Page 23 - Carbonate Facies in Geologic History

10 Principles of Carbonate Sedimentation

To repeat: because of the local origin of carbonate particles and the various
shapes and dimensions in which they may be produced, it is clear that textural
interpretation of detrital carbonates must differ from that of terrigenous clastics.
For example, in a mixture of65% lime mud with 35% sand to gravel size grains of
crinoids and bryozoan fragments, is the mud present because it was brought into
a site where crinoids and bryozoans were growing? Or, is so much mud present
because numerous algae or other potential contributors of fine material were
growing in the environment in competition with organisms which contribute
coarse particles? Such a question does not arise in terrigenous clastics but must
always be considered in carbonates.
The recognition that microcrystalline calcite (micrite) in limestones represents
what was originally lime mud and that many carbonate rocks in thin sections
could be studied texturally as detrital sediments should be credited to Folk (1959)
who proposed a classification and hierarchy of terms for limestones which is now
widely used. Many excellent and thoughtful essays on carbonate rock classifica-
tion, considering the philosophy behind textural groupings, may be found in the
American Association of Petroleum Geology Memoir I, edited by Ham (1962).
One of the best of these is the brief but significant paper by Dunham. Folk
reviewed and expanded his classification also in the AAPG Memoir I. The tex~
tural spectrum proposed in this paper embodies extremely useful concepts for
making environmental interpretations. Both classifications are given in Figs. 1-3
to 1-6 and are repeated in many general works on carbonate sedimentation. The
author generally uses Dunham's terms in this volume in order to avoid the
necessary rigidity involved in applying Folk's particle type prefixes and because
the packing concept of Dunham is important and should be designated by a
primary name.
Five main principles are significant in categorizing texture in carbonate sedi-
ments and are used in these classifications.

1. Presence or Virtual Absence of an Interpreted Very Fine-Grained Carbonate-
Lime Mud Matrix

The lithified equivalent is a fine mosaic called microcrystalline calcite (acronym
micrite) by Folk (particles 4-15 microns). Normally in sediment with some lime
mud the grain/micrite ratios or percent micrite are so variable as to make its
estimate almost meaningless. In nearby areas of the same bed, or even in the same
thin section, grain abundance may vary from 10% to 50% or more. Thus, it is
more important whether some mud exists than to know how much. Dunham's
concept (1962) is significant: focus on currents of removal, not delivery.
"The distinction between sediment deposited in calm water and sediment deposited in
agitated water is fundamental. Evidence bearing on this problem thus deserves to be in-
corporated in class names. This can be accomplished in several ways. One is to focus
attention on average or predominant size, which erroneously assumes that all sizes in a
sample are equally significant hydraulically. Another is to focus attention on the size,
abundance, and condition of the coarse material brought to the site of deposition. This
emphasis on what might be called currents of delivery has long been successful in dealing
with land-derived sediments. but does not work well in lime sediment because of the local
origin of many coarse grains. A third way is to focus attention on the fine material that