PISOLITIC IRONSTONE AND FERRICRETE IN THE 2.22 - 2.4 GA TIMEBALL HILL FORMATION, TRANSVAAL SUPERGROUP: IMPLICATIONS FOR THE HISTORY OF ATMOSPHERIC OXYGEN

The Timeball Hill Formation near base of the Pretoria Group (Transvaal Supergroup) hosts giant deposits of hematite-rich oolitic ironstone. The deposits are preserved over an area of ~ 100 000 km2 in beds 0.5 – 3.5m thick containing up to 45 wt. % Fe. The ironstones appear to cap coarsening-upward shale-quartzite parasequences in two deltaic lobes with sediment transport from the north. Three major facies types are present, namely a) oolitic ironstone capping delta front sands, b) sharp-based fining-upward quartzitic ironstone in delta distributary channels and c) pisolitic ferricrete along an old exposure surface. Pisolitic mudclast conglomerates are present in some of the channel deposits.

Oolites are composed of very finely laminated cortices of hematite and Fe-chlorite around cores of clay pellets composed of intergrown hematite and chlorite. Hematite-coated quartz grains are abundant in quartzitic ironstone. Pisolitic mudclast conglomerate is composed of Al-rich mudclasts (some of them coated by hematite), in-situ and reworked simple to composite hematite-coated pisolites, hematite-rich oolites and coated quartz grains in a hematite-rich matrix. The ferricrete bed which caps the ironstone succession locally, is composed of Al-rich mudclasts, clay pisolites and quartz grains cemented by hematite.

The Al-rich nature of clay clasts and the presence of reworked hematite and clay coated pisolites suggest lateritic weathering conditions in the source area. The size of the deposits are comparable to that of the largest known Phanerozoic ironstones. It is suggested that such large concentrations of terrestrial to deltaic hematite-rich ironstones could only have formed under an oxygenated atmosphere at 2.22 – 2.4Ga.