Australia: The Land Where Time Began
Great Australian Bight
The area that is usually referred to as the Great Australian Bight Marine Park is comprised of 2 sections, one administered by the federal government, the other by the South Australian government.
The Great Australian Bight stretches from Cape Catastrophe on the Eyre Peninsula of South Australia in the east to Cape Pasley, to the east of Esperance in Western Australia (Fig. 1, source 1). A very wide continental shelf is a feature of the Bight, in places more than 200 nautical miles wide. It is the longest stretch of east-west coastline in the world that is ice-free, facing the Southern and Antarctic Oceans.
The area has high biodiversity and a high level of endemism. Among the factors leading to these high levels of endemism and biodiversity are a long period of geological isolation, a high-energy environment that is persistent, in the east upwelling of nutrient-rich cold water and in the west intrusions of warm water brought by the Leeuwin Current down the West Australian coast and around the southern coast to the Bight region. Exceptionally high diversity is seen among some taxonomic groups such as red algae (sea weed), ascidians (sea squirts), bryozoans (lace corals), molluscs (shellfish) and echinoderms (sea urchins and sea stars).
The area is important for a number of endangered species, such as the southern right whale that comes to the region for the birth of its young, and the pupping area of the Australian Sea-lion, the only pinniped endemic to Australia. Humpback whales also visit the area, and there are a number of species of albatross. Great white sharks (previously known in Australia as white-pointer sharks), prowl around the seal colonies, seals being high on their list of favourite foods, said to be because of their high fat content, making them a rich source of energy.
The Nullarbor Plain forms the northern edge of the Bight, formed of ocean floor that has been uplifted. Mostly towards the eastern end, there are many islands and rocky headlands, embayments and surf beaches (James et al., 2001). Averaging about 80 m, the limestone cliffs stretch from the Head of Bight in South Australia, for 200 km to the West Australian border. No rivers or streams flow into the Bight from the Nullarbor Plain, as a result of the low rainfall and the limestone nature of the plain, the little rain that does fall on it either evaporates or in stored beneath the surface in holes and caverns that are present throughout the plain. There are also no true estuarine environments. The result of the lack of inflow from the land, with the associated lack of sediments being deposited, means the Pleistocene sand on the sea floor is not buried by later sediment.
The Great Australian Bight is part of a divergent, passive continental margin formed in the Cretaceous during a protracted period of extension and rifting associated with the separation of Australia from Antarctica at the final stage of the breakup of Gondwana (Willcox & Stagg, 1990; James et al., 2001; Norvack & Smith, 2000). The Gawler Craton in the east, and in the west the Yilgarn Block, both very ancient, form the basement rock of the margins of the Great Australian Bight. Submerged continental crust of Gondwana is present beneath the waters of the Bight, extending up to 500 km oceanwards in the centre of the Bight. This crust is much older than the Australian southern coastal margin, and incorporates some very old, poorly defined sedimentary basins, being more than 500 million years old (Willcox, 1998).
It has been found by studying the coastal region sediments that the area was initially a shallow embayment into which flowed large rivers, the full seaway between Australia and Antarctica forming about 30 Ma. This event is indicated by the replacement of the terrestrial and estuarine sediments by marine carbonate sedimentation that occurred from the Middle to Late Eocene, about 40 Ma, the resulting deposit being the Wilson Bluff Limestone that forms the lower strata, that are chalky, of the Nullarbor cliffs.
Large sedimentary basins, that have both onshore and offshore portions, were formed during the rifting of Australia from Antarctica (Li et al., 2003) (Fig. 3, source 1). One of these large basins that is partly offshore and partly onshore is the Eucla Basin, that extends about 350 km inland from the present coastline, and about 500 km seaward of the present coast, to the approximate foot-of-slope. The Eucla Basin is comprised on the Wilson Bluff Limestone and the hard crystalline limestone forming the upper part of the cliffs and the surface of the Nullarbor Plain, this crystalline limestone being younger, dating from the Miocene, 24-5 Ma.
Underlying part of the Eucla Basin, at depths ranging from 200-4000 m, is the Bight Basin, including the Eyre, Recherche and Ceduna Sub-basins. To the east is the Polda Basin, that extends beneath the Eyre Peninsula, and to the southeast is the Duntroon Basin. The Bremer Basin, south of the Yilgarn Block, lies to the west.
The ocean floor in this region has existed, essentially unchanged, for 20 million years, making it one of the most extensive of such areas in the world. Relict calcareous seafloor from the Pleistocene, about 1 Ma, is little affected by sedimentation, as the sedimentation rate is so low, is preserved in the region, from a time of low sea level. Features preserved include evidence of lagoon environments (Li et al., 1996) and bryozoan reef mounds (Holbourne et al., 2002).
Partly because of sealevel changes associated with climate change, such as glaciations, there is evidence of Pleistocene development in the coastal geology of the Bight.
A key bathymetric feature of the Great Australian Bight is the shallow continental shelf, a very large, arcuate, relatively flat submarine plain that is 80 km wide at either end and 260 km wide at the Head of Mouth (Harris et al., 2003; Willcox et al., 1988). 3 separate regions comprise the continental shelf (James et al., 2001), the inner shelf, where the water is <50 m deep, that includes all the waters of the State Marine Park, at depths of 50-120 m, the middle shelf and the outer shelf, that is 10-30 km wide and 125-170 m deep, extending to the shelf break.
There is a broad continental slope, including several terraces, in the Bight region that extends beyond the shelf. The Ceduna Terrace is the largest of these, being 700 km long and 200 km wide, in the central and eastern regions, beneath water that is between 200 and 3000 m deep (Hill et al., 2001; James et al., 2001). On the western margin, the Eyre Terrace is 200 km wide, at depths of 200-2000 m (Hill et al., 2001; James et al., 1994).
In the Commonwealth Park, the shelf margin, that is relatively featureless, apart from 2 pinnacles that are less than 100 m high and 700 m wide, situated near the 1750 m isobath, slopes gently down to 2500 m. The margin is steeper from this depth to 5000 m, and is faulted and carved by canyons, in some of which are large holes up to 500 m deep and 5 km wide (Hill et al., 2001). The southeastern apex of the Benthic Protection Zone extends to the edge of the South Australian Abyssal Plain.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|