Galgeberg Member
(Galgebergleddet)
(Erdtmann 1965)
(Previously termed: 3b-α (pars) 3b-β-ε 3bA (pars), 3b-B-C).
Main Lithology. Black shale.
Neostratotype. Oslo-Asker: Engervik (Fig. 6) (NM 843 302)
Hypostratotypes. Eiker-Sandsvær base - Skarakrysset, Vestfossen (NM 472 223), top - Råen (NM 459 176). Modum (Vikersund): Øvre Øren Farm (NM 556 444). Ringerike: Løkenåsen (NM 732 720). Hadeland: Helgåker (NM 847 945).
Thickness variation. Oslo Asker: Engervik (1 2m) (Fig. 6), Tøyen (1 2.43m) Eiker-Sandsvær Hals (>3.72m). Modum: Øvre Øren Farm (7.5m). Ringerike: Løkenåsen (>20m). Hadeland: Helgåker (?>15m), Tuv (12 m).
Definition. The base of the Galgeberg Member marks a sharp lithological change to black graptolitic shales from the striped shales and limestone nodules of the Hagastrand Member in parts of Oslo-Asker (including the neostratotype), Ringerike, Hadeland and Mjøsa or from the limestone and shales of the 'Vestfossen Member" in Eiker-Sandsvær and part of Oslo-Asker or from limestones of the Bjørkåsholmen Formation in Modum. The member is dominated by black shales with pyrite-bearing horizons. In Oslo-Asker isolated limestone nodules are developed in the uppermost metre and the shales become paler towards the base of the overlying formation. These upper beds were described by Spjeldnæs (1953 p.172) at Slemmestad, Asker and formed the basis for Erdtmann's (1965) Slemmestad Member. However, as Erdtmann (1965, p.488) noted, his base of this member was defined on faunal grounds not at an abrupt lithological change. These strata are here reassigned to the Galgeberg Member although they could eventually form the basis of a local biozone. Many of the exposures of the member in the Oslo Region show evidence of tectonism and thus the thickness estimates in Eiker-Sandsvær, Ringerike and Hadeland should be treated with caution.
Fauna and age. The rich graptolite fauna of the Galgeberg Member in Oslo-Asker has been documented by Monsen (1937), Spjeldnæs (1953) and Erdtmann (1965) and contains species indicative of the Tetragraptus approximatus to the lower part of the Didymograptus hirundo zones. It is thus equivalent to almost the entire Arenig Series. The faunas of the member in other districts of the Oslo Region have yet to be studied in detail but clearly contain the abundance of specimens of Didymograptus, Tetragraptus and Phyllograptus seen in the type district.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.