A spatial analysis. C Baker, A Potter, M Tran and A Heap, Geoscience Australia Department of the Environment, Water, Heritage and the Arts, 2008.

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Sedimentology and geomorphology cover

Executive Summary

This report contains a review of literature and the results of a study of the sedimentology and geomorphology of the North West Marine Region (NWMR) and nominated area of the North Marine Region (NNMR).

The study is a collaboration between Geoscience Australia and the Department of the Environment, Water, Heritage and the Arts (DEWHA).

Data generated by this study expands the national fundamental marine samples dataset for Australia’s marine jurisdiction, with analyses consistent with those completed on samples from the rest of the margin.

Information contained in this report will contribute to Geoscience Australia’s national work program through the creation of seascapes (surrogates for seabed habitats) for the NWMR and will be used by the Department of the Environment, Water, Heritage and the Arts as part of the Marine Bioregional Planning work program.

Geoscience Australia is the national repository and Custodian of marine sediment data including a national marine samples database that is a fundamental marine dataset for the Australian margin.

This study has significantly improved the distribution of quantitative textural and composition data stored in MARS for the NWMR and NNMR. To realise the principal aim of the study, the following three objectives were devised:

Analyse seabed sediment samples (nominally 200) for quantitative grainsize distribution and carbonate content
Identify sources of marine sediment samples and populate MARS with the data
Produce a report synthesizing and summarizing the oceanography, tectonic history, late Quaternary evolution, geomorphology and sedimentology of the NWMR and the NNMR based on this data and previous literature.

Results of the analyses are presented as a regional synthesis within the framework of the Integrated Marine and Coastal Regionalisation of Australia (IMCRA) and National Bioregionalisation of Australia 2005 and where possible within the constraints of geomorphic features identified in a recent study of the geomorphology of the Australian margin by Heap and Harris (in press).

Reporting the results in this way provides both an updated and quantitative analysis of the regional sedimentology from previous works and characterises the broad-scale management zones designed to support bioregional marine planning. Characterising sedimentology by geomorphic feature allows the resolution of relationships between feature and sediment type.

The NWMR is a tropical carbonate margin that comprises an extensive area of shelf, slope and abyssal plain/deep ocean floor. The northern shelf (Northwest and Sahul Shelves) is broad and gentle with an indiscernible shelf break, and the southern shelf (Dirk Hartog Shelf) is narrow.

The northwest margin is controlled by tectonic activity of the Triassic (250 Ma) to Jurassic (145 Ma) that occurred as a result of continental break-up.

The margin is influenced by the Leeuwin Current and Indonesian Throughflow, as well as seasonal up-welling events, internal tides and cyclone-induced storms. A series of reefs are located on the outer shelf/slope. Eight bioregions occur within the NWMR, and these include four offshore (~65% of total NWMR area) and four shelf (~35% of total NWMR area) bioregions.

The regional sedimentology is dominated by marine carbonates. Sediments show a broad zoning and fining with water depth.

Oceanography, tectonic history, late Quaternary evolution and geomorphology have established the sedimentary setting for the margin. Main sedimentary trends of the The regional sedimentology is dominated by marine carbonates.

Sediments show a broad zoning and fining with water depth. Oceanography, tectonic history, late Quaternary evolution and geomorphology have established the sedimentary setting for the margin. Main sedimentary trends of the NWMR include:

a tropical carbonate shelf that is dominated by sand and gravel to latitudes of around 15°
an outer shelf/slope zone that is dominated by mud
a relatively homogenous rise and abyssal plain/deep ocean floor that is dominated by non-carbonate mud because it occurs below the carbonate compensation depth.

Significant outcomes of this study include:

production of the most up-to-date and comprehensive representation of the seabed sedimentology for the northwest and the northern Australian margin, building on existing regional sediment models
production of a detailed synthesis and review of literature for the NWMR and the NNMR
quantification of regional seabed sediment characteristics and distribution in the NWMR and NNMR and assessment of the sediment variability at a NWMR, bioregion and geomorphic feature level
production of a robust, consistent quantitative dataset that permits defensible quantitative comparisons of the seabed sedimentology to be made between the northwest margin and the whole Australian margin
recognition and quantification of the spatial heterogeneity of seabed sedimentology within the NWMR and the NNMR that can be linked to seabed habitat complexity. Capturing the spatial heterogeneity of the seabed sedimentology will allow more accurate and precise mapping of seabed habitats (seascapes) and aid in more effective future sampling strategies.

A principal application of the study is to support research into the associations between physical seabed properties such as sediment texture and composition and the distribution of benthic marine habitats and biota.

This research contributes to Geoscience Australia’s work on the spatial representation of benthic marine habitats and biota for Australia’s vast marine jurisdiction.

This work is crucial for developing robust, defensible methods of mapping habitats over thousands of kilometres using spatially abundant physical data combined with sitespecific biological data.