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Lithology

An examination of the main structural elements around the Australia coast reveals that there are a number of areas of very old crystalline basement rocks which outcrop in Western Australia (Yilgam Block, Pilbara Block and Kimberley Block); Northern Territory (Litchfield/Pine Creek Block); Queensland (Coen Inlier); and South Australia (Gawler Craton and Mount Lofty Ranges). There is also the major influence of the Tasman Geosyncline stretching along the east coast of Australia through Queensland, New South Wales, Victoria and Tasmania. In places these old structural elements are exposed along the coast, such as in the Gawler Craton, which produces a number of rocky headlands in South Australia. Elsewhere, cliffs and shore platforms are found where consolidated rocks outcrop at the shore, such as in the Kimberleys, or where the eroded sandstone tablelands of the Sydney Basin run to the coast. The CS1RO survey (table 2.1) shows that cliffs and shore platforms make up about a fifth of the length of the coastline. These hard rock landforms have evolved slowly. When sea level was near its present level, as happened repeatedly during the Pleistocene, they evolved under coastal conditions; at other times subaerial conditions prevailed. As a result, the cliffs and shore platforms we see today are often relict features, the legacy of conditions changing over thousands of years, rather than simply the result of present processes.

Table 2.1 CSIRO aerial photographic survey of the Australian coast

Proportion of substratum types (%)

State

Sand

Rock

Mud

Aeolianite

Queensland

61

10

29

0

New South Wales

69

30

1

0

Victoria

68

19

10

4

South Australia

63

15

8

13

Western Australia

49

26

20

5

Northern Territory

45

7

47

0

Tasmania

49

50

1

0

Totals

54

20

22

3

Notes:

The above represents the materials at the waters edge, estimated per 10 km section on aerial photos

Sand: sand beach, backed by dune or aeolianite; sand in front of mangroves is sand; behind mangroves – mud

Rock: hardrock shores, except where the shore platform has a beach at the back (sand); fringing coral reefs are rock

Mud: mainly Northern Australia and associated with mangroves

Aeolianite: found along great stretches of coast, but usually fronted by a beach (thus sand)

Source: Galloway et al. 1984

The rapid rise in sea level from 20 000 to 6000-7000 years BP brought the sea into contact with a variety of geological structures, rocks and sediments that have had a profound influence on the modem coast. For example, in South Australia, Kangaroo Island and the Mount Lofty Ranges consist of old, uplifted, resistant rocks which typically produce rugged rocky coasts. In some cases, such as Backstairs Passage and Christmas Cove, resurrected glacial topography of the Permian period (around 250 million years ago, when Australia was still part of Gondwana) was drowned by the postglacial sea-level rise, producing very distinctive coastal features. A good illustration of this can be seen at Hallett Cove in South Australia, where glacial striations on exposed coastal rocks clearly show the movement of ice in a northerly direction relative to Australia's current position. This is also the cause of the erratic granite boulders found along the coast and mentioned at the start of this chapter.

In central and southern New South Wales the Hawkesbury Sandstone is an extensive feature along the coast. In South Australia and Western Australia many hundreds of kilometres of cliffs, shore platforms and reefs have been eroded in the Pleistocene dunerock, also known as aeolianite and calcarenite (see figure 2.44, page 122). The Tertiary limestones of the Nullarbor coast and the granites of south-west Western Australia are also extensively developed in cliffs. The spectacular cliffs and sea stacks at Port Campbell National Park in Victoria (see figure 2.9(l))are composed of friable sandstone of Cretaceous age.

 
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