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 prolong ed Cenozoic fluvial activity. Single thread, anabranching and distribu tary channels and floodplains, and associated aeolian dunes, character ize the uppermost sequences. Based on detailed interpretations of Late Quaternary fluvial sedimentation and surficial stratigraphy for this 77 000-km(2) basin, earlier 'prior stream' and 'ancestral stream' mode ls of fluvial deposition, deduced from limited stratigraphic and chron ological evidence, are replaced with aggradational palaeochannel and m igrational palaeochannel models. Thermoluminescence dating reveals fou r 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 t he fourth (Yanco phase) after the Last Glacial Maximum (LGM) in Stage 2. The first three of these phases were characterized by mixed-load la terally migrating sinuous palaeochannels with occasional transitions t o a straighter bedload-dominated mode, and vice versa. The first two p hases concluded with a bedload-dominated episode resulting in aggradat ional palaeochannels on the surface of the Plain, and the third phase (prior to the LGM) did also in its downstream reaches. The phase follo wing the LGM was characterized entirely by large mixed-load sinuous mi grational palaeochannels. These exhibited no terminating bedload episo de, because the onset of Holocene climates reduced the size of the flo od peaks, greatly diminished the supply of bedload from the upper catc hments and resulted in streams evolving to their present highly sinuou s suspended load form. The result is a complex stratigraphic architect ure consisting of vertically and laterally accreted units extending ov er hundreds of kilometres in the form of channel-sand stringers, sand sheets and derivative aeolian dunes partially or wholly encased in ove rbank fines.