SILICICLASTIC
SEDIMENTARY ROCKS
Prepared by Dr. F. Clark
Department of Earth and Atmospheric
Sciences, University of Alberta
August 06
INTRODUCTION - TEXTURES
Just as with igneous rocks, the textures of siliciclastic
sedimentary rocks are involved in their classification. As
a first pass, the rock name depends on the grain size,
but other aspects of texture, namely shape and
arrangement, are factors in further refinement of the
name. In gross terms, three grain sizes, namely 2 mm,
1/16 (0.0625) mm, and 1/256 (0.0039) mm, divide
grains, and thus siliciclastic sedimentary rocks, into four
size classes. Those four size classes correspond to
conglomerate, sandstone, siltstone, and claystone, in
order from coarsest to finest grain sizes.
Conglomerate.
A significant
proportion of
grains (some
sources
suggest over
30%) is larger
than 2 mm in
diameter.
Loose sediment with this grain size characteristic is referred to as
gravel. In the sample above, the larger gravel-sized grains [yellow
arrows] constitute the framework, whereas the smaller, sand- and siltsized grains constitute the matrix [blue arrows].
Conglomerate?
Perhaps no
more than
15% of the
grains exceed
the 2 mm
lower size limit;
most of the
grains are
sand-sized.
This sample shows that the distinction between the size classes can be
somewhat arbitrary. A subtle change in the current which produced the
lamination [green arrow] in these finer gravels, and from which this
sediment was deposited, could have resulted in all grains being sand
[purple arrows] or gravel [yellow arrows].
Conglomerate vs. Breccia
In more traditional usage, the term conglomerate applies to those
rocks with rounded clasts (left), whereas those rocks with more
angular grains (right) are referred to as breccia. The angularity of
the grains on the right specimen is not pronounced, so the term
breccia might not be appropriate in this case.
Conglomerate vs. Diamictite
In more current usage, the term conglomerate applies to those rocks
that are grain-supported, such as on the left; framework grains are
in contact [light blue arrows]. It is said to have an intact framework.
On the right, the rock is matrix-supported [dark blue arrows] and
the grains are not touching. This is called a diamictite, and is said to
have a dispersed framework.
Diamictite.
The high
matrix content
[blue arrows]
is best seen
where
framework
grains have
been plucked
[yellow stars].
The large amount of matrix is sufficient to form durable external molds
of missing framework grains. Such high matrix content is commonly
associated with glacial activity, which does not selectively remove the
finer matrix grains, or flood episodes.
Sandstone.
The highest
proportion of
grains lie in the
range between
2 mm and 1/16
mm. These
rocks are also
called arenites.
The yellow arrows point to individual grains which show up slightly
darker than their neighbouring grains. Virtually all the grains are of the
stable silicate mineral quartz, and so this is a quartz sandstone or
quartz arenite.
More Quartz Sandstones
The sample on the left shows lamination, parallel to the green arrow,
reflecting subtle changes in colour due to trace amounts of stain in
the cementing material that holds the grains together. Lighter
coloured layers, lacking the stain, are highlighted by light blue
arrows. Yellow arrows point to individual grains. The right sample
shows the uniform light appearance of many quartz arenites.
Lithic
Arenite.
Lithic (from the
Greek lithos,
meaning stone)
sandstone or
arenite is
characterized
by abundant
rock
fragments.
Because of the very high mechanical and chemical stability of quartz, it
will also usually be abundant in lithic sandstone. As a result, lithic
arenites characteristically have a “salt-and-pepper” look to them. This
example has traces of woody plant fossil matter [brown arrows].
Lithic Arenites/Sandstones
These examples are from the Belly River Formation of Cretaceous age
(on the order of 80 million years old), from the Foreland Basin of
Western Canada. In these classic salt-and-pepper lithic arenites, the
“pepper” is sand-sized grains of chert. Although chemically the same
as quartz, chert is classified as a lithic grain or rock fragment by
most sedimentologists, rather than as a variation of quartz.
Siltstone.
Silt-sized grains
are by
definition
between 1/16
and 1/256 mm.
This sample from the Spray River Group of Western Canada is quarried
near Canmore as “Rundle Rock”, and is used as a facing stone in
construction, especially common on upscale homes. Clearly, individual
grains are barely discernible without magnification.
Lamination and Bedding in Siliciclastics
These two views of the Spray River siltstone illustrate lamination
[parallel to green arrows], which is basically a synonym for layering.
This characteristic of many sedimentary rocks is produced by
discontinuities (e.g. grain size, grain type, colour) in sedimentation.
Discreet units of sediment are bounded by bedding planes [blue
arrows]; the layers are called beds if they exceed 1 cm in thickness.
Siltstone.
At these fine
grain sizes,
individual
grains can
barely be
detected even
with a hand
lens, and only
if they are
coarse silt.
The next grain size working down from silt is clay, less than 1/256 mm.
It is not generally practical, even with significant magnification, to
distinguish between fine silt- and clay-sized grains. This practical
limitation gives rise to the two siliciclastic rock types that follow.
Mudstone.
This term
embraces rocks
with grain sizes
less than 1/16
mm, and what
is called blocky
fracture,
without
distinguishing
silt vs. clay.
The sample above is relatively thick, bounded above and below by
bedding planes [blue arrows], and has broken along irregular failure
surfaces unrelated to bedding [purple arrows] into the three pieces,
producing relatively thick, irregular chunks of rock.
Mudstone.
At > 1 cm, the
mudstone slab
constitutes a
bed, whose
upper and
lower bounding
surfaces [blue
arrows] are
bedding
planes.
The tendency of mudstones is to break along fracture surfaces [purple
arrow] unrelated to both bedding [blue arrows] and lamination [green
arrow]. Our understanding is that mudstones do this because flat or
platy grains are not aligned parallel to each other and the lamination.
Shale.
The term shale
is applied to
those rocks,
with grains less
than 1/16 mm,
that are fissile,
or split into
thin sheets,
without regard
to silt vs. clay.
Again, we may not be able to distinguish siltstones from claystones
proper, so we classify the rock according to a gross textural
characteristic, namely how it breaks or splits. Our understanding is that
fissile rocks owe their character to parallel alignment of platy grains.
Fissility Expanded
In these two views of a shale, we see bedding planes [blue arrows]
being exploited as planes of weakness [yellow arrows] that make
this rock fissile. It must be pointed out that the parallel alignment of
mineral grains that produces these planes of weakness occurs at the
time of deposition, unlike the parallel alignment that produces slaty
cleavage in certain similar metamorphic rocks, in response to stress.
Fissility – Not Always Planar
These views of a shale illustrate that the lamination of a shale, the
bedding planes of that shale [blue arrows], and the resulting fissility
[purple arrows] are not necessarily planar. The sea or lake bottom is
often characterized by an irregular surface that is referred to as a
bedform (ripples and dunes are examples). Bedform development is
controlled by the interplay of sediment and waves or currents.
Colour as Environmental Indicator
The different colours of these shale samples tell us something about
the conditions at their environment of deposition. The black colour
of the left specimen is due to preserved organic matter in an anoxic
or anaerobic environment, whereas the red sample on the right
reflects oxidizing conditions that have turned the iron content red.