Definition of ‘Archaean’ — comment and a proposal on the recommendations of the International Subcommission on Precambrian Stratigraphy

The proposed definition of the Archaean-Proterozoic boundary at 2500 Ma (Sims, 1980) is unsatisfactory as it is difficult to use, contravenes the international stratigraphic guide and does not allow for revisions of isotopic decay constants. Instead it is proposed that the boundary be set at the cooling age of the Great Dyke of Zimbabwe (2.5 Ga), which provides an equivalent definition in the rock. The division of the Archaean at 2.9 Ga and 3.5 Ga (Sims, 1980) is also unsatisfactory and instead reference successions are proposed in the Rhodesian craton, the Pilbara block in Australia and in Greenland, to serve as possible future stratotypes for Archaean eras.