A short introduction to the geology around the inner part of the Oslo fjord:
The city of Oslo is located in a geologically interesting area in the middle of the Permian Oslo Grabensurrounded by Precambrian basement. Within the city and around the Oslofjord we find well exposed Permian igneous rocks and a down-faulted Lower Palaeozoic sequencepreserved from erosion by the graben structure. The lower Palaeozoic marine shales and limestonesform the low ground in the city centre and in Bærum and Asker to the SW while the Permianigneous rocks make up the high ground to the north and west. The landscape is also strongly influenced by glacial erosion and we find lakes dammed by terminal moraines to the north of the city.Even in the central part of the City there are many good geological exposures. The geology of the city of Oslo and its surrounding includea rather unique variety of rock types: 1)Precambrian basement rocks (900-1000 million yearsor older). 2)A Lower Palaeozoic marine sedimentary sequence from Middle Cambrian to late Silurian overlain by a late Silurian to lowermost Devonian sandstones deposited in a foreland Basin during Caledonian folding. 3)A relatively thin sequence of continental and partly also marine Late Carboniferous sediments unconformably overlyingthe folded Cambro-Silurian sediments. 4)Uppermost Carboniferous and Permian lavaes, intrusives and magmatic rocks. 5)Quaternary deposits deposited during and shortly after the retreat of the last glaciation includingterminal moraines and glacial-marine sediments. The maximum marine level in Oslo after the ice retreated was 226m. The Lower Palaeozoic sequence starts with the Middle Cambrian transgression covering much ofthe Precambrian Baltic shield. A shallow epicontinental sea existed on this stable cratonuntil the late Silurian Caledonian Orogeny. Land areas which could provide asourceof clastic sedimentson the Baltic Shield must have been rather small and thesequence was deposited during low sedimentation rates only a few meters per million years. The sediments are highly fossiliferousand of low metamorphic grade except where contact metamorphosed. The LowerPalaeozoicsequence was divided into 9 stages by Kjerulf and Brøggerrepresenting partly biostratigraphic,partly lithological units. Later more formallithostratigraphic units have been introduced (Worsley et al 1984, Owen et al 1990).During the Upper Cambrian and the earliest Ordovicianblack organic rich mud was deposited when the bottom conditions werestagnant. The Alum shalewas a good source rocks which matured during the Caledonian folding similar to other foreland type petroleum basins in the Middle East or east of the Rocky Mountains.The Ordovician sequence consists of a very typical sequence of shales and limestones.Much of the sequence consists of nodular limestoneswhich are limestone concretions in a matrix of calcareous (marly) shales.The regular alterations of thin limestone beds and shales may represent regular orbital (Milancovitch) climatic variations. |