Definition
This formation is best demonstrated on the coast and islands of Steinsfjorden in the Ringerike district. A single type section cannot be designated, but the type area comprises numerous overlapping coastal exposures in the northwestern part of Steinsfjorden, together with the adjacent islands of Herøya, Småøyene and Braksøya The basal stratotype is defined at Loretangen (NM728637) and representative sections through the formation are seen northwards along the coast to Brattstad (NM3668) and on Herøya (NM7363). The gradational contact to the overlying clastic red bed deposits of the Ringerike Group is well exposed at Ranberget (NM725604). The Steinsfjorden Formation is approximately 260 m thick in the type area (Fig. 18) and contains irregularly varying proportions of greenish grey shales and marls, red dolomitic shales, dolomites and limestones. Variation in these components forms the basis of a subdivision into three members proposed by Olaussen (1978). The formation corresponds to Stage 9 of Kiær (1908) and we here assign outcrops preserved in Ringerike, Asker, Eiker, Modum, Holmestrand and Skien to this unit.
The base of the Steinsfjorden Formation at Loretangen is defined at the first occurrence of thinly interbedded limestones and shales which overlie the thickly bedded limestones of the Braksøya Formation. Olaussen (19 7 8) assigned the basal 200 m of the Steinsfjorden Formation to the Sjørvoll Member (9a‑d and lower 9e of Kiær 1908). This member consists of variably interbedded dolomitic limestones and limestones with red and greenish grey shales and marls. Mudflake conglomerates, desiccation cracks and algal laminated dolomitic limestone are common; the fauna shows a low diversity dominated by euryhaline elements often showing high individual abundance. A noteworthy eurypterid fauna Størmer 1934b, 1938) occurs together with algae (Høeg & Kiær 1926) in beds assigned to 9d by Kiær (1908).
The overlying Brattstad Member (upper 9e and 9f of Kiær 1908) is 30 m thick and consists of medium to thickly bedded limestones in its lower and upper parts, with interbedded marls and dolomitic limestones in the middle of the member (Fig. 19). The limestones contain a relatively rich and varied fauna; 1 to 3 m thick massive favositid and oncolitic biostromes (the 'Favosites Limestone' of Kiær 1908) are characteristic of the uppermost beds of the member. The basal stratotype of the member is defined in a coastal exposure at Brattstad (NM745670) at the first occurrence of massive lumpy bedded limestones which lie above marls with desiccation cracks assigned to the Sjørvoll Member.
The overlying greenish grey marls with thin limestone beds and nodular horizons assigned to the Ranberget Member (30 m, 9g of Kiær 1908) display mudflake conglomerates, desiccation cracks and ripple marks. The fauna is restricted to bryozoans and ostracodes, with significant horizons containing agnathans and eurypterids. The base of the member is defined in the E68 road section between Vik and Sundvollen at the first appearance of marls over the massive lumpy bedded limestones of the Brattstad Member. A 4 cm‑thick bentonite occurs 2 m above the base of the member (Jørgensen 1964). The upper boundary is gradational, with interbedded limestones and greenish grey shales passing up into red mudstones and sandstones of the Ringerike Group. The junction is defined arbitrarily at the overall change in colour from greenish grey to red sediments; this transition is accompanied by the disappearance of the few remaining in situ marine faunal elements.
Other occurrences
Units assigned to the Steinsfjorden Formation in other districts show a thinner development than that of the type area (Asker 2 00 m, Holmestrand 115 m, Skien 140 m). The lower parts and the upper transition to the Ringerike Group are everywhere broadly similar to the type area. However, the middle parts of the unit in these other areas are dominated by limestones (which are quarried in the Skien and Holmestrand districts) with more normal stenohaline faunas. Both patch reefs and biostromes are found in these middle parts of the formation in Holmestrand, while biostromes also occur in Asker, Modum and Skien. The biostromal Favosites Limestone of Kiær (1908) can be found in all these districts in the upper parts of the formation.
Depositional environments
In its type area the formation contains mixed carbonate and mud lithofacies associations mostly deposited in supratidal, intertidal and restricted subtidal environments. Rhythmic lithological variations seen within the unit represent small‑scale transgressive and regressive events, the transgressive periods marked by the deposition of thin biostromal banks. The presence of early diagenetic dolomite and celestite suggests and conditions in the type area (Olaussen 1978 & 1981 b). The lower and upper parts of the formation in other districts were also deposited in peritidal environments, but the unit's middle parts suggest more open subtidal conditions which permitted the development of biostromes and patch reefs with a highly diverse flora and fauna on marginal areas of the inner shelf. The favositid bioherms and biostromes seen in the upper part of the formation in all districts mark a larger scale transgressive episode; for a short time this resulted in normal marine conditions in all districts, prior to a regional regression which culminated in the deposition of the overlying red beds of the Ringerike Group.
Age
Sequences which we assign to the Steinsfjorden Formation were originally correlated with the Ludlow. However, studies of agnathans (Heintz 1969), ostracodes (Martinsson 1969) and thelodonts (Gross 1967, Turner & Turner 1974) suggest that the transition from marine to red bed deposition at the top of the Steinsfjorden Formation approximates to the Wenlock/Ludlow boundary, thus placing the entire formation within the Wenlock. The age of the base of the Steinsfjorden Formation was first clarified by Bassett & Rickards (1971) using the occurrence of the brachiopod Eocoelia angelini. We have already noted the mid-Sheinwoodian age of this brachiopod: its occurrence in the lower parts of the Steinsfjorden Formation in Ringerike and in varying parts of underlying units in other districts suggests that the base of the formation youngs progressively from Ringerike through Holmestrand and Skien to Asker (Fig. 5). We therefore consider the lower parts of the Steinsfjorden Formation in its type area to be time equivalents of the entire Malmøya Formation of Asker and of the upper parts of the Malmøya and Braksøya Formations in Holmestrand and Skien respectively.
The ostracode and thelodont faunas described from the upper beds (9g) of the Steinsfjorden Formation in Ringerike (Gross 1967, Martinsson 1969, Turner & Turner 1974) suggest a correlation with the Halla Beds of Gotland (Gross 1968). Recent work on Gotland (Bassett et al. in press.) suggests that the Halla Formation lies within the nassa Biozone. The youngest marine faunas of the Steinsfjorden Formation may therefore have a slightly earlier age than that suggested near to the Wenlock/Ludlow boundary by among others Turner & Turner (1974:190). We have as yet little faunal evidence to date the upper parts of the Steinsfjorden Formation in other districts. The youngest faunas found at Holmestrand (in 9d) have a Sheinwoodian character, and faunas elsewhere from higher in the formation have a general Wenlock aspect. We have therefore no biostratigraphic data as yet to test the sedimentological models of Spjeldnæs (1966) and Bjørlykke (1974) which suggest a regionally diachronous contact with the overlying Ringerike Group, younging from Ringerike towards Asker and the south. Studies of thelodonts found in the Asker district (S. Turner pers. comm. 1982) may resolve this question.