²⁶Al/¹⁰Be ratios reveal the source of river sediments in the Kimberley, NW Australia

Abstract

We use cosmogenic 10 Be and 26 Al in both bedrock and fluvial sediments to investigate controls on erosion rates and sediment supply to river basins at the regional scale in the Kimberley, NW Australia. The area is characterised by lithologically controlled morphologies such as cuestas, isolated mesas and extensive plateaus made of slightly dipping, extensively jointed sandstones. All sampled bedrock surfaces at plateau tops, ridgelines, and in the broader floodplain of major rivers over the region show similar slow lowering rates between 0.17 and 4.88 m.Myr -1 , with a mean value of 1.0 ± 0.6 m.Myr -1 (n=15), whilst two bedrock samples collected directly within river-beds record rates that are one to two orders of magnitude higher (14.4 ± 1.5 and 20.9 ± 2.5 m.Myr -1 , respectively). Bedrock 26 Al/ 10 Be ratios are all compatible with simple, continuous subaerial exposure histories. Modern river sediment yield lower 10 Be and 26 Al concentrations, apparent 10 Be basin-wide denudation rates ranging between 1.8 and 7.7 m.Myr -1 , with a median value of 2.6 m.Myr -1 , more than double the magnitude of bedrock erosion rates. 26 Al/ 10 Be ratios of the sediment samples are lower than those obtained for bedrock samples. We propose that these depleted 26 Al/ 10 Be ratios can largely be explained by the supply of sediment to river basins from the slab fragmentation and chemical weathering of channel gorge walls and plateau escarpments that result in diluting the cosmogenic nuclide concentration in river sediments measured at the basin outlets. The results of a mass-balance model suggest that~60-90% of river sediment in the Kimberley results from the breakdown and chemical weathering of retreating vertical sandstone rock-walls in contrast to sediment generated by bedrock weathering and erosion on the plateau tops. This study emphasises the value of analysing two or more isotopes in basin-scale studies using cosmogenic nuclides, especially in slowly eroding post-orogenic settings.

Determining the origin and changing shape of landscape‐scale rock formations with three‐dimensional modelling: The Borologa rock shelters, Kimberley region, Australia

“…Blocks that had corresponding strata K–U thicknesses and morphologically matching surfaces could be merged to construct a tabular conjoin set that could be refitted to the cliff (Figure 14). The matching strata were consistent with their conjoining edge morphologies, a good indication of the reliability of strata correspondences among the hard and slow‐weathering quartzites of the Warton Sandstone formation (Cazes et al, 2020; Delannoy et al, 2013) (Figure 3). Conversely, many blocks could not be matched, often because vegetation or sediments covered important details relating to the thickness of individual strata and surface morphologies.…”

“…This emphasises the importance of determining when and how rock shelters formed while ascertaining occupational trends within sites and across landscapes. Such questions also lead us to further examine the mechanisms of landscape evolution for the Borologa hillside (see also Cazes et al, 2020; Young, 1987).…”

“…The question remains as to when in the past the blocks became detached from the cliff‐line, before sliding downslope to their current positions. Cosmogenic 10 Be nuclide measurements from the Drysdale River catchment provide critical insights on cliff‐line denudation rates (Cazes et al, 2016, 2020).…”

“…Fracture patterns on exposed parts of blocks can also assist with the interpretation of the shape of their buried parts (Figure 4). In the region around Borologa, quartzites of Warton Sandstone are compact and highly resistant to physical and chemical weathering, which means that significant irregularities in the fracture patterns are unlikely (physical impacts notwithstanding) (Cazes et al, 2020), as evident also by the frequent accurate conjoining of block surfaces and of detached blocks and their originating escarpment (see below).…”

Controls on denudation along the East Australian continental margin

“…A strong lithological control on denudation rates has been documented in numerous postorogenic landscapes, more recent salient examples including the Cape Mountains of South Africa [Scharf et al, 2013], the Pyrenees [Bernard et al, 2019], the Kimberley plateau in Western Australia [Cazes et al, 2020], and the Quadrilátero Ferrífero in Brazil [Peifer et al, 2020]. In contrast to the above, and despite well documented instances of a strong (localised) lithological control on escarpment retreat in south-eastern Australia [e.g., Nott et al, 1996], lithology does not seem to exert a first order control on 10 Be-derived denudation rates along the East Australian passive margin ( Figure 10).…”