Geoscience Reference
In-Depth Information
consideration. Classifying sedimentary deposits provides an
approximate quantifi cation of the rock components and
therefore information on the depositional process, sediment
source and environmental conditions. Use the following items
in Appendix A6 to describe and classify the rocks: checklist
(Table A6.1), table of properties of common minerals in
sedimentary rocks (Table A6.2), textures (Figures A6.1-A6.4)
and rock classifi cation charts (Figures A6.11-A6.13).
To test for small amounts of
quartz sand or silt in
sedimentary rocks you can
grind a small amount of rock
between your front teeth. The
quartz is gritty and
impossible to break whereas
siliciclastic clay minerals or
carbonate become a
fi ne-grained powder that
feels like a smooth paste.
6
Siliciclastic rocks
For siliciclastic rocks the classifi cation scheme depends on the
grain size and the composition of the major grains, except for
conglomerates and breccias where the clast shape is also
important. The general siliciclastic classifi cation process is
illustrated in the following fl owchart:
Classification of a
siliciclastic rock
Use general
classification
scheme for
sandstones
and mudrocks
(Figure A6.8)
Is
the average
grain size less
than 2 mm?
Are
more than
15% of the grains
<30
µ
m?
Are
more than
75% of the grains
<30
µ
m?
Y
Y
Y
Mudrock
Use classification
scheme for
mudrocks
(Figure A6.9)
No
No
No
Arenite
Wacke
Use classification
scheme for
conglomerates, breccias,
etc. (Figure A6.13)
Decide on the proportion of
feldspar, quartz and rock
fragments and use Figure A6.8
Arenite
classification,
e.g arkose, quartz,
arenite
Wacke
classification,
e.g. lithic
wacke
Mudrock (synonomous with mudstone) and coarse-grained
siliciclastic rocks can be subdivided further than shown in
Figure A6.6 and require slightly different observations than
siltstones and sandstones. These features are covered in the
following two subsections.
Mudrock
Fine-grained siliciclastic sedimentary rocks make up over 50%
of the sedimentary rock record. They are more diffi cult to work
with in the fi eld than coarse-grained siliciclastic rocks because
their features are harder to distinguish with the naked eye and
the variations are subtle. Carbonate mudrock with negligible
