Detrital zircon geochronology of lower Palaeozoic sedimentary rocks from the COSC-2 borehole, Scandinavian Caledonides

Detrital zircon geochronology is reported from the c. 1200 m thick Cambro-Ordovician sedimentary succession recovered in core from the COSC-2 continental drilling project in the Scandinavian Caledonides. Above a regolith marking the sub-Cambrian peneplain, a lower to middle Cambrian(?) succession comprises conglomerate, sandstone and shale overlain by gravity flows fining upwards into the Alum Shale Formation. First results of detrital zircon geochronology from the Cambrian(?) succession show that the basal section of the autochthonous cover is characterized by mainly late Paleoproterozoic – early Mesoproterozoic detrital grains. The middle part of the succession is dominated by late Paleoproterozoic detritus with minor Mesoproterozoic and Archean input. The upper part of lower Cambrian(?) succession is characterized by Archean to Cambrian detritus. The maximum depositional age is calculated to 530.5 ± 4 Ma for the upper part of the lower Cambrian succession. Two samples from the Lower Ordovician(?) succession above the Alum Shale Formation show predominantly Mesoproterozoic to early Neoproterozoic (1.5–0.9 Ga) ages.

The autochthonous lower Cambrian(?) passive margin succession in the lower section is dominated by local detritus, sourced exclusively from the Eastern Segment of the Sveconorwegian Orogen which includes the basement studied in COSC-2. Up-section, the provenance shifts towards the Transscandinavian Igneous Belt and Svecofennian Orogen sources, with the youngest part of the succession showing a notable input of Neoproterozoic –Cambrian active margin detritus. The Ordovician(?) succession is characterized by populations, likely derived from the Sveconorwegian Orogen, and a minor cratonic contribution.

Statistical analysis of detrital zircon datasets across Baltica suggests that the Southern Baltica/Sandomirian Arc, rather than the Timanian Orogen, was a significant source of detrital material across the paleocontinent. The influence of Timanian Orogen grains is limited to northernmost Scandinavia, whereas Sandomirian detritus reached central Scandinavia in the lower to middle Cambrian and remained prevalent in southern Scandinavia into the Lower Ordovician.