NAKKHOLMEN FORMATION
(Nakkholmformasjonen)
(Owen 1983, Harper et al. 1985)
(Previously termed: 4b-γ , Øvre Chasmopsskifer, Upper Chasmops Shale, Norderhov Formation).
Main Lithology. Shale (Nakkholmen Shale, Nakkholmskiferen).
Basal Stratotype. Oslo-Asker: Nakkholmen (Harper et al. 1985, Fig. 2) (NM 949 403).
Hypostratotypes. Oslo-Asker: Persteilene Spjeldnæs in Størmer 1953, Fig. 5, 45.6 to 23.4m in section) (NM 903 321; Ringerike: Norderhov (NM 710 666).
Thickness Variation. Oslo-Asker: Nakkholmen (13-14m), Persteilene (22.3m).
Definition. The base of the Nakkholmen Formation is defined by the development of thick dark shales which typify the unit in the eastern part of Oslo-Asker where they are more than 1 m thick at some horizons. Black limestone nodules, some septarian, occur at various levels: those in the lower part of the formation being large (up to 30cm thick), isolated and ellipsoidal in outline, those in the upper part being thinner (up to 10cm) and forming distinct horizons. Pyrite nodules are present throughout the unit. In the western part of Oslo-Asker the formation becomes calcareous with nodular limestone horizons more common and the shale thinner and paler in colour. A similar change occurs in Ringerike (Harper et al. 1985 pp.298-299). The formation thins eastwards from an estimated 3040m in Asker (Størmer 1953 p.65) to 12-13m in Oslo.
The unit in Ringerike was originally termed the Norderhov Formation (Owen & Harper 1982) but it is very poorly exposed here. Having taken advice from the Norwegian Stratigraphical Committee we here suppress the formation name in favour of the Nakkholmen Formation which is more firmly based, especially in terms of its upper and lower boundaries.
Fauna and Age. The Nakkholmen Formation contains a shelly fauna of brachiopods and trilobites which increases in diversity westwards and indicates a Woolstonian-Actonian age (Harper et al. 1985, Figs 3 & 4 and references therein). The brachiopod faunas share few elements in common with contemporaneous units elsewhere in the Oslo Region and even differ markedly from west to east within the formation. In contrast many of the trilobites are known also from the upper Furuberg Formation in Hadeland. The shelly fauna of the type Nakkholmen Formation is of very low diversity and is dominated by the brachiopods Hisingerella nana (Hadding), Onniella of. bancrofti Lindström and Sericoidea γ Spjeldnæs and the trilobite Broeggerolithus discors (Angelin). Graptolites are also known from the formation here and comprise Amolexograptus rugosus (Hadding), Climacograptus antiquus lineatus Elles & Wood and Corynoides incurvus Hadding indicative of the lower part of the Dicranograptus clingani Zone (Harper et al. 1985). Conodonts (Hamar 1966), ostracods (Henningsmoen 1953, Qvale 1980), gastropods (Yochelson 1963), bivalves (H. and T.Soot-Ryen 1960) and scolecodonts (Harper et al. 1985) are also known from the formation.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.