Stratigraphic architecture resulting from Late Quaternary evolution of the Riverine Plain, south-eastern Australia

The Riverine Plain of south-eastern Australia is the result of prolonged Cenozoic fluvial activity. Single thread, anabranching and distributary channels and floodplains, and associated aeolian dunes, characterize the uppermost sequences. Based on detailed interpretations of Late Quaternary fluvial sedimentation and surficial stratigraphy for this 77 000-km² basin, earlier ‘prior stream’ and ‘ancestral stream’ models of fluvial deposition, deduced from limited stratigraphic and chronological evidence, are replaced with aggradational palaeochannel and migrational palaeochannel models. Thermoluminescence dating reveals four distinct phases of palaeochannel activity between 105 and 12 ka; the first (Coleambally phase) late in Oxygen Isotope Stage 5, the second (Kerarbury phase) in Stage 3, the third (Gum Creek phase) before and the fourth (Yanco phase) after the Last Glacial Maximum (LGM) in Stage 2. The first three of these phases were characterized by mixed-load laterally migrating sinuous palaeochannels with occasional transitions to a straighter bedload-dominated mode, and vice versa. The first two phases concluded with a bedload-dominated episode resulting in aggradational palaeochannels on the surface of the Plain, and the third phase (prior to the LGM) did also in its downstream reaches. The phase following the LGM was characterized entirely by large mixed-load sinuous migrational palaeochannels. These exhibited no terminating bedload episode, because the onset of Holocene climates reduced the size of the flood peaks, greatly diminished the supply of bedload from the upper catchments and resulted in streams evolving to their present highly sinuous suspended load form. The result is a complex stratigraphic architecture consisting of vertically and laterally accreted units extending over hundreds of kilometres in the form of channel-sand stringers, sand sheets and derivative aeolian dunes partially or wholly encased in overbank fines.