Authors: Puga-Bernabéu, A., Webster, J.M., Beaman, R.J.
Publication: 29th IAS Meeting of Sedimentology, 10-13 Sep 2012. International Association of Sedimentologists (IAS), Schladming, Austria
Recent studies along the Great Barrier Reef (GBR) margin confirm it is heavily incised by canyons of various size and morphology that carry coarse?grained sediments into the adjacent Queensland Trough. However, less attention has focused on the slope failures and related debris that also shape this margin. These features are large enough to suggest potential hazard from landslide-generated tsunamis, and may also control the location of previously unknown deep-water benthic habitats.
We focus on the mass-wasting processes in the Noggin Passage region, north-eastern Australia. First, we used new multibeam and sub-bottom profile data to locate and characterize the morphology of the mass-wasting deposits and related features. Second, basic slope stability simulations and estimation of the maximum tsunami wave and runup height were performed to characterize the conditions for potential failure and tsunami generation. Finally, we present evidence of a cold-water coral bank on top of one large landslide block.
In the study area, several slope failures have modified the seafloor morphology. These failures include medium-scale landslides in the middle and lower slope. These landslides are a few km wide, with steep headscarps up to 250 m high. In the upper slope, a small circular area of about 5.3 km2 with well-defined margins has been identified. The margins correspond to different bounding seabed features indicative of sediment disruption, and can be considered as the precursors of a larger sediment failure. The largest landslide in the area is the Gloria Knolls Slide (GKS) which excavates a scarp failure area of about 178 km2 into the GBR margin. This scarp extends 20 km along and about 8 km across the slope, with a headscarp height up to 830 m. The net loss volume from the margin results in about 32 km3 of slope sediments. A cluster of eight knolls up to 179 m high and covering areas of up to 7.9 km2 occur 20 to 30 km downslope from the GKS.
Slope stability simulations indicate that the upper slope in the GBR margin is stable under normal present day gravitational conditions. However, instability conditions occurred when seismic loading was considered. Failure modelling shows that critical peak horizontal accelerations of 0.2 to 0.4 g could lead to the collapse of the Noggin slope. In north-eastern Australia these acceleration values would involve earthquakes generated at short hypocentral distances and short periods.
Semi-empirical equations based on numerical simulations suggest that the maximum initial tsunami amplitude potentially generated by the slope failures could be several meters in the case of the small failures to several 10s of meters for the GKS. Similar runup heights at the coast were also estimated, possibly consistent with previously identified tsunami deposits on the coast.
A dredge sample from the top of the largest knoll has revealed the presence of a thriving cold-water coral community, consisting of abundant live and fossil corals, mollusks, and barnacles. This type of community represents the first report of a cold-water coral bank in north-eastern Australia waters.
The results of this study represent a first order approach to assist in improving assessment of the potential for tsunami induced by submarine landslides in Australia. It also has important implications for locating similar cold-water coral bank habitats on other knolls or blocks derived from landslides on the GBR margin.