Page 37 - Sedimentology and Stratigraphy

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Stratigraphy
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Terrigenous Clastic Sediments: Gravel, Sand and Mud
24 Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 24 26.2.2009 8:14pm Compositor Name: ARaju
& ! 0 & & & 1 Fabric If a rock has a tendency to break in a certain
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C 2 1* direction, or shows a strong alignment of elongate
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B2 1*,2 * clasts, this is described as the fabric of the rock.

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B2 *,2 /.

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B2 /.,. 2 Mudrock that breaks in a platy fashion is considered
5
B. 2,) 2 to have a shaly fabric (and may be called a shale), and
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E) 2
sandstone that similarly breaks into thin slabs is
sometimes referred to as being flaggy. Fabrics of this
type are due to anisotropy in the arrangement of
particles: a rock with an isotropic fabric would not
show any preferred direction of fracture because it
consists of evenly and randomly oriented particles.
2.5.1 Granulometric and clast-shape analysis
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A B 2 1* @
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A B 2 *
Quantitative assessment of the percentages of different
grain sizes in clastic sediments and sedimentary rocks
is called granulometric analysis. These data and
measurements of the shape of clasts can be used in
the description and interpretation of clastic sedimen-
tary material (see Lewis & McConchie 1994). The
techniques used will depend on the grain size of the
material examined. Gravels are normally assessed by
direct measurement in the field. A quadrant is
laid over the loose material or on a surface of the
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A B . 2 @
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A B ) 2 conglomerate and each clast measured within the
area of the quadrant. The size of quadrant required
Fig. 2.15 Graphic illustration of sorting in clastic sedi- will depend on the approximate size of the clasts: a
ments. The sorting of a sediment can be determined precisely
by granulometric analysis, but a visual estimate is more metre square is appropriate for pebble and cobble size
commonly carried out. material.
A sample of unconsolidated sand is collected or a
a low sphericity. Sphericity is an inherited feature, piece of sandstone disaggregated by mechanical or
that is, it depends on the shapes of the fragments chemical breakdown of the cement. The sand is then
which formed during weathering. A slab-shaped passed through a stack of sieves that have meshes at
clast will become more rounded during transport intervals of half or one unit on the ‘phi’ scale (2.1.2).
and become disc-shaped, but will generally retain its All the sand that passes through the 500 micron
form with one axis much shorter than the other two. (phi ¼ 1) mesh sieve but is retained by the 250
D ! # & 5
9

5

?

5 Fig. 2.16 Roundness and sphericity

estimate comparison chart (from
Pettijohn et al. 1987).