Igneous Unit Definition and Nomenclature in Australia
Types of units
- The fundamental mapping unit of volcanic rocks is the formation, as is the case with other layered rocks.
- The fundamental mapping unit of intrusive igneous rocks is analogous to the formation, and is a defined body of predominantly intrusive rock, distinguished and delimited by rock characteristics.
- An igneous suite is a group of igneous units with common textural, mineralogical and compositional characteristics, or a sequence of such characteristics, based initially on field, then on petrographic, and finally on compositional data (White & others, 2001). It is the analogue of a lithostratigraphic group. [See 2.5]
- A supersuite is a grouping of suites that show similar characteristics but which also show distinct differences between the component suites. It is the analogue of a lithostratigraphic supergroup. [See 2.5].
- The term 'subsuite' as a subdivision of a suite is discouraged, but may be used in exceptional circumstances.
- A phase is analogous to a lithostratigraphic member, and is an entity within an intrusive unit that has lithological properties distinguishing it from adjacent parts of the unit.
- In contrast to lithostratigraphic units, intrusive units generally do not conform to the Law of Superposition.
- Volcanic formations and members can be regarded as both layered lithostratigraphic units and as components of an igneous suite hierarchy. A suite can embrace the volcanic components of one or more lithostratigraphic groups, exclusive of any intercalated sediments. Volcanic suites can exist in parallel with lithostratigraphic units. For example, on the south coast of New South Wales, the former Gerringong Volcanics have been replaced by the Budgong Sandstone and basal Illawarra Coal Measures, with the individual volcanic units being given member status. It would be possible to set up a Gerringong Volcanic Suite to include only the volcanic units, without affecting the names or status of the constituent units in the lithostratigraphic hierarchy.
- The criteria for allocating units to a suite/supersuite, within the constraints of the above definitions of these unit types, are at the discretion of the worker proposing the unit, just as is the case with allocating lithostratigraphic formation to a group.
- As in the case of a group, the definition of a suite/supersuite may later be revised to take account of new data.
- The ages of constituent units are not part of the criteria for allocating them to a suite/supersuite. However, setting up a suite/supersuite implies that the proposer believes the constituents are cogenetic, although not necessarily comagmatic. Age ranges that preclude cogenicity are therefore not found in suite/supersuites.
- Unlike a lithostratigraphic group, a suite/supersuite may contain as few as one constituent unit.
- The names of formation-level units should consist of a geographic name followed by a lithological name, e.g. Krackatinny Tonalite. Structure or form names such as batholith, pluton, dyke, sill, etc., are not part of formal unit nomenclature. In the case of volcanic units, the term 'Volcanics' may be used in the place of the lithological name. The use of the geographic name must not duplicate that of a lithostratigraphic name except as allowed under the rules for lithostratigraphic units.
- The names of suite should consist of a geographic name followed by the word 'Suite', e.g. Tunkillia Suite.
- The names of supersuite should consist of a geographic name followed by the word 'Supersuite', e.g. Qimbragill Supersuite.
- The geographic name used for a suite/supersuite should not be the same as one used for one of the constituent units. Notwithstanding this provision, it is recognised that in several regions it has been the practice to name a suite/supersuite after the constituent designated the 'type', and that it could lead to confusion rather than clarification if such units are renamed. To avoid confusion, existing names might have to stand. In future, a particular constituent specified as the 'type' should not share the name with a higher rank unit.
- If a suite/supersuite contains only one constituent unit, the geographic part of that unit's name should be applied to that suite/supersuite, as otherwise two or three different names would refer to exactly the same body of rock. If the suite is later enlarged to include another unit, then another name will have to be found for the suite.
- The names of phases should start with a geographic name and end with the word 'Phase'. Like a member, a lithological term is most commonly put in the middle, e.g. Bellabrinda Pegmatite Phase.
- Stratotypes for formation-level and member-level igneous units follow the same guidelines as for lithostratigraphic units. In the case of volcanic units, a measured type section should be specified wherever possible. In the case of intrusive units, the stratotype will usually be a type locality or type area.
- One of the constituents of a suite or supersuite should be designated the 'type' for that unit.
- The definition of formation-level and member-level igneous unit should follow the procedures for lithostratigraphic units as far as possible.
- For suite and supersuite, the essentials of the definition consist of the name and rank, derivation, synonymy, a listing of the constituent units, specification of the type constituent unit, and a statement clearly outlining the diagnostic characteristics on which the unit is based so that it can be recognised by other workers. This may be accompanied by relevant diagrams.
Suite nomenclature need only be used where it is possible to apply it, and where it has practical value. Intrusive units for which there is insufficient information to group them into suites should remain ungrouped.
There may be cases where field work establishes that there have been complex multiple intrusions, variable contamination, or diverse types of igneous rock irregularly mixed or with highly complicated relations, such that the components or their order of formation cannot be readily mapped. For such rock masses the lithostratigraphic term Complex may continue to be used as part of the formal name in the place of a rank term.
It is strongly recommended that International Union of Geological Sciences (IUGS) nomenclature be used in the lithological part of a formation-level unit name.
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- How to Define a Lithostratigraphic Unit
- Igneous Unit Definition and Nomenclature in Australia
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