A new upper Middle Ordovician–Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden: 2. Significance of δ¹³C chemostratigraphy

Abstract

A total of 239 isotope samples are used for establishing the δ¹³C chemostratigraphy in the upper Middle Ordovician to Lower Silurian succession in the approximately 70 m long Borenshult drillcore. The study interval starts in the upper Darriwilian Furudal Limestone and ends in the Rhuddanian Motala Formation. Four named δ¹³C excursions are recognized in 3–4 formations, namely the Guttenberg isotope carbon excursion (GICE) in the lower-middle Freberga Formation, the Kope (Rakvere) excursion in the uppermost Freberga Formation and possibly the Slandrom Formation, the Whitewater (Moe) excursion in the Lower Member of the Jonstorp Formation and the Hirnantian isotope carbon excursion (HICE) in the Loka Formation. The Middle Darriwilian isotope carbon excursion (MDICE) is missing in the drillcore and it is suggested that the study succession starts just above the interval of this excursion, which is in agreement with the range of MDICE in other Baltoscandic successions. The widespread Waynesville (Saunja) excursion may be cut out by the prominent unconformity below the Fjäcka Shale. The two most conspicuous excursions are the GICE (peak value ∼+1.9‰) and the HICE (peak value ∼+3.7‰), whereas the other excursions are represented by relatively minor perturbations in the δ¹³C curve. The Borenshult drillcore chemostratigraphy, which has been closely tied to conodont biostratigraphy, is quite similar to that of Estonian drillcores and is useful for not only trans-Baltic but also trans-Atlantic correlations. It is the first drillcore-based δ¹³C chemostratigraphy from the Swedish Ordovician.

Keywords:

• Middle and Upper Ordovician
• δ¹³C chemostratigraphy
• regional correlation
• Hirnantian excursion
• Guttenberg excursion
• Borenshult fauna

Acknowledgements

SMB is indebted to Lennart Andersson, Jan Bergström and Nils Malmsten for companionship during fieldwork excursions. MC wishes to acknowledge the receipt of the grant 20050748 from the Crafoord Foundation that defrayed the cost of the Borenshult drilling. OL is very grateful for support of his studies related to the CISP project “Circular Impact Structures in the Palaeozoic” of the Swedish Deep Drilling Program (SDDP). OL also acknowledges support from the International Continental Scientific Drilling Program (ICDP) by the Deutsche Forschungsgemeinschaft (DFG project LE 867 81). We thank Stefan Bengtson and Jonas Hagström for making some Borenshult fauna specimens kept at the Sektionen för Paleozoologi vid Naturhistoriska Riksmuseet, Stockholm available for δ¹³C study. We are also indebted to Mats Eriksson and Matthew Saltzman for reading the paper and offering useful comments. This paper is a contribution to the IGCP project 591 (The Early to Middle Paleozoic Revolution).