Climate and run-off
Australia is a dry flat continent, and rivers generally supply low volumes of water and sediment to the coast; significantly, some areas have little or no runoff to the sea. The average flow of the Murray-Darling Basin to the sea, under natural conditions, is about 12 000 GL/year, a tiny figure on world scales for a river system which drains one seventh of the continent. Moreover, all Australian catchments have great variability, in the swings from the dry El Nino years to the wet La Nina years. For example, in dry years the effective discharge of the Murray to the sea ceases, but in wet years the discharge rises to about 40 000 GL. Diversions for irrigation have reduced flows in many catchments; thus, the flow at the Murray's mouth now averages 21% of the natural annual flow (MacKay & Eastbum 1989).
Only the Eastern Highlands and some rivers of Northern Australia have rapid run-off, and hence regular sediment delivery to the sea; the arid South and West of the continent have virtually no sediment discharge to the sea.
Beach and dune sediments
Beaches are common on Australia's coasts (see table 2.1) and are constructed of sand-sized comminuted fragments of carbonate or quartz. Other fragments, including suites of heavy minerals, are found within beaches, but these make up less than 1% of the coastal sands. There are also some gravel beaches – for example, some Northern Territory beaches are of ironstone fragments from laterite – but gravel is not common in Australia, as it is on the coasts of Europe and other regions strongly affected by the debris of Pleistocene glaciation. A few shelly beaches are found in Australia, but they are rare.
A national review of beach materials by Bird (1978) described a broad contrast between the south and west of the continent where calcareous materials predominate, the east coast where quartzose sands are common and the Northern Territory where a variety of fine sediments are frequently found at the coast.
The cause of this continental scale contrast in beach sediments is climatic. In the south and west of the continent, long-term aridity has rendered fluvial transport of weathered rock fragments to the shore slight to non-existent. Between Broome and Bass Strait the only major river outlet is the Murray. At the present sea level, many of the minor streams reaching the coast are simply
Figure 2.2 Australian climatic regions and coastal wind systems
Source: modified from Colls & Whitaker 2001, and Short 1993
depositing their sediment load within the estuary; sediment from the Murray is infilling its terminal lakes, Albert and Alexandrina. The beach sediments of Australia's arid coast are predominantly shelf-derived biogenic carbonate material: fragmented shell and skeletal material. These have been delivered to the shore by the successive marine transgressions of the Pleistocene period. On
Figure 2.3 Coastal sedimentary environments
Source: after Atlas of Australian Resources (1980-90), Zatt & Kailola (1995), annotations from Bird (1978).
Figure 2.4 Spring tide range at locations around the continent
Source: data from Australian National Tide Tables 1993
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. Once at the shore the sediments have been redistributed by littoral drift into embayments and by wind into dune fields. Over time, many of the large carbonate-rich dunes, which have accumulated during the sea level oscillations of the Quaternary, have become lithified to aeolianite through ground water movement, solution and redeposition and calcrete formation. The resulting 'dunerock' may exhibit dune bedding, buried soils, root casts and multiple calcrete layers.
On the south-east coast of South Australia a series of aeolianite dunes or 'ranges' are arranged sub-parallel to the coast on a flat coastal plain (see figure 2.11). Because the coastal plain has been slowly rising tectonically, the dunes are arranged as a time series, with the oldest furthest inland. They bear witness to the repetition of the process of rising seas transporting quantities of carbonate sands to the highest shore of successive Pleistocene transgressions. On the Swan coastal plain in Western Australia aeolianite ranges are found, and they continue in succession offshore, with reefs and islands sub-parallel to the shore.
Quartzose sands of the south-east and east of the continent are, in contrast, of terrestrial origin.