Svartodden Member
(Svartoddleddet)
(New name)
(Previously termed: 3c-γ , Orthocerkalk, Der Orthocerenkalk, Endoceratid Limestone)
Main Lithology. Limestone.
Basal Stratotype. Oslo-Asker: Huk (Fig. 7) (NM 938 410).
Hypostratotypes. Modum (Vikersund): Øvre Øren (NM 556 444). Mjøsa (Nes-Hamar): Helskjer (PN 088 329). Eiker-Sandsvær Ringerike and Hadeland: as for formation.
Thickness Variation. Oslo-Asker: Bjørkåsholmen (2.4m), Killingen (2.6m), Vekkerø (2.7m). Eiker-Sandsvær: Råen (2.13m), Stablum (2.21m), Hals (2.64m). Modum (Vikersund): Vikersund ski jump (3.3m), Hovland (5.2m) n.b. both these sections contain faulted repeats. Ringerike: Løkenåsen Klekken (1.7m). Hadeland: Hovodden (>4.5m), Granvollen (4.66m), Hvattum (4.0m). Mjøsa (Nes-Hamar): Helskjer (4.0m).
Definition. The base of the Svartodden Member is an abrupt change from shales and nodular limestones to compact light grey limestone with irregular, thin, shale laminae or pressure solution seams. In the southernmost Mjøsa district, Nes-Hamar, the Svartodden Member overlies the Herram Member but in the other districts of the Oslo Region where it is developed, it succeeds the Lysaker Member. The lower half of the Svartodden Member is bioturbated and contains phosphoritic and oolitic horizons (Skaar 1972). In the Eiker-Sandsvær district the lower part of the member constitutes the so called 'Porambonites Beds' (e.g. Klemm 1982, Fig. 513). The upper part of the Svartodden Member is a massive limestone characterized, especially in the uppermost part, by abundant specimens of the nautiloid cephalopod Cycloendoceras commune.
Fauna and Age. As noted above, Cycloendoceras commune is the most characteristic fossil in the uppermost part of the member and both Skaar (1972) and Klemm (1982) have analysed the orientation of the conchs on individual bedding planes. Trilobites (notably asaphids), brachiopods (including Porambonites) and echinoderms also occur. Skaar (1972, p.443) ascribed the conodont fauna of the Svartodden Member to Lindström's (1971) Eoplacognathus variabilis Zone thus indicating a correlation with the Hunderum Substage of the Kunda Stage. As noted above, Jaanusson (1960, Table 9) argued that the trilobites of the Svartodden Member indicate a middle Kunda (Valaste) age. This is in agreement with the conodont studies of S. Stouge and J. Rasmussen of Copenhagen University who place the base of the variabilis Zone well below that of the Svartodden Member (pers. comms. 1987 & 1989 and Rasmussen 1989, see Fig. 8).
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.