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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