Cruise Proposal : AGSO Cruise 147 : Stratigraphy, Tectonic History and Palaeoclimate of the Offshore Tasmanian Region

The Tasmanian region's geological history bears on the history of the entire southern margin of Australia. The southern margin is already a major producer of petroleum from the Upper Cretaceous and Paleocene sequences in the Gippsland Basin, and has given 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. Both the west Tasmanian margin and the South Tasman Rise have petroleum potential. The targets on the forthcoming Rig Seismic sampling cruise of 31 days (AGSO Cruise 147) are on the west Tasmanian margin, the South Tasman Rise, the East Tasman Rise, and the adjacent oceanic areas. The general bathymetry of the region is shown in Figure 1. The targets were defined by the early 1994 Tasmante cruise (Exon et al., 1994), which used the French research vessel l' Atalante on an exchange basis to swath-map the sea bed. About two-thirds of both the South Tasman Rise (STR) and the west Tasmanian margin were mapped, an areal coverage exceeding 200 000 km 2. The very accurate 1:250 000 scale bathymetric maps and sonar images arising from the Tasmante survey provide an unequalled source of structural information. A simplified bathymetric map of the region, based largely on the Tasmante swath-mapping, is shown in Figure 2. The aim of the present sampling cruise is to ground truth the maps and ideas arising from the Tasmante cruise. It is designed to sample basement and Phanerozoic sedimentary rocks in all three continental blocks, Cainozoic volcanics in continental and oceanic terrains, and Cainozoic sediments on the continental blocks, by dredging and coring. This will help elucidate the Phanerozoic history of the area with emphasis on petroleum geology and plate tectonic hisory, and also the Cainozoic and especially Quaternary history with emphasis on plate tectonic history, and changes in climate and sedimentation patterns. Water column sampling is designed to provide more information about oceanographic controls on sedimentation. An additional aim is to sample manganese nodules in deep water to assess them as a potential resource, by the use of free-fall grabs. As a result of interest in the volcanic cones about 100 km of Hobart, mapped during the Tasmante cruise, which define major fishing grounds, two photographic profiles and one dredge are planned for there. The main sampling areas are discussed briefly below.