A multi-cosmogenic nuclide approach to assess sediment provenance and long-term denudation in the ancient Pilbara region, Western Australia
Ms Alissa Flatley1, Dr Jan-Hendrik May1, Dr Toshiyuki Fujioka2, Dr David Fink3, Dr Klaus Wilcken3, Professor Ian Rutherfurd1
1School of Geography, Earth and Atmospheric Sciences, University of Melbourne, Melbourne, Australia, 2Centro Nacional de Investigación Sobre la Evolución Humana (CENIEH) , Burgos, Spain, 3Australian Nuclear Science and Technology Organisation, Sydney, Australia
Detailed analysis of catchment morphometric properties and lithology have been combined with multi cosmogenic nuclide (10Be and 26Al) measurements to provide much needed long-term denudation rates for the region, in addition to improving our understanding of first-order controls on catchment wide erosion. Our results indicate that erosion rates within the Upper Fortescue catchments are between 0.94-4.04 m/Ma. This value is similar to channel bedrock erosion rates (2.5 ± 0.8 m/Ma; n = 4), but somewhat higher than mesa summit or alluvial fan surfaces (0.8 ± 0.6 m/Ma; n = 13) in the Pilbara region, previously determined by cosmogenic 53Mn measurements (unpublished data). A two-isotope plot (10Be vs 26Al/10Be) shows that most of our sediment samples plot below the steady-state erosion-exposure island (26Al/10Be = 4.7 – 5.8), indicating a complex exposure history (or perhaps a reduced production rate) despite the shallow and transitory nature of Quaternary deposits in the generally bedrock-controlled landscape.
The apparent offset between bedrock erosion rates (in-channel, mesa plateau) and basin-wide average denudation rates infers that vast areas of iron-rich outcropping rock surfaces in the region are not the major contributor of sediments to the system. We consider two scenarios – i) sediment samples, after an initial exposure on exposed parent rock, experienced episodes of deep burial for a minimum few hundred thousand years, and/or ii) sediments are derived from reworked river bank and floodplain deposits into the channel system following storage at shallow depth for a prolonged period of time. Most of our data can be explained by being sourced from the average depth of 0.5-2 m.
Surprisingly, our cosmogenic nuclide derived erosion rates also show an apparent inverse relationship against average basin slopes. Hence, we explore various interpretations of our data in the context of specific morphometric, lithologic and environmental settings in the Pilbara region, and discuss the contribution of retreating vertical faces (e.g., gully, cliff) as a potential sediment source. Ultimately, this approach thereby contributes to the question whether foundational relationships between underlying morphometric conventions for catchment analyses are appropriate with these ancient, quiescent dryland landscapes.
Alissa Flatley is a fluvial geomorphologist and recipient of an AINSE postgraduate research award to integrate cosmogenic nuclides into her PhD research. Her thesis investigates the geomorphology of arid zone streams in the Pilbara region.
The Pilbara region is characterized by extremely iron-rich Proterozoic bedrock geology which represents an enormous economic interest for Australia and the reason for extensive open-pit mining in the region. Investigating the role of terrigenous sediment supply is important to provide a baseline value of spatially averaged regional erosion rates across the landscape to help decipher in-channel responses of anthropogenically modified landscapes. In particular, quantifying long-term denudation rates within the region improves our understanding of sediment pathways, residence times and storage within the catchment setting. In this study, cosmogenic nuclides 26Al and 10Be from detrital samples are used to quantify basin-wide erosion rates and sediment flux within the Upper Fortescue River and to understand sediment provenance and its transport process in the area where mining operations are active.