GAMME FORMATION
(Gammeformasjonen).
(New name).
(Previously termed: Gagnum Limestone, Sphaeronidkalken (pars)).
Main Lithology. Nodular Limestone (Gamme Limestone, Gammekalken).
Basal Stratotype. Hadeland: Gammehaugen (NM 840 916).
Hypostratotype. Hadeland: Gagnum (NM 858 893 856894).
Definition. The top of the Gamme Formation is not known at outcrop but the change from shales to nodular limestone at the base is exposed on Gammehaugen and the typical development seen in the road section at Gamme. The lowest 25m is exposed at Tønnerud (e.g. see Bockelie 1984, Fig. 9B). The formation is composed of nodular limestones up to 20cm thick with little or no intervening shale. It has a maximum thickness of 85m near Grinda, thinning northwards to about 50m at Gammehaugen and tapering out southwards where it extends to within 800m of Lunner (Owen 1978, plate 1).
Discussion. This unit was formerly termed the Gagnum Limestone Størmer 1945) and this name was retained by Owen (1978) who recognised the undesirability of having the same geographical name as in another unit (the Gagnum Shale of Størmer 1945) but felt it important to retain an historically important term. However, following a recommendation by the Norwegian Stratigraphical Committee we replace the name Gagnum by Gamme herein.
Fauna and Age. Much of the Gamme Formation is unfossiliferous and where a fauna is present, it is dominated by the trilobites Tretaspis hadelandica hadelandica and Flexicalymene jemtlandica? Some 14 other trilobite species are known (Owen 1981 p.7, Table 1) but these are rare. Brachiopods, echinoderms (Bockelie 1984) and bryozoans are a little more common. The fauna indicates no more than an Ashgill age but the age constraints on the confining units indicate a maximum range from mid? Pusgillian to early/mid Rawtheyan.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.