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

Abstract

The southeast Australian passive continental margin is narrow, steep and sediment-deficient, and characterized by relatively low rates of modern sedimentation. Upper slope (<1200m) 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 cm 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 ka, 20.7 ka 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 shearstrengths 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.

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

“…These features are representative of slope failures that occurred in the two dominant slope morphologies present in the study area (see Figure 3) identified by Boyd et al (2010), Clarke et al (2012), and Clarke (2014): a) the relatively steep (3-7°) and canyon-incised slope (Bribie Bowl Slide and Bryon Slide); and b) the relatively gentle slope (1-3°) of the Nerang Plateau (Coolangatta-1, -2 slides and Cudgen Slide). Ten gravity cores from the upper continental slope (<1200 m) were examined in this work (Figure 2), with at least one gravity core recovered from each of the five submarine landslide scars investigated.…”

“…The margin is relatively narrow and typically presents a steep, thinly sedimented upper and middle slope with continental bedrock exposed on its lower slope ( Figure 5) (Boyd et al, 2010;Keene, Baker, Tran & Potter, 2008;Marshall, 1978Marshall, , 1979. The present day morphology of the slope is dominated by erosion and mass wasting (Boyd et al, 2010;Clarke et al, 2012;Glenn et al, 2008;Hubble et al, 2012).…”

“…Subsequent mapping of this margin with the next generation of high resolution, multibeam echo-sounding equipment between 2006 and 2013, has provided sufficient information and morphological data to establish that there are a surprisingly large number of submarine landslides evident on the continental slope (Boyd et al, 2010;Glenn et al, 2008;Hubble, Airey, Sealey, De Carli & Clarke, 2013) given this margin's well-recognised low sedimentation rates (Boyd, Ruming & Roberts, 2004) and passive margin tectonic setting. Instead of the relative quiescence and stability that might be expected, recent studies have demonstrated that submarine sliding should be considered to be a common and ongoing characteristic of the continental slope offshore eastern Australia (EA) south of Fraser Island (Boyd et al, 2010;Clarke et al, 2012;Hubble et al, 2012). This paper presents a study of upper continental slope sediments collected from the EACM and is part of a larger body of work intended to determine the frequency and consequences of submarine landsliding in this part of the margin (c.f.…”

“…Boyd et al, 2010;Clarke, 2014;Clarke et al, 2012;Hubble et al, 2012Hubble et al, , 2016. This paper aims to establish the geological and sedimentological characteristics of the materials in which geologically recent (<25 ka) submarine landsliding has occurred (Boyd et al, 2010;Clarke et al, 2012) and examines material sampled from ten gravity cores from the upper slope between 2-5 m long that were collected onboard RV Southern Surveyor in November 2008 (SS2008-V12; Boyd et al, 2010). Five cores that contain boundary surfaces are identified by a sharp, colour-change boundary, small increases in sediment stiffness, slight increases in sediment density, and distinct gaps in AMS 14 C age of at least 25 ka.…”

“…This paper presents a study of upper continental slope sediments collected from the EACM and is part of a larger body of work intended to determine the frequency and consequences of submarine landsliding in this part of the margin (c.f. Boyd et al, 2010;Clarke, 2014;Clarke et al, 2012;Hubble et al, 2012Hubble et al, , 2016. This paper aims to establish the geological and sedimentological characteristics of the materials in which geologically recent (<25 ka) submarine landsliding has occurred (Boyd et al, 2010;Clarke et al, 2012) and examines material sampled from ten gravity cores from the upper slope between 2-5 m long that were collected onboard RV Southern Surveyor in November 2008 (SS2008-V12; Boyd et al, 2010).…”