LANGØYENE FORMATION
(Langøyformasjonen)
(Brenchley & Newall 1975).
(Previously termed: 5b, Kalksandstein, Calcareous Sandstone, Calcareous Sandstone Formation).
Main lithology. Sandstone (Langøyene Sandstone, Langøysandsteinen).
Basal Stratotype. Oslo-Asker: South East Langøyene (Brenchley & Newall 1975, Fig. 4) (NM 968 385).
Definition. The base of the formation in the east is defined at the base of the shales, laminated sandstones and thin limestones which overlie the brown weathering sandstone at the top of the Husbergøya Formation. Brenchley & Newall (1975) produced a large number of measured sections in their original description of the formation showing the diverse, but dominantly arenaceous, facies present. They subsequently published detailed sedimentological evidence indicating that deposition took place within a major regressive phase on the Baltic craton (1980) and argued that this was of glacio-eustatic origin. Many of the sandstone beds are interpreted as having had a storm surge origin (Brenchley et al. 1979) and there is evidence of channelling (Brenchley & Newall 1975, 1980). Contorted bedding in both sandstones and shales indicates periodically high sedimentation rates and possibly seismic shocks related to basement faulting (Brenchley & Newall 1977).
The Langøyene Formation is 51 m thick in its type development and has a maximum thickness in the east of some 60m. Westwards all but the uppermost part passes laterally into the Langåra Formation. Thus the formation thins to 8m on Langåra; at Sandvika it is developed as only a metre of oolite breccia and at Haga there may be as little as 50cm of breccia. The formation is overlain with a distinct lithological change by shales, siltstones and limestones of the Silurian Solvik Formation (Worsley et al. 1983 pp. 12-13; Baarli 1985). These become more calcareous westwards.
Discussion. The uppermost Ordovician units to the west and south of Oslo-Asker, and that in Ringerike are here tentatively assigned to the Langøyene Formation. However, lithological heterogeneity is one of the characteristics of this part of the sequence and local formation (or at least member) names may eventually be established.
The largely arenaceous unit at the top of the Ordovician in Skien-Langesund was the subject of an unpublished thesis by Rønning (1979) who did not formally name it as he considered it may best be grouped with the lowest part of the overlying Silurian succession. Worsley et al. (1983 p. 17), however, assigned the lowest Silurian to their Sælabonn Formation and noted that the base of this formation is an erosive one. The base of the underlying unit (tentatively assigned to the Langøyene Formation herein) can be defined at the level where sandstone becomes markedly more common than in the upper part of the underlying Herøya Limestone Formation Rønning 1979 p.13, Fig. 3). As Rønning noted (1979 p. 10) there is no complete section through the formation but the best group of exposures are along a series of steep westward facing scarps between Versvika farm and Rød. The formation is approximately 40m thick and was divided into five informal members by Rønning From base to top these were termed: the lower siliciclastic member, the lower limestone member, the middle siliciclastic member, the upper limestone member and the upper siliciclastic member. Rønning interpreted the whole sequence as having been deposited in a wave and storm dominated shallow sea and he recognised two different depth regimes within the district. He considered the more calcareous successions in the north as having been deposited in shallower water than the largely siliciclastic sequence of the south with a fairly abrupt break between the two lying somewhere between Gimsøy and Herøya
The uppermost 40m of the Ordovician in Ringerike has been the subject of detailed study by N.-M. Hanken of Tromsø University who has described aspects of the sedimentology and faunas of this part of the succession (1974,1979,1979a) and proposes to establish a formal lithostratigraphy in due course. Hanken & Owen (1982 pp. 126-13 1, Figs 4-7) summarized the complex lithofacies distribution, diagenetic history and fauna of these beds and noted that the only complete section is on Store Svartøya where there is a gradational change from the limestones and shales of the Bønsnes Formation to sandstones. The 15m of sandstone at the base of the formation is overlain by limestones with a basal conglomerate composed of locally derived material. The limestones include a variety of types including small unbedded patches and, on the mainland at Ullerntangen, an extensive area of carbonate blocks. The various limestone facies pass northeastwards into sandstones (Hanken & Owen 1982 Fig. 4). The base of the overlying Silurian Sælabonn Formation is an irregular karst surface at the top of the limestones on Store Svartøya (Worsley et al. 1983, Fig. 8) whilst further north there is a planar erosion surface (Thomsen 1982 p.2).
Fauna and Age. The diverse sedimentary environments from inner shelf to tidal channels represented in the Langøyene Formation are also reflected in the faunas with Brenchley & Cocks (1982) recognising several different associations within the formation in Oslo-Asker. These range from the moderately diverse brachiopod dominated Hirnantia Association in the basal part of the formation in the east (see also Brenchley & Cullen 1984) to faunas rare in body fossils but containing a variety of trace fossils including Monocraterion, Planolites, Chondrites and Teichichnus. Stanistreet (1989) ascribed the trace fossils from most of the formation to the Diplichnites-Phycodes association.
Systematic studies have been carried out on the trilobites (Owen 1981, 1982) and brachiopods (Cocks 1982) which together indicate a Hirnantian age. Monoplacophorans (Yochelson 1977) and ostracods (Henningsmoen 1954) have also been described.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.