The lower boundary of the Second Stage of the Lower Ordovician Series, the Floian Stage, is defined by the first appearance of the graptolite Tetragraptus approximatus. In the stratotype section of Diabasbrottet at Hunneberg, southern Sweden, the boundary falls within the Megistaspis planilimbata trilobite Zone and within the Oelandodus elongatus–Acodus deltatus deltatus conodont Subzone of the Paroistodus proteus Zone, and within the Hunneberg Regional Stage. Graptolites, including the index species T. approximatus, are missing in the terrigenous sediment of Estonia. The earliest trilobites are poorly preserved in this terrigenous succession. They represent an interval older than the Megalaspides dalecarlicus Zone but probably younger than the M. planilimbata Zone, and are correlative to the Prioniodus elegans conodont Zone. Thus the lower boundary of the Second Stage can be detected using mainly conodonts. In accordance with conodont occurrence, the base of the Floian (i.e. somewhat higher than the base of the O. elongatus–A. d. deltatus Subzone) falls within the Joa Member of the Leetse Formation. Probable levels have been suggested in some localities. Thus the lower part of the Leetse Formation belongs to the Tremadoc Stage, and to the Hunneberg Regional Stage.
Bergström, J., Pärnaste, H. & Zhou, Z.-Y. 2013. Trilobites and biofacies in the Ordovician Öland Series (Tremadocian–mid Darriwilian) of Baltica and a brief comparison with the Yangtze Plate. Estonian Journal of Earth Sciences, 62 [accepted].
Bergström, S. M., Löfgren, A. & Maletz, J. 2004. The GSSP of the Second (Upper) Stage of the Lower Ordovician Series: Diabasbrottet at Hunneberg, Province of Västergötland, southwestern Sweden. Episodes, 27, 265–272.
Bergström, S. M., Chen, X., Gutiérrez-Marco, J. C. & Dronov, A. 2009. The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages and to δ13C chemostratigraphy. Lethaia, 42, 97–107.
http://dx.doi.org/10.1111/j.1502-3931.2008.00136.x
Cooper, R. A. & Lindholm, K. 1990. A precise worldwide correlation of Early Ordovician graptolite sequences. Geological Magazine, 127, 497–525.
http://dx.doi.org/10.1017/S0016756800015429
Jaanusson, V. 1960. Graptoloids from the Ontikan and Viruan (Ordov.) limestones of Estonia and Sweden. Bulletin of the Geological Institutions of the University of Uppsala, 38, 289–366.
Jaanusson, V. 1982. Introduction to the Ordovician of Sweden. In Field Excursion Guide. IV International Symposium on the Ordovician System (Bruton, D. L. & Williams, S. H., eds), Paleontological Contributions from the University of Oslo, 279, 1–9.
Lindholm, K. 1991. Ordovician graptolites from the early Hunneberg of southern Scandinavia. Palaeontology, 34, 283–327.
Löfgren, A. 1993. Conodonts from the lower Ordovician at Hunneberg, south-central Sweden. Geological Magazine, 130, 215–232.
http://dx.doi.org/10.1017/S0016756800009870
Löfgren, A. 1994. Arenig (Lower Ordovician) conodonts and biozonation in the eastern Siljan District, Central Sweden. Journal of Paleontology, 68, 1350–1368.
Löfgren, A. 1996. Lower Ordovician conodonts, reworking, and biostratigraphy of the Orrenholmen quarry, Västergötland, south-central Sweden. GFF, 118, 169–183.
http://dx.doi.org/10.1080/11035899609546251
Löfgren, A. & Bergström, S. 2002. A stratigraphically significant conodont fauna from the base of the Tetragraptus approximatus Zone at Holsbrotten, Mt. Hunneberg, SW Sweden. GFF, 124, 131–134.
http://dx.doi.org/10.1080/11035890201243131
Löfgren, A., Viira, V. & Mens, K. 2005. Conodont biostratigraphy and sedimentary history in the upper Tremadoc at Uuga, Pakri Cape, NW Estonia. GFF, 127, 283–293.
http://dx.doi.org/10.1080/11035890501274283
Maletz, J. & Ahlberg, P. 2011. The Lerhamn drill core and its bearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania, southern Sweden. Lethaia, 44, 350–368.
http://dx.doi.org/10.1111/j.1502-3931.2010.00246.x
Maletz, J., Löfgren, A. & Bergström, S. M. 1996. The base of the Tetragraptus approximatus Zone at Mt. Hunneberg, SW Sweden: a proposed global stratotype for the base of the second series of the Ordovician System. Newsletters on Stratigraphy, 52, 129–159.
Männik, P. & Viira, V. 2012. Ordovician conodont diversity in the northern Baltic. Estonian Journal of Earth Sciences, 61, 1–14.
http://dx.doi.org/10.3176/earth.2012.1.01
Nõlvak, J., Hints, O. & Männik, P. 2006. Ordovician timescale in Estonia: recent developments. Proceedings of the Estonian Academy of Sciences, Geology, 55, 95–108.
Nõlvak, J., Hints, O., Männik, P. & Pärnaste, H. 2007. Ordovician time scale in Estonia. Acta Palaeontologica Sinica, 46 (Suppl.), 351–356.
Pärnaste, H. 2003. The Lower Ordovician trilobite Krattaspis: the earliest cyrtometopinid (Cheiruridae) from the Arenig of the east Baltic. Special Papers in Palaeontology, 70, 241–257.
Pärnaste, H. 2006a. The Early Ordovician trilobite distribution and zonation of the East Baltic. Proceedings of the Estonian Academy of Sciences, Geology, 55, 109–127.
Pärnaste, H. 2006b. The earliest encrinurid trilobites from the East Baltic and their taxonomic interest. Palaeontology, 49, 1–16.
http://dx.doi.org/10.1111/j.1475-4983.2005.00535.x
Pärnaste, H., Popp, A. & Owens, R. M. 2009. Distribution of the order Proetida (Trilobita) in Baltoscandian Ordovician strata. Estonian Journal of Earth Sciences, 58, 10–23.
http://dx.doi.org/10.3176/earth.2009.1.02
Pärnaste, H., Bergström, J. & Zhou, Z.-Y. 2012. High-resolution trilobite stratigraphy of the Lower–Middle Ordovician Öland Series of Baltoscandia. Geological Magazine [accepted].
Schmidt, F. 1906. Revision der ostbaltischen Trilobiten. Abtheilung V: Asaphiden. Leif 4. Mémoires de l’Académie Impériale des Sciences de St-Pétersbourg VIII, 19(10), 1–63, pls 1–8.
Suyarkova, A. A. & Koren¢, T. N. 2009. Advances in the Ordovician graptolite biostratigraphy of the St Petersburg area, Russia. Estonian Journal of Earth Sciences, 58, 3–9.
http://dx.doi.org/10.3176/earth.2009.1.01
Tjernvik, T. E. 1956. On the Early Ordovician of Sweden. Bulletin of the Geological Institutions of the University of Uppsala, 36, 107–284.
Tjernvik, T. E. & Johansson, J. V. 1980. Description of the upper portion of the drill-core from Finngrundet in the South Bothnian Bay. Bulletin of the Geological Institutions of the University of Uppsala, n.s., 8 (for 1979), 173–204.
Tolmacheva, T., Koren¢, T. N., Holmer, L. E., Popov, L. E. & Raevskaya, E. 2001. The Hunneberg Stage (Ordovician) in the area east of St. Petersburg, north-western Russia. Paläontologische Zeitschrift, 74, 543–561.
Törnquist, S. L. 1901. Researches into the graptolites of the lower zones of the Scanian and Vestrogothian Phyllo-Tetragraptus beds, Part 1. Lunds Universitets Årsskrift, 37, 1–26, 3 pls.
Viira, V. 2011. Lower and Middle Ordovician conodonts from the subsurface of SE Estonia and adjacent Russia. Estonian Journal of Earth Sciences, 60, 1–21.
http://dx.doi.org/10.3176/earth.2011.1.01
Viira, V., Löfgren, A., Mägi, S. & Wickström, J. 2001. An Early to Middle Ordovician succession of the conodont faunas at Mäekalda, northern Estonia. Geological Magazine, 138, 699–718.
http://dx.doi.org/10.1017/S0016756801005945
