EVIDENCE FOR THE TRANSITION TO AN OXYGEN-RICH ATMOSPHERE DURING THE EVOLUTION OF RED BEDS IN THE LOWER PROTEROZOIC SEQUENCES OF SOUTHERN AFRICA

Strata > 2.5 to less-than-or-equal-to 1.8 Ga in southern Africa appear to span the transition from a non-oxidizing atmosphere to one characterized by free oxygen. Red beds in 2.3 to 2.1 Ga unconformity-bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks, ironstones, or banded iron formations, which evidently were sources of iron for the matrices. However, red beds in less-than-or-equal-to 1.9 Ga unconformity-bounded sequences exhibit not only matrix-staining but also hematitic coatings on grains; these beds also have far fewer interstratified ferruginous lithologies. Because red beds with stained clay matrices, banded iron formations, pyritic (Witwatersrand-type) conglomerates, and iron-depleted paleosols all occur in the approximately 2.2 Ga Pretoria Group, formation of these lithologies may not have been time-dependent until atmospheric oxygen approached 10(-2)atm. By 1.9 Ga atmospheric oxygen was greater-than-or-equal-to 10(-2)atm, causing lateritic weathering and red beds with hematitic coatings on grains.