Definition of New Stratigraphic Units - Unit Definition Form

Last updated:7 August 2023

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Defining a stratigraphic unit

Stratigraphic unit definitions involve characterizing the attributes and known distribution of stratigraphic units, so that they can be clearly recognised in the field by other geologists, and distinguished from other units in the area.

In order to create a formal definition of a new unit, a type section or type locality needs to be identified and described. Type sections serve as the defining benchmarks for lithostratigraphic units. If there is any uncertainty about what defines a unit, inspecting its type section should be able to tell you.

What information is required?

These are the essential pieces of information required in order to define any stratigraphic unit, as outlined by the International Commission on Stratigraphy:

  1. Name of unit: The names of most formal stratigraphic units consist of an appropriate local geographic name combined with an appropriate term indicating the kind and rank of the unit, e.g. Hawkesbury Sandstone or Canberra Formation. In order to avoid confusions, you will need to ensure that the geographic part of the unit name has not been used previously for another unit within the same state. You can check the suitability of a unit name by searching for it in the Stratigraphic Names Database. It is recommended you reserve the name before you write the definition to ensure that the proposed unit name is appropriate.
  2. Status of unit: Is it a new name; a variation of a published name; a redefinition of a unit?
  3. Rank: Stratigraphic rank of the unit: Supergroup, group, subgroup, formation, member, bed? Igneous groupings: Supersuite, suite?
  4. Derivation of name: The geographic feature (or other source) from which the name is taken, giving geographic co-ordinates where possible.
  5. Synonymy, unit name history (if any): Is this unit known by any other names prior to this definition, i.e. history of recognition and description of this unit.
  6. Distinguishing or identifying features:
  7. References: If published articles, reports or maps exist, give full bibliographic reference details.

If you are defining a formation or member, you will also need to nominate a type section or type locality for the unit:

  1. Type locality (including Lat. & Long): Where is the type section or type locality located? Please provide geographic co-ordinates, or appropriate and specified grid co-ordinates e.g. GDA94, plus an indication of the means of access to the site e.g. on private property? If the type section is part of a drill core, indicate who is responsible for managing the core, and where it was located at the time the definition was written.
  2. Description at type locality: What characterises the type section? Required information is described below, but you are also encouraged to provide an overall unit description that discusses any regional variations, mention the presence of fossils, diastems or hiatuses, or any other significant aspect of the unit.
    1. Lithology: What rock types are present in this unit at the type locality? In what proportions? Relationships between them, e.g. banding, lenses? You can also choose to include a description of the lithology of the unit as a whole.
    2. Thickness: At the type locality/type section. If it isn¿t known, explain why. Thickness range or variations are also useful information if known (This isn¿t relevant for intrusive igneous units.)
    3. Relationships & boundary criteria: At the type locality. How are the unit boundaries identified? What kinds of boundaries are there (sharp, transitional, unconformable, intrusive, etc.), and with which other units? If boundaries not exposed, this should be stated. You can also choose to describe boundary relationships observed in other outcrop or drill core where this unit is recognised.
    4. Distinguishing or identifying features: At the type locality. How is this unit distinguished from the surrounding units? You can also choose to describe any distinguishing features observed in other outcrops or drill cores of the unit.
  3. Age & evidence: The age range of the unit, and how that age was determined e.g. fossils in this unit or surrounding units, numerical age determinations made by isotopic or other means, age inferred from intrusive, structural or other kinds of relationships with other units, or by correlation with other units of known age. If the age is unknown, state that.
  4. Correlation with other units: Correlated or equivalent units and the reasons for relating them (eg, fossils, lithology, geochronology, structural style).

What is the procedure for submitting a unit definition?

All new definitions of stratigraphic units should be approved by the relevant State or Territory Stratigraphy Subcommission.

A definition may be completed by:

For assistance in completing definition forms please contact the database manager by email: stratnames@ga.gov.au, by phone on +61 2 6249 9535 or contact the relevant State or Territory Stratigraphy Subcommission Convenor.