FROGNERKILEN FORMATION
(Frognerkilformasjonen)
(New name)
(Previously termed: 4b-β Undre Chasmopskalk, Lower Chasmops Limestone, Rodeløkken Formation).
Main Lithologies. Bedded limestones and shales (Frognerkilen Limestone, Frognerkilkalken).
Basal Stratotype. Oslo-Asker: Rodeløkken Frognerkilen Bygdøy (Fig. 14) (NM 947 430).
Hypostratotypes. Oslo-Asker: Rabben (Fig. 15) (NM 840 308), Fornebu (Toni 1975a Fig. 1, 3.95-19.25m in section) (NM 915 416). Ringerike: Bratterud (NM 650 610), Norderhov (NM 710 666).
Thickness Variation. Oslo-Asker: Semsvann (> 18m), Fornebu, Persteilene (15.8m), Rodeløkken, Bygdøy (11.5m), Nakkholmen (ca. 10m), Alnabru (7m).
Definition. This unit was termed the Rodeløkken Formation by Harper (1986) and Owen (1987). The Norwegian Stratigraphical Commission consider the name inappropriate and hence it is changed herein. In the type area, the base of this formation marks an abrupt change from a shale dominated succession to rubbly limestones up to 10cm thick passing upwards into nodular limestones (up to 10cm) and shales (up to 35cm). The rubbly limestone is not-developed in Ringerike. The uppermost 2-3m of the formation comprises bedded limestones (5-10cm) and shales (5-30cm). A measured section through the formation on Persteilene was given by Størmer (1953 Fig. 5) including these beds within the Frognerkilen Formation but Toni (1975a Fig. 1) assigned similar horizons on Fornebu to the overlying Nakkholmen Formation. Re-examination of the latter profile indicates that the base of the Nakkholmen Formation should best be placed at the 19.25m level (see Toni 1975a Fig. 1) and thus Størmer's approach is adopted.
The Frognerkilen Formation thins eastwards in the Oslo-Asker district from over 18m at Semsvann to about 7m at Alnabru (Wale 1980 Fig. 22). Its thickness in Ringerike is difficult to assess but Størmer (1953 p.86) suggested that it may be of the order of 25m.
Fauna and Age. The Frognerkilen Formation contains a fairly sparse fauna, most elements of which occur also in the underlying unit (e.g. Spjeldnæs 1957, Henningsmoen 1960, Hamar 1966, Qvale 1980, McNamara 1980, Harper 1986) and thus indicate a mid-Caradoc age. Paul and Bockelie (1983) described a new species of cystoid, Sphaeronites (Peritaphros) pauciscleritatus from the middle part of the formation in the western (shallower) part of Oslo-Asker.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.