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Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland
Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995
ISBN 0 642 17399 0
Peter T Harris
Ocean Science Institute
University of Sydney
Sydney NSW 2066
Australian Geological Survey Organisation
University of Tasmania
GPO Box 252C
Hobart Tas. 7001
Australia's continental shelf covers approximately two million km2, an area equal to about 25% of the continent's land surface. Most of Australia's petroleum is derived from sedimentary deposits located on the continental shelf and the sea over the shelf contains some valuable fisheries. A growing tourist industry relies on a healthy marine environment: yet this same environment receives millions of tonnes (t) of eroded topsoil, fertiliser, sewage and industrial waste each year.
What do we know about Australia's continental shelf, about its origin, history and geology? What are the important factors controlling the functioning of its natural sedimentary systems? This chapter attempts to answer these questions as well as to explore the implications of shelf sedimentology for environmental monitoring and management.
The continental shelf is the sea floor which surrounds Australia (Figure 1). It is shallow, generally less than 200 m in water depth. Geologically, the continental shelf is similar to the rest of the continent in that its foundation is comprised of granitic crustal material - unlike the deep ocean bed which is underlain by basalt (Figure 2). Strictly speaking, the shelf extends from the beach (foreshore) environment, across the seaward dipping, low gradient (about 0.1°) shoreface to an offshore location where it rapidly changes in slope and is known as the shelf break. From there, the seabed forms a steeper continental slope which grades into the continental rise and abyssal plain at great depths (Figure 2). The continental shelf is thus bounded inclusively by the shoreface and the shelf break (Figure 2). The shoreface is generally a zone of active sediment reworking, delimited at its offshore margin by the so-called fairweather wave base, which is the downward limit of effective wave-induced sand movement during normal sea conditions.
In a global context, the depth of the shelf break (20-550 m water depth and defined as 200 m by international convention) and the width (2-1500 km) exhibit a wide variability. In the case of Australia (Figure 1), the shelf break is located about 10 km offshore from Fraser Island (east coast) and North West Cape at Exmouth (west coast) but at over 500 km distance offshore on the Arafura Shelf in the north. In vertical profile, most of Australia's shelf is a smooth, flat surface which dips gently seawards (Figure 2). However, some parts are rimmed by shelf edge barrier reef systems (eg the Great Barrier Reef shelf).
The shape of the Australian continent owes its origin to the rifting apart of a much larger super-continent known as Gondwanaland. About 65 million years ago Australia was still a part of Gondwanaland, connected along its southern margin to what is now Antarctica (Veevers 1984). Rifting and seafloor spreading caused the splitting away of Australia and its last connection with Antarctica - at the southern edge of Tasmania - was severed about 50 million years ago. Australia moved northward, pushed along by the sea floor-spreading 'conveyor belt'. About 10-15 million years ago, the northern margin of Australia collided with the Pacific Plate causing tectonic uplift, volcanic activity and eventually the creation of New Guinea.
Throughout Australia's geologic history - both prior to and after its separation from Gondwanaland - rivers and ice have eroded the mountains and transported sediment to the continental shelf, where sedimentary layers several km thick have accumulated (Figure 2). Australia's major oil and gas fields are located within these marginal, sediment-filled basins (Figure 1).
Figure 1: Location and regional bathymetry of Australian shelf seas. The locations of major oil and gas fields are indicated (stars).
Hence, the continental shelf as we see it today has evolved over many millions of years. Its surface morphology has been modified by processes related to sea level change, the rate and type of sediment supply and the energy available to erode, rework and disperse sediment. In the following sections the relative importance of each of these factors is assessed with reference to the Australian continental shelf.
Sea level rise (transgression) and fall (regression) relative to a given coastal province is the function of three different (though often contemporaneous) processes:
(1) the melting and/or formation of polar ice caps causing a change in the ocean's volume with consequent eustatic sea level changes;
(2) the loading of the earth's lithosphere with sediment, water or ice resulting in deformation causing isostatic changes; and
(3) the collision or rifting apart of continental plates or the subduction of oceanic crust beneath continental margins causing vertical tectonic movements.
The present position of eustatic sea level is high in relation to the record over the past 150 000 years (Figure 3). During previous lower sea level stands, coastlines would have occupied positions on what is now the outer continental shelf or upper slope. Therefore, the morphology of the present outer shelf and upper slope is partially the product of past, low sea level coastal sedimentary processes and partially the product of modern sea level shelf processes.
Figure 2: Diagran showing a typical shelf cross-section, illustrating the average gradients and positions of the continental shelf, shelf edge (depth 130-200m), continental slope, continental rise and abyssal plain. The general geological composition of the shelf is shown. It has a granitic basement with overlying sedimentary basins and underlying oceanic crust.
It has been shown in Australia that the lowering of eustatic sea level to at least -130 m occurred during the last glacial episode, 15 000-20 000 years ago (Chappell et al. 1983). Over the last 150 000 years, eustatic sea level has oscillated many times around 40-80 m below present sea level (Figure 3). Available sea level data indicates that the most recent sea level rise began about 15 000 years ago, reaching a maximum rate of about 2 cm per year between about 11 000 and 12 000 years ago. This high rate was followed by lesser rates of transgression until the position of the present sea level was reached about 6500 years ago (Figure 3).
Isostatic changes in relative sea level have caused only minor (about 4 m maximum) fluctuations around Australia in the Holocene period (Nakada & Lambeck 1989). Similarly, changes in sea level over the past 20 000 years or so due to tectonic effects are thought to be limited on the Australian continent which is considered to be 'stable' relative to other continents. Over longer time intervals (more than 20 000 years) there is evidence of tectonism in parts of Australia. Although the collision boundary in New Guinea forms a northern, tectonically active zone, the effect of this zone does not appear to extend southwards into the Cape York region of Australia in terms of late Pleistocene to Holocene sea level change.
Figure 3: Sea level curve showing the effect of eustatic (ie related to the melting of polar ice caps) sea level change over the last 140,000 years.
During Pleistocene periods of lower relative sea levels (Figure 3), the shelf was exposed subaerially and the Australian mainland was 'joined' by dry land to several of the adjacent large islands such as Tasmania and New Guinea (see Figure 4). Such 'land bridges' are considered to have facilitated the migration of animals and humans in the late Pleistocene 'ice age' (eg Peterson 1991). The shallow seas comprising the Sahul Shelf, Gulf of Carpentaria, Torres Strait and Bass Strait were thus subjected to erosion and sedimentation by rivers and wind on several occasions during the past 150 000 years (Figures 3 and 4). The basins in Bass Strait, Bonaparte Gulf and the Gulf of Carpentaria are considered to have been the sites of large fresh to brackish water lakes and lagoons during these periods of emergence (Van Andel & Veevers 1967; Blom & Alsop 1988; Jones & Torgersen 1988). At the peak of the last ice age, when sea level was about 130 m lower than at present (Figure 3), rainfall on the continent was also less than now and dust storms swept terrestrial clay into the adjacent oceans (Figure 4). Cores obtained from the deep ocean basins surrounding Australia contain discrete layers of these terrestrial clays (Thiede 1979).
Figure 4: Morphoclimatic map of Australia and Papua New Guinea at about 18 000 years ago (after Williams 1984). The approximate positions of lakes and lagoons in Joseph Bonaparte Gulf, the Gulf of Carpentaria and Bass Strait are shown.
In describing shelf sediments, the most widely used terminology is that proposed by Emery (1968) whose classification has a genetic basis. It distinguishes authigenic (chemically precipitated), organic (biologically produced shells, planktonic tests, etc.), residual (weathered from underlying rock), relict (sediments deposited in a different low sea level environment, such as a beach ridge, river delta or subaerial dune) and detrital (material presently supplied by rivers, glaciers or wind). Since most relict sediments were reworked during the post glacial transgression and/or are presently being reworked by modern shelf processes, the term palimpsest was later proposed to differentiate sediments which exhibit 'petrographic attributes of an earlier depositional environment and, in addition, petrographic attributes of a later environment' (Swift et al. 1971: 343). On the Australian shelf, residual and authigenic sediment types are generally minor, in terms of total volume, than are the other types named above. Detrital and organic (biogenic) sediment types are generally dominant although relict and palimpsest sediments may be locally important.