Integrated Upper Ordovician–lower Silurian biostratigraphy of the Grötlingbo-1 core section, Sweden

The biostratigraphical data obtained from the Grötlingbo-1 core section (southern Gotland, Sweden) enable recognition of most of the regional stages known from the Sandbian (Upper Ordovician) to lower Sheinwoodian (Wenlock, Silurian) interval in the East Baltic. The strata corresponding to the Upper Ordovician Keila–Nabala stages have markedly reduced thicknesses in the Grötlingbo-1 core section. Gaps of considerable duration were recognised in some parts of the section: lithological evidence suggest that only lower part of the Pirgu Stage (corresponds to all or part of the Jonstorp Formation) is represented in the section; in the Llandovery, strata corresponding to the Aspelundia? fluegeli and Distomodus staurognathoides Conodont Zones (CZs), and to the Pterospathodus eopennatus CZ (equivalent to the Conochitina alargada and Eisenackitina dolioliformisChitinozoan Zones (CtZs)) are missing. Graptolites from the core indicate that the sedimentary record above the Aeronian to middle Telychian gap recommences in the lower part of the Oktavites spiralis Graptolite Zone (GZ). Co-occurrences of conodonts, graptolites and chitinozoans agree with earlier data suggesting correlation between the lower boundaries of the Cyrtograptus lapworthiGZ and Pterospathodus amorphognathoides amorphognathoides CZ, and between the lower and middle parts of the Ireviken Event and the upper Cyrtograptus murchisoni GZ.