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ABSTRACT: GLORIA Acoustic Backscatter Imagery of the Steep and Thinly Continental Margin of Southeast Australia

Chris Jenkins

A brief GLORIA deployment in September 1989 obtained acoustic backscatter imagery of the continental slope southeast of Sydney, between 34° and 35°S on the western side of the Tasman Sea. Initial (shipboard) mosaic images are displayed.

This particular continental slope is steep and only thinly covered with post-rifting sediments. The distance from shelf-break to abyssal plain is only 35-80 km wide. The post-rifting sediments (Upper Cretaceous-Cenozoic) reach maximum thicknesses of 200-1200 m in the upper slope progradational wedge and in small intraslope basins. Continental basement rocks (chiefly Paleozoic) crop out over wide areas of the middle and lower slope; in these areas the slope topography still shows detailed features of the rifling geometry (transform faults and half-graben structures).

The GLORIA images revealed a distinct zonation of the slope: (1) Front face of the upper slope prograding sediment wedge, a gray

response zone known to have sediment thicknesses and marked at the seafloor with (a) numerous small, seaward-facing scarps parallel to contour and 1-2 km in length (small-scale mass-wasting?); (b) broadly curved ridges of gray reflectivity and many kilometers in extent (sediment waves under the East Australian Current?); (c) small, deeply incised submarine canyons, often cut to a planated basement surface; and (d) large, individual slope failures of up to 25 km down slope dimension. (2) Sedimented midslope zone of rough topography, but mostly gray acoustic response indicating significant sediment cover; fields of curved sediment ridges sometimes also present. (3) Reflective lower slope, showing rough topography, steep slopes, and large (10-30 km long) scarps--probably representing fau t structures imposed by rifting of this passive margin.

Other features highlight the sediment-starved nature of the margin. The larger submarine canyons frequently end at a small turbidite fan that is enclosed within the mouth of the canyon. Eddy structures of the East Australian Current are known to have strong effects to abyssal depths; this is evidenced not only by sediment wave fields, but also by current erosion of sediment from the continental slope.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990