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

Abstract

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 represent a mature stage of infill of a barrier estuary. The stratigraphy of sediments from more than 60 drillholes from the plains indicates that the area has infilled during the mid-Holocene as a result of input of marine sands associated with a sand barrier on the high-energy coast, and fluvial mud and sands from the Shoalhaven River. Molluscan assemblages indicate that marine influence was initially widespread throughout the eastern and southern parts of the embayment, and that most of the plains infilled under estuarine conditions. Prominent levees across the plains surface are interpreted to be part of a birdsfoot delta distributary complex, and individual floodbasins accreted progressively as they were encapsulated by distributary extension. These former tidal environments mean that there are extensive potential acid sulphate soil conditions beneath the plains. The upper occurrence ofNotospisula in drillholes records the cessation of tidal influence, and diatoms from the top of one drillhole record the transition from brackish conditions to freshwater alluvial sedimentation. Radiocarbon dating provides a chronology of mid-Holocene ecological changes on the plains.