A Global Stratotype Section and Point (gssp) for the Cryogenian System

The base of the Cryogenian Period is proposed to be defined at a Global Stratotype Section and Point (GSSP) on the island of Garbh Eileach, part of the Garvellach Island chain of the Inner Hebrides of Scotland, UK. The proposed primary marker lies at the first sedimentological evidence for cold conditions, coincident with an inflection in a major negative carbon-isotope excursion (the Garvellach anomaly), where δ¹³C values begin to recover toward 0‰. This horizon underlies the basal diamictite of the Port Askaig Formation, which marks the first unequivocal record of glaciation in the succession. The Garbh Eileach section is conformable and exceptionally well exposed, preserving a rare and continuous transition from pre-glacial carbonate sedimentation into glaciogenic deposition at low palaeolatitudes. Detrital-zircon U–Pb geochronology combined with global chemostratigraphic correlation constrains the marker horizon to approximately 717 Ma, consistent with independent radiometric estimates for the onset of Sturtian glaciation. The site satisfies all principal criteria for GSSP designation, including stratigraphic completeness, global correlation potential, accessibility, and long-term protection.