Page 30 - Carbonate Facies in Geologic History

Sedimentological Controls 17

aragonite needles, tiny detrital bits of high Mg calcite and coccoliths, and is 50%
water. The typical limestone micrite apparently equivalent to this is a mosaic of
3-4 microns or larger, more or less equidimensional, low Mg calcite crystals. It is
often dense, without measurable porosity, or perhaps chalky with the calcite
crystals loosely arranged. Modern lime sands deposited and cemented in entirely
marine environments contain interstitial fibrous aragonite or "dogtooth" Mg
calcite. Ancient lime sands are generally dense with intergranular space filled in
by later generations of calcite mosaic. Usually, ancient carbonate rocks show a
reorganization of crystal structure from the original grains and cement, as well as
complete mineralogical changes. Boundaries of former grains are recognized by
differences in their crystal sizes and shapes, or by color distinctions based on
inclusions and trace elements within replacement crystals or overgrowths. To
paraphrase St. Paul, "now we see through a glass darkly ... ", all the millions of
years of diagenetic alteration undergone by the typical carbonate rock. This is not
only because carbonate particles are very susceptible to solution· and mineralogi-
cal alteration when exposed to fresh or migrating connate waters, but also be-
cause in the sea, prior to final deposition, intense (bio) chemical and organic
activity can easily alter or destroy lime particles, or cement them into aggregates.
The importance of diagenetic study of carbonates is signified by the current
major literature on the subject. The contributions edited by Pray and Murray
(1965), Chilingar et aI., (1967), Purdy (1968), and Bricker (1971), are outstanding
modern references in this burgeoning research endeavor. Additionally, and inevi-
tably, classifications of carbonate porosity involve diagenesis. The outstanding
papers of Murray (1960), Powers (1962), Thomas (1962), and Choquete and Pray
(1970), may be cited. An outline of diagenetic events affecting calcium carbonate
sediment is appended to Chapter III.

Summary

This Chapter has briefly listed a few special qualities of carbonate sedimentation
which distinguish it from sandstone-shale deposition and which constitute impor-
tant principles to be considered in employing petrography aM stratigraphy in the
environmental interpretation of carbonate depositional patterns. Useful corollary
generalizations by Laporte (1969) have been added.

Sedimentological Controls

1. Most carbonate sediments are formed in a special depositional environ-
ment: warm, generally shallow, clear marine water. Despite the presence of shelly
calcium carbonate deposits in temperate and northern zones, lime sediment similar
to that found in the geologic record is today essentially a product of low latitudes.
2. Carbonate formation is essentially autochthonous. In Laporte's words
"intrabasinal factors control facies development."