Page 20 - Carbonate Facies in Geologic History

The Local Origin of Carbonate Particles - Textural Interpretation and Classification 7

in restricted environments for most marine animals. It also results in more vari-
able and extreme salinities when it occurs in an evaporative climate. PeUeting of
mud is important in creating some sand and silt in the lagoonal mud sediments
where mollusks, certain algae, foraminifera and ostracods dominate the biota and
contribute sand size-constituents. In the intertidal zone, a large group of distinc-
tive sedimentary structures forms in the lime mud by intermittent marine flooding
and drying due to subaerial exposure. In evaporative climates, calcium sulfate and
dolomite commonly form and in tropical rainfall areas, fresh water lenses provide
springs and swamps rich in brackish water plants which may precipitate non-
marine, low Mg calcite.

The Local Origin of Carbonate Particles -
Textural Interpretation and Classification

Because most carbonate sediment is organically produced, it is fundamentally
autochthonous, i.e., it is produced within a given basin and not introduced from
without by rivers or streams. Deposits of detrital carbonate sand or silt grains do
exist but are extremely rare because of the great solubility of CaC0 3 in fresh
water, particularly that with dissolved CO 2 • Most coarse carbonate grains do not
move very far unless spilled over the sides of steep shelf margins or transported by
longshore drift parallel to the coast. Most appear to accumulate as detrital parti-
cles fallen from growth positions or remain where the producing organism died
and decayed, and later moved very little laterally. This is demonstrated in biologi-
cal studies by Ginsburg (1956) across the Florida Reef tract and McKee et al.
(1959), in Kapingamarangi Atoll in the Pacific. In both cases biogenic particles in
the bottom sediment, despite extensive bioturbation, reflect rather faithfully the
general aspect of present day living biota and hence the environmental conditions
(salinity, circulation, temperature, depth, substrate, etc.) of the sea. Even ooids
created in tidal bars accumulate more or less where formed. The same constant
ebb and flow of water which causes accretion of the particles also constructs the
tidal bars in belts or in more or less fan-shaped areas. These build up and remain
at select places on the shelf margins, especially where water movement is in-
creased by vertical or lateral constriction. Some tiny particles comprising lime
mud are probably transported many miles by storm-wave currents and may
accumulate in especially protected localities in a given basin (deep or very shallow
water), but can also be produced and accumulated locally in wide lagoons or
shallow shelves.
Naturally the autochthonous origin of most carbonate sediment offers a
great advantage in its environmental interpretation and increases geological in-
terest and emphasis on identification of particle types, particularly in thin section
study. A whole discipline of carbonate petrography has developed beginning with
Henry Sorby (1879), and has increased in emphasis tremendously since about the
middle 1950's to the present day. Bathurst (1971) has pointed out that the sea
floor sediment represents but an insignificant remnant of the teeming variety of
organisms and ecological systems which contributed to it. The unravelling of