Geoscience Reference
In-Depth Information
Table 6.1 Sedimentary rock types and the sedimentary structures and features that they are commonly
associated with. Section 6.2.2 considers sedimentary structures more fully.
Sedimentary
rock type
Depositional features and
sedimentary structures
Post-depositional features
and sedimentary
structures
Commonly associated
features
Nodules : form below the
sediment-water interface
under particular
geochemical conditions.
Early diagenetic carbonate
nodules can be used to help
calculate the amount of
compaction. In some
successions they nucleate on
fossils
Cone-in-cone calcite : calcite
crystals that grow in cones
perpendicular to the
pressure during
overpressuring of mudrocks
Soft sediment deformation
features : common for high
sedimentation rate
interbedded mudrocks and
sandstones due to density
contrast of fresh sediment
(mud holds c . 80% water
whereas sand holds c. 50%)
Fossils : the low energy of
deposition, fi ne-grained
and non-porous nature
of mudrocks tends to be
excellent for fossil
preservation, both
mega- and microfossils.
Trace fossils are easy to
detect if there is a
colour change and the
fi ne detail is often well
preserved.
Pyrite : this is often
associated with euxinic
(free H 2 S in the water
column) depositional
conditions
Organic matter : marine
and terrestrial, which
infl uences colour and
can infl uence
lamination. It indicates
high productivity and/
or suitable preservation
conditions (e.g. anoxia)
Claystones,
mudrocks and
marls
Lamination : millimetre-scale
compositional variation
giving fi ne-scale rock banding
Fissility : rock breaks easily into
thin sheets parallel to
bedding (may be related to
lamination)
Colour : change in colour
represents compositional
variation, particularly iron
minerals, organic matter
and carbonate content
Fracture pattern : refl ects
compositional variation (see
above)
Weathering profi le : mudstones
with a higher calcium
carbonate content tend to
stick out as they are often
more resistant to
weathering. Similarly, those
with a high organic-carbon
content tend to weather
proud. When mudrocks are
interbedded with sandstones
or carbonates the mudrocks
always weather back.
6
Siltstones and
sandstones
Sedimentary structures : there is
a wide range (see Section
6.2.2), many of which
indicate the processes of
deposition
Nodules : as above. Late
diagenetic nodules are more
common
Liesegang rings : lines of iron
concentration that form as
later fl uids move through the
rock. Can appear at fi rst
glance to be sedimentary
structures
Fossils : often present but
can be reworked under
the high-energy
conditions of deposition
Carbonates
(non-organically
bound)
Sedimentary structures : there is
a wide range (see Section
6.2.2), many of which
indicate the processes of
deposition
Stylolites and other pressure
solution features : carbonates
are susceptible to
dissolution. Reprecipitation
can enhance, mask or
destroy primary features
Cementation : the soluble
nature of carbonates makes
carbonates particularly
susceptible to cementation
Fossils : bioclasts are
important in many
carbonates and provide
valuable information on
the site and conditions
of deposition
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