Definition
The name of the formation is taken from Solvik (NM981378), a bay on the west coast of Malmøya, 5 km south of Oslo city centre (Fig. 2a). The formation corresponds essentially to stage 6 of Kiær (1908) in the Oslo, Asker and Holmestrand districts. The Solvik Formation is characterised by dominant shales with very thin to thin siltstone and limestone intercalations (all quantitative terms which describe bedding and bed thicknesses used herein follow the scheme of Ingram 1954). Lithological variation (Fig. 6) suggests a twofold division into the Myren and Padda Members in the formation's type area on the Malmøya group of islands in the Oslo district. A complete type section cannot be designated and the basal stratotype of the Solvik Formation is defined on the south coast of Hovedøya (NM975412). Reference sections through the lower and upper parts of the formation are well exposed in two faulted but relatively continuous profiles on the east (NM988380) and west (NM981377) coasts of Malmøya, respectively; the top of the formation is seen in the latter section at Solvik. The formation is approximately 190 m thick in its type area, but faulting makes an exact determination difficult.
The lower boundary is defined by the sharp contact between underlying calcareous sandstones of the Langøyene Sandstone Formation (Brenchley & Newall 1975, equivalent to 5b of Kiær 1897, 1902) and the dark grey silty shales with minor siltstones of the Solvik Formation. A 60 cm‑thick nodular limestone occurs locally as a basal development of the overlying unit (Fig. 7a). The base of the lowermost Myren Member is, by definition, coincident with the base of the Solvik Formation. The Myren Member is approximately 160 m thick and includes 6a and 6b of Kiær (1908) in the type area of the Solvik Formation. The name of the member is derived from a locality on the east coast of Malmøya (NM988380) which shows the most complete development of the lower parts of this unit. Detailed correlations of several partial sections both here and on the west coast of the island suggest a minimum thickness of 120 m for Kiær's 6a (in contrast to his suggested thickness of 50 to 60 m) and a total minimum thickness of approximately 160 m for the whole Myren Member. The member consists of shales with minor laminae and very thin interbeds of siltstone. Siltstones are most common in 6a‑alfa and 6b‑beta where they comprise 30% of the succession (Fig. 6). Body fossils are generally rare in the lower parts of the member, but in situ brachiopod associations dominated by Leangella scissa and Protatrypa malmoeyensis characterize 6b‑alfa and 6b‑beta respectively (Worsley 1971).
The Padda Member corresponds in general to 6c of Kiær (1908) in the Malmøya area. Its name is derived from a small island northeast of Malmøya (NM992390) where the upper parts of the unit are well exposed, but its base is defined in a complete 30 m‑thick type section on the west coast of Malmøya (NM981377). Although shales also dominate this member, thin interbeds and lenses of both calcareous siltstone and limestone occur together with calcareous nodules. The base of the Padda Member is defined at the sharp introduction of these interbeds above the highly shaly upper parts of the underlying units. Characteristic faunal elements are both in situ and reworked populations of Stricklandia lens; other brachiopods, bryozoans, corals and stromatoporoids are also common.
The dominantly shaly Padda Member grades rapidly into the thinly interbedded limestones and shales (in approximately equal proportions) of the overlying Rytteråker Formation (Fig. 7b); the junction is taken at the first appearance of finely nodular limestone horizons assigned to the Rytteråker Formation. Bedding plane exposures show that many of these nodules are carbonate infills of endichnial burrows in the shales.
Other occurrences
The Solvik Formation is also seen in many exposures in the Asker and Holmestrand districts and its total thickness appears to be relatively constant.
The siltstones of the Myren Member thin and decrease in abundance westwards from the type area to Asker where beds which can be assigned to this unit interfinger with a more calcareous shale development throughout 6a and 6b of Kiær (1908). Marls and limestones in Kiær's 6c‑alfa at Spirodden (NM840338) contain both Stricklandia lens, abundant tabulate corals and stromatoporoids; these beds should be assigned to a new, as yet formally undefined, unit with member status (Baarli 1981). The occurrence of benthic algae in this unit at Spirodden was commented upon by Lauritzen & Worsley (1974) and Mørk & Worsley (1980). Kiær (1908) defined 6c‑beta in Asker as a 10 m‑thick shaly sequence which he assumed to be directly overlain by the limestones of stage 7. However, a series of new road cuttings not available to Kiær show a 60 m thick sequence of shales and siltstones which lie between Kiær's 6c‑alfa and the overlying Rytteråker Formation. A detailed study of the entire Solvik Formation in the Asker district now in progress will result in the definition of several new members in this area (Baarli 1981).
Only the uppermost 25 m of stage 6 are exposed in the Holmestrand district on the island of Bjørkøya (NL764986) and these were re‑examined by Mørk (1977). Both this exposure and cores from test boreholes through the entire lower Silurian succession in the subsurface of the neighbouring island of Langøya demonstrate a facies-development clearly assignable to the Solvik Formation. Small, highly metamorphosed, exposures through parts of the formation are also seen on the island of Tofteholmen and on the nearby coast of Hurumlandet (Brøgger 1931).
Depositional environments
The shale‑dominated sequence of the Solvik Formation reflects the rapid early Silurian transgression of the Oslo, Asker and Holmestrand districts and the establishment of sublittoral depositional environments in these areas. Siltstones of the Myren Member in the type area were storm generated and deposited in quiet muddy environments below normal wave base. Exposures through equivalent lower parts of the Solvik Formation in the Asker district show notably fewer siltstone intercalations than the type area and calcareous interbeds are more common. The transition to the shale and limestone facies of the Padda Member is abrupt in the type area. Both sedimentary structures and fauna indicate the establishment of shallower environments with intermittent periods of both current and wave reworking of the sea floor. Storm‑generated siltstones seen in 6c‑beta in Asker and Holmestrand are thought to be time equivalents of the Padda Member. In contrast to the depositional regime of the lower parts of the formation, the Asker and Holmestrand districts were now in a more proximal situation than the type area. Faunal associations seen throughout the Solvik Formation in all three districts correspond generally to Benthic Assemblages 5 and 4 of Boucot (1975).
Age
Graptolites and abundant brachiopods form the basis for our dating (Fig. 5). Both the basal nodular horizon and immediately overlying shales contain sparse and non‑diagnostic shelly faunas, but we have found Climacograptus transgrediens 11 m above the base of the formation on Ormøya (Fig. 2a). This graptolite ranges from the ?upper persculptus to lower acuminatus Biozones elsewhere and the base of the Solvik Formation therefore approximates to the Ordovician/ Silurian boundary as taken at the base of the persculptus Biozone. Occurrences of subspecies of Stricklandia lens in the Asker district indicate that the top of the formation there approximates to the Idwian/Fronian transition (Baarli 1981).