Detrital zircon geochronology and provenance in the Cambro-Ordovician sedimentary succession of the COSC-2 drillhole

The COSC-2 drilling project recovered ~2276 m of continuous core through the thin-skinned thrust system of the Caledonian Orogen in Sweden. We present a detrital zircon geochronological study of its Palaeozoic sedimentary section, which comprises a continuous Lower Cambrian(?) through Lower Ordovician(?) autochtonous to parautochtonous succession, marked by variable detritus sources along the profile.

The >350 m-thick Lower Cambrian portion was initially sourced locally from the Transcandinavian Igneous Belt, with a gradual shift toward a Sveconorwegian provenance. The upper part of the autochthonous sequence, just below the Alum Shale Formation, is marked by a flux of late Neoproterozoic-Cambrian grains. The maximum depositional age of the uppermost Lower Cambrian strata is estimated at 530.5 ± 4 million years. Despite tectonic overprint in its shaly middle part, the Alum Shale Formation remains continuous, encompassing a complete Middle Cambrian to Lower Ordovician sequence. Above, in the parautochthonous succession, detritus again reflects Sveconorwegian sources, resembling the provenance of Ordovician greywackes in the Lower Allochthon.

Comparison of detrital zircon spectra from the Lower to Middle Cambrian strata across Baltica shows a consistent influx of late Neoproterozoic-Cambrian detritus. Statistical analyses indicate that in Central Scandinavia, the likely source of this zircon population is the southern Santacrucian Arc. This flux reflects arc magmatism tied to subduction of the Mirovoi Ocean beneath Baltica.

Acknowledgements: We thank the COSC scientific drilling teams led by Henning Lorenz. The National Science Centre, Poland funded this research (projects 2018/29/B/ST10/02315 and 2019/33/B/ST10/01728) and the Deutsche Forschungsgemeinschaft supported the COSC-2 studies by OL (Le 867/13-2).