Holocene palaeoecology and formation of the Shoalhaven River deltaic-estuarine plains, southeast Australia

The Shoalhaven River is one of the largest rivers on the south coast of New South Wales, and the deltaic-estuarine plains associated with its mouth re present a mature stage of infill of a barrier estuary. The stratigraphy of sediments from more than 60 drillholes from the plains indicates that the a rea has infilled during the mid-Holocene as a result of input of marine san ds associated with a sand barrier on the high-energy coast, and fluvial mud and sands from the Shoalhaven River. Molluscan assemblages indicate that m arine influence was initially widespread throughout the eastern and souther n parts of the embayment and that most of the plains infilled under estuari ne conditions. Prominent levees across the plains surface are interpreted t o be part of a birdsfoot delta distributary complex, and individual floodba sins accreted progressively as they were encapsulated by distributary exten sion. These former tidal environments mean that there are extensive potenti al acid sulphate soil conditions beneath the plains. The upper occurrence o f Notospisula in drillholes records the cessation of tidal influence, and d iatoms from the top of one drillhole record the transition from brackish co nditions to freshwater alluvial sedimentation. Radiocarbon dating provides a chronology of mid-Holocene ecological changes on the plains.