Sedimentation history of the Mundi Mundi alluvial fans, Western New South Wales

Abstract

The Mundi Mundi alluvial fans lie along the Mundi Mundi fault scarp in arid far western New South Wales. The fans consist of a number of well-defined stratigraphic units which can be traced both across the fans and down the fans. The fans are presently dissected and sedimentation is proceeding at intersection points on the larger fans, and most of the small fans are incised so that a net sediment loss is occurring. The oldest stratigraphic unit recognised is the Umberumberka unit which was deposited by bedload transportation in partly braided distributary channels and by slurry-flow deposition over temporarily abandoned channel deposits. The accumulation of the unit was a response to a relatively wetter climate at a time of depressed temperatures prior to 16,000 years B.P., and the end of accumulation was caused by increasing aridity. The Umberumberka unit is capped by a calcareous palaeosol, the Belmont Palaesol, which formed on the stable surfaces of the fans immediately after the peak of desiccation. The Korkora unit overlies the palaeosol and was deposited by shallow channels and slurries. The Mundi Mundi unit overlies the Korkora unit and was deposited in the same way as the Umberumberka unit. The Mundi Mundi unit was deposited between about 6000 and 3000 years B.P. as a response to an increase in rainfall in the Australian interior. The end of accumulation of the Mundi Mundi unit was followed by dissection of the fans, and this change in fan regimen was a result of climatic deterioration. A minor period of alluviation in the trenches cut into the fans (the Tackaringa unit) began about 1000 years B.P. as a result of a slight increase in moisture. The modern streams are dissecting the Thackaringa unit, but floods with a low recurrence interval continue to contribute sediment to the unit. Climatic fluctuations have been responsible for the timing of periods of aggradation and degradation in the alluvial fans, but the major stratigraphic units record a progressive decrease in the mud (< 63 μm) content of the sediments. This decrease may reflect a long-term decrease in sediment yield from the fan catchments. The effect of this decrease in sediment yield on the behaviour of the fans is not yet clear.