Physical properties and age of continental slope sediments dredged from the eastern Australian continental margin - Implications for timing of slope failure

A large number of submarine landslides were identified on the continental slope of the southeastern Australian margin during voyages aboard the RV Southern Surveyor in 2008. Preliminary sedimentological, geotechnical and biostratigraphic data are reported for dredge samples of Neogene compacted, calcareous sandymuds recovered from submarine scarps located on the mid-continental slope. The scarps are interpreted to represent submarine landslide failure surfaces. Slope stability modeling using classical soil mechanics techniques and measured sediment shear-strengths indicate that the slopes should be stable; however, the ubiquity of evidence for mid-slope landslides on this margin indicates that their occurrence is a relatively commonplace event and that submarine-landsliding can probably be considered a normal characteristic of the margin. This presents an apparent contradiction that is probably resolved by one or both of the following: An as yet unidentified mechanism acts to reduce the shear resistance of these sediments to values low enough to enable slope failure; or geologically frequent seismic shaking events large enough to mobilise slides. It is hypothesised that the expansion of the Antarctic Icesheet in Mid-Miocene time and the consequent large-scale production of cold, equator-ward migrating, bottom water has caused significant erosion and removal of material from the mid and lower slope of the Australian continental margin in the Tasman Sea since the Mid-Miocene. It is also hypothesised that erosion due to equator-ward moving bottom water effectively and progressively removed material from the toe of the continental slope sediment wedge. This rendered the slope sediments that were deposited throughout the Tertiary more susceptible to mass failure than would have otherwise been the case.