A Cambrian–Ordovician boundary section in the Rafnes–Herøya submarine tunnel, Skien–Langesund District, southern Norway

Rock specimens and contained fossils collected in 1976 from a submarine tunnel driven between Herøya and Rafnes in the Skien–Langesund area of southern Norway, have been restudied. The contained fossils include olenid and agnostoid trilobites, graptolites and brachiopods, groups described in detail for the first time from the area and documenting a Cambrian–Ordovician boundary section unique in the district where the upper Cambrian Alum Shale Formation is elsewhere overlain by the Middle Ordovician Rognstranda Member of the Huk Formation (Kundan in terms of Baltoscandian chronostratigraphy). The hiatus at the base of the Huk Formation is thus smaller in the section described herein, beginning at a level within rather than below the Tremadocian. Estimated thickness of the Alum Shale includes 10–12 m of Miaolingian and 20–22 m of Furongian strata with trilobite zones identified, and a Tremadocian section of 8.1 m identified by species of the dendroid graptolite Rhabdinopora (unpublished) shot above the transect. The seismic data reveal the submarine continuation of the steep boundary escarpment of the Cambrian–Ordovician rocks typical of the landscape to the south of the tunnel location. The Lower Palaeozoic strata of the Skien–Langesund District typically dip eastwards towards the Permian plutons, and are cut by numerous faults and igneous dykes related to the Permian rifting of the Oslo Region. All the Middle–Late Ordovician formations described from outcrops in the Skien–Langesund District can be recognised in the tunnel section. However, a detailed stratigraphic investigation is beyond the subject of this work.