Page 102 - Carbonate Facies in Geologic History

Significance of Color in Carbonate Rocks 89

deposition probably below storm wave base in a zone where finer particles (i.e.,
lime mud) were being winnowed away. The area was a shoal above and on the
edge of an earlier formed algal plate mud mound. Settling of finer, mostly silt-size
sediment, occurred as it was deposited with the framework of larger grains, per-
haps during a single storm. Lithification began early with finer grain silt sediment
settling down through the pack, some moving to fill cavity floors. Calcite cement
filled pores of all sizes forming large areas of calcite in-fills or in places with more
lime mud, a generally clotted, grumelous matrix.

Significance of Color in Carbonate Rocks

In outcrop and core study much time can be occupied with a standard color chart
describing precise tone and hue, for color is an easily observable though variable
attribute of rock and is useful for environmental interpretation (Weller, 1960,
p.129-141; Krumbein and Sloss, 1963, p.123). However, variations due to grain
size of matrix, crystallinity, pigment content, and weathering are great, and de-
tailed color description is generally useless. For carbonate rocks only three basic
colors are important indications of environment: light, dark, and reddish hues. In
some fine-grained rock of even texture, only a trace of pigment is necessary to
create pronounced coloration. Absolutely black limestone of unusually fine crys-
talline texture (e.g., Marbre Noire of Belgium) contains on the average only 1-2%
insoluble matter of which but 0.2% is organic carbon. Light colored limestone
therefore is almost completely pure calcium carbonate lacking any trace of pig-
ment. Its color is commonly enhanced by oxidation and weathering, traces of
Fe 20 3 giving usually a buff or faded yellow or cream color. In quarries of shelf
limestone deposits, the water-saturated rock is normally gray, particularly if fine-
grained.
Shallows bays . and lagoons are surfaced with lime mud which is generally
oxidized during turn-over by burrowing and perhaps by oxygen produced by
photosynthesis of blue-green filamentous algae. Burial of this material under a
few cm of sediment and removal from the O 2 of sea water results in a rapidly
acquired reducing condition with production of H 2S, a gray color, and blacken-
ing of some organic rich particles. Decay of buried organic slime in the mud
causes the reducing environment which presumably contains active anaerobic
and sulfate-reducing bacteria. Such shallow water muds, however, are deposited
with only a few tenths of a percent of organic carbon. Their color is basically
medium gray. It is obvious that a truly dark gray, dark brown, or black carbonate
must result from much more reducing conditions and somewhat higher content of
organic carbon than normally observed in shallow shelf lagoons. Shelf limestones
traced into miogeosynclines or basins generally darken and become almost black
due to preservation of trace amounts of organic matter and ferrous sulfides thor-
oughly disseminated in the rock matrix. Only 1 or 2% is necessary for thorough
pigmentation. The preservation is probably due to more rapid burial and more
uniform marine conditions with no chance for periodic oxidation due to storm