Abstract
Carbonate-rich glacial deposits from two discrete Vendian glacial periods are described. The older is represented by the 24–40 m thick Petrovbreen Member (E2) of the Elbobreen Formation which contains abundant detrital dolomite. Clasts in E2 and their possible source rocks have positive δ13C and negative δ18OPDB values. In contrast the carbonate mud-fraction of E2 sediments has different cathodoluminescence characteristics from clasts, slightly negative δ13C values, and higher Fe and Mn concentrations than clasts. Oxygen isotopes vary from −2.5 to +4.5‰PDB, thought to be related to various seawater–meltwater mixtures in the depositional environment. Preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy (Ostwald's ripening) and coinciding with sulphate reduction.

The younger glaciation is represented by the Wilsonbreen Formation (170 m) which has a distinctive glaciolacustrine Middle Carbonate Member (W2). W2 contains precipitated periglacial carbonates (with high Mn/Fe ratio): both limestone (rhythmitic and stromatolitic) and dolostone (rhythmites, stromatolites and dolomite-rich sandstones) which are compared with carbonates in modern Antarctic lakes. Evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values (up to + 10.5‰PDB), high Na concentration and evidence for dissolved evaporites. High Mn concentration in detrital dolostones in W2 is suggestive of syn-sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones.

The carbonate–tillite association ultimately arises from the erosion of underlying carbonates which originated (in this case) under radically different climatic conditions. Glacial depositional waters then became carbonate-saturated as a result of dissolution of detrital carbonate. Massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis. In glacial lakes carbonate precipitated in response to evaporation or photosynthesis. Carbonate precipitation as the result of seawater freezing, or in warm interglacial conditions, is not yet established. Oxygen isotope value are inconsistent (too heavy) with the presence of high-latitude meltwaters, implying that glaciation extended to low latitudes as proposed by Harland.