Ordovician Unit, Oslo Region

Sjøstrand Member

(Sjøstrandleddet)

(New name)

(Previously termed: 4a-α-1-2 Upper Didymograptus Shale, Øvre Didymograptusskifer)

Main Lithology. Black shale.

Basal Stratotype. Oslo-Asker: Bjørkåsholmen (Fig. 11) (NM 836 286).

Hypostratotype. Oslo-Asker: Svartodden Vekkerø (NM 924 430).

Definition. The Sjøstrand Member comprises black shales with scattered limestone lenses. Its base is taken above the last continuous limestone bed of the Helskjer Member.

Størmer (1953 pp.55-57) gave lithological and faunal details of various sections through the Sjøstrand Member in Oslo-Asker and estimated its thickness to be approximately 32m on Huk, Bygdøy and 49m at Slemmestad. It must be stressed, however, that such thicknesses may be misleading as we have yet to encounter a section which is not tectonically disturbed.

Fauna and Age. Although the Sjøstrand Member contains shelly fossils in the limestone lenses, notably species of the trilobites Botrioides (see Owen 1987) and Ogygiocaris (see Henningsmeon 1960), the fauna of the shales is dominated by graptolites (see Størmer 1953). Berry (1964) described the diverse graptolite fauna of the member on the basis of his own and existing museum collections. He ascribed the fauna of the whole unit to the Didymograptus murchisoni Zone (1964 p.77) and thus did not confirm Størmer's reference to the lower part of the member ('4a a ') as the 'Didymograptus bifidus Zone' (1953 p. 119, Fig. 16). In a subsequent paper, however, Berry argued that the lower part of the D. murchisoni Zone in the Oslo Region may be equivalent to the upper D. 'bifidus' Zone in Britain (1968 p.226) (see Fortey & Owens 1987 for the renaming of this zone as the D. artus Zone).