Rognstranda Member
(Rognstrandleddet)
(New name)
(Previously termed: Endoceras Limestone, Endoceratid Limestone)
Main Lithology. Limestone.
Basal Stratotype. Skien-Langesund: Rognstranda (Fig. 9) (NL 409 411).
Definition. The Rognstranda Member is restricted to the most southerly district of the Oslo Region, Skien-Langesund, where it overlies Upper Cambrian (Peltura Zone) Alum Shales (Skaar 1972). This boundary has been interpreted as a tectonic junction, marking a plane of décollement above which the Ordovician and Silurian succession has been thrust southwards (e.g. Ramberg & Bockelie 1981, Fig. 2) but Ribland Nilssen (1985) has presented evidence for it being an unconformity. The Rognstranda Member is a 1.4m thick black limestone with abundant pyrite in its lower part. The dark colour and absence of shale horizons contrast markedly with the overlying Helskjer Member of the Elnes Formation.
Discussion. This limestone was formerly ascribed to the 'Endoceratid Limestone' (i.e. Svartodden Member) (e.g. Størmer 1953; Strand & Henningsmoen 1960) but it was considered sufficiently distinctive to warrant separate named status by Skaar (1972) and this view is upheld here.
Fauna and Age. Kohut (1972) extracted a conodont fauna from the upper part of the Rognstranda Member which included Eoplacognathus variabilis indicating a correlation with the Svartodden Member elsewhere in the Oslo Region.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.