Submarine landslides along the south-east Australian continental margin: an assessment of their tsunamigenic potential and tsunami hazard

- Mollison, Kendall Clare

Title
Submarine landslides along the south-east Australian continental margin: an assessment of their tsunamigenic potential and tsunami hazard
Creator
Mollison, Kendall Clare
Relation
University of Newcastle Research Higher Degree Thesis
Resource Type
thesis
Date
2021
Description
Research Doctorate - Doctor of Philosophy (PhD)
Description
Submarine landslides (SMLS) are a type of submarine mass movement that occur in a range of geological settings, including the continental slopes of passive margins. Extensive evidence of SMLS events is present along the south-east Australian continental margin (SEACM) and many of the SMLSs have been shown to have tsunamigenic potential. Despite this extensive evidence, the tsunami hazard posed by SMLSs along the SEACM and the role of sedimentology on their tsunamigenic potential is not well understood. This thesis presents an investigation of the tsunami hazard posed by modern SMLS events along the SEACM through the use of idealised and case study numerical modelling and analyses sediment core data from two SMLSs to better constrain their sedimentology, age, and recurrence interval. Sedimentological analysis and radiocarbon dating of six marine sediment cores collected from the upper continental slope offshore from Byron Bay, New South Wales, and Fraser Island, Queensland, indicate that the SEACM region is dynamic and provide strong evidence for mass movement processes occurring during the Late Pleistocene and Holocene. In particular, the three cores collected offshore from Fraser Island within the Wide Bay Canyon slide and adjacent open slope present intraformational, mud-clast conglomerates, comprised of sub-angular to rounded, pebbled and cobble-sized clasts of the hemipelagic muds that typically accumulate in this area. These conglomerates are interpreted to be debris flow deposits. One within-slide core and the adjacent open slope core also present coarse-grained, poorly sorted, unconsolidated sand layers that are interpreted to be grain flow deposits derived from continental shelf sands driven over the continental shelf edge by the East Australia Current during the Last Glacial Maximum. A similar grain flow deposit is observed in one of the three sediment cores analysed from within the Byron slide offshore from Byron Bay, but no debris flow deposits analogous to those observed in the Fraser Island region were identified in sediments from the Byron slide, which present as homogenous and uniform in nature. Radiocarbon dating of unique deposits identified in the Wide Bay Canyon slide cores indicates that abrasion and sediment removal during the emplacement of debris flows occurred during several separate events prior to ∼35.1 ka. The sand layers interpreted to be grain flow deposits contain Late Pleistocene age material that was likely re-deposited during the sea level low-stand associated with the Last Glacial Maximum. Paraconformities were identified in one core from the Byron slide and one from the Wide Bay Canyon slide by distinct colour changes which were found to represent distinct radiocarbon age gaps in each core. These paraconfomity surfaces are interpreted to represent the basal surface of these SMLSs, which occurred at least ∼35 ka ago for the Wide Bay Canyon slide and ∼25 ka ago for the Byron slide. The mass failure events documented in this study are separate, distinct events from other SMLSs observed in the region and are consistent with a large, relatively rare intra-plate earthquakes being a likely triggering mechanism of failure. The recurrence interval of dated SMLS events was refined in this thesis through the addition of the Byron and Wide Bay Canyon slide dating results, and is suggested to be 1 in every ∼6,500 years. This recurrence interval is within the range of previously suggested recurrence intervals for SMLSs along the SEACM, with a significantly longer recurrence interval suggested for hazardous tsunamigenic SMLS events. To assess the tsunami hazard posed by SMLS events along the SEACM, the opensource numerical model, Basilisk, was used to model both idealised and real world SMLS-generated tsunami along an idealised margin and four prehistoric SMLS sites. The two-layer Basilisk model was validated against two benchmark test cases, comprising laboratory and field data, and showed good agreement with observations. This model was applied to a Monte Carlo style analysis of SMLS-generated tsunami along an idealised SEACM to assess how a range of SMLS parameters affect tsunamigenic potential. It was found that SMLS volume was the most important parameter in determining SMLS hazard along the idealised margin, with SMLS thickness being the most important individual parameter. SMLS width, length, water depth, and density were shown to have smaller influences on tsunami hazard. When SMLSs were modelled within an idealised canyon, as compared with an idealised open continental slope, the spatial variation of tsunami hazard for adjacent coastlines was greatly increased. High resolution numerical modelling was conducted for four prehistoric SMLSs along the SEACM which had been previously identified as having hazardous tsunamigenic potential for adjacent coastlines. This modelling was done to understand the modern hazard posed by SMLS events such as those that have failed in the geologically recent past and to identify regions of the modern south-east Australian coastline vulnerable to tsunami impacts. Similar trends were observed between the prehistoric modelling results and idealised modelling results. SMLS volume and density were both shown to be important factors in determining tsunamigenic potential, as well as the presence of canyons along the continental slope. It was further confirmed that SMLS-generated tsunami pose a significant hazard for the south-east Australian coastline, with large population centers, such as Sydney, vulnerable to destructive tsunami impacts. In particular, estuarine environments were shown to be disproportionately exposed to the hazard when compared with the open coast. SMLS failure is considered an ongoing process along the SEACM and has been shown to be a dangerous tsunami source for the adjacent coastlines. The work presented in this thesis highlights the importance of understanding SMLS sedimentology and incorporating this information into high resolution site specific numerical modelling to better understand the tsunami hazard posed by these events. This information can be used to facilitate the development of appropriate hazard mitigation strategies for vulnerable coastal communities.
Subject
mass movements; submarine landslides; south-east Australia; continental slopes; sedimentation rates; numerical modelling; tsunami; natural hazards; radiocarbon dating; marine geology
Identifier
http://hdl.handle.net/1959.13/1495689
Identifier
uon:54036
Rights
Copyright 2021 Kendall Clare Mollison
Language
eng
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