Red beds assigned to the Ringerike Group have a conformable and gradational contact with the Steinsfjorden Formation in the Ringerike, Asker, Holmestrand, Eiker and Skien districts. The Ringerike Group is in turn uncomformably overlain by sediments (the Asker Group of Dons & Györy 1967, Henningsmoen 1978) and/or lavas. The Asker Group was previously thought to be Permian in age, but fusulines found recently have a mid/late Carboniferous aspect (Olaussen 1981a).
Kiær (1908) assigned the red beds of Ringerike to stage 10 and suggested a Devonian age. Subsequent work concentrated on the remarkable fauna of agnathans, crustaceans and eurypterids in the lower parts of this red bed sequence (Kiær 1911, Kiær 1924, Størmer 1934a, Størmer 1935, Heintz 1939 (and Hanken & Størmer 1975). Agnathans were also described from similar red beds on Jeløya on the eastern side of outer Oslofjorden in the Holmestrand district Kiær 1931, Heintz 1974). The entire red bed development was for many years assigned to stage 10 in most works (e.g. review of Henningsmoen 1960), but later workers called the sequence either the Ringerike Formation Spjeldnæs 1966), the Ringerike Sandstone Series (Whitaker 1966) or the Ringerike Sandstone (Heintz 1969). Knowledge of the lithostratigraphy and sedimentology of this red bed development has been greatly increased by the work of Whitaker (1964, 1965, 1966, 1980), Turner (1974a, b, c) and Turner & Whitaker (1976). These studies were chiefly directed towards the Ringerike district, although Turner (1974a) and Turner & Whitaker (1976) also presented information on Asker and Holmestrand.
Turner (1974a) proposed the Ringerike Group to embrace all red bed units earlier assigned to stage 10. The group comprises an approximately 1,250 m thick sequence of red siltstones and sandstones in its type area in the Ringerike district. Turner also defined the Sundvollen (lower 500 m) and Stubdal (upper 750 m) Formations within the Ringerike Group in its type area. The type section of the Sundvollen Formation on the western slopes of Kroksund (NM7259) consists mostly of red siltstones and sandstones arranged in fluvial upwards fining sequences. Interbedded marine mudstones and limestones of tidal inlet origin suggest coastal plain to estuarine environments. The Stubdal Formation's type section in Stuvdal (NM7766) is dominated by sandstones (with intraformational conglomerates), reflecting deposition by braided stream systems prograding over the more fine‑grained deposits of the underlying formation. Palaeocurrent indicators and the petrology of sandstones in both formations suggest drainage systems with sources in the developing Caledonides to the north and northwest of the present Ringerike district (Turner & Whitaker 1976). The maximum preserved thickness of the Stubdal Formation is seen at Stuvdal; when traced southwards the preserved thickness of the formation decreases rapidly because of pre‑Carboniferous tilting and erosion.
Units assigned by Turner (1974a) to the Ringerike Group in the Asker district at Kolsås show a broadly similar stratigraphy to that of the type area, although the total thickness of the group there is only 500 m. The Sundvollen Formation is 100 m thick and consists of siltstones and mudstones with minor sandstones; there is no clear organisation of these components. An estuarine environment was suggested by Spjeldnæs (1966) and Turner (1974a) also noted that the transition from marine to red beds was more gradual at Kolsås than in Ringerike. The Stubdal Formation consists of finer grained sandstones than those seen in the type area, although the mineralogical content is similar in both Ringerike and Asker. Thus the entire Ringerike Group in Ringerike and Asker is regarded as a south to southeastwards prograding unit.
The Holmestrand Formation of Turner (1974a) is exposed on both the east (Jeløya) and west coast of outer Oslofjorden in the Holmestrand district. Turner defined the type section of the formation between Engenes and Smørstein (NL723999) in the Holmestrand area. An approximately 600 m thick sequence is dominated by medium‑grained sandstones with extraformational conglomerate clasts, a development which Turner & Whitaker (1976) contrasted with the red beds seen in the Ringerike district. They suggested deposition of the Holmestrand Formation's sandstones by braided streams draining westwards, citing clast composition and sandstone mineralogy as suggesting the erosion of Precambrian basement complexes to the east of the Oslo graben. This interpretation contrasts with the results of other workers (F. Andreasen pers. comm. 1978) which suggest deposition of the Holmestrand Formation on coastal floodplains with palaeoccurrents from the northwest. An explanation of these conflicting views is suggested by recent studies of the junction between the Ringerike and Asker Groups (Olaussen 1981a); these indicate that some of the red beds assigned by Turner & Whitaker to the Holmestrand Formation may in fact be Carboniferous in age. More detailed work is clearly necessary; and studies of the metamorphosed 300 m‑thick sequence generally assigned to the Ringerike Group in Eiker and Skien are also needed before reliable palaeogeographical interpretations can be presented.
Early descriptions of the agnathan and arthropod faunas in beds assigned to the basal parts of the Ringerike Group in its type area followed Kiær's (1911) assumption of a Downtonian age. Later works (Heintz 1969:25, Turner & Turner 1974:190) suggested an early Ludlow age for the same units. However, as the youngest marine faunas found in the Steinsfjorden Formation are now thought to indicate a nassa Biozone age, the lowermost red beds could be also of Wenlock age. Turner (1974a) considered the Holmestrand Formation to be age equivalent to the Stubdal Formation of Ringerike, but we have no data on the age of the marine to red bed transition in Holmestrand. A fossiliferous horizon found in the lower parts of the Holmestrand Formation at Smørstein by Prof. G. Henningsmoen contains a plectambonitacean brachiopod and a single poorly preserved chitinozoan with a general Ludlow aspect (S. Laufeld pers. comm. 1980). Agnathans found on Jeløya (NL953923) suggest an early Downtonian age (Heintz 1974); although these finds are from the lower parts of the 230 m‑thick exposed sequence of the Holmestrand Formation, we do not know how far they are above the marine to red bed transition. Present evidence, albeit sparse, does not suggest any substantial diachroneity between the Ringerike and Holmestrand districts and we provisionally consider the Holmestrand Formation to be a lateral equivalent of the Sundvollen Formation of Ringerike (Fig. 5).