In early 1994, the Australian Geological Survey Organisation (AGSO) will use the multibeam sonarsystem of the French research vessel l'Atalante for 35 days mapping of 200 000 lcm2 of the continental margin (southern Otway Basin, Sore11 Basin, South Tasman Rise) and the adjacentabyssal plain. The cruise will start in Wellington on 16 January and finish in Adelaide on 27 February. Data will be recorded on the transits, as well as near Tasmania. The SIMRAD EM12D system provides bathymetric maps and acoustic imagery in real time, and can map an area alongeach track up to 20 km wide at a speed of 10 knots. The maps will define batliymetry and surface texture with a degree of accuracy and rate of coverage unobtainable in any other way. In addition we will record 6 channel seismic reflection, magnetic, gravity and bathymetric profiles. Thismapping of an area three times that of Tasmania will help to clarify the region's structural pattern and tectonic history, largely controlled by the separation of Australia and Antarctica about 40million years ago. We will also map large-scale sedimentary structures and patterns to help elucidate Tertiary sedimentary history, and morphology to help define Australia's Legal Continental Shelf for the day when Australia ratifies the Law of the Sea Convention (expected within a coupleof years).
The region's geological history bears on the history of the entire southern margin of Australia. This margin is already a major producer of petroleum from the Upper Cretaceous and Paleocene sequences in the Gippsland Basin, and has encouraging exploration results in several other basins.The Otway Basin, in particular, is the scene of major recent offshore gas discoveries in the Cretaceous sequence. The very accurate bathymetric maps and sonar images arising from thissurvey will provide an unequalled source of structural information of great value to AGSO and the petroleum exploration industry. The mapping of the fault patterns that come to the surface on the continental margin, believed to be part of a large, NNW to NW trending strike-slip system that firstformed in the Early Cretaceous, will lead to much better understanding of fault geometry. It also will aid the planning of future AGSO seismic reflection programs, aimed at mapping deep structurealong the margin.
The mapping of the abyssal plain to the west will bear closely on the breakup history of Australiaand Antarctica. It is generally believed that Late Cretaceous and Cainozoic spreading took place along north-south fracture zones that are clearly visible in satellite altimetry data, but also there isevidence of earlier, probably Early Cretaceous, oceanic crust in which the fracture patterns trend northwestward. Swath-mapping imagery will give us more detail of trends, and also locate goodsites for sampling basalts for dating and geochemical examination. In addition, it is uncertain howfar continental blocks extend out onto the plain, and the mapping of all blocks will enable future sampling is properly targeted to resolve this important scientific question.
The mapping of rocky outcrops, sedimentary structures, and sedimentary patterns will be invaluablein planning a forthcoming AGSO seafloor sampling cruise. This sampling will target Cretaceous, Paleocene and Eocene rocks to provide information on the pre-separation history of the margin, oceanic basalts to elucidate the spreading history, and Oligocene and younger sediments to provideinformation on changes in oceanic circulation and climate as Australia moved steadily north from Antarctica.
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