Page 21 - Carbonate Facies in Geologic History

8 Principles of Carbonate Sedimentation

clues left in this residue of debris and sewerage, for purposes of environmental
reconstruction, is a fascinating, if difficult, chore. Excellent descriptions and illus-
trations of typical carbonate particles offering ecologic clues are found in Ma-
jewske (1969) and Horowitz and Potter (1971).
The fact that many carbonate particles originate locally and are created in
many shapes and sizes, requires quite different classifications and interpretations
of textures from those of terrigenous clastic sediments. Much extremely fine-
grained material is produced in situ as algal or chemically precipitated 2-4 micron
aragonite needles, and by microplankton (coccoliths) and tiny detrital carbonate
grains. But also the breakdown of calcareous tests of organisms by physical
abrasion, organic abrasion or corrosion through ingestion, or simple collapse due
to skeletal decay, produces numerous silt, sand, and gravel size particles in a wide
variety of shapes. Additionally, these particles vary in mineralogy and in internal
structure, which may control the ultimate shape and size of particles much more
than the agents of breakdown. Based on the structure of resulting particles, one
may recognize several types of skeletons or tests and their resistance to further
abrasion. This classification somewhat overlaps but is different in scope from that
of Horowitz and Potter (1971, Table 7, p.36), which is concerned with ultimate
shapes of particles for the purpose of identification. The following grouping by
R. N. Ginsburg, University of Miami, recognizes six types of skeletons on the basis
of their relative resistance to breakdown.
1. Sheathed and spiculed skeletons are those in which the mineral matter is
small (silt-fine sand size) and loosely held by organic tissue. On death of the
organism the organic tissue decays and the particles are released as fine sediment.
Examples: Penicillus, alcyonarians, corals, sponges, tunicates, holothurians.
2. Segmented skeletons consist of mineral particles linked together by organic
tissue. Death and decay of the organisms most commonly yields sand-size parti-
cles to the sediment. Examples : Halimeda, articulated red algae and echinoderms.
3. Branched skeletons are composed of well-calcified cylindrical or blade-like
projections. The size offragments found in sediments depends on the original size
of the organism, the size and strength of the branches, and the nature and inten-
sity of organic and mechanical breakdown to which they are subjected. Examples:
some corals (Acropora), red algae and bryozoans.
4. Chambered skeletons include all those that are hollow or partly hollow.
Chambers persist after death of the organism, but there are wide variations in the
resistance of different types of chambers to breakdown, depending on their abso-
lute size, wall thickness, shape, and microstructure. In general, the arcuate shape
successfully resists breakage. Examples: gastropods, serpulid worm tubes, forami-
nifera, some crustaceans, pelecypods, some echinoderms and brachiopods.
5. Encrusted skeletons include all plants and animals that encrust surfaces.
The breakdown of the skeletons in most cases depends mainly on the organic
breakdown of the encrusted surface or substrate. These are mechanically resistant
structures. Examples: some algae, foraminifera, corals, bryozoans, worms, hydro-
corallines.
6. Massive skeletons are generally large and hemispherical in shape. They are
most resistant to breakdown because of their size and in some cases, their micro-
structure. Examples: corals and some coralline algae.