Estuarine infill and formation of Deltaic Plains, Shoalhaven River.

Abstract

The Shoalhaven River is an example of a mature, infilled estuary on a wave-dominated coast. The river presently flows between prominent levees towards the coast, periodically exiting at Shoalhaven Heads, but having much of its flow artificially diverted to exit at Crookhaven Heads. The alluvial plains east of Nowra are the result of estuarine infilling behind a sand barrier, represented by Comerong Island, a southern continuation of the Seven Mile Beach beach-ridge plain. Drilli has revealed marine sands adjacent to this eastern sand barrier, and extensive muds deposited in a barrier estuary beneath most of the plains. The molluscan composition of these sediments indicates deposition in brackish water conditions, with Notospisula trigonella especially widespread. Estuarine infill occurred progressively around deltaic channels, isolating smaller basins which accreted subsequently. Most of the estuarine sedimentation occurred 5500 – 3500 radiocarbon years BP. Brackish-water muds grade upwards into fresh-water alluvial muds deposited from fluvial overbank flows, and the transition is indicated from analysis of the diatoms within sediments in an augerhole near Jaspers Brush. Levees overlie these estuarine sediments in places and indicate natural changes in river channel location. There is an extensive potential acid sulphate soil hazard throughout the plains as a result of the occurrence of pyrite-rich estuarine muds up to an elevation close to mean sea level beneath most of the plains.