BJØRKÅSHOLMEN FORMATION
(Bjørkåsholmformasjonen)
(New name)
(Previously termed: 3a-α, Der Ceratopygen kalk, Ceratopygekalk, Ceratopyge Limestone).
Main Lithology. Limestone Bjørkåsholmen Limestone, Bjørkåsholmkalken)
Basal Stratotype. Oslo-Asker: Bjørkåsholmen (Fig. 3) (NM 844 294).
Hypostratotypes. Oslo-Asker: Hagastrand (Fig. 4) (NM 842 297), and Vekkerø (Fig. 5) (NM 925 432). Ringerike: Haug skole (NM 734 717). Eiker-Sandsvær Hals (NM 486 230). Vikersund: Øvre Øren Farm (NM 556 444).
Thickness variation. Oslo-Asker: Bjørkåsholmen (1.2m), Rortunet Shopping Centre (1 m), Vekkerø (1 m), Hagastrand (1.2m), Engervik (Fig. 6) (1.2m). Ringerike: Haug Skole (1.05m). Eiker-Sandsvær. Krekling (0.6m). Modum (Vikersund): Øvre Øren Farm (1.3m).
Definition. The base of the Bjørkåsholmen Formation is an abrupt change from the dark grey shales of the underlying Alum Shale Formation to pale weathering nodular limestone. The lower part comprises pale grey, irregularly bedded limestones with thin intercalations of shale. These limestones contain dark concretions at the base along with pale limestone clasts up to about 2cm long. The shales are locally graptolitic. At the Rortunet Shopping Centre the succeeding 74cm is composed of a single bed of grey limestone and is overlain by an 8cm 'glauconitic' (but see (Bjørlykke 1974, p. 13) limestone bed but at other localities (e.g. Bjørkåsholmen and Hagastrand) shales are developed in the uppermost part of the 'main' (thick) limestone and beneath the 'glauconitic' limestone (e.g. see Fjelldal 1966, Fig. 30 and Figs. 3, 4 herein). A detailed petrological study of the formation in Oslo-Asker and Eiker-Sandsvær was given by Fjelldal in an unpublished thesis (1966).
Age. The diverse shelly fauna of the Bjørkåsholmen Formation in the Oslo Region was listed and discussed by Brøgger (1882, pp.16-17; 1898). Some trilobite species have been redescribed more recently by Størmer (1940) and Henningsmoen (1959); the latter also providing a list of the whole trilobite fauna. The trilobites constitute the Euloma-Niobe fauna of Brøgger (1898 - see also Henningsmoen 1973) which is also present in the uppermost part of the Alum Shale Formation and is characteristic of the latest Tremadoc.
The Bjørkåsholmen Formation is widely developed in Sweden where Tjernvik (1956) discussed its fauna and distribution. Characteristic trilobites include Ceratopyge forficula, Triarthrus angelini, Euloma ornatum, Apatokephalus serratus and Niobe insignis. Other elements of the fauna include cephalopods, gastropods, echinoderms, brachiopods and bryozoans.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.