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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).

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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) Size classes

> 256 -8 to - ∞ Boulders

64–256 -6 to -8 Cobbles

2–64 -1 to -6

Gravel

Pebbles

1–2 0 to -1 Very coarse

0.5–1 1 to 0 Coarse

0.25–0.5 2 to 1 Medium

0.125–0.25 3 to 2 Fine

0.0625–0.125 4 to 3

Sand

Very fine

0.004–0.0625 8 to 4 Silt

< 0.004 ∞ to 8 Mud

Clay

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).

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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.

Intermediate diameter

Deviation from the mean |Xpebble – Xmean|

Square of the deviation from the mean

1

2

3

4

5

6

7

8

9

10

Mean diam. =

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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?

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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)

Weight of sediment on sieve (gm)

Weight % of sediment on sieve

0 25 1.25

1 47 2.35

2 160 8.00

3 790 39.5

4 771 38.5

5 145 7.25

6 42 2.10

7 20 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?

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d. Does this sample seem to be well sorted? Explain your answer.

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6 7

Phi size class

weig

ht

%

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?

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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?