Biostratigraphy

Biozonation and geological timescales

The Timescales project within Geoscience Australia maintains and develops geological timescales and fossil biozonations.

It places particular emphasis on those parts of the stratigraphic column where economic resources are important. The Timescales project team also provides biostratigraphic expertise to other Geoscience Australia projects to help with detailed age and palaeoenvironmental assessments.

What is biostratigraphy?

Biostratigraphy is the branch of stratigraphy that uses fossils to establish relative ages of rock and correlate successions of sedimentary rocks within and between depositional basins. A biozone is an interval of geologic strata characterised by certain fossil taxa. Such intervals are often defined by the first appearances (range bases), apparent extinctions (range tops/last appearances), or abundances of fossil index species. These key index species should be relatively abundant, short-lived taxa that are easy to recognise and as geographically widespread as possible. Widely used fossil groups include brachiopods, conodonts, dinoflagellate cysts, foraminifera, graptolites, nannofossil, spores and pollen and trilobites. Zonal schemes based on several different fossil groups can be used in parallel, and the zones can be calibrated to the absolute geological timescale using tie points to rocks which have been radio-isotopically dated.

Geological timescales

The geological timescale is one of the major achievements of geoscience. It has been developed by geologists over the past two centuries to describe and understand the history of the earth. Chronostratigraphic (relative-time) units, such as rock formations, biozones, and magnetostratigraphy, are calibrated against a chronometric scale (an absolute age in years) to build the timescale. Absolute ages (years before present) are usually measured using radio-isotopic dating techniques and in the Cenozoic and Mesozoic can be calibrated against high resolution orbital forcing events (that is, astronomical cycles). Modern techniques and instruments are delivering increasingly accurate ages (with precision down to ± 0.1 per cent), whilst biozonation schemes are continually refined and standardised on a global basis.

The Geologic Time Scale 2012 (GTS 2012 ), published by the Geologic Timescale Foundation , has been implemented as the standard geological timescale for use in Geoscience Australia. All Australian biozonation schemes have been or are being recalibrated to GTS 2012 and their relationships to it are stored in Geoscience Australia's Timescales Database. Geoscience Australia's Basin Biozonation and Stratigraphy Chart series, provides stratigraphic charts of Australian basins for download. Those with a publishing date of 2013 or later have been updated to GTS2012.

An 'Australian datapack' containing all these Australian biozonation schemes tied to the Geologic Time Scale 2012 has been pre-loaded into the public version of the TimeScale Creator software developed by Prof James Ogg and his colleagues at Purdue University. This software allows the user to quickly produce timescale, biozonation and stratigraphic charts and figures for a chosen interval of time. An Australian Public Mappack has been created to load into the TimeScale Creator software, which provides regional biostratigraphy, detailed lithostratigraphy and oil and gas well information for numerous Australian sedimentary basins.

Virtual centre of economic micropalaeontology and palynology (VCEMP)

The VCEMP allows Geoscience Australia to leverage both internal and external expertise in microfossil taxonomy and biostratigraphy to document and define biozones, and improve the correlation between those zones and the geologic timescale. Previously, experts from institutions including the British Geological Survey and the Palaeontological Institute (Moscow), have worked with Geoscience Australia on focused research projects to deliver improved biozonation schemes and timescales to industry.