Definition
This formation's name is derived from Skinnerbukta, a bay on the northwestern coast of Malmøya in the Oslo district. The Skinnerbukta Formation corresponds to stages 8a and 8b of Kiær (1908) in Oslo, Asker, Modum, Holmestrand and Skien. In spite of some tectonic disturbance a reasonably complete type section can be defined in cliffs on the east coast of Skinnerbukta (NM894381). The unit is approximately 80 m thick in its type section and consists of dark grey graptoliferous shales.
The base is defined immediately above a sequence of shales with both nodular and continuous limestone interbeds which constitute the base of the exposure. Although the nodules are larger, and the limestone beds thicker, than those usually seen in the Vik Formation, this sequence is assigned to that formation. The overlying dark grey shales of the Skinnerbukta Formation contain occasional thin marly horizons which are sometimes slightly bioturbated. The total calcareous content of the formation increases upwards and its upper parts consist of somewhat paler calcareous shales. These uppermost beds also show an increasing abundance of limestone interbeds and the gradational top of the unit is defined by the base of the overlying Malmøya Formation (p. 36). Graptolites and hyolithids dominate the fauna of the formation in the type section; some rare shelly elements have been found, including scattered lingulids, gastropods, crinoids and small specimens of Eoplectodonta transversalis.
Other occurrences
The formation is also exposed at several localities in Asker, Holmestrand and Skien. Exposures at Bærum in the Asker district (Overland, NM876449, Jong NM846404 and Gjettum NM850022) suggest a thickness of 90 m; shales there contain both graptolites and low diversity benthic faunas. Metamorphosed exposures on Kommersøya (NL750990) and at Sando (NM740013) in Holmestrand show a more diverse benthic fauna with several brachiopod and trilobite taxa, while graptolites are rare. Exposures in Skien (e.g. around Kapitelberget, NL367628) mark the continuation of this ecological gradient as benthic faunas there show a moderate diversity and approach a composition similar to that seen in the Bruflat Formation of Ringerike. Thin limestones are common, and form distinct horizons (often bioclastic lenses) in the upper part of the unit. However, the absence of coarse clastic material leads to the assignment of the Skien sequence to the Skinnerbukta, rather than to the Bruflat Formation.
Depositional environments
In its type area, the Skinnerbukta Formation's shales were deposited in low‑energy, usually anaerobic environments. The occasional presence of bioturbation and of (highly restricted) benthic elements suggests periodic flushing of a generally anoxic mud bottom. Several of these elements may represent either epiplanktic taxa drifted into the area attached to floating algae or possible giant spat. The only clearly benthic element is Eoplectodonta transversalis; the few individuals found are small and apparently stunted, suggesting restricted bottom conditions adverse to shelly colonisation. Bioturbation and benthic diversity increase upwards in the formation in the type area, suggesting a generally shallowing trend. Benthic faunal diversity also progressively increases from Oslo, through Asker and Holmestrand, to Skien and faunas found there may be assigned to Benthic Assemblages 5 to 4 of Boucot (1975).
Age
The faunal evidence suggests a marked regional diachroneity of the Skinnerbukta Formation's lower and upper boundaries, which became older from Malmøya to Skien. Graptolite faunas in the type section suggest that the base of the formation there is approximately correlative with the Llandovery/Wenlock boundary, although the basal 10 m (8a of Kiær 1908) may possibly be assigned to the latest Llandovery crenulata Biozone. Revision of the correlation of Bassett & Rickards (1971) suggests that elements typical of the Wenlock centrifugus, murchisoni and riccartonensis graptolite Biozones are found upwards through the formation on Malmøya and its upper parts extend into the rigidus Biozone. Both the base and top of the Skinnerbukta Formation become progressively older southwards, and benthic faunas near the top of the formation in the Skien district contain elements which, as noted under our discussion of the Bruflat Formation, are typical of the uppermost Llandovery (e.g. Costistricklandia lirata lirata, Palaeocyclus porpita and Isorthis mckenzii).
Comparison with Ringerike suggests that the Skinnerbukta Formation in its type area is laterally equivalent to the (combined) uppermost parts of the Bruflat Formation, the entire Braksøya Formation and the lowermost parts of the Steinsfjorden Formation of the latter district. This complex facies mosaic will be further discussed below.