Neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites

Neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large Cryogenian glaciations. Several thin dolomite units that have features resembling cap dolomites occur within the Cryogenian-Ediacaran succession of the Adelaide Geosyncline. While each of these dolomite units has distinctive features, they also share many similarities with cap dolomites including: stratigraphic thickness ranging from several centimetres to several metres; impure dolomicrite composition; graded beds with a sharp top that become more dolomite-rich upwards; and evidence of volume expansion in the form of sheet cracks, tepees or other soft-sediment deformation structures. The carbon isotopic compositions for these thin dolomite horizons have values ranging from similar to -5 to +2 per mil VPDB, which overlap those commonly found in both Sturtian and Marinoan cap dolomites. The generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate. Sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground, where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation. Dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds. Volume expansion structures are a product of this near-surface dolomicrite precipitation, brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited (marine) carbonate source for the precipitating dolomite. We suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of Cryogenian- early Ediacaran oceans and caused by alkalinity-producing anoxic bacterial reactions. When sedimentation rates slow sufficiently (e.g. during transgressions), thin dolomicrite horizons are produced. In this hypothesis, cap dolomites may largely be the result of sediment starvation, combined with a possible increase in carbonate saturation during and following the post-glacial transgressions associated with the Sturtian and Marinoan glacial events. This scenario explains most of the sedimentological features of these unusual carbonates. The widespread development of such authigenic carbonate would have profound implications for the interpretation of Precambrian marine carbon isotopic compositions.