Page 13 - Petrology of Sedimentary Rocks

Skewness and kurtosis tell how closely the grain-size distribution approaches the
normal Gaussian probability curve, and the more extreme the values the more non-
normal the size curve. It has been found that single-source sediments (e.g. most beach
sands, aeolian sands, etc.) tend to have fairly normal curves, while sediments from
multiple sources (such as mixtures of beach sands with lagoonal clays, or river sands
with locally-derived pebbles) show pronouned skewness and kurtosis. Bimodal sediments
exhibit extreme skewness and kurtosis values; although the pure end-members of such
mixtures have nearly normal curves, sediments consisting dominantly of one end
member with only a small amount of the other end member are extremely leptokurtic
and skewed, the sign of the skewness depending on which end member dominates;
sediments consisting of subequal amounts of the two end-members are extremely
platykurtic. (Folk and Ward). Plots of skewness against kurtosis are a promising clue to
environmental differentiation, for example on Mustang Island (Mason) beaches give
nearly normal curves, dunes are positively-skewed mesokurtic, and aeolian flats are
positively-skewed leptokurtic. Friedman showed that dunes tend to be positive skewed
and beaches negative skewed for many areas all over the Earth, but Hayes showed on
Padre Island that this is often modified by source of supply. Eolian deflation sediments
are commonly bimodal.

Fluvial environments consisting chiefly of traction load (coarse) with some
infiltrated suspension load (finer grains) are commonly positive-skewed leptokurtic;
glacial marine clays with ice-ratified pebbles are negative-skewed, etc. It would be
emphasized that faulty sampling may also cause erroneous skewness and kurtosis values,
if a worker samples two adjoining layers of different size - i. e., a gravel streak in the
sand. Each layer should be sampled separately.

Size analysis has been used practically in correlation of formations; in deter-
mining if a sand will contain oil, gas or water (Griffiths); in determining direction of
sediment transport; and an intensive study is being made to determine if characteristic
grain size distributions are associated with certain modern environments of sedimenta-
tion, so that such deposits may be identified by analysis of ancient sediments in the
stratigraphic column. Furthermore many physical properties of sediments such as
porosity, permeability, or firmness (Krumbein) are dependent on the grain size.

Particle Morphology

Under the broad term “particle morphology” are included at least four concepts.
Listed approximately in decreasing order of magnitude, these are (I) form, (2)
sphericity, (3) roundness, and (4) surface features.

Form is a measure of the relation between the three dimensions of an object, and
thus particles may be classed quantitatively as compact (or equidimensional), elongated
(or rodlike) and platy (or disclike), with several intermediate categories, by plotting the
dimensions on a triangular graph (Sneed and Folk: see p. 9).

Sphericity is a property whose definition is simple, but which can be measured in
numerous very different ways. It states quantitatively how nearly equal the three
dimensions of an object are. C. K. Wentworth made the first quantitative study of
shapes. Later, Waddell defined sphericity as
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