Samantha Clarke

Submarine Landslides on the Upper Southeast Australian Passive Continental Margin – Preliminary Findings

The southeast Australian passive continental margin is narrow, steep and sediment-deficient, and characterized by relatively low rates of modern sedimentation. Upper slope (<1,200 m) sediments comprise mixtures of calcareous and terrigenous sand and mud. Three of twelve sediment cores recovered from geologically-recent, submarine landslides located offshore New South Wales/Queensland (NSW/QLD) are interpreted to have sampled failure surfaces at depths of between 85 and 220 cm below the present-day seabed. Differences in sediment physical properties are recorded above and below the three slide-plane boundaries. Sediment taken directly above the inferred submarine landslide failure surfaces and presumed to be post-landslide, returned radiocarbon ages of 15.8, 20.7 and 20.1 ka. The last two ages correspond to adjacent slide features, which are inferred to be consistent with their being triggered by a single event such as an earthquake. Slope stability models based on classical soil mechanics and measured sediment shear-strengths indicate that the upper slope sediments should be stable. However, multibeam sonar data reveal that many upper slope landslides occur across the margin and that submarine landsliding is a common process. We infer from these results that: (a) an unidentified mechanism regularly acts to reduce the shear resistance of these sediments to the very low values required to enable slope failure, and/or (b) the margin experiences seismic events that act to destabilise the slope sediments.

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 sandy-muds 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.

Morphology of Australia’s Eastern Continental Slope and Related Tsunami Hazard

Morphologic characterisation of five distinct, eastern Australian upper continental slope submarine landslides enabled modelling of their tsunami hazard. Flow depth, run-up and inundation distance has been calculated for each of the five landslides. Future submarine landslides with similar characteristics to these could generate tsunami with maximum flow depths ranging 5–10 m at the coastline, maximum run-up of 5 m and maximum inundation distances of 1 km.

Submarine Landslides and Incised Canyons of the Southeast Queensland Continental Margin

An investigation conducted aboard the RV Southern Surveyor (SS2013-V01) in January 2013 offshore east Australia collected regional bathymetric data for the continental margin of southern Queensland between Noosa Heads in the south and Indian Head, Fraser Island in the north. This newly mapped area presents a particularly steep portion of continental slope (5–10°) that presents numerous submarine landslides, including two ‘whole-of-slope’ features (the Wide Bay Canyon, and Inskip Slides). The slope is also dissected by three large submarine canyons offshore northern Fraser Island, Wide Bay, and Noosa Heads (i.e. the Fraser Canyons, the Wide Bay Canyon and the Noosa Canyon). Dredge and core samples were collected from slide scars in the northern, central, and southern areas of the bathymetric survey area. The initial examination of the area’s bathymetry, the core and dredge sample sedimentology, and determination of biostratigraphic ages for these sediment samples indicates that the larger submarine slides present in this study area have probably been shed from the slope since the late Pliocene and that canyon incision is currently active on this portion of the slope. In one case, canyon incision is partly responsible for generating slides due to undercutting and removal of the toe of the slope. Slope sediments are dominantly comprised of hemipelagic muds but also include grain-flows and turbidites comprised of shelf-derived sands and upper slope sediment that have abraided the slope muds. The results confirm previous work that indicates that this margin is in an active phase of deconstruction dominated by mass failure.

Sedimentology, structure and age estimate of five continental slope submarine landslides, eastern Australia

ABSTRACT Sedimentological and accelerator mass spectrometry (AMS) 14C data provide estimates of the structure and age of five submarine landslides (∼0.4–3 km3) present on eastern Australias continental slope between Noosa Heads and Yamba. Dating of the post-slide conformably deposited sediment indicates sediment accumulation rates between 0.017 m ka–1 and 0.2 m ka–1, which is consistent with previous estimates reported for this area. Boundary surfaces were identified in five continental slope cores at depths of 0.8 to 2.2 m below the present-day seafloor. Boundary surfaces present as a sharp colour-change across the surface, discernible but small increases in sediment stiffness, a slight increase in sediment bulk density of 0.1 g cm–3, and distinct gaps in AMS 14C ages of at least 25 ka. Boundary surfaces are interpreted to represent a slide plane detachment surface but are not necessarily the only ones or even the major ones. Sub-bottom profiler records indicate that: (1) the youngest identifiable sediment reflectors upslope from three submarine landslides terminate on and are truncated by slide rupture surfaces; (2) there is no obvious evidence for a post-slide sediment layer draped over, or burying, slide ruptures or exposed slide detachment surfaces; and (3) the boundary surfaces identified within the cores are unlikely to be near-surface slide surfaces within an overall larger en masse dislocation. These findings suggest that these submarine landslides are geologically recent (<25 ka), and that the boundary surfaces are either: (a) an erosional features that developed after the landslide, in which case the boundary surface age provides a minimum age for the landslide; or (b) detachment surfaces from which slabs of near-surface sediment were removed during landsliding, in which case the age of the sediment above the boundary surface indicates the approximate age of landsliding. While an earthquake-triggering mechanism is favoured for the initiation of submarine landslides on the eastern Australian margin, further evidence is required to confirm this interpretation.