Process-based delineation of the Australian continental margin
Creators: name: Smith, Rick
Publisher: name: University Of Tasmania
Year: 2023

description: This study seeks to understand and explain the present-day gross morphology of the Australian continental margin. It defines a series of robust and quantifiable process-based delineations which extend from the coastline to the foot of the continental slope through an analysis of geomorphic forms. The form and structure of the continental margin (namely the coast, shelf and slope) are the result of the interplay between geology and formative processes. The geomorphic forms of Australia's margin are investigated with a strong emphasis on spatial and statistical analyses to define the relationships in geophysical data. Data included modelled estimates of wave and tide power, continental shelf sediment distribution, an elevation-bathymetric model and mapping of major geological structural elements and lithology. A geographical information system was used to integrate and compare these variables and map the results. The thesis is divided into three distinct bodies of work. The first considers the relative importance of waves and tides on the Australian continental shelf to produce a delineation of wave- and tide-dominated shelves based on quantitative measures. The second investigates the development of mesoscale coastal complexity through the modulation of geomorphic form by the aforementjoned marine processes over geologic time. The third examines the interactions between the continental slope and the continental shelf and deep ocean basin. A catchment-based approach is applied to characterise discrete drainage networks and the influence of marine processes at the shelf-slope interface. The Australian continental shelf is found to be mostly dominated by tides rather than by waves. This contrasts with empirical estimations that most of the world's shelves are dominated by waves, rather than by tidal currents. Additionally, the delineation of the shelf based upon modelled wave, tide and sediment entrainment demonstrated that the results contradict the conventional, graded shelf model, which suggests that mud content increases in an offshore direction with lower energy and increasing depth. The study also confirms that these coarse-grained sediments are not in hydraulic equilibrium with the prevailing tide and wave current regime. Terrestrial processes are the primary drivers of coastal complexity. Various geological regions respond differently to marine processes, depending on whether the lithology is homogeneous or heterogeneous. The hypothesis that wave action causes a coastline to become more complex over time is challenged by this study, which found that, for Australia, wave power is inversely proportional to coastal complexity. This is a reflection of the presence of homogenous lithology in wave-dominated waters around much of Australia and the opportunistic action of waves in attacking and eroding sections of coastline. Submarine canyon networks are demonstrated to be similar to terrestrial drainage systems, so that a catchment-based approach to delineating these canyons is appropriate. Lengthening of canyon path and compression of canyon width indicates that the drainage morphology in that particular catchment is being impeded or affected, therefore indicating control by geological inheritance. Within each of these submarine catchments, the derived drainage networks representing the dendritic canyon systems display to a greater or lesser degree characteristics associated with geological constraints through the derivation of Horton ratios (of bifurcation length and slope ratios) highlighting regional differences. This thesis provides crucial quantitative information about processes and form. The results of this investigation can be applied to revise the zonation of continental margins through the identification of bathomic structures, rather than the classical definitions of neritic and bathyal zonation which are currently used in regional marine planning. Consequently, this work will contribute to ongoing research in the classification of the Australian continental margin by utilising available geophysical datasets to establish process-based boundaries. There are many potential applications of such process-based, quantitative delineation to the continental margin, particularly in marine planning and management.