek Series has been correlated 1992; Wadleigh & Veizer 1992; Talent et al. 1993). with the uppermost part of the Oktavites spiralis grap- The pioneering data produced from the Swedish tolite Zone in the upper part of the Telychian Stage. island of and the East Baltic region during Above the lower part of the Chesnokovka Formation themiddletolate1990s(Samtlebenet al. 1996; graptolites do not occur in this region, and therefore Wenzel & Joachimski 1996; Bickert et al. 1997; Kaljo 13 the positions of the remaining regional boundaries et al. 1997, 1998) demonstrated three positive d Ccarb within the global scheme are less precise. Kul’kov excursions within the Wenlock to Ludlow interval, (1967) demonstrated that the Chagyrka Formation and these excursions have since been documented likely belongs to the upper part of the Wenlock Series. worldwide (Saltzman 2001; Pore˛bska et al. 2004; ThebaseoftheKuimovmaybecoincidentwiththe Cramer & Saltzman 2005; Noble et al. 2005; Cramer base of the Ludlow Series and the base of the Chernyj et al. 2006a; b; Jeppsson et al. 2007; Kaljo et al. 2007), Anui is correlated tentatively with the base of the Prid- even in non-tropical settings (Lehnert et al. 2007). 13 oli Series. A fourth Silurian positive d Ccarb excursion was also found to be associated with the Silurian–Devonian Other regions boundary interval in the type area of the Barrandian () by Hladikova et al. (1997). Since We have discussed only some regions with extensive then, this feature has been recognized in other parts of Silurian exposures although the mainland European the world as well (Saltzman 2002; Buggisch & Mann 13 sections in, for instance, Belgium, Sardinia, Germany, 2004), yet the d Ccarb record of the Pridoli Series and Austria, have been of significant historical impor- remains less well documented globally than that of tance as well. Their absence here, along with many the Wenlock and Ludlow Series. The most significant 13 other regions, is simply the result of limited space in positive d Ccarb excursions appear to occur within the this article and the fact that most of these other Wenlock, Ludlow, and Pridoli Series however impor- regions now employ the global standard chronostrati- tant isotopic features have been recognized within the 13 graphic classification. The reader is referred to the Llandovery Series as well. Positive d Ccarb excursions summary of Krˇı´zˇ et al. (2003) for data from classical during the early Aeronian, late Aeronian, and early Europeanregions.Inthesamevolume(Landing& Telychian (each approximately +2.0& change) have Johnson 2003) good summaries of Silurian strata from been documented (Kaljo & Martma 2000; Kaljo et al. many other regions not discussed in this article are 2003; Po˜ldvere 2003; Melchin & Holmden 2006; provided as well. Another useful overview of the Silu- Munnecke & Ma¨nnik 2009), but the Llandovery is the 13 rian stratigraphy of Europe was provided by Verniers least known portion of the Silurian d Ccarb curve on et al. (2008). It should be noted here that the excellent the global scale. At present, only the early Aeronian 13 Silurian exposures on the Swedish Island of Gotland and early Telychian d Ccarb excursions have been doc- span an interval from the uppermost part of the Tely- umented from more than one paleobasin, although chian Stage to the upper part of the Ludfordian Stage thelateAeronianexcursionhasbeendocumentedin 13 and serve as the type area for Wenlock and Ludlow d Corg records from multiple paleobasins. 13 13 conodont biostratigraphy and d Ccarb chemostratig- Previous versions of a composite d Ccarb curve for raphy. It has been primarily through comparison with the Silurian (e.g. Azmy et al. 1998) suffered from poor Gotland that global correlation of Wenlock and Lud- biostratigraphic control and low stratigraphic resolu- 13 low carbonate successions has begun to improve. tion, which complicated the use of Silurian d Ccarb Samtleben et al. (1996, 2000), Calner et al. (2004), chemostratigraphy as a chronostratigraphic tool (Kaljo 13 Eriksson & Calner (2005), and Jeppsson et al. (2006) & Martma 2006). An improved d Ccarb curve for the provide a summary overview of the lithostratigraphy, entire Silurian is included here that allows direct corre- 13 biostratigraphy, and chemostratigraphy of Gotland. lation between d Ccarb chemostratigraphy and the Silurian graptolite and conodont biozonations. It is beyond the scope of this report to discuss the Silurian 13 13 A new Silurian d Ccarb d Ccarb record in detail, but there are a few points worth mentioning here. chemostratigraphic curve 13 The middle Ludfordian (Lau) d Ccarb excursion 13 Starting in the late 1980s to early 1990s, it was becom- appears to be the highest magnitude positive d Ccarb excursion in the post- Phanerozoic. Middle ing apparent that the stable carbon isotopic ratio of 13 Silurian seas was highly variable and several discrete to late Ludfordian d Ccarb values typically reach 194 Cramer et al. LETHAIA 44 (2011)

+8.0& Vienna PeeDee Belemnite (VPDB) (Samtleben Theterm‘stageslice’,asusedintheOrdovician et al. 1996, 2000; Martma et al. 2005; Jeppsson et al. System (Bergstro¨m et al. 2009), refers to biochemo- 2007) and values in excess of +10.0& VPDB have stratigraphically defined units most closely resembling been documented (Wigforss–Lange 1999; Jeppsson the concept of assemblage zones (Salvador 1994). At et al. 2007). By the middle Ludfordian it had been present there is no accepted term for a stratigraphic 13 more than 150 million years since d Ccarb values had unit that is defined on the basis of more than one 13 been at such an elevated level and this d Ccarb excur- taxon and⁄or more than one faunal group together sion appears to be unique among those of the Silurian. with the chemostratigraphic content of a given set of 13 The Silurian d Ccarb curve indicates relative instability strata, but that is effectively what stage slices are. in the Silurian global carbon cycle, and the presence of Clearly, further discussion about the rank, utility, defi- 13 at least seven discrete positive d Ccarb excursions in nition, and scope of stage slices is warranted, and their <30millionyearsdemonstratesthattheSilurian introduction to the Silurian here is informal. Period was among the climatically least stable intervals In this article, the levels defined as boundaries of of Earth history. the stage slices are selected based on their chrono- stratigraphic utility. That is, they are levels that can be reliably identified and correlated on several different Stage slices . Following Bergstro¨m et al. (2009) we have used a two letter-number designation for each In regions that are well constrained within the global stage slice where the two letters refer to the global chronostratigraphic classification, intercontinental cor- stage (global series in the case of the Pridoli) and the relation far finer than the stage level is possible. As number refers to the relative position within the stage. noted above, the terms Gleedon and Whitwell were With the exception of Sh3, the base of each stage slice introduced from the Homerian of the United King- is defined by the base of a graptolite or conodont zone dom (Bassett et al. 1975), and the use of allostrati- (Fig. 3). The definition of each stage slice and their 13 graphic units finer than the stage level in correlations of relationship to the generalized Silurian d Ccarb curve Silurian strata within North America is common prac- is provided here. The graptolite biozones referred to tice (e.g. Brett et al. 1998; McLaughlin et al. 2008), below are slightly modified and refined from Melchin whilst biozones (especially graptolite, conodont, and et al. (2004) and Sadler et al. (2009), and the cono- ) are routinely used in intercontinental dont biozones are a composite of several different correlation. Frequently in stratigraphic studies, it is biozonations combined here for global (as opposed to necessary or useful to apply units between the stage regional) utility. The first or last appearances (FADs level and the biozone level. In particular, Silurian car- and LADs) used to define the base of each Silurian bonate sequences that have as yet yielded little or no conodont biozone are shown in Figure 3, and the total conodont biostratigraphic information often can be biozonation follows, in stratigraphic order: Ma¨nnik correlated only at a level coarser than the biozone, (2007); Jeppsson (1997); Jeppsson & Calner (2003); and providing a level of correlation between the stage Jeppsson & Aldridge (2000); Jeppsson et al. (2006); level and biozone level would be useful. Here we Corradini & Serpagli (1999); and Corriga & Corradini informally introduce the ‘stage slice’ for the Silurian (2009). as such a unit.

13 Fig. 3. Silurian graptolite and conodont biostratigraphic zonations, shown in relation to global stages and a generalized d Ccarb curve for the Silurian System. With the exception of Sh3, the base of each stage slice is coincident with the base of either a graptolite or conodont zone. 13 The generalized d Ccarb curve is based on a composite from the following sources: Rhuddanian-Aeronian: Po˜ldvere (2003); Uppermost Aeronian: 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 Sadler et al. (2009) and the FADs and LADs shown to the right of the conodont zonation refer to the definition of the conodont zones utilized in this 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. – ; Asp. – Aspelundia; Pr. – Pranognathus; Pt. – Pterospathodus; am. – amorpho- gnathoides; procerus – Pterospathodus pennatus procerus; K. – ; ranuli. – ranuliformis; O. – Ozarkodina; s. – sagitta; v. – variabilis; A. – Ancoradella; Po. – Polygnathoides; Ou. – ; w. – woschmidti; S.Z. – SuperZone; I.Z. – Interval Zone; s.l. – sensu lato. A slash between names in the graptolite zonation indicates a single ‘combined’ zone (e.g. Co. cyphus ⁄ M. revolutus). Dashed lines in the graptolite column denote uncertainty in the placement of that boundary with respect to Silurian series and stage boundaries and the relative duration of the zone. 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 Wenlock Series. See text for discussion of this boundary. LETHAIA 44 (2011) Silurian chronostratigraphic correlation 195

Ep- grapt- cono- Ma Age s.s. och olite dont 415 M. I. w. DEVONIAN uniformis woschmidti 416.0 Ou. M. transgrediens elegans Pr2 / M. perneri detortus FAD Ou. e. detortus M. bouceki M. branikensis / O. eostein- M. lochkovensis hornensis Pr1 M. ultimus s.l. I.Z.

Pridoli M. parultimus 418.7 LAD O. crispa Lu3 M. formosus O. crispa FAD O. crispa Ne. kozlowskii / O. snajdri 420 Lu2 Po. podoliensis I.Z. Lau LAD P. siluricus Bohemograptus Po. siluricus Lu1 Sa. leintward. / FAD P. siluricus Sa. linearis A. ploeckensis 421.3 FAD A. ploeckensis Lo. Go2 K. v. vari- Ludlow scanicus abilis I.Z. N. nilssoni / LAD K. crassa Gor.Go1 Lud. Lo. progenitor K. crassa 422.9 FAD K. crassa

Ho3 Col. ludensis K. ortus absidata Col. deubeli Col. praedeubeli FAD K. ortus absidata Mulde 425 Ho2 Pri. parvus / O. G. nassa bohemica longa Cy. FAD O. bohemica longa Ho1 lundgreni O. s. sagitta 426.2 FAD O. sagitta sagitta Cy. rigidus / K. ortus ortus Sh3 FAD K. ortus ortus M. K. walliseri

Wenlock FAD K. walliseri Sh2 belophorous O. s. rhenana FAD O. sagitta rhenana Ireviken M. riccarton. K. ranuli. S.Z.

Shein. Homer. LAD Pt. penn. procerus Sh1 procerus S.Z. 428.2 Cy. murchisoni LAD Pt. am. amorph. Cy. centrifugus Te5 Cy. insectus Pt. am. amorpho- 430 Cy. gnathoides Te4 lapworthi FAD Pt. am. amorph. Pt. am. O. lithuanicus spiralis FAD Pt. am. lithuanicus Pt. am. lennarti Te3 FAD Pt. am. lennarti Pt. am. Mcl. angulatus Telychian crenulata FAD Pt. am. angulatus Mcl. grieston. Pt. Te2 eopennatus S. crispus S.Z. Valgu 435 FAD Pt. eopennatus Sp. turriculatus Te1 Sp. guerichi D. stauro- 436.0 S. gnathoides Ae3 sedgwickii LAD Pr. tenuis Late Aeronian L. convolutus Pr. tenuis FAD Pr. tenuis Ae2 M. argenteus / Pr. leptotheca Llandovery D. tri- Asp. Ae1 angulatus / expansa Early Aeronian Aeronian D. pectinatus 439.0 Co. cyphus / FAD Asp. expansa Rh3 440 M. revolutus

C. Rh2 vesiculosus D. kentucky- P. acuminatus ensis Rh1

Rhuddanian A. ascensus 443.7 N. A. ORDOVICIAN persculptus ordovicicus –2 –1 0 +1 +2 +3 +4 +5 +6 +7 +8 +9 +10 445 196 Cramer et al. LETHAIA 44 (2011)

Definitions of potential Silurian stage slices Ae2. – Base of the Monograptus argenteus ⁄Pribylo- graptus leptotheca graptolite Zone to the base of the Sti- Rh1. – Base of the Silurian (first appearance of the mulograptus sedgwickii graptolite Zone. This includes graptolite Akidograptus ascensus)tothebaseofthe the M. argenteus ⁄Pr. leptotheca and Lituigraptus convo- Cystograptus vesiculosus graptolite Zone. This includes lutus graptolite zones as well as the upper part of the the Parakidograptus acuminatus graptolite Zone and is Asp. expansa and lower part of the Pranognathus tenuis within the Distomodus kentuckyensis conodont Zone, conodont zones of the middle part of the Aeronian 13 roughly covering the lower half of the Rhuddanian Stage. Generally, non-excursion or ‘baseline’ d Ccarb 13 Stage. The end of the Hirnantian d Ccarb excursion is values have been recovered from strata within this stage well within the Hirnantian and baseline values persist slice. The base of the Clinton Group in North America throughout this stage slice. is likely located within this stage slice.

Rh2. – Base of the Cystograptus vesiculosus graptolite Ae3. – Base of the Stimulograptus sedgwickii grapto- Zone to the base of the Coronograptus cyphus⁄Monog- lite Zone to the base of the Spirograptus guerichi grap- raptus revolutus graptolite Zone. This includes the tolite Zone. This includes the St. sedgwickii graptolite Cystograptus vesiculosus graptolite Zone and is within Zone, the top of the Pr. tenuis and the lower part of the Distomodus kentuckyensis conodont Zone, roughly the Distomodus staurognathoides conodont zones and covering the middle of the Rhuddanian Stage. A dis- represents the uppermost part of the Aeronian Stage. 13 tinct negative excursion near the top of this stage slice A positive d Ccarb excursion has been recorded in is known from the eastern Baltic but the global pres- Ae3 strata from the Canadian Arctic and appears to ence and precise chronostratigraphic position of this be restricted to this stage slice. This excursion has yet 13 excursion has not been demonstrated. This excursion to be reproduced elsewhere in d Ccarb records, but 13 mayoccureitherwithintheC. vesiculosus graptolite occurs in d Corg records from (Melchin & Zone or the lower part of the Co. cyphus ⁄M. revolutus Holmden 2006) and Bohemia (Sˇtorch & Fry´da 2009). 13 graptolite Zone and may correspond to one of two Aprotractedintervaloflowd Ccarb values has been weak negative excursions present in this interval in recoveredfromtheuppermostpartofthisstageslice Arctic Canada (Melchin & Holmden 2006) but it has (and the lowermost part of Te1) from Arctic Canada yet to be studied in detail elsewhere. that appears to follow immediately the positive 13 d Ccarb excursion. The base of the Syrovaty Forma- Rh3. – Base of the Co. cyphus ⁄M. revolutus graptolite tion and the Gromotukha Series in Altaj coincide with ZonetothebaseoftheDemirastrites triangulatus ⁄D. the base of this stage slice. A similar position (or pectinatus graptolite Zone. This includes the C. cy- within Ae3) is likely for the base of the Adavere Stage phus ⁄M. revolutus graptolite Zone and most of the in the East Baltic. stage slice is within the upper part of the Distomodus kentuckyensis conodont Zone. The lowermost part of Te1. – Base of the Spirograptus guerichi graptolite the Aspelundia expansa conodont Zone is included in Zone to the base of the Pterospathodus eopennatus the upper part of this stage slice that represents the conodont Superzone and the Pterospathodus eopenna- upper part of the Rhuddanian Stage. The base of the tus ssp. n. 1 conodont Zone. This includes all of the Raikku¨la Stage in the East Baltic probably lies low Sp. guerichi and almost all of the Sp. turriculatus grap- within this stage slice. tolite zones, is within the upper part of the D. stauro- gnathoides conodont Zone, and represents the Ae1. – Base of the Demirastrites triangulatus ⁄D. pec- lowermost part of the Telychian Stage. tinatus graptolite Zone to the base of the Monograptus argenteus⁄Pribylograptus leptotheca graptolite Zone. Te2. – Base of the Pterospathodus eopennatus cono- This includes the D. triangulatus ⁄D. pectinatus grap- dont Superzone and the Pt. eopennatus ssp. n. 1 cono- tolite Zone and is within the Asp. expansa conodont dont Zone to the base of the Monoclimacis crenulata Zone roughly corresponding to the lower third of the graptolite Zone. This includes the uppermost part of 13 Aeronian Stage. There is a positive d Ccarb excursion the Spirograptus turriculatus, and all of the Strepto- recorded in strata from this interval in the East Baltic graptus crispus, Streptograptus sartorius,andMcl. grie- (upper part of the D. triangulatus graptolite Zone stoniensis graptolite zones, as well as all of the in the Ruhnu core; Po˜ldvere 2003), which was also Pterospathodus eopennatus ssp. n. 1 conodont Zone observed in lower Aeronian strata from Arctic and most of the Pt. eopennatus ssp. n. 2 conodont Canada by Melchin & Holmden (2006), but this has Zone (most of the Pt. eopennatus conodont Superz- 13 yet to be recognized elsewhere. It is restricted to this one). A positive d Ccarb excursion (the Valgu excur- stage slice. sion) has been known from Te2 strata in the eastern LETHAIA 44 (2011) Silurian chronostratigraphic correlation 197

Baltic for more than a decade and this excursion has respectively), Lower and Upper Pterospathodus penna- recently been identified in North America (Munnecke tus procerus (Datum 3-4 and Datum 4-6 respectively), &Ma¨nnik 2009). This excursion appears to be and Lower Kockelella ranuliformis (Datum 6-8) cono- restrictedtothisstageslice.ThebaseofthePolaty dont zones. The onset of the well-known early Shein- 13 Formation in Altaj may correlate with a position woodian (Ireviken) positive d Ccarb excursion lies 13 within this stage slice (Sennikov et al. 2008, p. 13 within the lower part of this stage slice and d Ccarb record graptolites of the Mcl. griestoniensis Zone). values continue to rise throughout the rest of Sh1. At present, the base of the Cyrtograptus murchisoni grap- Te3. – Base of the Monoclimacis crenulata graptolite tolite Zone is tentatively considered to correspond to Zone to the base of the Pterospathodus amorphognatho- the base of the Wenlock Series (see Loydell 2008; ides amorphognathoides conodont Zone. This includes Cramer et al. 2010), and the base of this stage slice the uppermost part of the Pterospathodus eopennatus correlates with the base of the Sheinwoodian Stage ssp. n. 2 conodont Zone, all of the Pt. amorphognatho- and the Wenlock Series. ides angulatus, Pt. am. lennarti, and Pt. am. lithuanicus conodont zones, as well as all of the Mcl. crenulata and Sh2. – Base of the Upper K. ranuliformis conodont Oktavites spiralis graptolite zones. Stage slice Te3 covers Zone (within the K. ranuliformis conodont Superzone) roughly the middle third of the Telychian Stage. The to the end of the early Sheinwoodian (Ireviken) 13 base of the Tigerek Series and Chesnokovka Formation d Ccarb excursion. This includes the remainder of the in Altaj are probably within this stage slice. Monograptus riccartonensis graptolite Zone and the lower part of the Cyrtograptus rigidus ⁄Monograptus Te4. – Base of the Pterospathodus amorphognathoides belophorus graptolite Zone as well as all of the upper K. amorphognathoides conodont Zone to the base of the ranuliformis, Ozarkodina sagitta rhenana and lower 13 Cyrtograptus insectus graptolite Zone. This includes part of the Kockelella walliseri conodont zones. d Ccarb the lower part of the Pterospathodus amorphognatho- values first reach their maximum near the base of this ides amorphognathoides conodont Zone and probably stage slice and return to a level consistently at or below all of the Cyrtograptus lapworthi graptolite Zone corre- pre-excursion values by the top of Sh2. The base of the sponding to an interval within the upper part of the Lockport Group in North America correlates with a Telychian Stage. position in the middle of Sh2 and may be equivalent to the base of the Jaagarahu Stage in the East Baltic. Te5. – Base of the Cyrtograptus insectus graptolite Zone to the base of the Cyrtograptus murchisoni grapto- Sh3. – End of the early Sheinwoodian (Ireviken) 13 lite Zone. This includes the Cyrtograptus insectus and C. d Ccarb excursion to the base of the Cyrtograptus centrifugus graptolitezonesaswellasmostoftheupper lundgreni graptolite Zone. This includes the remainder part of the Pterospathodus amorphognathoides amorpho- of the Cyrtograptus rigidus ⁄Monograptus belophorus gnathoides conodont Zone of the uppermost Telychian graptolite Zone, the Cyrtograptus perneri graptolite Stage. The base of the Jaani Stage in the East Baltic likely Zone, as well as the upper part of the K. walliseri,all correlates with a level within this stage slice. of the K. ortus ortus, and the lowermost part of the Ozarkodina sagitta sagitta conodont zones. This stage Sh1. – Base of the Cyrtograptus murchisoni graptolite slice coincides with the upper part of the Sheinwoo- 13 Zone to the base of the Upper Kockelella ranuliformis dianStageabovetheIrevikend Ccarb excursion. conodont Zone within the K. ranuliformis conodont Superzone. At present, the base of the C. murchisoni Ho1. – Base of the Cyrtograptus lundgreni graptolite graptolite Zone is tentatively considered to correlate Zone to the base of the Pristiograptus parvus ⁄Gotho- with the base of the Wenlock Series (see Loydell 2008; graptus nassa graptolite Zone. This includes most of Cramer et al. 2010), and the base of this stage slice the Ozarkodina sagitta sagitta and the lower part of correlates with the base of the Sheinwoodian Stage the Oz. bohemica longa conodont zones. This stage and Wenlock Series as defined by the current GSSP slice is equivalent to the Cy. lundgreni graptolite Zone (see discussion above). This stage slice includes all of and approximates to the Gleedon Chronozone of the Cyrtograptus murchisoni, Monograptus firmus,and Bassett et al. (1975) (see discussion in Zalasiewicz & lower part of the M. riccartonensis graptolite zones. Williams 1999). The Mulde Event (conodonts) and This stage slice contains the uppermost part of the the ‘big crisis’ (graptolites) begin near, and at the top Pterospathodus amorphognathoides amorphognathoides respectively, of this stage slice. conodont Zone and the entirety of the Ireviken Event (Datum 1-8), which includes the Lower and Upper Ho2. – Base of the Pristiograptus parvus⁄Gothograptus Pseudooneotodus bicornis (Datum 1-2 and Datum 2-3 nassa graptolite Zone to the base of the Colonograptus 198 Cramer et al. LETHAIA 44 (2011) ludensis graptolite Zone. This includes the Pri. Par- GSSP remains uncertain and these three positions are vus ⁄G. nassa, Col. praedeubeli,andCol. deubeli grapto- tentatively correlated at the same level here. It may litezonesaswellastheremainderoftheOzarkodina however be that the base of the A. ploeckensis conodont bohemica longa,andlowerpartoftheKockelella ortus Zone is below the base of the Sa. leintwardinensis ⁄Sa. absidata conodont zones. The first peak of the Homeri- linearis graptolite Zone, which says nothing of their 13 an (Mulde) d Ccarb excursion appears to be restricted correlation to the GSSP. A low amplitude (but strati- 13 to Ho2. The base of the Rootsiku¨la Stage in the eastern graphically consistent) positive d Ccarb excursion asso- Baltic lies within Ho2. ciated with the underlying Linde Event has been found on Gotland from Gorstian–Ludfordian boundary Ho3. – Base of the Colonograptus ludensis graptolite strata, but this feature remains to be reproduced from Zone to the base of the Neodiversograptus nilssoni ⁄ other regions. The onset of the much higher amplitude 13 Lobograptus progenitor graptolite Zone. This includes middle Ludfordian (Lau) d Ccarb excursion begins in the upper part of the Kockelella ortus absidata cono- the uppermost part of this stage slice. dont Zone to the base of the Kockelella crassa cono- dont Zone and is equivalent to the Col. ludensis Lu2. – Base of the Ozarkodina snajdri conodont graptolite Zone. Ho2 and Ho3 combined approximate Interval Zone to the base of the Ozarkodina crispa to the Whitwell Chronozone of Bassett et al. (1975). conodont Zone. This includes the upper part of the This stage slice represents the uppermost part of the Bohemograptus,alloftheNeodiversograptus kozlow- Homerian Stage and includes the second peak of the skii ⁄Polonograptus podoliensis, and lowermost part of 13 Homerian (Mulde) d Ccarb excursion. the Monograptus formosus graptolite zones. This stage slice is equivalent to the Oz. snajdri conodont Interval Go1. – Base of the Neodiversograptus nilssoni ⁄Lobo- Zone spanning the middle part of the Ludfordian 13 graptus progenitor graptolite Zone to the base of the Stage. The middle Ludfordian (Lau) d Ccarb excur- Lobograptus scanicus graptolite Zone. This includes all sion is largely restricted to this stage slice where 13 of the Kockelella crassa conodont Zone and lowermost d Ccarb values rise over most of the lower part of this part of the Kockelella variabilis variabilis conodont stage slice, peak values are restricted to the middle part 13 Interval Zone. This stage slice is equivalent to the N. of Lu2, and d Ccarb values decrease through the upper nilssoni ⁄L. progenitor graptolite Zone covering the part of this stage slice. lower part of the Gorstian Stage. The base of Go1 is coincident with the base of the global Gorstian Stage Lu3. – Base of the Ozarkodina crispa conodont Zone and Ludlow Series, with the base of the regional to the base of the Monograptus parultimus graptolite Paadla Stage in the East Baltic, and potentially with Zone. This includes the remainder of the M. formosus the base of the Salina Group and Cayugan Series in graptolite Zone and is equivalent to the O. crispa North America. conodont Zone coinciding with the upper part of the Ludfordian Stage. Primarily, this stage slice contains Go2. – Base of the Lobograptus scanicus graptolite Ludfordian strata above the middle Ludfordian (Lau) 13 Zone to the base of the Saetograptus leintwardinen- d Ccarb excursion.ThebaseoftheKuressaareStagein sis ⁄Saetograptus linearis graptolite Zone. This includes the East Baltic region may be coincident with the base the remainder of the Kockelella variabilis variabilis of Lu3 or lies somewhat below it. conodont Interval Zone of the upper part of the Gors- tian Stage. Go2 is equivalent to the Lobograptus scani- Pr1. – Base of the Monograptus parultimus graptolite cus graptolite Zone and contains the biotic event Zone to the base of the Monograptus bouceki graptolite termed the Linde Event (Jeppsson 1993). Zone. This includes all of the M. parultimus, M. ulti- mus,andM. branikensis ⁄M. lochkovensis graptolite Lu1. – Base of the Saetograptus leintwardinensis ⁄ zonesaswellasthemajorityoftheOzarkodina Saetograptus linearis graptolite Zone to the base of the eosteinhornensis sensu lato conodont Interval Zone and Ozarkodina snajdri conodont Interval Zone. This represents roughly the lower half of the Pridoli Series. includes all of the Sa. leintwardinensis⁄Sa. linearis and The base of the regional Kaugatuma Stage in the East the lower part of the Bohemograptus graptolite zones as Baltic is considered to be coincident with the base of well as all of the Ancoradella ploeckensis and Polygna- the global Pridoli Series. The top of the Salina Group thoides siluricus conodont zones. The position of the in North America is likely within this stage slice. base of the A. ploeckensis conodont Zone with respect to the base of the Sa. leintwardinensis ⁄Sa. linearis grap- Pr2. – Base of the Monograptus bouceki graptolite tolite Zone, and the position of the base of either of Zone to the base of the Monograptus uniformis grapto- these biozones with respect to the base Ludfordian lite Zone. This includes all of the M. bouceki and M. LETHAIA 44 (2011) Silurian chronostratigraphic correlation 199 transgrediens ⁄M. perneri graptolite zones, the remain- Azmy, K., Veizer, J., Bassett, M.G. & Copper, P. 1998: Oxygen and carbon isotopic composition of Silurian : Implica- der of the Ozarkodina eosteinhornensis sensu lato cono- tions for coeval seawater and glaciations. Geological Society of dont Interval Zone, all of the Oulodus elegans detortus America Bulletin 110, 1499–1512. conodont Zone, and likely the lowest part of the Icrio- Bancroft, A.M. 2008: Silurian conodont biostratigraphy and car- bonate carbon isotope stratigraphy of the Eramosa of South- dus woschmidti woschmidti conodont Zone. The onset western Ontario, Canada. Unpublished MSc thesis. The Ohio and most of the rising limb of the Silurian–Devonian State University, Columbus, 45 pp. 13 boundary (Klonk) d Ccarb excursion is within Pr2 Barrick, J.E., Kleffner, M.A. & Karlsson, H.R. 2009: Conodont fau- nas and stable isotopes across the Mulde Event (late Wenlock; strata. This stage slice represents the uppermost part Silurian) in southwestern Laurentia (south-central Oklahoma of the Pridoli Series and the base of the Ohesaare Stage and subsurface west Texas). In Over, D.J. (ed.): Conodont Studies in the East Baltic probably lies within this stage slice. Commemorating the 150th Anniversary of the First Conodont Paper (Pander, 1856) and the 40th Anniversary of the Pander Society, 41–56. Palaeontographica Americana 62. Allen Press, Lawrence. Conclusions Bassett, M.G. 1985: Towards a common language in stratigraphy. Episodes 8, 87–92. Bassett, M.G., Rickards, R.B. & Warren, P.T. 1975: The type The chronostratigraphic correlation of the Silurian Wenlock Series. 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With the new Silurian and to d13C chemostratigraphy. Lethaia 42, 97–107. 13 Berry, W.B.N. & Boucot, A.J. 1970: Correlation of the North d Ccarb curve and the recent improvements in Silu- American Silurian rocks. Geological Society of America Special rian conodont biozonations there remains little justifi- Publication 102,1–289. cation for retaining regional chronostratigraphic Bickert, T., Pa¨tzold,J.,Samtleben,C.&Munnecke,A.1997:Paleo- classifications of the Silurian System. As more data environmental changes in the Silurian indicated by stable iso- topes in shells from Gotland, Sweden. Geochimica et become available, refinements to this curve will be Cosmochimica Acta 61, 2717–2730. required and the definitions of the stage slices will Brett, C.E., Tepper, D.H., Goodman, W.M., Loduca, S.T. & Eckert, remain similarly open to revision. B.-Y. 1995: Revised stratigraphy and correlation of the Niagaran Provincial Series (Medina, Clinton, and Lockport Groups) in the type area of western New York. United States Geological Sur- Acknowledgements. – The authors thank the Lethaia editors, vey Bulletin 2086,66pp. Dimitri Kaljo, and an anonymous reviewer whose detailed and Brett, C.E., Baarli, B.G., Chowns, T.M., Cotter, E., Driese, S. & insightful comments made this a stronger manuscript. This work is Johnson, M.E. 1998: Early Silurian condensed horizons, iron- a contribution to the IGCP 503 project and was partially funded by stones, and sequence stratigraphy in the Appalachian Foreland NSF grants # EAR 0518511 to CEB and W. Huff as well as # EAR basin. In Landing, E., Johnson, M.E. (eds): Silurian Cycles – Link- 0517929 to MAK and J. Barrick; NSERC (Canada) Discovery Grant ages of Dynamic Stratigraphy with Atmospheric, Oceanic, and Tec- to MJM; Estonian Science Foundation grants #7138 and #7640 to tonic Changes, 89–144. 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