Ordovician Unit, Oslo Region

ALUM SHALE FORMATION

(Alunskiferformasjonen)

Discussion. The Alum Shale Formation can be recognised over a large portion of Baltoscandia both in the platform sequences and the Caledonian nappes. It extends from the Middle Cambrian to lowest Ordovician and ranges in thickness from about twenty to almost a hundred metres and has been the subject of detailed reviews by Martinsson (1974) and Andersson et al. (1985). The formation has yet to be formally defined and this is beyond the scope of the present work. It is composed largely of black, organic rich shales, which give a black streak, together with discontinuous beds and concretions, some several cubic metres in size, of dark bituminous limestone ('stinkstone'). The shales have a very low CaCO3, content and are enriched in a variety of trace elements including uranium, vanadium, molybdenum and nickel (Bjørlykke 1974, Andersson et al. 1985, Berry et al. 1986).

Although at the time of writing an international stratotype for the base of the Ordovician System has yet to be selected, it will almost certainly lie at a point equivalent to a level within the uppermost part of the Alum Shale. Bruton et al. (1982, 1988), have suggested that this level may be at the first appearance of the olenid trilobite Boeckaspis hirsuta in the section at Nærsnes, Røyken south of Asker. The level approximates to the base of what has historically been termed the Dictyonema Shale (Strand & Henningsmoen 1960, Henningsmoen 1973) but this is a purely biostratigraphical unit and it lies in a uniform sedimentary sequence of Alum Shale. 'Dictyonema Shale' has been described from many parts of Scandinavia and the Baltic area (see Erdtmann 1982, Jaanusson 1982, Jaanusson & Mutvei 1982, Karis 1982, Henningsmoen 1973).

Near the top of the Alum Shale Formation in Oslo Asker is a 15-40cm thick black, planar limestone (the 'Platypeltoides incipiens Limestone'- see Henningsmoen 1973 p.431 for discussion). It is well exposed in a road cutting near the Rortunet Shopping Centre in Røyken (Fig. 2) where it is overlain by up to 6cm of black shale with a black streak, identical to that of the underlying shales and then a 12-20cm black planar limestone. At the top of this is a prominent pyrite horizon forming the base of the succeeding 6.5m of blocky dark grey shales which have a black streak in the lower half metre above which the streak becomes progressively paler up succession. In an unpublished thesis, Gjessing (1976) assigned the 'Platypeltoides Limestone' (his Vækerø Member) and the overlying shales with a black streak to the Alum Shale Formation. He termed the shales with a white streak the Breidablikk Shale Member of the Nærsnes, Formation within which he also included the Ceratopyge Limestone as a second member (our Bjørkåsholmen Formation, see below). The use of the name Vækerø was most unfortunate as in an earlier thesis Fjelldal (1966) had defined a Vækerø Formation which comprised the Ceratopyge Limestone and overlying Vestfossen Member (see below). Most of Gjessing's measured sections are incomplete, faulted or contain igneous intrusions. Moreover, his approach differed markedly from the traditional inclusion of all the beds between the base of the 'Platypeltoides Limestone' and the base of the 'Ceratopyge Limestone' as the 'Ceratopyge shales' (see Henningsmoen 1973, p.431). As the shales immediately above the 'Platypeltoides Limestone' are indistinguishable from those underlying it we include the whole sequence up to the base of the 'Ceratopyge Limestone' in the Alum Shale Formation. However, the 'Platypeltoides Limestone' marks an important faunal change (Henningsmoen op. cit) and is a useful informal unit. It is present throughout Oslo-Asker and also at Vikersund (Wandås 1982, p. 136). It may be absent from Eiker-Sandsvær as indicated by work on borehole cores by Gjessing (1976) and outcrop mapping by Gjessing (1976) and Cadow (1985) although Brøgger (1882 pp.28-29, text-fig. 1) suggested that a 30cm horizon of nodules at Vestfossen may represent this limestone. The 'Platypeltoides Limestone' contains the trilobites Platypeltoides incipiens, Ceratopyge forficula, Peltocare norvegica and species of Conophrys, Orometopus and Apatokephalus (see Størmer 1920, Henningsmoen 1973, Gjessing 1976). The overlying shales are poorly fossiliferous but have yielded C. forficula, and the graptolites Bryograptus ramosus, Kiaerograptus kiaeri and Triograptus (Spjeldnæs 1985). Gjessing (1976, pp. 112, 114) collected a large fauna from a limestone nodule in the shales at Killingen. This comprised C. forficula, Euloma ornatum Conophrys pusilla, Hypermecaspis inflecta, Parabolinella limitis, Asaphellus sp., agnostids, Broeggeria salteri, Conotreta sp. and Orbithele ceratopygarum. These faunas indicate a late Tremadoc age. Equivalent beds are largely absent from mainland Sweden (but see Karis 1982, p.55 for the allochton in Jämtland) but are well known from the island of Øland where the 17-2.7m thick 'Ceratopyge Shale' is poorly fossiliferous but contains a fauna similar to that in Norway (see Tjernvik in Magnusson 1956, Jaanusson & Mutvei 1982, pp.4-5).