GEOG376 Geomorphology I
6.2
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Spring 2005
Physical properties of sediments
physical properties of sediments (e.g., mineralogy, texture, sorting) or bulk properties
relating to arrangements in a deposit or landform (e.g., porosity, shear strength,
imbrication, fabric, structures) can control the rate & type of geomorphic processes
6.2.1 Grain size & texture
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texture: size and shape of sediment particles including surface features on grains
and their bulk arrangement (e.g., fabric, packing, orientation, porosity…).
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a fundamental property of sediments… useful description for classification &
interpretation purposes
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measured using the Udden-Wentworth size scale (Fig. 1): a geometric scale using a
log2 transform of grain sizes (D in mm) in phi (φ) classes, where φ = -log2 Dmm...
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as such, the -ve log makes φ positive for small (most abundant) particle sizes
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the log2 transform means that classes are ½ or 2x the mm value of the next class
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avoids using fractions of mm & log-normal graph paper
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terms such as sand, gravel, silt, clay-loam all have specific textural contexts
Table 1: common size classes and φ intervals (see also fig. 2)
size class
D
phi
boulder
> 256 mm
< - 8.0
pebble
64-4 mm
- 6.0 to -2.0
medium sand 0.5 - 0.25 mm
1.0 to 2.0
medium clay 0.00195-0.00098 mm 9.0 to 10.0
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an excellent online text on the properties of sediments and sedimentary rocks
published by a leading sedimentologist/geomorphologist, Robert Folk (1999).
Sedimentary Petrology. Check it out… it’s free!
www.lib.utexas.edu/geo/FolkReady/folkprefRev.html
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GEOG376 Geomorphology I
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Spring 2005
Figure 1: Udden – Wentworth grain size scale (source: Lewis & McConchie, 1994:
fig. 5.1)
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GEOG376 Geomorphology I
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Figure 2: Qualitative (verbal), metric (mm), Wentworth (φ) & graphic
representations of grain size scale (source: Boggs 2001: fig. 3.2)
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GEOG376 Geomorphology I
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Spring 2005
the texture triangle below is a graphical way of expressing the % sand – silt – clay in
any sediment or soil sample
Figure 3: Soil texture triangle (source: Christopherson, 1999)
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GEOG376 Geomorphology I
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variation in grain sizes of a sediment sample can be quantified & summarized in
tabular format by size groups (fig. 4A) or using a frequency distribution (figs. 4B-C)
where ‘frequency’ is expressed as a weight of sediment in individual size classes
(4b) or cumulative weight of size classes (4c,d)
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otherwise known as the ‘grain size’ distribution or gsd
Figure 4: Methods for summarizing grainsize data (source: Boggs 2001: fig. 3.3)
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GEOG376 Geomorphology I
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Figure 5: Normal and cumulative frequency distributions of grainsizes (source:
Boggs 2001: fig. 3.6, 3.4)
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GEOG376 Geomorphology I
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6.2.2 Sorting & skewness
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various moment statistics can be derived from the frequency distribution either
graphically (Folk & Ward, 1957; see fig. 5, 6) or statistically (Krumbein & Pettijohn,
1938) to describe sediment properties…
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1st moment = mean, 2nd = standard deviation (SD, σφ), 3rd = skewness (skφ), 4th =
kurtosis (kφ)…
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mean (first moment): decribes the average size of sediment particles in the sample…
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In general, mean grain size indicates energy of the transporting or depositing
environment… telling either of the competence of the process (i.e., can move a certain
size of sediment14) or the fall velocity below which that sediment can no longer remain
in transport
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standard deviation (SD or σφ, 2nd moment) or sorting is the range of grain sizes in a
sediment sample & how widely scattered these are about the mean
σφ
< 0.35 φ
0.35 – 0.50 φ
0.50 – 0.71 φ
0.71 – 1.00 φ
1.00 – 2.00 φ
2.00 – 4.00 φ
> 4.00 φ
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sorting
very well sorted
well sorted
moderately well sorted
moderately sorted
poorly sorted
very poorly sorted
extremely poorly sorted
in reality, most natural sediments are not normally distributed (i.e., mean, median and
mode do not coincide)… most are ‘asymmetrical’ & have coarse or fine tails to the
distribution… so we need some other measure of grain size distribution shape…
provided that the maximum size is available!
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GEOG376 Geomorphology I
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Spring 2005
skewness (skφ, 3rd moment) describes the shape of the grainsize distribution… e.g.,
is it skewed toward larger or smaller values (i.e., tails of fine or coarse sediments)?
o reflects sorting toward the tails in the distribution e.g., positive skewness =
mode of coarser sediments (- φ) with a tail of finer sediments (+ φ)… or the
distribution is skewed toward +φ values (see fig. 6A)
Figure 6: Skewness of grainsize distributions (source: Boggs 2001: fig. 3.7)
skφ
> +0.3
+0.3 – +0.1
+0.1 – - 0 .1
-0.1 – -0.3
< -0.3
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skewness
strongly fine skewed
fine skewed
near symmetrical
coarse skewed
strongly coarse skewed
¾ What would cause tails of fine or
coarse sediment in nature?
it is also possible to have bimodal distributions (i.e., two peaks in the gsd)… that
reveal clues about the sediment history (i.e., source, subsequent reworking) and
possibly, the contemporary processes acting in a sedimentary system
o e.g., coarse ‘lag’ or deflation deposits in deserts are negatively skewed with
few coarser sediments… larger particles (fine gravels delivered by fluvial
action) are left behind as they exceed the competence of the fastest winds
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grain size and texture variations reflect modes of transport, deposition and changes
in these processes through time (e.g., stratigraphic sequences, upward fining of
lacustrine deposits) and/or space (e.g., grainsize variations on a beach from fine to
coarse to fine, etc.) (Figs. 7,8,9)
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sorting reflects variations in the energy of the transporting or depositing environment
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e.g., most aeolian dune sands are well-sorted because wind action continuously
‘winnows’ away finer particles…
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most fluvial sands are poorly sorted… why?
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GEOG376 Geomorphology I
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Figure 7: Texture as a function of transport modes & environments of
deposition (source: Lewis & McConchie, 1994: fig. 5.5)
Figure 8: Bivariate plot of skewness vs. SD
showing regions typical for beach & fluvial sands
(source: Boggs, 2001: fig. 3.8)
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