Geoscience Reference
In-Depth Information
siliciclastic content are covered by the carbonate classifi cation
schemes. Figure A6.9 provides a general classifi cation scheme
for siliciclastic mudrocks that is applicable to most
successions. However, in some instances it may be necessary
to devise a classifi cation based on the particular succession
that you are examining and the major components within it.
To do this, decide what the major components are (e.g. clay
minerals, carbonate, phosphate and silica) both visually and,
if you are on a return visit, through chemical analysis
completed before you return to the fi eld. Then, using these
results devise a classifi cation scheme based on the average
percentages of each component that it is possible to detect in
the fi eld. Listed below are some useful common features of
mudrocks to look for in the fi eld. As with all geological
observations, use several different lines of evidence to form
your interpretation. The features listed below are particularly
useful for mudrocks:
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Colour:
As for other sedimentary rocks this primarily
refl ects composition. Most marine mudrocks are various
shades of grey. Use a Munsell colour chart and/or carry
small rock chips of each of the major colours identifi ed in
the succession for comparison. Mudrocks with a higher
carbonate or silica content, or less organic matter, tend to
be paler. Mudrocks rich in organic matter (i.e. organic-
carbon compounds) are a brownish grey (Figure 2.17d).
Non-marine mudrocks are often red or green depending on
the oxidation state of the iron; they can also be white and
various yellows. Bentonites (montmorillonitic clays of
volcanic origin) are a distinctive bluish or greenish grey
when freshly exposed.
Wet rocks, particularly
mudrocks, are usually darker
(have a lower value; Figure
2.17) than dry rocks.
However, the moisture
content tends not to alter the
hue or chroma. If the colour
is critical record either wet or
dry rocks and avoid areas
that are weathered.
•
Fracture:
The fracture pattern also provides a clue to the
composition and subtly changes with the composition.
Mudrocks mainly composed of clay minerals have an even,
blocky fracture. Increasing amounts of carbonate (e.g. marly
clays and marlstones) tend to give the rock a conchoidal
fracture pattern (Figure 6.2). Mudrocks with a high silica
content are harder.
Mudrocks rich in marine
organic matter that have
been buried and moderately
heated, such as most
Mesozoic strata, have a
distinct bituminous smell
when they are freshly broken.
Those with a very high
organic-carbon content are
very light in weight due to
their lower density.
•
Fissility:
Mudrocks with a fi ssility (i.e. break into thin
(millimetre-sized) layers) are termed shales. They can
develop a fi ssility for two reasons: (1) laminae scale
variation in composition; (2) compaction and weathering.
Shales with compositional lamination often have a higher
overall organic-carbon content and/or some coarser-grained
material. Not all compositionally laminated mudrocks are
fi ssile (e.g. Figure 2.19b).
