Conodont biostratigraphy, and δ¹³C and δ³⁴S isotope chemostratigraphy, of the uppermost Ordovician and Lower Silurian at Osmundsberget, Dalarna, Sweden

Abstract

The previously established graptolite and chitinozoan Hirnanian-Telychian biostratigraphy in the unique Osmundsberget North outcrop in the Siljan region, south-central Sweden, is integrated with new conodont biostratigraphy and δ¹³C_org, δ¹³C_carb and δ³⁴S_pyr chemostratigraphy. At this locality, the middle Hirnantian (latest Ordovician) topmost part of the Boda Limestone is overlain by the latest Hirnantian Glisstjärn Formation, and the late Aeronian–early Telychian (Llandovery) Kallholn Formation rests unconformably on the Glisstjärn Formation. Previous conodont work showed that the Glisstjärn Formation belongs to the lower Ozarkodina hassi Zone. New samples from calcareous interbeds in the dominantly shaly Kallholn Formation, some of which contain hundreds of condont elements, yielded Distomodus staurognathoides, Aspelundia fluegeli and other taxa indicating the D. staurognathoides Zone. In the East Baltic succession, the coeval interval, which is in the uppermost Raikküla–lowermost Adavere stages just below the geographically widespread Osmundsberg K-bentonite, has yielded a conodont fauna similar to that of the lower Kallholn Formation. A regional review of the D. staurognathoides Zone shows that there are possible equivalents to our study interval also in Norway and the Welsh Borderland, but equivalent strata are missing in large parts of North America, or have not produced diagnostic conodonts. The δ¹³C_org values from the study section are relatively uniform (mostly ranging between − 29‰ and − 30‰), and the late Aeronian and Valgu positive excursions have not been recognized.

Keywords:

• conodont biostratigraphy
• δ¹³C and δ³⁴S chemostratigraphy
• Llandovery
• Hirnantian
• Sweden
• regional correlation

Acknowledgements

Laboratory costs associated with processing and picking of most of the limestone samples by Eva-Marie Widmark at the Department of Geology, Lund University in the late 1960s, were paid by a grant to Stig M. Bergström from the Royal Physiographic Society of Lund. The research of Mats E. Eriksson is funded by the Swedish Research Council. We are indebted to Git Klintvik-Ahlberg and Carsten Tell for processing and picking the samples collected in 2010. Special thanks are due to Lars Holmer for providing several excellent conodont samples from the Osmundsberget North succession. We thank Nils Malmsten and Birgitta Bergström for fieldwork assistance during the collecting of shale and limestone samples in 2010. Isotope analyses were supported by grants from SIRF, Indiana University at Bloomington under the supervision of Lisa M. Pratt and Edward M. Ripley. We are also indebted to Benjamin Underwood and Peter Sauer at Indiana University for technical assistance with isotope analyses, and to the journal reviewers, Oliver Lehnert and Jan Ove R. Ebbestad, for useful comments on the manuscript.