Sedimentology and evidence for early Archaean life in the North Pole Chert-Barite unit, East Pilbara, Western Australia

This thesis provides a retrospective account of research into the sedimentology of the chert-barite unit at North Pole in the East Pilbara of Western Australia. It touches on the evolution of life, the oxygenation state of the hydrosphere, and the early development of greenstone belts. The similarity between the Onverwacht Group of South Africa and the Warrawoona Group is highlighted, suggesting that they developed in a similar manner.

Sedimentological studies of the c. 3,500 Myr old Warrawoona Group in the eastern Pilbara Block, Western Australia indicate evidence for deposition in a shallow water environment. At North Pole, silicification has preserved structures that are interpreted as slightly reworked sands derived from spalled mafic-ultramafic pillow lava fragments, carbonate muds, volcanic ash muds and baritised gypsum evaporites, all deposited in shallow basins. Primitive microorganisms lived in the shallow water and built stromatolites and microbial mats that are well preserved due to silicification of originally fragile organic structures. Microbes probably oxidised juvenile sulphur by photosynthesis. Sulphur isotope ratios from barite are consistent with evaporation from seawater sulphate of primitive isotopic composition. Thus, biogenic processes were already filling oxygen sinks at this time. The greenstone belts of the Pilbara are interpreted as having been laid down in shallow water basins formed between rising granitoid diapirs that controlled both late volcanism and sedimentation.