Sedimentology and carbon isotope stratigraphy of Lower–Middle Ordovician successions of Slemmestad (Oslo-Asker, Norway) and Brunflo (Jämtland, Sweden)

Sammanfattning: Lower through Middle Ordovician strata are described with respect to sedimentology and carbon isotope stratigraphy from two areas, viz. two outcrops at Slemmestad in the Oslo-Asker district of southern Norway and a core section from the Brunflo area in Jämtland, central Sweden. The strata at these locations are compared and further correlation is done with the Tingskullen core section from Öland, south-eastern Sweden. Regionally important hardground complexes and beds such as ‘Blommiga bladet’ and ‘Blodläget’ and the ‘Volkhov-Kunda boundary bed’ are recognised in Jämtland and Slemmestad for the first time. The presented δ13Ccarb-values from Slemmestad indicate subsequent diagenetic alterations, which hampers regional correlation of the Huk Formation. δ13Corg from the Tøyen Shale Formation at this locality gives a more reliable signal and the BFICE has been tentatively recognised as 3.5 ‰ positive excursion in the uppermost part of the Hagastrand Member, although a higher data resolution would be needed to confirm it. The δ13Ccarb datasets from Brunflo and Tingskullen are both reliable and of high resolution. Several of the minor excursions and trends that characterise the generally stable carbon isotope record of the Lower and Middle Ordovician have been recognised, starting with the LTNICE, the BFICE, the Floian-Darriwilian rise and the BDNICE. A fast shift in the carbon isotope data is correlated to the ‘Täljsten’ interval which resents a regional biotic crisis. A negative excursion precedes the rising limb of the MDICE, which is clearly expressed (1.4 ‰) in the upper member of the Holen and Segerstad limestones in Brunflo. The correlation shows that the sedimentation rate was considerably higher in Slemmestad than in coeval strata in Jämtland and Öland during the Tremadocian and Floian; the Tremadocian is e.g. represented by almost 13 m of strata in Oslo as compared to the 2.5 m of coeval strata in Jämtland. The difference in sedimentation rate levelled out during the Dapingian and Darriwilian stages as a response to higher sea level at the time of deposition.