Page 169 - Petrology of Sedimentary Rocks

The writer has not yet adapted this classification to rocks in which recrystalliza-
tion has been important; any such attempt would be quite premature until these rocks
have been better studied and their importance evaluated. One recrystallized rock type
appears to be of rather frequent occurrence in limestones, however. These rocks would
be classified as micrites, type III, or biomicrites, type Ilb, (i.e., nearly pure microcrys-
talline ooze, or fossils in an ooze matrix, respectively) were it not for the fact that the
grains are still equidimensional and uniform in size but microcrystalline calcite is
coarser than normal--average 5 to I5 microns instead of 2 to 5 microns. Because this
relatively coarser material occupies large areas or makes up the entire specimen, it
cannot have formed as a cement and probably represents aggrading recrystallization of
a “normal” microcrystalline ooze matrix. These rocks the writer has designated as
microsparite (corresponding to micrite), and biomicrosparite (corresponding to biomi-
trite), with symbols respectively Rlllm and Rllb.

Grain Size Scale for Carbonate Rocks

Carbonate rocks contain both physically-transported particles (oolites, intraclasts,
fossils, and pellets) and chemically-precipitated minerals (either as pore-filling cement,
primary ooze, or as products of recrystallization and replacement). Therefore the size
scale must be a double one, so that one can distinguish which constituent is being
considered (e.g. coarse calcirudites may be cemented with very finely crystalline
dolomite, and fine calcarenites may be cemented with coarsely crystalline calcite).
The size scale for transported constituents uses the terms of Grabau but retains the
finer divisions of Wentworth except in the calcirudite range; it is most useful for
limestones where transported particles are most obvious. For obviously allochemical
dolomites, the terms “dolorudite”, “dolarenite” and “dololutite” are substituted for these
shown. The scale for authigenic constituents also follows the Wentworth divisions; it is
most useful for dolomites, where transported particles are usually obliterated by
replacement, and crystal size is one of the few describable characteristics. Most
dolomites fall in the medium crystalline range. For symbolic abbreviation, use numbers
as shown.

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