The sediments of the Floian GSSP: depositional history of the Ordovician succession at Mount Hunneberg, Västergötland, Sweden

Abstract

The succession at Mt. Hunneberg consists of intercalated siliciclastic mudstones and carbonates of Tremadocian to Floian age. Above an unconformity with underlying Furongian shales, siliciclastic mudstones with graptolites and overlying glauconite packstones of the upper Alum Shale Formation are exposed showing a sharp top contact to the carbonates of Bjørkåsholmen Formation. Above, the Tøyen Shale Formation consisting of siliciclastic mudstones and intercalated carbonate beds forms the stratigraphically youngest Ordovician unit at Mt. Hunneberg. The Tøyen Shale Formation is characterized by a lower marl- and carbonate-rich part, exclusively present in the southwest of Mt. Hunneberg, and an upper portion consisting of siliciclastic mudstones, extending from the Tetragraptus phyllograptoides graptolite Biozone on upwards. The siliciclastic mudstones of the Alum Shale Formation represent open shelf sediments reflecting sea-level highstands of two trans- and regressions. Overlying glauconite packstones indicate a transgression of Adelograptus zone age or younger. The Bjørkåsholmen Formation reflects a relative sea-level lowstand. The Tøyen Shale Formation records a deepening of the sedimentary environment during sea-level rise initially establishing offshore conditions, with the upper Tøyen Shale Formation siliciclastic mudstones indicating open shelf deposition. Abundant burrows throughout the succession reflect hospitable living conditions in the Mt. Hunneberg area, also during deposition of the Floian black shales. A pronounced decrease in thickness of the Hunneberg succession toward the northeast reflects erosion in the proximal compared to distal Hunneberg areas. The succession shows that alternating offshore to open shelf conditions is an ideal sedimentary environment to establish a Global Boundary Stratotype Section and Point with abundant and detailed biostratigraphic information.

Keywords:

• Mt. Hunneberg
• Ordovician
• sedimentology
• depositional environment

Acknowledgements

This study forms part of a larger project on Ordovician shale depositional history of Scandinavia. The authors thank the donors of the American Chemical Society Petroleum Research Fund for their support of this research. Prof. Mikael Calner, University of Lund, encouraged us to submit this manuscript to GFF. Both Prof. Mikael Calner and Prof. Per Ahlberg, Lund University, significantly helped us during the course of this project with organizational issues, stimulating discussions about sedimentology in the Early Paleozoic as well as providing access to local literature on the Ordovician of Scandinavia. We would like to thank Warren Newby, Houston, for help in the field. Reviewers Prof. Per Ahlberg and Dr. Oliver Lehnert are thanked for helping to improve the manuscript and guiding us back on the safe path of adequate use of stratigraphic terms.