Page 34 - Petrology of Sedimentary Rocks

conglomerate can be subdivided into sandy granule conglomerate, sandy pebble con-
glomerate, sandy cobble conglomerate, adn sandy boulder conglomerate. For the sand
fraction, the median is also estimated separately, using the standard Wentworth grades
of very coarse sand, coarse sand, medium sand, fine sand, and very fine sand. This can
be done very easily by reference to a comparison set of sand grains of the several sizes.

For muds a somewhat different procedure is used, the name depending on the
relative proportion of silt versus clay. This proportion is usually very difficult to
determine with a hand lens, and the only really satisfactory way is to make a thin-
seciton (preferable) or a grain-size analysis by pipette or hydrometer. In many samples
it might be best just to use the broad term “mud” and not attempt to split it any
further. But the mud fraction of many sediments is obviously composed dominantly of
silt, while the mud fraction of others is just as certainly composed largely of clay;
therefore, it is considered worthwhile to make an attempt, if at all practicable, to
estimate this ratio. A threefold division is suggested: if the mud fraction contains
more than 67 per cent silt (i.e., silt-to clay ratio greater than 2:l),he material should
be called “silt” or silty”; if more than 67 per cent clay is present, it should be called
“clay” or “clayey” and for intermediate mixtures, the term “mud” or “muddy” (used in a
restricted sense) is proposed. Thus the major group of muddy sandstone may be divided
into clayey fine sandstone, silty very fine sandstone, muddy coarse sandstone, and so on,
since both the grain size of the sand fraction and the mud composition are to be
specified.

The complete series of major textural groups is presented in the table. Both
unconsolidated and consolidated equivalents are given for each group. At the right of
the group name is given an example of how the terms are to be further specified,
depending on the median grain size of each fraction present. Sand, silty sand, and
slightly conglomeratic sand may be further described by mentioning the degree of
sorting.

The major textural groups of “mudstone” and “sandy mudstone” should be modified
according to their structure, specimens with a well-developed closely spaced parting
parallel with the beds being termed “shale,” regardless of whether they are composed of
clay, silt, sandy clay, mud, or any other mixture of materials. This additional
modification is presented in table 2.

TABLE 2
Classification of Fine-Grained Rocks Based on
Grain Size, Induration, and Structure

Textural Indurated, lndura ted and
Class Unindurated Not Fissile Fissile

Z Silt (> 67 per cent silt) Si I tstone Silt-shale
M Mud (intermediate) Mudstone Mud-shale
C Clay (> 67 per cent clay) C laystone Clay-shale
sz Sandy silt Sandy si I tstone Sandy si I t-shale
sM Sandy mud Sandy m udst one Sandy mud-shale
SC Sandy clay Sandy claystone Sandy clay-shale

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