ARNESTAD FORMATION
(Arnestadformasjonen)
(New name)
(Previously termed 4b-α, Skifer med Chasmops conicophthalma, Undre Chasmops-Skifer, Lower Chasmops Shale).
Main Lithology. Shale (Arnestad Shale, Arnestadskiferen).
Basal Stratotype. Oslo-Asker: Vollen (Fig. 13) NM839310).
Hypostratotypes. Oslo-Asker: Rabben (NM 840 308), Rodeløkken Bygdøy (Fig. 14) (NM 947 432). Ringerike: Bratterud (NM 650 610).
Definition. The formation name was published as a replacement for 'Lower Chasmops Shale' by Harper (1986) and Owen (1987) but without formal definition. Kvingan (1986) also used the name in an unpublished thesis on the formation. The base of the Arnestad Shale Formation is defined at the development of thick dark shales with subordinate limestone horizons. This coincides with the occurrence of abundant specimens of Echinosphaerites aurantium Hecker (see Bockelie 1981) in many localities but at others, e.g. Huk and Killingen, this cystoid is present in the upper beds of the underlying unit. Shale horizons within the formation are commonly only 30-40cm thick with a maximum of about 70cm. Nodular and bedded limestone horizons are almost invariably less than 10cm thick. In Oslo, the shales are dark grey weathering to light grey whereas in the west in Asker and Ringerike they are green-grey in colour and this becomes more pronounced on weathering. Bentonites are developed in the formation at various localities in Oslo-Asker (Hagemann & Spjeldnæs 1955). Measured sections through the upper part of the Arnestad Formation and succeeding units (= the 'Chasmops Beds') on Persteilene and Fornebu were given by Størmer (1953 Fig. 5) and Toni (1975a Fig. 1) respectively. The thickness of the formation is difficult to estimate owing to tectonism and poor exposure and was estimated to be up to about 45m by Brøgger although most recently Kvingan (1986) gave a figure of about 22m.
Fauna and Age. The Arnestad Formation contains a rich shelly fauna with both diversity and abundance generally increasing westwards from Oslo into Asker.
The ostracods were described by Qvale (1980) who noted the presence of a large number of endemic taxa and suggested a correlation of the upper part of the formation with the lowest part of the Skagen Limestone (mid Caradoc) of Sweden. Jaanusson (1964 p.53) also argued that ostracods from the lower part of the Arnestad Formation indicate a correlation with the Upper Dalby Formation (lower Caradoc) in Sweden (see Bruton 1976 p.716). Over 30 trilobite species are known from the Arnestad Formation and include remopleuridids (Nikolaisen 1983), asaphids (Henningsmoen 1960), illaenids, proetids (Owens 1970), trinucleids (Owen 1987), cheirurids (Nikolaisen 1961), encrinurids (Nikolaisen 1961), calymenids (Siveter 1977), pterygometopids (McNamara 1980) and lichids. In describing a new species of Phillipsinella, P. fomebuensis, from the uppermost 5m of the formation, Bruton (1976 p.716) noted that the associated trilobites are known also from the Skagen Limestone in Sweden. This supports the correlation indicated by Qvale (1980). The supposed proetid Analocaspis ursina described by Owens (1970) from the Arnestad Formation was subsequently reassigned by him to the aulacopleurid genus Rorringtonia Whittard (Owens 1981). Rorringtonia is restricted to Llandeilo and lower Caradoc horizons in Britain and an early Caradoc age for the Norwegian species would conform to this.
Over 56 brachiopod species occur in the Arnestad Formation (pers. comm. D.A.T.Harper 1983 and Harper 1986) of which only strophomenids have so far been described Spjeldnæs 1957). One of the most characteristic elements of the fauna is Kullervo cf. lacunata Öpik which, again, indicates an early Caradoc age for the formation (pers. comm. D.A.T.Harper 1983). Many of the immigrant brachiopod taxa are of Scoto-Appalachian affinity (Harper 1986). Cystoids from the formation were described by Bockelie (1984).
Hamar (1966) extracted a conodont fauna from near the top of the formation at Rodeløkken Bygdøy. He noted (1966 p.42) that it marked the disappearance of species of Eoplacognathus and Polyplacogna thus characteristic of older units and the appearance of Ambalodus and Amorphognathus. He included the fauna with that of the succeeding three formations within his 'middle zone' and suggested (1966 p.43) that the Arnestad Formation may be broadly equivalent to the Dalby Formation (= D. multidens graptolite Zone, Lower Caradoc - Jaanusson and Karis 1982) in Jämtland, Sweden.
The tremendous increase in diversity of shelly fossils seen in the Arnestad Formation compared to earlier units may reflect the 'Skagenian' immigration documented by Jaanusson (1976) elsewhere in Baltoscandia but the precise level of this change has yet to be determined.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.