U-Pb zircon age for a tuff in the Campbell Group, Griqualand West Sequence, South Africa

An ion-microprobe U-Pb age of 2552+ or -11 Ma has been obtained on zircon separated from a regional banded-tuff horizon in the Nauga Formation (Beukes, 1980b) (upper Campbell Group, Griqualand West Sequence, South Africa). This age permits time constraints to be placed on lithologically correlated units within the adjacent Transvaal Sequence and correlations to be made with the further removed Hamersley Group in Australia. Calculated rock accumulation rates of 2 to 4 m/m.y. for these strata of mainly shale and banded iron formation suggest that sedimentation rates were significantly slower in the late Archean-Early Proterozoic than is generally assumed.