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.
Read more about this topic: Geological Unit
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