Conodonts from the type region of the Oandu Stage (Katian, Upper Ordovician) in NE Estonia were studied. Here, the lower boundary of the stage corresponds to a discontinuity surface at the base of the Hirmuse Formation and, in conodont succession, is marked by the disappearance of Semiacontiodus sp. and Besselodus? sp. The most characteristic taxa of the Oandu Stage are Phragmodus undatus, Icriodella cf. superba and Plectodina sp., which are rare or missing below and above this stratigraphic interval in Estonia. The comparison of conodont faunas from North and South Estonia suggests that the strata in North Estonia correspond to the upper part of the Oandu Stage only as identified in sections in South Estonia. However, the position of the lower boundary of the stage in South Estonia is highly problematic. The boundary between the Amorphognathus tvaerensis and A. superbus conodont zones in Estonia lies within the (upper) Oandu Stage.
Ainsaar, L. & Meidla, T. 2001. Facies and stratigraphy of the Middle Caradoc mixed siliciclastic-carbonate sediments in Eastern Baltoscandia. Proceedings of the Estonian Academy of Sciences, Geology, 50, 5–23.
Ainsaar, L., Meidla, T. & Martma, T. 1999. Evidence for a widespread carbon isotope event associated with late Middle Ordovician sedimentological and faunal changes in Estonia. Geological Magazine, 136, 49–62.
https://doi.org/10.1017/S001675689900223X
Ainsaar, L., Meidla, T. & Martma, T. 2004. The Middle Caradoc facies and faunal turnover in the Late Ordovician Baltoscandian palaeobasin. Palaeogeography, Palaeoclimatology, Palaeoecology, 210, 119–133.
https://doi.org/10.1016/j.palaeo.2004.02.046
Eichwald, E. 1854. Die Grauwackenschichten von Liv- und Ehstland. Bulletin de la Société Impériale des Naturalistes de Moscou, 27, 1–111.
Epstein, A. G., Epstein, J. B. & Harris, L. D. 1977. Conodont color alteration – and index to organic metamorphism. Geological Survey Professional Paper, 995, 1–27.
Hints, L. Meidla, T., Nõlvak, J. & Sarv, L. 1989. Some specific features of the Late Ordovician evolution in the Baltic Basin. Proceedings of the Academy of Sciences of the Estonian SSR, Geology, 38, 83–87.
Hints, L., Paškevičius, J., Martma, T., Männik, P. & Nõlvak, J. 2016. Upper Sandbian–lower Katian bio- and chemostratigraphy in the Pajevonys-13 core section, Lithuania. Estonian Journal of Earth Sciences, 65, 85–97.
https://doi.org/10.3176/earth.2016.08
Jeppsson, L. & Anehus, R. 1995. A buffered formic acid technique for conodont extraction. Journal of Paleontology, 69, 790–794.
https://doi.org/10.1017/S0022336000035319
Kröger, B., Hints, L. & Lehnert, O. 2014a. Age, facies and geometry of the Sandbian/Katian (Upper Ordovician) pelmatozoan-bryozoan-receptaculitid reefs of the Vasalemma Formation, northern Estonia. Facies, 60, 963–986.
https://doi.org/10.1007/s10347-014-0410-8
Kröger, B., Hints, L., Lehnert, O., Männik, P. & Joachimski, M. 2014b. The early Katian (late Ordovician) reefs near Saku, northern Estonia and the age of the Saku Member, Vasalemma Formation. Estonian Journal of Earth Sciences, 63, 271–276.
https://doi.org/10.3176/earth.2014.30
Männik, P. 2003. Distribution of Ordovician and Silurian conodonts. In Ruhnu (500) Drill Core (Põldvere, A., ed.), Estonian Geological Sections, 5, 17–23.
Männik, P. 2004. Recognition of the Mid-Caradoc Event in the conodont sequence of Estonia. In WOGOGOB-2004 Conference Materials (Hints, O. & Ainsaar, L., eds), pp. 63–64. Tartu University Press, Tartu.
Männik, P. 2010. Distribution of Upper Ordovician, Llandovery and Wenlock conodonts. In Viki Drill Core (Põldvere, A., ed.), Estonian Geological Sections, 10, 21–24.
Männik, P. & Viira, V. 2005. Distribution of Ordovician conodonts. In Mehikoorma (421) Drill Core (Põldvere, A., ed.), Estonian Geological Sections, 6, 16–20.
Männik, P. & Viira, V. 2012. Ordovician conodont diversity in northern Baltic. Estonian Journal of Earth Sciences, 61, 1–14.
https://doi.org/10.3176/earth.2012.1.01
Männil, R. 1958. Stratigrafiya i paleogeografiya oanduskogo gorizonta (DIII) v Éstonii [Stratigraphy and palaeogeography of the Oandu Stage (DIII) in Estonia]. In Tezisy dokladov nauchnoj sessii, posvyashchennoj 50-j godovshchine so dnya smerti F. B. Shmidta [Abstracts of the Presentations at the Scientific Session Dedicated to the 50th Death Anniversary of F. B. Schmidt], pp. 34–37. Tallinn.
Meidla, T. 1996. Late Ordovican ostracods in Estonia. Fossilia Baltica, 2, 1–222.
Meidla, T. 2003. Distribution of Ordovician ostracods. In Ruhnu (500) Drill Core (Põldvere, A., ed.), Estonian Geological Sections, 5, 25–28.
Meidla, T., Ainsaar, L. & Hints. O. 2014. The Ordovician System in Estonia. In 4th Annual Meeting of IGCP 591, Estonia, 10–19 June 2014, Abstracts and Field Guide (Bauert, H., Hints, O., Meidla, T. & Männik, P., eds), pp. 116–122. Tartu University Press, Tartu.
Nõlvak, J. 2003. Distribution of Ordovician chitinozoans. In Ruhnu (500) Drill Core (Põldvere, A., ed.), Estonian Geological Sections, 5, 23–25.
Öpik, A. 1934. Über Klitamboniten. Acta et Commentationes Universitatis Tartuensis (Dorpatensis), A XXVI, 5, 1–239.
Põldvere, A. 2003. General geological setting and stratigraphy. In Ruhnu (500) Drill Core (Põldvere, A., ed.), Estonian Geological Sections, 5, 6–12.
Põldvere, A. 2005. General geological setting and stratigraphy. In Mehikoorma (421) Drill Core (Põldvere, A., ed.), Estonian Geological Sections, 6, 6–13.
Põlma, L., Sarv, L. & Hints, L. 1988. Lithology and Fauna of the Caradoc Series Type Sections in North Estonia. Valgus, Tallinn, 101 pp.
Rõõmusoks, A. 1970. Stratigraphy of the Viruan Series (Middle Ordovician) in Northern Estonia. Valgus, Tallinn, 346 pp. [in Russian, with English summary].
Schmidt, F. 1881. Revision der ostbaltischen silurischen Trilobiten nebst geognostischer Übersicht des ostbaltischen Silurgebiets. Abtheilung I. Phacopiden, Cheiruriden und Encrinuriden. Mémoires de l’Academie Impériale des Sciences de St.-Pétersbourg, 7th Series, 30, 1–237.
Sweet, W. C. & Schönlaub, H. P. 1975. Conodonts of the Genus Oulodus Branson & Mehl, 1933. Geologica et Palaeontologica, 9, 41–59.
Viira, V. 1974. Ordovician Conodonts of the East Baltic. Valgus, Tallinn, 142 pp. [in Russian, with English summary].
