Estuarine sediment regimes, Far South Coast, N.S.W.

This dissertation presents the results of an integrated regional study of sediment exchanges between land and sea along a relatively unspoiled section of coastline in southern N.S.W. The regional climate is a temperate one with no seasonal moisture deficiency, while the shoreline is subject to a microtidal, high energy swell regime. Grainsize distribution, quartz roundness and sand mineralogy of sediments collected from fourteen rivers, their estuaries and the impounding barrier sands are analysed in order to determine the source(s) of local coastal sands. The factors and processes which control sediment distribution patterns in the three environments are also considered. Comparatively angular, poorly sorted river sands and gravels with significant proportions of non-quartz minerals are deposited at the heads of smaller estuaries as deltas. River mud accumulates on estuary floors as prodelta deposits or is flushed through the system and dispersed at sea. Estuary basin floors have been partly buried by landward prograding flood tide deltas whose sands are derived from the rounded, well sorted, predominantly quartzose sands which dominate most of the coastal embayments. Four estuaries have been infilled to the extent that terrigenous sands now travel through them to the sea. At two localities, advanced infilling reflects the dominance of fluvial activity, at another two, flood tide currents have performed the same role. Rates of terrigenous infilling are explained mainly by runoff and lithological characteristics of the drainage basins. Maximum infilling has occurred where large rivers drain granite catchments. Rates of infilling from seawards are greatest where inlet closure is least likely, a situation favoured by large tidal prisms, persistent stream flow and lower levels of incident wave power. In essence, most of the estuaries are sediment traps for both terrigenous and marine sands at the present time. Delivery of sand to the beaches from the continental shelf appears to have ceased; headland erosion is an unimportant source. Carbonate sands are a minor component of the coastal sediments. As estuary infilling nears completion, the potential for delivery of terrigenous sand to the sea by coastal rivers may be realised.