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Two-stage erosion and deposition in a continental margin setting: an ⁴⁰Ar/³⁹Ar laserprobe study of offshore detrital white micas in the Norwegian Sea

¹ The BAT Project, The Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 – Trondheim, Norway.
² Present address: Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK (e-mail: s.sherlock@open.ac.uk)

Detrital white micas from a restricted region of the Norwegian Sea have been analysed by the ⁴⁰Ar/³⁹Ar laserprobe. Examining the ages of individual white mica grains from offshore Triassic, Jurassic and Cretaceous sandstones reveals a narrow range of detrital white mica ages (369–424 Ma). Moreover, there is an inverse relationship between sediment age and detrital white mica age, with the Triassic sands containing a larger proportion of younger white micas, and the Cretaceous sands containing the oldest white micas. In addition, residence times of the detritus, which is the age of the sediment subtracted from the age of the detrital white mica, range from c. 150 to c. 300 Ma (for the Triassic and Cretaceous sands respectively). The inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion-deposition model involving at least two stages. Taking into account previous provenance studies in the Lofoten region of the Norwegian Sea, these new data suggest that detritus shed initially from the exhuming Caledonian nappes of NW Norway in late Silurian-early Devonian times was deposited in intramontane basins that were themselves, subsequently unroofed and re-eroded from Permo-Triassic times. This caused the recycling of the Caledonian detritus from the Devono-Carboniferous basins and led to the inverted detrital age distribution ovserved in the Triassic-Cretaceous sediments examined in this study. While sedimentary recycling is not a surprising feature for Mesozoic basins in the North and Norwegian Seas, this study documents a previously untapped method, ⁴⁰Ar/³⁹Ar laserprobe provenance studies, by which to identify its effect and extent.

Keywords: Ar/Ar, provenance, erosion, deposition, continental shelf.

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