Page 171 - Petrology of Sedimentary Rocks

Cryptocrystalline 0 under .OOl mm
Aphanocrystalline : .OOl - - .0039 mm
.0039
mm
.Ol56
Very finely crystalline
Finely crystalline 3 .0156 - .0625 mm
Medium crystalline 4 .0625 - .25 mm
Coarsely crystalline 5 .25 - 1.00 mm
Very coarsely crystalline 6 I .oo - 4.00 mm
Extremely coarsely crystalline 7 over 4.00 mm

Examples of replacement dolomite rock names are Medium Crystalline lntraclastic
Dolomite, Finely Crystalline Biogenic Dolomite, or for a rock with no visible allochems,
Coarsely Crystalline Dolomite.

Terrigenous Admixture. So far we have ignored the content of terrigenous
particles. If the rock contains over 50 percent terrigenous material, it is a Terrigenous
rock and not further considered here. If it contains less than IO percent terrigenous
material, it is a Pure Chemical rock and the terrigenous content is so low that it is not
mentioned in the classification.

However, if the rock contains between IO and 50 percent terrigenous material,
that is regarded as important enough to be mentioned in the name and in the
classification symbol. These rocks as a class are known as Impure Chemical rocks; a
specimen of this type is classified just as previously described (i.e., as a biomicrite,
oospar i te, etc.), but to identify it as an Impure Chemical rock the terrigenous
admixture is described.

The following list shows examples of this usage:
Clayey Biopelmicrite
Silty Coarsely Crystalline Dolomite
Sandy Dolomicrite
Sandy Dolomitized lntrasparite

The Classification used here necessarily is determined by relative rates of
formation of each constituent, not on absolute rates. Thus an abundance of terrigenous
material in a limestone may mean (I) that uplift or proximity of the source area caused
a more rapid influx of detritus; (2) a change of conditions in the depositional basin
suppressed chemical activity, so that terrigenous minerals accumulated by default; or
(3) current velocities were such as to concentrate terrigenous material of a certain size
in preference to allochemical material of different size.

Abundance of theatypes. Some remarks may be made as to the relative abundance
of these various rock types in the stratigraphic section as a whole. These observations
are based on examination of several thousand thin sections of carbonate rocks from
many regions.

lntraclastic rocks usually have a sparry calcite cement, inasmuch as currents that
are strong enough to transport fairly large carbonate rock fragments are also usually
capable of washing away any microcrystalline ooze matrix. Thus, rocks of type Ii
(intrasparite) are common, whereas type Iii (Intramicrite) is relatively rare. Texturally,
intraclastac rocks are about equally divided between calcirudites and calcarenites.

Oolite rocks with their high degree of sorting imply fairly vigorous current action,
therefore Oosparite (type lo) is much more abundant than Oomicrite (type Ilo).
Texturally these rocks are nearly always calcarenites, although in some specimens the

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