LANGÅRA FORMATION
(Langårformasjonen)
(Brenchley & Newall 1975)
(Previously termed: 5a (pars), 5b (pars), Gastropodkalk, Gastropod Limestone).
Main Lithologies. Limestone and shale (Langåra Limestone-Shale).
Basal Stratotype. Oslo-Asker: Langåra (Brenchley & Newall 1975, Fig. 7) (NM 856 356).
Definition. The Langåra Formation is restricted to the western part of Oslo-Asker and is a lateral equivalent of the upper part of the (Husbergøya Formation and all but the uppermost part of the Langøyene Formation. The sandstone bed at the top of the (Husbergøya Formation in the east can be recognised within the Langåra Formation at many localities. It was interpreted as an isochronous level by Brenchley & Newall (1975) who used its top as the boundary between their informal 'stages' 5a and 5b (Brenchley & Cocks 1982 pp.784-5).
The base of the Langåra Formation was chosen at a fairly arbitrary level within a transitional sequence at the type locality by Brenchley & Newall (1975) who noted that the top of the formation here and at most other localities is an abrupt change to massive sandstones of the Langøyene Formation. At Sandvika, however, the Langåra Formation is overlain by a limestone breccia ascribed to the Langøyene Formation.
The Langåra Formation is 13m thick on Langåra and attains a maximum thickness of 35m at Sandvika. In the southern part of its outcrop, the formation comprises shales and thin limestones but further north thick nodular limestones commonly containing the calcareous alga Palaeoporella are developed as are horizons rich in the pentameroid brachiopod Holorhynchus. Limestone conglomerates within the Langåra Formation at some localities represent channel fill deposits (Brenchley & Newall 1975,1980).
Fauna and Age. Much of the Langåra Formation contains a low density but high diversity fauna which Brenchley and Cocks (1982) referred to as the Onniella Association. The fauna is dominated by brachiopods but trilobites are present and more diverse than at equivalent levels further east. Molluscs, bryozoans, echinoderms and corals occur amongst the rest of the fauna and as noted above, the alga Palaeoporella is present in the upper part of the formation in its more northerly outcrops. The Onniella Association is interpreted as a mid-shelf equivalent of the deeper partly coeval Tretaspis Association found in the (Husbergøya Formation. A similar relationship within an inner shelf environment was proposed for the Hindella-Cliftonia Association recognised by Brenchley & Cocks in the Langåra Formation at a broadly equivalent level to the Hirnantia Association further east. The uppermost part of the Langåra Formation in the north west contains brachiopod-dominated faunas termed the Holorhynchus Association by Brenchley & Cocks (1982) and interpreted as representing an inner shelf environment. The complex distribution of trace fossil assemblages in the Langåra Formation was described by Stanistreet (1989).
Trilobites (Owen 1980a, 1981; Owens 1973), brachiopods Kiær 1902; Holtedahl 1916; Wright 1965; Boucot et al. 1971; Cocks 1982), ostracods (Henningsmoen 1954), cephalopods (Strand 1933), bivalves (Toni 1975), corals (Neuman 1975) and echinoderms (Bockelie 1984) have been described from the Langåra Formation which is late Rawtheyan to Hirnantian in age.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.