VOLLEN FORMATION
(Vollformasjonen)
(New name)
(Previously termed: 4a-β Ampyxkalk, Ampyx Limestone).
Main Lithology. Limestone (Vollen Limestone, Vollkalken).
Basal Boundary Stratotype. Oslo-Asker: North of Engervik (Fig. 12) (NM 845 306).
Hypostratotypes. Oslo-Asker: Vollen (Fig. 13) (NM 839 310). Huk Bygdøy (NM 932 412), Ildjernet (NM 350 355). Ringerike: Kullerud (NM 724 694).
Definition. This formation name was introduced without definition by Owen (1981). The disappearance of calcarenite horizons and the reappearance of the trace fossil Chondrites mark the base of the Vollen Formation (Fig. 12) and whilst a complete section across this boundary is exposed north of Engervik, only the lowest 13m of the formation can be studied here. This comprises rhythmically alternating limestone horizons (beds and nodules) with interbedded calcareous shale. Bjørlykke (1974 p. 17) noted that there are two cycles with approximate average thicknesses of 10cm and 40cm. He suggested that they may represent cycles of 15,000 and 60,000 years respectively and alluded to unpublished work by O.A.Christophersen relating them to astronomically controlled climatic changes. There is no known complete section through the whole formation. Approximately 40m is exposed on Bygdøy where the base is faulted and about 44m crop out at Vollen but the base is not seen here.
Discussion. The Vollen Formation is also developed in Ringerike Størmer 1953 pp.83-84) although the distinct rhythmicity seen in Oslo-Asker is not as evident. The base of the formation locally comprises a 1020cm thick conglomeratic horizon Størmer 1953 pp.83-84, Hamar 1964 pp.251-252). This is now only seen at Kullerud in a tightly folded sequence and a polished section of the conglomerate from here was illustrated by Hamar (1964 Fig. 3) who also published photomicrographs of thin sections from Gomnes (1964 Fig, 2). Hamar noted that the conglomerate comprises two horizons of coarse material and two of cross-bedded sand grade material. Clasts in the coarse horizons include shales and quartzite as well as fossil fragments.
Fauna and Age. The Vollen Formation contains a sparse but diverse shelly fauna dominated by trilobites, notably: asaphids (Henningsmoen 1960), cheirurids (Nikolaisen 1961), the trinucleids Reedolithus carinatus (Angelin) throughout the formation and species of Botrioides (in the lower part of the unit - Owen 1987) and three species of chasmopines (McNamara 1980), Chasmops tumidus (Angelin), Scopelochasmops conicophthalmus (Sars & Boeck) and Bolbochasmops bucculentus (Schmidt). Amongst the brachiopods are several strophomenids described by Spjeldnæs (1957) including species of Alwynella, Grorudia and Christiania. Other elements of the shelly fauna include cephalopods (Sweet 1958), gastropods (Yochelson 1963), bivalves (H. and T.Soot-Ryen 1960, H.Soot-Ryen 1966) ostracods (Henningsmoen 1953) and cystoids (Bockelie 1981, 1984). The correlative value of the shelly fossils has yet to be adequately tested.
Hamar (1964, 1966) described conodont faunas from the Vollen Formation in Oslo-Asker and Ringerike and distinguished three 'sub-zones' within the formation. He suggested a late Llandeilo or early Caradoc age for the formation which he equated with the Nemagraptus gracilis Zone and which is now considered largely of Llandeilo age (e.g. Williams et al. 1972). In a re-evaluation of Hamar's faunas, Bergström (1971 p. 104) ascribed the fauna from the basal Vollen Formation in Ringerike to the lower Pygodus anserinus Zone (= upper G. teretiusculus Zone) and that of the middle Vollen Formation in Oslo-Asker to the upper P. anserinus Zone (= lower N. gracilis Zone).
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.