Coastal dune types

Incipient dunes are formed at the back of the beach when sand is blown across the beach and deposited in the lee of minor obstructions such as wrack. Once formed, the incipient dune acts to slow down winds and encourage sediment deposition. A low, undulating ridge vegetated by small shrubs, grasses and herbs may form parallel to the shoreline landward of the incipient dunes, termed a foredune. A series of low foredunes may develop at the coast; these have been termed relic foredune plains (Hesp 1984). On an accreting shore these ridges represent the depositional history of the coast, in the same fashion that tree rings represent the growth history of a tree.

The loss of vegetation exposes the dune surface to wind action. Funnelling of the winds into these low dune areas can result in the rapid removal of sands and the development of an exposed section of dune, termed a blow-out. In time, as the blow-out grows, it may form into a horseshoe-shaped parabolic dune in which the horns of the dune are 'anchored' by vegetation and face into the wind, and the centre of the dune is blown downwind. The low-lying areas between coastal dunes have been termed swales or dime slacks. These areas are generally at or near the level of the watertable. As a result, a swale may be characterised by swamp vegetation, and the damp sands of swales inhibit sediment transport by wind.

Large mobile dunes with little or no vegetation are termed transgressive dune sheets. These dune forms are commonly found inland from beaches where the sand supply is great. The sand is free to move with the wind and often forms long straight or sinuous crests perpendicular to the wind. On the leeward side of this crest a precipitation ridge may form, which is a slip face whereby the dune sheet moves forward over established vegetation or other objects. Peaked dunes may also occur on these transgressive dune sheets. The precipitation ridge is so named

Figure 2.29 Some dune types commonly seen along the coast of south-eastern Australia. The occurrence of different dune types is associated with variations in mean breaker height and sand supply

Some dune types commonly seen along the coast of south-eastern Australia. The occurrence of different dune types is associated with variations in mean breaker height and sand supply

Source: modified from Short 1988

because sand 'precipitates' down onto the lee slope. The movement of these transgressive dune sheets may not be rapid, usually less than 10 m per year, but where large sand masses are involved it may be hard to stop. Near Beachport in South Australia the Ten Mile Drift was mobilised by grazing from 1860 onwards. By 1890 the Beachport to Robe coach road was blocked by the advancing sand, and today the Drift spills into Lake George down a steep 30-metre-high lee slope.

The coastal sand dunes of Queensland and New South Wales vary in form, from low ridged bay barriers to giant sand masses with parabolic dune forms up to 8 km long and 300 m high. Massive parabolic dune forms are found in southern Queensland at North Stradbroke, Moreton and Fraser Islands and the Cooloola sand mass. Large parabolic dune fields are also found in the far north, on the eastern coast of the Cape York Peninsula. These dunes are well vegetated and stable, attaining a climax of rainforest in some locations; however, at Cape Flattery in the far north, some active parabolic dunes are found.

Figure 2.30 The movement of the Ten Mile Drift near Beachport, South Australia

The movement of the Ten Mile Drift near Beachport, South Australia

Source: based on data in Armstrong 1977

In New South Wales, low-lying embayments of Quaternary barriers and swamps are more characteristic. The most notable dune forms are two sets of foredune ridges, parallel with the present shore: an inner barrier (c. 125 000 BP) of low ridges, widely spaced and set back in the embayments, separated from a younger outer barrier (c. 7000-3000 BP) by low ground and swamps. The ridges of the outer barrier are higher, more tightly spaced, and fresher in form than the older foredune ridges; they usually back the modem foredune and have frequently been eroded by modem beach recession.

Bird (1978) noted that sediment supply from the inner continental shelf to the beach and dune is, along the southern and western margins of the continent, dominantly carbonate, with local mineral additions near those outcrops from which sand-sized regolith has been weathered. He pointed out the association of hinterland aridity (and hence reduced fluvial delivery of terrestrial sediments), biogenic dominance of shelf sediments, and carbonate sands in coastal deposits of the south and west. Conversely, near the mouth of the one large river to flow to the southern coast, the River Murray, there is an increase in the proportion of mineral sands. On southern and western coasts large carbonate dune accumulations are found at the more exposed, high-energy shores, where large swells have been able to transport the carbonate detritus to the beach.

The large carbonate-rich dunes have accumulated during the sea level oscillations of the Quaternary, and these have, over time, become lithified to calcarenite (dunerock or aeolianite; see figure 2.44) through groundwater movement, solution and redeposition, and calcrete formation. Through these processes of many cycles of dune formation and movement followed by induration, the resulting dunerock may exhibit dune bedding, buried soils, root casts and multiple calcrete layers.

 
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