Hagastrand Member
(Hagastrandleddet)
(Erdtmann 1965)
(Previously termed: 3b?, 3b-α (pars) 3b A (pars)).
Main Lithology. Black and grey striped shale.
Neostratotype. Oslo-Asker: Hagastrand (Fig. 4) (NM 842297).
Thickness variation. Oslo-Asker: Hagastrand (1 0.6m) (Fig. 4), Tøyen (7.32m), Engervik (1 0.68m) (Fig. 6). Ringerike: Haug Skole (2.2m). Hadeland: Tuv (>3m).
Definition. The base of the Hagastrand Member marks the change from the limestones of the Bjørkåsholmen Formation to a sequence dominated by shales. At the neostratotype locality (Fig. 4) the lowest 2.3m comprises grey and rusty-weathering shales with limestone beds up to 15cm thick. There are thin strings of pyrite nodules in the lowest 50cm and pseudomorphs of calcite after ?barytes (Bjørlykke 1974, p. 14) occur in the succeeding 1.55m. Barium feldspar is also present in the pseudomorphs (Bjørlykke & Griffin 1973). The overlying 4.6m of the member comprises grey and black striped shales followed by 1.5m of grey shales with thin, rusty weathering limestone beds and nodules. The uppermost part of the member comprises 2.2m of striped shale with a horizon of 20cm thick isolated limestone nodules at the top. An identical succession is well exposed at Engervik, 500m to the north (Fig. 6).
Discussion. The Hagastrand Member is present in Eiker-Sandsvær, Oslo-Asker, Ringerike, Hadeland and the Mjøsa districts but is considerably thinner outside the type district.
In parts of Oslo and in Eiker-Sandsvær up to 1.15m of limestone with subordinate shale (the Vestfossen Member of Fjelldal 1966) is developed within or above the Hagastrand Member. Thus at Vekkerø (Fig. 5) some 0.96-1.3m of grey and rusty weathering shales are developed between the Bjørkåsholmen Formation and these limestones which in turn are overlain by black shale of the Galgeberg Member. On nearby Killingen however, approximately a metre of grey shales with a few limestone nodules is developed between the limestones and shales and the Galgeberg Member.
Erdtmann (1965, p.525) noted the presence of the "Vestfossen Member" at Vekkerø which he interpreted provisionally as a 'facies island' not connected with the development in Eiker-Sandsvær It seems reasonable, however, to postulate a zone of carbonate accumulation encompassing the two areas, perhaps situated on a topographic high above the periodically abiotic sites of mud accumulation.
A grey shale with limestone lenses is locally developed in the lower part of the Tøyen Formation in Ringsaker (Strand 1929; Skjeseth 1952, 1963; Erdtmann 1965) and was termed the Steinsodden Shale and Limestone by Skjeseth (1963, p.70). It may prove to be an equivalent of part of the Hagastrand Member or may, as Skjeseth (1963) suggested, best be included with the underlying formation. Detailed examination of the Ringsaker exposures is required before this problem can be resolved.
Fauna and Age. The shales of the Hagastrand Member have yielded a few graptolites, including Kiaerograptus, Didymograptus protobalticus and Tetragraptus phyllograptoides (Erdtmann 1965, 1982). Erdtmann (1965, Fig. 3) also recorded the trilobite Megistaspis planilimbata from a limestone horizon near the top of the unit suggesting a correlation with the upper Hunneberg Substage of Sweden. The difficulties of correlating the graptolite faunas with the British Tremadoc and Arenig series were discussed recently by Bruton & Erdtmann (1980) and Erdtmann (1982) who noted that the Hagastrand Member equates to levels within the overlap of the various definitions of the two series and is equivalent to the Hunneberg Substage in Sweden.
The trilobite fauna from the limestones and shales at the base of the formation in Eiker-Sandsvær includes Megistaspis (Ekeraspis) armata, Niobella sp. Varvia sp. and Symphysurus sp. (Klemm 1982, p. 142). The first of these is the eponymous species of the Megistaspis (Ekeraspis) armata trilobite Zone in Sweden which is equated with the lower Hunneberg Substage of the Latorp Stage, the earliest dichograptid faunas and the lower Paroistodus proetus deltifer conodont Zone (see Tjernvik & Johansson 1980, Jaanusson 1982, Löfgren 1985, Bergström 1986). Erdtmann (1965, pp.524-5, Fig. 9) argued that there may be an hiatus at the base of the Tøyen Formation in Eiker-Sandsvær and that the graptolites in the overlying shales indicate a correlation with levels within the lower part of the Galgeberg Member at Tøyen Oslo.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.