Australian Stratigraphic Type Sections
Last updated:7 June 2023
What is a type section?
A type section, type locality or type area (stratotype) is an essential part of the definition of a lithostratigraphic unit. Without it, others may not be sure what exactly the unit is supposed to be. Without it, the unit is invalid and has no official standing or priority. Other workers can refuse to recognise an invalid name, although in practice when such a name has become widely used, it is often in the best interests of the science for the invalid unit to be validated by publishing an appropriate type section or area.
Why have type sections?
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.
It follows that there can be only one type section for any stratigraphic interval. If two or more 'type sections' are set up and it later turns out that there has been a miscorrelation, how can anyone know which one is meant to represent the real unit?
A properly set up type section is also very useful for geologists new to the stratigraphy of an area for examining a typical example of a unit.
There is, therefore, no point in having a type section that no-one can look at. It must be generally accessible. Usually type sections are set up in areas of well-exposed outcrop, but if, as in the case of subsurface units, they are defined in drill holes, the drill core must be held in a place where any geologist can come and see it.
- It must be representative of the unit. It should have the typical lithologies and boundary relationships of the unit. Ideally, all the lithologies of the unit should be present, although they are commonly not all present in a single section, and supplementary reference sections elsewhere may be needed.
- Good exposure is required. Usually the best exposure of the unit without structural complications is selected, that also meets the other necessary criteria. Many units do not have 100% exposure anywhere, so their type sections have to take advantage of the best available. Sections may be offset so as to include more outcrop. If different intervals of a unit are only adequately exposed in different places, it may be necessary to designate one section as the type for part of the unit and another section as the type for another part. In this case, one of the component sections has to be designated the holostratotype or prime reference for the unit. If an unconformity is later found between the component sections, or the unit is otherwise split up, the holostratotype remains in the original unit by definition. In cases of very poor outcrop, it may be necessary simply to specify the stratigraphic interval between designated exposures (boundary stratotypes) of the top and bottom contacts. These contacts should be within reasonable proximity: if they only exist a long way apart, the bottom one should be designated the type locality, and the top one as a reference locality. For subsurface units, an intersection in a publicly accessible drill core can be specified.
- Boundary relations with adjacent units must be clear, and where possible, exposures of the contacts should be selected. A boundary should be based on a single point in a section or locality.
- Accessibility to all who are interested is essential if the type section is to fulfil its role as a standard of reference. For example, don't specify a type section in prohibited areas such as a closed Aboriginal sacred site, or in physically inaccessible areas such as half-way up a sheer cliff. Drill core type sections must be available for inspection by any geologist.
- There must be a reasonable assurance of long-term preservation. Don't choose areas about to be submerged or covered over, mine or quarry faces that will be destroyed, areas that will be built over, etc.
- For non-stratified units, or for some very poor exposures of stratified units for which there is no alternative, a type locality or type area has to be chosen instead of a section.
How to set up a type section
- Choose a section that meets the requirements of the Essentials of a Type Section above.
- Record the latitude and longitude of the base and top of the unit in the section (which may be a single point, as in the case of a cliff section). If the section has any offsets, the co-ordinates of the start and finish of each leg must also be given. If only a type locality or area can be specified, a single co-ordinate will usually suffice. Others must be able to find the location and be in no doubt about the exact position. You may need to describe how to get to the section, and a map or photograph showing the location might be necessary. Grid references may be given as well, but by themselves are not sufficient. Grid systems are not international, they have changed in the past, and can change again in future. Latitude and longitude, on the other hand, are internationally understood, although even these will be affected by the spheroid change from the Australian Geodetic Datum (AGD) to the Geocentric Datum of Australia (GDA) in 2000, causing a co-ordinate shift of about 200 m NE throughout Australia. If your co-ordinates are this accurate, you will have to specify whether the AGD or GDA datum has been used. Think long term when defining a type section, and consider succeeding generations.
- Measure the thickness of the unit in the section wherever possible, or at least estimate it.
- Describe the lithologies of the strata in the section, or the rock types present in the type area, in enough detail for the unit to be clearly distinguishable from adjacent ones. Measured columnar sections and photographs may accompany the description, but are not essential.
- The distinguishing or identifying features that justify separating the unit from adjacent ones must be clear. Sometimes this will be obvious as a markedly different lithology or an unconformity: other times it will need to be spelled out.
- Describe the relationships and boundary criteria with adjacent units. This includes conformable, unconformable, faulted or intrusive relationships, how the boundaries with underlying and overlying units are recognized, and why they were chosen.
- Fossils (if present) are essential to the description only if they are a diagnostic or noteworthy component. Detailed species lists are not required, but may be given. The degree of detail necessary will vary from only giving only the phylums present to naming particular species that have a crucial stratigraphic significance.
- Other non-essential but desirable elements to include in the description (where applicable and known) are the structural attitude, geomorphic expression, depositional environment, and diastems or hiatuses.
- The type sections of groups, subgroups and supergroups are the composite of the type sections of the constituent units. They do not have separate type sections of their own.
- The type sections of members are defined in the same way as those of formations. Often a member will be defined some time after a formation has already been established, but if it is defined at the same time, the type section of the member does not have to be in the same locality. The member may be geographically restricted, and not present in the formation type section. However, if the member is present in the formation type section, the latter section must include the member even if the type section of the member is somewhere else.
- Include the above details in your definition of the new stratigraphic unit.
In some cases, the type section does not contain or expose all the significant features of a unit, particularly if the unit varies geographically. Where necessary, supplementary reference sections or localities can also be specified. Reference sections can be set up at the same time as the type section or later, in the region of the type section or elsewhere. These sections remain at all times subordinate to the type section as the standard for the unit.
If a top or bottom contact is inadequately exposed in the type section, a better exposure of the contact at a reference locality elsewhere can be designated as the boundary stratotype.
Rarely, there may be cases where the original type section has been destroyed, submerged, covered over, or otherwise made inaccessible. If the accessibility and long-term preservation criteria are taken into account when the type section is chosen, this risk should be small, but if it has happened, a replacement type section can be selected.
Revisions of type sections
Advances in knowledge may require changes to the definition of a unit and its type section. An example is the discovery of a significant unconformity within a defined unit.
Such revisions require as much justification and the same kind of information as for establishing a new unit, and generally involve the same procedures.
If you have any questions about type sections, or other aspects of defining a lithostratigraphic unit, contact your local State or Territory Stratigraphic Names Subcommission, or the National Convener.