Abstract
The Sælabonn Formation belongs to the Bærum Group, and is recognized in the western part of the Oslo Region, from Mjøsa in the north to Skien in the south. The Sælabonn Formation is the lowermost unit in the Silurian succession in the Western Districts of the Oslo Region bound to an erosional surface, the Ordovician-Silurian boundary. In the Central Oslo Region the Solvik Formation is the eastern equivalent, displaying deeper marine conditions. Overlying the Sælabonn and Solvik formations is the carbonate dominated Rytteråker Formation. The Sælabonn Formation represents a shallow shelf environment dominated by storm processes. The relationship between the epicontinental sea and the advancement of the Caledonian orogeny, during the time of deposition, has previously been vaguely defined.
Outcrops in the Ringerike District and the Modum District have been logged during the summer and autumn of 2011. Samples of both siliciclastic and carbonate material have been collected, and various laboratory and microscope techniques, including point counting analyses have been performed.
The sedimentary logs display three units of the Sælabonn Formation. The laterally equivalent lower units, Store Svartøya Member and Sylling Member, are dominated by mudstone occasionally interbedded by sandstone and biosparitic limestone beds. The middle unit (Djupvarp Member) is the sandiest, and represents isolated sand shoals in the Ringerike District, and tempestites in the Modum District. The upper unit (Limovnstangen Member) has a decreasing content of siliciclastic material with mudstones interbedded with tempestites containing sand and bioclastic material, occasionally occurring in couplets. The sedimentary structures and lithology indicate a sedimentary environment dominated by storm processes where siliciclastic material is brought out into the basin and deposited together with winnowed bioclastic material. Results from tidal processes have not been recorded. The members belonging to the Sælabonn Formation are all positioned in the offshore-transitional environment on the shelf. Palaeocurrent measurements from the Djupvarp and Limovnstangen members suggest a stable palaeoshoreline, with a SW to NE strike.
Grain size of the tempestites range from silt to very fine sand. Mineralogical composition suggests a quartz rich source, “Telemark land” or possibly Valdres Thrust Sheet. The development of the Sælabonn Formation represents an overall transgressive setting, where the Djupvarp Member represents a progradation. In the developing Caledonian foreland basin, the Sælabonn Formation is suggested to have been formed in the back-bulge depozone.