Hirnantian (latest Ordovician) δ¹³C chemostratigraphy in southern Sweden and globally: a refined integration with the graptolite and conodont zone successions

Abstract

The δ¹³C_org chemostratigraphy of the Hirnantian and lower Rhuddanian in the biostratigraphically well-controlled Röstånga-1 drillcore from west-central Scania is used for an improved integration of the Hirnantian Isotope Carbon Excursion (HICE) with the standard graptolite zonation. In this drillcore succession, the end of the HICE corresponds to the top of the range of Metabolograptus persculptus. Baseline δ¹³C_org values occur in the uppermost Hirnantian Avitograptus avitus Faunal Interval as well as in the Rhuddanian Akidograptus ascensus Zone, and the isotope curve is also tied to the Swedish uppermost Katian and Hirnantian trilobite zonation. Chemostratigraphic data from sections in Västergötland confirm that the beginning of the HICE is at, or very close to, the base of the Skultorp Member of the Loka Formation. The biostratigraphically less precisely controlled end of the HICE is at least locally in the Upper Member of the same formation. The graptolite biostratigraphy in the Mt. Kinnekulle succession indicates that the lowermost Kallholn Formation, which has long been known as the Leonaspis (formerly Acidaspis) Shale, is of Hirnantian rather than earliest Silurian age which is consistent with the age of the lowermost Kallholn Formation in the Röstånga-1 drillcore. Comparisons with Hirnantian sections in the United Kingdom, North America and China make it possible to improve the calibration of the HICE with conodont and graptolite biostratigraphy and confirm the usefulness of δ¹³C_org chemostratigraphy for detailed correlations. The upper Katian carbon chemostratigraphy in key sections in North America and eastern Baltoscandia indicates that the Elkhorn and Paroveja excursions are the same. Available data are used for a new Hirnantian eustasy-climate-faunal evolution model.

Keywords::

• Sweden
• chemostratigraphy
• Hirnantian
• uppermost Ordovician
• graptolite and conodont biostratigraphy
• δ¹³C excursion (HICE)

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Erratum

Acknowledgements

Ahlberg and Bergström acknowledge a grant from the Swedish Natural Science Research Council that defrayed the cost of the Röstånga-1 drilling. Bergström is indebted to Stanley Finney and the late Reuben J. Ross, Jr for guidance to Great Basin sections and to Chen Xu and his colleagues in Nanjing for field assistance at Chinese sections. Eriksson thanks the Swedish Research Council for funding his research. The authors are also indebted to Dimitri Kaljo and an anonymous reviewer for useful comments on this manuscript. This paper is a contribution to IGCP project 591.