DEVELOPMENT, SEDIMENTATION AND ECONOMIC-POTENTIAL OF PALAEORIVER SYSTEMS OF THE YILGARN CRATON OF WESTERN-AUSTRALIA

The Yilgarn Craton, covering an area of 720,000 km2 is composed of Archaean rocks, dominantly granitoids, with narrow northerly-trending fold belts of older rocks. The surface of the northern part of the craton has been eroded to that approaching the nearly level Lower Proterozoic depositional surface. A well preserved palaeodrainage is present on the craton and forms part of the extensive palaeodrainage system of the Australian continent. Several interacting factors have shaped the palaeodrainage system and these include, initially, the land form prior to the break-up of Gondwana shaping the initial river systems, the break-up of Gondwana from Cretaceous to Early Cainozoic, the migration of the craton northward and a number of distinct climate cycles caused by global events independent of plate movement. The existing palaeochannels were incised during an Early Eocene erosion event. Sedimentary sequences contained in the palaeochannels relate to a series of depositional and erosional cycles from Late Eocene to present day. Conditions of peneplanation have persisted throughout the Cainozoic, and also probably from much earlier times, with the resultant preservation of a regolith of great antiquity. Climatic change since deposition has affected the chemical reactions between palaeochannel sediments and groundwater systems. The palaeoclimates include more extreme conditions, both arid and pluvial, than those at present, and have resulted in considerable chemical modification of the sedimentary sequence. The palaeochannels of the northern Yilgarn Craton have developed isolated but similar hydrochemical cells as a response to local rainfall recharge in a semi-arid climate. The palaeochannels in the southern parts of the craton, in more temperate conditions, form single hydrogeological systems along their linear flow paths. This hydrogeological style separation between northern and southern parts of the craton seems to have also existed in palaeoclimates. Valuable mineral deposits have resulted from hydrochemical reaction with the sediments and these include gold, uranium, alunite and gypsum. The sedimentary sequence also contains valuable water resources and potentially valuable lignite and limestone (calcrete) deposits.