A new upper Middle Ordovician–Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden: 1. Stratigraphical review with regional comparisons

Abstract

A recent drilling at Borenshult near Motala resulted in discovery of the stratigraphically most complete succession through the upper Darriwilian–Rhuddanian interval known in Östergötland. The approximately 70 m long drillcore succession is subdivided into eight formations, the oldest being the late Darriwilian Furudal Limestone and the youngest being the Rhuddanian Motala Formation. Conodonts are used for a detailed biostratigraphic classification of the Borenshult drillcore into three subzones of the Pygodus serra Zone, two subzones of the Pygodus anserinus Zone, and three subzones of the Amorphognathus tvaerensis Zone. The base of the Amorphognathus superbus Zone is taken to be ∼10 m above the Kinnekulle K-bentonite, that of the Amorphognathus ordovicicus just below the Fjäcka Shale, and that of the Ozarkodina hassi Zone at the base of the Middle Member of the Loka Formation. Because of its unique lithology and paleontology and its wide geographic occurrence, this member is formally named herein the Skultorp Member. The previously uncertain stratigraphical position of the internationally known “Borenshult fauna” is shown to correlate with the Skultorp Member. A regional comparison of the Borenshult drillcore succession shows it to be most similar to coeval successions in Västergötland and Dalarna but there are some significant regional differences. The average rate of net rock accumulation during late Darriwilian and Sandbian time is calculated to be ∼3–4 mm/ka.

Keywords:

• Middle and Upper Ordovician
• Rhuddanian
• stratigraphy
• conodont biostratigraphy
• Borenshult fauna
• Sweden

Acknowledgements

SMB is indebted to Lennart Andersson, Jan Bergström, Sven Laufeld, and Nils Malmsten for companionship during fieldwork excursions. MC acknowledges Crafoord Foundation Grant 20050748 for recovery of the Borenshult drillcore. OL acknowledges support of his studies in the Early Palaeozoic of Sweden related to the CISP project (Circular Impact Structures in the Palaeozoic) of the Swedish Deep Drilling Program and the International Continental Scientific Drilling Program by the Deutsche Forschungsgemeinschaft (DFG project LE 867/8-1). The authors wish to acknowledge valuable and insightful comments by reviewer Jan Ove Ebbestad. This paper is a contribution to IGCP 591 (The Early to Middle Paleozoic Revolution).