HERØYA FORMATION
(Herøyformasjonen)
(Dahll 1857)
(Previously termed: 5a, 4g, 4h, 4c-β-5a, Venstöb oder Herö-kalkstein, Isotelustalk, Gastropodenkalk, Isotelus Limestone, Gastropod Limestone, Upper Tretaspis Beds).
Main Lithology. Limestone Herøya Limestone, Herøykalken).
Basal Stratotype. Skien-Langesund: Steinvika (NL 429397).
Definition. Little is known in detail of the Herøya Formation but its base is here defined as being at the sharp change from dark shales with planar limestones of the Venstøp Formation to pale coloured, tightly nodular, limestones with little intervening shale. This is a similar change to that seen in Oslo-Asker where the Venstøp Formation is succeeded by the Grimsøya Formation. However, the latter formation is only one of several Ashgill limestones in Oslo-Asker which alternate with calcareous shale units whereas the Herøya Formation extends almost to the top of the Ordovician in Skien-Langesund and Eiker-Sandsvær The thickness of the Herøya Formation is unknown but is in excess of 50m and above the tightly nodular lower part it comprises interbedded limestones and shales with subsidiary sandstones. As noted by Owen (1979 p.256) and Harland (1981 p.33) the Herøya Sørbakken and Gamme (formerly Gagnum) limestones may belong to a single, carbonate facies belt. Once the Herøya Formation is better known, there may be a case for reducing the Sørbakken and Gamme limestones to member status and using Dahll's unit as the formation name in all the relevant districts. However, there are differences between the three units and separate formations are provisionally retained herein. The Ashgill limestones of Oslo-Asker probably represent 'tongues' of this carbonate belt.
Fauna. Gastropods, cephalopods (Strand 1933) and bivalves (Toni 1975) are known from the upper part of the Herøya Formation but otherwise the fauna of the unit has yet to be studied.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.