Evidence for Proterozoic primary CaCO3 precipitation from the McArthur Group of northern Australia

Radiating fans of acicular 'Coxco needles' are the characteristic feature of the Coxco Dolomite Member (≈1640Ma), a subunit of the Teena Dolomite within the Palaeoproterozoic McArthur Group. In the past, 'Coxco needles' have been variably interpreted as dolomitic pseudomorphs after aragonite, gypsum or trona. More recently, gypsum and an emergent brine pool depositional setting has been the favoured interpretation. New work reported here has found that crystal morphology, geochemistry, petrographical and sedimentological relationships are more consistent with a subaqueously deposited aragonitic precursor. The widespread occurrence of 'Coxco needles' at a confined stratigraphic interval is thought to be a function of a subtle change in the HCO₃^– concentration within the water body during deposition of the Teena Dolomite. Although elevated atmospheric CO₂ during the Proterozoic supports increased carbonate precipitation (including aragonitic fans), it can not satisfactorily explain the apparently synchronous precipitation of Coxco fans. Therefore, changes in the bathymetry of the McArthur Basin coincident with deposition of a broadly transgressive sequence is inferred to have triggered the widespread chronostratigraphic precipitation of carbonate (i.e aragonite fans) within a variety of lithofacies.