The continental margin off east Tasmania and Gippsland: structure and development using new multibeam sonar data

Abstract

The continental margin facing the southern Tasman Sea off southeastern Australia is of high petroleum and geological significance because it contains Australia's richest oil province, the Gippsland Basin. Exploration activity in the region has concentrated on the shelfal part of this basin. Off Tasmania the upper margin is host to important deep sea trawl fisheries, and regional geophysical data suggest some long-term petroleum potential. In early 1997, 20,000 km2of the margin were surveyed by the Australian Geological Survey Organisation, in co-operation with Scripps Institution of Oceanography and the University of Sydney. The SeaBeam 2000 multibeam sonar system of the US RV Melville was used to map the topography and character of the seafloor in unprecedented detail. The SeaBeam 2000 is a 12 kHz, 121-beam echo-sounder that collects both bathymetry and backscatter (sidescan) data across a swath of ~3.4 times the water depth. The survey also collected 3500 km of magnetic and gravity profile data. The margin off Gippsland is dominated by a large embayment 100 km across and floored by a ESE-trending chasm, the Bass Canyon, which is 60 km long and 10?15 km wide. The canyon has cut down about 2 km into the margin and is bounded to the north and south by very steep inner canyon walls 1000 m high. Sediments transported down the canyon debouch at its mouth, at ~4000 m depth, and spread onto the abyssal plain via a network of distributary channels and levees. Sediment from the shelf is channelled into the entrance of Bass Canyon by three major, deeply-incised tributary canyons and a number of smaller ones. Bass Canyon has probably acted as a conduit for clastic sediment to the deep sea floor since breakup at ~80 Ma. Seismic profiles over the Bass Canyon complex show that canyon erosion has exposed sections of almost the entire Gippsland Basin sequence down to the Early Cretaceous and underlying basement. Numerous suitable dredge and core sampling sites in the canyons have been identified for a geological sampling program. Off east Tasmania, the survey showed the margin to be generally steep and rugged, with relatively little sediment having been deposited on the continental slope, except in narrow rifts, since breakup. The adjacent abyssal plain is commonly underlain by 2 km of Late Cretaceous-Cainozoic sedimentary section. The continental slope is cut by an extensive system of canyons, some more than 30 km long and 500 m deep. Large fault blocks on the lower slope show structudral rends consistent with a NW-NNW rift direction and NE-ENE transfer direction. The upper slope appears to Volcanics, of probable late Cainozoic age, are fairly widespread on the seafloor off northeast Tasmania. Such volcanic terrain includes a number of scattered cones, the largest 450 m high.