Chemostratigraphy in the Swedish Upper Ordovician: Regional significance of the Hirnantian delta C-13 excursion (HICE) in the Boda Limestone of the Siljan region

摘要

Samples from the Boda Limestone and immediately overlying strata at Osmundsberget in de Siljan region have produced an excellent Hirnantian isotope excursion (HICE) curve, in which the excursion interval is about 22 m thick and the HICE reaches maximum delta C-13 values of between +5 parts per thousand and +6 parts per thousand. Both the HA and HB stratigraphic unconformities are recognized in the study succession as well as in the coeval stratigraphic interval in the Loka Formation in Vastergotland and at Meifod in central Wales. A comparison between these and some other shallow-water successions in North America and northern Europe shows striking similarities in the delta C-13 curve, the lithology, and the stratigraphic sequence development. This is taken as support for the idea that glacio-eustacy exercised a major control on the depositional pattern in shallow-water successions during Hirnantian time. This applies also to the Osmundsberget succession where the formation of carbonate mounds was stopped by the HA regression that was caused by a major glaciation in Gondwana.