Definition
The name of this formation is derived from Rytteråker Farm (NM699594) on the west coast of Tyrifjorden in the Ringerike district. The formation's character is well demonstrated in this type area, especially in coastal exposures around Limovnstangen, a peninsula 1 km south of Rytteråker Farm.
The Rytteråker Formation is essentially equivalent to 7a and 7b of Kiær (1908) throughout the Oslo Region. Our formational proposal replaces the term 'Pentamerus Limestone' which has been widely used subsequent to Kiær's work. Whereas many workers (e.g. Henningsmoen 1960) have applied this term in a sense which generally conforms to our Rytteråker Formation, Kiær applied it only to those restricted parts of our Rytteråker and overlying Vik Formations which contain abundant pentamerids. This varying usage prompts our rejection of the name 'Pentamerus Limestone' in spite of its priority. Pending the results of our work, Owen (1978) and Høy & Bjørlykke (1980) used the informal terms 'Engen limestone 1' and 'Limovnstangen formation' in the Hadeland and Hamar/Ringsaker districts respectively. Sequences assigned to those units are here grouped within the Rytteråker Formation.
The limestone dominated sequence of the Rytteråker Formation is approximately 50 m thick in the type area; Kiær's tripartite division (7a, 7b-alfa, 7b‑beta) was based both on faunal composition and on varying proportions of limestone and calcareous shale within the unit. A single complete type section is not seen, but the basal stratotype is defined on the northern tip of Limovnstangen (NM691591) and reference sections through the middle and upper parts of the formation are exposed along both coasts of this peninsula. The top is seen under the basal stratotype of the overlying Vik Formation on the limb of a small syncline whose axis defines the northern coast of Limovnstangen (NM695596).
The basal stratotype of the Rytteråker Formation shows a gradational development: interbedded thin to medium siltstones and shales of the Sælabonn Formation show intercalations of limestone beds upwards. The base of the Rytteråker Formation is defined at the quantitative dominance of limestone over siltstone (Fig. 11). Our formational base is therefore 5.5 m higher in the section than the junction between 6c and 7a defined by Kiær (1908) and corresponds to Kiær's bed XIV (Kiær 1908: Profiltafel 11). Limestones with pentamerids become increasingly abundant through the basal 10 m of the formation and grade upwards into thickly bedded biosparites in the middle 2 5 m of the unit (7b-alfa of Kiær 1908). These beds are composed of finely comminuted pentamerid and crinoid debris; small bioherms are found at the top of this sequence throughout the Ringerike district (Hanken, Olaussen & Worsley 1979). The uppermost 17 m of the formation (7b‑beta of Kiær 1908) consist of planar limestone beds and well‑bedded calcareous nodules with interbedded shales; the shale content increases towards the contact with the overlying Vik Formation.
Other occurrences
Although the character and content of carbonates in the Rytteråker Formation vary from district to district, the unit can be traced throughout the Oslo Region. The formation thins markedly northwards from the type area (Fig. 4); it thickens southeastwards and southwards to 65 m in Oslo/Asker and 80 m in Skien.
Exposures in Hadeland show a gradational contact with the Sælabonn Formation, interbedded limestones and shales passing up into a thickly‑bedded pentamerid biosparite unit. This is erosively overlain by interbedded limestones and shales with both bioclastic and general carbonate content decreasing upwards to the contact with the Ek Formation (see p. 25). The Rytteråker Formation thins progressively northwards through the Toten, Hamar and Ringsaker districts. Northernmost exposures at Torsæter Bridge in Brumunddalen (PN076586) show a 15 m‑thick sequence of interbedded thin limestone beds, nodular horizons and shales.
Sequences assigned to the Rytteråker Formation share many common features in Asker, Oslo, Holmestrand and Skien. The base is gradational in all cases, either from the more sandy Sælabonn or more shaly Solvik Formations. Thin, finegrained limestones and shales occur in equal proportions in the lower parts of the unit in all these areas. Limestone beds thicken and coarsen upwards and shale content decreases into the middle and upper parts of the unit. The transition to the overlying Vik Formation is often rapid, marked by the renewed development of interbedded limestones and shales.
Depositional environments
The Rytteråker Formation represents the establishment of relatively shallow carbonate depositional environments throughout the Oslo Region at a time when earlier source areas for coarse clastic material were submerged. The pentamerid biosparites of Hadeland and Ringerike represent high‑energy shoals, with crests which may have been intermittently emergent (e.g. in Hadeland). Biohermal development on the upper surfaces of the shoals in Ringerike was probably initiated subsequent to a relative rise in sea‑level which resulted in stabilization of the shoals' highly mobile substrates. Other districts show more distal marine carbonate sequences. However, in all these other districts a general shallowingupwards trend throughout the lower and middle parts of the formation is indicated by an increase of better sorted and thick bedded biosparites at the expense of shale and fine‑grained limestones. Abundant pentamerids, corals and stromatoporoids together with benthic algae in all districts indicate that the formation was not deposited in water of any great depth. The fauna is typical of Benthic Assemblage 3 of Boucot (1975).
Age
Occurrences of the pentamerids Borealis and Pentamerus suggest a somewhat diachronous base for the Rytteråker Formation, ranging from the middle to late Idwian in Ringerike and Hadeland to the late Idwian or early Fronian in central and southern districts Mørk 1981). The transition to the Vik and Ek Formations is dated by the graptolite and brachiopod faunas of these overlying units (pp. 25 and 27).