Page 48 - Petrology of Sedimentary Rocks

Trask’s Sorting Coefficient (So) is used only with millimeter values, and is given
Mm25/Mm75. Most beach sands have So = I .3-I .5. In the past, it has been almost
the only measure of sorting used, but its use is declining because it measures only the
sorting in the central part of the curve. It should be abandoned.

Phi is the exact analogue of So but adapted for the 4
scale; it is given by ($75~$25)/2. This measure, like So, fails to give a good indication of
sorting because they indicate only the sorting in the middle of the curve and ignore the
ends, where the differences between samples are most marked. For example, a beach
sand consisting of nothing else but fine and medium sand might have the same QD$ and
So as a sediment consisting of sand with 15% pebbles and 10% clay!

Therefore these measures should be no longer used.

Graphic Standard Deviation (aG) is (484~@l6)/2. It is very close to the standard
deviation of the statistician (see the method of moments) but is obtained by reading two
values on the cumulative curve instead of by lengthy computation. This sorting
measure embraces the central 68% of the distribution, thus is better than QD@ but not
as good as al. If a sediment has aG of 0.50, it means that two thirds (68%) of the grains

fall within I@ unit or I Wentworth grade centered on the mean - i.e., the mean + one
standard deviation.

Inclusive Graphic Standard Deviation (a,) (Folk). The Graphic Standard Deviation,

is a good measure of sorting and is computed as ($84-4 16)/2. However, this takes in
aG,
only the central two-thirds of the curve and a better measure is the inclusive Graphic
Standard Deviation, al, given by the formula

This formula includes 90% of the distribution and is the best overall measure of sorting.
It is simply the average of (I) the standard deviation computed from $16 and $84, and
(2) the standard deviation as computed from $5 and @95-- since this interval (from 5 to
95%) embraces 3.3Oa, the standard deviation is found as ($195~$5)/3.30. The two are
simply averaged together (which explains why the denominators are both multiplied by
2).

Note that the standard deviation here is measure of the spread in phi units of the
sample, therefore the symbol 4 must always be attached to the value for aI.
Measurement sorting values for a large number of sediments has suggested the
following verbal classification scale for sorting:

u under .35@, very well sorted I .O-2.041, poorly sorted
I
.35-.50$, well sorted 2.0-4.0$, very poorly sorted
.50-.7 i@, moderately well sorted over 4.041, extremely poorly sorted
.7l-l.O+, moderately sorted
The best sorting attained by natural sediments is about .20-25@, and Texas dune and
beach sands run about .25-.35@ Texas river sediments so far measured range between
.40-2.5@, and pipetted flood plain or neritic silts and clays average about 2.0-3.5$. The

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