SØRBAKKEN FORMATION
(Sørbakkformasjonen)
(Owen 1979)
(Previously termed: 4c-β, 4d, Trinucleuskalk, Trinucleus Limestone, lsotelus Limestone).
Main Lithology. Limestone Sørbakken Limestone, Sørbakkalken).
Basal Stratotype. Ringerike: N.W. Frognøya (Williams & Bruton 1983, Fig. 7) (NM 652 582).
Hypostratotype. Ringerike: W. Frognøya (top) (NM 653577).
Definition. Although there is no complete section through the formation, Owen (1979 p.235) estimated that it is of the order of 100m thick. The lowest 34m crop out on northwest Frognøya and 37m at Norderhov. The base marks a sharp change to limestone with subordinate shale although the uppermost part of the underlying Venstøp Formation can be regarded as transitional to this. The Sørbakken Formation comprises grey, nodular and bedded limestones up to 30cm thick alternating with thinner, calcareous shales. The lithology was outlined by Owen (1979 pp. 253-254) who noted that there is no palaeontological or lithological evidence for the disconformity within the unit suggested by Strand (1933 p. 102).
Fauna and Age. The Sørbakken Formation contains a diverse shelly fauna including trilobites (Owen 1979, 1980a, 1981; Bruton & Owen 1988), brachiopods (Owen 1979 p.255, Harper in prep.), cephalopods (Strand 1933), bivalves, gastropods, echinoderms, ostracods and bryozoans. Many of the trilobites range throughout the formation and six species occur in the underlying unit. However, Calymene (s.l.) cf. marginata Shirley occurs at 35-37m above the base of the formation suggesting a correlation with the Lower Drummuck Group (Cautleyan) of Girvan, SW Scotland and occurrence of Isotelus frognoensis Owen, Toxochasmops aff. eichwaldi (Schmidt) and Stygina latifrons (Portlock), extensa (Reed) in the upper part of the unit indicates a correlation with Rawtheyan units in Oslo-Asker and Hadeland.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.