Hostname: page-component-54dcc4c588-sdd8f Total loading time: 0 Render date: 2025-10-02T17:10:50.313Z Has data issue: false hasContentIssue false
  • English
  • Français

Redefinition of the Sandbian–lower Katian conodont biozonation of Baltoscandia

Published online by Cambridge University Press:  30 September 2025

Tõnn Paiste Open the ORCID record for Tõnn Paiste [Opens in a new window] ,
Peep Männik ,
Svend Stouge Open the ORCID record for Svend Stouge [Opens in a new window] ,
Leho Ainsaar  and
Tõnu Meidla
Tõnn Paiste*
Affiliation:
Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia
Peep Männik
Affiliation:
Department of Geology, Tallinn University of Technology, Tallinn, Estonia
Svend Stouge
Affiliation:
Natural History Museum of Denmark, University of Copenhagen, Copenhagen K, Denmark
Leho Ainsaar
Affiliation:
Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia
Tõnu Meidla
Affiliation:
Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia
*
Corresponding author: Tõnn Paiste; Email: tonn.paiste@ut.ee
Rights & Permissions [Opens in a new window]

Abstract

Recent development in the Upper Ordovician conodont biostratigraphy of Baltoscandia highlights the mismatch between the traditionally used conodont zonation and ranges of the eponymous species. Practical application of the zonation is further complicated by the fact that the morphology of the long-ranging species Amorphognathus tvaerensis, the key taxon of the eponymous conodont zone, changes through its distribution interval, and its older and younger representatives are quite different. The latter one was recently described as a new species, A. viirae. Also, it appeared that the specimens assigned earlier to A. ineaqualis in the northern Baltoscandian region are conspecific with A. tvaerensis and A. ineaqualis is missing here. As a result, the A. ineaqualis Conodont Zone has to be abandoned from the regional zonal scheme. Restudy of conodont collections from the Bliudziai-150 (Lithuania) and Kovel-1 (Ukraine) core sections demonstrated the absence of A. ineaqualis and the presence of A. viirae also in the southern Baltoscandian area and Ukraine. This paper contains a formal description of the new, emended conodont zonation for Sandbian and the lowermost Katian of the Baltoscandian Palaeobasin and its correlation to the regional chemostratigraphic standard.

Keywords

ordovicianbiostratigraphyconodont zonationcarbon isotopeschemostratigraphy

Information

Type
Original Article
Information
Geological Magazine , Volume 162 , 2025 , e39
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2025. Published by Cambridge University Press

1. Introduction

Correlation of the Ordovician strata in the Baltoscandian Palaeobasin is based on various lithological, biostratigraphical and geochemical criteria. Most reliable correlations are based on K-bentonites (Bergström et al. Reference Bergström, Huff, Kolata and Bauert1995; Kiipli et al. Reference Kiipli, Dahlqvist, Kallaste, Kiipli and Nõlvak2015) and distribution of different faunas, for example, graptolites (Männil, Reference Männil, Kaljo and Koren1976; Bergström, Reference Bergström, Hughes, Rickards and Chapman1986; Kaljo et al. Reference Kaljo, Borovko, Heinsalu, Khazanovich, Mens, Popov, Sergeyeva, Sobolevskaya and Viira1986), chitinozoans (Nõlvak & Grahn, Reference Nõlvak and Grahn1993; Vandenbroucke, Reference Vandenbroucke2008), conodonts (Bergström & Löfgren, Reference Bergström and Löfgren2009; Paiste et al. Reference Paiste, Männik and Meidla2022, Reference Paiste, Männik and Meidla2023) and ostracods (Meidla, Reference Meidla1996; Meidla et al. Reference Meidla, Truuver, Tinn and Ainsaar2020). During the last decades, δ13Ccarb data from different sections have been also successfully used for dating and correlation (Kaljo et al. Reference Kaljo, Hints, Martma and Nõlvak2001, Reference Kaljo, Hints, Martma, Nõlvak and Oraspõld2004, Reference Kaljo, Martma and Saadre2007; Ainsaar et al. Reference Ainsaar, Kaljo, Martma, Meidla, Männik, Nõlvak and Tinn2010).

The Upper Ordovician conodont zonation introduced by Bergström in 1971 and updated later by Dzik (Reference Dzik1978) and Bergström (Reference Bergström1983) is based on data from the Baltoscandian region and has remained unchanged until recently. However, the newest information about the composition and distribution of conodonts in the Sandbian Stage (Paiste et al. Reference Paiste, Männik and Meidla2023) demonstrates that a revision of this conodont zonation is needed. It appeared that the Amorphognathus inaequalis Conodont Subzone, as depicted in the latest regional stratigraphic charts (Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023; Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023), is missing in the region. Earlier this zone was identified in the Ruhnu-500 and Mehikoorma-421 core sections in Estonia (Männik, Reference Männik and Põldvere2003; Männik & Viira, Reference Männik, Viira and Põldvere2005), but the recent re-investigation of the probable specimens of A. inaequalis from these core section demonstrated that, in reality, they belong to A. tvaerensis. Additionally, a detailed study on the evolution of A. tvaerensis has uncovered notable morphological differences between elements in the lower and upper parts of its range and led to the erection of a new species, A. viirae Paiste, Männik et Meidla (Paiste et al. Reference Paiste, Männik and Meidla2023). This newly established species was widespread in the Baltoscandian Palaeobasin and serves as an index species for a new, eponymous conodont zone. The revised conodont biostratigraphy allows higher precision of the conodont-based correlations of the Sandbian successions but also increases the reliability of the correlations based on the δ13Ccarb curves.

The above conclusions have been verified in the course of re-investigation of the published Ordovician conodont key successions from Estonia and Sweden (see Paiste et al. Reference Paiste, Männik and Meidla2022, Reference Paiste, Männik and Meidla2023). Moreover, as new information about conodonts in the Bliudziai-150 (Lithuania) and Kovel-1 (Ukraine) core sections revealed that A. inaequalis, reported earlier from these sections (Stouge et al. Reference Stouge, Bauert, Bauert, Nõlvak and Rasmussen2016 and Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004, respectively), in reality is A. tvaerensis and A. viirae is present in both sections, it is evident that the enhanced conodont zonation is applicable also in these regions. The aims of the present paper are to update conodont biostratigraphy data in the sections from Latvia and Ukraine, to discuss correlation of these sections with those from the northern Baltoscandia and to provide formal definitions of the zones included in the proposed emended conodont zonation.

2. Geological setting

2.a. Regional palaeogeography

The Baltoscandian Ordovician Palaeobasin was located on the western shelf of the Baltica Palaeocontinent (Torsvik & Cocks, Reference Torsvik and Cocks2017). The Lower Palaeozoic deposits of this palaeobasin are well preserved and widely exposed in Eastern Europe, around Denmark, in the southern part of the Baltic Sea and smaller areas in Sweden, Norway, Finland (Åland) and the Bothnian Sea (Fig. 1). Based on the prevailing lithology of the deposits, three distinct facies belts are recognized in the palaeobasin (Fig. 1; Harris et al. Reference Harris, Sheehan, Ainsaar, Hints, Männik, Nõlvak and Rubel2004; Dronov et al. Reference Dronov, Ainsaar, Kaljo, Meidla, Saadre, Einasto, Gutiérrez-Marco, Rábano and García-Bellido2011). Limestones of nearshore facies are attributed to the Estonian and Lithuanian Shelves sensu Harris et al. (Reference Harris, Sheehan, Ainsaar, Hints, Männik, Nõlvak and Rubel2004) (= ‘North Estonian and Lithuanian Confacies’ sensu Jaanusson, Reference Jaanusson1995). Marls and argillaceous limestones dominate the Livonian Basin (= ‘Livonian Tongue’ sensu Jaanusson, Reference Jaanusson1995) and the eastern part of the Scandinavian Basin (= ‘Central Baltoscandian Confacies’ sensu Jaanusson, Reference Jaanusson1995).

Figure 1. General Ordovician palaeogeography in the Baltoscandian Palaeobasin (modified from Harris et al. Reference Harris, Sheehan, Ainsaar, Hints, Männik, Nõlvak and Rubel2004; Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004; Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023). (1) Distribution of the Lower Palaeozoic strata in the eastern part of the Baltoscandian region (after Nielsen & Schovsbo, Reference Nielsen and Schovsbo2011); (2) circles mark the locations of the sections discussed in the text. Filled circles mark the sections along the profile in Figure 5, while empty circles mark the additional sections that correlate the Sandbian δ13Ccarb data and conodont zonation in Figure 7.

2.b. Sandbian in the main study regions

Conodonts are well documented in the Sandbian of numerous sections in Estonia, whereas data about the coeval faunas from other parts around the Baltic Sea and most of Scandinavia are limited (Paiste et al. Reference Paiste, Männik and Meidla2022 and references therein). The best-studied sections in Sweden are located in the Siljan district (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a). Outside Estonia, Sandbian conodont data are available only from two core sections, one of them, Bliudziai-150, located in the central part of Lithuania (Stouge et al. Reference Stouge, Bauert, Bauert, Nõlvak and Rasmussen2016) and the other one, Kovel-1, in the western part of Ukraine (Fig. 1; Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004).

The Fjäcka main section, the best-studied outcrop section in the Siljan district, Sweden, exposes the upper Darriwilian, Sandbian, and lower to middle Katian strata (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a and references therein). The Siljan area is palaeogeographically located in the eastern part of the Scandinavian Basin (Fig. 1). The Furudal, Dalby, Skagen, Moldå and Slandrom limestones and the Fjäcka shale (Fig. 2) are exposed in this section. The section is located in a protected nature area and serves as the type locality for several conodont and chitinozoan (sub-) zones but also for the Dalby Limestone, Moldå Limestone and Fjäcka Shale (Jaanusson, Reference Jaanusson, Bruton and Williams1982). The Sandbian in this section comprises the Dalby and |Skagen limestones separated by the Kinnekulle K-bentonite (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a).

Figure 2. Comparison of the global (Goldman et al. Reference Goldman, Sadler, Leslie, Melchin, Agterberg, Gradstein, Gradstein, Ogg, Schmitz and Ogg2020), Scandinavian (Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023) and Eastern Baltic (Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023) stratigraphic schemes. Abbreviations: Jiangxigr. – Jiangxigraptus, Hustedogr. – Hustedograptus, Diplacanthogr. – Diplacanthograptus, grapt. – graptolite, P. – Pygodus, A. – Amorphognathus, B. – Baltoniodus, S. – Sagittodontina.

The Sandbian differs considerably between the Estonian Shelf and the Livonian Basin facies zones in Estonia. Weakly argillaceous to pure limestones dominate the Estonian Shelf, while argillaceous limestones are characteristic of the northern part of the Livonian Basin (Harris et al. Reference Harris, Sheehan, Ainsaar, Hints, Männik, Nõlvak and Rubel2004; Dronov et al. Reference Dronov, Ainsaar, Kaljo, Meidla, Saadre, Einasto, Gutiérrez-Marco, Rábano and García-Bellido2011). The Ordovician strata are exposed in the historical type region of northern Estonia, where the majority of the Baltic regional stages have been defined (Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023 and references therein). The Sandbian succession in this region comprises the Pihla, Viivikonna, Tatruse and Kahula formations (Fig. 2). In southern Estonia, in the northern part of the Livonian Basin, the interval is represented by the Dreimani, Adze, Blidene, Variku and Mossen formations (Fig. 2).

In central Lithuania, the Sandbian Stage consists of various limestones of the Kriaunos, Sartai, Šventupys, Auleliai and Vilučiai formations representing the Livonian Basin (Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023). The Sandbian strata in the western Volyn (western Ukraine) comprise grey and mottled or nodular limestones and marls of the Pischa Formation (Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023). Based on the lithological composition of rocks and their palaeontological characteristics (e.g. occurrence of Platystrophia lynx lynx, Horderleyella kegelensis and large crinoidal columnals – Pomyanovskaya, Reference Pomyanovskaya and Shulga1972; Konstantinenko, Reference Konstantinenko and Gozhyk2013), the strata in western Volyn are similar to those, characteristic of the North Estonian shelf.

3. Stratigraphic framework

The Sandbian Stage is the lowermost stage of the Upper Ordovician Series. The base of the Sandbian Stage is defined by the appearance of Nemagraptus gracilis (Hall; = the base of the eponymous graptolite zone), and its upper boundary by the first appearance of the Diplacanthograptus caudatus (Bergström et al. Reference Bergström, Finney, Chen, Pålsson, Wand and Grahn2000). In the stratotype section of the Sandbian Stage at Fågelsång, Nemagraptus gracilis appears in the Pygodus anserinus Conodont Zone, below the First Appearance Datums (FADs) of Baltoniodus variabilis (Bergström) and Amorphognathus tvaerensis Bergström (Bergström et al. Reference Bergström, Finney, Chen, Pålsson, Wand and Grahn2000). The lower boundary of the Katian Stage correlates with a level within the uppermost part of the A. tvaerensis Zone (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007, Reference Goldman, Sadler, Leslie, Melchin, Agterberg, Gradstein, Gradstein, Ogg, Schmitz and Ogg2020, Reference Goldman, Leslie, Liang, Bergström, Harper, Lefebvre, Percival and Servais2023).

Two successions of regional stages are proposed for the Ordovician in the Baltoscandian region (Fig. 3). The Estonian regional stages have been in use, with some modifications (Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023), in the eastern part of the palaeobasin (Latvia, Lithuania, Poland, Belorus, Ukraine, NW Russia) for more than a century and were also partly adopted for Sweden (e.g., Jaanusson, Reference Jaanusson, Bruton and Williams1982) and Norway (e.g., Owen et al. Reference Owen, Bruton, Bockelie and Bockelie1990). A new scheme of Scandinavian regional stages was proposed by Nielsen et al. (Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023). According to this scheme, the Sandbian comprises the interval from the uppermost Segerstadian Regional Stage up to the top of the Dalbyan Regional Stage; in the Baltic scheme reviewed by Meidla et al. (Reference Meidla, Ainsaar, Hints and Radzevičius2023), it comprises the Kukruse, Haljala and the lower Keila regional stages.

Figure 3. Diagnostic elements and ranges of the zonal taxa used in this study. Outline drawings of Pa elements of Baltoniodus are from Bergström, Reference Bergström, Sweet and Bergström1971, plate 2 and M elements are from Paiste et al. Reference Paiste, Männik and Meidla2022, figure 5. Pa element of Amorphognathus tvaerensis is from Bergström, Reference Bergström1962, plate 4, A. viirae from Paiste et al. Reference Paiste, Männik and Meidla2023, figure 4 and A. superbus from Bergström, Reference Bergström, Sweet and Bergström1971, plate 2. M elements of Amorphognathus are from Paiste et al. Reference Paiste, Männik and Meidla2023, figures 3–5.

The Sandbian conodont zonations in these two schemes are largely identical. However, correlation of the individual zones to the global standard differs in details (Fig. 3). The lower boundary of the A. inaequalis Subzone is tentatively drawn at the lower boundary of the Sandbian Stage in the Scandinavian correlation chart (Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023). However, the lower boundary of the Sandbian Stage is drawn within this subzone in the Eastern Baltic correlation chart (Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023).

The lower boundary of the Sandbian Stage has been suggested to lie in the transitional interval of Baltoniodus alatus (Hadding) sensu Stouge et al. Reference Stouge, Harper and Parkes2024 (B. prevariabilis Fåhræus sensu Bergström) and B. variabilis (Bergström, Reference Bergström2007 b) or near the base of the A. inaequalis Subzone (Goldman et al. Reference Goldman, Sadler, Leslie, Melchin, Agterberg, Gradstein, Gradstein, Ogg, Schmitz and Ogg2020). However, B. variabilis is reported to appear as a gradual transition from its predecessor B. alatus (Dzik, Reference Dzik1978; Stouge et al. Reference Stouge, Harper and Parkes2024), and because of the lack of A. inaequalis demonstrated in the Estonian and Scandinavian successions (Paiste et al. Reference Paiste, Männik and Meidla2023), the exact position of the base of Sandbian in the Baltoscandian region remains problematic.

In the scheme by Nielsen et al. (Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023), the lower boundary of the Katian Stage correlates with the basal A. superbus zone. In the scheme by Meidla et al. (Reference Meidla, Ainsaar, Hints and Radzevičius2023), an unzoned interval is indicated for the same interval due to the scarcity of stratigraphically important taxa. The succession of A. ventilatus and A. superbus zones of the lowermost Katian overlies this interval. Possibilities of drawing this boundary within the area addressed in the present paper will be discussed in more detail below.

4. Material

Information from the Kovel-1 (Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004) and Bliudziai-150 (Stouge et al. Reference Stouge, Bauert, Bauert, Nõlvak and Rasmussen2016) core sections was re-examined. For comparison, collections from the Kerguta-565 (Viira et al. Reference Viira, Löfgren, Sjöstrand and Põldvere2006 a), Mehikoorma-421 (Männik & Viira, Reference Männik, Viira and Põldvere2005; Paiste et al. Reference Paiste, Männik and Meidla2023), Ruhnu-500 (Männik, Reference Männik and Põldvere2003), Taga-Roostoja-25A (Viira & Männik, Reference Viira, Männik and Põldvere1999), Tartu-453 (Stouge, Reference Stouge and Männik1998), Valga-10 (Männik, Reference Männik and Põldvere2001), Velise-V97 (Paiste et al. Reference Paiste, Männik and Meidla2022), Viki (Männik, Reference Männik and Põldvere2010; Hints et al. Reference Hints, Martma, Männik, Nõlvak, Põldvere, Shen and Viira2014) core sections from Estonia and Fjäcka main outcrop section from Sweden (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a) were also inspected.

5. Results of re-investigation of the conodont successions in Lithuania, Ukraine, Estonia and Sweden

Occurrence of A. inaequalis in the Bliudziai-150 and Kovel-1 core sections was not confirmed. The oldest specimens of Amorphognathus in these core sections, previously identified as A. inaequalis, in reality belong to A. tvaerensis. Currently, no elements of A. inaequalis are identified from the Baltica successions.

Occurrence of A. viirae was confirmed in the Bliudziai-150 and Kovel-1 core sections. All known successions from the Baltica where upper range of A. tvaerensis has been previously described have yielded elements of A. viirae.

The FAD of A. superbus in the Hirmuse Formation, in the Estonian part of the Baltoscandian palaeobasin, is based on M elements known to appear along with other typical Katian taxa (e.g., A. complicatus Rhodes) in the Hirmuse Formation (Männik, Reference Männik2017).

In the Fjäcka main section, the reappearance of the genus Amorphognathus is recorded at 1.5 m above the base of the Kinnekulle K-bentonite (Fig. 2). The M elements present are morphologically similar to some of the M elements occurring in the Mehikoorma-421 core section in the interval 292.8–294.4 m together with typical M elements of A. superbus (Paiste et al. Reference Paiste, Männik and Meidla2023, Supplementary File, Fig. S13e, j).

Distribution and correlational remarks of zonal species are covered in the discussion below. The FADs and LADs of the zonal species are provided in Table 1. Diagnostic elements of zonal species are illustrated in Figure 2. More detailed representations of diagnostic elements are provided in Paiste et al. Reference Paiste, Männik and Meidla2022, Reference Paiste, Männik and Meidla2023.

Table 1. FADs and LADs of stratigraphically important conodont species in the studied sections. Revised ranges of first (FAD) and last appearance datums (LAD) of species are shown in italics

6. Discussion

6.a. Revised Sandbian conodont zonation of Baltoscandia

A tentative revised version of the Baltoscandian conodont zonation was proposed by Paiste et al. (Reference Paiste, Männik and Meidla2023). Formal definitions of the zones in this emended zonation were not provided in that paper but are given below in this chapter. The main changes compared to the traditional approach are (1) abandoning of the A. inaequalis Subzone, (2) the inclusion of a new, A. viirae Zone, corresponding to the upper part of the former Baltoniodus gerdae Subzone, and (3) redefinition of the former B. variabilis, B. gerdae and B. alobatus subzones as zones. A major difference between the proposed approach and the old biozonation is uniform definition of zones: all zones are defined as interval zones, with their lower boundaries drawn according to FADs of the eponymous taxa (Fig. 4). This improves clarity of the conodont zonation and supports its unequivocal implementation during future studies.

Figure 4. Conodont zonations used in the Baltoscandian region. In the left-hand side of the figure, ranges of the key taxa used to define the zones in this study are indicated. Upper. – Uppermost B. alobatus range.

6.a.1. Baltoniodus variabilis zone

Definition. The FAD of B. variabilis marks the base of the zone.

Reference section. The Fjäcka main section in Dalarna, south-central Sweden, where the FAD of B. variabilis coincides with the lower boundary of the Dalby Limestone (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a). The zone corresponds to the lowermost 6.05 m of the Dalby Limestone. It is overlain by the A. tvaerensis Zone (Figs. 4 and 5).

Figure 5. Updated distribution of stratigraphically important conodonts and correlation of the sections. Legend: (1) Kinnekulle-K bentonite; (2) discontinuity surface = prominent sedimentary hiatus in the succession; (3) species identification of a taxon based on recognizable Pa element; (4) identification of a taxon based on other than Pa element; (5) distribution data from Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a. Abbreviations: Mold – Moldå; Bl – Blīdene; Mo – Mossen; Kk – Kõrgekallas; * – Hirmuse; Tat – Tatruse; Rä – Rägavere; Dru - Drukšiai; Šve - Šventupys.

Description. Additionally to B. variabilis, characteristic of the zone are Pygodus anserinus, Complexodus puginifer and Sagittodontina kielcensis which range into the zone from below; the FAD of Eoplacognathus elongatus occurs within the zone.

Remarks. The zone corresponds to the lower part of the total range of B. variabilis. The base of the redefined B. variabilis Zone is remarkably older than the base of the B. variabilis concurrent range Subzone of the A. tvaerensis Zone sensu Bergström (Reference Bergström, Sweet and Bergström1971; Fig. 4). The B. variabilis Subzone sensu Bergström (Reference Bergström, Sweet and Bergström1971) corresponds only to a part of the range of the eponymous species between the appearance of A. tvaerensis and that of B. gerdae. The same interval corresponds to the redefined A. tvaerensis Zone herein (see below).

Stratigraphic age. The B. variabilis Zone represents the upper part of the Darriwilian Stage and the lowermost part of the Sandbian Stage; it correlates with the uppermost part of the Segerstadian Regional Stage in Scandinavia; it comprises the upper Uhaku and lower Kukruse regional stages in the eastern Baltic region (Fig. 6).

Figure 6. Sandbian correlation chart of the Baltoscandian region. Correlation of the Siljan district is based on the Fjäcka main section, N Livonian Basin on the Ruhnu-500 and Valga-10 sections; W-C Estonia on the Viki and Velise-V97 sections; N Estonia on Kerguta-565 and Taga-Roostoja-25A sections; E-C Estonia on the Tartu-453 and Mehikoorma-421 sections; S Livonian Basin on the Bliudziai-150 section; W Volyn region on the Kovel-1 section. Boundaries of the RSs are drawn according to the results of the discussion below and in the chapter ‘Sandbian-early Katian conodont zones and Regional Baltoscandian stages’. Abbreviations: G. Stage – Global Stage; EB. Stage – East Baltic Stage; S. Stage – Scandinavian Stage; Dar. – Darriwilian; Kat. – Katian; Oa. - Oandu Segerst. – Segerstadian; Mo. - Moldåan.

6.a.2. Amorphognathus tvaerensis zone

Definition. The FAD of A. tvaerensis marks the base of the zone.

Reference section. The Fjäcka main section in Dalarna, south-central Sweden, where the FAD of A. tvaerensis is recorded 6.05 m above the base of the Dalby Limestone (Bergström, Reference Bergström, Sweet and Bergström1971, Reference Bergström, Ebbestad, Wickström and Högström2007). The zone is 4.45 m thick and is overlain by the B. gerdae Zone (Figs. 4 and 5).

Description. The zone is characterized by A. tvaerensis; B. variabilis and E. elongatus range into it from below.

Remarks. Initially, the lower boundary of the A. tvaerensis Zone was drawn 5.25 m above the base of the Dalby Limestone (Bergström, Reference Bergström, Sweet and Bergström1971). Restudy of the same collection from the Fjäcka main section did not confirm the appearance of Amorphognathus at this level. The first true elements of A. tvaerensis were found 6.05 m above the base of the Dalby Limestone. As the specimens of A. inaequalis reported from the sections of the Baltoscandian region are conspecific with A. tvaerensis, the former A. inaequalis Zone comprises a lower part of the revised A. tvaerensis Zone.

Stratigraphic age. The A. tvaerensis Zone corresponds to the lower part of the Sandbian Stage; to the lowermost part of the Dalbyan Regional Stage in Scandinavia; to the middle part of the Kukruse Regional Stage in the eastern Baltic (Fig. 6).

6.a.3. Baltoniodus gerdae zone

Definition. The FAD of B. gerdae marks the base of the zone.

Reference section. The Fjäcka main section in Dalarna, south-central Sweden, where the first appearance of B. gerdae is recorded in the middle part of the Dalby Limestone, 10.05 m above the base of the unit. The zone is 3.25 m thick and is overlain by the A. viirae Zone (Figs. 4 and 5).

Description. The zone is characterized by B. gerdae; A. tvaerensis and E. elongatus range from below into the zone.

Remarks. The lower boundary of the B. gerdae Zone was initially marked at 9.5 m above the base of Dalby Limestone by Bergström (Reference Bergström, Sweet and Bergström1971). Restudy on the same collection from the Fjäcka main section did not confirm the presence of this species in the lowermost 0.55 m above this depth. The original B. gerdae Subzone sensu Bergström (Reference Bergström, Sweet and Bergström1971) was equivalent to the total range of B. gerdae (Fig. 4), whilst the revised interval zone version corresponds only to the lower part of the range of this species (Fig. 4).

Stratigraphic age. The B. gerdae Zone represents the middle part of the Sandbian Stage; the lower part of the Segerstadian Regional Stage in Scandinavia; comprises the upper part of the Kukruse and the lower part of the Haljala regional stages in the eastern Baltic (Fig. 6).

6.a.4. Amorphognathus viirae zone

Definition. The FAD of A. viirae marks the base of the zone.

Reference section. The Fjäcka main section in Dalarna, south-central Sweden, where the first appearance of A. viirae is recorded 13.3 m above the base of the Dalby Limestone. The zone is 1.05 m thick and overlain by the B. alobatus Zone (Figs. 4 and 5).

Description. The zone is characterized by A. viirae; B. gerdae ranges from below into the zone.

Remarks. The holotype of A. viirae comes from the interval 314.9–315 m of the Mehikoorma-421 core section, Estonia (Paiste et al. Reference Paiste, Männik and Meidla2023). No outcrop section suitable for use as a stratotype of A. viirae Zone is available in Estonia. The reference section for all Sandbian conodont zones proposed by Bergström (Reference Bergström, Sweet and Bergström1971; Fig. 4) is the Fjäcka main section in Dalarna, Sweden. The particular value of this locality was created by extensive excavations in 1945–1946 that opened an almost complete succession through the uppermost Middle and Upper Ordovician. The section was intensively studied during subsequent decades and serves as the type section for several conodont and chitinozoan zones (Bergström, Reference Bergström, Sweet and Bergström1971; Nõlvak & Grahn, Reference Nõlvak and Grahn1993), lithostratigraphic units (Jaanusson, Reference Jaanusson, Bruton and Williams1982) and regional stages (Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023). Considering the possibility of cleaning this heavily overgrown section in future, we tentatively keep the Fjäcka main section as the reference section for all Sandbian conodont zones, including the A. viirae Zone. However, as information about conodonts from the Fjäcka main section is based on a limited collection (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a; Paiste et al. Reference Paiste, Männik and Meidla2023; Fig. 5), a detailed restudy of the succession is strongly advised.

Outside the Baltoscandian region, A. viirae has been recognized in the sections of Oklahoma, USA, and Holy Cross Mountains, Poland, based on the illustrations published by Goldman et al. (Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007) and Dzik (Reference Dzik1994), respectively. The A. viirae Zone corresponds to the upper part of the B. gerdae Subzone of the former A. tvaerensis Zone of Bergström (Reference Bergström, Sweet and Bergström1971; Fig. 4). The A. ventilatus Zone (Fig. 4; Dzik, Reference Dzik1999) was based on the elements later reidentified as A. viirae (Paiste et al. Reference Paiste, Männik and Meidla2023).

In the Fjäcka main section, A. viirae is not very common (Fig. 5; Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a; Paiste et al. Reference Paiste, Männik and Meidla2023), but its FAD is distinct and easily identifiable based on the morphological changes in the lineage of Amorphognathus (Paiste et al. Reference Paiste, Männik and Meidla2023, fig. 7).

Stratigraphic age. The A. viirae Zone corresponds to the upper part of the Sandbian Stage; to the middle part of the Dalbyan Regional Stage in Scandinavia; to the middle part of the Haljala Regional Stage in the eastern Baltic (Fig. 6).

6.a.5. Baltoniodus alobatus zone

Definition. The FAD of B. alobatus marks the base of the zone.

Reference section. The Fjäcka main section in Dalarna, south-central Sweden, where the first appearance of B. alobatus is recorded in the upper part of the Dalby Limestone, 14.35 m above the base of Dalby Limestone. The zone is 12.85 m thick and is overlain by the A. superbus Zone (Figs. 4 and 5).

Description. The zone is characterized by B. alobatus. However, the recorded B. alobatus range does not extend up to the FAD of A. superbus (Figs. 4 and 5). A. viirae ranges from below into the zone. Icriodella superba appears within the zone (Männik, Reference Männik2017).

Remarks. Lower boundary of the B. alobatus Zone equals that of the B. alobatus Subzone sensu Bergström (Reference Bergström, Sweet and Bergström1971). In all studied sections, B. alobatus disappears near but still below the Kinnekulle K-bentonite (Fig. 2). The ‘Uppermost B. alobatus range’ by Männik (Reference Männik and Põldvere2003; Fig. 4) marks the interval barren of Amorphognathus elements in the sections of the Baltoscandian region (Fig. 5).

Stratigraphic age. The B. alobatus Zone corresponds to the upper part of the Sandbian Stage and the lowermost part of the Katian stages; to the upper part of the Dalbyan Regional Stage in Scandinavia; to the upper part of the Haljala and the lowermost part of the Keila regional stage in the eastern Baltic (Fig. 6).

6.a.6. Amorphognathus superbus zone

Definition. The FAD of A. superbus marks the base of the zone.

Reference section. The Fjäcka main section in Dalarna, south-central Sweden, where the first appearance of A. superbus is recorded in the lower part of the Moldå Limestone at the depth of 27.2 m (Fig. 5). The zone is 12.3 m thick and corresponds to the main part of the Moldå Limestone and major part of the Slandrom Limestone. It is overlain by the A. ordovicicus Zone (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a).

Description. The zone is characterized by A. superbus. The FADs of A. complicatus, A. ventilatus, Belodina confluens and Hamarodus brevirameus are characteristic of the zone.

Remarks. The lower boundary of the A. superbus Zone was initially located at the top of the Dalby Limestone by Bergström (Reference Bergström, Sweet and Bergström1971). Restudy of the Fjäcka main section did not confirm the presence of elements of Amorphognathus at this level. The first diagnostic elements of A. superbus were found 0.9 m above the base of the Moldå Limestone (in the sample labelled as ‘131; 4.9–5.0 above base unit’). In a later paper (Bergström, Reference Bergström, Ebbestad, Wickström and Högström2007 a), the lower boundary of the A. superbus Zone was drawn in the middle of the Moldå Limestone. The definition of the A. superbus Zone is still the same as in Bergström (Reference Bergström, Sweet and Bergström1971), but additional findings of characteristic M elements (Paiste et al. Reference Paiste, Männik and Meidla2023) shift the boundary downwards. The previous A. ventilatus Zone indicated as underlying the A. superbus Zone (Männik, Reference Männik and Põldvere2003) is abandoned because the revised FAD of A. superbus occurs below the FAD of A. ventilatus in Mehikoorma-421, Ruhnu-500 and Valga-10 core sections (Paiste, Reference Paiste2023).

Stratigraphic age. The A. superbus Zone corresponds to the lower part of the Katian Stage; to the upper part of the Dalbyan and to the lower part of the Moldåan regional stages in Scandinavia; in the eastern Baltic area it comprises the interval from the upper part of the Keila Regional Stage up to the lower part of the Nabala Regional Stage (Fig. 6; Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023, Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023).

6.b. Revised conodont correlation

The first conodont zonation based on the phylogenetical successions of species in the genera Amorphognathus and Baltoniodus was introduced by Bergström (Reference Bergström, Sweet and Bergström1971) and, with some later modifications (Dzik, Reference Dzik1978; Bergström, Reference Bergström1983; Dzik, Reference Dzik1999; Männik, Reference Männik and Põldvere2003), was applied during many decades. The probable earliest species of Amorphognathus, A. inaequalis, has not been found in the Baltoscandia. The oldest representative of the genus in this area is A. tvaerensis. The origin of the Amorphognathus lineage is currently unknown (Dzik, Reference Dzik2024). However, as A. inaequalis is known from Avalonia only, it is possible that the Amorphognathus lineage originated in that region and, after some time, migrated to the other palaeocontinents. So far, A. inaequalis s.s. is reliably identified from the sections located on Avalonia only (Ferretti & Bergström, Reference Ferretti and Bergström2022).

The correlations in the Figure 5 are based on the FADs of B. variabilis, A. tvaerensis, B. gerdae, A. viirae, B. alobatus and A. superbus in Baltoscandia. Although details of morphological changes during the transition from B. alatus to B. variabilis require a further taxonomic study, the transition level still represents a usable marker, like also suggested in earlier publications (Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004; Bergström, Reference Bergström2007 b). The usability of the transition level as a stratigraphic marker is also supported outside Baltoscandia (Bagnoli & Qi, Reference Bagnoli and Qi2014; Albanesi & Ortega, Reference Albanesi, Ortega and Montenari2016).

The redefined conodont zonation also harmonizes the practice of using Ordovician conodont zones on a global scale. For example, Goldman et al. (Reference Goldman, Sadler, Leslie, Melchin, Agterberg, Gradstein, Gradstein, Ogg, Schmitz and Ogg2020) use B. variabilis as a zonal taxon for correlating the sections in Baltica and South China Platform. This correlation is misleading as the B. variabilis Subzone sensu Bergström (Reference Bergström, Sweet and Bergström1971) represents the upper part of range of the eponymous species in Baltica (Bergström, Reference Bergström, Sweet and Bergström1971), whilst the B. variabilis Zone in South China is defined by the FAD of B. variabilis (Wang et al. Reference Wang, Zhen, Bergström, Wu, Zhang and Ma2019) and thus comprises the lower part of the range of this species. Redefining the B. variabilis Subzone and raising it into rank of a zone support the use of B. variabilis Zone as a global correlation marker and facilitate the stratigraphic correlation of the successions of Baltica with Scotland (Armstrong, Reference Armstrong1997), Wales (Bergström & Ferretti, Reference Bergström and Ferretti2018), South China (Zhang et al. Reference Zhang, Zhan, Zhen, Wang, Yuan, Fang, Ma and Zhang2019), North America (Bergström, Reference Bergström, Sweet and Bergström1971) and Argentina (Albanesi & Ortega, Reference Albanesi, Ortega and Aceñolaza2002).

The late appearance of P. anserinus in the Kovel-1 and Bliudziai-150 core sections requires additional comment. The FAD of P. anserinus above the FAD of B. variabilis in these sections suggests that the real FAD of P. anserinus was not recorded. The late appearance of P. anserinus in the Kovel-1 core section may be due to gaps, particularly considering the numerous discontinuity surfaces recorded in the core section (Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004). An alternative explanation for this section could be facies difference. According to Saadre et al. (Reference Saadre, Einasto, Nõlvak and Stouge2004), the strata in the Kovel-1 section represent shallow shelf or even shoal facies and P. anserinus is not common in shallow facies (Viira & Männik, Reference Viira, Männik and Põldvere1999; Viira et al. Reference Viira, Löfgren, Sjöstrand and Põldvere2006 a; Hints et al. Reference Hints, Nõlvak and Viira2007; Viira, Reference Viira2008).

Amorphognathus viirae has been identified in all studied sections, and its FAD provides a reliable level for basin-wide correlation (Table 1; Fig. 5). An exception is the appearance of A. viirae in the Eisenackitina rhenana Chitinozoan Subzone, clearly below the Lagenochitina dalbyensis Chitinozoan Zone (Stouge et al. Reference Stouge, Bauert, Bauert, Nõlvak and Rasmussen2016), in the Kovel-1 core section. This discrepancy remains currently unexplained.

The FAD of A. tvaerensis (Table 1) falls within the E. rhenana Chitinozoan Subzone in all Baltoscandian sections where both taxa are identified (Tartu-453 – Männik, Reference Männik1998; Taga-Roostoja-25A – Põldvere, Reference Põldvere1999; Valga-10 – Põldvere, Reference Põldvere2001; Kovel-1 – Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004; Ruhnu-500 – Põldvere, Reference Põldvere2003; Kohtla/Viru – Hints et al. Reference Hints, Nõlvak and Viira2007; Mehikoorma-421 – Põldvere, Reference Põldvere2005; Fjäcka – Grahn & Nõlvak, Reference Grahn and Nõlvak2010; Viki – Hints et al. Reference Hints, Martma, Männik, Nõlvak, Põldvere, Shen and Viira2014; Bliudziai-150 – Stouge et al. Reference Stouge, Bauert, Bauert, Nõlvak and Rasmussen2016). In the majority of the studied section (Tartu-453, Taga-Roostoja-25A, Mehikoorma-421, Fjäcka, Viki, Bliudziai-150), the FAD of A. viirae lies within the L. dalbyensis Chitinozoan Zone. However, as identified in the Valga-10 core section (Põldvere, Reference Põldvere2001), the FAD of A. viirae coincides with the base of Belonechitina hirsuta Chitinozoan Zone. Latter discrepancy may result from about a 2+ m unsampled interval below the FADs of these taxa.

Based on the data above (Fig. 5; Table 1), an updated correlation chart of the Sandbian in Baltoscandia was compiled (Fig. 6).

This chart (Fig. 6) also reflects the recent data about conodonts from the Oandu Regional Stage (Männik, Reference Männik2017; Paiste et al. Reference Paiste, Männik and Meidla2022). The lower boundary of the stage is based on a significant change in the faunal succession (Männil & Meidla, Reference Männil, Meidla, Webby, Ross and Zhen1994) in the lower parts of Variku and Mossen formations or corresponds to the sedimentary hiatus at the base of the Hirmuse Formation (Fig. 5; Jaanusson, Reference Jaanusson and Basset1976; Meidla, Reference Meidla1996; Ainsaar et al. Reference Ainsaar, Meidla and Martma2004). A restudy of elements of Amorphognathus in the lower Katian revealed that the FAD of A. superbus occurred earlier than previously suggested, in the Blīdene Formation and in the lower part of the Variku Formation of the Keila Regional Stage (Fig. 6) but not in the Oandu Regional Stage as suggested earlier (Männik, Reference Männik2017; Paiste et al. Reference Paiste, Männik and Meidla2022). Following Bergström (Reference Bergström, Sweet and Bergström1971) and Nielsen et al. (Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023, figs. 3 and 5), correlation of A. superbus Zone with the Skagen Topoformation (Fig. 6) is tentatively accepted here, although these papers provide no direct information which supports this statement.

6.c. The bases of the Sandbian and Katian stages in the conodont succession

The lower boundaries of the Sandbian and Katian stages are based on the FADs of graptolites and cannot be determined precisely in the conodont succession (Paiste et al. Reference Paiste, Männik and Meidla2022). In its stratotype section at Fågelsång, the base of the Sandbian Stage lies within a 4-m interval that is barren of conodonts (Bergström, Reference Bergström2007 b). Conodonts below and above this interval point at some levels within the P. anserinus range. As B. alobatus occurs below and B. variabilis is identified above the barren interval, the real FAD of B. variabilis and its relationship to the lower boundary of the Sandbian Stage cannot be identified directly in this section.

According to Vandenbroucke (Reference Vandenbroucke2004), the base of Sandbian corresponds to a level in the lower part of the E. rhenana Chitinozoan Subzone in the Fågelsång section. Available information from sections where conodonts and chitinozoans occur together (Tartu-453 – Männik, Reference Männik1998; Taga-Roostoja-25A – Põldvere, Reference Põldvere1999; Valga-10 – Põldvere, Reference Põldvere2001; Ruhnu-500 – Põldvere, Reference Põldvere2003; Kovel-1 – Saadre et al. Reference Saadre, Einasto, Nõlvak and Stouge2004; Mehikoorma-421 – Põldvere, Reference Põldvere2005; Kerguta-565 – Põldvere, Reference Põldvere2006; Smedsby Gård – Bergström et al. Reference Bergström, Calner, Lehnert and Noor2011, Grahn & Nõlvak – Reference Grahn and Nõlvak2010; Viki – Põldvere, Reference Põldvere2010, Hints et al. Reference Hints, Martma, Männik, Nõlvak, Põldvere, Shen and Viira2014; Bliudziai-150 – Stouge et al. Reference Stouge, Bauert, Bauert, Nõlvak and Rasmussen2016; see also Table 1) shows that the FAD of B. variabilis is always below the FAD of E. rhenana. Hence, it can be concluded that the lower boundary of the Sandbian Stage lies within the lower part of the B. variabilis Zone.

In the stratotype section of the base of the Katian Stage, the boundary lies 4.0 m above the base of the Bigfork Chert (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007). This level falls within the uppermost range of the A. tvaerensis, being located 1.7 m below the appearance of Amorphognathus sp. cf. A. superbus (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007). Section D, the supplementary stratotype section also indicates that the base of the Katian Stage falls within the range of A. tvaerensis. Here, the FAD of Diplacanthograptus caudatus lies within the upper range marked by A. tvaerensis? (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007, fig. 11). Revised identifications of some conodonts from the stratotype section of the Katian Stage are possible using the specimens illustrated by Goldman et al. (Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007, fig. 7: 17–22). The Pa element 0.3 m below the top of the Womble Shale (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007, fig. 7: 18) does not show the extra posterolateral process on the outer side of its posterior process. Absence of the extra posterolateral process is the primary feature distinguishing Pa elements of A. viirae from those of A. tvaerensis (Paiste et al. Reference Paiste, Männik and Meidla2023). Additionally, the small lateral lobe on the outer side of the posterior process and the similar lengths of the anterior process and the anterior branch on the inner lateral process are the characteristic features of A. viirae (Paiste et al. Reference Paiste, Männik and Meidla2023). This suggests that the specimens identified as A. tvaerensis by Goldman et al. (Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007) in reality belong to A. viirae, and the stage boundary falls within the range of A. viirae. Furthermore, illustrated specimen of Amorphognathus sp. occurring 5.7 m above the base of the Bigfork Chert was suggested to represent A. superbus (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007, fig. 7: 15–16). A re-examination of these photos confirms the absence of the extra posterolateral process and shows an almost straight main denticle row with its distal end pointing towards the outer side, and the equal lengths of the anterior process and the anterior branch on the inner lateral process. All these are characteristic features of A. superbus (Bergström, Reference Bergström, Sweet and Bergström1971). This suggests that the base of the Katian cannot be younger than the A. viirae Zone in terms of the zonation proposed in the present paper. This level occurs in the upper part of the B. alobatus Zone in Figure 20.2 by Goldman et al. (Reference Goldman, Sadler, Leslie, Melchin, Agterberg, Gradstein, Gradstein, Ogg, Schmitz and Ogg2020), where the previous definition of this zone was applied.

6.d. Sandbian conodont distribution and C stable isotopic data

Two distinct δ13Ccarb excursions are known in the Sandbian of Baltoscandia. The Lower Sandbian Negative Isotopic Carbon Excursion (LSNICE; Bauert et al. Reference Bauert, Ainsaar, Põldsaar and Sepp2014; Upper Kukruse Low by Kaljo et al. Reference Kaljo, Martma and Saadre2007) is a negative excursion during which δ13Ccarb values decrease to about 0‰ in the upper part of the Kukruse Regional Stage. The Guttenberg Isotopic Carbon Excursion (GICE), recognized in the upper Keila Regional Stage (Ainsaar et al. Reference Ainsaar, Meidla and Martma1999), is characterized by an increase of δ13Ccarb values close to or higher than +2% in Estonia for a short time interval, followed by a decline to about +1% (Ainsaar et al. Reference Ainsaar, Kaljo, Martma, Meidla, Männik, Nõlvak and Tinn2010). The GICE was originally recognized in North America (Harch et al. Reference Harch, Jacobson, Witzke, Risatti, Anders, Watney, Newell and Vuletich1987) and is also recorded in the stratotype of the Katian Stage (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007), directly above the base of the stage. In the stratotype section (the Black Knob Ridge section: Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007; Fig. 4), the GICE starts below the FAD of Amorphognathus sp. cf. A. superbus. Further studies of this interval in the type region are clearly needed as Carlucci et al. (Reference Carlucci, Goldman, Brett, Westrop and Leslie2015) have alternatively proposed, based on the occurrences of trilobites and chitinozoans, that the isotopic excursion in this area may represent a younger excursion, ‘the Kope excursion’ by Bergström et al. (Reference Bergström, Schmitz, Saltzman and Huff2010). If this hypothesis would be confirmed, the lower boundary of the Katian Stage in the whole Baltoscandian region may require a revision.

Conodont and δ13Ccarb data from the strata of the Sandbian are available from several Baltoscandian sections: Ruhnu-500 (Kaljo et al. Reference Kaljo, Hints, Martma, Nõlvak and Oraspõld2004; Ainsaar et al. Reference Ainsaar, Meidla and Martma2004), Kerguta-565 (Kaljo et al. Reference Kaljo, Martma and Saadre2007), Mehikoorma-421 (Kaljo et al. Reference Kaljo, Martma and Saadre2007), Valga-10 (Kaljo et al. Reference Kaljo, Martma and Saadre2007), Tartu-453 (Bauert et al. Reference Bauert, Ainsaar, Põldsaar and Sepp2014) and Viki (Hints et al. Reference Hints, Martma, Männik, Nõlvak, Põldvere, Shen and Viira2014) core sections in Estonia, the Fjäcka main section in Sweden (Ainsaar et al. Reference Ainsaar, Kaljo, Martma, Meidla, Männik, Nõlvak and Tinn2010) and the Kovel-1 core section in Ukraine (Hints, O., Martma, T., unpublished data, Supplementary Table S1, see also Fig. 7).

Figure 7. Correlation of the Sandbian δ13Ccarb curves and conodont zonation. Kk. – Kõrgekallas; * – Hirmuse; T – Tatruse; Va – Variku; Bl – Blīdene; Mo – Mossen.

The LSNICE correlates with the A. tvaerensis Zone in all studied sections (Fig. 7). In the Kerguta-565 and Viki sections, the GICE interval is evidently cut off by a gap at the lower boundary of the Hirmuse Formation (Fig. 7), but it is preserved in the Tartu-453, Mehikoorma-421, Valga-10, Ruhnu-500 and Fjäcka sections. In the latter successions, the lower boundary of the GICE interval (in sense of Ainsaar et al. Reference Ainsaar, Kaljo, Martma, Meidla, Männik, Nõlvak and Tinn2010) lies above the Kinnekulle K-bentonite and the lower boundary of the A. superbus Zone lies within the GICE interval, near the GICE peak (Fig. 7). This agrees with the data from the Katian stratotype (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007) where the first appearance of Amorphognathus sp. cf. A. superbus is recorded within the GICE interval, above the K-bentonite.

6.e. Sandbian-early Katian conodont zones and regional stages in Baltoscandia

In Estonia and adjacent areas, the Ordovician succession contains numerous sedimentary hiatuses. The historical boundaries of regional stages defined in the Ordovician outcrop belt in Estonia were often confined to more prominent discontinuities. Gaps in the succession made these boundaries biostratigraphically clear and distinct. In order to improve the chronostratigraphic correlation between the type region and more complete offshore sections, numerous attempts, mainly based on the distribution of microfossils, have been made during the last decades. Although conodonts represent a useful correlation tool, they cannot be effectively used for tracing the stage boundaries in the studied interval as not a single one of the zonal boundaries in the Sandbian strata correlates with a boundary of a regional stage in the eastern Baltic area (Meidla et al. Reference Meidla, Ainsaar, Hints, Bauert, Hints, Meidla and Männik2014; Meidla et al. Reference Meidla, Ainsaar, Hints and Radzevičius2023).

The lower boundary of the Kukruse Regional Stage lies distinctly below the FAD of the A. tvaerensis (Fig. 6) in the stratotype area of the stage (Viira et al. Reference Viira, Aldridge and Curtis2006 b), within the B. variabilis Zone (Hints et al. Reference Hints, Nõlvak and Viira2007). The zonal graptolite Nemagraptus gracilis, the generally accepted primary marker of the base of the Sandbian Stage, has been recorded in the Mehikoorma-421 (at the depth 319.5–324.7 m; Männik et al. Reference Männik, Lehnert, Nõlvak and Joachimski2021), Valga-10 and Ruhnu-500 drillcores (at the depths 407.8 and 662.8 m, respectively; Nõlvak & Goldman, Reference Nõlvak and Goldman2007). The FAD of A. tvaerensis is located below the first N. gracilis in the Mehikoorma-421 and Ruhnu-500 sections (Fig. 5) and above it in the Valga-10 section (Männik, Reference Männik and Põldvere2001). This fact seems to point at a discrepancy between the eastern Baltic succession and the GSSP of the Sandbian Stage, where the FAD of the A. tvaerensis is located distinctly higher than the first N. gracilis. However, as the finds of N. gracilis in Estonian sections are very rare, their real positions within the range of this species, but also in relation to the base of Sandbian, are unknown. Most probably, this explains this apparent discrepancy. The lower boundary of the Haljala Regional Stage in North Estonia is marked by a remarkable gap corresponding to a part of the B. gerdae Zone (Hints & Meidla, Reference Hints, Meidla, Raukas and Teedumäe1997).

In the recently proposed succession of the Ordovician regional stages for the Scandinavian part of the basin, the lower boundary of the Dalbyan Regional Stage, by definition, corresponds to the FAD of A. tvaerensis (Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023, p. 288). Results of the recent restudy of conodonts from the Fjäcka main section (Paiste et al. Reference Paiste, Männik and Meidla2023) suggest that the lower boundary of the Dalbyan Regional Stage in this section actually lies 6.05 m above the base of the Dalby Limestone, 0.8 m higher than stated in the original definition of the boundary stratotype by Nielsen et al. (Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023).

The lower boundary of the overlying Moldåan Regional Stage is marked by the appearance of trilobite Toxochasmops extensus (sensu lato) within the Freberga Formation, at the hiatus between the Skagen Limestone and Moldå Limestone in the Fjäcka main section (Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023). The current correlation of the base of the Katian with the base of the Moldåan Regional Stage (Nielsen et al. Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023) is based on indirect evidence, considering ties between the conodont and graptolite successions in the Scandinavian region as indicated by Bergström (Reference Bergström, Hughes, Rickards and Chapman1986, tie 41 referring to Bergström, Reference Bergström1973, ties 17, 18). The latter ties conflict with the current Ordovician timescale (Goldman et al. Reference Goldman, Sadler, Leslie, Melchin, Agterberg, Gradstein, Gradstein, Ogg, Schmitz and Ogg2020) and the faunal evidence from the Katian boundary stratotype section (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007). Comparison of our data with that from the Katian stratotype section (Goldman et al. Reference Goldman, Leslie, Nõlvak, Young, Bergström and Huff2007) suggests that the base of Katian is older than the FAD of the A. superbus and the starting point of GICE. As a result, the base of the Katian Stage correlates with a level in the upper part of the Dalbyan Regional Stage but does not coincide with the base of the Moldåan Regional Stage as suggested by Nielsen et al. (Reference Nielsen, Ahlberg, Ebbestad, Hammer, Harper, Lindskog, Rasmussen and Stouge2023, fig. 3).

7. Conclusions

- A. inaequalis is missing in the Baltoscandia as well as in the western Ukraine; hence, the use of the eponymous conodont zone in these regions is misleading and should be avoided.

- Base of the Sandbian Stage correlates with a level within the B. variabilis Zone. However, the exact position of the FAD of B. variabilis is still problematic. Further taxonomic study of specimens from the transitional interval between B. alatus and B. variabilis is needed to clarify differences between these taxa.

- The presence of A. viirae is confirmed in all studied sections within the Baltoscandian region. Study of the published illustrations of conodonts from the Katian stratotype section indicated that the base of the Katian Stage corresponds to a level in the upper part of the range of A. viirae, below the FAD of the A. superbus.

- Based on the previous considerations, the Sandbian conodont zonation of Baltoscandia was redefined. The emended zonation includes the following zones (from below): B. variabilis, A. tvaerensis, B. gerdae, A. viirae and B. alobatus. Lower boundaries of all zones are defined as FADs of the eponymous taxa. This improves clarity of the conodont zonation and supports its unequivocal implementation during future studies.

- The available data indicate that the lowest δ13Ccarb value of the LSNICE falls within the A. tvaerensis Zone, below the FAD of the B. gerdae, and the lower boundary of the A. superbus zone lies in the GICE interval (in sense of Ainsaar et al. Reference Ainsaar, Kaljo, Martma, Meidla, Männik, Nõlvak and Tinn2010), close to the level of its highest values.

Supplementary material

The supplementary material for this article can be found at https://doi.org/10.1017/S0016756825100241

Acknowledgements

We thank the editor, Guillermo L. Albanesi and an anonymous reviewer for their constructive reviews and comments that led to improvements in the manuscript. The authors would also thank Olle Hints and Tõnu Martma for providing δ13Ccarb data for the Kovel-1 core section. Tõnn Paiste, Peep Männik, Leho Ainsaar and Tõnu Meidla acknowledge support from the Estonian Research Council, grant PRG1701.

Competing interests

None.

Footnotes

†

Author is deceased

References

Ainsaar, L, Kaljo, D, Martma, T, Meidla, T, Männik, P, Nõlvak, J and Tinn, O (2010) Middle and Upper Ordovician carbon isotope chemostratigraphy in Baltoscandia: a correlation standard and clues to environmental history. Palaeogeography, Palaeoclimatology, Palaeoecology 294, 189–201.10.1016/j.palaeo.2010.01.003CrossRefGoogle Scholar
Ainsaar, L, Meidla, T and Martma, T (1999) Evidence for a widespread carbon isotopic event associated with late Middle Ordovician sedimentological and faunal changes in Estonia. Geological Magazine 136, 49–62.10.1017/S001675689900223XCrossRefGoogle Scholar
Ainsaar, L, Meidla, T and Martma, T (2004) The Middle Caradoc facies and faunal turnover in the Late Ordovician Baltoscandian Palaeobasin. Palaeogeography, Palaeoclimatology, Palaeoecology 210, 119–33.10.1016/j.palaeo.2004.02.046CrossRefGoogle Scholar
Albanesi, GL and Ortega, G (2002) Advances on conodont-graptolite biostratigraphy of the Ordovician System of Argentina. In Aspects of the Ordovician System in Argentina (ed Aceñolaza, FG), pp. 143–66. INSUGEO, Série Correlación Geológica 16.Google Scholar
Albanesi, GL and Ortega, G (2016) Conodont and graptolite biostratigraphy of the Ordovician System of Argentina. In Stratigraphy & Timescales (ed Montenari, M), pp. 61–121. Amsterdam: Elsevier.Google Scholar
Armstrong, HA (1997) Conodonts from the Ordovician Shinnel Formation, southern Uplands, Scotland. Palaeontology 40, 763–97.Google Scholar
Bagnoli, G and Qi, Y (2014) Ordovician conodonts from the Red Petrified Forest, Hunan Province, China. Bollettino della Società Paleontologica Italiana 53, 93–104.Google Scholar
Bauert, H, Ainsaar, L, Põldsaar, K and Sepp, S (2014) δ13C chemostratigraphy of the Middle and Upper Ordovician succession in the Tartu-453 drillcore, southern Estonia, and the significance of the HICE. Estonian Journal of Earth Sciences 63, 195–200.10.3176/earth.2014.18CrossRefGoogle Scholar
Bergström, SM (1962) Conodonts from the Ludibundus Limestone (Middle Ordovician) of the Tvären area (S.E. Sweden). Arkiv för Geologi och Mineralogi 3, 1–61.Google Scholar
Bergström, SM (1971) Conodont biostratigraphy of the Middle and Upper Ordovician of Europe and eastern North America. In Symposium on Conodont Biostratigraphy (eds Sweet, WC & Bergström, SM), pp. 83–157. Boulder, CO: Geological Society of America.Google Scholar
Bergström, SM (1973) Correlation of the late Lasnamägian Stage (Middle Ordovician) with the graptolite succession. GFF 95, 9–18.Google Scholar
Bergström, SM (1983) Biogeography, evolutionary relationships, and biostratigraphic significance of Ordovician platform conodonts. Fossils and Strata 15, 35–58.10.18261/8200067378-1983-05CrossRefGoogle Scholar
Bergström, SM (1986) Biostratigraphic integration of Ordovician graptolite and conodont zones—a regional review. In Palaeoecology and Biostratigraphy of Graptolites (eds Hughes, CP, Rickards, RB & Chapman, AJ), pp. 61–78. Geological Society, Special Publication no. 20.Google Scholar
Bergström, SM (2007a) The Ordovician conodont biostratigraphy of the Siljan region south-central Sweden. A brief review of an international reference standard. In WOGOGOB 2007. Field Guide and Abstracts (eds Ebbestad, JOR, Wickström, LM & Högström, AES), pp. 26–41. Sveriges Geologiska Undersöokning (Geological Survey of Sweden), Rapporter och Meddelanden 128.Google Scholar
Bergström, SM (2007b) Middle and Upper Ordovician conodonts from the Fågelsång GSSP, Scania, southern Sweden. GFF 129, 77–82.10.1080/11035890701292077CrossRefGoogle Scholar
Bergström, SM, Schmitz, B, Saltzman, MR and Huff, WD (2010) The Upper Ordovician Guttenberg δ13C excursion (GICE) in North America and Baltoscandia: occurrence, chronostratigraphic significance, and paleoenvironmental relationships. The Geological Society of America Special Paper 466, 37–67.Google Scholar
Bergström, SM, Calner, M, Lehnert, O and Noor, A (2011) A new upper Middle Ordovician - Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden. 1. Stratigraphic review with regional comparisons. GFF 133, 149–71.10.1080/11035897.2011.622049CrossRefGoogle Scholar
Bergström, SM, Huff, WD, Kolata, DR and Bauert, H (1995) Nomenclature, stratigraphy, chemical fingerprinting, and areal distribution of some Middle Ordovician K-bentonites in Baltoscandia. GFF 117, 1–13.10.1080/11035899509546191CrossRefGoogle Scholar
Bergström, SM and Ferretti, A (2018) Deciphering the geology of some Darriwilian- Sandbian (Ordovician) ‘ghost’ formations in the UK and North America using olistoliths in marine debris flows. Geological Magazine 155, 1507–22.10.1017/S0016756817000383CrossRefGoogle Scholar
Bergström, SM, Finney, SC, Chen, X, Pålsson, C, Wand, ZH and Grahn, Y (2000) A proposed global boundary stratotype for the base of the Upper Series of the Ordovician System: the Fågelsång section, Scania, southern Sweden. Episodes 23, 102–09.10.18814/epiiugs/2000/v23i2/003CrossRefGoogle Scholar
Bergström, SM and Löfgren, A (2009) The base of the global Dapingian Stage (Ordovician) in Baltoscandia: conodonts, graptolites and unconformities. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 99, 189–212.10.1017/S1755691009008081CrossRefGoogle Scholar
Carlucci, JR, Goldman, D, Brett, CE, Westrop, SR and Leslie, SA (2015) Katian GSSP and Carbonates of the Simpson and Arbuckle Groups in Oklahoma. Stratigraphy 12, 144–202.10.29041/strat.12.3.02CrossRefGoogle Scholar
Dronov, AV, Ainsaar, L, Kaljo, D, Meidla, T, Saadre, T and Einasto, R (2011) Ordovician of Baltoscandia: facies, sequences and sea-level changes. In Ordovician of the World. Cuadernos del Museo Geominero 14 (eds Gutiérrez-Marco, JC, Rábano, I & García-Bellido, D), pp. 143–50. Madrid: Instituto Geológico y Minero de España.Google Scholar
Dzik, J (1978) Conodont biostratigraphy and paleogeographical relations of the Ordovician Mójcza Limestone (Holy Cross Mts, Poland). Acta Palaeontologica Polonica 23, 51–72.Google Scholar
Dzik, J (1994) Conodonts of the Mojcza Limestone. Acta Palaeontologica Polonica 53, 43–128.Google Scholar
Dzik, J (1999) Evolution of the Late Ordovician high-latitude conodonts and dating of Gondwana glaciations. Bolletino della Società Paleontologica Italiana 37, 237–53.Google Scholar
Dzik, J (2024) Faunal dynamics and evolution of Ordovician conodonts on the Baltic side of the Tornquist Sea. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 115, 39–76.10.1017/S1755691024000070CrossRefGoogle Scholar
Ferretti, A and Bergström, SM (2022) Middle-Upper Ordovician conodonts from the Ffairfach and Golden Grove groups in South Wales, United Kingdom. Historical Biology 34, 462–85.10.1080/08912963.2021.1929952CrossRefGoogle Scholar
Goldman, D, Leslie, SA, Nõlvak, J, Young, S, Bergström, SM and Huff, WD (2007) The Global Stratotype Section and Point (GSSP) for the base of the Katian Stage of the Upper Ordovician Series at Black Knob Ridge, Southeastern Oklahoma, USA. Episodes 30, 258–70.10.18814/epiiugs/2007/v30i4/002CrossRefGoogle Scholar
Goldman, D, Sadler, PM, Leslie, SA, Melchin, MJ, Agterberg, FP and Gradstein, FM (2020) Chapter 20 - The Ordovician Period. In Geologic Time Scale 2020 (eds Gradstein, FM, Ogg, JG, Schmitz, MD & Ogg, GM), pp. 631–94. Amsterdam: Elsevier.10.1016/B978-0-12-824360-2.00020-6CrossRefGoogle Scholar
Goldman, D, Leslie, SA, Liang, Y and Bergström, SM (2023) Ordovician biostratigraphy: index fossils, biozones and correlation. In A Global Synthesis of the Ordovician System: Part 1 (eds Harper, DAT, Lefebvre, B, Percival, IG & Servais, T), pp. 31–62. London: Geological Society, Special Publications no. 532.Google Scholar
Grahn, Y and Nõlvak, J (2010) Swedish Ordovician Chitinozoa and biostratigraphy: a review and new data. Palaeontographica Abteilung B 283, 5–71.10.1127/palb/283/2010/5CrossRefGoogle Scholar
Harris, MT, Sheehan, PM, Ainsaar, L, Hints, L, Männik, P, Nõlvak, J and Rubel, M (2004) Upper Ordovician sequence of Western Estonia. Palaeogeography, Palaeoclimatology, Palaeoecology 210, 135–48.10.1016/j.palaeo.2004.02.045CrossRefGoogle Scholar
Harch, JR, Jacobson, SR, Witzke, BJ, Risatti, JB, Anders, DE, Watney, WL, Newell, KD and Vuletich, AK (1987) Possible late Middle Ordovician organic-carbon isotope excursion — evidence from Ordovician oils and hydrocarbon source rocks, Midcontinent and East-Central United-States. AAPG Bulletin—American Association of Petroleum Geologists 71, 1342–54.Google Scholar
Hints, L and Meidla, T (1997) Sedimentary cover, Ordovician, Viru Series (Middle Ordovician), Keila Stage. In Geology and Mineral Resources of Estonia (eds Raukas, A & Teedumäe, A), pp. 74–76. Tallinn: Estonian Academy Publishers.Google Scholar
Hints, O, Martma, T, Männik, P, Nõlvak, J, Põldvere, A, Shen, Y and Viira, V (2014) New data on Ordovician stable isotope record and conodont biostratigraphy from the Viki reference drill core, Saaremaa Island, western Estonia. GFF 136, 100–04.10.1080/11035897.2013.873989CrossRefGoogle Scholar
Hints, O, Nõlvak, J and Viira, V (2007) Age of Estonian kukersite oil shale – Middle or Late Ordovician? Oil Shale 24, 527–33.10.3176/oil.2007.4.04CrossRefGoogle Scholar
Jaanusson, V (1976) Faunal dynamics in the Middle Ordovician (Viruan) of Baltoscandia. In The Ordovician System: Proceedings of a Palaeontological Association Symposium (ed Basset, MG), pp. 301–26. Cardiff: University of Wales Press.Google Scholar
Jaanusson, V (1982) The Siljan District. In IV International Symposium on the Ordovician System. Field Excursion Guide (eds Bruton, DL & Williams, SH), pp. 15–42. Oslo: University of Oslo.Google Scholar
Jaanusson, V (1995) Confacies differentiation and upper Middle Ordovician correlation in the Baltoscandian basin. Proceedings of the Estonian Academy of Sciences, Geology 44, 73–86.10.3176/geol.1995.2.01CrossRefGoogle Scholar
Kaljo, D, Borovko, N, Heinsalu, H, Khazanovich, K, Mens, K, Popov, L, Sergeyeva, S, Sobolevskaya, R and Viira, V (1986) The Cambrian–Ordovician boundary in the Baltic–Ladoga clint area (North Estonia and Leningrad Region, USSR). Estonian Journal of Earth Sciences 35, 97–108.Google Scholar
Kaljo, D, Hints, L, Martma, T and Nõlvak, J (2001) Carbon isotope stratigraphy in the latest Ordovician of Estonia. Chemical Geology 175, 49–59.10.1016/S0009-2541(00)00363-6CrossRefGoogle Scholar
Kaljo, D, Hints, L, Martma, T, Nõlvak, J and Oraspõld, A (2004) Late Ordovician carbon isotope trend in Estonia, its significance in stratigraphy and environmental analysis. Palaeogeography, Palaeoclimatology, Palaeoecology 210, 165–85.10.1016/j.palaeo.2004.02.044CrossRefGoogle Scholar
Kaljo, D, Martma, T and Saadre, T (2007) Post-Hunnebergian Ordovician carbon isotope trend in Baltoscandia, its environmental implications and some similarities with that of Nevada. Palaeogeography, Palaeoclimatology, Palaeoecology 245, 138–55.10.1016/j.palaeo.2006.02.020CrossRefGoogle Scholar
Kiipli, T, Dahlqvist, P, Kallaste, T, Kiipli, E and Nõlvak, J (2015) Upper Katian (Ordovician) bentonites in the East Baltic, Scandinavia and Scotland: geochemical correlation and volcanic source interpretation. Geological Magazine 152, 589–602.10.1017/S001675681400051XCrossRefGoogle Scholar
Konstantinenko, LI (2013) Part 4. Ordovician System. In Stratigraphy of Upper Proterozoic, Paleozoic and Mesozoic of Ukraine (ed Gozhyk, PF), pp. 167–75. Kyiv: National Academy of Sciences of Ukraine. [in Ukrainian].Google Scholar
Männik, P (ed.) (1998) Estonian Geological Sections Bulletin 1, Tartu (453) Drill Core. Tallinn: Geological Survey of Estonia, 48 pp.Google Scholar
Männik, P (2001) Distribution of conodonts. In Estonian Geological Sections Bulletin 3, Valga (10) Drill Core (ed Põldvere, A), pp. 10–12. Tallinn: Geological Survey of Estonia.Google Scholar
Männik, P (2003) Distribution of Ordovician and Silurian conodonts. In Estonian Geological Sections Bulletin 5, Ruhnu (500) Drill Core (ed Põldvere, A), pp. 17–23. Tallinn: Geological Survey of Estonia.Google Scholar
Männik, P (2010) Distribution of Ordovician and Silurian conodonts. In Estonian Geological Sections Bulletin 10, Viki Drill Core (ed Põldvere, A), pp. 21–24. Tallinn: Geological Survey of Estonia.Google Scholar
Männik, P (2017) Conodont biostratigraphy of the Oandu Stage (Katian, Upper Ordovician) in NE Estonia. Estonian Journal of Earth Sciences 66, 1–12.10.3176/earth.2017.02CrossRefGoogle Scholar
Männik, P and Viira, V (2005) Distribution of Ordovician conodonts. In Estonian Geological Sections Bulletin 6, Mehikoorma (421) Drill Core (ed Põldvere, A), pp. 16–20. Tallinn: Geological Survey of Estonia.Google Scholar
Männik, P, Lehnert, O, Nõlvak, J and Joachimski, MM (2021) Climate changes in the pre-Hirnantian Late Ordovician based on δ18Ophos studies from Estonia. Palaeogeography, Palaeoclimatology, Palaeoecology 569, 110347.10.1016/j.palaeo.2021.110347CrossRefGoogle Scholar
Männil, R (1976) Распространение граптолоидей в карбонатных отложениях ордовика Прибалтики (Distribution of graptoloids in the Ordovician carbonate deposits of the East Baltic). In Graptolites and Stratigraphy (eds Kaljo, D & Koren, D), pp. 105–18. Tallinn: Institute of Geology, Academy of Sciences of the Estonian SSR, Valgus. [in Russian]Google Scholar
Männil, R and Meidla, T (1994) Estonia, Latvia, Lithuania, Byelorussia, parts of Russia, the Ukraine and Moldova (East European Platform). In The Ordovician System of the East European Platform and Tuva (South-eastern Russia) (eds Webby, BD, Ross, RJ & Zhen, YY), pp. 1–52. Paris: International Union of Geological Sciences.Google Scholar
Meidla, T (1996) Late Ordovician Ostracodes of Estonia. Tartu: University of Tartu, 221 pp.Google Scholar
Meidla, T, Ainsaar, L and Hints, O (2014) The Ordovician system in Estonia. In 4th Annual Meeting of IGCP 591, Estonia, 10–19 June 2014. Abstracts and Field Guide (eds Bauert, H, Hints, O, Meidla, T & Männik, P), pp. 116–22. Tartu: University of Tartu.Google Scholar
Meidla, T, Ainsaar, L, Hints, O and Radzevičius, S (2023) Ordovician of the eastern Baltic palaeobasin and the Tornquist Sea margin of Baltica. Geological Society, London, Special Publications 532, 317–43.10.1144/SP532-2022-141CrossRefGoogle Scholar
Meidla, T, Truuver, K, Tinn, O and Ainsaar, L (2020) Ostracods of the Ordovician–Silurian boundary beds: Jūrmala core (Latvia) and its implications for Baltic stratigraphy. Estonian Journal of Earth Sciences 69, 233–47.10.3176/earth.2020.20CrossRefGoogle Scholar
Nielsen, AT, Ahlberg, P, Ebbestad, JOR, Hammer, Ø, Harper, DAT, Lindskog, A, Rasmussen, CMØ and Stouge, S (2023) The Ordovician of Scandinavia: a revised regional stage classification. Geological Society, London, Special Publications 532, 267–315.10.1144/SP532-2022-157CrossRefGoogle Scholar
Nielsen, AT and Schovsbo, NH (2011) The Lower Cambrian of Scandinavia: depositional environment, sequence stratigraphy and palaeogeography. Earth-Science Reviews 107, 207–310.10.1016/j.earscirev.2010.12.004CrossRefGoogle Scholar
Nõlvak, J and Goldman, D (2007) Biostratigraphy and taxonomy of three-dimensionally preserved nemagraptids from the Middle and Upper Ordovician of Baltoscandia. Journal Paleontology 81, 254–60.10.1666/0022-3360(2007)81[254:BATOTP]2.0.CO;2CrossRefGoogle Scholar
Nõlvak, J and Grahn, Y (1993) Ordovician chitinozoan zones from Baltoscandia. Palaeobotany and Palynology 79, 245–69.10.1016/0034-6667(93)90025-PCrossRefGoogle Scholar
Owen, AW, Bruton, OL, Bockelie, JF and Bockelie, T (1990) The Ordovician successions of the Oslo Region, Norway. Norges Geologiske Undersøkelse Special Publication 4, 3–54.Google Scholar
Paiste, T (2023) Early evolution of the genus Amorphognathus and updated Sandbian (Upper Ordovician) conodont biostratigraphy in Baltoscandia. PhD thesis, Department of Geology, University of Tartu, Tartu, Estonia. Published thesis.10.1017/S0016756822001005CrossRefGoogle Scholar
Paiste, T, Männik, P and Meidla, T (2022) Sandbian (Late Ordovician) conodonts in Estonia: distribution and biostratigraphy. GFF 144, 9–23.10.1080/11035897.2021.2020333CrossRefGoogle Scholar
Paiste, T, Männik, P and Meidla, T (2023) Emended Sandbian (Ordovician) conodont biostratigraphy in Baltoscandia and a new species of Amorphognathus. Geological Magazine 160, 411–27.10.1017/S0016756822001005CrossRefGoogle Scholar
Põldvere, A (ed.) (1999) Estonian Geological Sections Bulletin 2, Taga-Roostoja (25A) Drill Core. Tallinn: Geological Survey of Estonia, 47 pp.Google Scholar
Põldvere, A (ed.) (2001) Estonian Geological Sections Bulletin 3, Valga (10) Drill Core. Tallinn: Geological Survey of Estonia, 50 pp.Google Scholar
Põldvere, A (ed.) (2003) Estonian Geological Sections Bulletin 5, Ruhnu (500) Drill Core. Tallinn: Geological Survey of Estonia, 76 pp.Google Scholar
Põldvere, A (ed.) (2005) Estonian Geological Sections Bulletin 6, Mehikoorma (421) Drill Core. Tallinn: Geological Survey of Estonia, 67 pp.Google Scholar
Põldvere, A (ed.) (2006) Estonian Geological Sections Bulletin 7, Kerguta (565) Drill Core. Tallinn: Geological Survey of Estonia, 43 pp.Google Scholar
Põldvere, A (ed.) (2010) Estonian Geological Sections Bulletin 10, Viki Drill Core. Tallinn: Geological Survey of Estonia, 56 pp.Google Scholar
Pomyanovskaya, GM (1972) The Volyn elevation. In Stratigraphy of the USSR, Vol. III, Pt. 1, Cambrian, Pt. 2, Ordovician (ed Shulga, PL), pp. 159–69. Kiev: Naukova Dumka. [in Ukrainian]Google Scholar
Rhodes, FHT (1953) Some British Lower Palaeozoic conodont faunas. Philosophical Transactions of the Royal Society of London Series (B. 647) 237, 261–334.Google Scholar
Saadre, T, Einasto, R, Nõlvak, J and Stouge, S (2004) Ordovician stratigraphy of the Kovel-1 well (Volkhov-Haljala) in the Volynia region, northwestern Ukraine. Bulletin of the Geological Society of Denmark 51, 47–69.10.37570/bgsd-2004-51-04CrossRefGoogle Scholar
Stouge, S (1998) Distribution of conodonts in the Tartu (453) core. In Estonian Geological Sections Bulletin 1, Tartu (453) Drill Core (ed Männik, P). Tallinn: Geological Survey of Estonia.Google Scholar
Stouge, S, Bauert, G, Bauert, H, Nõlvak, J and Rasmussen, JA (2016) Upper Middle to lower Upper Ordovician chitinozoans and conodonts from the Bliudziai-150 core, southern Lithuania. Canadian Journal of Earth Sciences 53, 781–87.10.1139/cjes-2015-0205CrossRefGoogle Scholar
Stouge, S, Harper, DAT and Parkes, MA (2024) Late Darriwilian (Middle Ordovician) conodonts from eastern and southeastern Ireland. Irish Journal of Earth Sciences 42, 15–60.10.1353/ijes.2024.a935020CrossRefGoogle Scholar
Torsvik, LR and Cocks, LR (2017) Earth History and Palaeogeography. Cambridge: Cambridge University Press, 327 pp.Google Scholar
Vandenbroucke, T (2004) Chitinozoan biostratigraphy of the Upper Ordovician Fågelsång GSSP, Scania, southern Sweden. Review of Palaeobotany and Palynology 130, 217–39.10.1016/j.revpalbo.2003.12.008CrossRefGoogle Scholar
Vandenbroucke, T (2008) An Upper Ordovician chitinozoan biozonation in British Avalonia (England and Wales). Lethaia 41, 275–94.10.1111/j.1502-3931.2007.00090.xCrossRefGoogle Scholar
Viira, V (2008) Conodont biostratigraphy in the Middle-Upper Ordovician boundary beds of Estonia. Estonian Journal of Earth Sciences 57, 23–38.10.3176/earth.2008.1.03CrossRefGoogle Scholar
Viira, V, Aldridge, RJ and Curtis, S (2006b) Conodonts of the Kiviõli Member, Viivikonna Formation (Upper Ordovician) in the Kohtla section, Estonia. Proceedings of the Estonian Academy of Sciences, Geology 55, 213–40.10.3176/geol.2006.3.02CrossRefGoogle Scholar
Viira, V, Löfgren, A and Sjöstrand, L (2006a) Distribution of Ordovician conodonts. In Estonian Geological Sections Bulletin 7, Kerguta (565) Drill Core (ed Põldvere, A), pp. 11–13. Tallinn: Geological Survey of Estonia.Google Scholar
Viira, V and Männik, P (1999) Distribution of conodonts. In Estonian Geological Sections Bulletin 2, Taga-Roostoja (25A) Drill Core (ed Põldvere, A), pp. 9–10. Tallinn: Geological Survey of Estonia.Google Scholar
Wang, ZH, Zhen, YY, Bergström, SM, Wu, RC, Zhang, YD and Ma, X (2019) A new conodont biozone classification of the Ordovician System in South China. Palaeoworld 28, 173–86.10.1016/j.palwor.2018.09.002CrossRefGoogle Scholar
Zhang, YD, Zhan, RB, Zhen, YY, Wang, ZH, Yuan, WW, Fang, X, Ma, X and Zhang, JP (2019) Ordovician integrative stratigraphy and timescale of China. Science China Earth Sciences 62, 61–88.10.1007/s11430-017-9279-0CrossRefGoogle Scholar
Figure 0

Figure 1. General Ordovician palaeogeography in the Baltoscandian Palaeobasin (modified from Harris et al.2004; Saadre et al.2004; Meidla et al.2023). (1) Distribution of the Lower Palaeozoic strata in the eastern part of the Baltoscandian region (after Nielsen & Schovsbo, 2011); (2) circles mark the locations of the sections discussed in the text. Filled circles mark the sections along the profile in Figure 5, while empty circles mark the additional sections that correlate the Sandbian δ13Ccarb data and conodont zonation in Figure 7.

Figure 1

Figure 2. Comparison of the global (Goldman et al.2020), Scandinavian (Nielsen et al.2023) and Eastern Baltic (Meidla et al.2023) stratigraphic schemes. Abbreviations: Jiangxigr. – Jiangxigraptus, Hustedogr. – Hustedograptus, Diplacanthogr. – Diplacanthograptus, grapt. – graptolite, P. – Pygodus, A. – Amorphognathus, B. – Baltoniodus, S. – Sagittodontina.

Figure 2

Figure 3. Diagnostic elements and ranges of the zonal taxa used in this study. Outline drawings of Pa elements of Baltoniodus are from Bergström, 1971, plate 2 and M elements are from Paiste et al.2022, figure 5. Pa element of Amorphognathus tvaerensis is from Bergström, 1962, plate 4, A. viirae from Paiste et al.2023, figure 4 and A. superbus from Bergström, 1971, plate 2. M elements of Amorphognathus are from Paiste et al.2023, figures 3–5.

Figure 3

Table 1. FADs and LADs of stratigraphically important conodont species in the studied sections. Revised ranges of first (FAD) and last appearance datums (LAD) of species are shown in italics

Figure 4

Figure 4. Conodont zonations used in the Baltoscandian region. In the left-hand side of the figure, ranges of the key taxa used to define the zones in this study are indicated. Upper. – Uppermost B. alobatus range.

Figure 5

Figure 5. Updated distribution of stratigraphically important conodonts and correlation of the sections. Legend: (1) Kinnekulle-K bentonite; (2) discontinuity surface = prominent sedimentary hiatus in the succession; (3) species identification of a taxon based on recognizable Pa element; (4) identification of a taxon based on other than Pa element; (5) distribution data from Bergström, 2007a. Abbreviations: Mold – Moldå; Bl – Blīdene; Mo – Mossen; Kk – Kõrgekallas; * – Hirmuse; Tat – Tatruse; Rä – Rägavere; Dru - Drukšiai; Šve - Šventupys.

Figure 6

Figure 6. Sandbian correlation chart of the Baltoscandian region. Correlation of the Siljan district is based on the Fjäcka main section, N Livonian Basin on the Ruhnu-500 and Valga-10 sections; W-C Estonia on the Viki and Velise-V97 sections; N Estonia on Kerguta-565 and Taga-Roostoja-25A sections; E-C Estonia on the Tartu-453 and Mehikoorma-421 sections; S Livonian Basin on the Bliudziai-150 section; W Volyn region on the Kovel-1 section. Boundaries of the RSs are drawn according to the results of the discussion below and in the chapter ‘Sandbian-early Katian conodont zones and Regional Baltoscandian stages’. Abbreviations: G. Stage – Global Stage; EB. Stage – East Baltic Stage; S. Stage – Scandinavian Stage; Dar. – Darriwilian; Kat. – Katian; Oa. - Oandu Segerst. – Segerstadian; Mo. - Moldåan.

Figure 7

Figure 7. Correlation of the Sandbian δ13Ccarb curves and conodont zonation. Kk. – Kõrgekallas; * – Hirmuse; T – Tatruse; Va – Variku; Bl – Blīdene; Mo – Mossen.

Supplementary material: File

Paiste et al. supplementary material

Paiste et al. supplementary material
Download Paiste et al. supplementary material(File)
File 32.5 KB