Definition
Kjerulf (1857) introduced the term Malmøya Limestone and Shale for exposures of his stage 8 on Malmøya in the Oslo district. Kiær (1908) assigned the lower (more shaly) part of Kjerulfs unit to 8c and the upper massive limestones to 8d; he applied the term Malmøya Limestone both to the latter limestones and to units he assigned to 8d in Asker, Holmestrand and Skien. Kiær's practice was also followed by Strand & Størmer (1955) in their definition of the 'Malmøya Limestone'. Our redefinition and use of the Malmøya Formation is based on Kjerulfs original designation; both Kiær's and subsequent applications of this term only to the upper part of Kjerulf a unit are rejected. Our use of the Malmøya Formation therefore generally corresponds to Stages 8c and 8d of Kiær (1908) in Oslo, Asker, Eiker and Holmestrand. Only the lower and middle parts of the Malmøya Formation are exposed on Malmøya and these represent the youngest Silurian strata preserved in the Oslo district. The basal stratotype defined on Malmøya (NM985382) is therefore supplemented with a parastratotype from Gjettum in the Asker district (NM875449). The formation is approximately 3 5 m thick at Gjettum (Fig. 17), and both its base Malmøya and top Gjettum are gradational. The base is defined on Malmøya at the first development of continuous limestone beds above the graptoliferous shales with marly horizons of the underlying Skinnerbukta Formation. The lower part of the formation (8c of Kiær) shows varying proportions of continuous, nodular and lenticular limestone beds, all with interbedded shales. These grade up into thickly to massively bedded biosparitic limestones and faunal diversity increases correspondingly. The lower and middle parts of the formation at Gjettum show a similar development to beds preserved on Malmøya the massive limestones seen in both areas grade upwards in the Gjettum section into coral and stromatoporoid biostromes and biosparrudites high in the formation. These show a gradational contact with the thinly interbedded micritic limestones and shales (with a more restricted fauna) which we assign to the overlying Steinsfjorden Formation.
Other occurrences
The Malmøya Formation is also exposed on Kommersøya (NL7499) and at Sando (NM738015) in the Holmestrand district; its base there is obscure and the Junction with the underlying Skinnerbukta Formation may be faulted. As in the type area, the unit shows a gradation upwards from interbedded limestones and shales into thickly bedded limestones with coral biostromes and biosparrudites.
Depositional environments
Thinly‑bedded limestones in the lower parts of the formation are interpreted as storm‑generated units deposited distally to the carbonate shoals which are represented by the overlying thickly bedded biosparites and coral biostromes. The entire sequence reflects progressively shallowing depositional environments in the Holmestrand and central districts, with progradation of carbonate shoals over the basinal shales of the underlying Skinnerbukta Formation. High energy environments on the shoals are suggested by the reworked corals of the biostromes; however, all the units assigned to the formation were deposited in subtidal environments and no evidence of emergence is seen. The fauna is reminiscent of Benthic Assemblages 4 to 3 of Boucot (1975).
Age
The base of the Malmøya Formation is younger than the riccartonensis (possibly early rigidus) Biozone graptoliferous shales in the upper parts of the Skinnerbukta Formation. Faunal elements within the Malmøya Formation have a general, but nondiagnostic Wenlock aspect. The brachiopod Eocoelia angelini occurs in the lower parts of the formation (8c) at Gjettum and on Kommersøya. This brachiopod suggests a correlation with the mid-Sheinwoodian riccartonensis Biozone (Bassett & Rickards 197 1, Bassett & Cocks 1974, Bassett 1979). Available evidence therefore suggests a diachronous base for the Malmøya Formation, ranging from the riccartonensis Biozone in Holmestrand and Asker to the rigidus Biozone on Malmøya. Basal units in Holmestrand and Asker (80 are considered to be time equivalents to the upper parts of the Braksøya Formation of Skien (8d) and to the basal unit of the Steinsfjorden Formation in Ringerike (9a). The basal units of the Malmøya Formation on Malmøya are somewhat younger and are correlated with 8d in Holmestrand and Asker, 9a in Skien and 9b in Ringerike. The age of the transition between the Malmøya and Steinsfjorden Formations in Asker and Holmestrand is still uncertain.