Sediment transport of fine-grained alluvium from Cooper Creek, Central Australia

The ability of mud aggregates to form depositional bedforms is of considerable sedimentological importance for explaining the geomorphology of the Channel Country of Central Australia as well as the depositional environment of certain argillaceous fluvial sequences in the rock record. The sediment transport and bedform development of an aggregated sediment obtained from the floodplain of Cooper Creek, Central Australia, was examined in a laboratory flume over a range of flow conditions. The aggregates were found to be clay-rich (>60% clay), fine sand-sized (d50=0.10 mm), low density (2300 kg.m-3), water-stable and contained very low salt levels (<0.02%). The presence of smectite in the clay mineralogy of the sediment is an important factor in the development of the aggregates. Disaggregated sediment could be reaggregated in a laboratory after 2-3 wetting/drying cycles under simulated field conditions. Bedforms of aggregated mud ranging from lower-regime plane beds to upper-regime antidunes were observed. In the flume, the aggregates moved predominantly as bedload with measured peak bedload concentrations being high compared to other flume studies. The highly mobile nature of the sediment is due to the ready entrainment of low-density aggregates. The occurrence of braid bars formed of mud in an extensive low-gradient arid environment evident in the Channel Country of Central Australia, can be attributed to steeper braid-channel gradients across the floodplains during the passage of a flood; the highly mobile nature of the low-density sediment aggregates; the abiUty of the aggregates to be transported as bedload and their durable nature within the flow.