Page 18 - Carbonate Facies in Geologic History

Principal Hydrographic Controls on Carbonate Accumulation 5

are derived from biological or mechanical attrition of shell material, but the
origin of aragonite, which constitutes generally more than half and in places up to
95% of lime mud, is much debated. The arguments focus particularly on the
extent to which tiny (4 micron) aragonite needles are produced from the break-
down of codiacean algae or to whether they can be inorganically precipitated.
Data indicate that whatever its origin, much, if not most or all, fine lime sediment
is organically derived.

Most Coarser Lime Particles also Have an Organic Origin

Much sand- to gravel-size carbonate sediment forms from breakdown of shells
and tests (bioclasts). For example, mollusks, green algae, modern corals and many
foraminifera today contribute aragonite particles; other foraminifera (miliolids
and peneroplids), red algae, and echinoderms contribute high Mg calcite. A small
amount oflow Mg calcite is contributed by breakdown of brachiopod, bryozoans,
ostracods, foraminifera, and additionally by trilobites and rugose corals in the
Paleozoic. Lime mud may be moulded or aggregated by organisms into fecal
pellets and grapestone lumps and transported as very low density sand grains.
Organisms also contribute indirectly to construction and modification of sand
size particles. Bioclasts are rotted by microboring fungi, algae and sponges, and
altered into peloids (rounded homogeneous micritic sand-size particles). Carbon-
ate particles are also formed as larger fragments by collapse of burrows or desic-
cation of algal mats. Thus, organisms contribute directly or indirectly to construc-
tion of practically all major carbonate particle types. This is even true in ooids;
coating blue-green algae playa role in the construction of the concentric laminae
which are probably formed from precipitated aragonite.

Biologically Precipitated Carbonate Masses Are Abundant
in the Geologic Record

In addition to forming both fine and coarse detrital particles of carbonate, many
primitive sessile invertebrates, as well as algae, are capable of direct secretion of
carbonate in or around their tissues to form various types of massive and rigid
frameworks and encrustations. Coelenterates (hydrozoans and anthozoans),
sponges, coralline red algae, bryozoans and mollusks are important in this regard.
This ability to create rigid boundstone (Dunham, 1962) or biolithite (Folk, 1959)
accounts for considerable local buildups of carbonate (organic-ecologic reefs) and
is a unique feature of carbonate sedimentation quite different from any mode of
terrigenous clastic deposition.

Principal Hydrographic Controls on Carbonate Accumulation

Once carbonate sediment is formed, it is subject to the same processes of marine
sedimentation which affect terrigenous clastic sediments. These are especially
operative in open sea positions or along the shelf edges of marginal (pericratonic)