Page 103 - Carbonate Facies in Geologic History

90 Outline of Carbonate Petrography

wave action. Limestone found with siliceous or argillaceous basinal deposits is
generally (but not always) dark.
Not only is an environmental distinction between light and truly dark carbon-
ate sediment useful, but attention to reddish-purple colors is also important. It is
well documented that the more or less resistant ferric oxides are the main pigmen-
tation of terrigenous clastics and are oxidation products either inherited from a
previous environment or of the direct depositional environment of the sediment.
Carbonates in shelf redbed sequences are usually light-colored unless externally
stained on weathering. On the other hand, reddish-purple carbonates, particu-
larly encrinites, are not uncommon in basinal and geosynclinal strata. The iron
pigment may have formed in oxidizing conditions on submarine swells and
washed down into geosynclinal troughs. Deeper water carbonates may be red or
pink or purple through preservation of Fe and Mn oxide pigments, just as occurs
during formation of radiolarites and siliceous shales. Preservation of hydrous
Fe 2 0 3 in deep basins with slow sedimentation is probably a function of the rate of
burial of decaying organic matter (Fischer in Mesolella et aI., 1974, p.54). Limo-
nite is produced widely in trace amounts in sea water. Normal sedimentation
entombs enough organic matter to reduce the Fe to pyrite within the substrate,
but in areas of very slow sedimentation, common in deep starved basins, bacterial
decay oxidizes the organic matter before its burial. Mocassin Ordovician lime-
stone of Tennessee, Hierlatzkalk (Jurassic) of Northern Calcareous Alps, Chapel
(Mississippian) limestone of the Llano Uplift of Texas, and the Cretaceous-Juras-
sic boundary beds of the Oman geosyncline of Arabia are all examples of pink
basinal crinoidal beds. The Ammonitico Rosso and Adneterkalk (Jurassic) are
conglomerate beds of red color produced on swells in the Alpine geosyncline.

Clastic Content in Carbonates

The small amount of terrigenous or biogenic siliceous clastic material in carbon-
ate sediment may be very significant in environmental interpretation. It may be
disseminated in the carbonate matrix, or present as grains, or concentrated in thin
discrete layers. The normal techniques of multiple acid etching on thin sections
and manufacture of insoluble residues in transparent finger plates for examina-
tion with a stereomicroscope have been previously described. X-ray defraction
mineralogical study is also a standard technique which may be employed al-
though not as a substitute for petrographic study. Since most carbonate rocks are
relatively pure, an increase in trace amounts of insolubles, particularly in the
coarser fraction, may indicate the landward direction. A specialized study of
carbonate insoluble residues was developed in the 1930's by McQueen (1931) of
the Missouri Bureau of Geology and Mines and reviewed by Ireland (1947). This
technique was used chiefly to discern correlation markers in Cambro-Ordovician
and Mississippian strata in the Midcontinent and Texas. Types of authigenic
chert and quartz and detrital sand grains were described in detail according to
texture, color, and included impressi9ns of fossils or dolomite rhombs. A com-
plete roster of descriptive terms for these is found in Ireland's paper. The causes of