TheBlackwell Publishing Ltd new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages and to δ13C chemostratigraphy
STIG M. BERGSTRÖM, XU CHEN, JUAN CARLOS GUTIÉRREZ-MARCO AND ANDREI DRONOV
Bergström, S.M., Chen, X., Gutiérrez-Marco, J.C. & Dronov, A. 2009: The new chrono- LETHAIA stratigraphic classification of the Ordovician System and its relations to major regional series and stages and to δ13C chemostratigraphy. Lethaia, Vol. 42, pp. 97–107
The extensive work carried out during more than a decade by the International Subcommission on Ordovician Stratigraphy has resulted in a new global classification of the Ordovician System into three series and seven stages. Formal Global Boundary Stratotype Section and Points (GSSPs) for all stages have been selected and these and the new stage names have been ratified by the International Commission on Stratigraphy. Based on a variety of biostratigraphic data, these new units are correlated with chronostratigraphic series and stages in the standard regional classifications used in the UK, North America, Baltoscandia, Australia, China, Siberia and the Mediterranean- North Gondwana region. Furthermore, based mainly on graptolite and conodont zones, the Ordovician is subdivided into 20 stage slices (SS) that have potential for precise correlations in both carbonate and shale facies. The new chronostratigraphic scheme is also tied to a new composite δ13C curve through the entire Ordovician. Chemostratigraphy, chronostratigraphy, classification, Ordovician System, stage slices.
Stig M. Bergström [stig@geology.ohio-state.edu], School of Earth Sciences, Division of Geological Sciences, The Ohio State University, 155 Oval Mall, Columbus, Ohio 43210, USA; Chen Xu [[email protected]], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; Juan Carlos Gutiérrez-Marco [[email protected]], Instituto de Geológia Económica, CSIC-UCM, Facultad de Ciencias Geológicas, 28040 Madrid, Spain; Andrei Dronov [[email protected]], Geological Institute of the Russian Academy of Sciences, Pyzhevsky per. 7, Moscow 119017, Russia; manuscript received on 1/11/2007; manuscript accepted on 20/2/2008.
As described by Webby (1998), Finney (2005), and shown by the fact that it was used in virtually all Bergström et al. (2006b), one of the principal goals of presentations at the 10th International Symposium the International Subcommission on Ordovician on the Ordovician System that was held in Nanjing, Stratigraphy has been to establish a globally applicable China, in June 2007. The purpose of the present series and stage classification of the Ordovician contribution is to summarize how the new global System. The conspicuous palaeobiogeographic terms correspond to terms used in various regional differentiation of Ordovician faunas has led to the classifications. Because many of the regional stages development of different regional chronostratigraphic are based on fossils with a very limited geographic classification schemes for virtually all the major range, the position of such chronostratigraphic units outcrop areas of Ordovician rocks, and terms such as is in some cases difficult to establish with adequate the Middle Ordovician have had a vastly different precision in the global scheme. We also introduce stratigraphic scope in different regions. This has subdivisions of the global stages, referred to as stage caused confusion, not only to the non-stratigraphers, slices (SS), which are biostratigraphically defined and there has been an urgent need for a globally units. Finally, we present the first modern δ13C curve applicable chronostratigraphic classification of the through the entire Ordovician and illustrate its Ordovician. After more than a decade of work, the relations to the new global stages and stage slices. goal of establishing such a global classification has been successfully reached, and the ratified new stage and series terms are shown in Figure 1. All the new Notes on the correlation of the new stages have an approved Global Boundary Stratotype chronostratigraphic units Section and Point (GSSP), the locations of which are summarized in Bergström et al. (2006b). It is of interest It is outside this brief summary to discuss in detail to note that the new chronostratigraphic scheme has the relations between all the new global units and the already gained a wide approval internationally as regional series and stages and we will herein restrict
DOI 10.1111/j.1502-3931.2008.00136.x © 2008 The Authors, Journal compilation © 2008 The Lethaia Foundation
98 Bergström et al. LETHAIA 42 (2009)
Fig. 1. Chart showing proposed correlation between the new global series and stages and regional chronostratigraphic units recognized in major outcrop areas of Ordovician rocks. Also shown are the stratigraphic positions of the stage slices (SS) defined in the present study. The Siberian and Iberian columns were compiled by Andrei Dronov and Juan Carlos Gutiérrez-Marco, respectively. The Global Boundary Stratotype Section and Point (GSSP) boxes in the North American column refer to the base of the Ordovician System and the base of the Katian Stage, respectively; those in the Baltoscandian column to the bases of the Floian and Sandbian Stages, respectively; and those in the Chinese column to the bases of the Dapingian, Darriwilian, and Hirnantian Stages, respectively. ourselves to some major points. For convenience, we Fortey et al. (2000) suggested that the base of the will deal with each region in the same order as shown Arenig in northern England may correlate with the in Figure 1. In the compilation of Figure 1, we have base of the Baltoscandic Tetragraptus phyllograptoides used much information from Webby et al. (2004) Graptolite Zone and the global Tetragraptus approx- together with new data. The scheme presented here imatus Graptolite Zone, that is, the base of the global differs in important respects from that of Webby Floian Stage. The same authors considered the base et al. (2004) as well as that published by Cooper and of the Llanvirn Series to be equivalent to the base of Sadler (2004). For a general reference to graptolite the Didymograptus artus Graptolite Zone but the and conodont zones mentioned below, see Webby precise global correlation of the base of this graptolite et al. (2004, figs 2.1 & 2.2). zone is currently not without problems although it is likely to correspond to a level in the lower–middle United Kingdom global Darriwilian Stage. The base of the Caradoc Series, although currently lacking an officially The series and stage classification in the UK column designated reference section, is taken to be the base follows Fortey et al. (2000), who recognized five of the Nemagraptus gracilis Graptolite Zone, that is, series (the Tremadoc, Arenig, Llanvirn, Caradoc, and it is the same as the base of the global Sandbian Stage. Ashgill Series) and 15 stages. The majority of these The base of the Ashgill Series is defined on shelly chronostratigraphic units are based on shelly fossils fossils in the type area, and the precise correlation of with a limited geographic distribution and their precise this level into the graptolite and conodont zone correlation to standard graptolite and conodont successions has been problematic and controversial. zones is in most cases not well established. However, recent chitinozoan investigations in the Ashgill In many areas the base of the Arenig Series type sections (Vandenbroucke 2005; Vandenbroucke coincides with a significant stratigraphic gap but et al. 2005) suggested that the base of the Pusgillian based on somewhat non-diagnostic faunal data, Stage, the lowest stage in the Ashgill Series, corresponds
LETHAIA 42 (2009) Ordovician chronostratigraphic classification 99 to a level close to the base of the Baltoscandic Nabala cently exposed in its type area in western Utah, and Stage. This horizon is in Baltoscandia taken to be at, contains abundant and diverse shelly faunas and or very near, the base of the Pleurograptus linearis conodonts, these carbonate sequences contain Graptolite Zone. Previously, the base of the Ashgill relatively few stratigraphically diagnostic graptolites has been correlated with a significantly higher level useful for correlation with the global Tremadocian in the graptolite zone succession, namely the middle and Floian Stages. Graptolite studies indicated that to upper part of the P. linearis Graptolite Zone. The the base of the Whiterockian Series in its classical new interpretation is in general agreement with the reference section at Whiterock Canyon in central fact the important zone conodont Amorphognathus Nevada is considerably younger than previously ordovicicus, which elsewhere appears in the middle of thought and the base of the Whiterockian is now the P. linearis Graptolite Zone and at a corresponding defined in the Ibex area succession. As shown by level in the North American Amplexograptus mani- sections in other areas, that level approximates the toulinensis Graptolite Zone (Goldman & Bergström base of the Isograptus v. lunatus Graptolite Zone and 1997), has its first occurrence in the Ashgill Series hence is equivalent to a horizon in the uppermost type area in the lower to middle Cautleyan Stage Floian Stage. A succession of standard stages has not (Orchard 1980). It should be noted that Fortey et al. yet been defined through the Whiterockian Series (2000) correlated this interval with substantially but the Chazyan Stage falls in the uppermost White- younger strata (uppermost Dicellograptus complanatus rockian Series if the base of the overlying Mohawkian to D. anceps Graptolite Zones), which is in conflict Series is taken to be the base of the Baltoniodus with both the conodont and the chitinozoan evidence. gerdae Conodont Subzone as suggested by Ross et al. In terms of the new global stage classification, the (1982). That level approximates the base of the base of the Ashgill Series would correspond to a level Atlantic Province D. foliaceus Graptolite Zone as well in the middle Katian Stage. as the base of the Pacific Province Climacograptus Most of the many stages recognized in the UK bicornis Graptolite Zone, and it corresponds to a succession are based on shelly fossils but unfortunately, level in the lower to middle Sandbian Stage. In recent there is very little direct evidence how these units years, the Mohawkian Series has been subdivided correlate precisely to standard graptolite and conodont into two stages, the Turinian and Chatfieldian Stages zones and hence, their relations to the new global (Leslie & Bergström 1995). The base of the latter stages remain uncertain. The general correlation stage is just below the base of the global Katian Stage. presented in Figure 1 agrees in most cases with those The base of the Cincinnatian Series has not been presented by Webby et al. (2004) and Cooper & closely defined faunally in the type area but it is just Sadler (2004) but it must be stressed that it is highly above the base of the Amorphognathus superbus preliminary in nature and involves many problems. Atlantic Conodont Zone and the Belodina confluens Midcontinent Conodont Zone (Bergström & Mitchell North America 1986, 1990, 1994) and near the top of the Diplacan- thograptus spiniferus Graptolite Zone. This level In the current standard North American classification, corresponds to a horizon in the lower Katian Stage. the Ordovician is subdivided into four series, the The four standard stages recognized in the Cincinnatian Ibexian, Whiterockian, Mohawkian and Cincinnatian Series are largely based on shelly fossils but the graptolites Series (Ross et al. 1982). The reference sections of the present (Bergström & Mitchell 1986, 1990, 1994) are Ibexian and Whiterockian Series are located in Utah useful for correlation to standard graptolite zones. A and Nevada in western USA, that of Mohawkian key level for global correlation is the first appearance Series in New York State and adjacent Ontario in of the zone conodont A. ordovicicus in the lower, but eastern USA, and that of the Cincinnatian Series in not lowermost, Richmondian Stage (Bergström & Ohio, Kentucky, and Indiana in the eastern Midcon- MacKenzie 2005), a level that corresponds to the tinent. For a long time, the very different location of lower–middle Cautleyan Stage in the UK and the the series reference sections, along with significant upper Nabala to lowermost Vormsi Stage in Baltoscandia faunal differences, led to difficulties establishing the (Nõlvak et al. 2006). The base of the North American close relations between these series but most of these Gamachian Stage is currently not well defined problems have now been solved. biostratigraphically in its type area on Anticosti Available information suggests that the base of the Island, Quebec (McCracken & Nowlan 1986) where Ibexian Series is slightly older than the base of the its base is taken to be at the base of the Ellis Bay Iapetognathus fluctivagus Conodont Zone, that is, Formation. The fact that this level is substantially the ratified base of the Ordovician System (Ross et al. below the beginning of the Hirnantian δ13C excursion 1997). Although the Ibexian succession is magnifi- (Bergström et al. 2006a) that elsewhere approximates
100 Bergström et al. LETHAIA 42 (2009) the base of the Hirnantian Stage suggests that the and more than 30 standard graptolite zones are base of the Gamachian Stage is well below the base of currently distinguished in Australia and New Zealand the Hirnantian Stage. Currently, available biostrati- (VandenBerg & Cooper 1992). The graptolites provide graphic and chemostratigraphic (Bergström et al. useful ties to the global stage classification and there 2007) information indicates that the Gamachian are few problems in correlating the Australian stages Stage is not represented in the Cincinnatian Series with those in the new global classification. reference sections in the Cincinnati region. China Baltoscandia Several chronostratigraphic classifications have been Compared to those of the UK and North America, the proposed for the Chinese Ordovician (see, for regional series and stage classification in the Ordovician instance, Mu et al. 1979, 1980; Zhang et al. 1982; of Baltoscandia (see, for instance, Männil & Meidla Wang et al. 1992; Chen et al. 1995). In the present 1994; Nõlvak 1997) has been remarkably stable for paper, the global Tremadocian, Floian, Dapingian, decades and the changes, such as the replacement of Darriwilian, and Hirnantian Stages are used for the Idavere and Jõhvi stages with the Haljala Stage China in their ratified stratigraphic scopes. Three (Jaanusson 1995), have been minor. The current Ordovician GSSPs are situated in South China (Chen chronostratigraphic classification (for a recent review, & Bergström 1995; Wang et al. 2005; Chen et al. see Nõlvak et al. 2006) was mainly developed in the 2006a,b). Also, an auxiliary GSSP section for the base East Baltic region (especially Estonia) but much of of Upper Ordovician is located in Xinjiang, China this scheme has proved to be applicable also to the (Bergström et al. 1999). Two regional stages, successions in Sweden and elsewhere in Baltoscandia. Neichiashanian and Chientangkiangian, remain for Three series are recognized, the Öland, Viru, and the current usage in China. The base of Katian Stage Harju Series. The base of the Viru Series corresponds in China is mainly within carbonate strata that are to a level in the middle of the Darriwilian Stage, and dominated by shelly faunas. However, the base of the the base of the Harju Series, which is taken to Chinese Neichiashanian Stage is the base of the coincide with the base of the Pleurograptus linearis N. gracilis Zone, hence the same level as the base of Graptolite Zone, to the middle of the Katian Stage. the global Sandbian Stage. The base of the overlying Most of the 18 regional Ordovician stages in Chientangkiangian Stage is the base of the Dicel- Baltoscandia are based on shelly fossils (particularly lograptus complanatus Zone. This stage is coeval with trilobites) but the presence of graptolites at many the middle Katian Stage. It should be noted that most levels, particularly in Sweden, provides useful ties to of the Chinese stages are defined in terms of graptolite the standard graptolite zone succession. Conodonts zones, a fact that facilitates their international corre- are common and taxonomically diverse and many lation. Thus, the correlation of the Chinese Ordovician conodont zones (more than 20) and subzones stages to the global classification is much easier than (Bergström 2007) have been recognized and proved is the case in some other major regions. very useful for local and long-distance correlations. A level in the middle of the Baltoscandian Siberia Hunneberg Stage is coeval with the base of the Floian Stage and a horizon in the upper Volkhov Stage Ordovician strata are quite widespread in vast areas corresponds to the base of the Darriwilian Stage. The of the Siberian Platform and adjacent regions. Although base of the Kukruse Stage is coeval with the base of they are commonly covered by bogs and forests, there the Sandbian Stage, a level just above the base of the are numerous outcrops along many of the rivers. Keila Stage corresponds to the base of the Katian Modern information about the Ordovician geology Stage, and the base of the Porkuni Stage correlates of the Siberian Platform was recently published with the base of the Hirnantian Stage. As a whole, by Kanygin et al. (2006). In the current stratigraphic there are few problems in applying the global classification (Fig. 1), the Ordovician is subdivided chronostratigraphic classification to the Baltoscandic into 12 regional stages, which are based on shelly fossils, succession and it should be noted that two of the predominantly brachiopods, trilobites, and ostracodes. global GSSPs are situated in Sweden. It should be noted that the stage classification in Figure 1 is, with the exception of two Tremadocian Australia units, the same as that employed by Ross & Talent (1988), and that the widely used designation ‘horizon’ The nine standard stages recognized in the Australian in Russian literature corresponds to ‘stage’ as the Ordovician successions are all based on graptolites latter term is used elsewhere in the world.
LETHAIA 42 (2009) Ordovician chronostratigraphic classification 101
The Siberian palaeocontinent was rather isolated that the base of the Chertovskian Regional Stage from the other continental plates during the Ordovician, corresponds to the base of the Sandbian Global Stage and the fauna of the epicontinental seas of the and to the base of the Upper Ordovician Series. Siberian Platform was mainly endemic and dominated Recognition of the boundaries between the Baksian, by shelly taxa with a limited geographic distribution. Dolborian, Nirundian, and Burian Regional Stages is This creates serious problems for the correlation of based on conspicuous changes in the shelly faunas, units in the Siberian succession with those in the but the correlation of these stage units to global global chronostratigraphic classification. The principal stages remains uncertain. Conventionally, the base of ties to the global stages are provided by graptolites the Dolborian Regional Stage is taken to correspond from the surrounding territories of Taimyr, Vercho- to the base of the British Ashgill Series and it might jansk Range, Chukotka, Altai, and Sayany, and correlate with a level in the middle of the Katian conodonts, which in most regions show closer Stage. affinities to coeval North American Midcontinent Across most of Siberia, the upper boundary of the species associations than to those of Baltoscandia. Ordovician succession is marked by a regional Recent information about the stratigraphic hiatus, which corresponds to most of the Hirnantian distribution of conodonts of the Cordylodus lineage Stage. Stratigraphically virtually complete boundary suggests that the base of the Ordovician Series should successions are known from some localities in north- be placed somewhere within, or at least not lower eastern Siberia, such as those along Mirny Creek and than, the base of the Nyaian Regional Stage. The key Ina River in the Omulev Mountains region of the species Cordylodus proavus has been identified from palaeogeographically enigmatic Kolyma Terrane, the lowermost part of the latter stage, and C. aff. where the level of the systemic boundary can be angulatus from its upper part in the important precisely located based on pandemic graptolites and section along Kulumbe River on the northwestern conodonts (Zhang & Barnes 2007). part of the platform (T. Tolmacheva, personal com- munication, 2006). Mediterranean-North Gondwana Recognition of the precise level of the Lower/ Middle Ordovician Series boundary is difficult due to This region includes southwestern and central the highly endemic conodonts and absence of Europe (Ibero-Armorica, Sardinia, Bohemia) to the graptolites in this interval in the Siberian sections. Balkan and the vast area from Maghreb in northern However, it appears that this level corresponds Africa to Saudi Arabia and part of the Middle East. approximately to the boundary between the Ugorian The general scarcity of graptolites and almost total and the Kimaian Regional Stages. absence of conodonts in the Lower and Middle On the Siberian Platform, the most useful correla- Ordovician, and the largely endemic shelly faunas, tion level within the Middle Ordovician Series is the impose serious difficulties to correlate the successions base of the Volginian Regional Stage. This level in this region with the new global chronostratigraphy. coincides with a well-developed erosion surface and The latter is illustrated by the fact that only two of the sequence boundary, and it also represents the taxa used for the definition of the global stages and beginning of a prominent transgression, which was series have been recorded in this region and none of associated with significant biotic changes across the them near formal stage boundaries. entire platform. The Volginian assemblage of The regional biostratigraphic scheme (Havlicek & brachiopods and ostracodes shows close affinities to Marek 1973; Gutiérrez-Marco et al. 1995, 1999, 2002) the apparently coeval one of the Verchojansk– is largely based on endemic shelly fossils combined Chukotka region, where it is associated with graptolites with some graptolites and a good organic microfossil of the Hustedograptus teretiusculus Graptolite Zone record. The North Gondwanan chitinozoan zonation of the upper Darriwilian Global Stage (Oradovskaya (Paris 1990) is useful for correlation with other 1988). regions, and sporadic occurrences of graptolites and The other regionally important correlation level in shelly faunas of Baltic or Avalonian affinities (Gutiérrez- the Siberian Ordovician succession is the base of the Marco et al. 1995) allow for indirect correlation with Chertovskian Regional Stage, which coincides with a some global stage levels. sequence boundary and marks a most prominent The global Tremadocian Stage faunas are rather deepening event. The brachiopod and ostracode uniform across much of the Gondwana and typical faunal assemblage of this regional stage is easily Tremadocian graptolites, trilobites, and palynomorphs recognizable also in the Verchojansk–Chukotka fold have been documented at several stratigraphic levels. belt where it is associated with graptolites of the For the next regional stage, the designation N. gracilis Zone (Oradovskaya 1988). This suggests ‘Arenigian’ is tentatively used because of the general
102 Bergström et al. LETHAIA 42 (2009) absence of chronostratigraphically diagnostic fossils slices’. The designation ‘time slice’ implies that these in the areas of the widespread Armorican Quartzite are to be considered geochronologic units but their facies. Floian graptolites and chitinozoans are rather original definition is not entirely clear. Nevertheless, common, but the successions that are broadly coeval these units have proved to be useful for a refined with the Dapingian and lower Darriwilian global subdivision of the Ordovician System. stages are sparsely fossiliferous. During the Oretanian In the present contribution, we introduce subdivi- Stage, which corresponds to the ‘pendent didy- sions of the global stages referred to as stage slices mograptid’ interval, the Avalonian affinities of the (Fig. 2). In terms of stratigraphic scope, a stage slice shelly faunas decreased and these faunas were falls between a stage and a faunal zone, that is, it replaced by Gondwanan endemics with great local corresponds to a substage or superzone. The correlation potential. The following Dobrotivian stratigraphic scope of each stage slice is not the same Stage contains few stratigraphically useful graptolites and varies from a single to several faunal zones. As and its top is marked by the disappearance of the recognized here, the stage slices are informal, but widespread trilobite Neseuretus in northern Gondwana. defined, chronostratigraphic units, most of which are This level is also characterized by the appearance of based on geographically widespread graptolite or the trilobite Dalmanitina and the Drabovia brachiopod conodont zones. Each of the global stages is fauna, which is characteristic of the Berounian Stage. subdivided into two to four stage slices that for In the absence of the key graptolite N. gracilis, the clarity are designated by a letter and figure prefix, base of the Upper Ordovician can be only which are based on an abbreviation of the stage name approximately located within the Dobrotivian Stage followed by a figure indicating the general position of by rare occurrences in the upper third of this stage of the unit within the stage. Some of the stage slices some graptolites and chitinozoans (Lagenochitina have a similar range as some time slices recognized ponceti Zone), which are characteristic of the N. gracilis by Webby et al. (2004) but others are different in Zone elsewhere. The Berounian Stage, which corre- scope. Except Hi2, the base of each stage slice is sponds broadly with the British Caradoc and earliest coeval with the base of a conodont or graptolite zone Ashgill Series, contains in Spain some brachiopod as shown in Figure 2. assemblages of Anglo-Welsh affinities but overall, the Upper Ordovician benthic faunas in Spain compare Definitions of stage slices more closely with those of Bohemia and northern Gondwana regions (Morocco, Sardinia). The base of Tr1. Base of the Ordovician (first appearance of the Kralodvorian Stage is marked by the sudden Iapetognathus fluctivagus) to the base of the appearance of a new shelly fauna that includes Paltodus deltifer Conodont Zone. This unit Avalonian immigrants and cosmopolitan taxa. A includes the Rhabdinopora flabelliformis, brief global amelioration, with a decrease of latitudinal Adelograptus hunnebergensis, Adelograptus temperature differences (the Boda Event of Fortey & tenellus, and Psigraptus Graptolite zones in the Cocks 2005), resulted in the deposition of mudstones lowermost Tremadocian Stage. and conodont-bearing shelf limestones (Bergström & Tr2. Base of the Paltodus deltifer Conodont Zone to Massa 1992) in regions close to the South Pole. The the base of the Paroistodus proteus Conodont latest Ordovician (Hirnantian) glacial cycles led to a Zone, which is approximately the same level as drastic change in both sedimentation and faunas. All the base of the Araneograptus murrayi Graptolite the biostratigraphic and sedimentological criteria Zone. This stage slice includes the middle used to characterize the formerly used regional Tremadocian Stage. Kosovian Stage are shared with the Hirnantian Tr3. Base of the Paroistodus proteus Conodont Zone Stage elsewhere. This justifies the adoption of the to the base of the Tetragraptus approximatus latter global stage term in the Mediterranean- Graptolite Zone. This interval corresponds to North Gondwana regional chronostratigraphic the upper Tremadocian Stage. It includes the classification. Hunnegraptus copiosus Graptolite Zone and the upper part of the Araneograptus victoriae and A. murrayi Graptolite Zones. Stage slices Fl1. Base of the Tetragraptus approximatus Graptolite Zone to the base of the Oepikodus evae In their monumental summary of the Ordovician Conodont Zone. This interval corresponds to biodiversification, Webby et al. (2004) recognized six the lower part of the Floian Stage and includes primary (labelled 1–6) and 19 secondary (labelled 1a the T. approximatus, T. fruticosus 3–4 br., and to 6c) Ordovician units that they referred to as ‘time Didymograptus nitidus Graptolite Zones. It is also LETHAIA 42 (2009) Ordovician chronostratigraphic classification 103
Fig. 2. Ordovician graptolite or conodont zones used in the definition of stage slices, and the relationships of each stage slice to global stages and a generalized composite δ13C curve. The base of each stage slice is defined as the base of a particular faunal zone except in the case of Hi2. Note that for clarity, all the stage slices are drawn to the same size although they clearly have different stratigraphic ranges. (g), graptolite zone; (c), conodont zone. The Hirnantian segment of the δ13C curve is based on Finney et al. (1999) and Berry et al. (2002), the Katian one on Bergström et al. (2007), the Sandbian, Darriwilian and Dapingian one on Kaljo et al. (2007), and the Floian and Tremadocian one on Buggisch et al. (2003).
equivalent to the lower part of the Arenig Series triangularis Conodont Zone, which defines the of the UK, and the upper Lancefieldian and base of the Dapingian Stage. This stage slice corre- Bendigonian Stages of Australia. sponds to the middle part of the Arenig Series of Fl2. Base of the Oepikodus evae Conodont Zone to the UK, the lower Whiterockian Series of North the base of the Didymograptus protobifidus America, the lower Volkhov Stage of Baltoscandia, Graptolite Zone. This interval corresponds to the and part of the Castlemainian Stage in Australia. middle part of the Floian Stage, and the lower Dp2. Base of the Isograptus v. maximus Graptolite and middle part of the Oe. evae Zone. Zone to the base of the Oncograptus Graptolite Fl3. Base of the Didymograptus protobifidus Zone. The latter level approximates the base of Graptolite Zone to the base of the Isograptus v. the Paroistodus originalis Conodont Zone. This victoriae Graptolite Zone. This interval corre- stage slice corresponds to the middle Dapingian sponds to the upper part of the Floian Stage, the Stage and the upper part of the Australian upper part of the North American Ibexian Series, Castlemaininan Stage. the upper Chewtonian and lower Castlemainian Dp3. Base of the Oncograptus Graptolite Zone to the Stages in Australia, and the Upper Billingen base of Undulograptus austrodentatus Graptolite Stage in Baltoscandia. Zone. This stage slice corresponds to the upper Dp1. Base of the Isograptus v. victoriae Graptolite part of the Dapingian Stage, and the Australian Zone to the base of the Isograptus v. maximus Yapeenian Stage as well as the P. originalis Graptolite Zone. The base of this stage slice is Conodont Zone and the lower part of the also at, or very near, the base of the Baltoniodus B. norrlandicus Conodont Zone. 104 Bergström et al. LETHAIA 42 (2009)
Dw1. Base of the Undulograptus austrodentatus in the UK, and the middle Cincinnatian Series in Graptolite Zone to the base of the D. artus North America. Graptolite Zone. This stage slice corresponds to Ka3. Base of the Amorphognathus ordovicicus the lower portion of the Darriwilian Stage and Conodont Zone to the base of the Dicellograptus includes the U. austrodentatus Graptolite Zone. complanatus Graptolite Zone. It includes the middle The top of this unit is at about the same level as Richmondian Stage of the Cincinnatian Series in the base of the Llanvirn Series in the UK. North America, the uppermost Eastonian Stage Dw2. Base of the Didymograptus artus Graptolite Zone in Australia, part of the Ashgill Series in the UK, to the base of the Pygodus serra Conodont Zone. the uppermost Nabala and Vormsi Stages of the This stage slice includes most of the interval from Harju Series in Baltoscandia, and the uppermost the Lenodus variabilis Conodont Zone to the top part of the Neichiashanian Stage in China. of the Eoplacognathus suecicus Conodont Zone. Ka4. Base of the Dicellograptus complanatus Dw3. Base of the Pygodus serra Conodont Zone to Graptolite Zone to the base of the Normalograptus the base of the N. gracilis Graptolite Zone. The extraordinarius Graptolite Zone (=base of the base of this stage slice approximates the top of Hirnantian Stage). This stage slice includes the the Histiodella kristinae Conodont Zone and the uppermost Katian Stage, the uppermost Rich- middle of the Pterograptus elegans Graptolite Zone mondian Stage of North America, the middle (Chen et al. 2006b). The stage slice corresponds Ashgill Series in the UK, the upper Eastonian and to the upper part of the Darriwilian Stage and the most of the Bolindian Stages in Australia, and the upper part of the Llanvirn Series of the UK. Pirgu Stage of the Harju Series in Baltoscandia. Sa1. Base of the Nemagraptus gracilis Graptolite Hi1. Base of the Normalograptus extraordinarius Zone to the base of the C. bicornis Graptolite Graptolite Zone to the end of the global Hirnantian Zone. This stage slice ranges from the middle δ13C excursion (HICE). This stage slice corresponds part of the P. anserinus Conodont Zone to the to the N. extraordinarius Graptolite Zone and base of the B. gerdae Conodont Subzone. It part of the N. persculptus Graptolite Zone. It includes the lower part of the Sandbian Stage and includes the lower and middle Hirnantian Stage, corresponds to the lower Gisbornian Stage in the lower and middle Gamachian Stage in North Australia, the lower Caradoc Series (Aurelu- America, the lower and middle Porkuni Stage in cian Stage) in the UK, the Kukruse Stage of Baltoscandia, and the middle Bolindian Stage in Baltoscandia, and the lower Neichiashanian Stage Australia. of China. Hi2. Extends from the end of the Hirnantian δ13C Sa2. Base of the Climacograptus bicornis Graptolite excursion (HICE) to the top of the Ordovician Zone to the base of the Diplacanthograptus (base of the Akidograptus ascensus Graptolite caudatus Graptolite Zone. It includes essentially Zone). This stage slice represents the upper part the Dicellograptus foliaceus Graptolite Zone and of the Hirnantian Stage and probably corresponds corresponds to the upper part of the Sandbian to the middle and upper parts of the N. persculptus Stage, the upper part of the Gisbornian Stage in Graptolite Zone as well as the upper part of the Australia, the Mohawkian and lowermost Chat- Gamachian Stage in North America. fieldian Stages in North America, and the Haljala and lowermost Keila Stages in Baltoscandia. 13 Ka1. Base of the Diplacanthograptus caudatus Graptolite A new Ordovician δ C curve Zone to the base of the Peurograptus linearis Graptolite Zone. The latter level approximates the base of the Recent studies in particularly the Baltoscandian and North American Amplexograptus manitoulinensis North American successions have demonstrated the Graptolite Zone. The stage slice corresponds to great value of δ13C chemostratigraphy for both local the lower part of the Katian Stage, the upper and long-distance correlations (Buggisch et al. 2003; Caradoc Series of the UK, most of the Chatfieldian Ainsaar et al. 2004; Kaljo et al. 2004, 2007; Ludvigson through Maysvillian Stages in North America, et al. 2004; Young et al. 2005; Melchin & Holmden the lower Eastonian Stage in Australia, and part 2006; Bergström et al. 2006a, 2007, in press; Barta et al. of the Keila through Rakvere Stages in Baltoscandia. 2007; Schmitz & Bergström 2007). Most of these Ka2. Base of the Pleurograptus linearis Graptolite studies have been centred on two major, apparently Zone to the base of the Amorphognathus ordovicicus global, positive δ13C excursions, namely the Hirnantian Conodont Zone. This stage slice corresponds to the excursion (HICE) and the lower Katian Guttenberg middle Katian Stage, the upper part of the Australian excursion (GICE). Several other positive excursions Eastonian Stage, part of the lower Ashgill Series have been recognized in the Katian of Estonia and LETHAIA 42 (2009) Ordovician chronostratigraphic classification 105
North America and used for trans-Atlantic correlations named excursions. This curve is a composite one (Bergström et al. 2007). In the pre-Katian portion of based on the Hirnantian curve from Monitor Range, the Ordovician, several local excursions have been central Nevada (Finney et al. 1999; Berry et al. 2002), recorded, but thus far, only one significant positive the Katian curve from the eastern Midcontinent of excursion, which is recognized across a wide region, North America (Bergström et al. 2007), the has been documented. This is the mid-Darriwilian Sandbian–Darriwilian curve from Estonia (Kaljo excursion (Kaljo et al. 2004; 2007; Ainsaar et al. 2007) et al. 2007) and the Dapingian–Tremadocian curve that has been traced from eastern Baltoscandia to Sweden. from Argentina (Buggisch et al. 2003). The The pioneer attempts to compile a δ13C curve covering stratigraphical relations between these curve segments the entire Ordovician System suffered from lack of are well understood and we believe that the curve is adequate data which resulted in non-descript curves representative of the changes in δ13C values through of very limited utility. More recently published δ13C the system. This curve is closely controlled curves, which are based on extensive data sets and chronostratigraphically and it is tied to the stage covering large parts of the Ordovician, have been slices. Although it is outside the scope of the present published by Buggisch et al. (2003) and Saltzman & paper to discuss the many interesting details in Young (2005). The former curve lacks data from this composite curve, one feature stands out, namely post-Darriwilian strata. The latter curve is strati- the fact that there are more distinctive positive graphically incomplete in that most of the Katian excursions in the Darriwilian through Hirnantian interval is not covered. Furthermore, because few stratigraphic interval than in the older parts of the biostratigraphic data are included in that paper, the Ordovician. interpretation of the curve is not straightforward. Based on drill-core data from Estonia, Kaljo et al. (2007) presented an important δ13C curve through Conclusions much of the Ordovician but their curve is incomplete in that it does not include information from the The goals of the present study have been threefold. Tremadocian and Floian Stages. First, we have attempted to elucidate the relations There are several problems in compiling a reliable between the new global series and stages and the δ13 C curve through the entire Ordovician. The fact various regional, often quite different, chronostrati- δ13 that a continuous C record through the whole graphic classifications employed in some of the System is currently not available from any continent major Ordovician outcrop regions in the world. of the world makes it necessary to employ a composite Although the general relations between the new and section based on information from several, geographi- previous classifications appear reasonably well cally widely spaced, regions. This creates some established, there are many remaining problems that problems in that the marked provincialism in require further study. This is especially the case in Ordovician faunas complicates inter-regional Siberia and the Mediterranean–North Gondwana biostratigraphic correlations that may affect inter- regions where much of the regional classifications are pretations of the proper relations between curve based on more or less endemic shelly fossils. Second, segments from different continents. Also, in some we have introduced a new and refined subdivision of stratigraphic intervals there are notable differences the stages into stage slices that can be used when δ13 between C values from different regions. These there are difficulties to recognize particular faunal differences may be due to several factors, such as gaps zones. Third, we have tried to present a generalized δ13 in the succession and regionally different baseline C δ13C curve through the Ordovician that is closely tied values. One example of this is the apparent absence to the new chronostratigraphy. The compilation of in the Great Basin (Saltzman & Young 2005) of the this curve is an attempt to illustrate the principal distinctive Mid-Darriwilian excursion recognized in fluctuations in δ13C values through the Ordovician, Baltoscandia (Kaljo et al. 2007). but we expect that when further information has Despite these potential problems, we have attempted become available, the curve will be modified in δ13 to produce what we believe is the first modern C detail. curve covering the entire Ordovician and this curve is calibrated to individual stage slices (Fig. 2). It is Acknowledgements. – Most of the present paper was written obvious that because of printing format limitations during the senior author’s stay in Nanjing in June, 2007, and he such a composite curve will have to be generalized in gratefully acknowledges the receipt of support from the National many details but we have tried to illustrate the Science Foundation of China research grant no. 40532009, which δ13 was awarded to Chen Xu and made this stay possible. We thank currently known significant fluctuations in C I.G. Percival, A.W. Owen, B. Webby, and A.H.M. VandenBerg values, especially those that have been recognized as for valuable comments. 106 Bergström et al. LETHAIA 42 (2009)
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