The δ13Ccarb and 87Sr/86Sr secular variations in Neoproteozoic seawater have been used for the purpose of ‘isotope stratigraphy’ but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for ‘blind dating’. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local δ13Ccarb fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative δ13Ccarb excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and ‘blind dating’. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590–544 Myr interval, and two age‐groups at 660–610 and 740–690 Myr can be resolved.

Rb–Sr dating of clay subfractions from three sedimentary formations of Late Riphean to Vendian age from Varanger Peninsula, northern Norway, has helped to refine the geochronology of the Late Neoproterozoic–Cambrian time interval in this region. Eighteen subfractions in six narrow size-ranges from 1–2 μm to <0.1 μm were separated from shales of the Stangenes, Nyborg and Stappogiedde formations. The coarser subfractions include some 2M1 illite as well as the 1M polymorph. The finer subfractions contain only the 1Md illite. After leaching with ammonium acetate, Rb–Sr ‘leachochrons’ for the untreated subfraction, leachate and residue were derived for all subfractions. As the clay particle size decreases from 1–2 to 0.1–0.2 μm: (1) the proportion of chlorite, where present, is reduced; (2) the Crystallinity Index Standard (CIS) increases; (3) the 87Rb/86SR ratio in the residues increases; and (4) the apparent Rb–Sr age decreases. In addition, the data points of the residues are aligned linearly in both the 87Rb/86Sr–87Sr/86Sr and the 1/Sr–87Sr/86Sr diagrams. A similar pattern in the 87Rb/86Sr–87Sr/86Sr coordinates is observed for the data points of leachates. This suggests that mixtures of at least two, non-cogenetic, illite generations are present in all the shales, and that these illites crystallized in environments with dissimilar 87Sr/86Sr ratios. The minimum Rb–Sr ages of early burial diagenesis are c. 650 Ma for the Stangenes, and 560–530 Ma for the Nyborg and Stappogiedde formations. These results indicate that: (1) the age of the Riphean–Vendian boundary is <630 Ma; (2) the age of the Varangerian glaciation on the Varanger Peninsula is bracketed between 630 and 560 Ma; (3) a c. 560 Ma burial diagenesis age for the intra-tillite Nyborg Formation and post-tillite Stappogiedde Formation may result from subsidence and sedimentation associated with the Late Vendian Timanian (Baikalian) deformation in adjacent areas; and (4) the ages of the finer, authigenic illite subfractions range from 440 to 390 Ma and thus appear to reflect phases of Scandian deformation and uplift.

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