How to Define a Lithostratigraphic Unit
Last updated:7 June 2023
Lithostratigraphy is concerned with the organization of rock strata into units based on their lithology. Other categories of geological units, not dealt with here, include biostratigraphic, chronostratigraphic, sequence stratigraphic, magnetostratigraphic and igneous geochemical units. Before geologists define new lithostratigraphic units, they should first become familiar with the principles involved. These are explained in two references:
- Salvador, A 1994, International Stratigraphic Guide: A guide to stratigraphic classification, terminology, and procedure. 2nd edition. International Union of Geological Sciences and the Geological Society of America, Boulder, Colorado.
- Staines, HRE 1985, 'Field Geologist's Guide to Lithostratigraphic Nomenclature in Australia', Australian Journal of Earth Sciences, vol.32, pp. 83-106.
The second reference contains notes on how some Australian practices differ from the International Stratigraphic Guide (ISG), and these take precedence wherever the two differ.
How to define a unit
Once a new unit has been decided on, the following steps need to be taken to define it:
- An Australian stratigraphic type section or locality must be set up. The type section is at the heart of the definition, and is so important that a separate page is devoted to it.
- A name has to be chosen for the unit. This must consist of a geographic name plus either the dominant lithology (e.g. Sandstone) or a rank term (Formation, Group, Member, etc.). A geographic name cannot be used for more than one unit in the same (or an adjacent) state or territory. In non-adjacent states/territories, the same geographic name may be used if the ages and lithologies of the units are sufficiently different so as to cause no confusion. Please read the procedure for checking the availability and reservation of stratigraphic unit names on the availability of a name, and reserving it for your use.
- Complete a stratigraphic unit definition form and use the headings provided as a guide to help you describe the unit as well as possible. Not all headings will apply to every unit (e.g. Thickness and Depositional Environment will not apply to a granite). Be sure to give the latitude and longitude of the type section and the locality from which the name was derived. The information on Depositional Environment, Structural Attitude and Geomorphic Expression is not essential to the definition, but is desirable if known.
- The definition must be approved before publication. The completed stratrigraphic unit definition form should be submitted by mail or email to the Stratigraphy Subcommission for the State or Territory in which the type section or locality of the unit occurs. Once the definition is approved, it is lodged by the Convener of the Subcommission with the Central Register of the Stratigraphic Index, where it is available for general reference.
- Publication of the definition is the essential last step, because reservation of a name and getting approval for the definition of a unit by themselves give no standing to that unit. Priority and standing are established only by publishing in a recognised scientific or technical publication. Definitions should be published, if possible, in the form approved by the State or Territory Stratigraphic Names Subcommittee, and preferably in the same publication in which the name of the unit is first used.