TØYEN FORMATION
(Tøyenformasjonen)
(Erdtmann 1965)
(Previously termed: 3b, Phyllograptusskifer, under Didymograptusskifer, Untere Graptolithschiefer, Unteren Didymograptusschiefer, Der Phyllograptusschiefer, Lower Didymograptus Shale, Phyllograptus Shale).
Main Lithology. Black and grey shale Tøyen Shale, Tøyenskiferen).
Neostratotype. Oslo-Asker: Hagastrand, (Fig. 4) (NM 842297).
Thickness variation. Oslo-Asker: Engervik (20.53m), Tøyen (1 9.75m). Eiker-Sandsvær Hals (?min 4.29m), absent in south of district. Modum (Vikersund): 7.5m Øvre Øren Farm (7.5m). Ringerike: Løkenåsen (-20m). Hadeland: Tuv (> 1 5m).
Definition. The original stratotype for the Tøyen Formation (Erdtmann 1965) was a temporary tunnel section at Tøyen Oslo. This is no longer exposed and as there are no other sufficiently complete sections within Oslo, a neostratotype (see Hedberg 1976, p.26) is here designated immediately to the west, at Hagastrand in Asker (Fig. 4). The lowest member (the Hagastrand Member) is complete along the foreshore here as is the base of the overlying Galgeberg Member. This uppermost member is fully exposed in a shore section at Engervik only 500m to the north (Fig. 6). The base of the Tøyen Formation marks an abrupt change from the limestones of the Bjørkåsholmen Formation to shales with subordinate limestone horizons. The top of the formation is similarly distinctive, contrasting with the thick limestones of the Huk Formation. Only in the Mjøsa districts do the overlying basal beds of the Huk Formation include a significant amount of shale.
Discussion. The Tøyen Formation is known from all but the southerly parts of the Oslo Region and extends into Västergötland, Jämtland and Skåne in Sweden (Erdtmann 1965, Jaanusson 1982, Jaanusson, Larsson & Karis 1982, Bergström 1982) although its stratigraphical range in Sweden is locally more restricted than in Norway. Within the Oslo Region the formation is dominated by shales although limestones, (the Vestfossen Member of Fjelldal 1966) are developed at or near the base in northern Eiker-Sandsvær and locally in Oslo-Asker. Two distinct shale members are recognised over most of the region with the black shales of the Galgeberg Member succeeding the rusty weathering striped shales of the Hagastrand Member. In the thin developments of the formation in Eiker-Sandsvær and Modum, however, only the Galgeberg Member is present. It is not yet known whether this indicates an hiatus at the base of the formation in these areas as suggested by Erdtmann (1965).
In his original description of the Tøyen Formation, Erdtmann (1965) defined a member at the very top of the formation, the Slemmestad Member. As is noted below, this was done on faunal grounds rather than at a distinct lithological change and these strata are here reassigned to the Galgeberg Member.
Age. Although the Galgeberg Member is the only richly graptolitic member of the formation, the Hagastrand Member has yielded a few graptolites and the locally developed limestones at or near the base of the formation contains a shelly fauna dominated by trilobites. Erdtmann (1982) and Bruton & Erdtmann (1980) have discussed the problem of correlating the Hagastrand Member with the British series at around the Tremadoc-Arenig boundary and concluded that it equates with the Hunneberg Substage in Sweden. The fauna of the Galgeberg Member in Oslo-Asker undoubtedly indicates a range from the Tetragraptus approximatus to the lower part of the Didymograptus hirundo zones (Spjeldnæs 1953, Erdtmann 1965) and thus is equivalent to virtually the entire British Arenig (see Fortey & Owens 1987, Fig. 5).
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.