Helskjer Member
(Helskjerleddet)
(Skjeseth 1955 in Strand & Størmer
(Previously termed: basal transition beds, 3c-γ (pars), Helskjær Shale and Limestone).
Main Lithologies. Limestones and shales.
Basal Stratotype. Mjøsa (Nes-Hamar): Helskjer (formerly Helskjær) (Fig. 10 and see (Wandås 1984, Fig. 2) (PN 088 329).
Hypostratotypes. Mjøsa (Nes-Hamar): Nydal-Furnes Church (PN 099 472) (Wandås 1984 Fig. 2). Skien Langesund: Rognstranda (NL 409 411). Eiker-Sandsvær Stablum (NM 443 160) (see (Wandås 1984, p.212). Modum (Vikersund): Vikersund ski jump (NM 563 453) (Wandås 1984, Fig. 2). Oslo-Asker: Bjørkåsholmen (NM 844 294) (Fig. 11). Ringerike: Klekken (NM 741 713). Hadeland: Hovodden (NM 785 952) (Wandås 1984, Fig. 2).
Thickness Variation. Mjøsa (Nes-Hamar): Helskjer (3.75m), Road Section Nydal-Furnes Church (-3.4m). Skien-Langesund: Rognstranda (1.0m), Tangvalkleven (1.13m). Eiker-Sandsvær Hals (1.58m). Modum (Vikersund): Vikersund ski jump (3.3m). Oslo-Asker: Bjørkåsholmen (0.8m). Ringerike: Klekken Post Office (0.35m). Hadeland: Hovodden, Hvattum (3.5m).
Definition. The Helskjer Member comprises bedded and nodular limestones interbedded with dark shales. The base is taken as the first shale above the compact limestones with Cycloendoceras of the Svartodden Member of the Huk Formation except in Skien-Langesund where the Helskjer Member marks a change to light limestones and thin shales from the dark limestone of the Rognstranda Member (Fig. 9). The top of the Helskjer Member is a change to shale as the predominant lithology.
Discussion. The Helskjer Member was formally defined by Skjeseth (1963 p.63) although the name was introduced by him earlier in the Lexique Stratigraphique (Strand & Størmer 1955). Although Nydal-Furnes was given as the type locality in the earlier publication, Skjeseth (1963) later designated Helskjer on the island of Helgøya, as the stratotype and this is followed here in view of the superiority of exposure there. Lithological logs for the Nydal-Furnes church section (now largely overgrown) were given by Nikolaisen (1963 p.360 Fig. 3) and (Wandås (1984 Fig. 2). The member has long been considered as a lithological transition from the 'Orthoceras Limestone' to a dominantly shale succession and tentatively included with the underlying unit (e.g. Holtedahl & Schetelig 1923, Størmer 1953, Skjeseth 1963, Skaar 1972, Kohut 1972). The Helskjer Member is here assigned to the Elnes Formation leaving the massive limestones packed with Cycloendoceras as an easily recognisable top to the underlying Huk Formation.
A 1 m thick red-brown weathering dolomite contrasting strongly with the light coloured Huk Formation below is present in parts of Oslo at the base of the Elnes Formation. Skaar (1972 p.A142) recognised a local intraformational conglomerate near the base of the limestone consisting of 1-5cm blocks of the underlying grey limestone set in a dolomitic matrix. He termed the dolomite the Killingen Dolomite and included it with the underlying limestone unit, giving it the Etasje term 3c-γ '. It is not formally defined herein but is considered an equivalent of the Helskjer Member and it is here assigned to the Elnes Formation.
Fauna and Age. The shelly fauna of the Helskjer Member was recognised by Størmer (1953) as being transitional between that of the underlying formation and that which is more typical of the shales of the Elnes Formation. Evidence for this was also presented by, amongst others, Skjeseth (1963) and Nikolaisen (1963). A detailed assessment of the rich trilobite fauna of the member has recently been made by (Wandås (1984) who has demonstrated a very high degree of similarity at both genus and species level with faunas from the 'Gigas Limestone' in Sweden. The Norwegian faunas include Megistaspis (M.) giganteus (Wandås a species very close to M. (M.) gigas (Angelin) and indicates a latest Kunda age.
Kohut extracted conodonts from the Helskjer Member ('transition beds' outside Nes-Hamar) from most districts of the Oslo Region but with the exception of the sample from the Rognstranda section, diagnostic elements were absent (1972 p.441). He concluded that the Rognstranda conodonts indicated an horizon no older than the Valaste Stage and thus he placed the Arenig-Llanvirn boundary in the lower 50cm of the member there (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.