Hydrology and sediments of a temperate estuary - Mallacoota Inlet, Victoria

Mallacoota Inlet is a drowned river valley that is almost cut off from the sea by a barrier-beach complex. Additionally, a flood-tidal delta chokes the mouth of the estuary. Except for fluvial sand in the upper reaches of the estuary, sandy sediments are restricted to the seaward end where hydrodynamic activity is greatest. Muds are confined to basinal and deep-channel environments; silts occur where fresh-water discharge contributes to water circulation, and clays where circulation is restricted. In the coarse fraction of the sediments, feldspar and mica contents are highest, and quartz and carbonate are lowest, where fresh-water discharge contributes. The clay mineral assemblage in the estuarine sediments is generally similar to that of the soil and stream sediments in the source area. Geochemical results show that Fe, V, Cr, Ni, Zn, Cu, and P2O5 levels are highest in stagnant areas where organic-rich clay prevails, whereas Mn and Ti are highest in silts where fluvial discharge is dominant and water circulates relatively freely. The results of the study suggest that the restricted basinal areas of the estuary are similar to fjords in terms of physical and chemical controls on sedimentation. The coastal barrier at Mallacoota was evidently formed by landward progradation during the Holocene, and overlies apparent remnants of a similar Pleistocene barrier. Many similar restricted estuaries exist along the south coast of New South Wales.

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