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Exercise 3
Texture of siliciclastic sediments
Siliciclastic sediments are derived from the weathering and erosion of
preexisting rocks. Once a sedimentary particle is loosened from its “parent”
rock, it can be transported by water, wind or ice before ultimately settling
in a depositional environment. Textural aspects of siliciclastic sedimentary
rocks can be used to make inferences about ancient depositional
environments.
Texture refers to properties of a sediment such as particle size, shape,
roundness, and sorting. A well sorted sediment is one in which the grains are
all about the same size. In contrast, a poorly sorted sediment contains a
chaotic mixture and large, intermediate and small grains. Shape is a measure
of the sphericity of a grain. Some grains are almost spherical, whereas
others may be elongate or flattened. Particle roundness refers to the
smoothness of a grain, regardless of its shape. Grains may be rounded (i.e.,
no sharp corners), subangular or angular. The concepts of sorting, roundness
and shape are illustrated in Figures 1–3.
Figure 1—Roundness. Angular grains have sharp corners and they probably have not been transported a great distance
from their source. Rounded grains are smooth and probably have travelled quite a distance from their source (from Brice
et al. 2001).
Figure 2—Roundness and
shape are independent textural
properties. A spherical grain
may be angular or smooth.
Similarly, an elongate grain
may be angular or smooth
(from Boggs 2001).
3–1
Figure 3—Sorting. Two sandstones as seen
under the microscope. Example A is poorly
sorted with angular grains of variable
composition. Example B is much better sorted,
and consists almost exclusively of quartz grains
(from Brice et al. 2001).
Particle size is usually described in terms of the Wentworth scale, which
comprises three main divisions — mud, sand and gravel — and many
subdivisions (Table 1).
Table 1—Wentworth scale for sediment size.
Size (mm)
Size (Φ)
> 256
-8 to - ∞
-6 to -8
-1 to -6
0 to -1
1 to 0
2 to 1
3 to 2
4 to 3
8 to 4
64–256
2–64
1–2
0.5–1
0.25–0.5
0.125–0.25
0.0625–0.125
0.004–0.0625
< 0.004
Size classes
Boulders
Gravel
Sand
Mud
∞ to 8
3–2
Cobbles
Pebbles
Very coarse
Coarse
Medium
Fine
Very fine
Silt
Clay
Part 1
In this portion of the exercise we will analyze the size distribution of grains
in a gravel sediment. Most sedimentary particles are not perfectly spherical,
so their shape may be described in terms of three mutually perpendicular
axes, a, b, and c. By convention, a is the longest axis, c is the shortest axis,
and b is the intermediate axis. When a grain lies on a flat surface its c axis
is vertical, as illustrated in Figure 4. Also by convention, the diameter of a
sedimentary particle is taken to be the length of its b axis.
Figure 4—The three mutually perpendicular axes of a non-spherical grain. Grain size, by convention, is the length of the
intermediate axis (b axis) (from Brice et al. 2001).
a. You will be given a sample of 10 pebbles. Measure the intermediate
diameter (b axis) of each pebble and record your measurements in Table 2.
Once all the measurements are recorded, calculate the mean diameter of
the sample, and then calculate the deviation from the mean and the square
of the deviation from the mean.
Table 2
Pebble No.
1
2
3
4
5
6
7
8
9
10
Intermediate
diameter
Deviation from the mean
|Xpebble – Xmean|
Mean diam. =
3–3
Square of the deviation from
the mean
Sorting of a sediment can be described in terms of the “spread” or
“dispersion” about the mean. Sorting can be expressed quantitatively by
determining the range of sizes in a sample, the mean deviation, and the
standard deviation.
b. What is the range of pebble diameters? [Range is the difference
between the largest and smallest diameters in the sample population.]
c. What is the mean deviation of the pebble diameters? The mean
deviation (MD) is obtained by first determining the deviation from the mean
for all of the pebbles in the sample. You already did this in column 3 of Table
2. Add the absolute values of these differences and then divide this total by
the number of pebbles in the sample. For example, if Xmean is the arithmetic
mean, N is the number of pebbles, and each individual pebble diameter is
Xpebble, then the mean deviation is as follows:
Σ |Xpebble – Xmean|
MD = _________________
N
MD =
d. What is the standard deviation of the pebble diameters? The standard
deviation (SD) is obtained by the equation below. For example, if the sum of
the squares of the individual deviations is 1600 mm2 for a population of ten
pebbles, the standard deviation is:
Σ (|Xpebble – Xmean|)2
SD = _________________ = 1600 = 12.65 mm
N
10
SD =
e. Which would be better sorted, a gravel with a standard deviation of 30
mm or a gravel with a standard deviation of 15 mm?
3–4
Part 2
Unlike gravel, individual grains in sand and silt sized sediments usually are
not measured, but rather the sediment is washed through a stacked set of
sieves in which the sieve mesh is progressively finer from top to bottom. A
particle will drop through the mesh of a given sieve only if its intermediate
diameter (b axis) is smaller than the mesh opening. Mesh sizes correspond to
Φ size units of the Wentworth scale (Table 1). Note that Φ values are
geometric and they become larger as the sediment size becomes smaller.
Once a sediment sample has been washed through a set of sieves, an
estimate of sorting can be obtained by plotting the results in the form of a
histogram. Table 3 contains results of size analysis performed on a sand and
silt sample weighing 2 kg.
Table 3—Grain size distribution of a sand and silt sediment
Mesh size
(Φ)
0
1
2
3
4
5
6
7
Weight of sediment on sieve (gm)
25
47
160
790
771
145
42
20
Weight % of sediment on sieve
1.25
2.35
8.00
39.5
38.5
7.25
2.10
1.00
total
2000
100%
a. Plot the data in Table 3 as a histogram on the following chart (Figure 5).
Use pencil to shade in the weight % of sediment (vertical axis) for each
Φ size class (horizontal axis).
b. By examining the histogram, what is the approximate mean Φ grain size in
this sediment sample?
c. What name is given to this size class in the Wentworth scale?
3–5
d. Does this sample seem to be well sorted? Explain your answer.
45
40
35
weight %
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
Phi size class
Figure 5—Plot the data in Table 3 on this chart.
Part 3—Short answer questions
a. How would you expect sediment derived from weathered igneous rocks in
a mountainous region to differ in texture from sediment derived from the
weathering of a preexisting sandstone?
3–6
b. How might one account for the presence of both angular and rounded
grains within a given sandy sediment sample?
c. Why are the grains in windblown sand dune deposits usually well sorted?
d. How do you explain the observation that in marine sedimentary
environments, offshore deposits are usually finer grained that deposits near
the shoreline?
3–7