Northward sediment transport from the late Carboniferous Variscan Mountains: zircon evidence from the Oslo Rift, Norway

Proto-rift, late Carboniferous fluvial and deltaic sandstones from the Asker Group are locally preserved in the upper Palaeozoic Oslo Rift. High-precision U-Pb ages of single detrital zircon grains from a Moscovian (late Westphalian to early Stephanian) sandstone and overlying tuffaceous sediments, that predated initial basaltic volcanism, have been used to study the provenance of the detritus and to evaluate the tectonic significance of the deposits. We find late Neoproterozoic (of Avalonian affinity), Cambro-Ordovician and early Carboniferous detrital zircons requiring that one of the source regions was located along, or south of, the axis between southern England and northeastern Germany and implying sediment transport from the south, probably as a consequence of uplift during the Asturic compressional phase of the Variscan Orogeny. Other age groups reflect typical Baltic sources, possibly from the Ringkobing-Fyn Nigh. Detrital zircons (and monazite) from underlying Silurian sandstone (Wenlock Ludlow) yield Svecofennian to Sveconorwegian as well as late Ordovician ages but lack late Neopotero-zoic (Avalonian) ages. It is thus unlikely that the bulk of the Carboniferous detritus could have been derived by recycling of preexisting Silurian sandstones. These data imply that the rivers feeding the Asker Group originated in the Variscan mountains and entered the Oslo proto-rift from the south, possibly via the Horn Graben or along the Sorgenfrei Tornquist Zone.