Page 134 - Petrology of Sedimentary Rocks

Notes: The garbage-can words “Litharenite,” “Sublitharenite,” etc. should only be
used as group terms, or when (as in many megascopic descriptions) it is not possible to
identify the specific rock fragments. The more exact terms “phyllarenite,” “feldspathic
phyllareni te,” I’ chert-arenite” should be used if at all possible. Instead of “Sublith-
arenite” one can use “Subphyllarenite” (or Quartzose Phyllarenite), “Quartzose Volcanic
Areni te,” etc. -If more specific information is needed, one can use “rhyolite-arenite,”
“andesi te-areni te,” etc. If rock fragments are subequally mixed, perhgps “polylith-
arenite” is a useful term.
For rocks that do not logically fit on this triangle, which is intended for
volumetrically important sandstones, such names as “glauconite-arenite,” “phopsphate-
areni te,” ” augi te-areni te,” ‘I gypsum-arenite” are perfectly satisfatory.
The unfortunate characteristic of this plot is that it is no longer possible to show
provenance by the position of one dot on the main triangle alone. Of course, two dots
(one on the main triangle, one on the RF triangle) do pretty well indicate provenance,
but this becomes messy with a lot of samples. The only way to indicate provenance
satisfactorily is by a separate graph. One good way to show this is by a star diagram,
such as this:
Unknown

Sedimentary

Plutonic

Metamorphic

\ Volcanic
Here, at the “sedimentary” pole, one might place together quartz with overgrowths,
chert, super-rounded quartz, limestone fragments; at the “Volcanic” pole Quartz
phenocrysts, plagioclase and VRF’s and biotite; at the “Metamorphic” pole, meta-
quartzite, schist and muscovite.
Opi ni on on Graywac ke. Originally, the term “graywacke” was applied to a hard,
dark, semi-metamorphosed sandstone that was rich in mixed rock fragments and
chloritic clay matrix, a type of rock that many now think has been deposited by
turbidity currents, and is often somehow metamorphosed. Modern workers have each
seized some one aspect of the type “graywacke” and have used it to define their
particular kind of “graywacke.” One group considered the graywacke to be a rock-
fragment sandstone, later specifically one rich in metamorphic rock fragments,
emphasizing a source-area approach to the definition. A second group considered the
graywacke to be a clayey, ill-sorted sandstone regardless of mineralogy, essentially a
textural approach. A third group considered the graywacke to be characterized by
those sedimentary structures thought to be formed by turbidites, essentially a deposi-
ti onal environment approach. Others insisted that it be dark, dirty, and highly
indurated (a “know-nothing” approach). Because of these vastly different usages, the
writer now follows McBride (1962 and 1963 JSP) and has discarded the term “gray-
wacke” from any seat in a quantitative, mineralogically-oriented classification of
sandstones. It should revert to a status like that of “trap rock” as a very loose, field
term--essentially a very hard, ugly, dirty, dark rock that you can’t tell much about in

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