A geological unit is a volume of rock or ice of identifiable origin and 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% or more".
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The component units of any higher rank unit in the hierarchy need not be everywhere the same.
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 correlattions 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 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.