The Upper Ordovician of Estonia: facies, sequences, and basin development

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.