Deciphering the role of terrigenous sediment supply for headwater channels in the Pilbara, WA

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Australasian Quaternary Association Inc.
Using a multi cosmogenic nuclide approach (10Be and 26Al) we investigate terrigenous sediment supply in catchments to provide much needed baseline denudation rates in the Pilbara region. Detailed analysis of catchment morphometric properties and lithology have been combined with cosmogenic nuclide measurements to improve our understanding of sediment pathways, residence times and storage in headwater catchments. Our results suggest that this region has some of the lowest erosion rates in the world, between 0.94-4.04 mMyr-1, a range similar to previously measured channel bedrock rates in the region, but somewhat higher than results from outcrops on mesa summits previously measured using 53Mn (0.8 ± 0.6 mMyr-1). This apparent offset in erosion rates between horizontal bedrock surfaces and basin wide averages infers that the vast areas of iron-rich rock surfaces within the region are unlikely a major contributor of sediment to the system. Instead, vertical faces in the catchments have a role as a dominant sediment source and there is a complex erosional history of in-channel sediments within the headwater streams. Our results show 26Al/10Be ratios lower than the nominal production ratio, suggesting that the channel sediments are provided from either (or combination of) long-term, shallow buried regolith, or non-horizontal surfaces in the landscape (e.g., gorges and exposed cliffs). Erosion at the margins of the river channels is hence a key source of sediment supply and broadly the hillslope-headwater channel sediment conveyer is a minor contributor. Base-level stability and highly resistant nature of the prevailing lithology in the region results in these extremely low erosion rates in a high slope environment. Within these headwater catchments, sediment contributions are compounded by topographic inversion, increased albeit episodic fluvial activity and highly erosion resistant Banded Iron Formation. The study demonstrates that a multi cosmogenic nuclide approach is useful, not only to evaluate denudation rates, but also to decipher the complex history of sediment production and transport.
Watersheds, Isotopes, Erosion, Sedimentary basins, Topography, Rocks, Western Australia, Beryllium 10, Aluminium 26, Lithology
Flatley, A., May, J.-H., Fujioka, T., & Fink, D. (2022). Deciphering the role of terrigenous sediment supply for headwater channels in the Pilbara, WA. Paper presented to the AQUA 2022 Conference, 6-8th December, Adelaide (pp. 146-147). Retrieved from: