SKJERHOLMEN FORMATION
(Skjerholmformasjonen)
(New name)
(Previously termed: 4c-γ , Øvre Trinucleusskifer, Upper Trinucleus Shale, Upper Tretaspis Shale).
Main Lithologies. Calcareous shales with limestones, siltstones and fine sandstones.
Basal Stratotype. Oslo-Asker: Søndre Skjerholmen (Fig. 20) (NM 968 365)
Hypostratotypes. Oslo-Asker: Grimsøya (base) (NM 889 384). Skogerholmen (top) (Fig. 21) (NM 862 353).
Definition. As with the underlying and overlying units, the Skjerholmen Formation was named but not defined by Owen (1981 Fig. 1). The formation comprises light grey, rusty weathering calcareous shale beds up to 50cm thick but commonly less than 20cm alternating with nodular silty limestones, calcareous siltstones and fine calcareous sandstone the majority of which are less than 1 Ocm thick. In the basal stratotype on S. Skjerholmen (Fig. 20) the base of the formation is taken at the first thick shale (20cm) above nodular limestones of the Grimsøya Formation. The siltstones and sandstones are more common towards the top of the formation and some are cross-bedded. Many horizons are strongly bioturbated and small scale channelling is evident at some levels.
The distinction between the Skjerholmen Formation and the underlying limestone unit is clear in the east but westwards towards Asker the boundary lies within a transitional sequence (e.g. Spjeldnæs 1964 p.9). Thus it is defined on Grimsøya at a 5cm siltstone bed above which shale beds become thicker and siltstones more common than in the underlying unit. The lower 16.5m of the Skjerholmen Formation are exposed above this boundary on Grimsøya
The thickness of the Skjerholmen Formation varies considerably but this may in part reflect tectonic disturbance. It is 35.6m thick on Søndre Skjerholmen and Brøgger (1887 pp.28-29) gave thicknesses between 35 and 43m for sections around Oslo and 2248m in Asker although it is not clear how he defined the base of the unit in the west.
Fauna and Age. The Skjerholmen Formation is very poorly fossiliferous and although Brøgger (1887 p.29) listed trilobites, cephalopods and indeterminate gastropods and brachiopods, only trilobites (Owen 1980a, 1981) a tabulate coral (Spjeldnæs 1964) and a monoplacophoran (Yochelson 1977) have been described from the formation and indicate no more than an Ashgill age.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.