The Late Jurassic Bjorøy Formation: A provenance indicator for offshore sediments derived from SW Norway as based on single zircon (SIMS) data
The Late Jurassic Bjorøy Formation is located in a fracture zone in granodioritic gneisses of the Øygarden Complex in the Bergen area, SW Norway. An ion-microprobe (SIMS) U-Pb zircon study has been performed on a sand sample and the granitic basement, and the data are integrated with Nd and Sr whole rock isotopes in order to identify the provenance components to the Bjorøy Fm. The detrital zircons reflect a source area that was dominated by rocks with zircon ages in Caledonian (450 and 520 Ma), Sveconorwegian (900 to 1 0 1 0 Ma) and mid-Proterozoic (ca. 1450 Ma) times. One Archaean zircon at ca. 2700 Ma is recorded. The sand sample has a relatively low Nd model age of 0.5 Ga, which can be connected to an important sediment source in Caledonian rocks of intermediate and mafic compositions. These were probably derived from units related to the Min or and Major Bergen Arcs. Thus, rocks of the Upper Allochthon of the Caledonides were probably important in the sediment source area together with Proterozoic gneisses and late-Proterozoic platform sediments, which appear as tectonically intercalated slivers in the upper allochthonous units. Ordovician sediments in the provenance may explain the elevated time-corrected Sr isotope composition of the Bjorøy Fm, but it cannot be excluded that this signature is a result of interaction with seawater and/or crustal fluids during and after sediment deposition. The Bjorøy sand gives a zircon age signature of SW Norway in Mesozoic times. This is important since offshore sands with distinctly different zircon age distribution have previously been connected to a source in SW Norway.
t.l.knudsen @toyen. uio. no