INTRODUCTION The Pilbara region is situated north of the Tropic of Capricorn, in the arid North-West of Western Australia. It covers an area of 178,000 km2. The region is bordered by the Indian Ocean to the west, Ashburton River to the south, Little Sandy Desert to the east, and Great Sandy Desert to the north (fig. 1). The Pilbara is a part of the Western Shield that has emerged since the Precambrian (600 Mya - 400 Mya). The Western Shield is the oldest known land surface in the world (Fox, 1999), and next to the Pilbara Craton, it includes Yilgarn Craton to the south (Beard, 1998). The Pilbara Craton lies between two sedimentary basins: the Canning to the northeast, and Carnavon to the southwest (Beard, 1975). It consists of the Proterozoic and Archaean rocks (fig. 2A), the latter constituting a block known as the Pilbara Block, being overlaid by Proterozoic deposits, which are exposed in the northeastern part of the region. Lower Proterozoic rocks are 9000 m thick and mainly consist of basalt, shale, dolomite, and jaspilite. Middle Proterozoic rocks are composed mainly of sandstone and conglomerate. The underlying Archaean basement consists of essentially the same type of rocks as the Lower Proterozoic, but these are much more folded, and in some parts metamorphosed and intruded by masses of granite (Beard, 1975). The regolith in the Pilbara comprises a fine red blanket over much of the region (fig. 2B). Minor Tertiary limestone outcrops occur across the plain (Reeves et aI., in press). The highest point in the Pilbara is Mount Meharry (1250 m), which is also the highest peak in Western Australia. Mount Meharry is a part of the Hamersley Range, which lies in the central Pilbara. Another plateau, the Chichester Range, lies north of the Hamersley and has a peak of 618 m. The rest of the Pilbara consists of wide floodplains and a coastal zone. The Pilbara encompasses five hydrographic basins: the Ashburton, De Grey, Fortescue, Port Hedland, and Onslow basins (Eberhard et aI., 2005). The Hamers1ey Range is drained by the Ashburton and Fortescue rivers, while the Chichester Plateau is drained by numerous rivers in the De Grey and Port Hedland hydrographic basins. The drainages of most rivers in the region were formed during the Late Cretaceous to Early Tertiary. They are well preserved because of the tectonic stability and because a change from a humid to an arid .j:>. o~ S I <: -§ ~ i o·~ N + "OO~ Fig. 1. Map of the Pilbara region with the sampling localities. .