Geological unit
A geological unit is a volume of rock or ice of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize it.
Units must be mappable and distinct from one another, but the contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%".
Lithostratigraphic units
- Supergroup – two or more groups and lone formations
- Group – two or more formations
- Formation – primary unit of lithostratigraphy
- Member – named lithologic subdivision of a formation
- Bed – named distinctive layer in a member or formation
- Flow – smallest distinctive layer in a volcanic sequence
The component units of any higher rank unit in the hierarchy need not be everywhere the same.
- Supergroup
- The term "supergroup" may be used for several associated groups or for associated groups and formations with significant lithologic properties in common.
- Group
- A succession of two or more contiguous or associated formations with significant and diagnostic lithologic properties in common. Formations need not be aggregated into groups unless doing so provides a useful means of simplifying stratigraphic classification in certain regions or certain intervals. Thickness of a stratigraphic succession is not a valid reason for defining a unit as a group rather than a formation. The component formations of a group need not be everywhere the same.
- Subgroup
- Exceptionally, a group may be divided into subgroups.
- Formation
- Formations are the primary formal unit of lithostratigraphic classification. Formations are the only formal lithostratigraphic units into which the stratigraphic column everywhere should be divided completely on the basis of lithology. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region and the detail needed for geologic mapping and to work out its geologic history. No formation is considered justifiable and useful that cannot be delineated at the scale of geologic mapping practiced in the region. There is no formal limit to how thick or thin a formation may be.
- Member
- The formal lithostratigraphic unit next in rank below a formation.
- It possesses lithologic properties distinguishing it from adjacent parts of the formation.
- No fixed standard is required for the extent and thickness of a member.
- A formation need not be divided into members unless a useful purpose is thus served.
- Formations may have only certain parts designated as members. A member may extend from one formation to another.
- Bed
- Beds are the smallest formal unit in the hierarchy of sedimentary lithostratigraphic units, e.g. a single stratum lithologically distinguishable from other layers above and below. Customarily only distinctive beds (key beds, marker beds) particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units.
- Flow
- A discrete extrusive volcanic body distinguishable by texture, composition, or other objective criteria. The designation and naming of flows as formal lithostratigraphic units should be limited to those that are distinctive and widespread.
Chemostratigraphic units
Whenever lithological grounds fail to provide significant ability to distinguish mappable rock units, it is possible to map lithology using geochemistry to identify stratigraphy with the same or similar geochemical composition.
Chemostratigraphy can also be the basis for defining a member, bed or subdivision of a geologic unit. For instance, a shale unit may be more sulphidic in the base, and less so in the upper portions, allowing a subdivision into a sulphidic member.
The mapped chemostratigraphic units need not follow stratigraphic or lithostratigraphic units, as the chemical stratigraphy of an area may be independent of lithology. Any geochemical criteria could be used to define chemostratigraphic units; gold, nickel, carbonate, silica or aluminium content, or a ratio of one or more elements to another.
For instance, it would be possible to map a regolith feature such as carbonate cement in a sandstone and siltstone area, which is independent of lithology. Similarly, it is possible to identify fertile nickel-bearing volcanic flows in heavily sheared greenstone terranes by utilising a chemostratigraphic approach.
In mapping of igneous rocks, particularly volcanic rocks and intrusive rocks, particularly layered intrusions and granites, chemical stratigraphy and chemical differentiation of phases of these intrusives is warranted and in many cases necessary.
Chemical stratigraphy is useful in areas of sparse outcrop for making correlations between separate, distant sections of stratigraphy, especially in layered intrusions and granite terrains which have poor outcrop. Here, chemical trends in the stratigraphy and between intrusive phases can be used to correlate individual sections within the larger intrusive stratigraphy, or group outcrops into their respective intrusive phases and make rough correlative maps.
Chemical stratigraphy is often used with drilling information to assist in correlating between drill holes on a section, to resolve dips and pick formation boundaries. Downhole geophysical logging can produce a form of chemical stratigraphy via logging of radioactive properties of a rock.
Often when compared to lithostratigraphic units, chemostratigraphic units will not always clearly match. Thus, it is wise to map both lithology and geochemistry and provide separate interpretations and map units.
Biostratigraphic units
Biostratigraphic units are defined by the presence of biological markers, usually fossils, which place the rock into a chronostratigraphic sequence.
Biostratigraphic units are defined by assemblages of fossils. Few biostratigraphic intervals are entirely distinctive as to an age of a rock, and often the best chronostratigraphic resolution that can be provided by biostratigraphy can be a range in ages from a maximum to a minimum when that fossil assemblage is known to have coexisted.
Biostratigraphic units can be defined by as little as one fossil, and can be marker beds or members within a formally identified Formation, for instance an ammonoid bearing bed. This can be a valuable tool for orienting oneself within a stratigraphic section or within a thick lithostratigraphic unit.
Biostratigraphic units can overlap lithostratigraphic units, as the habitat of a fossil may extend from areas where sediment was being deposited as sandstone into areas where it was a being deposited as a siltstone. An example would be a trilobite.
Other biostratigraphic markers are restricted to certain environments, for instance, graptolites are generally only found in shales.
Biostratigraphic units can also be used in archaeology, for instance where introduction of a plant species can be identified by different pollen assemblages through time or the presence of the bones of different vertebrate animals in midden heaps.