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ABSTRACT: Submarine Slope Failures on the Steep, Thinly Sedimented Continental Margin of Southeastern Australia

Chris Jenkins

The Southeastern Australian continental margin, on the western side of the Tasman Sea, 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 rifting geometry (transform faults and half-graben structures).

The younger Cenozoic sediments have undergone extensive slope failure despite the seismically inactive, nonglacial, and thinly sedimented nature of this margin. On the upper slope, ^sim300 m thick rotational-translational slides of up to 25 km downslope length are developed, which commonly involve ^sim1 km of sediment movement and sediment volumes of up to 300 km3. The evidence suggests that the major slides are geologically recent (?Pleistocene or younger). Slope areas at the seaward edge of the late Cenozoic shelf edge progradation have been the most unstable, especially where the sediments are relatively thick (200-400 m) and are not buttressed by a basement high. In a number of cases, large masses of disturbed sediment have traveled 10-20 km distance downslope, to be em laced in lower slope basins or at the foot of slope.

Detailed morphometric measurements from seismic profiles of the slides provide information on the slope stability in this area, where geotechnical investigations have not yet been conducted. The data relate to large-scale and in situ (deep subsurface) properties of the slope sediments (layered sand-silt-clays and sands) and are also related to methods of geotechnical slope stability analysis, i.e., the "infinite slope" and "Taylor 1948" models. They are used to identify: (1) the geological situations where the slope movement has been most frequent, and (2) parameters that may allow slope stability to be compared between continental margins of contrasting sediment type, slope conditions, and history.

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