Geoscience Reference
In-Depth Information
commonly possible only after you have examined the main
rock types in the area, although any scheme can be amended
later.
It is perhaps worth remembering that the defi nition of a
geological formation is a unit mappable over a reasonably large
area, with characteristics that distinguish it from adjacent units.
Mapped units generally have distinct contacts with
neighbouring units, though some unit boundaries may be
transitional or indistinct; there are no limits to the thickness of
a formation. Two or more formations with features in common
may be designated as a group. Most igneous and many
sedimentary contacts are quite sharp (e.g. dyke margins,
bedding planes). However, some unit boundaries are less
obvious (e.g. metamorphic zone boundaries, boundaries
between sedimentary units that obey Walther's Law). The
following criteria may be useful in defi ning a range of
lithological units:
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major rock type (igneous, sedimentary, metamorphic);
compositional change (indicated by changes in colour,
mineralogy, weathering, etc.);
grain-size changes (e.g. siliciclastic sedimentary deposits,
volcaniclastic rocks, intrusive igneous facies);
textural or structural change (generally indicating a change
in
process
);
change in fossil assemblage (biostratigraphy);
change in mineral assemblage (metamorphic zones, igneous
petrography).
Lithological subdivisions and marker layers
The type and scale of mapping control how detailed a scheme
has to be to classify lithological units. A large area may have a
wide range of rock types that are easy to categorize. Mapping
smaller areas, or those with little variation in rock type,
demands discrimination on subtler criteria. For example,
volcanic units might be defi ned by the abundance of different
phenocrysts, or sedimentary units by the relative proportions of
sand and silt. Such subdivisions may require detailed logs of
certain sections to defi ne them.
The basic mappable unit is the formation. Formations may
contain one or more subdivisions known as members, which
are units with characteristics that distinguish them from the
adjacent parts of the formation. In monotonous sections of
strata, the widespread occurrence of a distinctive single
member or even layer can be critical to interpreting the geology
of the area. Examples of such a marker layer could include a
