The formation of fluvio-lacustrine ferruginous pisoliths in the extensive palaeochannels of the Yilgarn Craton, Western Australia

Iron pisoliths and nodules are common within the fluvio-lacustrine kaolinitic clays of Tertiary paleochannels developed in the Yilgarn Craton of southern Western Australia. Deposition within these channels began in the Late Eocene with sediments comprised a fining upward sequence, from sands at the base, to lacustrine clay deposits above. Iron pisoliths are found randomly distributed in the clays in deposits more than 300 km apart. The pisoliths are composed of goethite and hematite and minor quartz and clays, they are 2 to 30 mm in diameter and formed of an inner nucleus surrounded by up to 200 iron oxide laminations. There is no evidence for transportation of pisoliths into the channels but the internal fragmented nature of some of the pisoliths suggests some movement has occurred and multiple generations of pisoliths have developed. The interface between river water and sediment was a redox boundary between reduced slightly acidic sediments and oxidised river water. Pisoliths formed on, or just beneath the surface of the sediment by redox reactions likely mediated by microorganisms. Pisoliths have undergone cycles of exposure, growth and movement linked to the height and course of the palaeoriver system.