Stratigraphy of the Ordovician–Silurian boundary interval in Östergötland, Sweden, based on ostracod distribution and stable carbon isotopic data

Abstract

The Hirnantian glaciation event at the end of the Ordovician brought along major biological and oceanographic changes. The Harpabollia harparum ostracod assemblage characterized by its distinct species composition has been described only from localities where the presence of the Hirnantian strata is proven. This paper focuses on the distribution of this association from the Baltic area to Scandinavia (Sweden) and compares the faunal composition between these two areas. Altogether 4733 ostracods of 69 species were collected from the Ordovician–Silurian boundary interval in the Råssnäsudden section in Östergötland, SE Sweden. Pre-Hirnantian and Llandoverian assemblages are present, but there is no evidence of any H. harparum association or any HICE (Hirnantian carbon isotope excursion) peak in δ¹³C values. This evidence suggests a gap in the Råssnäsudden outcrop that corresponds to most of the Hirnantian.

Keywords:

• Upper Ordovician
• Hirnantian
• ostracods
• Baltoscandia
• Sweden
• Råssnäsudden

Acknowledgements

The authors wish to thank the reviewers of this article, Prof. Stig M. Bergström and Dr Mark Williams for very valuable comments. We are also indebted to Ruth Bengel for processing the ostracod samples and to Holar Sepp for conducting the δ¹³C carbon isotope analysis. This study was supported by the Estonian Target Financing Projects SF0180051s08 and Estonian Science Foundation grant 8049. This paper is a contribution to the IGCP project 591 (The Early to Middle Palaeozoic Revolution).