The Caledonides of the Oslo Region, Norway – stratigraphy and structural elements

In geological terms, the Oslo region is a graben structure containing downfaulted fossiliferous, Lower Palaeozoic rocks in a belt 40-70 km in width and extending 115 km north and south of the city of Oslo. Shelly, graptolitic and early vertebrate faunas together with microfaunas and -floras offer a detailed biostratigraphy and time scale for the Caledonide tectonics and associated events. The provided correlation charts reflect a preferred Baltoscandian terminology for the Cambrian and Ordovician successions and a standard British system for the Silurian. Reference to recent biostratigraphic and sedimentological studies allows speculation on changes in sedimentary rates having both global and local causes based on the fact that the Oslo Region occupied an intermediate position between the stable platform to the east and the developing orogen to the west. Sedimentary rates, were high with dominantly mudstones and limestones and local thicknesses up to 1 km in the Ordovician and nearly twice this amount in the Silurian where siliceous rocks in a red-bed facies first appear around the Wenlock-Ludlow boundary. Caledonian tectonics in the Oslo Region activated the Osen-Røa detachment along which the major displacement was to take place. This structure underlies the entire Oslo Region, but dies out to the south in the Skien-Langesund area. In the Oslo Region, the Osen-Røa detachment lies within the late Cambrian Alum Shale and is developed as an intensely deformed thrust plane, from which numerous faults splay up-section into the Ordovician and Silurian strata, forming a duplex structure. Although the strain intensity decreases towards the south and towards the upper part of the Cambro-Silurian section, three major structural levels, in addition to the basal Osen-Røa detachment are identified. These are partly associated with flats of semi-regional significance in the nappe pile. The bulk transport is towards the south-southeast, but areas of southerly (Klekken area) and southeasterly transport (southern Ringerike) are also prominent. The detailed timing of the deformation is not well established, but the first sedimentary response to the growing mountain chain to the northwest is believed to be the fine-grained sandstones and siltstones of the Elnes Formation of late Middle Ordovician (Darriwilian) age, whereas the first siliciclastic sediments of more significant thickness date to the latest Ordovician. Finally, it is evident that the up to 1250 m-thick, Upper Wenlock-Lower Ludlow sandstones of the Ringerike Group have been affected by the contractional deformation, defining a maximum age for the latest Caledonian (Scandian) orogenic movements.

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