Håkavik Member
(Håkavikleddet)
(New name)
(Previously termed: 4a-α-4, Ogygia (= Ogygiocaris) zone (pars), Trinucleus bronnii zone, Bronnii Beds).
Main Lithologies. Grey shales, limestones and calcarenites.
Basal Stratotype. Oslo-Asker: North of Engervik (Fig. 12) (NM 845 306).
Hypostratotype. Oslo-Asker: Huk Bygdøy (NM 938 410).
Thickness Variation. Oslo-Asker: North of Engervik (14m), Killingen (ca. 12m).
Definition. As described by Størmer (1953 pp.58-61) the unit consists of alternating calcarenites up to 1 Ocm thick, bedded and nodular limestones also up to 1 Ocm thick and grey calcareous shales up to 30cm thick. Many of the calcarenites show cross and convolute lamination. The base is defined at the first calcarenite bed and this level also marks the disappearance of Chondrites which reappears at the base of the succeeding unit.
Discussion. The strata of this unit formed the basis for Størmer's Trinucleus bronnii Zone (1953 pp.58-61) although as Owen has pointed out (1987 p.84) the characteristic trinucleid is Botriodes impostor Owen, not B. bronnii (Boeck). Størmer's zone was essentially a biostratigraphical concept but he chose Huk on Bygdøy as the type section for the sedimentary development. This section is less complete than that near Engervik and the latter is here designated as the stratotype for the Håkavik Member. The unit is 14m thick here, including a 65cm gap in exposure 4m from the top. The calcarenites of the Håkavik Member were first noted by Brøgger (1887 p. 17) and their sedimentary structures discussed by Størmer (1953), Seilacher & Meischner (1964) and Bjørlykke (1965, 1974). In contrast to Seilacher & Meischner's interpretation, Bjørlykke concluded that the laminated calcarenites are not turbidites but represent fairly shallow water deposits laid down by storm-induced currents. He argued that the size of the quartz grains (less than 30-40mm) indicates reworking within the basin rather than an influx of extrabasinal material.
Fauna and Age. In addition to the trinucleid Botrioides bronnii and the very abundant B. impostor (see Owen 1987), other trilobites from the Håkavik Member include asaphids, raphiophorids and cybelines. The rest of the shelly fauna is sparse but includes brachiopods Spjeldnæs 1957). Berry (1964 pp.78-79) included the graptolites of the member entirely within the Glyptograptus teretiusculus Zone.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.