Page 172 - Petrology of Sedimentary Rocks

oolites may reach an average diameter larger than I mm, in which case the rock would
be a calcirudite. Pisolite rocks might be classified as Pisosparite.
Biogenic rocks may occur just as frequently with a microcrystalline ooze matrix
(Biomicrite, type Ilb) as with a sparry calcite cement (Biosparite, type lb). Chalk is
usually foraminiferal biomicrite or foraminiferal micrite.
Type Ilb rocks indicate either that the fossils are sedentary or else that currents
were calm in the depositional area and the microcrystalline ooze did not get winnowed
out from the shell material. Type lb rocks indicate deposition under vigorous current
action where the microcrystalline material was washed away. Note that both
intraclastic rocks and oolitic rocks require vigorous current action in order to form,
thus are usually -sparites; while biogenic rocks do not, hence may have either
microcrystalline matrix or sparry calcite cement. Biogenic rocks are most commonly
calcirudites or calcarenites, although calcilutites occur sometimes if the fossils are
very fine-grained foraminifera.

Pellet rocks are quite common, but are often mistaken in the field or even under
the binocular microscope for microcrystalline rocks. Usually they have a sparry calcite
cement, thus belong in type Ip (Pelsparite) although sometimes they have a microcrys-
talline matrix (type Ilp, Pelmicrite). Texturally they are borderline between very fine
calcarenites or coarse calcilutites, but they are all of such uniform size that the writer
designates them all as Pelsparite or Pelmicrite regardless of the precise average
diameter of the allochems.

Microcrystalline rocks, type III (Micrites, type III, or Dismicrites, type IIIX)occur
frequently in the section. They quite often contain more than IO percent clay, thus
type Tclll (Clayey Micrite) is common; Fossiliferous Micrite (Illb) is another rather
frequent type.

Biolithites (type IV) are uncommon but interesting. A rock should not be called
this unless it consists of organisms essentially in situ and forming a rigid framework; it
should not be applied to broken and redeposmragments. Coral biolithi te, Blue-
Green-Algal biolithite, Red-Algal biolithite, and bryozoan biolithite are most common.
These may form either mounds or more blanket-like bodies.

Genetic Significance. A host of papers and books are now available on this
subject (e.g. Bathurst, 197 I; Milliman, 1974; Wilson, 1975).

In general, type I limestones (Sparry Allochemical Limestones, or -sparites for
short) are those limestones deposited in environments of vigorous winnowing acton and
pretty efficient sorting; they often show good bedding or cross-bedding, close packing
and good orientation of allochems. Thus they form as eolianites beaches, shallow
neritic sediments, submarine raised banks, swift tidal channels, barrier bars, etc. To
these sediments the concept of textural maturity can be fruitfully applied (this idea
stimulated by R. J. Dunham and C. 8. Thames). Of course all type II limestones
(-micrites) would be classed as immature, and the -sparites would be submature or
better. The poorly sorted -sparites would be submature, well sorted ones mature, and
sorted and rounded ones supermature. The maturity concept would be without much
significance for pelsparite and oosparite, which are virtually by definition well sorted
and well rounded. lntrasparites could be divided into well-sorted versus poorly sorted
types, but intraclasts are almost always rounded so that “supermature” would have no
meaning. The most cogent application would be to the biosparites. Supermature
biosparites (finely broken and heavily abraded and rounded fossil fragments get ground

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