MARINE MAGNESITE IN THE NEOPROTEROZOIC? EVIDENCE FROM THE SKILLOGALEE DOLOMITE, ADELAIDE GEOSYNCLINE, AUSTRALIA

Sedimentary magnesite is abundant in the Neoproterozoic Skillogalee Dolomite of the Adelaide Geosyncline, South Australia, where it occurs mainly as clasts in laterally extensive, intraformational conglomerates that comprise up to 30% of the unit. Although the magnesite has been interpreted as an evaporation product, the absence of CaSO₄ minerals that would be expected to form upon seawater evaporation has led previous workers to envisage magnesite formation in alkaline lakes or lagoons fed by continental groundwaters. Here we consider new data from fresh quarry exposures within the context of a Proterozoic ocean characterized by low sulfate and extreme carbonate saturation. Marine C-O isotope signatures, elongation of stromatolites on bedding planes, and considerable evidence of periodic emergence, a lower flow regime origin for most physical sedimentary structures, and reversing flows are best reconciled by an environment in which large expanses of intertidal flats flanked a flat, low-energy marine shelf. Coarse-grained facies are interpreted as the products of sheet deltas that delivered sediment from continental drainage systems to the coast. Their sand-dominated load was supplanted in the coastal realm by intraformational debris eroded from semi-lithified peritidal flats and entrained into high concentration outflows to form conglomerates. Intermixing of magnesite and dolomite intraclasts, microcrystalline textures of clasts, preservation of algal microstructures, and the presence of bedded magnesite mudstones directly beneath conglomerate units suggest that dolomite and magnesite formation was penecontemporaneous with deposition. In this evaporative environment, progressive dolomite precipitation would utilize Ca²⁺ and thereby increase the fluid Mg²⁺/Ca²⁺ ratio. In a system rich in HCO^3- but SO₄^2--poor, as envisaged for the Proterozoic oceans, further evaporative concentration of seawater would result in magnesite precipitation. By contrast, increased marine sulfate availability in the Phanerozoic oceans creates conditions in which significant magnesite deposition is relatively rare and occurs only under unique circumstances.