Sequence stratigraphic analysis of the Nabala–Porkuni regional stages (RSs; upper Katian–Hirnantian) of Estonia clarifies the Late Ordovician evolution of the Estonian Shelf–Livonian Basin. The integration of depositional facies, biostratigraphy, carbon isotope chemostra- tigraphy, karst surfaces, and hiatuses indicates seven sequences: (1) Nabala (Paekna and Saunja Formations (Fms)); (2) Vormsi (Kõrgessaare, Tudulinna, and Fjäcka Fms); (3) Lower Pirgu (Moe and Jonstorp Fms); (4) Middle Pirgu (most of the Adila and Halliku Fms, and Jelgava and Parovėja Fms); (5) Upper Pirgu (Kabala Member (Mb), part of the Halliku Fm); (6) Lower Porkuni Sequence (most of the Ärina Fm); and (7) part of the Upper Porkuni–Juuru Sequence (Kamariku Mb and Saldus Fm). A lowstand systems tract is only identified in the uppermost sequence. Transgressive units are marked by onlapping depositional packages. Highstands consist of one or more shallowing-upward shelf packages (notably in the Vormsi, Lower Pirgu, and Middle Pirgu sequences). The sequences record the progradation of shallow-to-middle ramp facies as sediment infilled the northern edge of the Livonian Basin, leading to an open shelf (Porkuni RS). Eustasy was the major factor in sequence boundary formation with larger amplitude sea level oscillations associated with Hirnantian (Porkuni) glaciations. A shift to more strongly differentiated ramp facies at the Nabala–Vormsi transition coincides with the initial collision of Baltica and Avalonia.
Ainsaar, L. and Meidla, T. 2001. Facies and stratigraphy of the middle Caradoc mixed siliclastic-carbonate sediments in eastern Baltoscandia. Proceedings of the Estonian Academy of Sciences. Geology, 50(1), 5–23.
https://doi.org/10.3176/geol.2001.1.02
Ainsaar, L., Kaljo, D., Martma, T., Meidla, T., Männik, P., Nõlvak, J. et al. 2010. Middle and Upper Ordovician carbon isotope chemostratigraphy in Baltoscandia: a correlation standard and clues to environmental history. Palaeogeography, Palaeoclimatology, Palaeoecology, 294(3–4), 189–201.
https://doi.org/10.1016/j.palaeo.2010.01.003
Ainsaar, L., Truumees, J. and Meidla, T. 2015. The position of the Ordovician–Silurian boundary in Estonia tested by high-resolution δ13C chemostratigraphic correlation. In Chemostratigraphy: Concepts, Techniques, and Applications (Ramkumar, M., ed.). Elsevier, 395–412.
https://doi.org/10.1016/B978-0-12-419968-2.00015-7
Bassett, M.G., Kaljo, D. and Teller, L. 1989. The Baltic region. In A Global Standard for the Silurian System, Geological Series, 9 (Holland, C. H. and Bassett, M. G., eds). National Museum of Wales, Cardiff, 158–170.
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(4), 195–200.
https://doi.org/10.3176/earth.2014.18
Burchette, T. P. and Wright, V. P. 1992. Carbonate ramp depositional systems. Sedimentary Geology, 79(1–4), 3–57.
https://doi.org/10.1016/0037-0738(92)90003-A
Calner, M., Lehnert, O. and Nõlvak, J. 2010. Palaeokarst evidence for widespread regression and subaerial exposure in the middle Katian (Upper Ordovician) of Baltoscandia: significance for global climate. Palaeogeography, Palaeoclimatology, Palaeoecology, 296(3–4), 235–247.
https://doi.org/10.1016/j.palaeo.2009.11.028
Cocks, L. R. M. and Torsvik, T. H. 2005. Baltica from the late Precambrian to mid-Palaeozoic times: the gain and loss of a terrane’s identity. Earth-Science Reviews, 72(1–2), 39–66.
https://doi.org/10.1016/j.earscirev.2005.04.001
Cooper, R. A. and Sadler, P. M. 2012. The Ordovician period. In The Geologic Time Scale 2012 (Gradstein, F. M., Ogg, J. G., Schmitz, M. D. and Ogg, G. M., eds). Elsevier, Amsterdam, 489–524.
Dronov, A. V. and Holmer, L. E. 1999. Depositional sequences in the Ordovician of Baltoscandia. Acta Universitatis Carolinae. Geologica, 43(1–2), 133–136.
Dronov, A. V. and Holmer, L. E. 2002. Ordovician sea-level curve: a Baltoscandian view. In Extended Abstracts: The Fifth Baltic Stratigraphic Conference “Basin Stratigraphy – Modern Methods and Problems”, Vilnius, Lithuania, 22–27 September 2002 (Satkūnas, J. and Lazauskienė, J., eds). Geological Survey of Lithuania, Vilnius, 33–35.
Dronov, A. V., 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 (Gutiérrez-Marco, J. C., Rábano, I. and Garcia-Bellido, D., eds). Instituto Geológico y Minero de España, Madrid, 143–150.
Einasto, R. 1986. Main stages of development and facies models of the East Baltic Silurian pericontinental basin. In Theory and Practice of Ecostratigraphy(Kaljo, D. and Klaamann, E., eds). Valgus, Tallinn, 37–54.
Einasto, R. 1995. „Liivi keele“ omapärast Baltika arenguloos. In Liivimaa geoloogia (Meidla, T., Jõeleht, A., Kalm, V. and Kirs, J., eds). University of Tartu, Tartu, 23–32.
Goldman, D., Sadler, P. M. and Leslie, S. A. 2020. The Ordovician Period. In Geological Time Scale 2020 (Gradstein, F., Ogg, J. G., Scmitz, M. D. and Ogg, G. M., eds). Elsevier, Amsterdam, 631–964.
Goldman, D., Leslie, S. A., Liang, Y. and Bergström, S. M. 2023. Ordovician biostratigraphy: index fossils, biozones and correlation. In A Global Synthesis of the Ordovician System: Part I, Geological Society London Special Publications, 532 (Harper, D. A. T., Lefebvre, B., Percival, I. G. and Servais, T., eds). Geological Society, London, 31–62.
https://doi.org/10.1144/sp532-2022-49
Haq, B. U. and Schutter, S. R. 2008, A chronology of Paleozoic sea-level changes. Science, 322(5898), 64–68.
https://doi.org/10.1126/science.1161648
Harris, M. T., Sheehan, P. M., Ainsaar, L. Hints, L., Männik, P., Nõlvak, J. et al. 2004. Upper Ordovician sequences of western Estonia. Palaeogeography, Palaeoclimatology, Palaeoecology, 210(2–4), 135–148.
https://doi.org/10.1016/j.palaeo.2004.02.045
Hints, L. 1990. Ordovician articulate brachiopods. In An Excursion Guidebook: Field Meeting Estonia 1990 (Kaljo, D. and Nestor, H., eds). Estonian Academy of Sciences, Tallinn, 58–61.
Hints, L., Meidla, T. and Nõlvak, J. 1994. Ordovician Sequences of the East European Platform. Geologija, Vilnius University, 17, 58–63.
Hints, L., Oraspõld, A. and Kaljo, D. 2000. Stratotype of the Porkuni Stage with comments on the Röa Member (uppermost Ordovician, Estonia). Proceedings of the Estonian Academy of Sciences. Geology, 49(3), 177–199.
https://doi.org/10.3176/geol.2000.3.02
Hints, L., Oraspõld, A. and Nõlvak, J. 2005. The Pirgu Regional Stage (Upper Ordovician) in the East Baltic: lithostratigraphy, biozonation, and correlation. Proceedings of the Estonian Academy of Sciences. Geology, 54(4), 225–259.
https://doi.org/10.3176/geol.2005.4.02
Hints, L., Hints, O., Nemliher, R. and Nõlvak, J. 2007. Hulterstad brachiopods and associated faunas in the Vormsi Stage (Upper Ordovician, Katian) of the Lelle core, central Estonia. Estonian Journal of Earth Sciences, 56(3), 131–142.
https://doi.org/10.3176/earth.2007.16
Hints, O., Martma, T., Männik, P., Nõlvak, J., Põldvere, A., Shen, Y. et al. 2014. New data on Ordovician stable isotope record and conodont biostratigraphy from the Viki reference drill core, Saaremaa Island, western Estonia. GFF, 136, 100–104.
https://doi.org/10.1080/11035897.2013.873989
Hints, O., Ainsaar, L., Lepland, A., Liiv, M., Männik, P., Meidla, T. et al. 2023. Paired carbon isotope chemostratigraphy across the Ordovician–Silurian boundary in central East Baltic: regional and global signatures. Palaeogeography, Palaeoclimatology, Palaeoecology, 624, 111640.
https://doi.org/10.1016/j.palaeo.2023.111640
Jaanusson, V. 1973. Aspects of carbonate sedimentation in the Ordovician of Baltoscandia. Lethaia 6(1), 11–34.
https://doi.org/10.1111/j.1502-3931.1973.tb00871.x
Jaanusson, V. 1976. Faunal dynamics in the Middle Ordovician (Viruan) of Balto-Scandia. In The Ordovician System: Proceedings of a Palaeontological Association Symposium, Birmingham, September 1974 (Bassett, M. G., ed.). University of Wales Press and National Museum of Wales, Cardiff, 301–326.
James, N. P., Kendall, A. C. and Pufahl, P. K. 2010. Introduction to biological and chemical sedimentary facies models. In Facies Models 4 (James, N. P. and Dalrymple, R. W., eds). Geological Association of Canada, 323–339.
Kaljo, D. 1990. An introduction to the geology of Estonia. In Field Meeting Estonia 1990. An Excursion Guidebook (Kaljo, D. and Nestor, H., eds). Estonian Academy of Sciences, Tallinn, 6–10.
Kiipli, E., Kallaste, T. and Kiipli, T. 2004. Metabentonites of the Pirgu Stage (Ashgill, Upper Ordovician) of the East Baltic. In WOGOGOB-2004, 8th Meeting of the Working Group on the Ordovician Geology of Baltoscandia, 13–18 May 2004, Tallinn and Tartu, Estonia. Conference Materials, Abstracts and Field Guidebook (Hints, O. and Ainsaar, L., eds). University of Tartu Press, Tartu, 1–52.
Kröger, B., Hints, L. and Lehnert, O. 2017. Ordovician reef and mound evolution: the Baltoscandian picture. Geological Magazine, 154(4), 683–706.
https://doi.org/10.1017/S0016756816000303
Männik, P. and Nõlvak, J. 2023. Boundary between the Porkuni and Juuru regional stages in the Neitla section, Estonia. Estonian Journal of Earth Sciences,72(1), 66–69.
https://doi.org/10.3176/earth.2023.52
Männik, P. and Viira, V. 1990. Conodonts. In Field Meeting Estonia 1990. An Excursion Guidebook (Kaljo, D. and Nestor, H., eds). Estonian Academy of Sciences, Tallinn, 84–89.
Männil, R. 1976. Distribution of graptoloids in the Ordovician carbonate rocks of the East Baltic area. In Graptolites and Stratigraphy (Kaljo, D. L. and Koren, T. N., eds). Academy of Sciences of the Estonian S.S.R. and Institute of Geology, Tallinn, 105–118.
Männil, R. 1990. The Ordovician of Estonia. In Field Meeting Estonia 1990. An Excursion Guidebook (Kaljo, D. and Nestor, H., eds). Estonian Academy of Sciences, Tallinn, 11–20.
Männil, R. and Meidla, T. 1994. The Ordovician System of the East European Platform (Estonia, Latvia, Lithuania, Byelorussia, parts of Russia, the Ukraine and Moldova). In The Ordovician System of the East European Platform and Tuva (Southeastern Russia) (Webby, B. D., Ross, R. J., Jr. and Zhen, Y. Y., eds). International Union of Geological Sciences Publication, 28A, Trondheim, and Geological Society of America, Denver, 1–52.
Mazur, S., Porębski, S. J., Kędzior, A., Paszkowski, M., Podhalańska, T. and Poprawa, P. 2017. Refined timing and kinematics for Baltica–Avalonia convergence based on the sedimentary record of a foreland basin. Terra Nova 30(1), 8–16.
https://doi.org/10.1111/ter.12302
Meidla, T. 1996. Late Ordovician ostracodes of Estonia. Fossilia Baltica, 2. Institute of Geology, Tallinn, and University of Tartu Press, Tartu.
Meidla, T. and Sarv, L. 1990. Ostracodes. In Field Meeting Estonia 1990. An Excursion Guidebook (Kaljo, D. and Nestor, H., eds). Estonian Academy of Sciences, Tallinn, 68–71.
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(4), 233–247.
https://doi.org/10.3176/earth.2020.20
Meidla, T., Ainsaar, L. and Hints, O. 2024. The Ordovician System in Estonia. In 11th Baltic Stratigraphical Conference, 19–21 August 2024, Tartu and Abavere, Estonia. Abstracts and Field Guide (Hints, O., Männik, P. and Toom, U., eds). Geological Society of Estonia, Tallinn, 54–59.
Nestor, H. 1990a. Some aspects of lithology of Ordovician and Silurian rocks. In Field Meeting Estonia 1990. An Excursion Guidebook (Kaljo, D. and Nestor, H., eds). Estonian Academy of Sciences, Tallinn, 27–32.
Nestor, H. 1990b. Basin development and facies models. In Field Meeting Estonia 1990. An Excursion Guidebook (Kaljo, D. and Nestor, H., eds). Estonian Academy of Sciences, Tallinn, 33–36.
Nestor, H. and Einasto, R. 1997. Ordovician and Silurian carbonate sedimentary basin. In Geology and Mineral Resources of Estonia (Raukas, A. and Teedumäe, A., eds). Estonian Academy Publishers, Tallinn, 192–204.
Nielsen, A. T. 2004. Ordovician sea level changes: a Baltoscandian perspective. In The Great Ordovician Biodiversification Event (Webby, B. D., Paris, F., Droser, M. L. and Percival, I. G., eds). Columbia University Press, New York, 84–93.
Nielsen, A. T. 2011. A re-calibrated revised sea-level curve for the Ordovician of Baltoscandia. In Ordovician of the World. Cuadernos del Museo Geominero,14 (Gutiérrez-Marco, J. C., Rábano, I. and Garcia-Bellido, D., eds). Instituto Geológico y Minero de España, Madrid, 399–401.
Nõlvak, J. 1999. Ordovician chitinozoan biozonation of Baltoscandia. Acta Universitatis Carolinae. Geologica, 43(1–2), 287– 290.
Nõlvak, J. and Grahn, Y. 1993. Ordovician chitinozoan zones from Baltoscandia. Review of Palaeobotany and Palynology, 79(3–4), 245–269.
https://doi.org/10.1016/0034-6667(93)90025-P
Perens, H. 1995. Ülemordoviitsiumist Põltsamaa-Jõgeva-Ruskavere joonel. In Liivimaa geoloogia (Meidla, T., Jõeleht, A., Kalm, V. and Kirs, J., eds). University of Tartu, Tartu, 45–50.
Raukas, A. and Teedumäe, A. 1997. Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn.
Read, J. F. 1983. Carbonate platform facies models. AAPG Bulletin, 69(1), 1–21.
https://doi.org/10.1306/AD461B79-16F7-11D7-8645000102C1865D
Simmons, M. S., Miller, K. G., Ray, D. C., Davies, A., van Buchem, F. S. P. and Gréselle, B. 2020. Phanerozoic eustasy. In Geologic Time Scale 2020(Gradstein, F. M., Ogg, J. G., Schmitz, M. D. and Ogg, G. M., eds). Elsevier, Amsterdam, 357–400.
Torsvik, T. H. and Rehnström, E. F. 2003. The Tornquist Sea and Baltica–Avalonia docking. Tectonophysics, 362(1–4), 67–82.
https://doi.org/10.1016/S0040-1951(02)00631-5
Tuuling, I. and Flodén, T. 2000. Late Ordovician carbonate buildups and erosional features northeast of Gotland, northern Baltic Sea. GFF, 122(2), 237–249.
https://doi.org/10.1080/11035890001222237
Winchester, J. A., Pharaoh, T. C. and Verniers, J. 2002. Palaeozoic amalgamation of Central Europe: an introduction and synthesis of new results from recent geological and geophysical investigations. Geological Society London Special Publications, 201 (Winchester, J. A., Pharaoh, T. C. and Verniers, J., eds). Geological Society, London, 1–18.
https://doi.org/10.1144/gsl.sp.2002.201.01.01
Zhang, A., Yang, C., Sahy, D., Condon, D. J. and Li, X.-H. 2026. New high-precision U–Pb zircon age constraints on the Sandbian stage of the Ordovician System from the Guanzhuang section in North China. Palaeogeography, Palaeoclimatology, Palaeoecology, 683, 113468.
https://doi.org/10.1016/j.palaeo.2025.113468