Ordovician Unit, Oslo Region
Husbergøya Formation (Husbergøyformasjonen)
(Brenchley & Newall 1975)
(Previously termed: 4b-γ, 5a, Øverste Chasmopsnivå, Gastropodkalk (pars)).
Main Lithology. Shale (Husbergøya Shale, Husbergøyskiferen).
Basal Stratotype. Oslo-Asker: NW (Husbergøya (Oslo) (Brenchley & Newall 1975, Fig. 4) (NM 962 377).
Definition. The Husbergøya Formation was defined and described in detail by Brenchley & Newall (1975) who also discussed the historical development of the terminology applied to this part of the Norwegian succession. The base of the formation marks an abrupt change from nodular limestones of the Spannslokket Member to a shale dominated succession. Thin calcareous sandy horizons become progressively more common up through the formation and the top of the unit in the east is developed as a brown weathering bedded sandstone up to 5m thick. Bioturbation is common with Chondrites occurring throughout the unit and a horizon containing a large trace fossil referred to ‘Tricophycus‘ by Seilacher & Meischner (1964) (but see Brenchley and Newall 1975 p.257) can be recognised a short distance above the base of the formation in many sections. The formation was interpreted as representing a deep shelf environment by Brenchley et al. (1979) and Brenchley & Cocks (1982).
The (Husbergøya Formation is 18.5m thick in the type section and shows an overall thinning to as little as 10m in Asker although in its northernmost development in the west, at Sandvika (Bærum) it thickens to 35m (Brenchley & Newall 1975). The upper part of the formation in Oslo passes westwards into the Langåra Formation. The brown bioturbated sandstone at the top of the (Husbergøya Formation in the east can be traced into the Langåra Formation thus enabling fairly precise correlation across the Oslo-Asker district (Brenchley & Newall 1975 p.259).
Fauna and Age. The Husbergøya Formation contains a sparse but fairly diverse fauna which was listed by Brenchley & Cocks (1982 pp.792-3) who termed it the ‘Tretaspis Association’. These authors also illustrated some of the trace fossils from the formation (1982 P1.85, Figs 3-5) (assigned to the Thalassinoides association by Stanistreet 1989). Brenchley & Cocks (1982 p.793) ascribed some samples from the upper part of the formation to a ‘Tretaspis-Onniella Association, in which brachiopods characteristic of the Onniella Association occur along with typical Tretaspis Association elements. These authors also assigned the fauna of the bioturbated sandstone at the top of the formation in Oslo to this ‘mixed’ association. Bockelie (1984 p. 10) termed this latter fauna the ‘Tetreucystis tetrabrachiolata association’ which also includes the trilobites Mucronaspis mucronata (Brongniart) kiaeri (Troedsson) (Owen 1982) and Platycoryphe sp. Owen (1986 p.233) has noted that the occurrence of these trilobites along with an Hirnantia brachiopod fauna at one locality represent an early stage in the spread of Gondwanan elements onto lower latitude shelves. Elements of the (Husbergøya Formation fauna which have received taxonomic attention include the trilobites (Owen 1980,1980a, 1981,1982), ostracods (Henningsmoen 1954), cephalopods (Strand 1933), brachiopods (Holtedahl 1916, Cocks 1982), cystoids (Bockelie 1984) and monoplacophorans (Yochelson 1977).
The trilobites include several species which indicate a Rawtheyan age for the unit (Owen in Cocks 1982 p.756) and the presence of Tretaspis sortita (Reed) broeggeri Størmer in the uppermost part of the (Husbergøya Formation suggests a correlation with the Lady Burn Starfish Beds of Girvan, Scotland and thus a late Rawtheyan age (Owen 1980a, Harper 1982).
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.
Publisert 19. feb. 2009
UiO: Naturhistorisk Museum / Natural History Museum, Oslo