SUBMARINE SLOPE FAILURES OF THE SOUTHEAST AUSTRALIAN CONTINENTAL-SLOPE - A THINLY SEDIMENTED MARGIN

Large-scale slope failure structures are observed in geophysical profiles from the southeast Australian continental slope. On the upper slope, almost-equal-to 25-400 m thick rotational-translational slides, up to 25 km in downslope length, are developed on areas of 2-6-degrees gradient and commonly involve almost-equal-to 1 km of sediment movement. Evidence suggests that they are geologically recent (Quaternary?). 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). Most upper slope slides have a tensionally deformed upper part and a compressionally deformed lower part. The most common situation for formation of tensional failure surfaces is at 100-150 m subbottom depth with 5-6-degrees gradient of failure surface. A threshold, that depends on both failure surface slope and depth, is defined. Compressional deformations occur at and upslope from buttressing of the slides by basement highs on the stope. In lower slope basins and at the foot of slope, large masses of disturbed sediment have been emplaced as a result of sliding. Photographic evidence illustrates seafloor instabilities at the seafloor after emplacement of a debris flow on the lower slope.