Controls on denudation along the East Australian continental margin

Abstract

We report a comprehensive inventory of 10Be-based basin-wide denudation rates (n = 160) and 26Al/10Be ratios (n = 67) from 48 drainage basins along a 3000 km stretch of the East Australian passive continental margin. We provide data from both basins draining east of the continental divide (n = 37) and discharging into the Tasman and Coral Seas, and from basins draining to the west as part of the larger Murray-Darling and Lake Eyre river systems (n = 11). 10Be-derived denudation rates in mainstem samples from east-draining basins range between 7.7 ± 1.9 (± 1σ; Mary) and 54.6 ± 13.7 mm kyr−1 (North Johnstone). Denudation rates in tributary samples range between 3.0 ± 0.7 (Burdekin) and 70.2 ± 18.9 mm kyr−1 (Liverpool). For west-draining basins, denudation rates are overall lower and with a more restricted range of 4.8 ± 1.2 (Barcoo) to 15.4 ± 3.6 mm kyr−1 (Maranoa) in mainstem samples, and between 4.4 ± 1.0 (Murrumbidgee) and 38.5 ± 7.8 mm kyr−1 (Murray) in tributary samples. East Australian denudation rates (median = 14.5 mm kyr−1) are similar to those found in other postorogenic landscapes (global median = 12.4 mm kyr−1) and the medians of the top 10% denudation rates recorded here (46.5 mm kyr−1) and in other passive margin settings are also similar, despite differences in topography and precipitation. These median denudation rate values are close to the 95th percentile denudation rate for all tectonically passive basins (≈53 mm kyr−1) and are very similar to the global silicate weathering speed limit (≈58 mm kyr−1) calculated as the 95th percentile of global soil weathering rates. The above suggests that in post-orogenic terrain, the overall rates of topographic decay have a ‘speed limit’ that is imposed by the rate at which rock is converted to soil by chemical weathering. Denudation rates along the East Australian margin correlate with topographic metrics at both the mainstem basin scale and at the smaller tributary basin scale suggesting that topography exerts the main control on rates of landscape lowering in this setting. An important link between denudation rate and rainfall is also inferred: the highest mainstem 10Be denudation rates all occur in basins which also have the highest rainfall amounts and there is a strong correlation between the distance knickpoints have travelled upstream from the river mouth and basin area — a proxy for discharge and so to some extent also rainfall. On both sides of the divide, in all but a few of the samples, 26Al/10Be ratios are consistently lower than what is expected in a setting where sediments experience a simple and continuous exposure history. East of the continental divide, the lowest 26Al/10Be ratios are found in basins that also experience increased flood variability. We posit that there is a causal link between the hydrological variability that characterises the coastal rivers of eastern Australia and the observed non-steady state 26Al/10Be ratios: the periodic stripping of vertically accreted floodplains by large floods means that deeper and potentially older material is periodically incorporated into the sediment mix transported by the modern river.