Southeast Australia: A Cenozoic Continental Margin Dominated by Mass Transport

Abstract

The Southeast Australian continental margin extends for 1,500 km northward from Bass Strait to the Great Barrier Reef. Mass transport dominates the continental slope, which stretches from the shelf break around 150 m depth to the abyssal plain around 4,500 m depth. The continental slope has average slopes of 2.8–8.5° and extends seaward from the shelf break an average distance of 50 km. Margin structure results from Late Cretaceous rifting, producing exposed fault blocks and igneous complexes on the lower slope, and an overlying sediment wedge around 0.5 km thick, centered at the shelf break. Recent collection of multibeam echosounding and high-resolution seismic data provide a detailed view of mass-transport features over a 900 km length of the margin. The features are mostly slab slides, box canyons, and linear canyons. They are ubiquitous along the steep rifted margin, but absent in regions of gentler slopes such as submarine plateaus and failed rift arms. Submarine landslides range in scale from hundreds of small slides of <0.5 km³ volume, up to the largest documented slide of 20 km³. However, potential future slide masses of basement blocks up to 105 km³ have been identified. Cores that penetrated the basal-slide surface show variable sediment accumulation, since the mass-movement event, but four penetrations show accumulations of <2 m, and one of <0.6 m. At current accumulation rates, these data indicate that many landslides occurred less than 25 ka, with some as recent as 6 ka. Mass movements appear to follow a pattern of box canyon development exploiting structural trends in pre-rift and syn-rift strata, until the canyon head intersects the toe of the Tertiary sediment wedge. Once this occurs, sediment creep, faulting and failure of the wedge toe migrates up slope, finally reaching the upper slope and Quaternary deltaic depocenters.