Identifying late Neoproterozoic glaciation in the Scottish Dalradian using stable isotope techniques
Following the identification of a third Neoproterozoic glacial episode in the Irish Dalradian, and preliminary isotopic data suggesting a negative d13Ccarb anomaly in the Scottish Easdale Subgroup, new d13Ccarb, d13Corg, d34Ssulphide, 87Sr/86Sr and geochemical data are presented here for a section from the Easdale Slate through to the Shira Limestone. This section preserves a negative excursion in both carbonate and organic carbon isotopic data within the Craignish Phyllite, falling to a minimum d13Ccarb of c.-10‰. Values lighten over ~200m of section, and persist for up to 1.5km, before returning to values of 0‰-2‰ in the Shira Limestone. A potential ‘cap carbonate’ is identified within the phyllites, with characteristic recovery of d13Ccarb values. d34Ssulphide rises steadily from c.8‰ to c.20‰ in the Degnish Limestone, prior to the d13C excursion; data is unavailable for the section preserving the excursion. 87Sr/86Sr data is comparable with previously published data for the Easdale Subgroup, with least altered values of 0.70965 for the Degnish Limestone. Major and trace element data suggest oxidising conditions at the sea floor during sedimentation and do not support the idea that deposition within a restricted basin generated the observed excursion. While direct evidence of glaciation is lacking within the section, the identification of this excursion and its possible correlation with the Marinoan glaciation lends support to the hypothesis that the Dalradian preserves all three major Neoproterozoic glaciations in the Port Askaig Tillite (Sturtian), Stralinchy Conglomerate-Craignish Phyllite (Marinoan) and Inishowen-Loch na Cille boulder beds (Gaskiers). This correlation relies on the correlation of the studied section with the glaciogenic horizon and isotopic data observed within equivalent stratigraphy in Ireland.
Royal Holloway, University of London
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