Local and trans-Atlantic chemostratigraphic significance of new δ¹³C_carb data from the Sandbian and Katian Stages (Middle–Upper Ordovician) of the Oslo region, Norway

Late Sandbian to early Katian δ¹³C_carb chemostratigraphy has in Norway been described in only two previous reports that dealt with two geographically rather widely separated areas, namely the Oslo-Asker district and the Nes-Hamar district. No data have been available from the Ordovician outcrop areas between these districts that could help clarify the partly unclear regional stratigraphic relations across the Oslo region. A chemostratigraphic study of the classical road section at Tønnerud in the northwestern Hadeland district about halfway between the previously investigated districts resulted in the recognition of two δ¹³C excursions, namely one in the uppermost Furuberget Formation tentatively identified as the Guttenberg Isotopic Carbon Excursion and one in the lower Solvang Formation that is classified as the KOPE (RAKVERE) excursion. Based on these results, the Furuberget and Solvang formations are interpreted to be separated by a significant gap in the study area corresponding to the Oandu and lower Rakvere regional Baltoscandic stages. This suggests that the Tønnerud succession is less complete stratigraphically than those in the Oslo-Asker district. Similar gaps are not uncommon in this interval in Baltoscandia and in North America and probably reflect both eustatic and local epeirogenetic movements that make it difficult to establish a regionally applicable sequence stratigraphy.