Towards linking long-term denudation and modern fluvial dynamics in the Pilbara, Western Australia

The Pilbara is one of the oldest subaerially exposed surfaces in Australia, and has been slowly eroding since at least the late Palaeozoic. The region is also characterized by extremely iron-rich Proterozoic bedrock geology which represents an enormous economic interest for Australia. Headwater river channels within the Fortescue Catchment in the Pilbara are under increasing anthropogenic pressure from contemporary mining practices, increasing intensity of land-use and fire. In addition, rivers are also subject to anthropogenic diversions around operating mine sites, even though the role that these catchments play in producing, transferring and supplying sediments to downstream reaches is currently not well understood. While increasing effort has been concentrated on understanding the modern processes and fluvial geomorphology of these first and second order rivers, our knowledge regarding the long-term sediment production, denudation rates and their implications for river dynamics is extremely limited.

This paper presents one of the first systematic dataset on long-term landscape evolution in the Pilbara based on analysis of cosmogenic nuclides. This project determines and compares long-term catchment-wide denudation rates among key sub-catchments in the Upper Fortescue. Detailed mapping and quantification of modern catchment morphometric properties and lithology have been achieved via rigorous GIS analysis. We use the multi cosmogenic nuclide approach (with 10Be and 26Al) to examine variations in long-term denudation rates and then assess first order controls on catchment scale denudation and late quaternary landscape evolution in the Pilbara. Specifically, rates of erosion, exposure ages and sediment flux were examined from headwater channels draining the Hamersley Ranges, active watercourses, outwash alluvial fans draining into the Fortescue Marsh. Quantifying these geomorphic parameters at longer timescales (10s to 100s of thousand years) will improve our Quaternary interpretation of this environment. A catchment wide, source to sink investigation will enhance our understanding of the late Quaternary landscape evolution in the Pilbara, which will provide a guide to mining rehabilitation and the future trajectory of this landscape under a changing climate.