GRIMSØYA FORMATION
(Grimsøyformasjonen)
(New name)
(Previously termed: 4c-β Trinucleuskalk, Trinucleus Limestone, Tretaspis Limestone).
Main Lithology. Limestone Grimsøya Limestone, Grimsøykalken).
Basal Stratotype. Oslo-Asker: Grimsøya (Asker) (Fig. 19) (NM 889 384).
Hypostratotypes. Oslo-Asker: Raudskjer (NM 864 347), Kalvøya (NM 864 401), Nakkholmen (NM 949 405).
Definition. The term Grimsøya Formation was introduced without detailed description for this unit by Owen (1981 Fig. 1). The lower part of the formation (1 0.85m in the basal stratotype) comprises thin (rarely more than 6cm) nodular limestone horizons with rusty weathering shale partings. This contrasts with the shale-limestone alternations of the top of the underlying Venstøp Formation (Williams & Bruton 1983, Figs. 2-6). The upper part of the Grimsøya Formation is 29.Om thick on Grimsøya and is made up of alternating bedded limestones and shales. Siltstone beds are also present in this upper development, the first lying at 18.8m above the base of the formation in the basal stratotype. As Brøgger (1887 pp.25-6) indicated, the formation thins eastwards from above 46m at Odden on the Asker coast to as little as 10m on Rambergøya, Oslo. Spjeldnæs (1964 p.9) also noted that the unit thins to about 0.5m in Nittedal northeast of Oslo.
Fauna and Age. Much of the Grimsøya Formation is unfossiliferous but trilobites (Owen 1980a, 1981), corals (Spjeldnæs 1964), echinoderms (Bockelie 1984) and cephalopods (Strand 1933) have been described from various horizons. None of the fauna is particularly diagnostic in terms of the position of the unit in the Ashgill Series.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.