Geological investigations for the ROGFAST project in the area between Kvitsøy and Bokn
The Geological Survey of Norway (NGU) has previously been in involved in two reports describing the geological conditions along the trace of the ROGFAST tunnel. There it was concluded that the geological information for the area between Kvitsøy and Bokn was too scattered to give a reliable interpretation of the geology. New information in the form of 8 samples from the seafloor collected by ROV, and three additional sub-sea drill cores reaching down to the level of the tunnel has made it possible to revise the geological model : The area close to Kvitsøy is occupied by black shale and greenstone as could be expected according to the drill core from Sauholmen. The deepest part of the tunnel transect consists of a granite that most likely is a part of the Precambrian basement. The boundary to the shale and greenstone on Kvitsøy could either be a thrust, or it could be a fault. Three samples of phyllite or mica schist in the northern part of the tunnel transect probably belong to the Ryfylke schist and are interpreted as a thin cover on top of the Precambrian gneisses. This interpretation implies that most of the planned ROGFAST tunnel north of Kvitsøy will run in crystalline rocks of granitic composition. In the report other possible models are also briefly presented.
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