Ordovician Unit, Oslo Region

Gagnum Member

(Gagnumleddet)

(Størmer 1945)

(Previously termed: Sphaeronidskifer, Sphaeronid Shale)

Main Lithology. Calcareous shale with limestone nodules.

Basal Stratotype. Hadeland: Field outcrops east of Nerby (see Owen 1978, Fig. 8) (NM 859 887).

Hypostratotypes. Hadeland: Tønnerud shore section (see Bockelie 1984, Fig. 9B for schematic section) (NM 780 922). Gammehaugen (NM 841 912), Roko (NM 840 916).

Definition. The type locality for the Gagnum Member was given by Strand & Størmer (1955) as being near Nerby. As Owen (1978 p.13) noted, the exposures here are poor but the boundary with the underlying Solvang Formation can be defined to within 0.5m. The, member is fully exposed only in the road and coast sections at Tønnerud and the latter is here designated as a hypostratotype.

At Tønnerud the Gagnum Member is some 7m thick but mapping of the unit has shown that this increases to 17m at Gammehaugen (where all but the lowest part crops out) and 40m west of Lunner. It becomes more calcareous northwards where, although the base is a sharp boundary, the upper part marks a transition to the Gamme Formation and the member is essentially a marl with limestone nodules.

Fauna and Age. West of Lunner, where the Gamme Formation is at its thinnest, the thick development of the Gagnum Member is indistinguishable lithologically and faunally from the Lunner Formation around Lunner. North of the Kjevlingen Fault (Owen 1978 p. 13), however, the shelly fauna becomes much more diverse in the more calcareous development. Some 22 trilobite species are known from the member (Owen 1981 pp.6-7, Fig. 2, Table 1) and whilst the trinucleid Tretaspis hadelandica hadelandica occurs throughout most of the unit, the lowest part in the north contains T. ceriodes (Angelin) angelini Størmer (Owen 1978,1981). Thus the base of the member may be diachronous with the oldest, T. ceriodes angelini bearing beds in the north being latest Caradoc in age whilst the rest of the unit, including the base further south is early Ashgill. Other elements of the diverse fauna include brachiopods (Bassett in prep.; Harper in prep.), echinoderms (Bockelie 1984), corals, cephalopods and bryozoans.