Definition
The Sælabonn Formation is best demonstrated in the Ringerike district (Fig. 2b) in a series of small coastal exposures around Sælabonn a small bay in the northern part of Tyrifjorden. This formation corresponds in general to stage 6 of Kiær (1908) in the Ringerike, Skien, Hadeland, Toten, Hamar and Ringsaker districts. Our proposal incorporates the Helgøya Quartzite of Skjeseth (1963), reduced to member rank, in the three latter districts. The Sælabonn Formation is approximately 110 m thick in its type area, where it is characterised by varying proportions of sandstone, siltstone and shale. Sandstones dominate the middle part of the unit and lithological variation was clearly the basis for Kiær's (1908) tripartite subdivision into 6a, b & c. Formal proposals for subdivision into three members approximately equivalent to Kiær's units will result from work in progress in the Ringerike area (Thomsen 1981).
The entire Sælabonn Formation is not exposed in any one section in the type area; the basal stratotype for the formation is defined on Store Svartøya (NM678585) where the lower 11 m of the formation are also exposed (Fig. 8). A reference section through the middle of the unit is exposed on the east coast of Sælabonn (NM696606) and its upper parts are well displayed in the basal stratotype for the overlying Rytteråker Formation at Limovnstangen (p. 20). The base of the Sælabonn Formation is defined on Store Svartøya where crinoidal biosparites of 5b show a karstic upper surface and are overlain by silty shales with minor thin limestones and siltstones (Fig. 8). This 20 m‑thick lower development of the formation coarsens up into medium to thickly bedded sandstones with minor siltstones and shales (6b of Kiær 1908, 50 m thick). These further fine up into thin to medium interbeds of siltstone and shale with increasing limestone intercalations towards the junction with the overlying Rytteråker Formation.
Other occurrences
Exposures elsewhere in the Ringerike and Skien districts indicate a similar tripartite development to that of the type area. The base is exposed at several localities in the southern parts of the Skien district (e.g. Skrapekleiv NL381523) where silty shales of the formation overlie a thin (0.5‑1 m) very coarse sandstone which has an erosive contact with small biohermal structures and biosparites of upper 5b (Rønning 1979). Exposures north of Skien at Jønnevall in Gjerpendalen (NL321708) show an erosive contact with 5b, but the base of the formation there consists of sandstones which grade up into the silty shales assigned to 6a by Kiær (1908). Metamorphosed outcrops assigned to this formation are also seen in the Eiker district.
In Hadeland, Owen (1978) defined the Skøyen Sandstone Formation as a sandstone and shale unit with a gradational base (to his Kalvsjø Formation, 5a of earlier workers) and top (to our Rytteråker Formation). This unit is 120 m thick and correlates with Stages 5b and 6 of earlier workers. Because of poor exposure Kiær (1908) and Major (1946) were unable to define the boundary between 5 b and 6 in Hadeland, and Owen's approach is eminently justifiable as present exposures do not display the precise contact between these units. However, sections examined by us suggest an important change in sedimentological development in the middle of Owen's Skøyen Sandstone and the upper 60 m of this unit show a tripartite development similar to that of the Sælabonn Formation in the adjacent Ringerike district. Pending further detailed analysis we suggest as an interim measure that Owen's Skøyen Sandstone be regarded as an informal unit, the upper part of which is clearly correlative with the Sælabonn Formation at Ringerike.
Further north, in the Toten, Hamar and Ringsaker districts, the Sælabonn Formation is represented in toto by the thin Helgøya Quartzite Member which rests on eroded surfaces of the Mjøsa Limestone. Sandstone infills of the karstic topography marking the top of this limestone are seen at several localities in Toten. The Helgøya Quartzite's original type section at Eksberget (PN087355) on Helgøya (Skjeseth 1963) is poorly exposed and a hypostratotype is proposed in a disused quarry at Vestby near Gjøvik (NN921379). The member is 9 m thick there (Fig. 10). The lower 5 m consist of medium to thickly bedded sandstones with thin shaly partings; these grade up into thinly interbedded sandstones and shales, with limestone intercalations uppermost marking the transition into the overlying Rytteråker Formation. Other exposures further north in Brumunddalen (PN076586) and on Helgøya indicate an essentially similar, but thicker development (15 to 30 m).
Depositional environments
Basal sandstones and shales in the Ringerike and Skien districts reflect the early Silurian transgression of these areas. Sandstones in the middle of the formation represent a subsequent progradational episode and deposition in coastal environments. The upper parts of the unit in both areas suggest renewed transgression, with a gradual transition from clastic to carbonate‑dominated sedimentation. Exposures in Hadeland suggest a similar depositional history. The thin sandstones of the Helgøya Quartzite Member in the Toten, Hamar and Ringsaker districts reflect the transgression of an area which had been emergent throughout the late Ordovician. This development appears to correlate with the upper part of the Sælabonn Formation in its type area. In all districts the transition to the overlying Rytteråker Formation suggests a complex and irregularly diachronous cut‑off of coarse clastic supply as a result of continuous transgression. The faunas of the formation are reminiscent of Benthic Assemblages 1 to 3 of Boucot (1975).
Age
A general Rhuddanian to Idwian age is suggested in the type area by brachiopods (Thomsen 1981) and conodonts (RJ. Aldridge, pers. comm. 1981). The occurrence of Borealis around the junction with the overlying Rytteråker Formation indicates a middle Idwian age for the uppermost beds of the Sælabonn Formation in its type area (Mørk 1981). However, the base of the formation has not yet been definitively dated.