POSTOROLAND · The evolution of post-orogenic landscapes: bedrock rivers, lithology and relief development

The evolution of post-orogenic landscapes: bedrock rivers, lithology and relief development

The aim of this project is to use analysis of digital terrain data, terrestrial cosmogenic nuclides (TCNs), and long profiles in the Lachlan River – a large (20,000 km2 ) Australian drainage basin with a well-constrained Cenozoic evolution – to assess lithological control of the rate and style of landscape evolution in a typical post-orogenic setting. The project achieved this aim (i) by quantifying rates of landscape evolution, and (ii) by assessing how landscape evolution is slowed by resistant lithologies, a key but neglected issue in understanding post-orogenic setting (Bishop 2007). The project engages with enduring questions in geomorphology, namely, does lithology (rock type) influence landscape morphology, and if so, how does it exert this influence? In summary, the objectives were: 1. To map the spatial distribution of knickpoints in the bedrock drainage net of the Lachlan catchment and their association with lithology; 2. To use TCN analysis of modern bedrock channels and fluvial sediments to determine the spatial pattern of bedrock channel incision and catchment lowering; and 3. To use TCN analysis of Early Miocene fluvial sediments to test the hypothesis that the spatial distribution of denudation rates revealed in 2 has pertained since before the Early Miocene. As part of meeting objective 2, a sub-objective was set up after the first field campaign to use optically stimulated luminescence (OSL) to assess the ages of widespread valley-bottom sediments that may have shielded channel beds from cosmic radiation (and hence compromised the record of terrestrial cosmogenic nuclides (TCNs) that underpin this work). Results All three objectives have been addressed, but, as noted in the periodic report, we await the results of the TCN analyses, the delays to which have arisen from events completely beyond our control or influence. The work has shown that steep bedrock river reaches do indeed coincide with resistant lithologies, as we had hypothesized. We hypothesize – and will test using TCN analysis – that the rates of denudation and bedrock incision downstream of the lithologically controlled knickpoints will be equivalent to the rates of trunk stream incision (i.e., 1-10 m Myr-1, driven by denudational isostatic rebound – Bishop 1985), whereas the rates upstream of the lithologically controlled knickpoints will reflect the ‘top-down’ process rates governed by discharge, sediment flux and slope. These rates upstream of the lithologically controlled knickpoints may match those downstream of the knickpoints but we hypothesize further that they will be lower than the downstream rates, reflecting the low gradients and low sediment fluxes in these upstream catchments. Those remain our expected results and we await the results of the TCN analyses. OSL analyses were undertaken on the swampy meadow (SM) sediments that underlie the valley-bottom-blanketing sediments commonly known as post-settlement alluvium (PSA). This ‘PSA’ designation reflects the almost-universal interpretation that these blanketing sediments were mobilised when Europeans settled the area in the 19th century, extensively clearing the catchments, triggering catastrophic catchment erosion and downstream movement of massive amounts of sediment that is now stored in valley bottoms (e.g., Prosser 1991; Wasson et al. 1998). Butzer and Helgren (2005) have recently questioned this interpretation, using early explorers’ accounts and diaries to argue that inland SE Australia was quite extensively gullied and ‘disturbed’ when Europeans arrived. Resolution of this issue is important here: massive valley-bottom sedimentation substantially earlier than the 19th century means that there is the potential to perturb the acquisition of TCNs, invalidating the approach we are taking here. The objective of the dating exercise was to determine the depositional age of the PSA, but because of the clearly unbleached character of the PSA (Munoz-Salinas et al. 2011), we concentrated the OSL dating exercise on the SM sediments underlying the PSA and then projected the sedimentation rate of the SM to its contact with the base of the PSA. Kinnaird et al. (2011) report that the samples exhibit good OSL sensitivity and produced acceptable SAR internal quality control performance. Age estimates of 5.17 ± 0.30 ka (163 cm depth) and 2.43 ± 0.14 ka (103 cm depth) were obtained for the SM deposit. A series of profiling samples (yellow samples on Fig. 1) were converted to ages, by combining the stored dose in these profiling samples and the field-measured dosimetry of the full dating samples (the red samples in Fig. 1). A linear extrapolation through the profiling apparent ages gives an estimate the age of the SM/PSA contact at ~ 390 yrs. Thus, it is apparent that the contact between the SM and PSA, which we interpret to be the onset of the PSA sedimentation in the valley bottom, is associated with a date of about 400 years. Intriguingly, this age pre-dates the arrival of Europeans in inland southeastern Australia but, that conclusion notwithstanding, an age of four centuries (or less) creates few issues in relation to shielding of the channel bed from cosmic radiation.

Data: CORDIS, © European Union