The Dalradian Supergroup of largely clastic metasediments in Scotland and Ireland is a well-known record of Neoproterozoic to Cambrian sedimentation on the Laurentian margin. Its apparent long duration, from ∼ 800 to ∼ 500 Ma, has encouraged a search for tectonic breaks and unconformities within the sequence. One suggestion is that a break in sedimentation occurred within the upper part of the Argyll Group, specifically in the Easdale Subgroup. Here we report dates for demonstrably synsedimentary barite mineralization hosted by the Easdale Subgroup Ben Eagach Schist Formation. Bedded pyrite from the Foss and the Ben Eagach–Duntanlich barite deposits, stratigraphically < 100 m apart, yields Re-Os ages of 604.0 ± 7.2 Ma and 612.1 ± 18.6 Ma respectively with two-sigma overlap. These ages are similar to the previously constrained 601 ± 4 Ma Tayvallich Volcanic Formation lying 2–5 km stratigraphically above. The new ages suggest continuous sedimentation through this period and, in combination with existing time markers, increase the likelihood that the basal-Argyll, Port Askaig Formation diamictite represents the Marinoan glaciation rather than the Sturtian glacial episode which may not be represented in the Dalradian sequence. As such, we urge caution in using the Dalradian in global compilations of Neoproterozoic history until further geochronological constraints are obtained for the Cryogenian-Tonian parts of the stratigraphy, which is now a high priority.
Bibliographical noteFunding Information:
We thank Owen Dickinson and Ian Hughes of M-I Drilling Fluids UK Ltd. and Dr Rachael Bullock for assistance with sample collection. DS acknowledges technical support from Geoff Nowell, Antonia Hoffman and Chris Ottley. NM thanks the University of Brighton School of Applied Sciences for technical support and part-funding of the Duntanlich Re-Os analyses, and the Scottish Universities Environmental Research Centre (SUERC) for sulfur isotope analyses. The Editor and anonymous reviewers are thanked for their many suggestions that helped to improve the manuscript.
© 2022 The Author(s)
- Dalradian Supergrou
- Marinoan and Sturtian panglaciations
- Global Boundary Stratotype Section and Point for the Tonian-Cryogenian boundary
- Re-Os geochronology
- Aberfeldy stratiform barite deposits