FURUBERGET FORMATION
(Furubergformasjonen)
(Skjeseth 1963)
(Previously termed: 4a, 4b-γ ?, Coelosphaeridium Zone, Cyclocrinus Zone, Coelosphaeridium Beds, Cyclocrinus Beds, Coelosphaeridium Shale, Cyclocrinus Shale and Limestone).
Main Lithologies. Shale with limestones and calcareous siltstones.
Basal Stratotype. Mjøsa (Nes-Hamar): Furuberget Railway Section (PN 092 443).
Hypostratotypes. Mjøsa (Ringsaker): Fangberget (PN 021527). Hadeland: 600m east of Raknerudtjern (NM 860 905), Lunner road section (see Lauritzen 1973, Fig. 4) (NM 865 863).
Definition. In the Mjøsa districts the base of the Furuberget Formation is in a gradational sequence from the grey shales of the Hovinsholm Formation to green/grey shales with limestones and calcareous siltstones. The formation was defined originally in the Mjøsa area (Skjeseth 1963 pp.65, 74) but is now also recognised in Hadeland (Harper & Owen 1984, p.21) where its base is also marked by the development of calcareous sandstones although these are restricted to 2.2m of the succession.
Spjeldnæs (1982 pp.150-152) noted that cross bedding in the siltstones and limestones of the lower Furuberget Formation in Nes-Hamar indicates a palaeo-slope from west to east. He also argued that the limestones of the formation were channel-fill sediments derived from a carbonate platform in the west.
The upper part of the formation contains thicker limestone horizons (> 10cm) in the Mjøsa areas. In addition in Hadeland, 1.2m thick bed of tightly nodular limestone very near the top of the formation is considered anticipatory of the overlying Nerby Member of the Solvang Formation. Limestones and shales which Kiær (1897) and Holtedahl (1909) ascribed to the Mjøsa Limestone were assigned to the uppermost Furuberget Formation by Størmer (1953 p.104) and Skjeseth (1963 p.65) and this view is followed here. Moreover, Spjeldnæs (1982) has argued that the base of the overlying Mjøsa Formation is diachronous.
Fauna, Flora and Age. Størmer (1953) and Skjeseth (1963) recognised two "zones" in the Furuberget Formation on the basis of fossil calcareous algae. The 'Coelosphaeridium zone' overlain by the 'Cyclocrinus zone'. These have yet to be defined in any formal way although Skjeseth (1963 p.65) referred to them as informal members of the Furuberget Formation. The limestones and to a lesser extent the shales of the formation contain a diverse shelly fauna which Spjeldnæs (1982 p. 150) argued had been washed in from the west. Trilobites (Henningsmoen 1960; Nikolaisen 1961, 1963, 1983; Bruton 1965; Owens 1970), brachiopods (Holtedahl 1916; Spjeldnæs 1957) and a possible alga (Nitecki & Spjeldnæs 1989) have been described from the Furuberget Formation in the Mjøsa districts. Here Spjeldnæs (1982 p.152) suggested that the presence of the brachiopods Christiania holtedahli Spjeldnæs and Kullervo sp. in the lower part of the unit may indicate a correlation with the Arnestad Formation of Oslo-Asker and thus an early Caradoc age. The brachiopods and trilobites from the uppermost part of the formation in Hadeland were described by Harper & Owen (1984) who suggested a Marshbrookian - early Actonian age for this part of the formation and a similar age is indicated by Spjeldnæs (1982 p.152) for the uppermost part of the unit at Furuberget (Nes-Hamar) which is similar to that of the Oandu Stage in Estonia on the basis of the bryozoan fauna. Spjeldnæs also suggested that these beds at Furuberget may be equivalent to the lower part of the Mjøsa Formation at Bergevika south.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.