Page 168 - Petrology of Sedimentary Rocks

So far, gross texture (whether Sparry Allochemical, Microcrystalline Allochemi-
Cal, or Microcrystalline), and composition of allochems (whether intraclasts, oolites,
fossils, or pellets) have been included in the classification, but nothing has been said
about grain size of the allochems. If the allochems average coarser than I mm, the
rock is a calcirudite (or dolorudite); if they lie between .0625 and I mm, the rock is a
calcareni te or dol areni te; if finer than .0625 mm, calcilutite or dololutite. In
determining the grain size name, only the size of the allochems is considered;
percentage and crystal size of microcrystalline ooze or sparry calcite and grain size of
terrigenous material is ignored. Thus a rock consisting of 20 percent brachiopod shells
embedded in microcrystalline ooze and quartz sand is descriptively a calcirudite, just as
much as a rock consisting of 80 percent limestone pebbles cemented by sparry calcite.
Genetically, of course, there is a great difference between them and the writer seldom
uses the term “biomicrudite”.

In theory, the three fold size classification just given is valid; but in practice,
rocks with allochems averaging in the calcilutite range are very rare. The only
allochem rock types with representatives in this size class are pellet rocks or biogenic
rocks, and in both of these the pellets or fossil fragments average no smaller than .04 or
.05 mm, just barely under the limit of calcarenite; pellet rocks nearly always hover on
the borderline between calcilutite and calcarenite (since the average pellet size is
between .05 and .I 0 mm). Setting a new rock class apart on such an artificial and
insignificant boundary seems to be an unnecessary complication, hence the writer has
lumped these rare allochem calcilutites together with the calcarenites in the classifica-
tion scheme. The only common calcilutites are the pure microcrystalline oozes,
although many pellet rocks appear as calcilutites in the field and under a binocular
microscope.

All the rock characteristics discussed above are combined in a single name, shown
in the table and diagrammatically in the second figure. At first the writer used such
cumbersome terms as “sparry intraclastic calcarenite” for intrasparite, “microcrystalline
biogenic calcirudite” for biomicrudite, etc.; but these names, although self-explanatory,
were too awkward to use in descriptions. As an alternative he thought of introducing
locality terms, but the localities would be difficult to choose and the terms in
themselves would be entirely meaningless and difficult to memorize. Finally, the
writer decided to use single composite words, each portion of which referred to a
specific rock characteristic: thus “intra” for intraclastic rocks, “00” for oolitic ones,
“bio” for biogenic types, and “pel” for pellet rocks. Whether the rocks were type I or
type II was shown by the second part of the name, “spar” for those with sparry calcite
cement and “micr” (pronounced with a short i, as in “mick”) for those with microcrystal-
line ooze matrix. Type III limestones, almost entirely ooze, were designated “micrite”
(pronounced “rnick-rite”) without any allochem prefix. Inasmuch as most limestones are
calcarenites, no further syllable was added if the size fell in that category, and as
exlained above, the rare and somewhat artificial calcilutites are lumped together with
the calcarenites in this table. It is important to differentiate the calcirudites, though,
and the word segment “rudite” may be added if the size falls in that class. Examples,
together with the symbols used, are presented in the table.

Some rocks classified as Oosparite, Intramicrudite, etc. may have significant
amounts of other allochems which do not appear in the name. These may be specified
at the discretion of the worker, such as Fossiliferous Oosparite, Oolitic Intramicrudite,
etc. Biogenic rocks, if composed largely of one or two types of organism, should always
be described as Brachiopod biomicrudite, Gastropod biosparite, Foramcrinoid biomi-
trite, Oyster-echinoid biosparite, etc.

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