Lysaker Member
(Lysakerleddet)
(New name)
(Previously termed: 3c-β Expansusskifer, Der Expansuschiefer, Expansus Shale and Limestone, Asaphus-skifer, Asaphus Shale).
Main Lithologies. Limestone and calcareous shales
Basal Stratotype. Oslo-Asker: Huk (Fig. 7) (NM 938 410).
Hypostratotypes. Modum (Vikersund): Hovland (NM 560 445). Eiker-Sandsvær Ringerike and Hadeland: as for formation.
Thickness Variation. Oslo-Asker: Huk (2.56m) Vekkerø (1.5m), Killingen (2.5m), Engervik (3.5m), Tøyen (4.5m). Eiker-Sandsvær Hals (9.75m). Modum (Vikersund): Hovland Hovland Farm (2.6m). Ringerike: Løkenåsen Klekken (2.15m), Smeden (ca 6.3m). Hadeland: Granvollen (1.6m).
Definition. The light grey calcareous shales or marls and nodular limestones of the Lysaker Member contrast markedly with the compact limestone members above and below. Some of the variation in thickness within and between districts may be a tectonic effect reflecting the difference in competence between the Lysaker Member and its confining units. Shales may have been squeezed out locally and strata may also be cut out or repeated by faults.
Discussion. Skaar (1972) introduced the name 'Bjerkås Shale and Limestone Member' for this unit but to avoid confusion with Fjelldal's (1966) term 'Bjørkåsholmen Member' for the 'Ceratopyge Limestone' (= Bjerkåsholmen Formation herein) a completely new name is used here.
Fauna and Age. The Lysaker Member is richly fossiliferous: The asaphid trilobite Asaphus expansus occurs in abundance along with other asaphids (possibly including Megistaspis (M) limbata (see Jaanusson 1956), Cybele bellatula (see Owen & Tripp 1988), illaenids and pliomerids. The stratigraphical distribution of these trilobites has yet to be determined but will have great potential for correlation with uppermost Arenig - lowest Llanvirn in the Swedish successions. There is also an extremely diverse brachiopod fauna (Öpik 1939; Harper 1986) along with ostracods (Öpik 1939), echinoderms (Regnéll 1948, Bockelie 1981) and bryozoans.
Jaanusson (1960, pp.343-345) and Skevington (1965, p.70) equated most, if not all, of the member with the uppermost part of the D. hirundo graptolite zone whilst Kohut (1972) assigned conodonts from the member to the uppermost originalis to very lowest variabilis conodont zones. He argued therefore that the uppermost part of the D. hirundo Zone lies above the top of the Lysaker Member. As shown on Fig. 8, Jaanusson (1982, Fig. 4) equated the base of the D. 'bifidus' Zone with a level within the lowest part of the variabilis conodont zone (see also Bergström 1986, p.65). He also correlated the Asaphus expansus trilobite zone in Sweden with the uppermost parva and basal variabilis zones; a view also adopted by Löfgren (1985, Fig. 1) and Stouge & Rasmussen (pers. comms. 1987 & 1989 and Rasmussen 1989). These last workers also suggest that the base of the parva Zone lies within the underlying, Hukodden Member.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.