LUNNER FORMATION
(Lunnerformasjonen)
(Owen 1978)
(Previously termed: 4c, 4c-α , Trinucleusskifer, Tretaspis Shale, Sphaeronidskifer, Gagnum Shale).
Main Lithology. Shale (Lunner Shale, Lunnerskiferen).
Basal Stratotype. Hadeland: Lunner Railway Section (Owen 1978 Fig. 7) (NM 877 862).
Hypostratotype. Hadeland: Road south of Lunner (NM 869 859 - 874 859). (See also stratotypes for individual members.)
Definition. The Lunner Formation is dominantly a shale unit which is thick (?of the order of 185m - Owen 1978 p. 11) in the southeast of Hadeland around Lunner but which thins north westwards in which direction it is divided into two tongue-like members by the Gamme (formerly Gagnum) Formation. The lowest of these, the Gagnum Member, is present in the most northerly outcrop at Tønnerud where it is 7m thick. The upper member, the Grinda Member, wedges out completely.
Around Lunner the formation is composed largely of dark calcareous shales which become grey and friable. About 9m of nodular and bedded limestones are developed in the lowest part (the Lunner Kirke Member of Owen 1978; a term here considered unnecessary) and near the top but are fairly rare in most of the formation in the southeast. Thin siltstone horizons occur at various levels. Limestones are much more abundant in the Grinda and, especially, Gagnum members.
Fauna and Age. The Lunner Formation around Lunner contains a low diversity shelly fauna dominated by the trilobites Tretaspis hadelandica Størmer and Flexicalymene trinucleina (Tullberg)? (Owen 1981 p.7) and species of the brachiopods Onniella and Chonetoidea. Graptolites in the lower part of the formation here probably belong to the upper Pleurograptus linearis Zone and thus indicate a Pusgillian age. The Gagnum and Grinda shale members contain much more diverse shelly fauns (Owen 1981 Fig. 2). The Gagnum Shale contains T. ceriodes in its lowest part and T. hadelandica at higher levels indicating that the base of the Lunner Formation is slightly older (latest Caradoc) in the north than in the south (Owen 1978). The Grinda Member contains a fauna of Rawtheyan age which compares closely with that of its lateral equivalent in the south, the Kjørrven Formation.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.