Page 159 - Petrology of Sedimentary Rocks

types). Metamorphic rock fragments: (rank; size and shape with
respect to quartz). Authigenic minerals (time and mode of
origin).

Petrographic Nomenclature for Con Iomerates Sandstones, and Siltstones.
Progress in a science can 0ftenTe - measure y the increasein precision of its
nomenclature. Much more information is communicated if we say “Spirifer mucro-
natus” rather than “brachiopod” or simply “fossil” as was done in the 17th century.
Describing an igneous rock as “phanerite” or “trap rock” long ago passed out of style,
and we now use such terms as “pink, fine-grained hornblende quartz monzonite.”
Unfortunately, primitive and simple (thereby meaningless) terminology still prevails
among geologists for sedimentary rocks, and most stratigraphic sections are still
described simply as sandstone, shale, or limestone. To do this is to gloss over the
tremendous variation which occurs in these rocks, and to obscure stratigraphic and
regional changes, most of which would become of obvious importance if proper
terminology were in use. How far would geologic work in igneous and metamorphic
areas progress if areas and sections were described as covered with outcrops of “fine,
pink rock,” ” course rock,” or “crystalline rock” as is now done with sediments?

The more information a term communicates, the longer it must become. A term
including all the important information about a sandstone, then, is bound to be lengthy
because there are so many important ways in which sandstones can vary. Five
sandstone properties are held to be important enough to be included in the rock name:
the grain-size name, the chemical cements, the textural maturity, the miscellaneous
transported constituents, and the clan designation.

I. The grain-size name is the most obvious property to many geologists, and
often is the only description given to a sandstone. First determine the percentage of
gravel (grains larger than 2 mm) if posible from a hand specimen because the thin
section is too small in area to give a representative sample of the gravel content. Then
estimate the median size of the gravel fraction alone. From this point on, the process
is the same whether the rock contains any gravel or not. Estimate the ratio of sand to
mud (silt plus clay). This can be easily done by selecting a detrital grain .062 mm in
diameter and placing it under the cross-hairs to use as a standard for comparison, then
estimating what proportion of the rock consists of grains larger than this. Next,
estimate the median size of the sand fraction, alone. Finally, estimate the ratio of silt
vs. clay matrix and name according to the triangle on page 28.

2. The chemically-precipi.tated cements offer important clues to the chemical
conditions prevailing when the rock was deposited or afterward. Clay is not
considered as a cement, but as a detrital matrix. If one considers clay as a cement, he
gets into difficulties with the shales which then become entirely “cement.” Only the
prominent cements (usually one, almost never more than two) are included in the five-
fold name. Wherever there is a possibility of confusion in that one mineral can be
present in a sandstone either as a cement or as a transported constituent, use the
construction “-cemented” as opposed to “-bearing”. The following usage is suggested:

Quartz: If the rock is so tightly cemented with abundant quartz overgrowths that
no pore space is left (and the rock consequently breaks through the grains), use “highly
si I iceous.” If there are only a few small overgrowths, use “slightly siliceous.” Most
rocks may be called simply “siliceous.”

Chert: “Chert-cemented” (not “cherty” because this could mean a chert-bearing
sandstone with chert sand grains). If the cement is well-developed fibrous chalcedonic
quartz, then “chalcedonic” should be used instead.

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