Late quaternary evolution of riverine plain paleochannels, southeastern Australia
The Riverine Plain of southeastern Australia is the result of prolonged Cenozoic fluvial activity associated with the Murray River and its major southern tributaries, the Murrumbidgee, Goulburn and Lachlan. Single thread, distributary and anabranching channels and floodplains with associated eolian dunes and lunette-bordered lake basins characterise the uppermost sequences of the Late Quaternary. Following 30 years of detailed mapping and stratigraphic investigation, more recently supported by luminescence dating, the Pels sequential model of prior streams and ancestral rivers on the Riverine Plain was replaced with the Page & Nanson model of alternating migrational and aggradational paleochannels. Despite some sub-catchment variability the emerging picture of climatic and hydrological change since the Last Interglacial shows many common themes that are in accord with findings in other parts of Australia including the Lake Eyre Basin and the coastal rivers of New South Wales. Enhanced fluvial activity is apparent in much of Oxygen Isotope Stage (OIS) 5 between 110 and 80 ka and in OIS 3 from 55 to 25 ka. The intervening OIS 4 represents a definite pause between pluvial episodes with an almost complete absence of luminescence dates from both riverine and lacustrine environments supported by evidence of enhanced dune mobilisation and high Antarctic dust flux. OIS 2 from 24 to 12 ka remains somewhat enigmatic with evidence for intense aridity in the Willandra Lakes and elsewhere at the Last Glacial Maximum apparently contradicted by evidence of higher flows in the Darling and Lachlan Rivers and high water levels at Lakes Tandou, Urana and Cullivel. The question of the persistence of the Lachlan River's connection with the Willandra Lakes at this time remains an unresolved issue in urgent need of further study. Despite uncertainty about the nature of the Last Glacial Maximum, it is now clear that higher stream discharges and lake levels occurred later in OIS 2 from about 20 to 12 ka. Dating evidence shows that the transition to essentially modern river regimes occurred early in the Holocene, but may have been somewhat asynchronous between the various Murray Basin sub-catchments.