Hovedøya Member
(Hovedøyleddet)
(New name)
(Previously termed: 4d-α , Undre Isoteluskalk, Lower Isotelus Limestone).
Main Lithologies. Limestone, siltstone and shale.
Basal and Hypostratotypes. As for formation.
Definition. The Hovedøya Member comprises alternating limestone, siltstone and shale beds. In contrast to the underlying unit, the combined thickness of limestone and siltstone beds exceeds that of the shales although some individual shale horizons are thicker (up to 30cm) than the thickest (ca. 15cm) limestone or siltstone beds. All the lithologies are dark grey when fresh but whereas the shales weather dark blue-grey, the limestones and siltstones become a light grey or even creamy colour. The basal boundary is well exposed in the stratotype on Skogerholmen (Fig. 21) and is defined at the first continuous limestone horizon. The Hovedøya Member here is 12.48m thick and this slight westward thinning of the member is consistent with the thickness estimates given by Brøgger (1887 pp.29-30).
Fauna and Age. Very few fossils are known from the Hovedøya Member of the Skogerholmen Formation and although Brøgger recorded eight trilobite genera from the unit, only five are preserved in existing museum collections (Owen 1981, table 1).
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.