Terminal Ordovician stratigraphy of the Siljan district, Sweden

Integration of new isotopic data and earlier biostratigraphic information from eight sections through the terminal Ordovician (Pirgu and Porkuni stages) of the Siljan district, Sweden, allows a more precise correlation of sections in terms of biostratigraphy and carbon isotope dating. Four levels with positive d13C excursions are identified (from bottom) – the Moe, an unnamed excursion, Paroveja and Hirnantian Carbon Isotope Excursion (HICE). The d13C values through the Boda Limestone are 1–2‰ higher than usual in Baltica, only the values for the HICE remains within what is expected. Background values increase from 1.5‰ in the bottom of the core of the Boda Limestone up to 3‰ in the top of it. The HICE is identified in five of eight sections and the main peak falls according to inferred correlation within the Metabolograptus persculptus Biozone, at or close to the Hindella beds in the Upper Boda Member. The late Katian (Pirgu) age of Holorhynchus in the Siljan district is clear and its co-occurrence with the chitinozoan Belonechitina gamachiana in Estonia supports a Katian age for this zone. The base of the Ozarkodina hassi Biozone may occur within units B–C of the Upper Boda Member and in the upper part of the Loka Formation and most likely is correlated with the M. persculptus Biozone. The Hirnantia–Dalmanitina faunas reported from the lowermost part of the Loka Formation and units B–D of the Upper Boda Member seem to range through all the Hirnantian, but detailed morphological studies allow to distinguish an older (¼ extraordinarius) and a younger (¼ persculptus) fauna.