Sedimentology

Sedimentology is the study of sediment grains in marine and other deposits, with a focus on physical properties and the processes which form a deposit. Deposition is a geological process where geological material is added to a landform. Key physical properties of interest include:

• the size and shape of sediment grains
• the degree of sorting of a deposit
• the composition of grains within a deposit
• sedimentary structures.

These properties together provide a record of the mechanisms active during sediment transportation and deposition which allows the interpretation of the environmental conditions that produced a sediment deposit, either in modern settings or in the geological record.

Within marine and coastal environments, sediments are broadly divided into two generic groups, allochthonous sediment and autochthonous sediments. Allochthonous sediments include grains which have been transported to a site of deposition. Most marine and coastal sediments of this type are rock fragments having been transported by waves, wind, tidal currents or ocean currents. Autochthonous sediments are produced locally and include biogenic sediment such as carbonate (e.g. shell, foraminifer and coral fragments) and silica (e.g. sponge spicules and diatoms). In many marine and coastal settings, the deposits on the seafloor are a mix of allochthonous and autochthonous sediments.

Marine samples database

As the national repository for marine sediment samples in Australia, Geoscience Australia holds sediment data from a wide range of environments in the Marine Sediments Database (MARS). MARS contains a record of mud, sand, gravel and/or carbonate content as well as spatial information for more than 40 000 samples. In addition, mineralogy, age determinations, geochemical properties, bulk density, p-wave velocity, porosity and magnetic susceptibility data are provided for sediment cores. New data are added on a regular basis.

Geoscience Australia also permanently archives thousands of seafloor sediment samples for use in future scientific study which have been collected on surveys since the 1960s, including many contributed by external scientists and institutions.

Sediment data are used at Geoscience Australia to characterise areas of the seafloor within Australia's marine jurisdiction at a range of spatial scales. For example, using information from the MARS database, a national map of marine sediment types has been produced. It shows the distribution of mud, sand and gravel content at a spatial resolution of 0.01 degrees, or approximately 5km. These data in turn have been combined with other data layers such as water depth, slope, seafloor temperature and primary productivity to develop a classification of national seascapes as a technique for identifying unique and distinctive areas of the seafloor.

Sediment data are being used at a local scale also to study relationships between the physical character of the seafloor and benthic organisms. For this work, sediment samples and biota are collected from co-located sites and the resulting information is correlated with high resolution images of bathymetry and seafloor hardness, the latter being a proxy of sediment type. By understanding the physical characteristics of sediments at sample sites the information can be extrapolated to apply in areas where samples are not available and help to better predict the distribution of benthic marine organisms in those areas. For more information on this work go to surrogacy and spatial analysis.