Page 31 - Petrology of Sedimentary Rocks

Grain Size Nomenclature

The basis of the classification is a triangular diagram on which are plotted the
proportions of gravel (material coarser than Zmm), sand (material between 0.0624 and
2mm), and mud (defined as all material finer than O.O625mm, i.e., silt plus clay), as
shown in the triangular diagram. Depending on the relative proportions of these three
constituents, fifteen major textural groups are defined--for example, sandy conglom-
erate, slightly conglomeratic mudstone, or sandstone. This classification is presented in
detail by Folk (Jour. Geol. 1954); and also Folk, Andrews & Lewis (NwZd).

To place a specimen in one of the fifteen major groups, only two properties need
to be determined: (I) how much gravel (material coarser than 2mm.) it contains--
boundaries at 80, 30, 5 per cent, and a trace; and (2) the ratio of sand to mud (silt plus
clay) with boundaries at 9: I, I : I, and I :9.
The proportion of gravel is in part a function of the highest current velocity at
the time of deposition, together with the maximum grain size of the detritus that is
available; hence even a minute amount of gravel is highly significant. For this reason
the gravel content is given major emphasis, and it is the first thing to determine in
describing the specimen. This is best done on the outcrop by naked-eye examination,
perhaps aided by a percentage comparison chart; thin sections and hand specimens
commonly give too small a sample to be representative of the gravel content. Using
this scheme, a specimen containing more than 80 per cent gravel is termed “conglom-
era te”; from 30 to 80 per cent gravel, “sandy conglomerate” or “muddy conglomerate”;
from 5 to 30 per cent gravel, “conglomeratic sandstone” or “conglomeratic mudstone”;
from a trace (say 0.01 per cent) up to 5 per cent gravel, “slightly conglomeratic
sandstone” or “slightly conglomeratic mudstone”; and a specimen containing no gravel
at all may range from sandstone through mudstone, depending on the sand:mud ratio.

The proportion of sand to mud is the next property to be determined, reflecting
the amount of winnowing at the site of deposition. Four ranks are defined on the basis
of the sand:mud ratio; in the nonconglomeratic tier, these are sandstone (ratio of sand
to mud over 9: I), muddy sandstone (ratio I: I to 9: I), sandy mudstone (ratio I :9 to I: I),
and finally, mudstone (ratio under l:9). The ratio lines remain at the same value
throughout the triangle: e.g. sandy conglomerate is also divided from muddy sandy
conglomerate by a sand:mud ratio of 9:l. This division is fairly easy to make with a
hand lens, unless a large amount of coarse silt and very fine sand is present.

These two simple determinations are sufficient to place a specimen in one of the
fifteen major textural groups shown in Table I. One might simply stop at this point and
say no more about the grain size; yet a great deal of information is gained by
specifying, whenever practicable, the median diameter of each of the fractions present.
Thus two specimens belonging to the conglomeratic sandstone group have quite
different significance if one is a bouldery fine sandstone and the other is a pebbly very
coarse sandstone. The detailed breakdown will be used in all our class work.

These fine subdivisions are determined by specifying the median diameter of
each fraction considered independently of any other fraction that may be present. For
some specimens this is, of course, not possible; but in most it can be accomplished
sufficiently well for field purposes, especially if the material is bimodal. The size
terms of Wentworth (1922) are used for the various classes. Thus, if gravel is present,
one determines whether the median of the gravel fraction considered alone falls in the
granule, pebble, cobble, or boulder class; for example, the major group of sandy

25