Page 109 - Petrology of Sedimentary Rocks

percentiles is gone through, except the grains are measured in millimeters. The
millimeter values can be converted to phi values by using the chart on page 23 and
treated as above; or more roughly one can divide the two millimeter values and use the
ratios in the following table (for example if the one-sixth limits are .40 mm and .I5
mm, the diameter ratio is 2.7, and the rock is moderately sorted and submature).

I 6%/84%
Diameter ratio, Phi Standard
millimeters Deviation Verbal Scale

Moderately Sorted
4.0 1.00

8.0 I .50 Poorly Sorted
16.0 2.0
Very Poorly Sorted

Unfortunately, it is a common tendency to overestimate the standard deviation
values of well-sorted sands. Hence these values can be more conveniently obtained by
comparison with the set of standard deviation comparison images, previous page.

In determining maturity, there is one exception to the numerical limit a = .5$
advocated above. Some sands that would be considered poorly sorted as whole, consist
of two distinct modes which are in themselves well sorted. These examples should be
called “bimodal mature” or “bimodal supermature” (depending on the rounding), consid-
ering them to be genetically well sorted for the purpose of maturity terminology,
although numerically they would not be. Many of these are desert deflation products
(Folk, I968 XXIII Int. Geol. Congress, Praha Cedskoslovensko).

Textural Inversions occur when well-sorted or well-rounded grains occur in a clay
matrix, or when a sediment is composed of poorly sorted but well-rounded grains.
These are very valuable in interpretation because they indicate mixing of the products
of two energy levels. Sorted or rounded sand grains in a clay matrix often occur in
lagoons behind barrier bars, where the sand grains are blown off the beaches or dunes
(where they achieved their sorting and rounding) and mixed with the lagoonal clays by
storm winds or waves. Final deposition occurs in a low energy environment, therefore
texturally inverted sediments are classified as to the lowest stage of maturity present,
which normally represents the latest environment. Textural inversions also occur when
older sandstones are eroded to produce a new sediment, for example poorly sorted river
sands made up of rounded grains from outcrops of much older sandstone. Some textural
inversions may be caused by burrowing organisms; for example, pelecypods or worms
could burrow through a nicely interlayered series of well-sorted sands and interbedded
clean clays, and make the whole thing into a homogeneous mass of clayey, immature
sand. But the presence of these immature sands would indicate that the final
environment was one of low energy, or else the currents would have re-sorted the
material after burrowing.

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