MJØSA FORMATION
(Mjøsformasjonen)
(Kiær 1908)
(Previously termed: 5a (pars) 5b, 4b-δ , ?4b- δ +, der Mjøsen Kalk, Mjøskalk).
Main Lithology. Limestone (Mjøsa Limestone, Mjøskalken).
Basal Stratotype. Mjøsa (Toten): Eina (Opalinski & Harland 1981, Fig. 5) (NN 876 238).
Definition. The Mjøsa Formation is a varied limestone-dominated unit which maintains a thickness of about 1 00m over the Mjøsa districts. Its base in the Toten district is an abrupt change to bioclastic limestone from the shales and calcareous siltstones of the underlying Furuberget Formation (Opalinski & Harland 1981 p.61). The same is probably true in Ringsaker (Størmer 1953, Skjeseth 1963) but the boundary lies within a transitional succession. The top of the Mjøsa Formation is a distinct karst surface overlain by Llandovery sandstones of the Helgøya Member of the Sælabonn Formation (Skjeseth 1963; Opalinski & Harland 1981; Worsley et al. 1983).
Opalinski & Harland (1981) subdivided the Mjøsa Formation in Toten and Nes-Hamar into five named members and these are summarized here (Fig. 18). Spjeldnæs (1982) however, preferred not to use a formal lithostratigraphy in his excursion guide to these areas. The development in the Ringsaker district has yet to be described in detail but it is broadly divisible into two parts (Størmer 1953 p.111; Skjeseth 1963 p.75). The lower 30m of compact limestone rich in the alga Solenopora may be broadly equivalent to the Furnesfjorden Member further south whereas the overlying 60-70m of red calcareous shales appears to have no lithological equivalent in Toten or Nes-Hamar.
Fauna, Flora and Age. The Mjøsa Formation contains a diverse fauna and algal flora and locally includes the development of 'reefs' (Harland 1981 a). Algae from the unit were described by Kiær (1922) and Høeg (1927) and stromatoporoids by Webby (1979). Faunal elements include: brachiopods (Holtedahl 1916; Spjeldnæs 1957; Harper in prep.), trilobites (Nikolaisen 1961; Henningsmoen 1960; Siveter 1977), corals (Hill 1953), ostracods (Henningsmoen 1953), cephalopods (Sweet 1958) and bivalves (H. & T. Soot-Ryen 1960).
The Mjøsa Formation was originally thought to be late Ordovician in age (Kiær 1897, 1908; Holtedahl 1909) but Raymond (1916) and Kiær (1 922,1922a) recognised the hiatus at the top of the formation and equated the Mjøsa Limestone with late Caradoc units elsewhere in the Oslo Region. The described faunas generally confirm this correlation although some elements are endemic to the unit and reflect environments otherwise unknown in the Oslo Region. Spjeldnæs (1982 p. 152) argued that the lower part of the Mjøsa Limestone at Bergevika south is equivalent to the upper part of the Furuberget Formation at Furuberget and that both have a bryozoan fauna indicating a correlation with the Oandu Stage (mid-Caradoc) of Estonia. Hamar (in Størmer 1967 p. 204; in Bjørlykke 1974 p. 15), noted the existence of conodont evidence indicating that the formation extends into the lower Ashgill but details have not been published. The very thin development of the overlying Sælabonn Formation compared to elsewhere in the Oslo Region has led Worsley et al. to suggest that the hiatus above the Mjøsa Limestone may also include the lowest Llandovery (1983 Fig. 5).
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.