Dryland Sediments from Source to Sink – Lake Eyre Basin, Australia.

Abstract

Most of the Australian continent lies within a dryland arid zone, dominated by the Lake Eyre basin (>1M km2), a low relief interior drainage system in the centre of Australia. The Lake Eyre basin contains numerous dryland rivers inset into wide alluvial plains that terminate in saline playa lakes. Lake Eyre is the largest of these, and lies in the basin centre. The lake is surrounded by relict Pleistocene aeolian dune fields overlying older fluvial-lacustrine sediments. The present-day main coarse clastic input is along the western side of Lake Eyre (Neales Fan), sourced mainly from recently uplifted basement rocks (Peake and Denison Inliers) and eroded cover successions (Eromanga Basin). The purpose of this study is to investigate the predictability of variation in grain size, sorting and composition from source to sink in a dryland setting. A relatively small, discrete catchment (100km²) on the fringe of the Neales Fan (Umbum Creek) was selected for detailed analysis. Recent sediments were sampled from strategically located stream confluences (90 samples) and then a quantitative textural and compositional dataset generated from 34 selected samples. Each sample includes a complete half-phi sieve analysis (4mm to 32 microns), and an associated detailed petrographic description recognizing 72 categories of grain composition of varying grain-size. Results show that the sediment provenance comprises uplifted metasedimentary and igneous rocks (Neo-Proterozoic), marine mudstone and sandstone (Mesozoic), and semi-consolidated fluvial and aeolian sands and silts (Tertiary-Quaternary). A mantle of Tertiary-Quaternary duricrust over bedrock lithologies (comprising mainly Fe-rich silcrete) forms remanent tablelands, mesas, and gibber plains and these provide a distinct contribution to the sediment flux. A key outcome is the recognition of apparent downstream coarsening of river sediments. This is partly due to the high influx of resistant silcrete fragments derived from erosion of the duricrust over the adjacent bedrock and low relief plains, probably via major flood events. At least five different styles of silcrete are recognized, and grain size and composition vary strongly with the local bedrock lithology. This has broad implications for controls on sediment supply type and the routing of local sediment sources from interfluves. During climatic wet-phases, higher rainfall and water infiltration assist the mobilization of coarse sediment available for downstream runoff. In contrast, during climatic dry-phases, aeolian processes are dominant, resulting in the removal of the very fine- to fine-grain size fraction. Apparent downstream coarsening is therefore the net effect of a cyclical pattern of wet (flooding) versus dry (aeolian) phases driven by climatic changes in the Late Quaternary, probably in response to shifts in the monsoonal belt. Reservoir implications include changes in the expectation of initial porosity and permeability along the river network, and in particular within the distal zone near the lake margin.