The Albany Canyon complex off southwest Australia extends 700 km from Cape Leeuwin to east of Esperance. The Canyons head on the uppermost continental slope and extend up to 90 km offshore, to the lowermost slope and onto the abyssal plain. The largest have cut down 1500-2000 m in places. In general, on the upper slope they have cut down into harder, older rocks: Canyon walls are steep, thalwegs slope at up to 15?, and ancient structures control their orientation. On the lower slope the Canyons generally have not reached harder rocks, Canyon walls are less steep, thalweg slopes are less, and they are generally oriented down slope. The Canyons have exposed Jurassic and younger sequences: their nature and information from seismic profiles, have helped us build an understanding of Canyon history. Flood plain deposition rather than erosion occurred during Australia-Antarctic rifting in the Late Jurassic, not supporting cutting of river Canyons. Shallow marine sedimentation characterised the Early Cretaceous, when gradients were low and Canyon cutting unlikely. Deep river Canyons were probably cut during uplift and erosion immediately before the Santonian break-up from Antarctica, and their paths controlled later marine Canyons. Only with the onset of rapid seafloor spreading and subsidence in the Middle Eocene (~43 Ma) did gradients steepen and major marine Canyon cutting become possible. The major sea level fall at the Middle/Late Eocene boundary (~40 Ma) brought sediment to the edge of the continental shelf, which may have initiated the Canyons. Carbonate sedimentation replaced siliciclastic sedimentation in the Late Middle Eocene, but carbonate grains from the outer shelf could cut the Canyons, largely during periods of low sea level.

Map showing the Geomorphic Features of the Australian Margin and Island Territories. The features were interpreted from Geoscience Australia's 250 m horizontal bathymetry model and other published data, and include those specified in the International Hydrographic Office definitions.

The Eltanin Geophysical datasets are a major resource for examining the geology of the Southern Ocean. Unfortunately, the data have been loaded into Geoscience Australia databases in a rather piecemeal fashion over two decades leading to a number of inconsistencies and duplication. This report documents what Geoscience Australia?s data holdings are, the nature of the problems found and their solution. The report hopes to provide a level of confidence to previous and future users of the data by showing that the status of the data is well understood and documented. This report is intended to be the first of a series of reports examing the various datasets that make up the Geoscience Australia marine, geophysical database OZMARII.

This report presents the results of a regional seafloor mapping study carried out during 2000/2001 as part of Geoscience Australia's South and Southwest Regional Project. The aim was to support future Regional Marine Planning in the Great Australian Bight (GAB) by underpinning biological, environmental and economic assessments with basic information on geomorphology and the seabed character. Four major geomorphological features are present on the margin in the South and Southwest (SSW) region: a continental shelf, marine terraces (including the Eyre and Ceduna Terraces in the GAB), a continental slope and a continental rise. The boundaries of these geomorphological features have been delineated and captured in a Geographical Information System (GIS). The GIS also includes the location of sedimentary basins, plateaus, terraces and canyons previously mapped in the region. Seabed character mapping was carried out for the GAB area only. Five echo facies have been defined in the GAB area based on the interpretation of available 3.5kHz echo-sounding records and high-resolution seismic profiles in terms of acoustic facies, and their groundtruthing against seafloor samples. The interpretation of these facies has been digitised and captured into a GIS. The GIS includes key attributes for every echo facies. The acoustic facies distribution on the GAB margin and offshore in the South Australian abyssal plain shows the importance of geological inheritance to the geomorphology and sea-bed character of the region. Facies I, which represents undisturbed, layered sediments is mainly localised on the shelf, the Eyre and Ceduna Terraces, and in the greater part of the abyssal plain. Facies II, which may represent more disturbed sediments, is confined to the Ceduna Terrace and along two elongated E-W trending areas on the abyssal plain near the continent-ocean boundary. Facies III, associated with extreme (IIIA), moderate (IIIC) and low (IIID) topography, underlies scarps, canyons, and depressions on the continental slope and the abyssal plain. The distribution of acoustic facies from the upper slope down to the abyssal plain indicates that the major sedimentary process in the deep water GAB is deposition of pelagic sediments. Reworking of sediments by both bottom currents and gravity flows is probably limited to submarine canyons.

Ausgeo News Article for the release of the Australian Bathymetry and Topography Grid June 2009

The definition of Australia?s marine jurisdiction, under the 1982 United Nations Convention on the Law of the Sea (LOS), may need to take into account the location of the 2500m seafloor contour. A large amount of water depth data from a variety of sources have been used to produce bathymetric grids and contours around the Australian region by the LOS Study Group at Geoscience Australia. It is well known that the data are far from perfect, containing problems in depth soundings as well as navigation. The LOS Group wished to determine a quantitative measure for the accuracy of the data they have used. This study took the data used in the Naturaliste Plateau area and determined the intersection points of all the survey tracks. From this it computed interpolated water depths at the intersections and calculated mistie values between the intersecting surveys. In all 12411 intersections were found. Not all were useable, mainly due to the long intervals between soundings. Some surveys contained numerous intersections in very shallow water (~ 40 m) with excellent mistie statistics (+/- 1 m), but it was felt that these could unfairly bias the derived statistics. Taking these considerations into account reduced the number of intersecting points and mistie values that went into the statistical analysis to 5606. The procedures and programs employed in this report can easily be extended to all the other areas investigated by the LOS Group. This will be done in due course, with a future report summarising the statistics for each of the LOS study areas and making recommendations for the improvement of the bathymetric data held by Geoscience Australia.

Legacy product - no abstract available

Australia's marine jurisdiction is one of the largest and most diverse in the world and surprisingly our knowledge of the biological diversity, marine ecosystems and the physical environment is limited. Acquiring and assembling high resolution seabed bathymetric data is a mandatory step in achieving the goal of increasing our knowledge of the marine environment because models of seabed morphology derived from these data provide useful insights into the physical processes acting on the seabed and the location of different types of habitats. Another important application of detailed bathymetric data is the modelling of hazards such tsunami and storms as they interact with the shelf and coast. Hydrodynamic equations used in tsunami modelling are insensitive to small changes in the earthquake source model, however, small changes in the bathymetry of the shelf and nearshore can have a dramatic effect on model outputs. Therefore, accurate detailed bathymetry data are essential. Geoscience Australia has created high resolution bathymetry grids (at 250, 100, 50 and 10 metres) for Christmas, Cocos (Keeling), Lord Howe and Norfolk Islands. An exhaustive search was conducted finding all available bathymetry such as multibeam swath, laser airborne depth sounder, conventional echo sounder, satellite derived bathymetry and naval charts. Much of this data has been sourced from Geoscience Australia's holdings as well as the CSIRO, the Australian Hydrographic Service and foreign institutions.Onshore data was sourced from Geoscience Australia and other Commonwealth institutions. The final product is a seamless combined Digital Bathymetric Model (DBM) and Digital Elevation Model (DEM).The new Geoscience Australia grids are a vast improvement on the existing publicly available grids.

The RV Franklin sailed from Brisbane on 17th January 2002 and returned to Cairns on 9th February, 2002. The cruise discovered that a zone of strong tidal currents at the northern end of the Great Barrier Reef prevents the southward advance of sediment that would otherwise bury the coral reefs. The Fly River, located in close proximity to the northern end of the Great Barrier Reef, discharges about 120 million tonnes/yr of sediment. This sediment does not penetrate as far south into the reef area as might be expected because, over glacial-interglacial cycles of sea level change, the southward-prograding deposits are eroded by tidal currents. Deployment of an instrumented current meter and suspended sediment measurement frame on the seabed, offshore from the Fly River Delta, recorded a net sediment advection southwards. Sediment transport was greatest following a northerly wind event, which caused high bottom stress and increased turbidity levels. Swath sonar mapping and underwater video equipment were used to map a series of channels up to 220 m deep extending from eastern Torres Strait across the northern end of the Great Barrier Reef. Channels in the north are clearly relict fluvial channels, exhibiting lateral accretion surfaces and incised channels that intersect and truncate underlying strata. Over-deepened channels in the south, however, appear to have formed by tidal current scour. They exhibit closed bathymetric contours at both ends and are floored with well-sorted carbonate gravely sand. Oceanographic observations indicate that the channels provide a conduit onto the shelf for up-welled Coral Sea water. The deepest channels form isolated depressions, and possibly were the sites of lakes during the last ice age. Preliminary modelling indicates that the strongest tidal currents occur when sea level is about 40m below present, suggesting that the channels are Pleistocene or older in age and of relict origin.

Large barchan shaped sand deposits have been observed in the north west of Torres Strait. These deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks. Satellite imagery shows that the deposits migrate in the direction indicated by their horns (10-15 m west per year), and that sediment is shed from their horns, features that are characteristic of subaerial barchan dunes. However the orientation of sandwaves superimposed upon the sand banks indicate the presence of mutually evasive channels and circulation of sediment around the sand bank, a characteristic of subaqueous sand banks. The presence of mutually evasive channels is the criteria used to categorise the deposits as sand banks. Barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes. In the Torres Strait both these criteria are met. Previous work has demonstrated the presence of a net westward current through the Torres Strait that is driven by the southeast trade winds. The relatively high displacement of the wind driven currents during the trade wind season relative to the monsoon appears to provide the necessary "unidirectional" regime to form barchans. The low, and typically eastwards, displacement of the residual monsoon season current appears to have a negligible affect on the barchan form.