Topographically associated but chronologically disjunct late Quaternary floodplains and terraces in a partly confined valley, south-eastern Australia
The Bellinger River catchment in the New England Fold Belt on the mid-north coast of New South Wales is characterized by an assemblage of stepped late Quaternary alluvial units. Late Pleistocene terraces were formed by large, more competent rivers that eroded almost entire valley floors; however, a decline in discharge prior to the Holocene has resulted in the abandonment of these deposits as elevated terraces or residual alluvium, onlapped by contemporary floodplains. Intrinsic controls on floodplain formation appear to be superimposed over an early–mid-Holocene climatic signature. A fluvially active period, known as the Nambucca Phase, from 10 to 4·5 ka, eroded Late Pleistocene terraces. Two floodplain surfaces, one higher than the other, both started to accrete vertically from 4 ka but with some valley locations remaining vulnerable to episodes of erosion, resulting in substantial units of even younger basal alluvium. The high floodplain is dominated by horizontally laminated, vertically accreted sequences, while the low floodplain, which overlaps in age, is characterized by pronounced cut-and-fill stratigraphy. Terraces and floodplains in partly confined settings can have similar elevations but be polycyclic, with very different basal ages. In such landscapes the classical assumption that individual terrace or floodplain profiles along a valley represent periods of coeval formation is shown to be frequently invalid.