Case study: Cliffs near Robe, South Australia

From Robe south-east to Cape Banks, low but spectacular cliffs are a feature of the coast (figure 2.44). Many of the attractive features of the coastal scenery here are associated with rapid erosion: jagged overhanging rocks, caves, stacks, islands and arches give the coast a dramatic quality which draws visitors to this cool, windy place.

The cliffs are made of lithified calcareous sands of Pleistocene age, where the sea has cut into the youngest of a series of parallel dune ranges which line the Naracoorte coastal plain. Each ridge is a former barrier dune coastline, constructed of calcareous sands, transported from the continental shelf by rising seas. The barrier forming the present cliffs is called the Robe Range and has been dated at 80 000 to 100 000 BP (Schwebel 1983). Following deposition the barrier, sands have been hardened under subaerial conditions by dissolution (of the carbonate) and redeposition (of the bicarbonate) due to groundwater fluctuation. Hardening also occurs where evaporation draws limerich groundwater to the surface to leave a hard crust, sometimes thickening to a tough, 10 to 15 cm thick 'calcrete'. These hardened sands are known as aeolianite or calcarenite. The aeolianite shows wind-blown sedimentary structures such as cross-bedding, and curved 'aerodynamic' former dune surface forms. In the eroded cliff faces, former soil layers may also be seen, together with calcified root cavities.

Active erosion has breached the Robe Range, notably at Guichen Bay and Rivoli Bay, created numerous offshore reefs and islands, and formed large platforms and reefs. Although storm waves are slowly abrading the seaward edge of the platforms, it appears that dissolution under subaerial conditions has been a major process in forming these platforms. Together with other weathering processes, dissolution appears to have lowered the platform to low tide level. Once the platform is permanently under water this process stops (seawater is saturated for lime), and platform extension at the landward edge continues. In this way remarkably flat, extensive, platforms at low tide level are formed.

Resistant calcrete layers ensure that many aeolianite cliffs are steep or overhanging. Cliff recession proceeds by wave quarrying, water layer and solution weathering, leading to undercutting and collapse. At Robe, high wave energy ensures the rapid removal of collapsed material and further undercutting.

Figure 2.44 Aeolianite cliffs at Robe, South Australia. The aeolianite cliffs are part of older lithified dune barriers associated with high sea level stands during the Quaternary. The original dune bedding is preserved within the cliffs and an indurated calcrete layer can be seen at the top of the cliff

Aeolianite cliffs at Robe, South Australia. The aeolianite cliffs are part of older lithified dune barriers associated with high sea level stands during the Quaternary. The original dune bedding is preserved within the cliffs and an indurated calcrete layer can be seen at the top of the cliff

(photography Nick Harvey)

Records of a detailed town survey near Cape Dombey in 1896 and a careful resurvey by the Coast Protection Board of South Australia in 1987 have allowed the cliff recession there to be measured (table 2.10).

It is clear that average recession rate of the aeolianite cliffs in the exposed open ocean sites is rapid, and also that there is great local variation in erosion speed. However, the most exposed places, such as promontories, are not eroding most rapidly; rather, erosion appears to follow lines of weakness within the rock to further recess inlets and bays. In this way the coast is becoming more indented.

Table 2.10 Average erosion of Cape Dombey cliffs, 1896-1987

Category of site

Average erosion (m)

Average rate of erosion (cm/year)

Straight

6.1

6.8

Protected sites

1.14

1.3

Open ocean sites

6.6

7.3

Embayments

7.38

8.2

Promontories

3.89

4.3

All sites

5.9

6.6

 
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