Conodont biostratigraphy, and δ_(13)C and δ_(34)S isotope chemostratigraphy, of the uppermost Ordovician and Lower Silurian at Osmundsberget, Dalarna, Sweden

摘要

The previously established graptolite and chitinozoan Hirnanian-Telychian biostratigraphy in the unique Osmundsberget North outcrop in the Siljan region, south-central Sweden, is integrated with new conodont biostratigraphy and δ~(13)Corg, δ~(13)C_(carb) and δ~(34)S_(pyr) chemostratigraphy. At this locality, the middle Hirnantian (latest Ordovician) topmost part of the Boda Limestone is overlain by the latest Hirnantian Glisstj?rn Formation, and the late Aeronian-early Telychian (Llandovery) Kallholn Formation rests unconformably on the Glisstj?rn Formation. Previous conodont work showed that the Glisstj?rn Formation belongs to the lower Ozarkodina hassi Zone. New samples from calcareous interbeds in the dominantly shaly Kallholn Formation, some of which contain hundreds of condont elements, yielded Distomodus staurognathoides, Aspelundia fluegeli and other taxa indicating the D.staurognathoides Zone. In the East Baltic succession, the coeval interval, which is in the uppermost Raikküla-lowermost Adavere stages just below the geographically widespread Osmundsberg K-bentonite, has yielded a conodont fauna similar to that of the lower Kallholn Formation. A regional review of the D. staurognathoides Zone shows that there are possible equivalents to our study interval also in Norway and the Welsh Borderland, but equivalent strata are missing in large parts of North America, or have not produced diagnostic conodonts. The δ~(13)C_(org) values from the study section are relatively uniform (mostly ranging between-29‰ and-30‰), and the late Aeronian and Valgu positive excursions have not been recognized.