Definition
The development of this formation is best demonstrated in widespread but partial exposures throughout the Ringerike district. Easily accessible sections are seen in roadside cuttings immediately east of Vik (NM715609), prompting our use of this formational name. The formation generally corresponds to stage 7c of Kiær (1908) in the Ringerike, Oslo, Holmestrand and Skien districts and to Kiær's 7b‑beta) and 7c in Asker. The Vik Formation is approximately 80 m thick in its type area and shows a tripartite development with varying proportions of both red and greenish grey shales, marls and limestones. This tripartite development was the basis of Kiær's (1908) subdivision into 7c‑alfa 7c‑beta and 7c‑gamma the members proposed by Aarhus (1978) generally correspond to Kiær's subdivisions. We here modify the provisional nomenclature of Aarhus and propose the establishment of the Storøysundet, Garntangen and Abborvika Members (in ascending order) in the Vik Formation in its type area. A single type section for the formation cannot be designated but a basal stratotype is defined in a coastal section northwest of Rytteråker Farm (NM695596). This section shows the gradation from thinly interbedded limestones and greenish grey shales of the underlying Rytteråker Formation into predominantly red shales. The base of the Vik Formation is defined at the sharp transition from limestone to shale dominance 4 m below the lowermost occurrence of red shales.
The base of the Storøysundet Member is, by definition, coincident with the base of the Vik Formation. The basal stratotype displays 12.5 m of the member, but a total thickness of approximately 20 m in the type area of the Vik Formation is suggested by other coastal exposures around Storøysundet (NM6958). The red shales of the Storøysundet, Member contain minor bioclastic limestone lenses, small calcareous nodules and occasional greenish grey shale interbeds. A diverse but sparse fauna includes crinoids, brachiopods, stromatoporoids and tabulate corals.
The middle 13 m of the Garntangen Member are exposed in the E68 road section near Garntangen (NM720609) on the coast of Steinsfjorden. The member is also well exposed on the west coast of Storøya (NM690576 to NM694582) where its basal stratotype is defined. Its base is taken at the marked increase in limestone interbeds 3 m above the uppermost red shales of the underlying unit. The Garntangen Member is approximately 25 m thick and consists of thinly bedded limestones and calcareous nodules with minor greenish grey marls. Fresh exposures in the road section near Garntangen display 19 bentonitic horizons which vary from a few mm to 12 cm thick (Fig. 12a, see also Hagemann 1966). This part of the Vik Formation has a more rich and diverse fauna than the adjacent members and both corals and stromatoporoids are abundant; brachiopods (especially Pentamerus oblongus, Pentameroides cf. subrectus and Costistricklandia lirata) are also common in certain beds.
The upper Abborvika Member consists of greenish grey to red shales with interbedded finely nodular limestones. A sparse fauna of crinoids, cephalopods and brachiopods is seen. The member is estimated to be 35 m thick in its stratotype on Purkøya (NM688571); the base is defined by the transition from the abundant limestones of the underlying unit into greyish green calcareous shales with sparse evenly bedded and calcareous nodular horizons. These beds pass up into red shales with irregular very finely nodular horizons about 18 m above the base of the member. The upper 3 m of the member consist of greenish grey shales; these are sharply overlain by siltstones of the Bruflat Formation and the junction is well exposed on Purkøya (Fig. 12b).
Other occurrences
Although exposures in the Holmestrand district show a similar tripartite development to that of the type area, the middle unit in Holmestrand (well exposed on Killingholmen, NL753995) consists of thickly bedded biosparites with corals, stromatoporoids and coquinas of Pentameroides sp. This unit was assigned to 7c‑beta or the 'Upper Pentamerus Limestone' by Kiær (1908). In the Skien district a 10 m thick grey shale sequence Kiær's 7c‑alfa passes up into nodular limestones and greenish grey marls which apparently are directly overlain by shales of the Skinnerbukta Formation (8a/b of Kiær 1908).
Exposures assigned to the Vik Formation on Malmøya in the Oslo district consist of irregularly varying proportions of greenish grey shales and marls with finely nodular limestones. A notable feature is the occurrence of exceptionally well preserved articulated crinoid stems up to several metres long. Occasional thin intraformational conglomerate horizons are also found on Malmøya these contain eroded and redeposited corals and calcareous nodules, reflecting the early diagenetic formation of these nodules. Occasional easily weathered interlaminae may represent bentonitic horizons. Exposures in the Asker district have still not been examined in detail, but we provisionally include both (7b‑beta and 7c of Kiær in the Vik Formation in this district. We further note that sections seen here appear transitional between the Ringerike and Malmøya developments.
The Vik Formation thins southwards from 80 m in the type area to 45 m in the Skien district. Red coloured shales are only developed in Ringerike and Asker; bentonites have not yet been found in Holmestrand and Skien.
Depositional environments
Sediments and faunas of the Vik Formation suggest somewhat deeper depositional environments than those of the underlying Rytteråker Formation, with greater (although still fine‑grained) clastic influx. Shales near the base and top of the formation in Ringerike, Holmestrand and Skien contain faunas characteristic of Benthic Assemblages 4 and 5 of Boucot (1975). The presence of abundant corals and stromatoporoids (and associated benthic algae) typical of Benthic Assemblage 3 suggests the development of shallow marl banks in the Garntangen Member of the type area. A comparable unit in Holmestrand comprises a bioclastic shoal rich in pentamerid and coral debris. Interpretations of the local red colour of the Vik Formation's shales will be discussed further below (p. 50).
Age
Our correlation of the unit is based both upon occurrences of the brachiopods Pentamerus, Pentameroides and Costistricklandia and on conodonts (R. J. Aldridge pers. comm. 1981). The brachiopods suggest that the Garntangen Member may be equivalent to the C4/C5 transition in the British sequence and conodonts indicate a slightly older (C4) age for the base of the Vik Formation. A markedly diachronous ‑top is suggested, ranging from mid‑C5 equivalent in Ringerike to late C6 in Oslo (Fig. 5).