Ordovician Unit, Oslo Region

SOLVANG FORMATION

(Solvangformasjonen)

(Owen 1978)

(Previously termed 4b-δ-1-2, Øvre Chasmopskalk, Upper Chasmops Limestone, Sphaeronidkalken (pars), Dasyporella - kalken, Sphaeronid Limestone (sensu Størmer 1945), Lower Trinucleus Limestone, Høgberg Member).

Main Lithologies. Nodular limestone and bedded limestones with shale (Solvang Limestone, Solvangkalken).

Basal Stratotype. Hadeland: 0.45km southeast of Nerby (Owen 1978, p.8, pl.1) (NM 859 887).

Hypostratotypes. Hadeland (top): Lunner Railway Section (Owen 1978, Fig. 7) (NM 878 862), (whole unit): Tønnerud shore section (NM 780 922). Ringerike: Norderhov (Owen 1979, Fig. 4) (NM 709 667), Frognøya (NM 652 581). Oslo-Asker: Ø. Raudskjer (Fig. 16) (NM 865 347). Fornebu (Toni 1975a, Fig. 1 above 44.95 in section) (NM 915 416). Rodeløkken Bygdøy (Fig. 14) (NM 947 430). Nakkholmen (Harper et al. 1985, Fig. 2 for base (NM 949 404), Williams & Bruton 1983, Fig. 4 for top (NM 948 404)).

Thickness Variation. Hadeland: Tønnerud (17m), Lunner (ca. 70m). Ringerike: Norderhov (ca. 12m). Oslo-Asker: Rodeløkken (12 m), Nakkholmen (14.5m), Fornebu (15.5m), Raudskjer (ca. 20m), Persteilene (23.3m).

Definition. The Solvang Formation is a limestone unit containing varying amounts of interbedded calcareous shales but contrasting markedly with the dominantly shale units both above and below. The formation was defined originally in Hadeland (Owen 1978 pp.8-10) where it comprises a nodular development with little shale (the Nerby Member) and interbedded limestones and shales (the Lieker Member). Owen subsequently applied the formation name to the equivalent limestone unit in Ringerike (1979 pp.248-253) although only the bedded limestones and shales of the uppermost part on Frognøya were given named member status as the Høgberg Member (see below). The name Solvang Formation is formally used herein for the unit in Oslo-Asker which has historically been termed (inter alia) the 'Upper Chasmops Limestone' although Owen (1981 p.5) has already informally introduced the new formation name to the succession in this district; a practice continued by Williams & Bruton 1983; Harper et al. 1985. Preliminary analysis indicates that the Solvang Formation in Oslo-Asker should eventually be divided into several members.

The base of the Solvang Formation in Oslo is taken at the conspicuous pyrite band which is overlain by nodular and bedded limestones with subordinate shale horizons Størmer 1953 p.65, Fig. 5; Toni 1975a Fig. 1; Bruton & Owen 1979 p.216). This basal boundary is unequivocal on Nakkholmen, Persteilene and Bygdøy but further west, e.g. at Fornebu and on Raudskjer, several pyrite horizons are present and the base of the Solvang Formation is drawn at the top of the last thick shale of the underlying Nakkholmen Formation. There is a pyrite horizon within 1 m above this at most localities.

On Nakkholmen, and at Bygdøy and Fornebu, the formation is composed of bedded and irregularly nodular limestone up to 20cm thick with intervening calcareous shales up to 60cm thick. On Raudskjer (Fig. 16), such a development is only seen between 1-7m below the top of the formation; the underlying portion containing some rubbly limestone beds but mostly composed of small, closely spaced, limestone nodules. A similar development is seen on Persteilene Størmer 1953 Fig. 5). The uppermost 1 m on Raudskjer comprises planar limestone beds up to 15cm thick with very thin, light weathering, shale partings. This is reminiscent of the uppermost beds (formerly termed the Høgberg Member) on Frognøya Ringerike (Owen 1979) and the underlying 6m on Raudskjer broadly similar to the Lieker Member in Hadeland (Owen 1978).

Distinctive bioclastic limestones, rich in trilobites, notably illaenids (Owen & Bruton 1980 Fig. 3), can be traced over the whole Oslo-Asker district. In the east, on Nakkholmen, only one such bed is present, 1m below the top of the formation. This corresponds to a similar bed 5m below the top on Ostøya, 5.5m on Fornebu, 6m on Persteilene and 8m on Raudskjer. Thus this bed occurs at successively greater distances below the top of the formation as it is traced eastwards. Similarly, a second bed is present at the top of the formation at Fornebu and on Persteilene, 1 m below the top on Ostøya, and 2m on Raudskjer.

The upper 1 m of the Solvang Formation on Nakkholmen above the 'illaenid bed' has been included in the overlying Venstøp Formation by some authors (e.g. Owen 1979,1981, see also Størmer 1953 pp.65, 68). However, Williams & Bruton (1983 Fig. 4) have reassigned these dark bioturbated shales and limestones to the Solvang Formation. This was done largely on palaeontological grounds but it does mean that the overlying phosphorite conglomerate, which represents a demonstrable hiatus, is taken as the base of the Venstøp Formation. This phosphorite is equated with other such horizons which rest directly on the unequivocal top of the Solvang Formation elsewhere in Oslo-Asker.

The uppermost 2.88m of the Solvang Formation on Frognøya Ringerike comprises planar limestones and interbedded shales. These overlie tightly nodular (stylonodular) limestone and were termed the Høgberg Member by Owen (1979). The geographical component of this name was a local term, not on any published map, and could be confused with other names applied in the nappes to the north east of the Oslo Region (e.g. Nystuen 1980, Rasmussen & Stouge 1989). The term is therefore abandoned herein.

Fauna and Age. The Solvang Formation is one of the most fossiliferous units in the Oslo Region. The trilobites have been described by Nikolaisen (1983), Owen & Bruton (1980) and Owen (1 980a, 1981), strophomenid brachiopods by Spjeldnæs (1957), ostracods by Qvale (1980), echinoderms by Bockelie (1981, 1984) and conodonts by Hamar (1966); see also Bergström (1971). Other elements of the shelly fauna yet to be fully explored include the remainder of the brachiopod groups (Harper 1986 and in prep.), cephalopods (Sweet 1958), gastropods (Yochelson 1963), bivalves (H. & T. Soot-Ryen 1960), and bryozoans. Bruton & Owen (1979) interpreted the trilobite distribution in the Solvang Formation in terms of the progressive immigration of species with an abrupt faunal shift being marked by the base of the overlying Venstøp Formation. They argued that the Solvang Formation trilobites in Oslo-Asker indicate a westward younging of the top of the unit and an overall correlation with the Actonian and Onnian stages in Britain. There are several species in common between the two countries, the most important being Tretaspis ceriodes (Angelin) (see also Owen 1980a, 1987a). This occurs in the uppermost part of the formation in Bærum and Asker and also in the dark limestone in the shales between the illaenid bed and phosphorite on Nakkholmen (see above). The shale below this dark limestone (i.e. below the T. ceriodes occurrence) has recently yielded a graptolite fauna which belongs to the middle? Dicranograptus clingani Zone (Williams & Bruton 1983). The uppermost part of the Solvang Formation on Frognøya Ringerike (formerly the Høgberg Member) contains trilobites (Owen & Bruton 1980, Owen 1980a) indicating an earliest Ashgill (Pusgillian) age for this part of the unit (see also Owen 1979 pp. 250-252, Bruton and Owen 1979 pp. 219-221). This and lower parts of the formation also contain conodonts including Amorphognathus complicatus Rhodes (see Hamar 1966) which Bergström (1971 p.105) interpreted as indicating the upper part of the A. superbus Zone.